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1978 GS750 Cold exhaust pipe

  • Thread starter Thread starter ZakDanger
  • Start date Start date
Z

ZakDanger

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So the other day I noticed fuel dumping out of my pod on the #1 carb when I give it gas. Checked my pipes after running for a bit and sure enough #1 is cold in comparison to the other three. Checked spark, she's bright and blue. Checked my float needle, she springs back no problem. Even checked to see if my floats were still floaty. In the middle of checking the carbs I notice a screw plugging up the overflow drain tube. Great. So apparently this was an issue before, and this is how the guy fixed it.

The slide works, which would let gas in... so... huh? Anyone have this issue? Whats goin on??
 
Take out your jets and clean them (BTW, you did clean your carbs?)

The gas is running out the carb because it's not going into the cylinder
 
ZakDanger said:
So the other day I noticed fuel dumping out of my pod on the #1 carb when I give it gas. Checked my pipes after running for a bit and sure enough #1 is cold in comparison to the other three. Checked spark, she's bright and blue. Checked my float needle, she springs back no problem. Even checked to see if my floats were still floaty. In the middle of checking the carbs I notice a screw plugging up the overflow drain tube. Great. So apparently this was an issue before, and this is how the guy fixed it.

The slide works, which would let gas in... so... huh? Anyone have this issue? Whats goin on??
Click to expand...
Not sure on your root problem, but if the pipe is still cold a few minutes after riding, the pilot jet could be clogged, that jet and the choke tube get crud in them easy.

Not sure of whether there could be other reasons on your overflow, but if the float needle valve seat, float needle valve, and petcock aren't all working together the way they're supposed to, that will cause overflow.

Carb dip and rebuild with new o-rings is usually recommended, probably not so much because everything needs to be fixed, but because you have to get a good baseline and cross a lot of little gremlins off the list, unless you do it, it's hard to know for sure.


(pic is from the VM rebuild pdf)
vmcarb.jpg
 
Is there any way for me to just get the o-rings and gaskets so I don't have to buy a full $ 25 rebuild kit for each one? About 125 to rebuild my carbs? Ick.
 
Usually all you need are o rings, unless some PO glued the gaskets down.
Robert Barr has what you need.
Search him on this forum.
 
Wow. This guy is a friggin saint. Thanks alot man, you and he saved me some serious coin.
 
Ok. Pulled the carbs apart, dipped each one individually, carb spray and compressed air through the jets, replaced the float needles (actually found some K&L for $5 each), replaced the gaskets, replaced the o-rings, including the ones connecting the carb boots to the engine... These things could NOT get any cleaner. Lookin, workin, feelin GOOD.

Started her up after kicking the hell out of her and some ether, keep her at a choppy idle (needs a sync), pipes 2,3,4, nice and hot. #1. Still cold. Pull the plug, no change in idle. Unscrew the plug, she fires with a gorgeous but disheartening purple spark. Pulling the 3 & 4, get a blue spark, pull #2, get a mostly blue, somewhat purple spark.

I think I might buy an ignition module, but I wanna ask the experts first. Whatchoo think?
 
Looked dry to me, but then again I wasn't really checking for that. Also clean. Way too clean. Didn't think to check how wet it was tho... good call...
 
Dry means its not getting any fuel

Wet means the plugs not firing

You don't have an ignition module, you have points or an aftermarket electronic ignition
 
Did you remove the jets and and soak them with each carb? It's possible #1 is still blocked.
 
Take the screw out of the overflow tube so it stops holding the float up..If the floats are adjusted properly then theres no need for the overflow to be blocked. Id be interested to see if the screw head is hitting on the float hinge somewheres. Look at the brass area for any tell tell witness marks from the screw head rubbing.
 
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I actually have all new main and pilot jets, doing pods. Dipped every part on the carb that wasn't made of plastic or rubber. The overflow tube doesn't need/have a screw in it anymore, took care of that when I rebuilt the carbs. So its not getting fuel huh... Checking compression on it tonight. 8.7 to 1. I will check out the plug again, is it possible that I am getting a weak spark, enough to burn the gas but not enough to fire ignition?
 
Got the 1 and 2 carbs transposed?? Someone just went thru that mistake as well. Note that you should have the air screws pointing to the left on 1 and 2 and to the right on 3 and 4..If they are all going the right way then look to see that carb 1 DOES NOT have a brass nipple on it...should look like this...

 
Nah, I read up on those carbs quite a bit on here and other resources. Definitely made sure that they are in the correct position. 1 no nipple 2 tube goes to airbox 3 tube goes to petcock 4 tube goes to airbox. I WISH my carbs looked like that Ha!
 
Check the No. 1 plug. You thought it was dry, but check it again after idling the bike with that cylinder failing.

If it is dry at that check, you have a fuel problem. Backtrack from the No. 1 cylinder to determine why there is no fuel getting through. If there is fuel in the bowl but your plug is dry, you might need to raise the float level so that your engine will pull that fuel in. If the bowl is dry, find the blockage in the fuel path. When you cleaned the carbs, did you synch them, at least a bench synch? If your slides are out of synch, your No. 1 slide might be too low to pull fuel, relative to the other three. Bench synch the slides, get the bike started and warm, then vacuum synch the carbs.


If the plug is wet, you have a spark problem. I don't know what a purple spark indicates, but usually timing and ignition problems show up in pairs -- 1/4 or 2/3 -- because the spark is delivered to the cylinders 2 at a time. If your problem is isolated to the No. 1 cylinder, it probably traces to the plug or connection.

This is easy to confirm. Swap the plug leads for 1 and 4. Start the engine. If the No. 1 pipe is hot and 4 is cold, then you know the boot connection or lead is the problem. If the cold pipe stays at No. 1, then you have a bad spark plug in that cylinder.

If none of this bears fruit, you can also check your compression. First, have you checked and adjusted your valve clearances? Get that sorted out, then do a compression test. If the No. 1 cylinder has bad compression . . .
 
You can put a rag under the bowl and loosen the drain screw to check for fuel at least that far too.
 
Got her running, idling, and rode her for a bit last night. Gonna need to vacuum sync the carbs but definitely a boost in my motivation hearing her run. Cleaned the points and I got a nice solid spark for my #1, flipped the float in #1 cuz apparently I put it in upside down (uh dehr). Thanks for all the help guys. Invaluable information abounds on this forum and its helped me not just with my GS, but my XS750, and a couple of my friends' bikes that they are working on. One thing that puzzles me, isn't an issue, just a curiosity. When I open up my drain screws to drain the gas out of my float bowls, the gas comes out of the drain screw hole instead of my overflow tubes. Not a deal breaker or even really an inconvenience, just... Huh?
 
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