Cylinder three running extremely hot

[decker7]

Hello fellow GS riders. I am in the process of restoring a 79 GS850 that I purchased last year from a co-worker. I am not a mechanic by any means, and everything I know about motors/bikes is all self taught. The bike is in great cosmetic condition as he had it in his shed for 15+ years. I went ahead and replaced all gaskets on the top end, and did a nice full cleaning and rebuild of the carbs. The issue I'm running into though has stumped me. I got the bike started and idling, however, within a matter of 60-90 seconds at idle, the pipe coming from cylinder 3 becomes extremely hot. Here is a bit of an update on what my bike has, and what I have done:

- Mikuni VM26 carbs (soaked 24 hours in carb dip, new gaskets and O-Rings)
- Stock exhaust (cut off mufflers)
- Upped the main jet to accommodate for shorter exhaust
- Stock air-box with new filter
- Updated to non-vacuum Pingle petcock (Capped vacuum port on Carb 3)
- Did a "Plug Chop" and found that cylinder 3 shows it is running lean
- Ensured carb boots didn't have any cracks
- Replaced O-Ring for carb boots at the engine
- While engine was running, sprayed carb cleaner around carb at air-box and engine to check for vacuum leaks (didn't notice any difference in idle)

Because I was able to determine cylinder 3 was running lean, I feel confident this is the cause of the extremely hot exhaust pipe. I disassembled the carbs last night, and I didn't see anything that would have clogged any of the jets. I took off the valve cover, and it appears that all of the valves are opening and closing properly. The spacing on them is in spec as well. As I stated earlier, I am no mechanic, and everything I know/understand about engines is all self taught. I am restoring this as a form of therapy for myself. I enjoy getting my hands dirty and enjoy working on the bike, so I am hoping that I can get some insight here rather than having to take the bike to a shop. Any input would be greatly appreciated!!

 

[zed1015]

Yes! The lean condition will be the cause of the overheating cylinder.
You should look further into number 3 carb for the cause.
Make sure the jets and passages really are clear and that the fuel mixture screw tip isn't broken off and blocking the outlet and that the float height is correct.

 

[decker7]

Thank you for the reply! I’m in the process now of reassembling the carbs. The passages are nice and clear and look ready to go. As for the list height, that’s something I never paid attention to. Can you explain to me why this is so important? I’ve seen other people post about it, so I was going to make sure to get that set this time as well. I just don’t know/understand why that makes such a big difference. Thanks in advance!

 

[decker7]

So bit of an update, carbs are completely cleaned (again) and back on the bike. As a troubleshooting tactic, I transferred all of the jets and other internals from Carb 4 to Carb 3. I figured if the Issue was with the carbs, this would then cause cylinder 4 to run lean rather than carb 3. Unfortunately, i am still having the same issue. Even with the different jets and other parts, cylinder 3 is still running very hot.

This makes me think there is something inside the engine causing the issue. I’ve tried everything I can think of to detect a vacuum leak or anything along those lines. My next step will be to go ahead and take off the top end of the engine. This will allow me to check all the valves and also check if there is potentially a crack in the cylinder head (god I hope that’s not the case).

if anyone has any other thoughts or things I can look for, please let me know!

 

[rphillips]

Did you re-try to sync. the carbs?? Last time you mentioned it, [NODE="3"]Blogs[/NODE] was very little vacuum, that vacuum is what sucks the fuel into that intake stream.

 

[rphillips]

OOhh, that wasn't you that said vacuum was low on #3, that was darrell3001 who was having the same problem... Sorry I got ya'll confused.

 

[Big T]

What are the pilot air and pilot fuel screw settings?

 

[decker7]

The Air screw is 1 1/4 turn out. The Fuel Screw is 1 turn out. I followed the steps on a carb rebuild guide I found on BassCliff's website (https://gsarchive.bwringer.com/bikecliff/images/vm_carb_rebuild.pdf). I made sure there were no broken tips or anything stuck in the carbs from the fuel screws. I even purchased new ones as the old ones were a little damaged.

 

[zed1015]

You should check out the valve clearances.

 

[decker7]

I'm going to look further into the valves today after work. I intend to take off the top end to ensure all valves are free and moving appropriately. It may be over kill to fully disassemble the head, but I would rather go ahead and clean/verify everything is working as it should. I will keep everyone posted with what I find. Thanks again for all of the replies! Good to see that there is a nice community I can come to for help

 

[allojohn]

Adjust the valves to the looser end of spec, then you'll need to vacuum sync again.

 

[zed1015]

With the carbs off, if you spin the motor on the starter and place your palm over the inlet rubber you will feel the suction.
momentarily stop the starter and pull your hand off the inlet, you will feel the suction and hear a light pop as the vacuum is released from the inlet tract..
Do this in quick succession a couple of times across all four cylinders.
With some practice you will be able to feel if all the cylinders have a similar pull and also leakage back into the cylinder.
This combined with any differences in the sound of the pops can help determine if any inlet valves are not seating fully or leaking.
Non seating inlet valves will produce little or no suction and no pop.
You can do a similar test with no special tools on the exhaust valves by removing the plugs and placing a thumb over the hole to feel for compression.
The inlet needs to be done first as the thumb test won't rule out leaking inlets unless they are proved to be seating first.

 

[pdqford]

Thank you for the reply! I’m in the process now of reassembling the carbs. The passages are nice and clear and look ready to go. As for the list height, that’s something I never paid attention to. Can you explain to me why this is so important? Thanks in advance!

The intake air going through the carburetor creates a vacuum that “lifts” the fuel out of the float bowl into the various channels in the carb. The higher the fuel level in the float bowl the easier it is to “lift” the fuel a very short height. The lower the fuel level, the vacuum has to “lift” the fuel a higher distance to get it where it needs to go.

Just to add to the confusion, when you check or set the float height, the carbs are upside down, so increasing the FLOAT” height will actually decrease the FUEL height when the carbs are installed.

 

[decker7]

The intake air going through the carburetor creates a vacuum that “lifts” the fuel out of the float bowl into the various channels in the carb. The higher the fuel level in the float bowl the easier it is to “lift” the fuel a very short height. The lower the fuel level, the vacuum has to “lift” the fuel a higher distance to get it where it needs to go.

Just to add to the confusion, when you check or set the float height, the carbs are upside down, so increasing the FLOAT” height will actually decrease the FUEL height when the carbs are installed.

Thank you for this. That really helps me understand the importance of the height! I will be sure to put the floats at the appropriate level when I put them back on the bike!

 

[Rob S.]

Blocked oil passage?