SOHC.co.uk Forums > CB750

jumping ...missing and backfiring

<< < (5/8) > >>

K2-K6:
I was looking at this last week but couldn't reply as my temp wi-fi link wouldn't keep me on long enough to write it, I was over in your neck of europe bitsa but a fair way from you in annecy/moutier/frejus area and drove back yesterday.
My initial thoughts barring any simple air leaks/fuel jet blockages etc are those that Bitsa says above in bold, are you sure the cam timing is right?
If the cam was timed a little in advance of spec then it would step toward what you have experienced I'm not saying it's the only thing you should look at but you'd need to be confident it's right to avoid chasing your tail through lots of other stuff.
I know this ignores the exhaust cam timing (which would also be out) but if you run the inlet cam in advance of the correct setting it would have the effect of compromising low speed running to potentially give gains at higher rpm, similar to using a wild cam in a race engine.
The inlet cam should open at low rpm late enough to prevent any mixture being drawn in and being fired by the wasted spark on the exhaust stroke of the engine, this would happen (the small amount of mixture being ignited and come out of the inlet because the valve is open too far) if you have got the cam in advance of the correct timing.
I was trying to think of a way to check the timing without removing the engine and the logical way is to use a degree disc on the crank to measure against spec if you are familiar with the method.
If not you could try a rough and ready setup to see if you get an improvement by backing off the inlet tappets to twice our three times manual spec's (should retard the inlet valve timing) and retarding the whole ignition plate by about 5 degrees (this would move the wasted spark further away from the inlet charge entering the cylinder) and then run the engine to see if the spitting back has been reduced, this is not for a solution overall but to see if the motor will run smoother at low rpm and give you more reasonable cause to remove the motor if required.
See what others on here think, please shout if anyone disagrees with the above.

billy192:
thanks Guys,lots for me to look at. when i put the cam back in i had the timing mark on the crank and the notch on the cam lined up. is there another way. surely if the notch is pointing up and the two lines are horizontal also 1-4 is at T then the cam is timed? or am I wrong? i can just about see the notch in the cam with the engine in the frame.
Billy

mike the bike:
That's correct; the 2 marks on the cam gear level with top of head while at TDC

fisjon:
Hi 'Billy'
This is john here from Caerphilly, is this the beast that I did the carbs for? If so then I can say that the carbs were definitely on the bike tidy, the carb balance correct and the ignition timing and points dwell spot on.

I do remember that the engine had a habit of racing up on it's own for no apparent reason and a it came back down when it felt like or after you blipped the throttle a few times, that might have been the fuel supply as it was a temporary arrangement at the time.

The only other thing I can think of that would cause that is a sticking advance retard mechanism. You say that you have checked this out, but did you remove it and clean up, lube and re-fit or did you just check with a strobe. If you checked with a strobe it still might be temporarily jamming up and that would cause misfire/popping but this would be on all fours pots. The other thing is the springs, are they the correct ones as I seem to remember they are different strengths.

The problem with these old Honda electrics is that they have a very weak spark to start with, if there is an electrical fault it won't take much to upset the spark output.

Just a thought.
john

mihai.i:
I had almost the same problem. Turn out there was air leak between carbs and admission. Also, i had problems with my points.
After new o-ring for the admissions, timing and tappets tune up all is good!

Also check for the following:
- good battery
- problems with the fuel line
- good sparks and spark plug
- good air filter
- carb sync

Navigation

[0] Message Index

[#] Next page

[*] Previous page