View Full Version : Cb won't start warm

06-04-2015, 6:03 AM
Just got on this site, seems like a decent bunch of folks. I have a 77 cb750 that I've been riding for years. Last year it started screwing with me a little. It starts & runs fine from a cold start, & runs great till you shut it off. Wait 5 minutes,& try to start it & you're shit out of luck. Wait an hour, starts right up. So far I've replaced the points & condensers adjusted the valves & cleaned the carbs. Also put a new acc. pump &float jets. Any thoughts?

06-04-2015, 9:21 AM
When you try to restart is it even turning over or not firing at all?

06-04-2015, 9:47 AM
It turns over fine, just won't fire. If I'm by a hill, I can pop start it & it will run fine again,just can't get on the throttle right away.

06-04-2015, 4:29 PM
Is there spark when this is happening? I don't mean the starter turning over, I mean the coils firing. It could be related to the RUN switch which shuts off the coils. It's not an uncommon problem with age since the RUN switch is taking all the current that flows to the coils all the time when it's ON. They sometimes develop intermittent problems in their solder joints because of the heat associated with handling that amperage to the coils.

06-05-2015, 5:17 AM
How are you testing for spark?
What do the spark plugs look like?
If its not your coils....
Fuel overflow issues like non-vented tanks, bad float needles or simply too high a float setting can flood while sitting. esp the left carb(s) while on the side stand.
A Shovel with an S&S and way too much lean came by the shop with hard hot start problems. I clamped it vert on a lift and demonstrated no starting problems till it was on it's side stand again.
My theory, when the bike sits long enough with the petcock closed, there's time for the excess fuel to evaporate/washdown.

06-05-2015, 8:49 AM
Spray starting fluid or carb cleaner on your intake boots between the motor and the carbs and listen for a surge in your idle. Could have a small air leak at one of the boots that get worse when warm. Do you leave the fuel valve on when you park it?
Try some experiments like park it on the center stand warm n re start it. Does cracking throttle a lil help it start when warm? Does using the choke make easier or harder to start warm , have you tried kicking it. Kicking tells u a lot more about the bikes state of tune , is it a 1-3 kicker cold or does it take a bunch of kicks?
Check fuel tank vent for a clog also.

06-06-2015, 7:00 PM
Typically I like to test the spark by having one of my buddies hold the plug wire while I crank it like a maniac. Failing that, I put a screwdriver that fits the cap & jump the spark to the plug. I haven't had a chance to work on it the last few days, but I will try some of your suggestions soon. Thanks dudes!

06-23-2015, 8:29 PM
Got a chance to work on it a little,found #1 cylinder is fouling the plug with too much gas. Going to reset the float level and check the compression