RPM ISSUES

If it's broken or just needs tweaked

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Cresty
Posts: 8
Joined: Fri Aug 10, 2018 4:52 pm

RPM ISSUES

Postby Cresty » Sat Apr 20, 2019 1:35 pm

I purchased a 1981 CB650C with the engine top end disassembled. I rebuilt the engine top end by refacing the valves, installing new piston rings and gaskets as required. All other parts were inspected and found to be in good shape, 16,000 total miles at the time. Engine runs great, but has a RPM problem. Any RPM over 4700 rpm the engine will start to miss and cutout until the rpm is reduced. Any rpm below that the engine operates find, at idle or hard acceleration up to 4700 rpm all is find. Timing is right on, valves were rechecked ok and all new plugs installed. When I say cutout, I don't mean the engine quit, just acts like a limiter on the rpm. The only other thing I have not look at is the ignition leads which are original and not the best. Can any one here give me some idea of what's going on with engine? I guess as long as I keep my speed below 60, I could live with it, but I live to get on it at times. Please help me, I just love this bike.

flatlander80
Posts: 23
Joined: Fri May 05, 2017 9:08 pm

Re: RPM ISSUES

Postby flatlander80 » Sun Apr 21, 2019 9:43 am

Check your timing advance weights under the points cover. If they're sticking, it won't advance your timing. It's an easy check.


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GeorgeSweety
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Joined: Tue Aug 08, 2017 11:47 am
Location: UK.

Re: RPM ISSUES

Postby GeorgeSweety » Sun Apr 21, 2019 2:14 pm

What flatlander said, my money is on your timing advance weights as well. They seem prone to rust so you should take them out, strip them then clean them up and lubricate them. My advice is to take pictures before doing it so that you put them back together correctly.

Cresty
Posts: 8
Joined: Fri Aug 10, 2018 4:52 pm

Re: RPM ISSUES

Postby Cresty » Sun Apr 21, 2019 9:49 pm

Never thought about that, I should be able to check the advance timing weight movement by checking the timing at the higher rpm. I think I read about doing that in the manual I only checked timing at idle. Thanks guy for the information, I'll post what I find.

Cresty
Posts: 8
Joined: Fri Aug 10, 2018 4:52 pm

Re: RPM ISSUES

Postby Cresty » Mon Apr 22, 2019 6:38 pm

GeorgeSweety wrote:What flatlander said, my money is on your timing advance weights as well. They seem prone to rust so you should take them out, strip them then clean them up and lubricate them. My advice is to take pictures before doing it so that you put them back together correctly.

I just checked my timing advance weights as you suggested and they were free and not binding. I lubricated them and checked timing again with a light and you could see the timing advance. The engine under no load will turn any rpm you desire. One thing I notice though is that the rpm will advance to a higher rpm easier in the lower gears rather than say 4th or 5th. So engine load must have something to do with it. Thanks for the information, if you think of any thing else give me a post.

Cresty
Posts: 8
Joined: Fri Aug 10, 2018 4:52 pm

Re: RPM ISSUES

Postby Cresty » Mon Apr 22, 2019 6:39 pm

flatlander80 wrote:Check your timing advance weights under the points cover. If they're sticking, it won't advance your timing. It's an easy check.


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I just checked my timing advance weights as you suggested and they were free and not binding. I lubricated them and checked timing again with a light and you could see the timing advance. The engine under no load will turn any rpm you desire. One thing I notice though is that the rpm will advance to a higher rpm easier in the lower gears rather than say 4th or 5th. So engine load must have something to do with it. Thanks for the information, if you think of any thing else give me a post.

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Volker_P
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Location: southern Germany

Re: RPM ISSUES

Postby Volker_P » Wed Apr 24, 2019 1:41 am

Possibly it does not run permanently on all four.
Do your plugs look equal and are your header pipes equally hot?
A CB650 on three cylinders seems not to run that poor up to 60mph if you do not know better how it should be.

Might be a fuel issue, too. Something like kinked fuel line, tank not clean, petcock clogged (the petcock has a filter screen inside the tank), ...
Try to pull the choke a little bit when the sputtering starts and see what happens. It should run better for a short time in case of a fuel issue.
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

flatlander80
Posts: 23
Joined: Fri May 05, 2017 9:08 pm

Re: RPM ISSUES

Postby flatlander80 » Sat Apr 27, 2019 1:02 pm

Since load seems to have something to do with it, I'd agree with Volker and say it's a fuel issue. I had this issue to a lesser extent on mine. It would "top out" at about 120kph. Turned out my in tank filter had come off, and partially plugged my fuel system. I took off the carbs, got rid of the sediment/crap in the fuel bowls, and changed to an inline filter.


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