RPMs go high when clutch pulled in + Right cylinder runs colder than left

edmond

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I took my 78 for its first voyage down the street and back. The idle RPM was around 1.5k, and I haven't yet adjusted with the vacuum gauge but it just arrived so I may do that today. I've also just rebuilt them with new seals and jets, and bench adjusted using two identical small screwdrivers to fit the throttle valves. It slipped into first gear well, but the revs seem twitchy.

The issue is when I took it out of first and held in the clutch, the RPMs went crazy to 4k and wouldn't go down until I started rolling it back into first. Could it be an air leak? How would I test for that? The choke slides are good and the choke was off. Could it be a stuck throttle cable? Not sure if that was it either.

The second issue is the right cylinder runs much colder than the left. The left side has exhaust I can see, whereas the right cylinder is compressing and pumping out exhaust but at a much lower temperature. The right sparkplug looks wet, whereas the left is dry and less dark. What should I check for? Could this be a carb sync issue, where the right side isn't getting enough air flow?

Out of these, which will help the most?
- Cam chain adjustment
- Valve adjustment
- Timing check/adjustment

I'll post a video in a bit.
 
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Video 1:
Here you can first hear the high idle, which was twitchy. As I pull out it settles into a nice low idle.

Video 2:
Now this was after I stopped to turn around and pulled in the clutch. RPMs went crazy and only lowered to 2-3k when I got back into first and came back. I turned off the bike as I pulled in. You can hear the difference in idle from video 1. It was janky, I forgot the license plate holder was rattling everywhere and the kickstand spring isn't in! LOL!
 
Ooooh - last thing first! I hear a "tinking, clanking" noise as you coast to stop. Possible drive chain issue; adjust and align per book (~1" up/down movement at middle). Danger!

2) check ignition timing with timing light - sounds to me as only running on one cyl. Must be diagnosed & spark verified

3) after 2 above, set idle as you can, synch carbs and re-adjust idle to 1100rpm

Report back please
 
No worries about the clanging - that was the license plate holder I forgot was hanging under the body. No problem there

2. Got to order a timing light; in the meantime can I do anything to verify the cylinder is running? Both pipes get hot.
 
The second issue is the right cylinder runs much colder than the left. The left side has exhaust I can see, whereas the right cylinder is compressing and pumping out exhaust but at a much lower temperature
I just test the header pipe temp with a wetted finger. The uninitiated, usually advise use an infra-red thermometer or a damp rag and listen for equal "hiss" sound
 
Yes, that bike has issues, lol. You said you rebuilt the carbs with new jets and seals. By seals do you mean butterfly shaft seals? If so, did you center the butterfly plates before tightening their mounting screws? When you assemble the butterfly shafts and put the return springs on, they pull the shaft towards them as it rotates. That can cause the side of the butterfly plate to bind up against the side of the main bore and stop it from closing all the way.

Also, I see you have mods (pipes at least). Are your new jets bigger or just stock size? For mods, you need to re-jet and install bigger ones.
 
I didn't see no pipes installed at all! lol
(saw stock headers, other thread)
 
Geez, I didn't notice that either, lol. These poor old bikes.
 
I saw that kickstand hanging down in the second vid. Lucky it didn’t catch and send you flying!
 
Yes, that bike has issues, lol. You said you rebuilt the carbs with new jets and seals. By seals do you mean butterfly shaft seals? If so, did you center the butterfly plates before tightening their mounting screws? When you assemble the butterfly shafts and put the return springs on, they pull the shaft towards them as it rotates. That can cause the side of the butterfly plate to bind up against the side of the main bore and stop it from closing all the way.

Also, I see you have mods (pipes at least). Are your new jets bigger or just stock size? For mods, you need to re-jet and install bigger ones.
Yeah, and I'm trying my best to fix them! Not the butterfly shaft seals. I kept that assembly as is, the only things I replaced were gaskets (not seals, sorry) and the jets. They are closing and opening without any scraping or issues. Does it sound like the issue is more likely a carb or an engine problem?

I didn't see no pipes installed at all! lol
(saw stock headers, other thread)
The exhausts are off because I wanted to clean them out and figure out the muffler situation. The previous owner cut them off. *OK so I now know its bad to run them without pipes, lol. I'll put them back on.
I saw that kickstand hanging down in the second vid. Lucky it didn’t catch and send you flying!
Lol! So many things could've gone wrong in that one video. I was just anxious to see if it could take me up and down the block.

I'm doing a carb sync later today. :notworthy:
 
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lol we're funn'in with you not at you:D

I've always heard a (maybe) wive's tale about possible valve damage from no pipes, not seen it. However, it won't run right without some exhaust.
 
2.8 Float needle and seat
#140 Main Jet & #27.5 Pilot jet

OK, with my running Uni Pod filters on my carbs and free flowing aftermarket single wall headers and commando style repop muffs, I had to go to #140 mains and #32.5 pilots to get rid of a hanging idle (1978 BS38's carbs) It's worked a treat for me. Thinking there's no change in the needle setting from stock.

Those pleated air filters can be problematic, and make sure the carbs are tightened down with the proper style clamps.
You'll get it.
 
Yes, I've heard that running with no pipe at all can burn up the exhaust valve. 140 mains should be good to start, that's 2 up from stock, but 27.5 pilots are stock. You're going to need to increase those one or two sizes as well. And with those bigger mains, especially on your year carb set, you're going to need to lean the needles a step. Now, the stock pilots may seem OK with the needles in the stock position (middle or #3 slot) but as soon as you lean them a step you will definitely need bigger pilots.
 
OK, with my running Uni Pod filters on my carbs and free flowing aftermarket single wall headers and commando style repop muffs, I had to go to #140 mains and #32.5 pilots to get rid of a hanging idle (1978 BS38's carbs) It's worked a treat for me. Thinking there's no change in the needle setting from stock.

Those pleated air filters can be problematic, and make sure the carbs are tightened down with the proper style clamps.
You'll get it.
The previous owner had these foam inserts in the pod filters. I've heard of issues with these filters having too much airflow, and people adding a restricting hole inside to lower that. I'll try adding them back in. I gotta get a pair of the OEM carb clamps too.

So I did some more tinkering and seeing how the bike responds to what. I unscrewed the idle screw almost all the way and got it to a point where I had a bit over 1000 RPM idle. It sounds pretty bad, but why it sounds bad I'm not sure - would love to hear thoughts...

Then the weird thing happens, I open up the throttle and after 3k or so it just starts climbing by itself. Could this be a stuck throttle valve as 5twins suggested? I did actually take off the bolt retaining the spring for the butterfly valve, maybe upon reinstalling it shifted and gets stuck in place? It's weird because I think it comes back down after restarting the bike (or maybe not, maybe the vibrations allow it to return to place after a bit). Final weirdness, after it got stuck in a RPM climb and peaked around 4.5k, turning on the fuel enrichener/choke brought the idle back down to 1.5. Choke off, back to 4.5k.

Video:

Choke turns RPM down

I'll spray carb cleaner around the intake/carb when its running to try and see if there is a vacuum leak. Then maybe take carbs out and check the butterfly plates.
 
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Keep the stock pilots and mains if you're going to get a stock air box and stock exhaust, otherwise get the ones needed for the changes made. Your mains should be good at #140, increase the pilots. Get the next two steps up.
Get yourself some JIS screwdrivers or bits, these Japanese Phillip head style screws take a specific bit.
 
Exhaust valves burn when they overheat. They sit in the exhaust stream so they pick up a lot of heat. That heat is dissipated when the valve closes against the seat. From there the heat radiates out to the cooling fins or coolant if you're liquid cooled. When that metal to metal contact is lost, usually due to carbon buildup, the ability to transfer heat is severely diminished. the valve overheats and burns. In days of old it was a fairly common occurrence. Better gas formulations and fuel injection have all but eliminated burnt valves.

Open exhaust's have nothing to do with it.

Rolls Royce Merlin's, Pratt & Whitney R4360's.... Lycoming O360's.... literally hundreds of engine designs in hundreds of aircraft types ran short stack open exhaust's. None had a habit of burning valves.


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Any clues in the videos? Would not-ideal jets cause something this severe, or more likely stuck butterfly valve?
 
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