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Mon May 12, 2025 6:41 am

So, I have an issue with my bugs engine, which I believe to be carb related.

Engine is a 1835cc, all new parts built last year. Magnaspark ignition system, dual EMPI HPMX 40's.

Car was running fine, but then started to develop this issue :
Whenever I am driving along at a constant speed, and then want to accelerate, it will stumble, sometimes backfire, but then will start pulling normally.
Also happens when I start to pull off from a full stop, like at a traffic light.

Last week I pulled the carbs, took them apart.
Found nothing wrong, except one idle jet had some crud in it.

While I was at it, also pulled the plugs, all look the same, nice light brown color.
Measured compression, 125-130 psi on all cylinders.

Cleaned, reinstalled carbs, problem persists...

Any ideas ?

Mon May 12, 2025 7:44 am

Sounds like possibly your accelerator pump isn't adjusted properly. Have you investigated that yet?

Mon May 12, 2025 8:09 am

If it was fine, and recently started the problem, then it sounds like more plugged idle jets. If you are using the thin chrome stamped steel air filter housings, those have a lot to do with it. They are really hard to get to seal the dirt out. I prefer the CB cast filter housings. They are ridged and don't flex when you tighten the top nuts down.

Brian

Mon May 12, 2025 8:56 am

No, have not messed with the accelerator pump adjustment, since it says in the installation and tuning instructions, that they are adjusted at the factory and should not be messed with.

Idle jets ?
Like I wrote, I had the carbs apart, and found one jet with some crud in it, cleaned them all..
But I will check them again.

Also, how would the air cleaners affect the fuel going through the idle jets ?
They could affect the idle air circuit, but I blew those out too when I had the carbs apart.

Mon May 12, 2025 9:05 am

MTT3107 wrote: No, have not messed with the accelerator pump adjustment, since it says in the installation and tuning instructions, that they are adjusted at the factory and should not be messed with.

Idle jets ?
Like I wrote, I had the carbs apart, and found one jet with some crud in it, cleaned them all..
But I will check them again.

Also, how would the air cleaners affect the fuel going through the idle jets ?
They could affect the idle air circuit, but I blew those out too when I had the carbs apart.

They affect the fuel going through the idle jets because they let so much dirt in, then the dirt blocks the idle jets, and stops the flow of fuel through them. You can keep cleaning them weekly, but they will keep letting dirt in till they are sealed up better. I lot of guys add grease around the top and bottom of the filter element to help seal it up more.

Gotta remember, those stamped steel housings, and those filter elements that come with them are the bottom of the barrel for quality.

Clean them all again and see if that helps. Its really the only thing that can change without making a deliberate adjustment.

You could also check the sync on the carbs again. Maybe one of the linkage arms moved without you noticing it? The threaded down rods should be vertical when looking at the engine from the back.

Brian

Mon May 12, 2025 9:08 am

It could also be something like a loose wire shutting down the ignition as the engine twists as you accelerate.

But with idle jets, sometimes the disturbance caused by fiddling with the carburettor picks up more dirt, so you have to check again.

Also I think the comments about air cleaners are that some of the filter mounting assemblies do not seal well and let in dust and dirt.

Mon May 12, 2025 9:43 am

Quote: You could also check the sync on the carbs again. Maybe one of the linkage arms moved without you noticing it? The threaded down rods should be vertical when looking at the engine from the back.

I did sync the carbs and adjust idle air after reinstalling them.
Rods were not touched.

Mon May 12, 2025 12:43 pm

I had a very similar issue and it was a plugged idle jet on #3 cylinder. At least they're easy to get out and clean. Of course mine are in the open on a buggy frame.

Mon May 12, 2025 2:15 pm

I'll check the idle jets again..

Tue May 13, 2025 2:27 pm

So, it turned out to be another clogged idle jet..
Cleaned it, reinstalled everything.

Synced the carbs again, and adjusted the idle mixture.

Cas runs great now.

Next project will be to install "weber windows", so it will be easier to get to the idle jets, and to adjust idle mixture.

Tue May 13, 2025 8:48 pm

You don't need CB anything to make it work right build your own jet doctors and a vent tube extension. Its stupid simple use steel fuel line drill for a press fit done deal. For the vent extension JB steel stick