Guys I need help solving this challenge.
I am finishing a K7 (PD41A carburettors) and it can’t get a vacuum reading at idle. I can only get it above 2.5-3k RPM. Yes the gauges work, and no there is no vacuum leak, I have sprayed wd40 everywhere and no sign of it being sucked in.
I actually connected the gauges to my other CB750 and they read 18-20 cmHg at idle.
So this bike has had valves, cam chain, timing, points gap adjusted. It’s all set correctly, or at least it should be. Valves are 2 and 3 thou as per the manual.
Carburettors have been cleaned very well, all settings are set to default apart from the needle jet clip which is on the second notch from the top. Main is 115 and slow is 35, mixture screw is 1 and 1/2 out.
Compression is good 120, 125, 130 and 125.
The bike starts easily and holds idle well but it sounds clunky like it’s out of sync. I have synced the carburettors as far as could but it still doesn’t sound right. When I rev it past 2.5k or 3k RPM I get vacuum readings.
When at 3k RPM it takes time to come back down after I close the throttle. And sometimes it races up to 3k RPM, yes it behaves like there is a vacuum leak but it can’t be that I have a generalised leak on all four carburettors.
Any ideas as to why this could be happening? I have gone over everything and can’t figure out why there is no reading at low RPM.
Thanks in advance!
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