Thread: Misfire?

Need a bit more info.

Are the plugs sooty when the engine is hot?

Sooty plugs happen on a cold engine, or if the mixture is too rich, or if the spark is not doing its job properly.

Have you checked the valve clearances and the ignition-timing before adjusting the carburettor? Always tune an engine in the order of: valves; ignition system; carburettor. Tuning an ignition system can take some time if you are using points and you really need an instruction book to help you get everything done properly,

Cheers Charlie

Just a thought but how old is the fuel in your tank ?

Cheers Chris

Thanks for the quock replies.

In aswer to your questions|

Fuel is definitely fresh

Drove for an hour earlier today - highway speed, so definitely operating temp

Will check tappet clearances and points, then carb - thanks Chazza

Will also test compression in each cylinder, while plugs are out

If fuel, spark and timing OK, could it be valves?

Thanks again

Roger

Originally Posted by dreamin'

Thanks for the quock replies.

In aswer to your questions|

Fuel is definitely fresh

Drove for an hour earlier today - highway speed, so definitely operating temp

Will check tappet clearances and points, then carb - thanks Chazza

Will also test compression in each cylinder, while plugs are out

If fuel, spark and timing OK, could it be valves?

Thanks again

Roger

If valves, it will show in the compression test. And yes, this is a possibility. Make sure that there is not a track on the distributor cap shorting one cylinder.

John

Originally Posted by JDNSW

If valves, it will show in the compression test. And yes, this is a possibility. Make sure that there is not a track on the distributor cap shorting one cylinder. John

Stick your head under the bonnet when it's dark, with the engine running you can see straight away where the sparks are jumping.

Like all engines that have alloy head's, you could have a blown head gasket.
But the 2.6 does have a tendency to burn numbers five and six exhaust valves.
It helps heaps if you are both ambidextrous and a contortionist to work on those valves
.

Originally Posted by dreamin'

Experimenting with mixture - probably richer than needs to be

5 plugs black and sooty, #6 grey and clean

Does this mean no ignition in this cylinder, therefore electrical problem?

Seeing that you have taken it for a good run, then it is probably sooty due to the mixture being fiddled with; however; stick with the plan and start with the valves and compression test. Remember to have the throttle wide open when you do the compression test; the engine hot and record the results wet and dry - good luck!

Incidentally - how did it run on the road? What made you decide it was mixture being the problem?

Cheers Charlie

OK

Compression test (dry) as follows:

1. 130
2. 125
3. 130
4. 140
5. 130
6. 120

Throttle was open (pedal to the floor), plugs out, battery charger attached for even turning, but engine probably cold and no wet test - I figured if there was a problem that could be rings or valves I will have to get the head off anyway. Also, my compression tester is a bit old and unknown accuracy.

Taking all that into account, can these results tell us anything? I was expecting higher figures generally but also looking out for one or two cylinders dramatically lower than the others. Low readings overall could be wear, but
maybe just my guage? #6 (the one with the non-sooty plug) is definitely lowest but only by about 10 psi.

I also adjusted inlet tappets, fitted and set new points and condenser, and cleaned the distributor cap - both looking a bit old and worn. I have new ones on order.

Started and idled fine but still a bit rough, 'lumpy'. Turned all the lights off and no visible sparks, so maybe they're staying where they should be, Arthur?

This all started when she suddenly started running very rough a couple of weeks ago. I think I fixed the main problem by replacing the carb diaphragm
(Zenith Stromberg) and then went through the manual procedure for setting idle speed and mixture. The engine seems to be very sensitive to these settings, but the adjustments seem very coarse and I just can't get it to run smoothly. Maybe the o-rings on the throttle adjuster?

Tomorrow I'll clean the carb and fit new gaskets and seals - I still think this is
where I'm having trouble getting the air/fuel mix right. She drives well enough - sits happily on about 105 kmh on the hwy - but 'pops' and runs rough at idle and overrun, with strong petrol fumes and I would like to know if I am actually using all cylinders!

Thanks again for your help and advice

Roger

Originally Posted by dreamin'

Compression test (dry) as follows:
1. 130
2. 125
3. 130
4. 140
5. 130
6. 120

Taking all that into account, can these results tell us anything? I was expecting higher figures generally but also looking out for one or two cylinders dramatically lower than the others. Low readings overall could be wear, but
maybe just my guage? #6 (the one with the non-sooty plug) is definitely lowest but only by about 10 psi.

I do think that the spread of those readings is borderline with that lowest reading of number six cylinder being 20 PSI below the highest reading of 140 PSI, the difference is 14.29%, but that's not enough of a problem to pull the whole engine down unless it is also breathing oil fumes heavily, the most likely problem that has been shown by that test is that the exhaust valves now need some attention.

According to my workshop manual for a 7:1 compression ratio engine the test should show 140 lbs / sq in, see if you can catch the attention of Blknight.aus for his opinion, as Dave would have worked on a lot more of those engines and more recently than I have.

Started and idled fine but still a bit rough, 'lumpy'. Turned all the lights off and no visible sparks, so maybe they're staying where they should be,

good
.