The Sooty Plug - Where to next?

Hi folks,
my 750 is suffering from a sooty right hand spark plug. The machine has been rebuilt with new pistons, rings and valves.
The Amals (930) have been refurbished with all new parts to standard (220 main.106 needle , circlip middle position.)
At a high idle (1200rpms) the left hand side fires nicely and warms up. The right hand side is cold and misfires slightly. The soot is dry and not oily.
The bike starts first kick and ticklers take exactly the same 5-6 seconds to flow.
So far I have:
Cleaned the pilot jet
Checked the float height. OK
Checked sychonisation. OK
Checked the timing. OK
Checked inlet and exhaust valve clearnances. OK
Compression 150psi dry
New spark plugs
Swapped plug leads over (single RGM coil and Pazon)

Any suggestions what to look at next? I'm going nowhere at present!

What grade of plugs and brand .... ?
Craig

We had the same with a racing Suzuki (sorry☺). Worked fine on startup started to miss fire as soon as ridden. It was the spark breaking down under load, in our case it was a coil on the stator plate. It was electronic ignition by electric world. Was tested fine by the company but broke down again on the bike. Cost us two race meetings! New stator plate and it's fine.
Hope this helps.
Suspect ignition, plug/ lead/ coil breaking down.
Also assume circling has not failed on needle, float failure.

Davo said:

Hi folks,
my 750 is suffering from a sooty right hand spark plug. The machine has been rebuilt with new pistons, rings and valves.
The Amals (930) have been refurbished with all new parts to standard (220 main.106 needle , circlip middle position.)
At a high idle (1200rpms) the left hand side fires nicely and warms up. The right hand side is cold and misfires slightly. The soot is dry and not oily.
The bike starts first kick and ticklers take exactly the same 5-6 seconds to flow.
So far I have:
Cleaned the pilot jet
Checked the float height. OK
Checked sychonisation. OK
Checked the timing. OK
Checked inlet and exhaust valve clearnances. OK
Compression 150psi dry
New spark plugs
Swapped plug leads over (single RGM coil and Pazon)

Any suggestions what to look at next? I'm going nowhere at present!

Click to expand...

You have switched plug wires so that rules out ignition problems with a wasted spark ignition.

Next obvious [and probably already done] switch out the spark plug.

Observe the valve motion to make sure you have not wiped a cam lobe.

Now the most likely thing. The carb probably has a damaged or plugged low speed jet in the carb body. The one that is difficult to get to behind the mixture screw.
Also make sure the pressed in cap visible in the carb body inside the bowl is sealed and make sure the float bowl is not bent to the point of causing a leak in the low speed fuel pickup at the gasket surface.
Sometimes replacing the carb is the only cure. Jim

Any suggestions what to look at next? I'm going nowhere at present!

Click to expand...

There is a fairly fast and easy way to verify if you have a specific carb problem or not

switch the carb bodies left to right, leave the cables attached to the slides, just switch the bodies, clean plugs again start and go for a ride

if the cold misfiring former right side now moves to the left then you know for sure it is a carb problem and that it is the left carb body that is not flowing/jet/etc

Carb!

If you can remove the air filter and see into the carb you can see the slides move as you lift the throttle, you should be able to see if one slide body is sticking in the open position, which would make one plug appear rich. Also you could have a sticky float, but usually that would cause your carbs to bleed uncontrolably. The other thing that could be amiss is that your slide needle clip could be displaced from it's position under the spring in the carb body... or you bumped the needle during your carb assembly and it jumped into the lower groove so the needle is now in the wrong groove.

All guesses based on my own bad experiences...

Thanks folks, all good tips.
I have checked the needle circlip, all good there, tried 3 x new plugs just to be sure (NGK BP7es).
I wet and dry sanded the float bowls on glass before re-assembly.
I suspect Jim has the answer, the idle jet being damaged. A previous owner may have been a bit heavy handed with jet probing.
I'll swap them over this weekend to confirm and report back.
All that money thrown at old carbs could have been spent on Premiers. Live and Learn!

Rather than all that work swapping the carb bodies over,
just tug on the throttle cable (with the engine running) for the carb not doing any work.

If it roars into life, as a one cylinder engine, you know its only the idle side of things in that carb that is wrong.
If nothing much happens, its something deeper....

Dry sooty stuff can be light oil burning however.
You may need to watch for light blue smoke in the exhaust, especially visible at night...

Nothing has been said about intake valve seals, would this be a possibility. I had a problem such as this on my MGB (oops) and some of the soot bulit up and got between the valve and the valve seat burning away a large portion of the valve from the burning jet of fuel (like a plasma cutter) now able to pass by. Only took a single drive from running good, but with one sooty plug, to very bad rebuild the head time. The valve seals were like rocks and the culprit one was half gone.

Maybe a time for Ludwig's technique here, but hopefully simply a carb issue. (Simply I say...)
Lance

Davo said:

the left hand side fires nicely and warms up. The right hand side is cold and misfires slightly. The soot is dry and not oily.

Click to expand...

I had this issue where I had a cold exhaust pipe, a rich looking plug and a slight misfire. When I twisted the throttle, and the bike's RPM's climbed to a certain point, the misfiring cylinder would "kick in" and the bike ran like normal above 2800 RPM's. I started a thread for it and finally I got enough feedback from people here that it hit me that the idle circuit was clogged and I was idling on 1 cylinder and that's why the exhaust pipe was not hot. IF you start the bike and have it running for a minute, you should be able to spit on the exhaust pipe and it should instantly smoke when it hits the pipe because it should be that hot or else it's not firing.....

Davo said:

All that money thrown at old carbs could have been spent on Premiers. Live and Learn!

Click to expand...

Ok,....... so you have given up on the amals... so you might as well try something desperate (like I did) I followed the directions to drill out the OPPOSITE side of the amal's idle MIXTURE screw's passage, so I could inspect the idle jet. Mine was clogged. I poked it clear, threaded the opening that I had drilled and installed a set screw with locktite to plug the new hole I made. I Reinstalled the carb and Fixed my problem...Done!

There's no reason you shouldn't drill the offending carb and do the same thing I did if you are giving up on the amal carbs anyway... Your symptoms sound just like mine. You can go to the link I've posted below and read the instructions. I didn't use a 2BA screw to seal the hole I drilled. I had a tap and set screw that was close enough in size so I used what I had...

http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

comnoz said:

Observe the valve motion to make sure you have not wiped a cam lobe.

Click to expand...

Been there! Sooty plug was definitely present.
Let's hope it's your carbs.

Substitution. Substitution.

Don't think you can swap the carbs over, the boss for the throttle stop and the pilot air screws will prevent the carbs getting close enough together allow the carbs to sit flat against the manifolds.

True. Known good components for substitutions to eliminate and/or narrow down the problem.

Solved!
I'm a little embarrassed to admit it but it seems to have been a clogged pilot all along.
I got my guitar string tool out for one last effort ( having already done this multiple times) and bingo no more sooty plug. Nice tan colour either side and spit test positive!
Thanks all for your suggestions. Lesson learned, the simplest explanation is most likely the cause.

Now for a new question: Could the crud I dislodged in previous cleaning efforts (with carbs on bike) re-block the jet straight away?
Should the bowls be dropped and compressed air forced through to do a thorough job?

Humbly yours
Dave

Davo said:

Solved!
I'm a little embarrassed to admit it but it seems to have been a clogged pilot all along.
I got my guitar string tool out for one last effort ( having already done this multiple times) and bingo no more sooty plug. Nice tan colour either side and spit test positive!
Thanks all for your suggestions. Lesson learned, the simplest explanation is most likely the cause.

Now for a new question: Could the crud I dislodged in previous cleaning efforts (with carbs on bike) re-block the jet straight away?
Should the bowls be dropped and compressed air forced through to do a thorough job?

Humbly yours
Dave

Click to expand...

Nothing to be embarrassed about... I poked my idle jet 10 times thinking each time I had made sure it was clear, then I moved on to other possible causes of the problem. It was after someone commented on my thread that their problem had the exact same symptoms as mine (and that problem turned out to be a clogged idle jet) that it dawned on me from reading the bushman's page that the idle jet can be difficult to clear and nearly impossible to visually inspect while it's on the bike... so I did the bushman modification and cleared the jet from it's backside.

If you decide to blow air down the idle air screw passage, you need to either remove the float bowl so the nugget actually exits your carb rather than just get blown down into your bowl. You could just remove the bowl's drain plug and hope it flys out the hole in the bottom of the bowl too...

.. Thanks for reporting back...