Cylinder 2/3 not Firing

[poppadre]

I recently rebuilt my GS750 engine. I had a bit of a hard time getting it to start until recently the bike fires up right away. Last night it took it for a spin and it was running quite well until half way though the ride the engine lost most of the torque and seemed like it was only firing on 2 cylinders. Sure enough today I took a look at the plugs and inside the cylinder.

Plugs 1/4 were a nice tan color and the cylinder inside looked dry. Plug 3 was pretty black and and the cylinder looked wet. Plug 2 was wet and the cylinder was wet as well.

I tested the resistance over the coil I got a reading between 3.6 and 4.0 ohms on the coils. All 4 plus sparked and all 4 HT wires produced a spark. I am not sure if this would be an electrical problem / fuel problem / engine problem. One thing I did change before the ride was I put on the case cover breather filter and the hose is pretty long it sits on the pods of carbs 2/3. But with the breather venting air I would assume it would be lean not rich.

I can definitely hear a back fire while riding and need to be at full throttle to really get any power. This all changed on me mid ride last night from having a well riding bike.

Any ideas what it might be or what to check?

[tom203]

Electrical sounds OK (since all plugs spark). could be floats set wrong on 2 and 3. but since it ran OK and changed mid ride, I'd suspect a mechanical issue. What did you "rebuild" here? Check compression in 2 and 3- I hope it ain't the head gasket between them.

[poppadre]

Electrical sounds OK (since all plugs spark). could be floats set wrong on 2 and 3. but since it ran OK and changed mid ride, I'd suspect a mechanical issue. What did you "rebuild" here? Check compression in 2 and 3- I hope it ain't the head gasket between them.

Thanks Tom,

I will double check the floats next week when I get back in town. I kicked over 90 psi on all cylinders but forgot to open the throttle so I will have to re do that. I am going to pick up a leak down tester and hope that will give me an indication of what could be wrong. I rebuilt the entire engine. New oil seals and piston rings. Honed cylinders. Lapped valves.

The points for cylinders 2/3 had some white build up on it (carbon?) since I last adjusted them, so I used some fine sand paper to take that off. Is that an indicator of anything.

I would like to upgrade to DYNA Ignition and coils but I want to make sure I fix the problem before I dump any money into it.

[poppadre]

So I ran a leak down test and all cylinders are only at 5% so I did a decent job on the rebuild. But why are the cylinders not firing? Any ideas? I am going to dish out the money for Dyna coils and ignition and hope that fixes the problem when I get the next pay check. Stampede is really making it hard to save money haha and it just started.

[chuck hahn]

Recheck the plug gaps and IF youve got a little extra wire, unscrew the caps and clip off about 1/4 inch and rescrew caps back on. Youll be amazed at what a nice fresh connection can do for it.

[chuck hahn]

And whats the ignition?? Points and condensors or electronic? Condensors going south can make it misfire and stutter as well.

[poppadre]

And whats the ignition?? Points and condensors or electronic? Condensors going south can make it misfire and stutter as well.

Thanks Chuck, they are points and condensers. All coils measure resistance well and fire. Not the best. I did see some white residue build up on the 2/3 points so maybe they are on their way out. Ill buy the new Coils, wires and ignition and hopefully that helps!

[poppadre]

Recheck the plug gaps and IF youve got a little extra wire, unscrew the caps and clip off about 1/4 inch and rescrew caps back on. Youll be amazed at what a nice fresh connection can do for it.

I'll recheck gaps. I gaped them before the ride and it was running great then all of the sudden it craped out.

[chuck hahn]

Sounds like I would be at the points and condensors first. Points shouldnt have any white chalky looking anything on them. Good sign that they may be shorting to ground. And burnt points make the gaps there larger also...which translates to ****ty running. See the manual here and note the trouble shooting section...

http://www.mtsac.edu/~cliff/storage/gs/GS750_Service_Manual_early8valve.pdf

[Chris G]

unscrew the caps and clip off about 1/4 inch and rescrew caps back on. Youll be amazed at what a nice fresh connection can do for it.

I would definitely do this