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I hate to post this, But Im dropping a cylinder

  • Thread starter Thread starter GSstiny
  • Start date Start date
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GSstiny

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I promise, I did a search. Actually I did about 15. Anyway, I keep dropping #3 . My fuel mileage is crap (<20 mpg), there is alot of vibration above, say 3500, and It just doenst have the power a 750 should. I know its dropping 3 because with it idling, I can pull the #3 plug wire and there is no change in the idle. #2 cylinder is firing normally. Odd part is that it seems to get worse as the engine gets hot, but still fires intermittently if i really whack the throttle.

The plugs, wires, caps, points, and condensers are all brand new and properly adjusted. I tested the coil, and got about 5 ohms across the primary, and about 21,500 across the secondary. Low for a stock coil, but I dont see how thats causing the problem. I pulled the wire out of the splicer, and there is continuity in the wire. I also have continuity across the coil primary. There is no continuity across the plug cap, so I dont get a continuity reading for the secondary.

Since it is the #3, I checked the petcock vaccum line, and the diaphragm, both of which were fine. I just rebuilt the carbs and had a shop check my work, so the float height is correct, and all the passages are clean.

Is it possible for one side of the coil to be kaputt and the other side normal? Any and all suggestions welcome. If it will help Ill go for a ride and post a pic of the #3 plug.
 
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So much for the test ride. I went out and the $%^&* key switch is locked up. I broke the little gray piece on the top, sprayed it full of everything i could think of, dumped it full of graphite powder, and i still cant even start the bloody thing. this thing is getting on my last nerve. SV650s are beginning to call my name.

and chef, ill swap caps as soon as i can get it started
 
Did you lock your front forks also? Try wiggling you forks while turning the key.
 
Two ways too by pass a key. I know been there done that. One is the jump the solonoid with a screw driver. Two is two jump the switch on the bottom of the iginition. They are four wires, red, brown, oragne and blue. Red is hot. brown is ground. I am not sure what orange and blue are. I am not sure the sequence but I was able to make conatct with two wires using red/? and getting it too fire up.

Now I had the number three fail on this Gs 750. Maybe what I learned can help you. I must of taken them carbs apart at least 20 xs. I found that the number three is what takes in gas from the diaphragm. Now if your plug is wet and you don't see any spark. Then your lead on 3 is bad or the spark plug is bad. Now if 2/3 are both not getting spark. Then its your coil. Swap out left for right and see. 3 for 2 4 for 1. This will tell you if the lead too three is not making contact in the plug or not. Sometimes its as simple as a bad wire in the plug connector. Cut it off and rethread it back on. Now if its the coil. It can be bad points or pitting on the points. I liltterly had too bend the fixed part on the point too make proper contact. Dont do that unless your familar with points. clean them good with a small file if they are pitted.

So it could pitting on 2/3 on the points. But this is only true it 2/3 are not working at all. Now if two is firing. and you find that three does not fire. Try this next test.
Too test that out. lay the plug on the engine block unscrewed with the lead connected make sure the tang is touching the block so it can ground out and then turn over the battery. Unplug the other three when doing this. You should see a small blue spark. If not then try a new plug or clean your old one. The gap should be 23ths.

The only thing left then once you get spark too three is the diaphragm. Just make sure its not rotted out. and make sure the tube doesnt have any holes in it either so it can suck like its meant too. Another test is too screw out the idle screw a 1/4 turns. if its getting gas and spark it should bog down.

Try those, if you need more help I will try my best too help
Remember this NEVER NEVER NEVER GIVE UP. Walk away from it, read up on it, and they try again. It took me two weeks too finally get it too fire up. 20 times apart and then reassembled. You are going to gain in knowledge with each reassembly. I can dismantle my carbs now down the floats in ten minutes.
 
Is the plug on #3 clean, sooty, wet? That will tell us alot. I had two cylinders go dead on the 550 when I first got it. One came alive after I rebuilt using a carb kit. The other had been foobar'ed by the PO so bad, I had to replace it (got lucky and found one through this forum). In both cases the problem was running rich. If it gets worse when the motor heats up, you're probably running too rich with soot on the plugs.

I'm assuming you've already adjusted idle mixture and vacume synch'ed.
 
Yes, the plug in question is wet. Sorry that wasnt clear.

Ive got the mixture screws pretty close, but I havent vaccum synch'd yet because I cant get it running on 4.
 
Is that number 3 as your sitting on the bike or from looking at the front?


The vacuum line should lead to the #2 cylinder....
 
I havent fixed the steering lock yet, but I took the whole shootin match out of it and hotwired it so i could test ride. Admittedly it was pretty funny having to get off the bike at a gas station and turn the "key" off with a screwdriver. :D I swapped the 2/3 plug leads, and the problem did not follow the leads.

SO, that leaves the petcock or something really crazy going on in that #3 carb. I had the petcock apart for cleaning purposes the other day and the diaphragm looked fine. I could see/feel it open and close by sucking on the vaccum line. I didnt see anything else out of the ordinary in there (bad orings, etc). Could it be just super super rich and there is so much fuel it keeps the plug from firing at all? Since engines richen up as they get warm, that would explain why the problem gets worse the longer I ride. What to do now? Again, the #3 plug is wet, and Ive got it narrowed down to a fuel problem.
 
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Tonight's update: Still down on power, still some vibration, and yes, crappy mileage, BUT the #3 plug is no longer wet. Its very rich, but its been firing. I guess it just needed to have the crap run out of it for a little while. It sputters, hesitates, and generally runs like crap, but if its running on all 4, its something I can work with. \\:D/
 
Another update: I pulled the carbs and checked the #3 float, somehow it was bent all crazy. I squared it up and made sure it was buoyant (no pinholes) and measured the height, 25mm. I also found that my NEW #3 needle valve had a bad wear groove in it. WTF!? :shock: I replaced the needle with new spare and put it all back together with a new #3 plug.

Absolutely no improvement. Around the end of my short test ride it starts billowing white smoke. Soon after parking I notice a puddle of black crap under the right pipe. Odd. Upon further ispection its OIL with an absolute crapload of gas in it. A quick inspection found the source of the drip at the oil filter cover. The needle valve obviously has been screwing around on the job and has let ALOT of gas into the crankcase. So much so that its popped the oil filter cover Oring.

I really dont know what to do next, aside from change the oil and filter, pronto. The petcock is closing fine. Should I just raise the float a few mm's and go from there?
 
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