One Hot Cylinder

[twerth]

I just got my basket case '78 GS750 started for the first time tonight and ran into a problem I'd like some input on. I noticed that cylinder number 2 gets really hot really fast. The rest of the cylinders seem consistent with each other. Is this an indication of a lean mixture or is it something more serious?

[BlackStar]

Curious how you determined #2 was hot. Blue pipe, IF thermometer?
At any rate it probably is due to being lean. How does the plug look?
Vacuum leak at intake boot, Not sure if your bike has o-rings between the boot and the head, mine does.
Cracked boot, or loose clamp?
Missing Sync. port plug?
Miss-adjusted fuel/air screws?
Low float level or plugged passages/jets?
Thats a good start

Are you on RSWarrior.com?

[twerth]

Curious how you determined #2 was hot. Blue pipe, IF thermometer?
At any rate it probably is due to being lean. How does the plug look?
Vacuum leak at intake boot, Not sure if your bike has o-rings between the boot and the head, mine does.
Cracked boot, or loose clamp?
Missing Sync. port plug?
Miss-adjusted fuel/air screws?
Low float level or plugged passages/jets?
Thats a good start

Are you on RSWarrior.com?

I could tell #2 was hot because it blistered the paint on my header in about 30 seconds. The other three are still ok. I know the blistering is from heat and not poor paint prep because after 30-60 seconds of running, I can still touch 1, 3 & 4 but 2 is hot as hades.

I'll pull the carbs back off and check the items you suggested. I was careful the first time, but I could have missed something. I'm just glad to hear that you think it's carbs and not something more sinister.

One other question. Another member on this site told me that the vacuum connections on #2 and #4 carbs should be tied together and vented to atmosphere. #3 is connected to the fuel petcock. This seems strange to me. If these are actually vacuum connections, wouldn't venting them reduce performance and potentially suck dirt into the carbs?

And BTW, I am on RSWarrior. Same moniker (twerth). I'll check your other bike out.

Thanks for the help.

[1980GS1000E]

One other question. Another member on this site told me that the vacuum connections on #2 and #4 carbs should be tied together and vented to atmosphere. #3 is connected to the fuel petcock. This seems strange to me. If these are actually vacuum connections, wouldn't venting them reduce performance and potentially suck dirt into the carbs?


Thanks for the help.

You are correct. If these are vacuum lines, they should NOT be vented to atmosphere. Perhaps he meant the float bowl vents. If you have any vacuum leaks, you will have problems.

[twerth]

You are correct. They should NOT be vented to atmosphere. Perhaps he meant the float bowl vents. If you have any vacuum leaks, you will have problems.

I think the confusion may be that the CV type carbs have vents, but the older versions (which I have) don't, or at least it's a different configuration. I have a Haynes manual, but it leaves quite a bit to the imagination. If anyone has a fuel system schematic, or photos of a properly connected system, it would be greatly appreciated.

[twerth]

Ok, this is driving me nuts. I took the carbs off to give them a good cleaning and adjustment (again) and to check the things BlackStar suggested. I adjusted float levels and slide travel, things I hadn't done the last time, but wasn't able to find any other problems.

While I had everything apart, I decided to run a compression test to rule out a burnt exhaust valve. Results were a little low, maybe due to a cold engine, but consistent, so I think mechanically, I'm good. I also double checked firing order and coil connections and everything looks good as well.

So, I put it all back together and started it up and this time #1 gets hot instead of #2. After listening to it run for a while and checking the temperature of the headers, I think the problem isn't that I have one hot cylinder, but that I only have one firing. With this in mind, I checked for fire at the plugs and that doesn't seem to be the problem. I even pulled plugs and grounded them to make sure there was spark in the gap. I also checked to make sure there was fuel in all four carb bowls.

So, what can it be. I can't think of anything I've done that would move the problem (or lack of problem) from #2 to #1 and I can't tell why the other three might not be firing. Any ideas? I'm about to tie a rope to it and use it to hold my boat in one place.

[Nessism]

Your '78 has VM type carbs. There is a link to a clean up article on the Garage section of the GSR homepage.

Your carbs have two vent lines - on carb #2 and #4. There should be sections of hose on these vents routed up and laying on top of the airbox - open to the atmosphere.

There is also a vacuum tube coming off the #3 carb running up to the petcock.

All these old GS bikes are ultra sensitive to vacuum leaks and pluged pilot jets. The rubber carb mounting boots are prone to cracking/leakage, particularly from the large o-ring between the boot and the head. Replace the o-rings and inspect the boots carefully.

Hope this helps.