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[Timmo]

Hi all, I’m looking for some help. I have a 1967 Interceptor Mk1 a and it has Developed a misfire on the left hand cylinder and is popping and banging in the exhaust.
I have stripped the carb and cleaned it, swapped the coils over, swapped the leads over, fitted new plugs and checked the valve clearances, all to no avail.
The right hand cyl is perfect.
I have compared compression on both cylinders by removing the plug and kicking it over and they seem comparable.
Oh it has electronic ignition fitted. Any ideas?
Thanks

[Daiwiskers]

Did you check the compression with a tester or just your thumb? it's almost impossible to tell if you have 150 or 60 with the thumb

If tested with tester and both showing within 10% I would check for air leaks both at inlet and exhaust

Hope this helps Dai

[Daiwiskers]

Too late to edit

I have a feeling that this is a exhaust valve problem on the left cylinder
Or a air leak on the inlet left side

Sure you'll find the problem Dai

[Timmo]

Thanks for the response Dai, I checked by removing each plug and feeling the resistance on the kick start so pretty approximate. I’ll invest in a compression tester before I go any further I think.
Cheers

[Daiwiskers]

Not sure what the test should show, but 50 odd years turning spanners I have found most engines will run with about 90psi, but that's a low reading I would think your looking for 120-130, don't worry if there not both exactly the same within 10% is near enough.

Do your test with throttle wide open or with carb removed both plugs out to make it easier to kick over, 4 or 5 kicks should get you there .

As I said it might be a air leak on the inlet left hand side, if you can get it started try spraying the inlet with WD40 or similar, the revs increase you have found the problem and may just need a new gasket made from a cereal box and a bit of sealer, two gaskets you will need to replace both if you need to take the manifold off

Good luck Dai

[Timmo]

Found it Dai. Did the compression with a tester and had 120 psi on both so borrowed a carb balancer.
The second I put the cone over the Carb inlet the left cylinder burst into life. So carb off again, removed pilot jet which was clear but the fuel passage was blocked, so airline and wd 40, put it back together and bingo.
Thanks for your help

[Daiwiskers]

Glad you got it sorted
Here was me thinking you had a single carb
Compression tester was well worth the money now you know the top end is all good

Take care Dai

[windmill john]

The thumb test almost got me last week. Both cylinders felt the same on my thumb (1976 CB500T) turned out one was 150 and the Other 60!
I knew I had an issue and have just replaced a valve.


John

[Bullet Whisperer]

The thumb test almost got me last week. Both cylinders felt the same on my thumb (1976 CB500T) turned out one was 150 and the Other 60!
I knew I had an issue and have just replaced a valve.


John

Was that due to one exhaust valve being very slightly bent?

[windmill john]

Yes.

Got the bike last year with no compression on the left. Replaced the inlet valve and put it together.
Some lunatic, I won’t mention who.... ahem... then bent the right exhaust valve when rotating the exhaust camshaft to attach the camchain when obviously in hind sight, the right cylinder was at TDC.
I see what I did wrong, I mean I know what the idiot did wrong!
I should have known by the pressure I exerted when turning the camshaft.

Stupidity, I’ve leaned my lesson.


ANOTHER... reason to love OHV


John