Smoking MK2a

MCP

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I have a 1974 850 Mk2a, balanced pipes with pea shooters, 25,000 original miles on it, new piston rings on original pistons, about 1000 miles since installed, compression tested Left cylinder 141 PSI, Right cylinder 125 PSI, smoking badly when the throttle is blipped, but not while riding. The right side muffler heats up instantly, the left is cool to the touch even after a 5 minute idle. The bike ran nicely, but used an incredible amount of oil in 1000 miles. The right side plug was fouled badly. New Premier carbs.

I removed the head and passed it on to an experienced mechanic. The valve seals were in good shape (now replaced with new ones), the right exhaust valve was sloppy, now repaired. Anything else suspect on the head will be dealt with.

What would you suggest as the next step since the head is off? Any thoughts on why the right side ran hot the left cool? Why the smoke at idle? I was hoping it was valve seals based on my reading here, may not be.

Thanks
 
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I had a similar problem on a newly rebuilt bike and it turned out to be the piston rings were not installed with the required ring gaps in relation to each other
 
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Tornado

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Had blip puffs on my mk2 850 on one side. I fit fresh viton seals without removing head and issue went away. But if head is at machinist, why not have the guides replaced, new valves and seats done?
Also, confirm the oil drainage hole under right side inlet spring seat is clear and the hole it connects to in the cylinder block. With head off, try poking thin wire down the block hole until it reaches a sharp bend. Blow some WD40 or similar through it into timing chest. Oil slow to drain can give smoking as it gets past seals.
 

hhh

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My '74 Mk 2 had a weird oil burning problem that had me wondering for a while. It turned out that the incompetent machine shop whacked the guides out when the head wasn't sufficiently heated and scored the shit out of the exhaust guide bores. The smoke was worst after sitting at a light for a minute or two and then taking off, and on first startup. I've temporarily cured it by using high heat silicone under the valve spring insulators. As long as you don't get carried away, there's little risk of plugging an oil way.
 

johnm

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My '74 Mk 2 had a weird oil burning problem that had me wondering for a while. It turned out that the incompetent machine shop whacked the guides out when the head wasn't sufficiently heated and scored the shit out of the exhaust guide bores. The smoke was worst after sitting at a light for a minute or two and then taking off, and on first startup. I've temporarily cured it by using high heat silicone under the valve spring insulators. As long as you don't get carried away, there's little risk of plugging an oil way.
Nasty. My favourite engineer in NZ actually sets the head up in a mill and machines away the exposed parts of the old guides before removing the remains of the guides.

You can get oversized guides if it's too bad.
 

marshg246

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I have a 1974 850 Mk2a, balanced pipes with pea shooters, 25,000 original miles on it, new piston rings on original pistons, about 1000 miles since installed, compression tested Left cylinder 141 PSI, Right cylinder 125 PSI, smoking badly when the throttle is blipped, but not while riding. The right side muffler heats up instantly, the left is cool to the touch even after a 5 minute idle. The bike ran nicely, but used an incredible amount of oil in 1000 miles. The right side plug was fouled badly. New Premier carbs.

I removed the head and passed it on to an experienced mechanic. The valve seals were in good shape (now replaced with new ones), the right exhaust valve was sloppy, now repaired. Anything else suspect on the head will be dealt with.

What would you suggest as the next step since the head is off? Any thoughts on why the right side ran hot the left cool? Why the smoke at idle? I was hoping it was valve seals based on my reading here, may not be.

Thanks
The left being cool to the touch after 5 minutes means that the cylinder is not running as idle. What color smoke? Since the right exhaust valve was "sloppy" that was probably the cause of the low compression.

I would pull the cylinders and make sure the rings are right side up and that the gap is reasonable (right side up very important, ring gap less so).
 

MCP

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The smoke is white, and the left cylinder was definitely firing on idle.

Wasn't sure if I should take a chance and just put the head back on when it's done, but now I'll tick all those suggestion boxes in the next day or 2 when we get back at it.

Thanks everyone
 

marshg246

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The smoke is white, and the left cylinder was definitely firing on idle.

Wasn't sure if I should take a chance and just put the head back on when it's done, but now I'll tick all those suggestion boxes in the next day or 2 when we get back at it.

Thanks everyone
Pretty tough to have a cylinder running for 5 minutes and not making heat! I've been tricked by Nortons idling on one cylinder and the cylinder "kicking in" as you twist the throttle.

Generally:
Black Spoke: Too much Fuel (way rich)
Blueish Smoke: Oil
White Smoke: Water (condensation)
 

Mart UK

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The smoke is white, and the left cylinder was definitely firing on idle.

Wasn't sure if I should take a chance and just put the head back on when it's done, but now I'll tick all those suggestion boxes in the next day or 2 when we get back at it.

Thanks everyone
The balance pipe might fool you into thinking it's firing?
 

MCP

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Pretty tough to have a cylinder running for 5 minutes and not making heat! I've been tricked by Nortons idling on one cylinder and the cylinder "kicking in" as you twist the throttle.

Generally:
Black Spoke: Too much Fuel (way rich)
Blueish Smoke: Oil
White Smoke: Water (condensation)
Your correct above Greg, I should have been more clear; my mistake, there was heat on the left pipe but nothing in comparison to the right. The right muffler would heat up instantly, the left would not.
 
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Did you have a machinist measure the bores as a minimum , or did you fit the new rings without a machinist to lightly hone the bores for these new rings ?
 

Mart UK

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New carbs, right plug fouling, left possibly not idling. Could the right carb be too rich and the left have a blocked pilot circuit?
 

HRD

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I have a 1974 850 Mk2a, balanced pipes with pea shooters, 25,000 original miles on it, new piston rings on original pistons, about 1000 miles since installed, compression tested Left cylinder 141 PSI, Right cylinder 125 PSI, smoking badly when the throttle is blipped, but not while riding. The right side muffler heats up instantly, the left is cool to the touch even after a 5 minute idle. The bike ran nicely, but used an incredible amount of oil in 1000 miles. The right side plug was fouled badly. New Premier carbs.

I removed the head and passed it on to an experienced mechanic. The valve seals were in good shape (now replaced with new ones), the right exhaust valve was sloppy, now repaired. Anything else suspect on the head will be dealt with.

What would you suggest as the next step since the head is off? Any thoughts on why the right side ran hot the left cool? Why the smoke at idle? I was hoping it was valve seals based on my reading here, may not be.

Thanks
What is the head ,the type number, on the bike as many years since bike was new and lots can change over time and who knows if the miles are correct ,people say lots of things and their memory is often sketchy and maybe not honest. Pea shooters were not original for a Mk2a so maybe other things have changed .
 

WZ507

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This might possibly shed a ray of light on the temperature difference from R to L side.

 
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MCP

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Thanks for all the input. I'll let you know what we find when its solved, but at this stage, the barrel is off and is being checked for tolerances and being honed. Likewise the pistons being checked for tolerance. The rings were installed correctly.

Just to be clear, the bike was running on both cylinders on idle (that was confirmed by a couple mechanics).

The original carbs were swapped this summer for premiers, and it ran smoother but still smoked.

The oil drainage hole was clear.

The miles are correct, zero motor work until I had new rings installed last year. I've owned it since new.

WZ507 - thanks for the link, really interesting and I'll follow that up.

Thanks again.
 
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