Topic: CB750 only starts with starter fluid, but runs great

[pdcombs]

So I just got done with the top end rebuild on my 78 cb750k and can't get it to start on its own. I decided to pop the air filter off and spray some starter fluid into the intake and boom, it fired right up. After that it runs great. I ran it for about 5 minutes changing the throttle/rpms and letting it idle on and off. After I hit the kill switch, it won't start back up unless a little starter fluid is sprayed back in there.

I replaced the spark plugs, all the gaskets and o-rings in the top end, lapped the valves, adjusted the valves, set the timing. It also has a Dyna S electronic ignition. The bike was running great when I parked it a month ago to start the rebuild. I cleaned and synced the carbs just 6 months ago. So I didn't bother with those since they were running really well before tearing it down. And it runs very well now, just can't get the initial start on its own.

After searching for a while I haven't found any solutions. Most of the posts I found had a couple suggestions that didn't really help, and the OP never posted the solution.
I'm stumped and would greatly appreciate any tips or knowledge you guys might have on this issue.

[540nova]

Sounds like it's too lean.


Sent from my iPhone using Tapatalk

[Grabcon]

check valve clearance. It sounds like maybe tight clearance.

[markreimer]

When you rebuilt the carbs did you confirm the float levels? Maybe they are too low and it's running lean. Also worth confirming the pilot jets are clean as can be


Sent from my iPhone using Tapatalk

[pdcombs]

I can check the valve clearance later this week. Also it wouldn't hurt to check the float levels, but yes, when I cleaned the carbs last, I verified all that was good. And it ran very well until I tore it down a month ago. Since then I haven't touched the carbs so I figured they should be the same as when I took them off.

Update: Tonight I adjusted my dyna s ignition timing just a hair, and it is able to start only if I am at WOT. No need for the starter fluid now. I need to continue to do some tuning and checking. But It seems that I am getting closer.

[scottly]

Sounds like your ignition timing was retarded, and when you adjusted it "just a hair" you advanced it. (rotated the ignition plate counter-clockwise) You could try rotating the plate a bit more, but you really should check the timing with a timing light.

[harisuluv]

So how did the carb choke sync go?

[pdcombs]

Just wanted to post back my solution:

I ran back though the valves and found one was just a little tight. After adjusting that and setting my electronic timing with a light, it started a lot easier. Just a couple kicks everytime. I also found that my battery is shot so I am assuming I am getting weak spark until that alternator can kick in. Even with a charger hooked up it won't start with the starter motor, but will if I kick it. So once I replace the battery it looks like everything is good to go. Thanks for the help.

[Airborne 82nd]

I thank you should revisit your accelerator pump It must not be working. With it setup right it does the same thing you are doing with the can of spray.