One dry sooty plug

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Interstate

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Jul 8, 2011
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Greetings,

This post concerns is my '73 850 Interstate.
I bought it in 1974 and have clocked about 12,000 miles of easy riding. The ignition system is fired by Boyer. I pulled the plugs and found the right side shows a light, even coating of dry, black soot. The left plug is exceptionally clean with only a very slight tinge of black on all surfaces (center porcelain is off white).

This plug condition has always been typical since I've owned the machine. I've used a few various plug types over the decades. I did search this forum for prior discussion but with no reference to a single plug foul. I've replaced both exhaust pipes twice in the past and recall the right side with a deeper blue color than the left side. Currently the newer pipes are quite good showing an even, medium straw coloration.

I'm simply looking for opinions on what I might do to make the right side fire as well as the left side does. My first guess is the throttle needle position on the right carb slide is set too rich.

It starts good and runs good but I'm still concerned.

Thanks for any help,
Dave
 
Been there got the t-shirt. Spent about 2 years fannying about trying to sort this exact same problem, carbs, ignition, valve seals, rocker spindle flats etc etc. The one thing I had which you don’t mention is the bike burnt a fair amount of oil. Anyway, torn into the top end, replaced everything, honed the liners, now both cylinders run the same and burns no, like zero oil. What was the actual problem, no idea
 
acadian said:
Need more info, is the soot localized to the base ring (indicating jetting), or is the ground strap and porceline covered as well?

http://members.uia.net/pkelley2/sparkplugreading.html
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Soot build up has coated all plug end surfaces with a thin, even layer of flat black, dry soot which will not wipe off with the finger tip. It looks as if all plug surfaces beyond the last screw thread have been sprayed with an even coat of flat black spray paint. No loose particles anywhere or build up of soot around the base of the porcelain or within the plug gap. Yes, ground strap and white porcelain are covered in the same manner.

Thanks for your reply,
Dave
 
Triton Thrasher said:
How old are the needle jets?

Have you tried weakening the pilot mixture on the sooty side?
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Needles & jets are original to the machine. My thought is the needle height adjustment may need to be lowered to decrease fuel intake, right?
Have not messed with the pilot air mixture screw yet. I wanted to gather some clues before I tear into the problem.

Thanks.
 
Onder said:
Compression check? Just curious.
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No, did not check compression yet. I do have a new tester. I'll look up the correct compression value unless you know what the '73 850 engine should read when cold. Thanks for the advice.
 
Steves said:
Been there got the t-shirt. Spent about 2 years fannying about trying to sort this exact same problem, carbs, ignition, valve seals, rocker spindle flats etc etc. The one thing I had which you don’t mention is the bike burnt a fair amount of oil. Anyway, torn into the top end, replaced everything, honed the liners, now both cylinders run the same and burns no, like zero oil. What was the actual problem, no idea
Click to expand...
I hope you like the T-shirt. At least you fixed the problem among other repairs at the same time. My machine never burned any oil since I've owned it EXCEPT on start up due to wet sumping. I solved that issue just a year ago with a ball valve in the oil line off the tank. It has an ugly red leaver on the valve that sticks out into the kick starter when it's closed as a reminder to open it before starting. If I forget to open it one day I'd do the work you did for the same T-shirt....no thanks!

Thanks for your reply,
Dave
 
I have good compression or rather acceptable compression but have the same black plug disease as you do.
No smoke but it uses oil. Therefore most likely it is lack of oil ring control. Yes, the jugs should come off and
with a hone and a re-ring. After I get my head back we shall see if I was correct.
 
Without knowing the jetting of the 2 cabs your description sounds suspiciously like oil contamination, oil could be getting in from up top or down below. Remember that you can get a false compression test reading with bad rings, even a leakdown test won't necessarily identify a bad oil control ring if the upper two are good
 
Just to add a bit more misery to the situation it is possible that a loose rod is spewing more oil than one with proper clearance and it is overwhelming the oil control ring. When you pull the jugs to see about the rings give a wiggle to the offending rod and compare it to the other side.
 
I wouldn't tear the motor apart for a little bit of soot on one plug. Get the carburetors synced up, and adjust the idle mixture screws that you have not touched. Make real sure your electrical system is putting out proper voltage, and the ignition timing is optimal. Dropping the needles would be a last resort if the bike starts easy and runs well currently. More than likely it'll fall on it face a little if you drop the needles. But who knows.

I've gone from 2 sooty plugs to plugs that look like they have never been taken out of the packaging with the same engine without tearing the motor apart. Point is I know it is possible to get a better looking plug without tearing a motor apart. How I did it is on the pricey side, and does not involve using Amal carburetion. You might be able to accomplish the same with a set of new Amal Premiers. I don't know for sure though.

You could also take the if it aint broke don't fix it approach.
 
Looking in through the spark plug hole with a good light, what does the top of the piston look like?
 
Interstate said:
Needles & jets are original to the machine. My thought is the needle height adjustment may need to be lowered to decrease fuel intake, right?
Have not messed with the pilot air mixture screw yet. I wanted to gather some clues before I tear into the problem.

Thanks.
Click to expand...
Needle jets wear out, causing richness at small throttle openings. You can’t fix that properly by lowering the needles.
 
i, too, have fought the dry, sooty plug monster as well documented on this forum........i blamed dark color on the 34mm mikuni so bought new 932 amals tried different needle clip positions and mains and plugs (Champion N4G, B7 Iridium NGKs) best i could get was chocolate brown going riding tomorrow with autolite AP63s if plugs still dark then so be it bike starts runs great maybe i just relax and enjoy the ride instead of chasing a color.............. (and further depleting my meager bank account).
 
Assuming the ignition timing is correct and you get a nice fat spark on that side with a fresh plug, there are any number of carb related problems that can make it run too rich. Worn needle/jets, leaky float, float height, float hung up on gasket, etc.

Seeing as it is original carb, and you don't do many miles, it may benefit from a strip down and ultrasonic clean at this point. I would buy an Amal carb rebuild kit and jet removal tool and replace the parts on that carb that come with the kit. Also buy a new main jet and one smaller than you currently have.

Rebuild to current spec will eliminate worn parts. If problem recurring, then you can start changing heights, etc.

amalcarb.co.uk

Notes On Rebuilding the Amal Mark 1 Concentric Carburetter | AMAL | AMAL Carburetters

The Notes On Rebuilding the Amal Mark 1 Concentric Carburetter page for the AMAL Carburetters website.
amalcarb.co.uk amalcarb.co.uk


 
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