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I'm working on a custom built trike based on a 1600cc Air-cooled VW engine.

I'm a specialist auto-electrician and these engines are a bit before my time so I'm struggling to work out what is wrong.

The engine has a Solex carb which requires two 12v feeds that are both present. Is has various vacuum ports that are blanked off.

The distributor doesn't have vacuum operated advance/retard. The points look new.

The engine runs fine when cold, leave it overnight and it runs, revs and drives perfectly.

When it is warm it has no power you can barely pull away, it also back fires extremely bad while accelerating.

The owner seems to think the engine has recently been rebuilt.

What would be causing this and why would it do it when hot?

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    Some things that jump to mind. When a coil is starting to go bad, it usually gets worse when hot. When it's cold it needs the choke. I'm wondering if there is something going on there - choke not coming off, or generally being out of tune but the choke covers up those issues while cold. Wondering if there may be a vacuum leak that gets worse when its hot. There are boots in the intake that can rip. Read vacuum while cold and hot. There could be issues with spark plug wires grounding. Watch it running in the dark and look for sparks. Make sure the ignition is advancing properly.
    – rpmerf
    Commented Jun 12, 2017 at 14:55
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    @rpmerf has some great advice. I would lean toward the coil or spark plugs first. What is the condition of them? Also, These engines do not run well when they overheat. Is the engine getting proper air flow over the fins from the fan? Does it have a temperature gauge? As far as timing advance, it is mechanical, so make sure if moves freely. Sometimes they get rusted and will not move.
    – CharlieRB
    Commented Jun 12, 2017 at 15:51
  • Replace the condenser in the distributor.
    – Moab
    Commented Jun 19, 2017 at 0:40

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This engine can be prone to detonation, which can be aggravated by lean mixture and hot spots in the cylinder. I would check out timing as well.

If that is not the issue, then I would be looking at the spark intensity (voltage) with a spark tester (that is adjustable to give voltage indications).

I find it interesting that there is no vac advance...how does the timing change for higher power draws?

Is there a centrifugal advance for the timing, which is perhaps binding a bit?

Poor timing and hot spots with an air cooled engine are frequent causes of poor operation at high engine temps. Lean mixture can cause higher than normal cylinder temps, but the fact that the engine runs OK when cool discounts mixture as being a primary issue.

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There were two things that I done that fixed this issue:

The automatic choke was well out of adjustment so I adjusted this and then readjusted the fuel mixture to suit.

There was still a problem that made me lean towards a fuelling issue, my theory was that the fuel was heating up where the fuel pipe runs across the exhaust manifold and where it had custom manifolds to suit the trike frame the carb was very hot so I rerouted the fuel pipe and made a heat shield for the carb and this cured the remaining issue.

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  • Why do you think the choke adjustment might cause the backfiring which was observed at full engine temp, when presumably any choke action would be null, and backfiring is associated with a lean mixture?
    – mongo
    Commented Oct 12, 2018 at 7:01

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