Exhaust pipe discoloration- one side only

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Richard Tool

Richard Tool

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The right exhaust pipe on my newly rebuilt 750 Combat has discolored immediately. The pipes are brand new and bought as a set . Timing is spot on and cylinder head temp is within one or two degrees of each other measured at the same spot above the exhaust on each side with a digital infrared thermometer. When measuring temperature of the pipes themselves the discolored side is actually a few degrees cooler. What’s up with this ?
Is it possible that the chrome job itself may be to blame ? The total amount of running time is no more than a few minutes.
Looking for help here - thanks all - RT
 
I venture that there are a few possibilities beyond thin chrome or pipe wall thickness under .062"

1) One cylinder's rings seated early and that cylinder is working harder.
2) An air leak on either side of the head.
3) Carb sync.
4) A slide that is more warn then the other, see #2.

Other members will be able to add/subtract from this, but in the days before I discovered stainless exhaust I'd keep a set pipes for break-in and a new set for after break-in. During the break-in period tuning an engine is like shooting at a (fast) moving target as engine conditions change rapidly.

Best.
 
Given that your temperature measurements don't show a big discrepancy between cylinders, I'm inclined to suspect lousy chrome on one pipe. If there was a timing or lean mixture issue the blued pipe would certainly be hotter than the other.
 
The infrared detector likes a flat black surface as opposed to shiny. Are you able to provide this? One would think that the error would be about the same off any surface of similar refleciveness but I don't know if that holds true as I've never explored the idea. As to how to provide that flat black, I took an old tee-shirt (black) and cut strips of it to wrap around the exhaust pipe to measure temps. Make sure its a cotton shirt! No poly. In theory it provides a better result.

You could of course spray it with some flat black stove paint but probably don't want to bake that stuff on and then try to clean it off. As for the head, you could make a metal tag with a hole in it, spray it flat black, remove a nut from a rocker cover and attach it there. Then use it for measuring temps from side to side.
 
Had the same issue.
Infrared temps. We’re close at idle but at 3k and holding the temp. Imbalance became clear.
Carbs out of sync, one way too rich the other crazy lean.
I never could get a good balance until I had them sleeved. After that it was a different engine.
 
Carbs are brand new Premiers - temp was taken on cylinder head which is soda blasted aluminum - hardly shiny but not flat black either . It would seem to me that regardless of the infrared devices preference for surface colors, I am only looking for temperature differential so as long as the surfaces I took the readings from were the same it should not matter . I took readings before starting and then at various times while warming up and also up to 3k rpm - of course the higher rpm readings were higher temperature but, again, they were within a few degrees of each other ( left cylinder to right cylinder) .
 
It would be interesting to see the numbers from either a leak-down test or an old fashion compression test.

Do the new carbs have #19 idle jets? are both air screws the same or close. How many turns out from them seating? And you did sync the slides?

As I mentioned earlier, new rings and valves take a while to enthusiastically join the team.

Best.
 
RoadScholar said:
It would be interesting to see the numbers from either a leak-down test or an old fashion compression test.

Do the new carbs have #19 idle jets? are both air screws the same or close. How many turns out from them seating? And you did sync the slides?

As I mentioned earlier, new rings and valves take a while to enthusiastically join the team.

Best.
Click to expand...
Not certain on idle jet # - carbs purchased as a pair for 750 Combat. Throttles and air slide are synced - air screws both @ 1 .5 turns out from seated.
 
Did you put 1/2 can of black BBQ heat paint down each pipe before intallation?
What is the temp of each pipe within 2" of the head.
Idle only did this coloring? or do you have "down the road" 12-15hp time?
These kind of tests are best during a dark night when you can pull over to see the "glow".
 
My pipes did the exact same thing. Pretty discouraging after spending so much on the pipes and shipping. Right side discolored immediately, the left did not. The right float level was pretty low when I pulled and checked it. Original Amal 932s.
 
dynodave said:
Did you put 1/2 can of black BBQ heat paint down each pipe before intallation?
What is the temp of each pipe within 2" of the head.
Idle only did this coloring? or do you have "down the road" 12-15hp time?
These kind of tests are best during a dark night when you can pull over to see the "glow".
Click to expand...
No paint - had no idea about this but it makes sense . The left pipe is fine - right pipe now gold ,no blueing. Bike has not been ridden as the tank is at painters . All running done stationary with a temporary fuel bottle.
The max temps I read at cylinder head directly above the exhaust rose were 220 degrees Fahrenheit. Pipes themselves measured between the rose nut and first bend were in the 185 degree Fahrenheit area.
Idle till warm then occasional blipping up to 3k but not sustained.( original cam and followers so no need for cam break in.)
 
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I'd be measuring on the pipe. When I run the dyno, the thermocouples are inside the hot air exhaust stream within 2" of the head. For me the head reacts way to slow. For you the exterior of the pipe would be cooler. I run for around 1300F EGT or less. Lots more power if hotter, but melting pistons suck....LOL
Did someone sneak in at night and steal your old tune up/break-in pipes?
Want to borrow my 9mm? LOL
 
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Take it for a ride and bed the rings in and make sure the carbs are set right under load, as well each carb can be set different from each other to have it tuned right.

Ashley
 
I’m thinking that the blame lies on the pipe itself- maybe thin walls or chrome or both . If both sides are showing essentially the same temps what else could it be ? Scout 63 had the same experience and in looking at older threads regarding the same it seems it is not uncommon. I will live with it for the time being. The pipes will have to come off at some point to replace one bronze exhaust rose as yours truly broke one fin off snugging them up after the head warmed up . Looking to re grind my C spanner so it engages the fin at the base and not the top . Live and learn. At that point I will decide what to do about the pipe.
Thanks all for the help - RT
 
As I was tuning an MKIII with everything stock I couldn't get the carbs to sync with vacuum gauges. I messed with the cables but nothing. Then I thought about the timing and sure enough, with points you can get a big difference between the sides. Got that set and everything came right into balance. Any way, you already said the timing was spot on but if it has points you have to check both sides. (retarded timing makes super hot exhaust when you give it the gas.)
 
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