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Intermittent loss of power / hesitation
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nic_
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PostPosted: Sun Jun 13, 2021 10:05 pm    Post subject: Intermittent loss of power / hesitation Reply with quote

Since finishing my oval I've had an intermittent stuttering / hesitating / loss of power issue. Every time it happens I read through various threads here, but haven't been able to find anything yet.

What happens is that I'll occasionally just start loosing power as I'm accelerating, and the engine sort of hesitates and stutters. If I don't take my foot off it just keeps doing it and hopping down the road (it's never completely lost power or stalled though). If I ease right off and then accelerate again sometimes it goes away. But other times it sticks around, and I sort of hop home (as happened yesterday). If I put the clutch in I can hit the accelerator and the engine revs perfectly.

This is a very intermittent issue - it happened yesterday afternoon, and hadn't happened for the last ~150 miles. So it makes it hard to diagnose! It seems to be more likely uphill, but does happen on the flat and even downhill when accelerating.

Since finishing the build I've adjusted points, timing and valves many times over the 600 miles I've driven. Plugs have been a bit black but I put that down to slow traffic driving to get home. Otherwise everything in that department seems fine. I haven't tested the vacuum advance yet, but it was fine when it went in (also the fact I can rev perfectly with the clutch in makes me think it's not the advance?).

It doesn't feel like a clutch issue. Given it's intermittent this also seems unlikely?

The heat riser is working fine. I'm in Melbourne so no real extremes of weather, and this has happened on both warm and cool days.

The carb was rebuilt, so only has ~600 miles on it. It's one part I left to an expert, so I have to assume it was done well. One thing I have noticed when adjusting it is that if I pump the throttle lever a few times a little fuel leaks out the main gasket. Not sure if this could be related or a sign it's a carb issue?

Possibly related to the above, I do get a bit of a fresh fuel smell in the engine bay. Fuel lines all look fine, and while I don't have a pressure gauge to test I figure it's running perfectly 90% of the time, so unlikely to be a pressure issue? One thing I did note yesterday was that I filled up with 91 RON. The last tank had been 95 RON. I'm hoping it's maybe something as simple as this!

The only other thought I've had is that it could be battery related? My generator is not great, and yesterday afternoon I was heading home at dusk and had the lights on. The drive out (no lights, filled up fuel tank half way through the trip) was fine, drive back I got shuddering (lights on, 91RON well and truly in the system).

Any thoughts appreciated.
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VW_Jimbo Premium Member
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PostPosted: Sun Jun 13, 2021 11:00 pm    Post subject: Re: Intermittent loss of power / hesitation Reply with quote

Sounds like a fuel delivery issue. Maybe a plugged up fuel filter, or maybe some debris in the tank.

Or maybe a vacuum leak. Throttle shaft? Vacuum can? Carb base gasket?

You got to get in there and look closely. At everything!

Best of luck! If you want further in sight, post up a few pictures of the engine.
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nic_
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PostPosted: Sun Jun 13, 2021 11:35 pm    Post subject: Re: Intermittent loss of power / hesitation Reply with quote

Since "carb base gasket" was on that list, I've grabbed a video from a few months back of the slight gasket leak. This is (obviously) with the engine not running. I haven't been able to make it leak when running, but I'm thinking perhaps it could be the cause?

Link


Full engine photo also...
Image may have been reduced in size. Click image to view fullscreen.
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PostPosted: Mon Jun 14, 2021 4:51 am    Post subject: Re: Intermittent loss of power / hesitation Reply with quote

Another thing to check is your plugs, a fouled plug can do this. How is the manifold heat? Now that we've moved into the colder weather you maybe getting icing in the manifold
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PostPosted: Mon Jun 14, 2021 8:30 am    Post subject: Re: Intermittent loss of power / hesitation Reply with quote

Yep! That’s another one.

In the video I see the throttle shaft drip fuel! That’s a vacuum leak on the right side!

And, is that a ported vacuum port to the distributor. I am just not recalling if that port was manifold or ported vacuum?

Also, is your coil properly wired? The (15 circuit) black wire, from the ignition switch connects to the 15 terminal. The 1 terminal is for the condenser connection. Might be worth a look!

Look at the coil bottom for embossed numbers.
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nic_
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PostPosted: Mon Jun 14, 2021 6:37 pm    Post subject: Re: Intermittent loss of power / hesitation Reply with quote

VW_Jimbo wrote:
In the video I see the throttle shaft drip fuel! That’s a vacuum leak on the right side!

Oh my I've watched that video quite a few times but always looking at the gasket. I have never noticed that drip. I feel like this is the prime suspect for the issue, including the fuel smell.

I'd love to have a go at fixing this. Obviously I can do a bit of harm pulling apart the carb but would this be an easy enough thing to do without a full rebuild?

I'll also double check the coil but pretty sure it's all fine, and do another check of fuel lines and filters.

@66brm I've pulled over when it was ahving some issues to check the manifold. It's cold and has some condensation, but I'm not sure how cold is too cold. Is there an easy way to know? (also this has happened on warm days back in Feb/March) Plugs look OK to me, and the fact it revs fine not under load makes me think not a problem there?
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PostPosted: Mon Jun 14, 2021 7:09 pm    Post subject: Re: Intermittent loss of power / hesitation Reply with quote

The throttle shaft needs to have new bushings inserted. Not to many guys doing this anymore, that I know of. I believe Volkzbitz does. I know he sells rebuilt carbs and gets great things said about him in the late Bug threads! Might want to send that carb to him for repairs or purchase a rebuilt one from him.

https://www.volkzbitz.com/
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PostPosted: Tue Jun 15, 2021 7:28 am    Post subject: Re: Intermittent loss of power / hesitation Reply with quote

Speaking from recent (and 30 years of) experience- the condenser can act just like you're running out of fuel.
Driving a kid to school about 6 weeks ago had the engine start to stumble. It would act *exactly* like it was staving for fuel. Down the road intermittent cut-outs over and over. Eventually I pull over and see if I can get it to clear up, and it does. But starts again.
Condenser was about a year old.
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PostPosted: Thu Jun 17, 2021 11:35 pm    Post subject: Re: Intermittent loss of power / hesitation Reply with quote

The fuel from the accelerator pump goes through the center gasket into the top half of the carburetor, so it looks like this is what is leaking. Gas will drip from the throttle shaft bushings if you pump all that gas down the carburetor with the engine off. Its pooling on the throttle plate and running across the shaft and out of the bushings. They probably are worn unless they were done when the work was done on the carburetor. Also check the fuel pressure, too much pressure will flood the carburetor, this could be why your plugs are black.
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PostPosted: Sun Jun 20, 2021 11:26 pm    Post subject: Re: Intermittent loss of power / hesitation Reply with quote

Thanks for the help all. I had a check over of things on the weekend (unfortunately no new condensers available nearby - so need to head for a drive to pick one up).

I cleaned everything off and dusted a bit of talc in the relevant spots to check for any fuel leaks. When running with a lot of revving I get the faintest trace of fuel around the gasket, and also the throttle shaft. This obviously isn't great, so it may be the issue.

One thing I did find was also a slight leak on the line going into the carb. When I first rebuilt the engine 3 years ago I posted this video of the rather epic leak. At the time I pulled the line off and reinstalled, and thought everything was fine. I'm wondering now though if perhaps it's not? Either the thread on the line or carb is leaky, or perhaps the pressure is too high?

My garage opens up basically to the front door of my neighbour, so there's a minimal amount of neighbourly-acceptable revving I can do. I'll head out to a friends out of town to do a bit more investigating next weekend.

Prior to pulling the carb and shipping it half way around the world, I figure I'll be sure it's nothing simpler. So the plan now is:
  • New condenser on, and double check the condition of wiring around the distributor
  • Remove the pump -> carb fuel line and double check threads and re-install
  • (assuming there's still an issue)... drain and clean out the fuel tank, check the screen and clean out lines to pump and install new filters
  • Check fuel pressure
If I'm still getting the issue after all this, you can expect a carb in the post VolkzBitz Wink

Thanks for all the help and ideas so far everyone. Nothing more frustrating than an intermittent and hard to replicate issue. Rolling Eyes (Although at least it hasn't left me stranded yet)
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