Misfire

[dandan62 26]

Hi there. My trusty pinto has a misfire. Removing no 4 lead from the spark plug reveals that cylinder is the problem as there is no change. Have changed lead, plug, distributor cap with no luck. Compression good. There is a spark when i hold it against the engine but it looks a little feeble. Its an accuspark dizzy with electronic module. Could that module or the rotor arm cause one weak spark? Engine seems ok at higher revs. Webber carb by the way. Any clues? Thanks in advance. Dave

[theduck 856]

If its only one cylinder with an issue its more likely to be the cap.  Try rotating the cap 180* and see if the misfire moves.

[dandan62 26]

I had a brand new replacement already. Same with lead and plug. I've decided to increase my spares selection and have ordered coil, ignition module and rotor. It must be one of those methinks. Always handy to have in the boot anyway.

[theduck 856]

oh sorry, missed you had tried a new cap.

[Longboarder 642]

Just in case, have you checked the strength of the spark on the other three plugs. Might not be as localised as it seems and they might all be weak and cylinder 4 (possibly the least efficient mechanicaly due to wear, sticky valve or whatever) just the first to show the problem.

[dandan62 26]

That was my thought but compression is very good in that cylinder. I tried testing another spark but the car won't run on 2 cylinders and i was single handed so couldn't just test on turning it over. My money is on coil giving generally weak spark. The fact that given some beans it seems to run ok points to that , I think. I'll replace one thing at a time and report back.

[theduck 856]

Next thing to check would be valve clearances imo

[richyb66 1,184]

Can you remove the distributor, rotate the shaft 90 degrees and move the HT leads round to the next terminal on the cap. 

If the bad spark moves to a different cylinder then the issue is the cap.

[dandan62 26]

New cap, leads and plug. I think I'll draw this to a close and let you know when everything is replaced and then come begging for help if its still there. Valve clearance good by the way. Thanks guys.

[dandan62 26]

Hold on to your hats.

New cap, new rotor, new plug, new lead, new accuspark stealth unit, new coil. Checked compression (175) within 10% of others. Checked valve clearances, checked timing. Good battery voltage and voltage at the coil.

Still not firing on no 4. Engine note doesn't change when i pull the lead. Pulling leads on others it misses more.  There is a spark but it looks weak. Orange and dancing about the gap. 

Took for a drive. Drives fine above about 2500 revs. Or maybe the problem is hidden above that? But seems to perform well. 

I'm stuck. All answers appreciated.

David

[dandan62 26]

Question.. could a damaged valve cause a misfire but still give good compression? Clutching at straws now.

More info. Letting it run on idle with misfire and removing the plug straight away shows a wet with fuel plug. Revving the engine then removing it shows a good dry plug..

[dandan62 26]

Question.. could a damaged valve cause a misfire but still give good compression? Clutching at straws now.

More info. Letting it run on idle with misfire and removing the plug straight away shows a wet with fuel plug. Revving the engine then removing it shows a good dry plug..

[dandan62 26]

Is everyone as stumped as me? I think its head off time...

[richyb66 1,184]

Clutching at straws now. Is the cap sitting square on the dizzy? If the cap isn't properly seated the rotor arm won't spin concentric to the terminals in the cap which will cause a weak spark.

Did you try rotating the dizzy and re-ordering the timing as I suggested previously to see if the misfire moves to another cylinder?

[brumster 706]

Could do - sticky valve? Mind you the pinto is a fairly simply beast in terms of valvetrain isn't it, so there's no hydraulic followers to blame