TheSamba.com :: View topic - Newest Chinese 34PICT-3 carb questions.

Sat Jun 21, 2025 8:31 pm

Hi all,

Thank you for taking the time to read this and offer suggestions. I'm well aware of the fairly bad reputation that the 34PICT-3 non-German knockoffs have but chosen the J-Bugs house version to help move the project forward more quickly than rebuilding my German carb as Volksbitz was about 6 weeks out. Also, I want to make clear I'm not throwing any shade on J-Bugs or any other provider of these carbs.

This carb is mounted on a 1600 DP motor. Stock compression ratio. Aftermarket air cleaner and exhaust, and a 123 electronic ignition. I'm also using a Malpassi fuel pressure regulator set to 1.5 PSI being fed by a rebuilt Pierburg mechanical pump.

Here's the problem: After shutdown (whether it's after just enough time for the choke to open or after 3 hours of driving) the accelerator pump discharge nozzle will drip onto the closed throttle plate. Some will exit through the throttle x-shaft, some will get around the slightly opened throttle plate and wind up in the manifold. It will do it even if I pull the fuel line (slight back pressure noticed) immediately after shutdown. Today I pulled the top of the carb off after shutdown and verified the fuel level is correct in the bowl. I've also verified the vents that go from the lower part of the float bowl to the top of the lower carb are clear. The carb top gasket is not covering any vent opening, it's correct. But...if I wait until it quits dripping (maybe 10-15 minutes) and pull off the top of the carb the fuel level will have dropped to the level of the vent drillings which are about 1/2" up from the bottom of the float bowl. I've also verified there is a check ball in the accelerator pump circuit.

I only have speculation as to what is causing this. Any ideas? Please note I've read about everything there is in The Samba tech forums, Aircooled.Net, and a couple of others. I suspect something isn't right either with the venting or the accelerator pump check valve, but nothing is presenting itself.

Thanks in advance, Gary

Sat Jun 21, 2025 11:25 pm

You seem to think this is a problem, and maybe it is
but I don't consider that to be abnormal.
probably most carbureted vehicles did that when you shut them off hot.

Some carburetors had a tiny bleed hole from the pump cavity to the float bowl for the purpose of preventing this percolation, but I don't think the vw solex carbs had that, tho I could be wrong.

It also has a bit to do with the gasoline itself, having the right vapor pressure for the temperature and altitude. It's never perfect.
There are folk remedies and products to dose your fuel with help either way, if it's not vaporizing enough or vaporizing too easily, and I don't really recommend any of that.
I think its best to figure out how to USE the fuel you CAN get.
And if that means shutting off the fuel pump a block or two before you get home, because somebody doesn't like the smell in the garage, then just do that. Or if the carb is getting too hot make it cooler somehow. Or whatever.
I really doubt many VW owners actually parked these things in a garage or on concrete. Modern problems.

Sat Jun 21, 2025 11:50 pm

Acc. pump activator lever is not releasing.

Sun Jun 22, 2025 12:36 am

To Modok:

Losing a shotglass or two out of the float bowl upon shutdown is neither a first world or old world problem. It's simply wrong. That much gas down the manifold wrecks oil, and therefore wrecks rings and bearings. I don't mean to suggest you're an idiot, but honestly this cannot be your best answer, is it? You're suggesting this is a fuel problem? Really? I have a mechanical fuel pump, how do you imagine I might shut off the fuel a block before stopping the car? That's really not a workable suggestion.

I appreciate you weighing in, but this doesn't offer really any help. Thanks anyway...

To Alstrup: the arm is definitely releasing....

Thanks all,

Gary

Sun Jun 22, 2025 2:50 am

I think the accelerator pump check ball could be the issue, or maybe the linkage is hitting the alternator or generator keeping pressure on the diaphragm. I would want this problem fixed for sure, good luck.
Tony

Sun Jun 22, 2025 8:19 am

Thanks for that last tip on the accelerator pump linkage. It's very close to the generator (about 1mm of clearance) but doesn't hit through it's range of motion.

I'm not 100% clear on the full function of the check ball. For sure one of its roles is to block fuel from flowing backward into the float bowl when you push the throttle open to accelerate. But is it also supposed to also keep fuel from flowing through the accelerator pump circuit when the throttle is closed? If so it's not working.

Also, given that the level of the accelerator pump discharge is higher than the fuel level in the float it seems like it shouldn't flow through the circuit when the engine is shutdown unless it was under pressure. Maybe I'm misunderstanding how the float bowl vents work as I thought they keep the fuel in the float bowl from being under pressure at any given time of operation or shutdown?

Anyway, thanks again for the tip!

Sun Jun 22, 2025 8:48 am

I cant help with the specific question at hand as i wont use pict carbs, just cant tune one to save my life and typically dont build anything theyd be very good on anyhow. but, easiest way ( for me ) to understand a carb and whats going on in them is to look at the diagrams for them and see how the circuits run, things make a lot more sense then

Sun Jun 22, 2025 9:25 am

Yes, it's a fuel problem.

Modern gasoline is designed with modern cars in mind: closed fuel tanks and vent systems to keep vapours from escaping to the atmosphere, and closed fuel injection systems that maintain positive pressure throughout even after shut down. What happens when you increase the pressure on a liquid? The boiling point goes up.

Components of modern gasoline, especially ethanol blends, start to boil at as low as 135°F at standard atmospheric pressure. When you shut your engine down and you get a natural heat soak condition in the engine, the temperature of the carburetor float bowl easily can exceed that. Ever look at how a drip coffee maker works? Compare how that system is to the design of the accelerator pump circuit in your carb. Notice the similarity?

Post shut down heat in the carb coupled with the low boiling point of modern fuels basically turns your accelerator pump circuit into a Mr Coffee. This isn't a problem on modern cars because the fuel rail retains enough pressure after shut down to raise the boiling point of the fuel high enough to prevent the issue from happening.

Post shut down heat soak and resulting fuel boiling issues always have been around to an extent, but older formulations of gasoline when carbs were the norm were less prone to the problem. About all you can do to lessen the effect is to either keep heat away from the carburetor post shut down somehow, or try using a different brand gas in the hopes it boils less readily.

Sun Jun 22, 2025 9:32 am

I would look at that fuel pressure. If the pump is correct it wont need a regulator you have something going on there.

I had a weird pump once with a full tank engine off the fuel would push past the pump build pressure and flood the carb just enough psi to get past the seat would drip fuel when parked. if I parked it on my angled driveway it was worse. under half tank it was fine. Changed to a BROSOL pump fixed.

Sun Jun 22, 2025 11:32 am

mukluk wrote: Yes, it's a fuel problem.

Modern gasoline is designed with modern cars in mind: closed fuel tanks and vent systems to keep vapours from escaping to the atmosphere, and closed fuel injection systems that maintain positive pressure throughout even after shut down. What happens when you increase the pressure on a liquid? The boiling point goes up.

Components of modern gasoline, especially ethanol blends, start to boil at as low as 135°F at standard atmospheric pressure. When you shut your engine down and you get a natural heat soak condition in the engine, the temperature of the carburetor float bowl easily can exceed that. Ever look at how a drip coffee maker works? Compare how that system is to the design of the accelerator pump circuit in your carb. Notice the similarity?

Post shut down heat in the carb coupled with the low boiling point of modern fuels basically turns your accelerator pump circuit into a Mr Coffee. This isn't a problem on modern cars because the fuel rail retains enough pressure after shut down to raise the boiling point of the fuel high enough to prevent the issue from happening.

Post shut down heat soak and resulting fuel boiling issues always have been around to an extent, but older formulations of gasoline when carbs were the norm were less prone to the problem. About all you can do to lessen the effect is to either keep heat away from the carburetor post shut down somehow, or try using a different brand gas in the hopes it boils less readily.

This,, and to add to it, california gas is worse than outside this state.

Sun Jun 22, 2025 1:03 pm

You can get the China ones to work but it can take a lot of work.

If you want to buy one off the shelf get Bocar made in Mexico. There's lots of NOS ones out there. I was told they were all hot tested at the factory before boxing them.

Sun Jun 22, 2025 1:30 pm

Send your German Solex to Tim.

Six weeks will go by fast.

Sun Jun 22, 2025 1:39 pm

I've had classics since I got my license in California never had a fuel boiling problem On my VW or ford V8's

Sun Jun 22, 2025 3:16 pm

Thanks again for the replies. To answer:

Mukluk, I take your points on modern fuels being fine for fuel injected cars but not for carb cars. I started down this road 2 months ago when I bought the carb. At that time it had a Brosal fuel pump. When the carb leaked, I thought "oh it's leaking past the fuel pump diaphragm upon shutdown and causing the fuel tank head pressure to flow past the pump right into the carb". When I talked with JBugs they thought it probably was that fuel pump was pumping at a pressure too high for the float valve and leaking past it and flooding the carb. That's what sent me down the road of installing a fuel pressure regulator and rebuilding a vintage Pierburg pump to replace the Brosal. I also put in a fuel rated ball valve just before (inline) before the pump.

What I've discovered is that the carb will leak through the accelerator pump circuit until the bowl is even or below the lower vent holes in the bowl whether or not I cut fuel with the ball valve at shutdown, whether or not I remove the fuel line at the carb immediately upon shut down, whether it's run for just long enough to fully open the choke or after it's truly warmed after an hour of driving. I will leak even if I pull off the top of the carb upon shutdown too.

While I also thought heat soak and fuel boiling might be an issue too, I should say now this motor is in a Manx style buggy with doghouse cooler, working thermostats, and full tins. After driving any length of time upon shutdown the carb body is cool to the touch and only slowly warms from heat being soaked up from the manifold or the rest of the engine. It will begin to leak within 10-15 seconds no matter what the circumstance.

That's why I was inquiring about the check ball function of the accelerator pump circuit. Does anyone know if it's supposed to literally stop flow through the circuit, or is it just a one way type valve that stops flow back into the float bowl but allows it to flood through to the inside of the accelerator pump diaphragm?

Also, I've got a new development here: Just for laughs I took the carb off, clamped it level, removed the top and filled the float bowl with gas to normal height. Within an hour it had dropped in level by 1/2". The throttle plate was wet, and it sure seemed to be from an occasional drip of from the accelerator pump nozzle. So obviously no fuel boil in this scenario.

Anymore thoughts?

Thanks all, Gary

Sun Jun 22, 2025 4:03 pm

I'm glad you didn't listen to the fuel boil load of crap gee wizz. I never liked Jbugs everything i bought is under the bunch as junk. Try the cheapest eBay carb you can get. Ebay will refund you if its junk J bugs will argue why bother its all the same china stuff its a gamble. If you look at it 60$ is penny's these days so you spent $120 to find a good one you now have parts.

https://www.ebay.com/itm/156682275386?_trkparms=am...1R71REKW7P

Put it in your cart wait a few days they will give you money off or make an offer for 43$ then wait for a counter off its the 21st century just play the game its the times we live in now.

Sun Jun 22, 2025 5:44 pm

Update. I had time this afternoon to repeat some of the tests I'd performed previously and have to change one item as I could not get it to repeat. Everything else did repeat. Here's what I found:

1) Only the first maybe 10 drops were coming from the accelerator pump discharge nozzle. I pulled off the air cleaner upon shutdown and watched.
2) The throttle plate remained wet even though I could not see obvious dripping from inside the carb throat past those first 10 or so drops.
3) The dripping coming from the right side throttle x-shaft remained constant. I counted over 100 drops coming at the rate of about 1 every 3 seconds. At that point I removed the top of the carb to have a look at the fuel level in the float. It was down to about 30mm below the top of the lower carb edge. It continued to drip.

So yes DesertSasquatchXploration, the fuel boil notion isn't applicable here.

I had a spare rebuild kit on hand so just for fun I changed the accelerator pump diaphragm. Made no difference.

The carb is definitely junk. I don't know how they managed to screw it up at the Chinese factory but I suspect one of the drillings just above the throttle butterfly is going someplace it shouldn't, like into the float bowl someplace I cannot see. Probably into the area where fuel enters the area to be pumped when you hit the gas pedal. That's not really fixable. I've renewed my contact with J-bugs asking for a better solution. We'll see.

Back in another life I did work in Taiwan to get cheaper parts for bicycles. You could get a range from absolute crap to really well made stuff, it all came down to how much you paid and how hard you worked with them to overcome QC issues. The Chinese are fully capable of making excellent carbs, but our importers here aren't doing the work needed to get them made right. I'd gladly pay $300 for a carb that actually worked.

I appreciate the Ebay tip! Thanks all,

Sun Jun 22, 2025 5:46 pm

GGable wrote: That's why I was inquiring about the check ball function of the accelerator pump circuit. Does anyone know if it's supposed to literally stop flow through the circuit, or is it just a one way type valve that stops flow back into the float bowl but allows it to flood through to the inside of the accelerator pump diaphragm?
The check balls in the accelerator pump circuit are only there to act as one way valves for pump function. It doesn't take much pressure or flow at all to unseat them.

Sun Jun 22, 2025 7:49 pm

mukluk wrote: GGable wrote: That's why I was inquiring about the check ball function of the accelerator pump circuit. Does anyone know if it's supposed to literally stop flow through the circuit, or is it just a one way type valve that stops flow back into the float bowl but allows it to flood through to the inside of the accelerator pump diaphragm?
The check balls in the accelerator pump circuit are only there to act as one way valves for pump function. It doesn't take much pressure or flow at all to unseat them.

Don't think its possible for the fuel bowl to have (pressure) that big tube out the top is called the vent tube.

Sun Jun 22, 2025 7:59 pm

DesertSasquatchXploration wrote: mukluk wrote: GGable wrote: That's why I was inquiring about the check ball function of the accelerator pump circuit. Does anyone know if it's supposed to literally stop flow through the circuit, or is it just a one way type valve that stops flow back into the float bowl but allows it to flood through to the inside of the accelerator pump diaphragm?
The check balls in the accelerator pump circuit are only there to act as one way valves for pump function. It doesn't take much pressure or flow at all to unseat them.

Don't think its possible for the fuel bowl to have (pressure) that big tube out the top is called the vent tube.
Pressure can also be negative, as in when the throttle lever is released, causing the accelerator pump diaphragm to move which pulls the check ball in the float bowl off its seat in order to fill the pump. :wink:

Sun Jun 22, 2025 8:08 pm

:roll: Your something thats for sure LOL at rest the check ball is open to the fuel bowl once you jab the throttle the ball moves UP to force fuel thru the pump jet circuit and not back into the bowl. very simple only 100YR old design.