inconsistant motor

[jaysihn]

78 gs750. I had the bike tore completely down to teh frame doing cleanup and painting. I got it all back together and it started and ran fine for the couple times I drove it. just recently I was riding and it started to sputter a bit on acceleration. It got to where as i was cruising along, it would bog down adn feel like I was only running on a couple cylinders. I could hold the throttle open and it would suddenly gain power and surge forward only to immediately bog back down. Then when put in nuetral and let idle it would die unless i held the throttle open enough. I took the carbs apart adn cleaned them all up and put in new plugs which at first seemed to fix it but after riding for 10 minutes or so it started to show the same symptoms. It always starts up fine and acts like its going to run ok but then shortly after starts acting up again.


any ideas where to start?

[nabrams]

Sounds like one one of your coils is starting to fail. Eventually it will fail altogether and the problem will be obvious. See my other posts about this.

[wiredgeorge]

Sounds like you are about to head off on a few rabbit trails. FIRST, there is not a single mention of the bike's state of tune. Second, the problem could be in a zillion different areas that require some systematic trouble shooting!

1. has the bike been tuned since your frame-off refurbish? That is, were the valves adjusted? Was the cam chain timing confirmed? Was the compression checked? If not, THAT would be a good place to start... making adjustments to carbs would be at the tail end of any troubleshooting.

2. Gas may not be getting to the carbs. Could be a vacuum lock. Next time it happens, open your gas cap. If the bike runs normally almost at once, the venting on your gas cap is plugged up. Are the carbs getting gas? Did you clean the inside of your tank? Clean the petcock filter screen? Do you use an inline filter? Could there be a sharp bend in the fuel hose? Not only should you check for gas in the bowls but do a "service fuel level" check following the steps in a service manual.

3. Could be electrical issue. Check your bike's ground. Is it solidly connected? What do your plugs look like? Did you check the wires in the coils? Did you measure the primary and secondary impedences on the coils? I would get a spray bottle of water and spray the pipes after the bike starts acting up. If a cylinder isn't firing, the water will get real hot but will run off. If it immediately is vaporized, the cylinder is firing.

[jaysihn]

sorry i didn't give more details on the state of tune.

when i had the bike tore down, i didn't touch the motor. It ran good before I tore it down so I put it back together and figured everfything should be the same in regards to it.


The coils are less than a year old and haven't seen much use so I realy hope they aren't the problem. I think the gas cap venting sounds plausible and will check that when i get home.

thanks

[SqDancerLynn1]

Check the petcock or try running on prime Sounds like a fuel problem

[brveagle]

check ur air filter...

Also, do u have a inline fuel filter? that could be it as well.