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Idles fine - but not in the mid range

  • Thread starter Thread starter phrode
  • Start date Start date
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phrode

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Regarding my GSX750E 1981 model..

My bike starts fine, idles fine.. But after driving 2-300 meters on the road it starts to misfire (kinda like running out of petrol). If I give it full throttle, it slowly wakes up, and at about 6-7000rpm it comes alive again (running fine). I can keep it at high revs, but as soon as I hit midrange, it starts to misfire.

If I turn on the choke, it works a little better for a while.

I thought perhaps it drew false air, but I tried with "start gas" around the carburetors and intake. Nothing happened. So to me it is not leaky.

Any ideas?

I took out the sparkplugs (new ones) and they are GRAY! :-( Too much air, and too little fuel I guess.. But from were? There are no leaks that I can find..
 
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Well it pretty much sounds like dirty carbs with plugged jets or passages. Time for a proper cleaning and rebuilt with new o-rings.

You could try a fuel additive like Seafoam or even some gasline antifreeze for a quick fix but really the only solution is a proper carb cleaning. Check out the tutorials on the main page and Cliff's website. It is not difficult but follow the tutorials closely if you want to solve this issue.

Good luck,
Spyug
 
If I turn on the choke, it works a little better for a while.
Click to expand...

That means it is running lean.

Are you using the stock air box or pods?
 
Thank you.

O-rings? Inside the carb? Is there part number for all items I need?
 
Yes gray means its lean and the stumble is that its gas starved. I would recheck the float heights again ( low float hts can also cause this symptom ) and be at the middle of the specified range as the start point. If it gets better but still a bit off, then LOWER the float hts by 1/2 MM and retry her.

LOWERING the float hts will make the fuel levels in the bowls rise..which in turn allows the carbs to drink easier. Also be sure the vents are not blocked or capped off. Put a piece of vac hose on the vents and just blow and see if air moves thru them.

The carbs need "make up air " to allow atmospheric pressure to move the gas up thru the needle jets as you roll on the throttle. Its all a chorographed dance in a way of speaking.

See this thread for some good carb info links.... http://www.thegsresources.com/_forum/showthread.php?t=206383
 
It sounds like you need to go through all the maintenance procedures for a bike of that age. I would also check the tank for possible rust that may end up affecting your carburetors. I had the pleasure of visiting your home about 40 years ago while I was in the Navy. What a beautiful place to live and ride a motorcycle. I certainly remember all the hills walking around the town.
 
OldVet66: You have been here :-) Yes this is an eldorado for riding bikes. From sea to summit.

I have taken out the carbs now. Guess I have some work to do the next weeks :-(
 
Check the petcock as well, make sure it isn't clogged and perform a flow test on it.
 
The flow is good from what I can see. All start working on the carbs this weekend. Bought all the o-rings from a local dealer today, carb claner + a sonic cleaner.
 
Here is a little analogy that you can do hands on for an understanding..

Take a plastic op bottle and fill it to the rim with water.

Now gently sip on it and you get water very easily with very little suction..right? Now keep sucking across the opening and try to keep getting water. The lower the water goes, the harder you need to suck in to get some water..right?

well, this is how the engine vacuum sips fuel from the bowls. The fuller the bowls are to the proper level.. but not over flowing..the easier the engine can get its proper supply of fuel.

Now do the same thing, but now squeeze the sides of the bottle so the water ( fuel ) rises as you sip. Now, as the water is kept up at the top, you can consistantly sip the water with the same littlest vacuum..right? Well this is the same thing that the correct float hts do in the bowls..they keep the fuel consistantly at the highest optimal level and the engine runs right thru ALL throttle positions and power bands.

That is why i said to check the float heights and see that they are at the optimal settings first...there " MAY " be nothing else needed other than this simple adjustment.
 
I do not have a manual....maybe bikecliffs website????

But what you can do is to check the hgts as they are now..then LOWER them by 1/2MM or 1MM and see how she runs then. Lowering the float hgts will raise the fuel levels in the carbs the same amout..so if you lower the hgts by 1/2MM, then the fuel will be 1/2 MM higher in the bowls..Follow me?
 
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I have now taken the carbs apart, but I cant find any place to adjust the hight??
 
but I cant find any place to adjust the hight??
Click to expand...

There is a little tab on the float that you need to GENTLY bend. This procedure is in the service manual. You have a manual don't you?
 
All I got is basscliff's carb manual.

I checked it with a micrometer, but there is no way I can adjust that 0,5mm. What if I get it 2mm out of position?
 
I checked it with a micrometer, but there is no way I can adjust that 0,5mm. What if I get it 2mm out of position?
Click to expand...

I really can't understand what you are trying to say. Here is a picture of what you should be trying to set:

float_height-measurement.jpg


Measure the height A and then gently bend the tab to set the float height.
 
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