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carb trouble?

  • Thread starter Thread starter Anonymous
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Anonymous

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ok i have a '79 gs 750e that has a kerker 4 into 1 pipe, individual k&n pod filters, dyna s ignition, new battery, and a new r/r. I recently rebuilt the carb's using a 115 main jet and a 17.5 pilot jet and now the bike seems to run good. the air screw and the fuel pilot screw are both turned out 1 1/2 turns. the problem that i am having is that when i first start the bike when it is cold (choke on, no throttle) only the #3 cylinder seems to work, then after letting it run for a minute the other three start to work. I am not sure what could cause this and any help would be appreciated. I also have not sync'd the carbs yet (waiting for manometer to arrive), but i don't think that would really affect this. thanks
 
I think that synching the carbs could help, my previous bike a 1000S was so far out of synch that when I first started it the bike only ran on 2 cylinders.

Do you have a inline fuel filter on the gasoline going to the carburators to prevent little particles of dirt and rust clogging up the idle jets :?:
 
I did have an inline fuel filter that i just put on, but i was thinking that maybe it was the cause of my problem so i took it out. i will put it back on as soon as i get this fixed.
 
Re: carb trouble?

On the 79 750E, the number 3 cylinder is the control cylinder. You synch 1,2, and 4 to match it. Robinjo is probably correct that your problem is the 1,3 and 4 cylinders are so far out of synch that they are the problem.

Earl

MattD said:
ok i have a '79 gs 750e that has a kerker 4 into 1 pipe, individual k&n pod filters, dyna s ignition, new battery, and a new r/r. I recently rebuilt the carb's using a 115 main jet and a 17.5 pilot jet and now the bike seems to run good. the air screw and the fuel pilot screw are both turned out 1 1/2 turns. the problem that i am having is that when i first start the bike when it is cold (choke on, no throttle) only the #3 cylinder seems to work, then after letting it run for a minute the other three start to work. I am not sure what could cause this and any help would be appreciated. I also have not sync'd the carbs yet (waiting for manometer to arrive), but i don't think that would really affect this. thanks
Click to expand...
 
I would try the carb sincing too. If you have a manual, make sure you manually sinc the carbs by adjusting their fully closed and then fully opened positions. That will get you running and your sinc' tool will do a better job.
Also, you may not want to hear this, but any motor that has pod's and a pipe installed, will also need a jet-needle adjustment. You will be running lean and hot. Surging and some hesitation are the usual problems. Take a plug read. No way can you increase the air intake (as pod's do) and not raise the needle at least 1 groove, if not more.
Running lean can burn your valves, pistons can melt, and plugs too.
Just advice from someone who's done a lot of jetting on the VM carbs.
By the way, you should remove the 2 floatbowl vent tubes if you have'nt already. They are 3/16'' tubes that come off of #2 and #4 carbs. With the increased draw on the carbs you need to minimize the resistance in venting the bowls. Removing the tubes decreases resistance and the bowls vent better, so the fuel can be drawn into the carb throats easier. I know this because Dynojet recommends it and I have tested for it .
 
The 115 main jet and 17.5 idle jet he is using sound pretty correct, in my 750 with the pods and 4-1 a 116 was used but I had it done at a Dyno shop for a lot of $$$$$ 8O , so don't know the settiings for the needle, stock is middle groove so I would atleast go down one groove and try that. But synching first...

If you refit the filter make sure the flow of gas isn't restricted by the filter, check the flow with and without the filter by setting it on prime.
 
I agree that carb synch is crucial before going further. Carbs a little out of synch will give big problems on a 4. I have had the bike a little out of synch, running like a 450! Even a manual carb synch to get it within the ballpark does not get it running right (at least on the slide carb models).

A Keith said, needles, slide cutoffs etc can also affect tuning. I probably cheated and put in Kawa 9 28mm carbs with different needle profiles. Seemed to solve all my tuning and running problems.
 
Thanks for all of your guys help. i'm going to sync the carbs as soon as i can. I also raised the needle one notch when i was rebuilding the carbs. so i think everything should be okay now.
 
IF YOUR CARB SINC' does not cure your hard starting, then I think you are running too rich in the pilot circuit. First and easiest, turn your side air screws out 1 full turn more than the 1 1/2 turns you are at. This alone may help your hard starting. But to finish this, warm up your engine, then adjust the air screws to achieve the highest possible idle. Turn 2 screws at a time in 1/2 turn increments to best hear the rpm's, then re-adjust the idle properly with the idle screwknob.
If this does'nt help enough, then I would put the stock pilot jets back in. I'm running a Dynojet needle, which does have a different taper than stock. They REQUIRE that you use the stock pilots and they are right in my case. But this advice may work for you too. The pod's increase the air intake a lot, but the pilot circuit is simply not effected AS MUCH as the needle and main circuits. I properly tuned my pilot circuit just by the air and pilot screw adjustments. If you do change the pilot jets, remember to re-adjust the air screws again. Also, take off the 2 floatbowl vent tubes and see if it runs better, especially at steady speeds. Let us know how it goes.
 
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