Topic: Why would 1 of 4 exhaust pipes have a different temperature?

[BudgetBiker]

Its been a while and my searching has brought up very old  posts so starting a new one.

I have been working for over a year restoring an old 1975 CB550F Supersport that was severely neglected. Have done a lot. But right now the engine seems to idle with a low tone knock and occasional stutter.

Question
I have rebuilt the carb, synced and tested it however i seem to have exhaust pipe one at under 200f at idle after warmed up while the other 3 seem to be normal. About 300F more or less. What would cause this? Poor timing? It seems this is the source of the knock. I have been using premium fuel. Should i switch back to regular?

Thanks.

I have done the following to this bike.

Replaced engine upper manifold gaskets, cleaned out a lot of carbon buildup on the cylinder heads.

Checked and adjusted the inlet/exhaust valves to spec
Changed coil packs, plugs.

Rebuilt oil pump
New petcock.
Rebuilt carb and synced. Adjusted air/fuel screws.
New exhaust
New ignition switch
New timing switch

[Flyin900]

What is a timing switch?

Many different possible issues could be the cause. It would be best to start with ignition to check the plug on that cylinder, the plug cap and wire to that cylinder and the coil. You can try switching the plug cap around off the coil with the wasted spark to see if it moves with the switch.
Could be carburation where you haven’t got the carb cleaned well enough for that cylinder...etc

It is going to be working through electrical and carburation to determine where the issue lies. Given it is a single cylinder it should be just a matter of working on the suggestions above to track down the problem.

If your plug caps are original and the wire is original that could be a starting point. Replace all the caps is just best practice on 45 year old parts, yet it may not be your issue. Cutting back 1/4 inch of the wire that connects to the cap will expose cleaner internal wire than the degraded end on there now.

[scottly]

check the plug on that cylinder

1+ Is it carbon fouled? (carburation) Is it clean but wet with gas? (ignition)

[DaveBarbier]

I would also check the plug, plug cap resistance and the plug wire going to that cylinder. And +1 with cutting back a little of the plug wire to expose fresh copper. Do a compression check as well; dry and wet.

I didn’t see you mention checking ignition timing with a strobe. Do that too.

How did you “adjust air fuel screw”? Did you go to the factory setting?

That being said, after you get this sorted you can safely go down to regular fuel. You’re just waisting money with the premium. It does nothing other than help prevent detonation.

[mrbreeze]

You say there is an occasional stutter. Do you have a timing light? Try hooking one up to the cylinder that has low temp and watch it. If you have an ignition misfire you will see the light flicker. I would go through and check everything that has been mentioned on that cylinder. It shouldn't take long and would be the easiest way to pinpoint where the problem is. Good luck with it.

[Don R]

 X2 on mrbreeze's post. I thought I was the only guy using a timing light that way. These guys have done a great job of listing the possibilities. I had one with a bent valve once due to me choosing parts poorly and missing a shift. A leakdown test should prove that to be false.