No spark on every other cylinder

Thomas-P6

Thomas-P6

Member
Hello,
I have the problem that all over sudden the engine started to run rough such as if one or more cylinders were not firing. The following was tested:
- changed all spark plugs
- every cylinder gives a spark (when checked with an Accuspark tester)
- changed the dizzy cap and the rotor
- checked compression: ok
- checked firing order: ok
- checked ignition timing: ok at 10° early

Then pulled the plugs with the engine running and found that cylinders 2, 3, 5, and 8 are not firing (although the Accuspark tester indicates a spark). So I am getting the following picture:

Firing Sequence: 1 - 8 - 4 - 3 - 6 - 5 - 7 - 2
Cyl working Y/N: Y - N - Y - N - Y - N - Y - N

As the ignition seems to be in order, I am suspecting a problem on one the carbs being clogged. Can somebody confirm which carb (left or right side) is responsible for the cylinders affected by the non-firing (2, 3, 5 and 8)?

Thanks for your input.

Thomas
 
Hi, The left hand carb.

Each carb feeds the middle two its own side and the outer two the other side.

Colin
 
It's the RH carb as Harvey said.

I removed the carb (HIF 6) and took it apart, but I could not find any obvious problem. The float chamber contains petrol and is absolutely clean. The needle valve could move freely. I used compressed air and blew through every bore, everything seems to be free, no clogging.

Put the carb back in, but no change. Cylinders 2, 3, 5 and 8 are still not firing altough they do have a spark.

I'm running out of ideas. Has anybody another suggestions where to look?

Thomas
 
a couple, valley gasket leaking, also if the servo or its pipe leak that can cause a lean mixture that knocks out no2 cylinder altogether and has a drastic effect on the others on the same carb
 
Checked the valley gasket and the servo/pipe: no leaks whatsoever.
Still suspecting the carb: what causes the complete refusal to deliver pertrol??

Thomas
 
If you want to prove it's the carb at fault, (although there isn't much doubt), spray some petrol down the carb throat with the engine running, if you're not getting petrol through the carb the engine will pick up onto 8 cylinders when you do it.

If you're not comfortable doing that you can look down the carb while revving the engine and you should see the piston rise and the petrol spray out of the top of the jet.
 
I had this problem when I first started up my car, It was caused by fuel starvation. Only one carb were fed. When I put some more petrol in the tank all was good. I noticed it due to very rough running and the headers were cold on 4 cylinders.
 
I tested the carb and found:
- a spray of petrol is visible when looking down the RH carb with the engine revving - looks the same as the spray on the other side's carb.
- additionally sprayed starter spray into the air intake - no change!
- the plugs of the non-firing cylinders look wet - still getting strong sparks on all of them.
- when blocking the air intake on the RH carb, nothing happens - doing the same on the LH carb, the engine starves immediately.
- engine starts nicely everytime - with just 4 cylinders working.

I'm lost!
Thomas
 
When you took the carb apart, did you also check the cold start device? The have a couple of O rings inside that when they get old and perished they give running problems.
 
you say 2,3,5 and 8 are'nt firing and the obvious fault is the right hand carb. Is it only on tickover ? When you saw fuel running into the engine through the right hand carb. did the four cylinders start firing?
 
@ Demetris:
Took the carb off again today and dismantled it completely including the cold start device:
- again plenty of petrol inside the carb
- there is 1 O-ring on the cold start device but it had a very loose fit, so I wonder if that was really sealing well. Fitted a new one.
- adjusted slightly the float level (it was level with the flange of the float chamber, so I raised it by 1 mm.

I haven't got the carb back in again, this will be done tomorrow.

@ Jim:
- non firing present all the time, i. e. on tickover and when revving the engine up
- fuel spray seems to be there all the time as well

Tank is full, and the fuel pump delivers plenty.

I have no logical explanation as to what the cause of the problem might be.

Thomas
 
Thomas, when you said that the plugs in the non-firing cylinders were wet i thought that a faulty cold start seals on the problematic carb could possibly flood these cylinders. This is why you have spark and fuel but not ignition. I had HIFs in the past with perished cold start seals and the car could barely run, or not at all. Anyway, this is just my theory. When you will put the carb on, we will learn if it is right or wrong!
 
Respect Demetris,
looks like you were right! After re-installing the carb with the replaced cold starter unit O-ring, everything is back in order!
All 8 cylinders are firing again. Apparently, the mixture was overrich and thus not igniting on the RH carb side.

A little finetuning is still needed, but then I should be back on the road.

Thanks again to everybody going through this with me!

Thomas
 
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