SOHC.co.uk Forums > CB750

1&2 not firing.

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Spitfire:
Hi Andy there should be some mesh on the inlet to the petrol tap, plus there are individual filters on each carb.
To check the individual filters remove the carb float bowls, then making sure that you do not mix them up remove the four floats and needle valves (or do it one at a time), then using a 10mm spanner (or socket) remove the needle valve housing, the rust etc will be on the filter and in the carb.
Don't ask me how I know but be very careful doing this as you do not want to break the little legs that the floats hinges off if you use a spanner !
Here's the filter

and this is what was in it


Cheers

Den

Andy1976:
Hi guy's
Thanks for your replies.  I've done some checks this aft, got a spark, got fuel getting in (plugs wet), but no bang (only on 4 now).

I'm going to do a compression test tomorrow, what should the readings be?

I've a funny feeling it maybe a head gasket problem.

Cheers
Andy

Andy1976:
Test results  No.1  135 psi
                  No.2  120
                  No.3  110
                  No.4  150

This was done on a cold engine as it won't run properly (only on No.4) to get it warm.

Could this be the head gasket blown?

Cheers
Andy

Spitfire:
Don't think this would be a head gasket, it's an air cooled motor so the pressure would find it's way out somewhere (breather or external head/barrels) and would be fairly obvious, more likely rings/bores, but the test may not be that accurate if the engine was cold.
You could still be having carb/timing issues, have you checked the valve clearances, they could be causing a lot of your problems including compression.

Cheers

Den

Andy1976:
Hya Den,
I'll double check valve clearances tonight, I also found out that I should've had the throttle wide open when I did the comp test so I'll re do that as well.

Andy

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