Any other ideas...

The length of the post is necessary for full description.

I need some ideas on what to try next for my 74' 850.

The bike runs great. I mean great, just pulls and pulls. I have been scolded by the police twice to back off. I think they give me a break because I am an official "old timer".

But, after a few hours, the plugs, once again, begin to foul. I have been dealing with this condition since I installed the carbs after the original overhaul. I have taken them down multiple times since to confirm all the components are in place, the floats are set and jets are clear and working...I'm confident the internals are correct.

I have installed the new hardened slides, needles and guides, floats and needles, fuel screens/filters, clean idle circuits. I run a K&N air filter, the intake and carb fasteners are tight, no air leaks. The intake manifolds have insulating spacers/gaskets in place, no gas leaks, etc. etc. etc.

There is a new Tri-Spark EI installed and timed out with a degree wheel...28 degrees BTDC. It is performing flawlessly.

I put in hotter plugs I got from Danno, changing from NGK BPR-7ES to Champion N-4G's which helped in terms of a cleaner burning plug but these too eventually foul. This might be an indication of what? Advance the timing a few more degrees?

The wet sumping is almost non-existent but I drain the case anyway, just takes a couple of minutes so WGAS.

When the engine is warm it usually fires first kick and if it won't fire from the 3rd kick on I know exactly where to look, spark plugs.
Fouled. As in Black.

I have really worked at tweaking the 932's for optimal performance and if I take them any further to the lean side performance suffers and idle falters along with all the popping and backfire racket....and yet the plugs still foul.

I try not to let it sit and idle for too long...but traffic conditions are not always conducive to this and I don't do too much long ride/highway riding...today's drivers are idiots. Could these fouled plugs simply be reflecting my recent riding style?

I'm really considering a new set of Amals but after spending a bunch of money on rebuild components I am really reluctant to do so...plus I did a good job on the rebuild/overhaul and I want the original carburetors on the bike. There is zero assurance that after dropping more money on new carbs this is the route to solving the issue (although I bet it will) so that gnaws on me as well.

Any ideas on where to dig next are certainly welcome.

Assuming the Tri Spark is functioning normally, the advance curve of the unit is fairly well sorted for most scenarios.

You don't describe where in the throttle range the problem occurs, might help to narrow it down to a specific jet.

You need to know for sure whether the fouling is from gas or oil. Do the pipes smoke sometimes? Both plugs look the same?

acadian said:

Assuming the Tri Spark is functioning normally, the advance curve of the unit is fairly well sorted for most scenarios.

You don't describe where in the throttle range the problem occurs, might help to narrow it down to a specific jet.

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The problem is not within a specific point of the throttle range. As stated "it runs great and pulls and pulls". The issue is spark plug fouling over a period of time, making it a beast to start and interfering with performance. With clean plugs it fires readily.
Thank you for the thoughts.

Do you have good compression in both cylinders ? What fuel are you using as far as octane and ethanol goes ? The higher the octane the better for my bike, less fouling. 73 850. Another possibility could be that as your engine heats up, your coils heat up and lose their ability to produce good spark.
If you have a good blue spark when the engine is cold and then turns to a weak yellow spark when it's heated up, that will cause fouled plugs after awhile.
You mentioned you use a K&N air filter....so do I.....make sure that its clean.

maylar said:

You need to know for sure whether the fouling is from gas or oil. Do the pipes smoke sometimes? Both plugs look the same?

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Zero smoke.
Head was rebuilt along with with new gaskets and seals when the engine was rebuilt. New rings (and pistons) are well seated. I really don't believe this condition is due to oil blow-by.
Plugs are almost duplicate in coloration, RC might be a hint blacker than LC.
Appreciate the thoughts.

jsnorton said:

Do you have good compression in both cylinders ? What fuel are you using as far as octane and ethanol goes ? The higher the octane the better for my bike, less fouling. 73 850. Another possibility could be that as your engine heats up, your coils heat up and lose their ability to produce good spark.
If you have a good blue spark when the engine is cold and then turns to a weak yellow spark when it's heated up, that will cause fouled plugs after awhile.
You mentioned you use a K&N air filter....so do I.....make sure that its clean.

Click to expand...

Good spark regardless of engine temp. I use an ignition pack.
93 Octane.

Thanks for consideration.

Can you elaborate a bit more on the following:

Lineslinger said:

But, after a few hours, the plugs, once again, begin to foul.

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Do you have a loss of performance or the bike just won't restart due to fouled plugs?
Are these fuel fouled or oil fouled - any hint of oily residue?
Any hint of smoke it the tail pipe?
Do both plugs foul together around the same time and same condition or is there a cylinder bias e.g. one before the other every time?

Lineslinger said:

Could these fouled plugs simply be reflecting my recent riding style?

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What are you alluding to here. You mention traffic? What should we know?

First, I would not change the heat range on the spark plugs except to experiment a bit. Stay the course with the recommended heat range.

It sounds like you freshened up the carbs a bit. What elevation do you ride at (Denver @ `5,280' or sea level)? Did you replace carb internals one-for-one or go with factory spec; the reason I ask this is if factory spec, it is typically for sea level.

Much of most peoples riding is on the slide/needle/needle jet. You might want to do a side by side comparison of the old slides and new slide to see if there was a change in the cutaway. Also check where the needle clips are at now and where they were at when you dismantled.

But before you jump into the carb component comparisons, make sure you are not chasing an oil foul issue. Oily plugs and smoke out the tail pipe are clues. Over oiling the cylinder contributes to oil consumption; this could be caused by such things as rocker spindles installed incorrectly, inoperative oil control ring or wrist pin clip failure allowing the cylinder to be scored.

It appears you've done everything right.
The older Amals have a Pilot Bushing in lieu of a replaceable jet. It's possible the ID has been expanded through cleaning or someone prior may have enlarged it.
I believe the fix for this is to drill the cap from the back side and replace it with a jet. (This is how the premiers are set up.)
I've never done this BTW. So, you'll need to research it. And it may not be the issue.
Myself, I would probably drop main jet sizes without touching anything, and see what happens.

MichaelB said:

It appears you've done everything right.
The older Amals have a Pilot Bushing in lieu of a replaceable jet. It's possible the ID has been expanded through cleaning or someone prior may have enlarged it.
I believe the fix for this is to drill the cap from the back side and replace it with a jet. (This is how the premiers are set up.)
I've never done this BTW. So, you'll need to research it. And it may not be the issue.
Myself, I would probably drop main jet sizes without touching anything, and see what happens.

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I thought of the pilot bushings, especially when considering the age of the carbs, 44 years, but wondered if anyone else might have the same idea. The jets a are 220,s...what are your thoughts on downsizing and I thank you for the input.

think you have to decide for sure what type of plug fouling is happening , either fuel (carb or elec.) or burnt oil , once you positive , lots of info to get whatever the issue is corrected ....until then just travelling in circles ..... if Ride style was cause a good run on open throttle should /would usually clean your plugs back to almost new ....

Very much doubt you have a main jet problem
Did you replace the needles and needle jets when you did your overhaul?
As others have said you need to know for sure if it's a carb/ignition problem
Or if the engine is burning oil?
If it's carb/ignition related I think you are going to have to substitute components until you find the culprit
But go back to the original grade of spark plug
Also make sure your bike is charging properly and the battery is ok etc

Most likely needles and needle jets. If you lower the needles one notch you should get a cough as you use the throttle when riding the bike and changing up through the gears. When you get that, raise the needles one notch. There has to be a lot of oil to foul #7 plugs. If your air-cleaner is blocked, you might not get enough adjustment on the needles to make the motor cough when you lower them. Main jets should not be your problem unless you are really fanging the bike over long distances. It is normal to have the main jets slightly too rich.

A 74 850 would normally have a 260 main jet, unless it's a MK2A.

When the motor is cold, it is normal to need choke to start it.

maylar said:

A 74 850 would normally have a 260 main jet, unless it's a MK2A.

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With modern fuels, the main jets might need to be the next size up. But if they are wrong, they would probably be too lean - that would not causse fouling, and main jets don't usually wear.

Are you sure there ;s no smoke while actually riding, accelerating, downshifting etc? Have an observer follow you along to monitor. I discouvered a leaking intake valve seal quite by chance. Just happened to be stopped up in the alleyway outside my home and blipped the throttle while off bike. In the dimly lit alley, I noticed a blue puff from left side. It only appeared when blipping, not when holding throttle on. Replaced original, crumbling hard black seals with fresh KibbleWhite Viton's and problem cured. Did the dasterdly deed without pulling head, as per a thread found in this forum.

Get your oil to the lab.

https://www.blackstone-labs.com/tes...abs.net/Bstone/(S(y3r00daciaj2dgnjv54c1j55))/

A long shot but easy to try

450 Ducati single
If I use Shell 98 highest octane fuel it fouls the plug.
If I use BP or any other brand 98 no problem

Graeme

If I run the 116 oct. race gas in my ‘72 Commando the plugs look like new after a decent ride .... if I run the 91oct. pump gas while bike runs fine the plugs are a little dis-coloured and bit of snap is missing from throttle ....