Bogging Down Around 5k RPM

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Bogging Down Around 5k RPM

Lochabar
Hey, fellas. Been doing a lot of my troubleshooting on here, and you guys are a fantastic help!

I ride an 81 CB750, and the problem I'm having right now is that I can't seem to get my bike to run over 5k RPM without it bogging down and almost cutting out on me. At first I thought it might be that the carbs were dirty, so I tore them down to take a look, and I cleaned them up (turns out they were in great shape, but I figured I might as well give them a wipedown while I was in there anyway). The I was seeing a lot on here about stator failure causing hesitation, which made sense after my battery died about an hour into a test ride around town. Well, I replaced the stator and rotor, but it's still bogging down. Haven't had the time to take it for an extended trip since I took care of that, but the charging system seems fine. Also, as a side note, changed out the starter, solenoid, and wires because I was getting a click from the solenoid, but it wouldn't turn over. At least it starts now XD. Now, the only thing I can think of being an issue is that the guy who previously owned it put a 4-1 exhaust on it, but I didn't check to see if he rejetted the carbs. That's something that I can take care of, but might be a bit until I can get t . I was wondering if anyone had any other ideas about why I'd be losing so much power that I'd be basically stalling out over 5000.

Thanks a bunch, gents.
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Re: Bogging Down Around 5k RPM

Truck
Did you check the voltage output after changing out the stator? the DOHC charging systems suck. I replaced my entire system then had to clean every connection to get it to charge right.

Assuming you are using the stock air box correct?
It's only illegal if you get caught.

If at first you don't succeed, use more lighter fluid

95% of Harley Davidsons ever made are still on the road... The other 5% made it home.

New Baltimore, Va '82 CB900c, 1980 CB985F/K 'Mutt"
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Re: Bogging Down Around 5k RPM

Lochabar
Didn't check the voltage output, but I'll get on that the next time I get a chance to get into the garage (hopefully after work tomorrow). I did clean everything that I could get my hands on when I changed the stator. And I do have the stock air box. I've taken it off and made sure it was cleaned out, too. New filter and all that jazz.
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Re: Bogging Down Around 5k RPM

big-nate
im in the same boat as you are. my bike is doing the same thing. ive replaced the whole charging system, coils and spark plugs,  checked valve clearance, compression, sonic cleaned the carbs, and re jetted it twice. the only thing i can think of it being is a head gasket problem. if you have any luck let us know.
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Re: Bogging Down Around 5k RPM

Re-run
Administrator
Well, if it were my sohc(different carbs though), I would check if my ignition pickups are set right. I don't know the dohc ignition, though. But many ignition problems don't show up till higher rpm. Honestly, your charging system really shouldn't matter as these bikes should run fine as a total loss system till the battery drains too much. The manual should describe some ignition tests that can be performed to make sure things are firing right at higher rpm.

I would also attempt a high speed plug chop. Bogging down like that can be a sign of not enough fuel. Your fuel goes through more than 1 jet at that throttle position.

Also, check float levels. A level that might be fine for lower rpm, might not be for higher rpm. Many people goof when setting float height. Best way to do it is to turn the carb just enough so that the float closes the needle valve, but not so much that the little pin in the needle valve gets pressed in. About a 45 degree angle is optimal, so turn the carbs upside down, with floats up. Then turn to about 45 degrees and the needle should be closed but the little pin in the needle should not be pressed in.
Then, do not measure from the from edge of the float, measure from the side of the float, in the middle. Low float measurement means more fuel in the bowl, higher means less. When you turn the carbs right side up, you'll see what I mean.
From there, you need to make sure the bowls do fill properly. Hook clear tubing onto the drain nipples and then open the screws. The fuel will rise to the same height as the fuel in the bowl. All 4 should be about even. This is a bit of a pain with the vacuum petcocks but it can be done.

I don't see why you would think a head gasket issue. If you don't have oil leaking out, the gasket should be fine. If in question, do a compression check with a gauge that can read motorcycles. Larger auto gauges do not always read smaller engine pressures, properly.
The ride IS the adventure. The destination is just to get gas!