Carb/fuel overheating

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peteinnit
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Carb/fuel overheating

Post by peteinnit »

Hi all,
I've got a 2l Aircooled with twin webers and it suffers from a little pre ignition and is very hard to start when hot, after a motorway run. For example if I stop for petrol on a motorway run, it will usually start first time, but then die, unless I rev it really hard to get it through a lumpy period. If it dies, it's flooded and takes a while to catch again, when it will need the high revving again to get back to normal running.

Initially I thought this was a carb problem, but speaking to a carb specialist he advised that it sounded more like the fuel was too hot, either because the carbs themselves were overheating, or the fuel in the pipe was. If that's the case, I assume the high revving is drawing new cooler fuel into the carb, hence returning to normal running.

Curious if anyone else has had this problem and if there are any cures to stop the carbs/fuel getting too hot when it's sitting in the carb during rest breaks. It's pretty annoying sitting at the pump for 5-10 minutes everytime I fill up on the motorway.

Cheers,
Peter
T25 2.0l Aircooled CI - 1982

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937carrera
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Re: Carb/fuel overheating

Post by 937carrera »

What's your ignition timing set to, vac connected or not ?

Just double checking, you said Webers, not standard Solex ?
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

peteinnit
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Re: Carb/fuel overheating

Post by peteinnit »

Yes webers, the ICT34's. Don't know the answer to the other 2 questions, but it has been regularly tuned, so it should have been set up properly, but not tuned after a long run on the motorway.
T25 2.0l Aircooled CI - 1982

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937carrera
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Re: Carb/fuel overheating

Post by 937carrera »

peteinnit wrote:Hi all,
it suffers from a little pre ignition

That's indicative of a weak mixture and / or an ignition timing issue.

Ignition was I thought the easiest for you to check.

Measuring at idle is just a starting point, you need to know if you are getting full advance from the distributor when accelerating and that the mixture is rich enough when the engine is at cruising speeds - say 3000-4000 rpm
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

peteinnit
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Re: Carb/fuel overheating

Post by peteinnit »

OK thanks, I'll get that checked out. I will say though that it drives fine, no problem with acceleration etc, it's just when I stop the engine for a few minutes and want to start again. If I stop and restart immediately it's fine, it's only when left to rest for 5 minutes.
T25 2.0l Aircooled CI - 1982

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matteo
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Re: Carb/fuel overheating

Post by matteo »

Hmm not sure about the overheated fuel explanation, I've had long runs with a hot engine and haven't experienced the symptoms you describe.
Could it be the carb fuel cut-off solenoids not functioning so you're getting an excess of fuel in the chambers?
1980 2l CU Aircooled; left-hand drive - now officially a historic vehicle!

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937carrera
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Re: Carb/fuel overheating

Post by 937carrera »

I considered that too, if that were the case the engine would continue to run when switching off at the ignition, that symptom wasn't mentioned.

I just did a quick google and there do seem to be a few reports of ICT34's suffering from vapour lock, in which case adding a return to the tank may help.

To the OP when restarting are you using the throttle at all ?

The mention of pre-ignition was the most significant to me. If the mixture timing isn't right any other issue can be amplified.

See also http://www.vw-resource.com/vapor_lock.html, not much mention of this issue with the Solex's on T25 though
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

peteinnit
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Re: Carb/fuel overheating

Post by peteinnit »

I've tried all combinations when re-starting, no throttle, full on, wait for it to catch and gun it etc. It's the restart thing that is most annoying. I will check out the other thread and also look at the return to the tank, since replacing the tank is already on the list of jobs to do! Cheers
T25 2.0l Aircooled CI - 1982

petethefeet
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Re: Carb/fuel overheating

Post by petethefeet »

We also surfer with a similar problem, cold the engine starts very easily, hot after filling up with fuel (5-7mins) the engine will crank quite fast but it is very slow to fire and run. I have had similar problems with motorcycle that did not have good enough insulation between the carb and the inlet tract and cylinder head.
From memory the carbs on the CU doesn't have a heat insulating plastic spacer between the carb and inlet tract, could this be a problem with modern petrol that is suppost to have more problems with boiling the fuel?
We are running Solex Brosal carbs with std filter air box set up
1982 2 litre CU Auto Devon Moonraker called Brian

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937carrera
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Re: Carb/fuel overheating

Post by 937carrera »

This is a really, really complex area.

The addition of ethanol means that the base fuel stock is reformulated so that the overall evaporative emissions are still within the same EN228 fuel spec, so shouldn't be so very different. Winter fuel is also different to summer fuel. :!:

Insulating the fuel line is one route, but makes inspection more difficult and much less likely
1981 RHD 2.0 Aircooled Leisuredrive project, CU engine
1990 RHD 1.9 Auto Sleeper with DF/DG engine

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