Linkert M74-B running too rich

[48HD]

I have a '48 Pan with '55 (or later) heads fitted with shovelhead valves, hardened seats and a Linkert M74B. With the original main jet, my spark plugs load up quick.
I have reduced the main jet from a #19 (.081") to a #17 (.076") and finally down to a .059". This bike runs great even with this small main jet however, after a short 20 mile ride, highway & city, the plugs are coated with black soot. The front plug is worse than the rear. I am assuming this is gas fouling.
Compression is 95-100 psi and engine has 3000k since rebuild. Plugs are new, battery is good, points are adjusted and timing has been set. I'm running 89-93 octane unleaded.
I'm aware our present unleaded gasoline molecules are smaller than the leaded gas molecules of yesteryear. Has anyone else figured out the right combination for unleaded gas vs. correct main jet size?

I'm open to suggestions!

[haggis]

48HD,
What plugs are you using? also do you have the correct Venturi in the Linkert and also possible float height problems....whew! Haggis
Ps. Welcome aboard

[Jonderson]

If the fouling is different on the front plug than it is on the rear, first thing you need to do is pressure test the intake manifold and see if it is leaking.

[sleeper]

48, Jonderson is Correct about your plugs color. they should be the same, or very,very close to same..
You sure you're not looking at oil & a little carbon ??
Haggis mentioned float level. Good Point also. How is your float ?? Is it gas logged. You would need to remove it & put it in a tin of gas & see how it floats, If it's riding low, that could be the whole problem.
Where's Cotten ??
You say it runs Great.
What heat range plugs are you using??

[Cotten]

So far, you guys were doin' fine without me!
Yes, all manifolds should be pressuretested; they cannot be left to chance.
But that normally results in a lean condition. (Although it is common to over-richen the idle circuit in order to get it to idle, and that over-laps the highspeed circuit, making it soot the plugs.)
And my third eye sees a boatanchor brass float.
I would return to the original jet if either of these conditions are corrected.
Other immediate things to inspect for are daylight around the outside of the venturi, and excessive wear from the throttleplate into the bore. Both of these conditions are repairable.
Another gas-sucking problem that 48HD faces is the chubblehead intake valves. Pan ports are just too twisty and small to accommodate them efficiently.
As the port widens at the huge valve, it is a pressuredrop, and fuel can actually "puddle out" from lowered velocity. The droplets come out of suspension and collect into larger ones, reducing efficiency of burn. The cooling effect is diminished as well, so in some ways it becomes the same as if the fuel was a lower octane.
The combustion chamber can only be "packed" well at the highest of RPM's, and low and midrange are robbed of torque.

[Rainjester]

I had this carbon coated black soot plugs problem for quite some time now.... until yesterday!
I was able to resolve my intake manifold leak (new intake nipples and brass seals for plumbing style intake) and siliconed my exhaust spigots for the paughco pipes. When I ran my bike for about 20 minutes at high idle and pulled the plugs, no soot! it was perfectly baked brown. I then rode it for about an hour then pulled them out, no soot!
If you want to do a quick test while not having the correct pressure test tool handy, get a can of carb/choke spray to squirt on the outside of the intake where the o-rings connect up while the bike is running to see if it stumbles or dies. If it does, you definitely have a leak and that's likely the source of the black soot on your plugs.

jester-

[sleeper]

Jester, Glad you got your intake nipple leak fixed!!
Also you said "When I ran my bike for 20 minutes at high idle" This sounds like you had it parked doing this.
That's enough to over-heat the motor. ANY motor.
With no- breeze blowing over it..
Just thought I'd mention it, as you do have a NICE scoot!!

& the quick test, isn't near as accurate & only puts off making the right test plate. Anybody riding ought to have a pressure test plate for their particular manifold. The're so simple to make. Mine has multi-bolt patterns in it to fit many different manifolds.

[Cotten]

Jester!

I stopped advising the carb cleaner trick when I was embarrassed over and over by how many leaks it failed to display. It only shows BIG leaks. Trusting to that test can leave you chasing your tail, pulling your hair out, or even burning a hole in a piston.
Pressuretesting shows ALL leaks, and WHERE, without having to rev the machine dangerously.

[48HD]

Thanks for all the good suggestions.
I've tried Champion J12YC in 2 heat ranges and A-C 43S & 45S. All foul the same. The float is OK and height is set at 1/4 inch/per the manual.
Yesterday I bought new O rings for the intake manifold and when I install 'em I'll check that float (again). I spoke with one of my local shops & he suggested I may have excessive back pressure. I removed & inspected my muffler and I have no baffles but the tall, narrow fin exhaust opening is 1.25 sq./in. and I calculate the area of the exhaust pipe opening entering the muffler is 2 sq./in. I'll keep everyone posted on my progress.

[Rainjester]

Billy: I live north of Seattle. It was a bit cool out with a breeze when I was running it idle. I hadn't been running it too high and was watching it to see how hot it gets. The bike actually seems to have run cooler since I fixed the leaks.
Cotten: I wouldn't rely on the carb choke spray method as a final check, but only as a quick check to see if there were any gaping holes. Besides, if a guy doesn't have a pressure tester made (I made one last week!), it's a quick way to see if there might be leaks.
Thanks for all your help guys. This is the best panhead site on the net.

jester-

[Cotten]

Uh,... I just gotta ask:
What kind of float are you trying to run?
If you intend to attempt a brass float, you had better weigh it. A 9 g Indian float requires a 3/8" setting, and a 12g Kokesh sometimes needs .400" if not a full 7/16", if it doesn't bottom out on the bowl stop cast inside.

[48HD]

'Sorry to take so long in getting back but here's what I found, so far:
I renewed the intake manifold O rings, both were in pretty bad shape but none of the cuts appeared to go to the outside. Afterwards, I noticed the engine ran slightly smoother throughout the power range.
When the manifold was off I noticed quite a bit of oil in the intake valve port in the front head. I may have opened the valve oiler too much the last time I had the head off. This explains the soot problem with the front spark plug. 'Easy fix once the head's off.
I weighed and float tested the the brass float. It weighed in at 11.2 grams and floated fine for 20 minutes. Noted no sign of gasoline intrusion. I did notice the float fingers were somewhat tight around the needle valve causing some sticking. 'Readjusted the fingers to allow free movement of the needle valve. Float set at 1/4".
I went back to a .070 main jet and took a 50 mile road test, the last 30 miles at 70 mph. Engine ran great but I noticed an occasional backfire coming off the throttle. Rear plug still somewhat dark from gas, front still highly sooted from gas & oil.
I corrected one problem (O rings) and found another (over oiling of the front intake valve) but still haven't solved my initial problem of running too rich. I strongly suspect the main jet should be reduced in size to run today's unleaded gas. I'm aware of the damage that may be caused by running too lean.
Is anyone aware of a service bulletin (possibly aftermarket) recommending a change in main jet size for today's unleaded gas?

Thanks for everyone's help!

[Cotten]

Your original jet should work fine.
If it doesn't:
Note that your brass float weighs nearly four times as much as the carb was designed to use.
And an untested manifold of any design can hide very small leaks that will make tuning a tail-chasing experience.