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Rear Carb out of control

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bob hughes

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Can the collective help me with this little problem on my BJ7?

The other day I experienced a problem after travelling in access of the speed limit of 70MPH on this side of the water - the engine started misfiring badly.

Checking the cylinder pressures (without oil) most cylinders were around 160psi +/- and number two was around 150psi. The plugs where white-ish. Thinking that I had burnt a valve I took the head off but all was OK there, changed the coil for another (not new) and checked over the points etc. Still no good. Changed the jet head on the front carb - due to a damaged rubber grommet - could not just by the grommet locally.

I took the car to my expert who changed the leads, end caps, points etc. and eventually had it running - with a strange note, not like the usual.

Brought it home and after 20miles into the journey - the misfiring started back again.

Took it around to me local garage and they kindly checked the exhaust gasses - the rear carb was running very very rich indeed, pulled the plugs and sure enough they were sooted up. Changed them for a new set and that temporarily solved the problem.

Tried to adjust the rear carb and now the jet head is up as tight as it will go and we are still running rich. The jet and the needle are both standard and the needle looks to be undamaged. On a whim, I have ordered up a new jet head for the rear carb and it should arrive tomorrow, but has anyone any other ideas as to why I can not control the carb? - Front carb is OK.

:cheers:

Bob
 
Could be that your piston is binding and the return spring cannot push it back down. Try removing the damper while the condition is happening and then pushing it back in to see if it lowers the piston. Also check the piston return spring to make sure it's not damaged. Good luck.
 
Just to update the collective on this one.

We think that there were two problems, one electrical - in the dizzy / plug leads / coil and the other was in the fuelling. My expert solved the electrical side of things and the purchase of the new rear jet head resolved the fuelling problem - will now purchase new needles to complete the carb overhaul. Just got to stop the popping and banging in the exhaust which I am told is due to a small air leak and we will be tickety boo again.

:cheers:

Bob
 
Hi Bob,

To clarify for my education, is your reference to the "Jet Head" the same as the needle seat?

I have HD8s and the seat has an attached diaphragm. A while back I had my rear carburetor get stiff and crack. This allowed additional fuel to enter and cause a rich condition. it also caused backfiring. Changing the pin and seat solved both problems.

Sorry for being late to respond and my experience may not be applicable to your carburetors.

Thanks,
Ray (64BJ8P1)
 
Hello Bob, I am also sorry that I couldn't contribute soon enough to help you, but for others in the future with a similar problem, I can concur with Ray. Some years ago, I had just cleaned up my carbs ( HD8s) without replacing any parts. they worked good but the idle was always alittle too fast and the plugs alittle too rich. It was because the diaphrams on the jet seat thru age had just gotten very stiff. Once I change to new jet seats the carbs work perfectly. Dave.
 
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