QuestionThanks for your response. Actually I've been approaching it in much the way that you say but the full history would take many pages, I gave you the short version.
Compression. Good to have confirmation the compression is OK. The figures I quoted before were throttle closed, throttle open I get 150-160 psi. That's OK then
Ignition timing. In the brief time when it was running properly I set it to 12 degrees BTDC with my ancient timing light, as old as the car, it's just a neon tube which is plugged in series with a plug lead. It lights for as long as there's HT current (so the timing marks blur in the direction of rotation and you have to time to the trailing edge). But it does show HT current.
Spark. Jumps about 1/2" between the lead and the plug if it has to. When it's not running properly it needs to be revved at up to 2500 rpm just to keep going - that varies, it started today at about 1000 rpm and got worse - and if you let the revs fall it slows and stops (you can sometimes get it back by blipping the throttle, see mixture). I'm sure it's intermittently not firing at speed and steadily not firing when it stalls. It's obvious that the brightness of the timing light varies, bright when it's firing properly and dim when it's not. My reasoning goes: if there's a flash there's some current, the plugs are newish and clean so there must be a spark. I seem to remember from way back that when the mixture fires the spark voltage drops so the spark current will keep on longer, hence the brighter flash. BICBW.
Mixture. It certainly sounds weak, popping and banging in the exhaust and occasionally spitting back throuh the carbs (the spit back doesn't smell of fuel much though). WD40 in the carbs made almost no difference - the behaviour is pretty random so it's difficult to tell - and I'm pondering something I could use to spray fuel into the inlets. In the meantime i've tweaked the float to get the level in the chambers as high as possible (the jets aren't easily adjustable, they're pressed into place and I don't want to disturb them because, after all, it was right). That helped a bit.
When it starts misfiring blipping the throttle usually gets it to catch again. All round it sounds like weak mixture - but the plugs are sooty.
There's only one bleed into the manifold, the emission control valve which sucks oil vapour from the crankcase. I've taken it off and blocked the hole in the past, it didn't make a lot of difference. There is a bimetallic strip controlled bypass to the the venturi which was sticky but I've fixed that (I think)
It's got to be something I've done/overlooked. It was running fine.
AnswerYour carbs don't have the 3mm Allen adjustment down in the bottom of the oil reservoir? What happens when you slowly cup your hand over one carb at a time when it is running poorly? Have you looked at the decel valve (3 screws close to the throttle shaft? Every one I looked at in the last 10 years were bad and the diaphragm was so hard the valve couldn't move and when it did it stayed where it was open or closed. It was just for emission control on decel so I run the adjustment so as to force the valve closed but first you need to clean the valve and it's seat. This would not effect mixture but could cause a high and low idle at times. Are the butterfly screws tight and the butterfly seated correctly? Has this got the coolant operated choke or manual?
What kind of oil are you using in the top carb pots?
Any smoke out the tail pipe when it is messing up? If so what color?
Let me know, this is a puzzle that needs to be solved.
Howard
