My Experience Setting up a Single HPMX 40 Carb
Disclaimer: By no means should any of this information be regarded as reliable or even factual. My experience with automotive mechanics is very limited - I'm simply stating my experience. I really don't want to feel responsible for anyone losing money or injuring themselves as a result of taking any of this as 'advice.'
I also want to point out that if you don't like the idea of potentially modifying expensive components and you aren't doing this for the love of it, then perhaps you should take your VW to a certified VW mechanic and have them work on your engine for you. I am not an expert at any of this. I just try to use common sense, and whatever logic and rationale I can conjure up in my own head. I am still a newbie and I will continue to be a newbie for another 50 years.
If any (more senior/experienced?) members would like to comment on something or point out inaccuracies, that would be appreciated and I'll try to update my post to include any corrections/modifications.
Motivation
I decided to get a single HPMX carb for a number of reasons. Cost, in fact, was not one of them. Although the cheaper price tag compared to an actual Weber IDF may be the deciding factor for most people who buy an HPMX, for me it was just about availability and naivete. I didn't realise they were crap until I bought one and tried to set it up. Once I realised it was going to be a pain, I started to do some more digging and found forum post after post saying how bad they were (poor build quality, sub-standard components, inaccurate jets, incorrect float level settings, manifold is too long, etc, etc...)
Despite a number of rather sane and good reccomendations to dump the carb and get a worthwhile setup (dual Kadrons, Dellortos or real Webers) I decided to press on. My desire to learn 'why' and 'how' things are broken outweighs my responsibility to my bank balance. So I dug deep and found a few posts from people who seem to have good luck with the HPMX. Some of the information seems to be way off, and some of it was very helpful. Most of it was for dual carb setups, so I attribute the different opinions in jet sizes and venturi to that.
I also purchased a Weber tuning manual which has a farily decent explanation of the basic concepts of how a carb works, as well as exploded diagrams and parts lists for the 40 IDF which EMPI copied to make the 40 HPMX.
I was terrified of tuning carbs beacuse I knew nothing about jets or floats - I didn't even know where they were. But between the Weber manual and google, I've learned quite a bit. If you're in the same boat and you're just afraid to tackle a performance carb setup because you don't know where to start, just get into it. Just do it. Yes, you will have to buy a bunch of jets that you may never use afterwards. Yes, you may have to dismantle your carb several times to change a venturi or clean it out. Yes, you will have to drive around and look like an idiot for a few minutes, bucking and coughing smoke when the jets are wrong. But these are all just parts of owning a classic car and a performance engine. Once you get the motivation to do it, its a rewarding experience.
Tuning a carb is an art form. Yes, you can use some basic math to get a "starting point" for jet sizes, but this will only get you close. You need to run the engine and use trial and error to find which combination will work for you.
In the end, I can't say that the HPMX carb is particularily good, but I can say that it works adequately for me. A *lot* of people have had a *lot* of shitty experiences with them but I'm not sure how many of them have bothered to take the time to fix the issues rather than just go online and post their rants about it. Perhaps I'm luckier than most (despite the fact that I do have some complaints about them) but the end product is that my engine runs without backfiring or choking, downshifts beautifully, and there are no flat spots.
I won't argue with anyone about it. I still agree that I could probably get better performance out of a dual setup of just about any other carb. I just want to do my bit to help save any other newbies who, for whatever reason, have wound up with an HPMX.
My Initial Setup
Type 1 Case (1776cc Displacement, Engle 110 Cam, Stock stroke crank shaft, Stock rocker arms)
Cylinder Head: 40mm Intake (Probably too large for the application) 35 Exhaust
'009' distributor.
Single HPMX 40 carb
Main Jet: 135
Idle Jet: 60
Air Corrector: 180
Float Level: 13mm
Venturi: 28mm
No Fuel Pressure regulator (stock fuel pump = 6+psi!)
My Final Setup
SVDA distributor connected to vacuum advance port on carb.
Single HPMX 40 carb
Main Jet: 160
Idle Jet: 55
Air Corrector: 200
Float Level: 10mm
Venturi: 28mm (ideal might be 30-32)
Pressure regulator set to 2.5 PSI
Heat riser tubes allowing exhaust gas through manifold
My Plan
First, I just put the carb in and hooked everything up. It idled irratically and when I actually got it onto the road, it was coughing and wheezing and hacking all over the place. I had big flat spots, acceleration was non existant, and it coughed and choked under load. It was useless.
I was beside myself with anger. It was spring and I wanted to drive my car. Not wanting to be defeated, I tore everything apart and decided to go over every piece of the equation bit by bit.
Below is the list of things that I identified as problems or potential problems, in addition to the procedures that I used to set things up.
Findings / Problems
Distributor
I was using a cheap imitation '009' distributor which is only mechanical advance. I'm not sure why these things are even made. I had set the timing at 30 degrees @ 3500 RPM with a timing light so I assumed everything was correct. While this may not be technically accurate, my feeling is that the mechanical advance carbs dont advance quickly enough or provide ENOUGH immediate advance to give me the kind of throttle response that I want. Since a Vacuum Advance distributor will advance as soon as there is extra air flow as opposed to 'reacting' to increased RPM, it seems like the way to go. Since the HPMX has a Vacuum Advance port in it already, I decided to get an SVDA just so I wouldn't have to worry about the potentiality of the '009' causing any of my performance problems.
If you want the most from this carb (or any carb, for that matter) get a decent distributor. Even if you dont end up keeping the HPMX, a good SVDA distributor is a wise investment.
Air leaks: Intake Manifold
The HPMX "installation instructions" (which are woefully inadequate) indicate that the manifold that connects to the end castings "may be too long." While I can expect that they do this so that the manifold could be used on larger (wider) engines, I still think its a rather ridiculous. Sure enough, I didn't check the length of the manifold before I installed it. When I put the end castings on, they were not sitting flush against the heads since the manifold was pusing them out. I didn't notice this, since the manifold boots were already on. This caused a massive air in-leakage around the cylinder heads and was responsible for a great deal of my initial performance issues (rough idle, etc.) Either shortening the intake manifold or the two end castings will allow them to sit flush against the head. I also got two new seals for the manifold to end casting transition to rule out any leak.
This allowed me to get a much smoother idle and I felt like my problems had been solved as the engine idled fine and revved fine under no load. I took it for another test run and found that, under load, it was just as useless.
Fuel Pressure at 6 PSI
My new stock fuel pump was putting out about 6psi. Not only is this too much for the carb (which likes about 3psi) but the pressure was bouncing all over the place. I ordered an adjustable fuel pressure regulator from California Import Parts and a fuel pressure gauge from JEGS.
I set the fuel pressure regulator at about 3psi. The pressure still 'spikes' slightly, but it isn't nearly as bad as it was without the regulator.
Please note that fuel pressure regulators seem to be assembled by hand and NONE of the screws and bolts are tightened. This includes the top adjustment screws, the bottom screw, the two inlet/outlet valves and the four screws that hold the casing together. To prevent leaks around the inlet/outlets, I removed them and used teflon tape - I'm not sure if the gas will eat away at this, but so far it does not leak. Please check the four case screws and make sure they are tight, otherwise you will get fuel bubbling up out of them, and spewing all over your hot engine.
Heat Riser Tubes: Intake Manifold
The EMPI intake manifold has holes for heat riser tubes which are not drilled all the way through. This means that the heat risers are basically there just to hold the carb in place. This doesn't seem to be something that any other forum post mentioned. Since the manifold is blocked, exhaust gases can't actually pass through the manifold and warm it. This will severely limit the amount of heat in the manifold, prolonging warm-up times and promote icing inside the manifold.
I got a foot long drill bit and opened the manifold through so that both sides were connected. This isnt something I'd reccomend you do, but I did it and it seemed to help. I'm not sure if this was just a defect with my particular manifold or not. I've seen some manifolds modified so that the tubes go in one side, then travel up the length of the manifold, then back down and out the other side, but this does not appear to have been the case with mine.
A lot of people (with much more experience than I have) are saying that the intake run is just too long and that causes too much of the fuel to fall out of suspension before reaching the cylinder. Assuming that this was an issue with the stock intake manifold (which is the same length) and that heat risers were the VW solution to the problem, then I feel like this should be adequate solution for an HPMX also. I'm not sure why I haven't heard anyone mentioning that the heat risers are inadequate (being completely blocked off in the middle of the manifold) or mentioning that there is a simple solution to that problem.
Heat Risers: Exhaust Manifold (4-1 Collector)
The heat riser tubes on the exhaust collector were not drilled through either. This (I am told) is common since a dual carb setup won't require them, manufacturers won't drill the riser tubes all the way through. Easily fixed. Now, exhaust gas can pass through them, into the manifold, giving adequate heat which will help keep the fuel in suspension and prevent icing.
Throttle Jet Threads Stripped
While disassembling the carb to inspect and clean it, I noticed some fairly horrible workmanship. I'm not a machinist, but I can tell when bolts are stripped. It was nearly impossible to remove one of the throttle jets (which is neccesary in order to pull out the venturi) When I finally managed to get it out, the first 3/4 of the bolt were clearly stripped - the threads in the throttle jet gallery were garbage. I measured the thread and bolt sizes and chased the threads several times. The amount of garbage that came out of the hole was *astounding.* Once it was cleaned and the bolt threads were chased, the bolt went back in without a problem.
There's no way of knowing if this actually was a performance issue but generally, if there is dirt and crud inside your carb, things aren't going to run as well as they should.
It sure is a bummer when you realise the shoddy craftsmanship that goes into a piece of equipment that you've paid a lot of money for.
Idle Jet Size
Stock setup was 60, which I left for a while. By the time I had gotten up to 155-160 main jets, the mixture adjustment screws were fully closed, meaning that my idle circuit was running way too rich. I decided to go down one size on the idle jet to 55. This allowed me to open the mixture screws back out so I could tune the mixture again.
If you need to turn your mixture screws OUT more than 2-2.5 turns for optimal RPM, then go up one size of idle jet.
If your mixture screws are bottomed out or only about 0.5 turns open for optimal RPM, then go down a size of idle jet.
Main Jet Size
The stock setup had 135 main jets. Since the engine idled well but began to choke and lack power under load, I decided to go up to 150 Main jets as a starting point.
Around 2000-3000 RPM is when the main jets start to kick in.
Assuming your idle mixture isnt too rich or too lean, and you are driving under load:
If the engine chokes and coughs, spewing black smoke / smelling of fuel: Its too rich, try going down one size on your main jet.
If the engine chokes and coughs, with no black smoke / no fuel smell: Its too lean, try going up a size on your main jet.
After some trial and error, I went up to 155, and eventually 160 main jets.
Air Corrector Size
As I increased the main jet size, I also increased the air corrector to 200.
My (very basic) understanding is that the air correctors come into play after the main jets. That is, at RPMs higher than 3500-4000ish. Since I hadn't really had the engine up that much, I decided just to put it up to 200 and see how things went. So far, I haven't changed this.
If you're too lean above 4000 RPM, then put in a smaller Air Corrector.
If you're too rich above 4000 RPM, then put in a larger one.
Venturi Size
Stock size is 28mm. I've read a few posts (ie: Mark Harney's post on theSamba) which indicated that one should subtract 4-6mm from the size of the intake and use that as a venturi size. He was, however, speaking of a dual carb setup and I'm not sure the advice really applies to a single carb setup at all. Since I had 40 mm intake ports on my heads, that would call for 34-36mm venturi which would be ridiculously too big for my 1776 engine. (One could argue that 40mm intakes are too big for a 1776cc engine already, but thats what I've got. So there.)
I think its probably a danger to choose venturi size based primarily on the size of an intake port since displacement (the AMOUNT of air that passes through) is also a factor in the airspeed through the venturi. A 2 Litre engine with 40mm intake ports will have a much greater airspeed through the venturi than a 1.6 Litre engine with the same intake size.
A smaller venturi will limit your top end response a bit, but if you notice this, you can start playing with a larger one. In terms of just getting the carbs running for normal use, I stuck with 28 mm for now with plans to try a 30 or 32 in the future. Venturi are a bit more expensive to swap out than jets and you have to dismantle the whole carb to get them out. Additionally, as soon as you make a change to your venturi, all of your jets are likely to need changing also.
Float Setting
Float setting is supposed to be about 10mm. When I measured mine, it was about 13-14mm. Ridiculous. You can't blame EMPI for this, however, since shipping and handling can cause the little metal prongs to bend. If thats the case, it just goes to show how much your carbs were smashed around during shipping.
(See below for how to check float level if you haven't done this before.)
Throttle Linkage Construction
Not exactly a performance problem, but the bushings for the throttle linkage and cable clamp all utilize very tiny allen screws. I'm not sure why they went this route since it makes adjusting anything a real pain. I used several bolts in place of the allen screws and swapped the throttle cable clamp with a bolt style clamp also. Adjusting these things in the field will now be easy since I'll always have an 8mm wrench while I may not have the two different sizes of allen screws that they required previously.
Throttle Return Spring
Again, while not exactly a problem with the carb, it was definitely annoying that the return spring didn't have nearly enough strength to return the throttle back to the stop (this is, after all, the fault of my nasty old throttle cable which defies my attempts to lubricate it.)
Using a bent piece of metal from a mechanical choke mounting kit, I rigged up an additional spring on the secondary throttle lever to pull it back into position.
Pump Bypass Valve
This isn't something I've tackled yet, although it appears to be one of the "fixes" that has been done to make the HPMX 'work'
My understanding of the bypass valve is that it allows fuel to return back into the carb rather than ejecting it all out of the accelerator pump jets. The fix appears to be meant to close this valve to allow for a longer jet of fuel to be injected during heavy acceleration.
The HPMX seems to be plagued with stumbles and flat spots, and I think the purpose of prolonging and maximizing the acceleration jet squirt is to combat the flat spots. If your accelerator adjustment doesn't give you the amount of squirt you need, then this might be the problem. (I believe Redline Weber offers a kit with a zero bypass valve which will do this.)
How-To's
Keep in mind that I'm not an expert by any means. These are just the procedures that I've used to set up my single 40 HPMX, and they appear to work for me. If they're wrong, please comment.
Check Float Level
The float level determines the amount of fuel that is stored in the carb. It's controlled by a floating lever which opens and closes a small ball-valve that allows fuel into the carb. If this setting is incorrect (or if there is too much fuel pressure) then the carb could be force-fed too much fuel, or it could be starved.
The measurement is taken by holding the cover plate vertically so that the floating armature is hanging down. You want to orient the cover plate so that the small ball valve is just in contact with the armature but not being depressed by it. You then measure the distance between the cover plate and the top of the float. The distance should be about 10mm with the gasket still in place.
There is a small tab of metal that contacts the ball valve, and it can be gently bent in or out to adjust this distance.
Disassemble / Reassemble
The best way to familiarize yourself with the carb is to take it apart. This image shows the relevant jets and adjustments as well as their locations. The only reason you'll need to take the cover off is to clean/swap the accelerator jets or to replace venturi. The odds are that your accelerator jets just need to be clean, and the stock size will be OK for average users.
(Lifted from Redline Weber and simplified - also available from the Weber Tuning Manual)
Tuning / Setup Procedure
(Lifted from Redline Weber - also available from the HPMX "Assembly Instructions")
Turn the idle speed screw out until its not touching the throttle linkage. Turn it in until it touches the linkage, and then go about 1/2 turn in.
Loosen the lock nuts on the air screws, lightly bottom them out and then tighten the lock nuts again.
Lightly bottom the mixture screws out and then turn them back out 1.0 - 1.5 turns.
If this is the first time the engine has run (or if you have replaced the distributor) make sure that you have the timing set at (at least) 7.5 degrees BTDC. You'll need to set the timing with a strobe light later - this is just to get the engine to run.
If you can get the engine to run here, it'll probably sound like a bag of hammers, but as long as it idles, thats fine. In order to get it to idle, you may have to sit there with your foot on the throttle for a while until it warms up. Don't use the idle speed screw to adjust anything yet, since you'll be "faking" the carb into running better than it should be, before you get a chance to tune it.
The engine should be warmed up completely before adjusting anything else. If you can't get it to idle on its own, try opening up the mixture screws a bit.
Once idling and warm...
Tighten the mixture screws in 1/4 or 1/2 turn at a time, until the engine starts to hesitate and it wants to die. When you can hear the RPM go down, back the screw off to the last position where it ran nicely. If the engine is already running rough, this may be hard to hear, but it will get easier as you progress. Once you do one mixture screw, go back and do the other. Each time you adjust one, check the other to see if anything has changed. Ideally, what you want is to set it up so that opening or closing either of the screws causes the engine to run worse (lower RPM.)
I think you can do this same procedure only in reverse, by opening up the mixture to find the highest RPM rather than closing it to find when the engine wants to die. Just use the same procedure for both mixture screws. (I'm not sure about which is best, but I assume that leaning the mixture screw in to find out when the engine dies will probably settle on a leaner overall mixture rather than opening them both up to find where the highest RPM is.)
If your mixture screws are turned out more than 2-2.5 turns, then go up a size of idle jet. Your idle is too lean.
If your mixture screws are bottomed out or are about a half turn open, then go down a size of idle jet. Your idle is too rich.
At this point you should be able to get the engine up to 3500 RPM (lets make noise!) and use a timing light to set the distributor at 30degrees advance.
This should get you onto the road and you can start testing the Main Jets under load.
In 2nd or 3rd (while holding RPM or *lightly* accelerating) if it chokes and coughs, watch in you rear view mirror for any smoke. If you have a tachometer hooked up, you want to get the engine up around 2500-3000 RPM so the main jets are in effect, while under load.
Spewing black smoke / Smell of fuel: Its too rich, try going down a size on your main jets.
No black smoke / no fuel smell: Its too lean, try going up a size on your main jets.
If you're pushing the throttle wide open, or pushing it down quickly, you might be getting different results just because of the accelerator pump. When you're trying to figure out what main jets you want, try to avoid quick bursts of throttle and work off a steady RPM range. If its choking and coughing, obviously it will be hard to maintain the RPM but you need to avoid quick bursts of throttle otherwise those accelerator pump fuel squirts might mislead you to think that your main jets need the wrong adjustment than they actually do. Once you've figured out the proper main jet, you can work on accelerator pump settings for a bit quicker throttle response.
If, when you stomp on it, the engine bogs but recovers, keep watching for that black smoke and smelling for fuel. If its too rich, try leaning out the accelerator pump by backing the nut off a few turns. If you don't notice any smoke or fuel, then it might already be too lean and you should try adjusting the nut in a few turns so it delivers a bit more fuel. If it doesn't bog or cough, then your accelerator pump setting is probably ok.
Accelerator pump adjustments:
Adjustment Nut In = more fuel
Adjustment Nut Out = less fuel
Conclusions & Recommendations For Other Newbies
Get over your fear of jets and not knowing anything about your carb. Once you've taken it apart four times, you'll have a much more intimate relationship and you'll be glad you got over your fear. The only way to maximize performance is to use a bit of trial and error to get the optimal jet setup for your particular engine and vehicle. Don't just get one set of jets based off of what someone else is running with a 'similar' setup. You can ask around to find a good 'starting point' but you'll need to test different sizes of jets to find what works for you. The HPMX allows easy access to your jets and its not very hard to get at them to swap out. Venturi are a bit harder, but you likely won't be swapping these in and out very much at all.
Don't even put the carb on until you've dismantled it and cleaned everything!
Use your air filters. PLEASE. All the cleaning you just did can be completely undone by driving down the road without an air cleaner on your carb.
Make sure that any rubber O-Rings are in good shape, not cocked to one side or broken.
Don't trust the factory jet sizes to be what they are stamped. The odds are, you're going to be changing them anyway so either measure them, or get jets from a reputable source.
Get a few sets of jets. Yes, its going to cost money, but you can sell them off for nearly what you paid for them as long as you dont damage them. If you plan on building more engines in the future, or making changes to your current setup, its always nice to have a selection of alternate jets.
When removing jets from holders, using a electric drill chuck to lightly grab cyilindrical surfaces works well. Just be sure to wrap it in something (fine grit sand paper?) to prevent damaging the surface of the holder/jet and dont use too much pressure. Working the jets out by trying to rotate them side to side first will help prevent wrecking them if your pliers/vice grips slide off prematurely. Putting a cloth around the jet will help prevent damage too. When swapping a lot of jets in and out, you don't want to wreck a bunch and end up with a pile of useless jets that you paid 7 bucks a piece for.
If you're at all interested in performance and maintaining your VW, invest in some tools. A strobe timing light is a neccesity! A good tap and die set will likely come in handy if you're pulling apart these carbs since the build quality is questionable at best. Being able to chase threads means you can salvage some parts that were mashed during rushed assembly in an EMPI sweat shop 
Take a look at your intake manifold's heat riser tube and see if it goes through both sides, or look for openings inside the tubes that indicate that the manifold draws the exhaust gases up towards the carb. If there are no holes (up or through) then the exhaust gas isnt going to go anywhere and you'll seriously limit the amount of heat thats coming up to the manifold. Again, check with someone who has more experience than I do before you go drilling it out. In my (limited) experience, it seems to help. I've driven the car in cold and warm weather and as long as its warmed up for a few minutes, it hasn't given me any problems.
Test fit your intake manifold (without boots) and bolt the end castings on to the heads so that you can see if they are touching the manifold. If they're touching, get out your grinder and start shaving down the manifold or the end castings until they sit flush on the heads and the manifold isn't holding them open.
Get a decent Vacuum Advance Distributor. If you're trying to get performance out of a '009', then you're more hard headed than me and I wish you luck. It will likely never react as fast as neccesary and since the HPMX already has a vacuum advance port installed, I can't see any reason not to utilize it.
Set your timing properly at 30 degrees @ 3500RPM with a good strobe light. I have a digital light that shows me my RPM, etc... Wise investment!
Invest in an adjustable fuel pressure regulator and a 1-15PSI gauge. Even if you end up ditching the HPMX, Kadrons and other carbs still only like about 2PSI of fuel pressure. Once the regulator is set, you can remove the guage and use it on another engine if need be. Any decent plumbing store should have barbed ends and a T-fitting that will fit the guage.
Air filters are there for a reason. Garage testing your engine without one is fine, but do you really want to drive down a dusty road, sucking particles into your nice clean carb, screwing up all the tiny adjustments you just made to regulate minute amounts of air and fuel mixtures? Yes it looks cooler without them, but!
If you have the money, go for a real Weber. If you're lucky, there won't be any build-quality problems with your EMPI, but the odds don't seem all that good. Any single carb setup will have the same issues with manifold icing and it'll have to be addressed regardless. If you have the money, go for DUAL Weber's.
Parts
http://www.cip1.ca/ProductDetails.asp?P … 26-127-005
Adjustable Fuel Pressure Regulator - California Import Parts (Canada)
http://www.jegs.com/i/VDO/918/153002/10 … tId=745270
0-15 PSI Fuel Pressure Gauge (which you'll need for the California Import Parts regulator to see where it's set.)
http://www.jegs.com/i/Trans+Dapt/969/2329/10002/-1
Adjustable Fuel Pressure Regulator - JEGS (Easy dial so you don't need a gauge!)
http://www.carbkitus.com/
Main Jets, Idle Jets, Venturi, Emulsion Tubes, etc... - Chirco Performance
Suggested Reading:
http://www.aircooled.net/gnrlsite/resou … etting.htm
Excellent explanation of Jetting Theory by John Connolly. In fact, you should have read that before you read my post, since he actually knows what he's talking about
http://www.redlineweber.com/html/Tech/i … ntrols.htm
More detailed explanation of setup and tuning from REDLINE Weber
Last edited by thebignic (2010-04-27 11:48:20)
-biggie
