1979 cb 750f Supersport turns over but won't start

Johnan · Nov 24, 2022

Everything electrical is new besides pulse generator and spark units I checked compression and it's good but when I try to start it gives a weird sound and occasional fire ball out of the exhaust pipe I have new carbs ,battery ect.... any advice would be greatly appreciated

jpdevol · Nov 24, 2022

Have you had this bike running before? New carbs; what make model (maybe pics)? Have you verified that you are getting spark by placing a spark plug in the cap, grounding to the head and rolling the motor while watching for spark (cyl 1 & cyl 2)? If you remove a spark plug, can you see or smell fuel present? Have you tried spraying a starter fluid into carb intake while trying to start? All that trying to determine if the issue is ignition or fuel related issue.

And...can you further describe the "weird sound" and is it once a crank revolution or half that frequency?

Johnan · Nov 25, 2022

jpdevol said:
Have you had this bike running before? New carbs; what make model (maybe pics)? Have you verified that you are getting spark by placing a spark plug in the cap, grounding to the head and rolling the motor while watching for spark (cyl 1 & cyl 2)? If you remove a spark plug, can you see or smell fuel present? Have you tried spraying a starter fluid into carb intake while trying to start? All that trying to determine if the issue is ignition or fuel related issue.

And...can you further describe the "weird sound" and is it once a crank revolution or half that frequency?

Yes I verified spark and yes I can see and smell fuel and the weird sound is like a warping sound and that's when I see the fire from the tail pipe like fuel buildup in the chamber. I have brand new Mikuni carbs from Murrays carbs and running 4 into 1 exhaust I will get pics soon 1979 cb750f Supersport and I will try the starter fluid thanks !!!

dirtdigger · Nov 25, 2022

What is "good" compression? Fireballs out the exhaust indicates possible timing issue....

Johnan · Nov 25, 2022

dirtdigger said:
What is "good" compression? Fireballs out the exhaust indicates possible timing issue....

Compression is at 125 on 3 cylinders and 110 psi on one I know it can be higher but should be enough to start right ? And I'll check timing once again I'm suspecting the pulse generator what you think? Thanks!

dirtdigger · Nov 25, 2022

Barely enough compression. 140 to 150 minimum on these motors for them to run ok. 110 is on the edge of missfire. I would highly suggest checking the valve clearances before going much further. They are more than likely too tight but if they are .oo5" then you have other mechanical issues.

1979 cb 750f Supersport turns over but won't start

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