Runs rough @ 4500+ RPM

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Hi Everyone!
I'm trying to get a '81 CB650C to purr. It runs well up until 4500-5000 RPM when it starts to cut out/run rough. Seems like it's running too rich. Carbs recently rebuilt by Honda shop. I've checked the timing and it seems fine. Plug wires are new and the caps are good. Air filter new. I haven't checked/adjusted valves. It has Keihin VB44A carbs. Any idea's? Thanks for any help!

[Pinhead]

How's your alternator charging? It takes more voltage/power to spark at higher RPM, so with a weak battery/alternator, you won't be getting the full spark power. When my alternator wasn't working it wouldn't hardly run over ~5k but once I got that sorted out it happily revs up to redline.

[holysmokes]

your charging rotor is your trouble

under the round left engine cover is where its located

get a multimeter, check the readings between the slip rinngs, there is a posting explaing these proceures, in one of the technical areas


http://www.cb900c.net/forum/viewtopic.php?t=899

[Bump Start]

Thanks Pinhead and holysmokes! After I read your posts I got to work and checked the rotor. The Clymer manual said it could be checked on the bike. I couldn't get a consistent reading. 4.8 held for a bit at one point but mostly way over that and moving all over the digital scale. Sounds like the likely culprit?

[holysmokes]

if it checked out at 4.8 ohms your okay

a rotor is either good or bad, nothing in between

have you checked the pulse generators with your meter, they can be weak..right round cover, check them at the plug in

if their okay, next step is plug wires ,, its only a few dollars to get new 7mm wire..$20 will buy caps and wires

[Bump Start]

holysmokes!
I'm not so sure it did check out okay at 4.8. It did hold at 4.8 for a bit, but mostly it bounced all over numerically. Each test I take seems to have a different result. Rarely will it settle down and stay at any given number, be it within functional range or not. Thanks!

[Ibsen]

If you get different readings each time it could also be your meter causing this. Try to measure with a different meter and see what happens.

[Bump Start]

I'll try and get a hold of another multi-meter and get a second reading. Mine however is new this year and has worked well so far. Thanks!

[Volker_P]

Welcome here!
Is the choice of your nickname related to recent experiences?
Well, your problem may be caused by charging problems only when the battery is very weak. Just check battery voltage while reving up. If it does not go down when it starts to sputter, it is most probably something else. Is there a difference with headlights on/off?
If you think it is running too rich, spark plugs should look black or even be wet. Check all four, do they look equal? Make sure your choke flaps open completely.
Checking the valves is always a good idea but it probably wont help you with your problem. Nevertheless it is simple and basic before further work.
How did it run before the carb rebuild was done?

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Thanks for the welcome, Volker_P and everyone's help so far. I haven't got her to purr yet but feel with all your help and my perseverance I will! After putting some new batts. in the multimeter I got a good read on the rotor so it seems okay. I followed the manuals instruction for checking the stator by checking resistance between each wire at the wiring harness connection. Couldn't get a good or bad reading there so I put everything back together and did a voltage test at the battery. Got a solid/consistent 14.5 volts @ 5000rpm with the high beam on. Does this eliminate any issues? I'll check the pickup coils on the CDI today and maybe get to the valves. The choke flaps are open as that was an issue that I took care of earlier in the project. Yes, spark plugs are sooty black. When we bought the bike it had this issue of cutting out at 4500-5000 rpm. I've been trying to track down the problem ever since!
Thanks again everyone!!

[Tunes81CB]

Have the exhaust pipes been changed recently? I put a set of after markets on a CB650 once and had to change the jets.

[Volker_P]

Yes, good point. In case someone changed exhaust or you changed back to original air filter from something less restrictive, the (adapted by PO) jetting might be not appropriate any more. I am sorry I do not have data to the diaphragm carbs. Ibsen?
With 14.5 Volts at 5000rpm and a bright high beam, charging system seems to be not the problem. In case of a rotor problem you would see a voltage step down (I would guess 1-2V depending on battery) at some rpm level when charging stops. You could rev up a bit higher above your problematic range to be really sure.
If all four plugs look equally black, it is most probably something that affects all cylinders. The ignition system is in many parts two independent systems for 1+4 and 2+3 (coils, pickup coils, CDI's/spark units) which should not fail simultaneously in such a strange way. So you probably have too rich mixture or weak ignition. You need a real strong, blue spark outside. One way to weaken an intact ignition is to use resitive spark plugs together (in series) with resitive caps. It could also be vibrations that causes a contact in the wiring harness or the ignition lock to get bad at a certain rpm level.
I do not know much about the CB diaphragm carbs, but looking for a loose, untight, misaligned or missing vacuum hose seems a straightforward thing to do.

[Bump Start]

Thanks again for everyones help. I'll get down to the garage after work today and keep plugging away at it. I believe the exhaust system is stock. I'll try the voltage test again icluding revving it up. I'm using NGK D8EA plugs. I checked the plug caps' resistance a while ago and if I remember, they all checked out at or below 5K ohms.
To be continued...............

[Ibsen]

Your carbs are the Keihin CV carbs without diaphragms. This is the parts diagram fro the carbs:

http://images.powersportsnetwork.com/fi ... 081-82.gif

One problem with this parts diagram is that it doesn't show all the parts that are shown in the parts diagram in the Clymer workshop manual.
According to the parts list at www.mrcycles.com the standard main jet size are #120 .

Btw, have you checked the spark plug colour and confirmed that the carbs are running rich?
Another thing that might be a problem if the carbs are running rich in the mid range are worn needle jets and jet needles. Also if the air jets located under the cover(#17 on the parts diagram) under the vacuum piston are clogged up the mixture will be rich. Another thing that can cause a problem is the rubber plugs that are covering the slow jets. If they are leaking fuel will bypass the main jets.

And I agree with Volker about the charging system. At idle you will probably only get approx 13 volts across the battery, but above 2500 it will increase to approx 14,5.