Runs strong 10 15 miles then cuts out?

[boris0612]

Hello everyone, I am new to the forum, sorry if do something wrong
I bought a 1972CB500, with a 1982 CB650 engine, long story short I was riding it home (200mile Ride) and the bike ran strong throughout the first 4 gears when I test drove it in town, when i got on the highway it would cut out at 7500rpm at 75mph. I thought I will just get it home and look at the carbs , however 120miles in to the ride it started to loose cylinders and died within 20 sec. Pulled the plugs they were pitch black, so of course it was running rich. I put new plugs in it the next day and rode to work 2 miles and she dies right in the gate to work. Pull the plugs they are white its not getting fuel. So I took the carbs off and they looked clean the floats were good, looked at all the jets they were clean. Put them back on, ran good even in 5th gear under a load didnt cut out... for like 5 miles, until I kept getting on it every stop light, and it did it again starved out of fuel. Note, I put it on the charger the night i cleaned the carbs. Is it electrical or fuel?

[bolo79]

Hi there!If after the engine has stopped and you can still rotate with the starter,this means you have a problem with the fuel.White spark plugs also indicate that.If you can not turn the engine, then check the rotor, brushes and battery.

[boris0612]

Yes, the engine will turn over and try to fire when i give it half choke. I worked at a Kawasaki shop as a helper a few years ago, and I have never ran into this problem. The bike will run for a long time as long as you don't get on the throttle hard, once you wrap out the rpm in every gear, it starts to miss after a few minutes, then starts loosing cylinder and finally dies. Im dumbfounded because how did it run super rich on my trip home? And the next day its starving for fuel? The carbs have a vacuum to pull the gas right? So it wouldn't be that my tank is getting low on a gas and there is not enough pressure in the tank to feed the gas to the carbs? And also there is a 3 inch incline in the fuel line when it comes out of the petcock and then it runs into a little filter and then it goes back down to the carb should that be there? It looks home made because of the 90degree elbow that is also in line with the whole setup. I cant really explain it correctly.

[onepieceatatime]

There should not be an incline in the fuel line, it should only go down from the tank. If your carb rack has the vacuum cutoff device still attached, that may have a problem as well.

[cb650]

What air filter?

[davesee]

i've had similar mysterious fuel delivery problems from a badly run fuel line and bad petcock.

get some fuel hose and build a new fuel line that avoids any bends...if the tank is clean and you are comfortable doing so, leave the filter out of the equation for the time being. test it on a full tank of gas.

along with cb650, i think the air filter may be part of the issue, so go ahead and have a look at it as well.

[boris0612]

Thanks guys for all your input, I took the advice and shortened the fuel line and took out the incline, however I left the filter inline, because when I took of the petcock bowl it was full of crud and the screen was super dirty, I hope this works. I also put a new battery in her, and so far so good, I rode it pretty hard last night, wrapping out every gear and so far she is running like never before!!! Also is there a smaller sprocket I can get anywhere, my bike is running at 8000r pm at 70mph in 5th?
Once again thank you all for the advice, this is a great forum!!!

[cb650]

Count your teeth and see if anybody has changed them already. Sounds hi in the rpm's. What year model?

[boris0612]

Update, the bike runs good now but still cuts out at 8000rpm under a load in 5th gear. Could it be weak ignition?
Its a 1972 cb500 frame with a 1981 cb650 motor and swing arm, and I counted 34 teeth.

[boris0612]

Update, the bike runs good now but still cuts out at 8000rpm under a load in 5th gear. Could it be weak ignition?
Its a 1972 cb500 frame with a 1981 cb650 motor and swing arm, and I counted 34 teeth.

[cb650]

Yes it could be a weak ignition. Also could be the fact a 500 electrics dont match a 650 motor. So depends what the Po did to make it work.
34 teeth is small for a 650 but right on for a 550. that is the rear I guess. What about the front?
Sounds like some sorting out is in order.

[boris0612]

That makes sense . Would anybody have any part numbers of their rectifier, to see if mine was converted. The guy said the ignition in cb650 nighthawk. And I will check what the front was. And again thank you.

[bolo79]

Checking the ignition timing sounds like a good idea,I mean for me was. My bike cut off in fifth gear at 70 mp / h.Unfortunately, I do not know rpm's,I have no tachometer.I traced the cause to the ignition timing, mine was retarded as Peter Griffin.Worth checking.

Its a 1972 cb500 frame with a 1981 cb650 motor and swing arm

Sounds interesting,show off a little.

[Volker_P]

Welcome here!

... when I took of the petcock bowl it was full of crud and the screen was super dirty, I hope this works.

Sounds like you should start with cleaning out your tank.

Then it might get enough fuel through the plugged filter to run at 8000rpm, too.

Btw, don't know for the 500, but the CB650 petcock filter screen is too coarse and usually requires an additional fine enough inline filter or a perfectly clean tank.

[cgswss]

There is realy no sense in chasing problems until you fix the tank. Get it completely cleaned out and I would still recommend a tank sealer-I used Cadwell with great success. Can you post some pics? I'd like to see a close up of the engine and check the serial number on the carbs. Mostly interested in the first 3 Like 44A or 44C. Because you have sorta a franknbike, it may take some guessing to figure out every thing you have

Far and away, the easiest way to change your gearing is by changing you front sprocket. Count the teeth on the front (there is no oil behind that cover-you can take it off) You should have a 16 tooth sprocket. I'm guessing you have a15 on there. A 16 tooth should give you 4500 rpms at 55mph. You can very easy go up to a 17 tooth to lower that RPM.

Also, give us a better picture of how the bike is set up. I'm guessing that you have pods (no air box) and an after market exhaust (maybe 4 in to one)

Do you have a compression gauge? I had problems like yours when my compression was very low. If you haven't already done it, change the oil and put 20-50 in it for now.

Looking forward to hearing more about you unusual bike!