Poor running when hot?

[GregMcN]

I have finally got my cb77 on the road (thanks for everyone's help) though this now allows me to find new problems (challenges!).

To recap the bike has a 350cc conversion and is running on CB750 carbs.

The bike seems to run quite nicely for about 10 miles of mixed running (cruising at 5- 6k revs and then stopping for traffic lights/junctions) but after about 10 miles it then will not idle and becomes increasingly difficult to restart. I have repeated this twice with exactly the same result. The plugs look quite nice so I am unsure if this is a carburation issue or something else.

Any thoughts/ideas appreciated?

Greg

[G-Man]

Tank vent blocked? Try taking the cap off as the bike starts to run rough.

Condenser? Find a replacement. Condensers begin to fail when hot...


G

I have finally got my cb77 on the road (thanks for everyone's help) though this now allows me to find new problems (challenges!).

To recap the bike has a 350cc conversion and is running on CB750 carbs.

The bike seems to run quite nicely for about 10 miles of mixed running (cruising at 5- 6k revs and then stopping for traffic lights/junctions) but after about 10 miles it then will not idle and becomes increasingly difficult to restart. I have repeated this twice with exactly the same result. The plugs look quite nice so I am unsure if this is a carburation issue or something else.

Any thoughts/ideas appreciated?

Greg

[jleewebb]

Might want to recheck your valve clearances. I had a similar issue, turned out to be a too-tight exhaust valve.

[GregMcN]

I have rechecked the valve clearances, redone the ignition timing, initially static then with a strobe. and the tuned the carbs at idle both manually and using a colortune.
I have also changed the condenser (though since not new I have no idea if better or worse that original).
I also checked the tank vent in the fuel cap which could possibly have blocked because the seal is not correct so I modified to try and keep the hole clear.

Went for a ride but same as before fine at first but after 10 miles would not idle or restart when stalled but found it would start if I gave it choke. I then vented the fuel tank and found it would idle so I thought problem solved! Did another 10 miles problem returned so vented the fuel tank but this time would still not run. It would restart on the choke but would not idle without choke on. I tried richening the idle mixture via the air screws but could not get it to idle that way.
Gave up.

Can anyone make any sense of this?

[DianneB]

If you need choke to get it running, that a sign you have an air leak in the intake.

[GregMcN]

Thanks Dianne

That was one of my thoughts but the plug colour does not imply it is running lean so I am confused as to whether it is too much air or too little fuel and why it changes when it gets hot / does some mileage.

I can not see any obvious problems with air leakage but will certainly look at it again.

Greg

[Geoff Hastings]

When it dies, try pulling the fuel line off the tap to check the flow. I had the same problem and ended up drilling an extra air bleed hole in the filler cap.