Topic: RPM question - Stock 72 CB750

[Atlantic]

So, I have been riding the thing for a little now.
I noticed that the bike seems to not be very rpm happy. When I first detected that, I replaced the air filter which didn't change much. Maybe it's just me not being used to the bike (I used to ride new bikes)? Here is what I feel.

- I have a very hard time to get it over 5000
- I don't think I ever had it at 6000
- I have to re-grab the throttle to get it to speed up on the freeway

Could there be any kind of restricter (back in Europe they sometimes put a washer between carb and cylinder)? I just feel it doesn't breathe right - or is this normal? Wish someone else could ride the bike and let me know (and fix it

THX !

[fire113]

Hi Atlantic,

...do a complete tune up (set the valves, clean the carbs & sync them, check the timing, new plugs) and try again.

Good luck...Georg

[andy750]

Its not normal. Check the plugs and let us know what you find - photos are best. Maybe running too lean?

[Atlantic]

great - your answers being so definite confirms my suspicion. I did the valves and the plugs looked good (not hot/lean). 
I might have to give the carbs and the timing a good look. First thing I'll try is to start/run it without the air filter to see if the restriction comes from there.

[Gordon]

That sounds like a 750 that's only running on three cylinders.  Lightly touch the headers as the engine is warming up, and I bet you'll find one that either stays cold or only gets slightly warm. 

[Johnie]

If you have not cleaned and synced the carbs I bet it needs it. Those items would be the last thing you do after you change out the points, condensors, and valves are done. Would not hurt to throw new plugs in and cut a little off the end of each plug wire and reinstall the plug cap. And of course be sure the tank is free of any crapola. Good luck and let us know how you come out.

[madmtnmotors]

- I have to re-grab the throttle to get it to speed up on the freeway

When I experienced this particular symtom it turned out that my throttle grip was coming loose from the throttle sleeve. At least verify that your throttle grip is still firmly connected to it's throttle sleeve.

[Atlantic]

okay guys - here I am back some 6 weeks after the initial posting of this thread. But, hey, I am a strong believer in wrapping these things up so they might benefit someone else some day.

Well, I was lazy and just parked the bike - too much other stuff going on. Last Saturday then, I started it up to see if and how I can address the issue. It ran so well, that I took it to a weekly classic car meet here. My drive includes a little freeway section ... bang, I could easily get it up to 7000. Something that never happens happened - it healed itself (usually problems get worse). I rode it yesterday again and put maybe 35mi on it - no problem.

I am thinking it was gunk in the carbs that got dissolved over time. To help this process I purchased some cheapo carb-fuel injection cleaner and pored in the tank.

Thanks for all the good ideas and input again!