Topic: 750 newly built 836 running troubles

[Sweep]

I have checked and rechecked timing and coil voltage but the problem is that the bike will not stay running without the choke on.  It starts and idles high with the choke and will stay started at around 2400 rpm but as soon as I disengage the choke it all peters out.  It won't even take any throttle, just choke.  I'm going to have to take off the carbs to sync them anyway but any ideas what the problem might be?

[736cc]

  I had a stock-carbed CB750 that did that once...the slides were installed backwards (it can be done) duh

[Sweep]

  I had a stock-carbed CB750 that did that once...the slides were installed backwards (it can be done) duh

It ran alright for a while and this just started on my last ride.  I thought it was that my dyna ignition was screwed up because my timing shaft was bent (and scratched the dyna rotor pickup).  There's always the chance that the ignition is still the problem but the symptoms are strange if that's the case.

[Sam Green Racing]

Sweep, you said it ran OK for a while then this problem started.  How long is a while ?

Sam.

[Sweep]

Sweep, you said it ran OK for a while then this problem started.  How long is a while ?

Sam.

2 long rides.

It sort of just seemed to stop functioning correctly at the end of my second ride, about 20 minutes in.  I attributed this to a problem with the dyna S due to the bent timing shaft.  The dyna rotor was pretty scored and there is a chance that one/both of the modules are damaged in some way but I can't see how since I've run all the voltage tests and it all seems ok.

[Sam Green Racing]

The only thing that I can think of is fuel starvation. If it came on sudenly you may have a fuel blockage, check the pet cock to see if it is flowing fuel.

Sam.

[Sweep]

The only thing that I can think of is fuel starvation. If it came on sudenly you may have a fuel blockage, check the pet cock to see if it is flowing fuel.

Sam.

I backed out the air screws another turn and it seems to help but I still cannot get any throttle.  I'm taking off the carbs and I will check the fuel flow.

[lrutt]

You getting full flow from the fuel tank?? Pull the lines, keep the gas cap on and sealed and see if you get a full stream from the line. If so, then next step would be checking low speed jets. If they are plugged you will have just what you describe.

[Sweep]

You getting full flow from the fuel tank?? Pull the lines, keep the gas cap on and sealed and see if you get a full stream from the line. If so, then next step would be checking low speed jets. If they are plugged you will have just what you describe.

I do seem to be getting flow and the carbs are now off, I'll check for plugged jets, thanks.  Lots of varnish in the carbs...

[Tower]

Since the carbs are off, you can easily check the intake manifold rubbers for cracks, brittleness, or poor/loose fit.  You may have had an air leak.

[Sam Green Racing]

I thought the CRs were new

Sam.

[mick750F]

You getting full flow from the fuel tank?? Pull the lines, keep the gas cap on and sealed and see if you get a full stream from the line. If so, then next step would be checking low speed jets. If they are plugged you will have just what you describe.

I do seem to be getting flow and the carbs are now off, I'll check for plugged jets, thanks.  Lots of varnish in the carbs...

   If you have varnish in the carbs then there's a real good chance you have clogged jets and/or passages. Time for a complete cleaning?

Mike

[MRieck]

 I wouldn't blame the carbs if they are really varnished up. Kind of like saying a blind man can't drive a car because he's not trying. You have to disassemble and soak in Yamaha carb cleaner (my favorite), set float height etc and start from a solid baseline. Rock hard carb boots have to go to. Those carbs can work perfectly....there are a few people on the board that run them and are very happy.

[Sweep]

I wouldn't blame the carbs if they are really varnished up. Kind of like saying a blind man can't drive a car because he's not trying. You have to disassemble and soak in Yamaha carb cleaner (my favorite), set float height etc and start from a solid baseline. Rock hard carb boots have to go to. Those carbs can work perfectly....there are a few people on the board that run them and are very happy.

I'm not blaming the carbs necessarily, just my tuning.  so far I cannot find anything wrong.  The varnish concerns me a little but I have no more than 2 hours running on these carbs and the boots.  I'm going to do the sync fittings and then do my next start with a remote gas tank and take it from there.

[Sam Green Racing]

I thought the CRs were new

Sam.

Are the carbs new Sweep ?

Sam.

[Sweep]

I thought the CRs were new

Sam.

Are the carbs new Sweep ?

Sam.

Yes sir.  I thought I had them pretty well tuned until something just "went".  So as of now my fuel line is clogged OR my ignition system is knackered.  I'm gone over the carbs and the worst they can be is rich or lean.  They're probably not the problem.  I think I fried my dyna s ignition module(s) or a coil or two.  Luckily I have a spare set of dyna coils to eliminate them as a candidate.

[Sam Green Racing]

I would be wondering how the varnish got into new carbs.

Sam.

[Sweep]

I would be wondering how the varnish got into new carbs.

Sam.

Exactly.  Any ideas?  They were clean as a whistle when I put them on there.  I'm wondering if the tank needs treatment, I'm wondering so many things!

[MRieck]

I would be wondering how the varnish got into new carbs.

Sam.

Exactly.  Any ideas?  They were clean as a whistle when I put them on there.  I'm wondering if the tank needs treatment, I'm wondering so many things!

Ethanol in the fuel breaking down varnish in the tank??

[edbikerii]

Are the pickups on the Dyna both still timed, and secured, properly?  They are adjustable individually, so one could be way out of whack.  I'd be especially suspicious because you've indicated a "bent timing shaft".  Can you explain that a little more?

Also, could there be a wire chafing near the ignition coils?  Maybe rubbing on the frame or the bottom of the tank or something?  Perhaps corroded or wet terminals on or near the ignition coils?

[Sweep]

Are the pickups on the Dyna both still timed, and secured, properly?  They are adjustable individually, so one could be way out of whack.  I'd be especially suspicious because you've indicated a "bent timing shaft".  Can you explain that a little more?

Also, could there be a wire chafing near the ignition coils?  Maybe rubbing on the frame or the bottom of the tank or something?  Perhaps corroded or wet terminals on or near the ignition coils?

Good suggestions and I'll make sure I redo the coils with dylectic (sic?) grease as well.  The bent timing shaft caused the center dyna rotor to rub on the pickup modules.  This I'm hoping did no permanent damage because I don't want to have to buy a new dyna ignition.  I'll go through everything and post my results.

[MRieck]

 The bent timing shaft must be the stud that holds the Dyna ignition rotor

[Sweep]

The bent timing shaft must be the stud that holds the Dyna ignition rotor

It is and I've bent it back and it's ok now, not contacting anything.  I tried to find a replacement because it came back badly bent with my crankshaft but to no avail.

[MRieck]

The bent timing shaft must be the stud that holds the Dyna ignition rotor

It is and I've bent it back and it's ok now, not contacting anything.  I tried to find a replacement because it came back badly bent with my crankshaft but to no avail.

Yes....that part has been discontinued.

[CB750F2]

Sweep, I suggest that you if you have a spare points ignition system and coils fit them and see what happens. If you still have trouble you can be sure that the trouble is nothing to do with the ignition system. Pat