Topic: 78 750 F Lost top end , bike will not exceed 6000rpm HELP!

[joelt85]

Hey guys, new to the forum really hoping for some help. I have a basically stock 78 CB750F , ran great all some , been riding her a little hard some times but do all the regular maintenance and take good care of it.

Problem : About a month ago it started sputtering and bogging down almost like it was running out of gas, that would happen on and off and go away , problem continued to get worse , never stalled , but when at a constant rpm anywhere from 3000 up it would seem like it was starving.
-Pulled the tank checked the gas flow, flows great ,
-changed spark plugs ,
-re-set the point gap (new points and condenser installed early summer this year) ,
-re-set the ignition timing (which was off a touch) .
-checked the valve clearances ,
-pulled the carbs tore em down and ran em through the ultrasonic cleaner and check the float heights etc ,
-took a known good set of coils from another bike and tried them on there , all this with no luck in fixing the problem.

Did a compression test last week on it , will build up to 150 psi or more on all cylinders.

Really not sure what else to do , hoping some one might have some answers or suggestions.

Only other thing I can think of right now is the condenser is not working properly and I will be replacing it shortly.

Its a mystery to me.
Thanks guys
Joelt85

[dave500]

air filter?

[joelt85]

Air filters good , second thing I checked , should have mentioned that.
Thanks

[phil71]

fuel is a good instinct, but that behavior can also be caused by an undervolt condition. If you dropped a leg on the stator, it would charge, but not at the voltages that coils like for hi revs. Just a thought.

[trueblue]

Possibly the ignition advancer is stuck, check with a timing light if it is getting full advance when you rev it up.

[Markcb750]

I suspect your float levels are set too low for the high speed jet to feed fuel properly.  the cross over from low to high jets occurs in the 3 to 4,000 rpm range.  sufficient fuel flow/depth will cause bogging as you describe.

There are several threads here that discuss using cleat tubing to properly set the floats.  I found setting them with a steel scale to the height indicated in the carburetor manual inadequate and had to adjust them a few mm lower to correct a flooding issue on my 750.

The 450 I did this spring had the same symptoms you describe, I had to set the floats a few mm lower then the book stated to reach full RPM. 

[MoMo]

Check the fuel cap vent...Larry

[joelt85]

ok , so thanks for your replies , I went to the shop today after really not doing anything to it other that the recent compression test , stuck a pin in the fuel cap vent to make sure there was no blockage , didnt seem like there was anything. but i fired her up and briefly no1 cylinder was not firing but then caught and I rode it all day with no problems , bike runs as good as it ever has , top end is perfect.

I dunno what i did but hopefully it stays fixed.
Thanks guys

[ABikeNamedTrouble]

+1 on trueblue's comment.

That sounds very much like a sticking spark advancer to me.  If it needs grease, it could freeze and then work itself loose, "magically" fixing the problem.

It's a quick check/fix anyway.  Just pull it off, lube it up, put it back on and get back on the road .