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Pinto Misfiring
davidimurray - 14/3/14 at 01:43 PM

Further to my previous post here - http://www.locostbuilders.co.uk/viewthread.php?tid=189530

I was wondering what thoughts people had around my misfire?

Car has done 300 mile since being put on the road. Wideband has been on it the whole time and AFR has been good. On the way back from work noticed a few pops and bangs and noticed that she was running a bit lean.

Checked when i got home and there was a definite rough missfire and the AFR had gone up by about 1.3 throughout the range - i.e. 12.7 to 14 as an example.

Missfire generally seems to be barely noticeable at idle, quite bad at light throttle openings and less noticeable at higher throttle/rpm.

Car is running megajolt, ZX6r carbs and has always popped on the overrun

Whipped the pilots out, quick spray with carb cleaner no difference.

As per advice I rebalanced the carbs, which were out and it is much better, but still has a missfire.

Put a new set of plugs in and no change.

Have soaked around the inlet manifold and carbs with WD40 and no change in RPM.

Current thought it is it could be exhaust manifold leak?

Any ideas/thoughts much appreciated

[Edited on 14/3/14 by davidimurray]

zammobhoy - 14/3/14 at 03:04 PM

Need to find out if it's related to one particular cylinder mate.

Pull of all the plug caps one by one. They will all make a difference but if the misfire is down to one cylinder, when you pull that cap, it will run evenly on three pots without misfire. If they're all the same then it's a general fueling/ignition thang.

cheers,

Paul.

ash_hammond - 14/3/14 at 05:16 PM

These have helped me out in the past when my pinto has developmed a miss fire:

Link

gregs - 14/3/14 at 05:22 PM

Worth checking valve stem seals if it's one cylinder - they can (and did on mine) come up and then leak oil into the cylinder snuffing the spark.

ash_hammond - 14/3/14 at 05:25 PM

quote:
Originally posted by gregs
Worth checking valve stem seals if it's one cylinder - they can (and did on mine) come up and then leak oil into the cylinder snuffing the spark.


+ 1 mine has done this once.

davidimurray - 14/3/14 at 05:59 PM

quote:
Originally posted by gregs
Worth checking valve stem seals if it's one cylinder - they can (and did on mine) come up and then leak oil into the cylinder snuffing the spark.


When you say come up I take it you mean raise up the stem away from the head? Would never have thought of that one! Did you get any smoke when that happened?mine is lovely and clean from the exhaust.

ash_hammond - 14/3/14 at 06:11 PM

When mine did it I got smoke on over run and adle.

gregs - 14/3/14 at 06:53 PM

quote:
Originally posted by davidimurray
quote:
Originally posted by gregs
Worth checking valve stem seals if it's one cylinder - they can (and did on mine) come up and then leak oil into the cylinder snuffing the spark.


When you say come up I take it you mean raise up the stem away from the head? Would never have thought of that one! Did you get any smoke when that happened?mine is lovely and clean from the exhaust.


Yes - they lift off the head and work up the valve.

jacko - 14/3/14 at 07:06 PM

Could it be spark plug leads [HT]
could it be one of the coils breaking down
Valve burnt ? do a compression test
Just a few ideas to think about
Jacko

old_timbo - 14/3/14 at 11:36 PM

As I suggested on your other thread I recon you are running too weak at below 2000 rpm and at light throttle openings. My Pinto does similar things with an AFR much more than 12:1. You can just about get it to idle at 14.7, but it is totally undriveable. I have also discovered it makes things a lot worse if you run a high octane slow burning fuel like Optimax which is needed for full throttle, high boost etc. when you are trying to avoid det. Guess it is down to a relatively unsophisticated combustion chamber, designed in the days of leaded petrol, and when fuel economy and emissions wern't the holy grail they are now.

davidimurray - 15/3/14 at 12:06 PM

quote:
Originally posted by old_timbo
As I suggested on your other thread I recon you are running too weak at below 2000 rpm and at light throttle openings. My Pinto does similar things with an AFR much more than 12:1. You can just about get it to idle at 14.7, but it is totally undriveable. I have also discovered it makes things a lot worse if you run a high octane slow burning fuel like Optimax which is needed for full throttle, high boost etc. when you are trying to avoid det. Guess it is down to a relatively unsophisticated combustion chamber, designed in the days of leaded petrol, and when fuel economy and emissions wern't the holy grail they are now.


Timbo- the car was running around the 12s and did so happily for 300 miles then suddenly started running around the 14s with no changes made to the car- hence why I suspect there is a problem. I don't want to mess with the fuelling as it was set nicely.

jacko - 15/3/14 at 12:51 PM

valve tappets adjustment to tight ?
jacko

davidimurray - 15/3/14 at 02:49 PM

Issue seems to be cylinder no 2, couldn't detect an issue pulling the plug leads, but at certain rpm it is spitting back through the carb on no 2 only.

Checked the cam belt hadn't slipped, but all looks OK.

Just been out to get a compression tester so that's the next check.

rusty nuts - 15/3/14 at 04:01 PM

Check the valve clearances

davidimurray - 15/3/14 at 05:16 PM

Compession a good 13.5 to 14 bar on all cylinders. Time to whip the cam cover off and inspect.

zammobhoy - 15/3/14 at 05:50 PM

Have a look in the mouths of the carbs whilst blipping the throttle and make sure that slide on no. 2 is lifting. If not, suspect the diaphragm.

davidimurray - 15/3/14 at 07:38 PM

Whipped can cover off. Checked valve clearances and all seemed OK bar cyl 3 and 4 inlets which were a smidge tight, I reset just in case. Also checked all the stem seals and they were all there and correctly seated.

Fired her up and as expected still misfiring

All the carb slides are raising properly, inc no 2.

Also tried richening up the carbs at idle to give me an AFR of 12 again and the burble was still there.

Think it might be exhaust off next to check and reseal.

davidimurray - 16/3/14 at 02:03 PM

She lives

Whipped the diaphragm assemblies out of 1 and 2 and noticed that no2 needle was sticking out further than the other. Stripped the needle and found the spacer washer under the circlip missing. Found one in my box of spares, fitted and all good.

Mystery is how has it been fine since I last touched the carbs 200 miles ago?I can only think I missed refitting it and the circlip has been sat on the ledge all this time until it dropped down?