Recurrent misfiring problem

[Oneball]

I use this Gunson timing lamp. With the engine running I simply fit the inductive pick-up on each HT lead in turn and pull the trigger. If the light doesn't flash then that cylinder isn't firing. If the flash is sporadic or intermittent there's an occasional misfire.

If that is the case then your problem is almost certainly in the electrical system. There are very few non electrical issues that test would show up (flooded plugs for example)

 

[antijam]

If that is the case then your problem is almost certainly in the electrical system. There are very few non electrical issues that test would show up (flooded plugs for example)

I was much of your opinion. However, I've tested and swapped all the ignition parts separately. Everything is in spec and each item seems to be functioning correctly. The fact that all plugs fire satisfactorily outside the cylinder and my spark tester verifies an adequate voltage on all HT leads is leading me to think that the problem must lie elsewhere.

 

[teamzr1]

If the sparkplug gap is correct and your testing showed high voltages from coil than
it would have to be a makeup of the charge to the cylinder or dirty exhaust pulled back into the cylinders

Above points, I made and using a borescope to closely look at top of pistons and face of valves and seats
IF thinking there are misfires then be interesting to see the difference of AFR of B1 and B2 using
an AFR tester at muffler tips

 

[Roscobbc]

I was much of your opinion. However, I've tested and swapped all the ignition parts separately. Everything is in spec and each item seems to be functioning correctly. The fact that all plugs fire satisfactorily outside the cylinder and my spark tester verifies an adequate voltage on all HT leads is leading me to think that the problem must lie elsewhere.

Remember the old Colourtune sparkplugs?. With it's glass (rather than ceramic) insulator you would adjust your ideal mixture to a perfect burn and see the 'burn' through the glass insulator. Of course you would have to repeat for multi-carb/choke engines but it could be used to 'visualise' and establish it individual cylinders had potential issues.

 

[teamzr1]

Interesting, never heard of this test tool before
Some info

Tuning the engine with a colortune plug

Although some experienced mechanics can tune a car's carburettor fairly well by ear, to do the job really accurately you need the help of special equipment.

www.howacarworks.com

Sold in the UK

Gunson Colortune Plug Kit (14mm)

Gunson Colortune Plug Kit (14mm) from Frost.co.uk - The Specialist of High Quality Tools & Equipment for the Classic Vehicle Restorer

www.frost.co.uk

Gunson G4074 Colortune Single Plug Kit 14mm

Gunson Colortune plug kit lets you tune 2

www.gunson.co.uk

 

[antijam]

Remember the old Colourtune sparkplugs?. With it's glass (rather than ceramic) insulator you would adjust your ideal mixture to a perfect burn and see the 'burn' through the glass insulator. Of course you would have to repeat for multi-carb/choke engines but it could be used to 'visualise' and establish it individual cylinders had potential issues.

Yes indeed Rosco! - I didn't know they still made them? A rummage through my 'Tune-up' tool kit revealed this....


...a near pristine Colourtune kit! It has to be something of a museum piece - the last time I used it must be more than 40 years ago! It was very useful in setting up old SU's - you could adjust the mixture very easily with the engine running.

 

[Roscobbc]

Yes indeed Rosco! - I didn't know they still made them? A rummage through my 'Tune-up' tool kit revealed this....

View attachment 33078
...a near pristine Colourtune kit! It has to be something of a museum piece - the last time I used it must be more than 40 years ago! It was very useful in setting up old SU's - you could adjust the mixture very easily with the engine running.

I was going to add that were great for setting-up SU's or Strombergs - can't remember the viewing tube and mirror gizmo.....and yes its also more than 50 years ago used one. Shame we didn't start this conversation a few days earlier - had you not stripped the inlet off the engine you could have tried this on the 'iffy' cylinder.
Irrespective of that it would be interesting to try this of each cylinder in sequence to see how the air/fuel mixture is on each cylinder at idle........even when all is 100% there will be variations between cylinders.

 

[teamzr1]

If your borescope has a record video mode
Insert that AFR tool into 1 of the cylinders you think has misfire
Rig the borescope so it could record the tools color and go for a drive and check AFR under real-world conditions
and then replay the recording

This might also trap if there is any small cracks in a sparkplug arcing to ground

 

[antijam]

I was going to add that were great for setting-up SU's or Strombergs - can't remember the viewing tube and mirror gizmo.....and yes its also more than 50 years ago used one. Shame we didn't start this conversation a few days earlier - had you not stripped the inlet off the engine you could have tried this on the 'iffy' cylinder.
Irrespective of that it would be interesting to try this of each cylinder in sequence to see how the air/fuel mixture is on each cylinder at idle........even when all is 100% there will be variations between cylinders.

Trouble is it only works if the plug is firing. Even then using the viewing tube and mirror could be tricky in tight places. I'm trying to imagine getting a sensible view of the plug in #7 of an sbc with headers !

 

[Oneball]

Have you pulled the plugs out and compared a good cylinder with a bad one?

 

[johng]

Don't know if your's is the same, but my old Colourtune won't fit into the spark plug hole, the body is too wide.

 

[antijam]

Have you pulled the plugs out and compared a good cylinder with a bad one?

I pulled all the plugs to do the compression test and noted which cylinder each came from. The only visible difference of those from the non-firing cylinders was that they were wet with fuel.