Running 3 out of 4...what's going on??

[chuckycheese]

My 1980 GS750L has been running 'poopy' for awhile and I'm ashamed to admit that I just realized that when it's idling...I can pull the plug wire on number 4...and it makes no difference in the idle or anything else...whereas, the bike will shut down if I do that with any other plug (after pulling #4).

I DO notice that I get a spark if I put them in the same proximity (the plug and the wire)...so what is going on???

I appreciate your thoughts and suggestions..as always!!:-D I think it's, no doubt, a simple problem but I'll be happy to provide additional information if necessary!!

[Jethro]

The cylinder only needs 3 things to create combustion- fuel (that being a combination of gas and air), spark, and compression. Sounds like you have spark, but I'd actually pull the plug and check spark at the firing end before going much farther. Could be a faulty plug. While the plug is out, crank the bike over and put your thumb or something over the plug hole. A cylinder doesn't need much compression to allow combustion, so if you feel a little pressure, you should be fine. If it's fuel, and since the other carbs are mixing well (we think) you need to pull the carbs and open them up. Could be a plugged fuel tube to that carb restricting gas, could be a float with a hole in it flooding with too much gas. Could be a float needle jet (actually the most likely casue). Could be a clogged needle seat too.

Does the pipe get hot at all, like, maybe it's not fireing on the idle circut, but on the mains it's ok?

[chuckycheese]

Thanks

Thanks, Jethro! You've given me a number of things to consider and I'll check them out. I DO think it must be 'kicking in' on the main because the bike runs really well...so I think it must be occurring only at idle. Also, the pipe only gets warm while idling (convection, I suppose) but it does get hot when I 'hit the road'.

If that's the case, then it must be a carb problem...right?:?

(By the way, it can't be the plug because I've already tried new ones.)

[flyingace]

Put new plugs in 1 and 4, then swap plugs wires 1 and 4. If the symptoms move over to #1, it's electrical (as in plug wire/cap). If that's not it, then it's likely the carb.

[chuckycheese]

Thanks

That sounds like good advice! Believe it or not, that idea popped into my head about 3:00am and I'll give it a try...and report back!:-D

[chuckycheese]

Update...

OK...I switched the wires around and swapped plugs, too. The problem remains with #4 and all else is well. The bike runs great when I'm hitting speeds above an idle so I'm supposing it's probably a clogged jet.

Since it runs so well and I did such an awesome job of rebuilding the carbs last time....is there any reason I can't fix this without removing the carb rack? It would seem that with patience and a mirror, I should be able to get this taken care of. Access is good (sort of). I think removing all of the carbs is 'over kill' but maybe I don't know what I'm talking about. All thoughts would be appreciated.

[chuckycheese]

Update

Well, I got up this morning and decided to try to rebuild #4 without removing it from the bike. I removed the gas tank, seat, tool kit, etc. and 'dug in'. I thought it would be difficult and tedious but it really wasn't bad at all. I have small, nimble hands which I think makes a lot of difference.

I removed everything I could including all jets (but the air jet), the diaphram, needle valve, float, etc. I cleaned everything in acetone and only replaced the "O" ring on the needle valve (which I don't think was the problem). Getting it back together was fairly easy although I had to modify a couple of screw drivers in the process. The only really difficult part was re-installing the needle valve pointed 'thingy' while putting on the float pin....it took 3 hands and a few foul words but worked out fine.

To make a long story a little longer, it now runs great and goes like 'the wind'. I always wondered if #1 and #4 could be effectively worked on without removal...and they can. I think the inside carbs could possibly be done, as well, but it would be much, much more difficult.:?

[Jethro]

Sweet, I love it when a plan comes together.

[chuckycheese]

Thanks

Many thanks, Jethro....You know you're my buddy...and I always respect your opinion and advice!:-D

[flyingace]

There have been a number of us with #4 carb problems. My theory is that because the side stand is on the opposite side, rain water gets into the pilot air screw of #4. The screw recess will let water just sit there and seep in. The resulting corrosion of the spring blocks the pilot air passage. (I've actually seen this on my bike, so I'm not just guessing). If you don't have CV carbs, ignore the rest of this message.

Because people don't know this, they tinker with #4 carb a lot trying to fix it. So over time other problems are created - most importantly the float pins get misallinged causing intermittant flooding when the float sticks open. My bike still has this intermittant flooding problem once in a while, but it usually cures itself if I drive it a while.

I would be sure the pilot air passage is clean and the spring is not corroded.