Misfire on cylinders 1 and 3

[Moseley]

MOT day tomorrow, so Sod's law the van throws a wobbly. Just got it out of the garage ready and it struggled to start, and when it eventually got going, it was idling low and very lumpily. Left it to warm up and the idle improved, but still didn't seem like it was firing properly. The engine has always shook a lot and is more noticeable at idle revs. I pulled the leads one by one and have found that both 1 and 3 have no effect on the idle. Both plugs are receiving a spark, albeit they don't look too strong. I need to pull one of the OK ones to compare. Either way, ive swapped the plugs from cylinder to cylinder and nothing changes.

The engine has only done a few thousand miles so I'd expect it to be in good nick. I guess what I'm looking for is some way of knowing what to check / change next. I have adjusted the valves since I last had the van out, but can't remember what it was like before - I say that because it seems to drive ok now, although can only do a limited run up my road because of lack of mot. The valves didn't need much tweaking which makes me think I haven't cocked this up. The compression tester I have unfortunately doesn't come with the right adapters. So, what next?!

 

[Moseley]

Should probably mention, the engine is an 1835 running twin empi carbs (assuming a Weber copy).

 

[Hilly_70]

I had something similar but just cylinder 1.. really lumpy on tick over ..

Turned out changing spark plug cleared it .. even tho it was a bosch, not very old and looked fine.

Just in case it helps.

Hilly

 

[Moseley]

Ta for the reply, although I've swapped plugs between 1 and 2, 3 and 4 and the 'misfire' remains on 1 and 3. I've just driven it to the MOT (which it passed, whoop!) and it is driving well when up to speed, so there's no way it is running on only 2 cylinders. Been doing some digging and some people have hinted that on some dual carbs at idle, that only 2 and 4 have any real input to keeping the engine turning over. I'm intrigued to learn more about this as I don't like not understanding things!


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[67westy]

Have you got a balance pipe fitted?

 

[mister]

Have you tried:
- swapping the HT leads, they could be faulty too, or not quite connecting with the plug or dizzy cap
- checking the dizzy cap is ok and seated properly on the dizzy body
- checked all the valves are opening and closing properly, I had a problem with an exhaust valve recently, at speed it was OK as the compression opened it, but not at idle it mis-fired.
^^^ all of these things have caused me issues in the past!

 

[Dave.P]

What twin carbs are they, twin dual choke or singles?
Your symptoms sound just like blocked idle jets to me. If it drives ok above 2.5k revs, popping a lot, crap idling and crap running at low revs I'd be checking them

 

[Middleton]

I am having the same issue. Misfiring on 1 and 3 at idle with dual 34ICTs. I haven't investigated it yet but I thought I'd mention it for a bit of solidarity!

 

[Trikky2]

Since the ict is a single barrel carb I can't see it being a carb problem - since any fault would affect both cylinders on that side of the engine.

Check the HT leads and plug caps. The original plug caps have a resistance in them to limit radio interference. You can check them with a multimeter. The resistance is quite low. If they read a high resistance or are open circuit then they will cause misfiring.

Next check the gaskets between the head and the inlet manifold for leaks.

If they check out as OK then you need to check the compressions.

 

[megaaircooledvw]

Was the cam burnned in properly.