Cold Case #001 - The Case of the Cold Pipe (2015)

The Case of the Cold Pipe

The following is a description of the suspect.

• Bone stock '74 850 Roadster with 10K mi
• Carburetion - stock Amal 932 Concentrics freshly rebuilt, new air cleaner, synchronized via vacuum gauges at idle and on the road.
• Ignition - points, auto-advance functioning properly, timing 28 deg both cylinders at 3000 rpm, idle timing identical R and L cylinders
• Valve lash - at specification
• Exhaust - stock with header x-over pipe and unobstructed 1-3/8" peashooters. Left pipe is blued considerably more than right pipe, but who knows how, when and where it got that way? Right pipe has welding in the vicinity of the x-over, but is nevertheless sealed tight.
• Compression - with hot engine R = 150 psi, L = 170 psi
• Sparkplugs - new set NGK BP7ES that look excellent (white porcelain, majority of ground strap clear of carbon, dark brown base circle, with slightly darker base on left cylinder, possibly due to modest oil consumption on this cylinder (uses some oil, but I don't know which cylinder or both cylinders?)
• Drivability - starts 1st kick, stable 1000 rpm idle at temperature, operates like an electric motor on a rheostat, easily pulls the ton anytime, pleasure to operate.

Modus Operandi
At an idle the discharge from the right muffler is cool (cone of muffler is literally cold), whereas the discharge from the left muffler is warm. After a 20 minute drive, the right muffler cone is warm, and the left muffler cone is much warmer than the right side.

Discussion
Upon making the initial observation, at idle speed, regarding the hot and cold muffler discharge from opposite sides of the bike, my 1st thought was that there is something wrong with the idle operation of the motor (fuel mixture, carb synchronization, ignition timing at an idle, etc). However I've back tracked through all the things recited above and find the bike to be in a nearly perfect state of tune - and it runs like it. I have no drivability issue whatsoever, just thought it really strange that EX discharge from one muffler is cool and the other is hot.

While futzing away investigating the phenomenon, I deliberately increased the slide height on the right carb to assure it was firing/pulling on the right cylinder, and dropped the left slide all the way down, thereby shutting off the left cylinder, which was verified by pulling the left plug wire and noting no change whatsoever in idle quality at 1500 rpm. Thus the left cylinder was dead and the engine was running at 1500 rpm solely on the right cylinder.

Of course with only the right cylinder firing and having the extra duty of pulling along the dead left cylinder, the trend would now have to be reversed, i.e., the right muffler discharge would have to be hot and the left would now have to be cold right? It has to be so doesn't it? NOPE! The right muffler discharge still had cool air coming out and the left muffler discharge had warm air coming out. Perhaps I didn't wait long enough, but one would think that operating like this for a couple minutes would at least produce some warm air coming out of the right muffler?

I suspect I'm not the 1st and won't be the last to make such an observation, but what I don't have is any sense whatsoever of why/how this occurs. In conducting a cursory search of the site here I did turn up some discussion about one blue pipe and one gold pipe, EX temps, etc, but did not find anything specifically related to what I am reporting here. Perhaps I'm a poor searcher and it has all been revealed long ago. If that is the case might you please direct me to that discussion?

As always, interested to hear any other thoughts related to the above described phenomenon.

Recently I had a misfire that I tracked down and it turned out to be a clogged idle jet on one carb. The exhaust pipe on the misfire side was cold by comparison to the cylinder that was running fine. I had the misfire at low RPMs, and the pipe was relatively cold until I throttled the bike and that cylinder kicked in and the exhaust heated up when it ran normally in the RPM range that wasn't effected by the clogged pilot jet...

You should see if your exhaust pipe is always cold or if it's only cold in a certain range. That might point to where your problem is occuring if it's carb related.

My method of testing the heat of the exhaust pipe was the "spitting method", which worked well.

What happens if you give the engine a bit of throttle, what happens to the air temp out each muffler ?

Can you tweak the throttle cables so each cylinder in turn does all the work for a bit of a rev.

I guess we'll have to ask if the mufflers are identical, including the insides ?
If one muffler was a bit blocked up inside....
Lotta work, but it would be possible to switch the mufflers over - probably for no gain.


We'd also have to comment on those compression numbers.
Done hot, with the throttle wide open ?
If so, one side with more compression shows that t'other cylinder is a little worn.
Shouldn't make much difference to the heat coming out of each muffler though.
Do both sides of the cylinder head feel equally hot - ie can you burn your hand on each side of the head !

Hi WZ507.
From your explanation, your RH header pipe is obstructed after the x-over and before the muffler.
Ta.

Check the cold cylinder is getting fuel at idle as it sounds like a blocked pilot jet or pilot jet system. Remove the plug from the hot cylinder an see if the engine will run on the bad cylinder only.

Using a vacuum gauge to balance the carbs will only tell you that amount of air going through them it will not indicate the amount of fuel mixture in that air. So if the idle system is not working the cylinder is not firing hence its cold. Therefore setting up the carbs one at a time with the other cylinders plug out as per manual will show this up as though both cylinders may not have the same adjustment on the pilot air screw but they should be close to each other.

You could carefully feed a length of stiff wire up the exhaust from the silencer end to check for obstruction but be aware that it may just get stuck on a bend. Or block off the hot end with a rag and as your system has the balance pipe then gas should be able to flow out of the other silencer. It is possible the weld repair has effected the flow of the system.

What you don't say is did the bike run ok before? If its has been run an this problem is a new one then the welded exhaust (probably) won't be the cause. So blocked carb is more likely. Also check each coil an plug lead by swapping them for other side (your running points so you can't just swap plug leads as each coil is timed to fire its own cylinder). If possible test the voltages at different parts of the ignition system and resistance of each coil, ballist resistor and capacitor.

I would check exhaust an ignition just to rule them out but my feeling is the carbs. As i say vacuum balancer can't tell you if mixture is wrong just the amount of air going through the carbs.

Mouse nest in right muffler? Crossover on '74 would allow running with decreased flow volume on the right. Happens a LOT in snowmobiles during summer storage. It's a bitch to remedy on the welded spiral mufflers. :evil:

A flashlight will help you solve the crime... :mrgreen:

concours said:

Mouse nest in right muffler? Crossover on '74 would allow running with decreased flow volume on the right. Happens a LOT in snowmobiles during summer storage. It's a bitch to remedy on the welded spiral mufflers. :evil:

A flashlight will help you solve the crime... :mrgreen:

Click to expand...

+1. Your test running on the right cyl. only, indicates something blocking the exhaust. See also <needing> post above.

Slick

Rohan said:

What happens if you give the engine a bit of throttle, what happens to the air temp out each muffler ?

Can you tweak the throttle cables so each cylinder in turn does all the work for a bit of a rev.

I guess we'll have to ask if the mufflers are identical, including the insides ?
If one muffler was a bit blocked up inside....
Lotta work, but it would be possible to switch the mufflers over - probably for no gain.


We'd also have to comment on those compression numbers.
Done hot, with the throttle wide open ?
If so, one side with more compression shows that t'other cylinder is a little worn.
Shouldn't make much difference to the heat coming out of each muffler though.
Do both sides of the cylinder head feel equally hot - ie can you burn your hand on each side of the head !

Click to expand...

If you give the engine throttle (say hold at 3000 rpm) the temp on the right discharge comes up, but the temp bias still exists between the right and left discharge.

I haven't tweaked the throttle cables since I did the same thing already with the slide height screws.

The mufflers are the same brand and appear to be the same.

Compression measured hot with WOT.

Can readily burn your hand anywhere you want on either side of the head.

o0norton0o said:

Recently I had a misfire that I tracked down and it turned out to be a clogged idle jet on one carb. The exhaust pipe on the misfire side was cold by comparison to the cylinder that was running fine. I had the misfire at low RPMs, and the pipe was relatively cold until I throttled the bike and that cylinder kicked in and the exhaust heated up when it ran normally in the RPM range that wasn't effected by the clogged pilot jet...

You should see if your exhaust pipe is always cold or if it's only cold in a certain range. That might point to where your problem is occuring if it's carb related.

My method of testing the heat of the exhaust pipe was the "spitting method", which worked well.

Click to expand...

Thanks for your thoughts. I have no performance issue as the bike runs extremely well, from idle to full throttle, like any well tuned 850 should. The pipes are both hot, it is the muffler and discharge therefrom that is the point of temperature differentiation.

toppy said:

Check the cold cylinder is getting fuel at idle as it sounds like a blocked pilot jet or pilot jet system. Remove the plug from the hot cylinder an see if the engine will run on the bad cylinder only.

Using a vacuum gauge to balance the carbs will only tell you that amount of air going through them it will not indicate the amount of fuel mixture in that air. So if the idle system is not working the cylinder is not firing hence its cold. Therefore setting up the carbs one at a time with the other cylinders plug out as per manual will show this up as though both cylinders may not have the same adjustment on the pilot air screw but they should be close to each other.

You could carefully feed a length of stiff wire up the exhaust from the silencer end to check for obstruction but be aware that it may just get stuck on a bend. Or block off the hot end with a rag and as your system has the balance pipe then gas should be able to flow out of the other silencer. It is possible the weld repair has effected the flow of the system.

What you don't say is did the bike run ok before? If its has been run an this problem is a new one then the welded exhaust (probably) won't be the cause. So blocked carb is more likely. Also check each coil an plug lead by swapping them for other side (your running points so you can't just swap plug leads as each coil is timed to fire its own cylinder). If possible test the voltages at different parts of the ignition system and resistance of each coil, ballist resistor and capacitor.

I would check exhaust an ignition just to rule them out but my feeling is the carbs. As i say vacuum balancer can't tell you if mixture is wrong just the amount of air going through the carbs.

Click to expand...

Thanks for your thoughts. As I noted in my description, the bike runs perfectly from idle to full throttle, thus the engine is sound, all carb circuits, ignition, etc are spot on. It is only the exhaust discharge out the muffler that exhibits a temperature bias. Blocking one muffler makes all the EX come out the other and vice versa.

needing said:

Hi WZ507.
From your explanation, your RH header pipe is obstructed after the x-over and before the muffler.
Ta.

Click to expand...

I don't think so, as both mufflers are passing very similar amounts of air as assessed by holding a hand behind either outlet.

What about if you block off the exit for the hot muffler, and force all the hot gases out the other one - can it breath OK ?
And get warm as a result. ??

Maybe the cold muffler has its absorbant layer still intact, and the hot one not... ?

In that case, suspect loss of sound deadening material from the 'hot' muffler.
Can your mufflers be dis-assembled to check? I use stainless steel 'wool' over a perforated inner tube.
Ta.

Rohan said:

What about if you block off the exit for the hot muffler, and force all the hot gases out the other one - can it breath OK ?
And get warm as a result. ??

Maybe the cold muffler has its absorbant layer still intact, and the hot one not... ?

Click to expand...

Yes, breaths just fine and warms up more.

Regarding the muffler layers, I'll just say you're getting warmer - no pun intended. I'm drafting something now to post shortly.

The Cold Case is now solved, and the cause was mindboggling/unbelievable to me (well maybe not that startling but unexpected anyway).

I removed the mufflers to inspect them even though I knew they were wide open, since a 1" broom handle could be sailed right through them all the way into the EX pipe. I don't know what brand they are, but have the word Norton stamped vertically at the end of the expansion just before the cone end. They have 1-3/8" inlet and outlet and the baffle is a full length piece of larger dia tubing that has 3 rows of forward facing stamped louvers evenly spaced around the circumference (noon, 4, 8 o'clock). So no obvious difference between the mufflers, with both wide open over their full length.

I started the bike with just the pipes on it and the discharge seemed very similar out of each pipe, i.e., there was no indication that a partial blockage existed. Went for a short spin and all was well with the exception that it didn't run as strong as it does with the mufflers.

So, with no smoking gun in hand or even in sight, reassembled it, but just for the hell of it swapped the mufflers side to side. BINGO!

The right side muffler that always had cold discharge was now on the left side and discharge was just as cold. The original left side muffler, now on the right side that always had hot discharge was as usual discharging hot exhaust on the right side of the bike. So even though the mufflers appear identical visually, there is obviously a difference in packing inside them. I never would have believed that muffler packing could make such a dramatic difference in a muffler that was wide open end to end.

I suspect the cold muffler must have less packing, thereby allowing the EX gas to freely flow into every nook and cranny, cooling along the entire length of the muffler and exhausting at a lower temperature? Perhaps the hot muffler has it's insulation intact, which hinders the ability of exhaust gas to flow through the entirety of the muffler, which tends to make it short circuit straight out of the muffler at a higher temperature?

Anyway that's what I found, and clearly would not have believed it if not seen with my own eyes.

Any noticeable sound difference between the mufflers? Packing, or lack of it, should affect sound.

Slick

texasSlick said:

Any noticeable sound difference between the mufflers? Packing, or lack of it, should affect sound.

Slick

Click to expand...

Not that I can detect, although I'm sure there has to be a difference. They just sound a little throaty and to my ear, provide a pleasant level of noise.

Aha. !
Or maybe thats - Bizarre !

WZ507 said:

I don't know what brand they are, but have the word Norton stamped vertically at the end of the expansion just before the cone end.

Click to expand...

If they have the curly Norton logo inscribed into them, they are of fairly recent origin.
That only appeared in the last decade or two...

I wouldn't be surprised if the "mice theory" proposed previously isn't the reason for the lack of insulation in one exhaust pipe. Every now and then I lift the hood of my truck and see that the damn mice have shred the insulation glued to the underside of the engine compartment hood and made a little nest on top of the engine again... Damn mice!

My 74 commando has always run colder on the L/H side. I have just fitted new unbalanced pipes and new mufflers, along with new Amals. The l/h side still runs colder than the right! It originally had balanced pipes,so this was the only difference from the original set-up. So work this one out, I might say the motor runs and idles beautifully.