Poor running when hot?

[DJM]

The extra vent hole in the filler cap is an official Honda service update, I have details somewhere. The theory is that the rubbber (originally cork) washer can move blocking the vent hole, if you drill a second hole exactly opposite the original the washer can't block both holes at once. Makes sense and I put the extra hole on both my bikes as a precaution.

Not sure it will solve this problem though but it's worth a try.

[GregMcN]

Thanks Dennis / Geoff

Mine has the cork gasket and I tried stopping it moving but not 100% sure I succeeded so will drill another hole. Is this necessary if you change to the rubber seal? Are the caps the same?

Greg

[GregMcN]

Thanks Dennis / Geoff

Mine has the cork gasket and I tried stopping it moving but not 100% sure I succeeded so will drill another hole. Is this necessary if you change to the rubber seal? Are the caps the same?

Greg

[DJM]

The style of the cap can vary a bit but the CB72/77 style cap is almost generic and different versions were used on lots of bikes over a long period of time.

I can't just locate the service bulletin relating to the extra vent hole but, from memory, it doesn't differentiate between rubber and cork gaskets. The extra hole would be a sensible precaution with either.

[flahotrodder]

You might want to check the petcock screen for debris. It sounds like it might be flowing just enough to keep the main jets covered but not the low speed jets which are higher in the bowls. Also if the carbs are mounted at the same angle as your originals you may need to compensate the float level settings. If I remember correctly those carbs sit close to level on the 750. I have those same carbs on my CB72 and I had to raise the float level slightly.

[GregMcN]

More work and investigation.

Second hole drilled in fuel cap so hopefully any issues resolved.
Carbs stripped down and some sediment in bowls and partial blocking of pilot jets, rest of carbs looked OK. Some deposit in petcock bowl and on screen.
Fit of carb to head rubberm joiners was not as good as I thought so possibility of leakage here ... though I am not convinced it was actually leaking.

Difficulty is knowing which was causing the problem or combination but will attempt to resolve all.

Finding better carb to head joiners is not easy, Tour Max appear to manufacture a range but only listed by application, I have been unable to find specifications. If anyone happens to know of any female / female applications with 37mm for one please let me know.

Greg