Low compression,black plugs and mis fire

[Asw]

Hi
I have been battling a miss fire and intermittent back fire. It only really shows when the engine is warmed up and running at 50 mph + or on hard overrun.
So the engine.
CU air cooled running power spark SVDA dizzy and twin Brosol 44 carbs. Hydraulic tappets.
What I have found so far.
The plugs on no 3 and 4 are black.. like it’s running really rich. 1 and 2 are a nice, almost lean “lean” brown.
Unplugging the HT Leeds on 3 or 4 has almost no effect on idle. Doing the same on 1 and 2 and the effect is dramatic.
Compression on 3 and 4 are the same ... around 105 psi.
Compression on 1 and 2 are the same... Around 145 psi.
I can turn the mixture jet on 3 and 4 right in and it doesn’t seem to affect idle... this kind of fits with disconnecting HT on 3 or 4 not really affecting idle.
I have double, triple, checked the timing, plug lead sequence, and for air leaks.

My feeling is the low compression and black plugs are linked, and the fact that the engine has to be properly warm implies expansion is involved.
My next check was tappets. But a. Why are two cylinders so similar and b. A thunderstorm has just hit.
So thought it was time to ask for help...

[937carrera]

My take:

The right hand bank has good compression and the carb that fuels that side is set up / working correctly

The left hand bank is very marginal on compression and the carb is not set up correctly

The overfuelling on the left hand bank is just within tolerance when the engine is cold and you need a richer mixture. When up to temperature it's way too rich, so

Check tappets and redo the compression test.
Look at the operation of the choke, make sure both sides are adjusted / operating correctly and come off when warm

I don't know enough about the setup on the Brosol carbs, but offer these thoughts

The standard Solex have a balance pipe and idle is led by one of the carbs - is that how the Brosol work
If the "44" is indicative of sizing in the same was as the Solex, then these appear to be for a much larger engine - even the 412 only had Solex 36/40

[Asw]

Yes the carbs are big but are sold as appropriate for a 2l Cu.
So I’m going to have a go at the tappets.. can hydraulic tappets be so far out as to have that effect on compression?
Also does low compression need a lot leaner mixture?
Presumable the low compression could be caused by valves not seating fully. And the back fire caused by unburnt mixture leaking into the exhaust?
The carbs have no choke. They do have a balance pipe. Im going to disconnect it (blank off, no air leak) disconnect throttle linkage, wind mixture on RH carb right in and try and get the engine idling on the left bank.

if all else fails I have an old centre mount twin Webber that I can install and see what happens.

All assuming the sun comes out again after work ....

[937carrera]

Tappets not being set correctly can cause either valves not closing properly, or them being open for such a short amount of time that not much air can get in, that's why you need to check those before doing the compression checks.

There is no relationship between compression ratio and mixture - one is the amount of air, the second is how much fuel is included along with the air. Look up stoichimetric

Well, no choke means no fault there than. If you have an idle stabiliser on the left hand side of the engine bay, unplug the connectors and join them together to defeat it. Make sure there is an idle adjustment screw on both carbs

Have you checked that both carbs are jetted the same ?


I'm very much an OEM guy, many "go faster" places recommend 009 dizzys as appropriate for VW Aircooled engines....... they're completely wrong for almost all applications