Tilotson Carb Adjustment

[MrMarty51]

The motorcyc!e is a 1957 Model B Hummer, 125cc.
The engine idles real nice, revs up as it should so all is fine there.
The problem is on the drive, the motorcycle gains speed very nicely up to a point.
Maybe about 30 MPH and then the exhaust pipe emits a sort of a rapppp pappp pappp noise, the engine will not gain any more speed.
I easily let out of the throttle and then the engine begins to run at an easier pace and will sort of begin to gain some RPMs.
This tells Me that the fuel condition is on the lean side and needs to be enrichened.
I pull over and give the main jet adjusting screw an 1/8th of a turn out , counter clockwise, and then those same conditions are met at a much lower engine speed, Me thinking that by screwing outwards on the main adjustment needle would enrichen the fuel mixture.
Stopping the bike, screw the main back in to where it was and ride off being satisfied that maybe this is the best it will be.
One thing I did fail to mention, the main adjusting screw is 1/4 of a turn from being fully seated.
I am a little fearful of going in too far with the main adjusting needle and causing a much leaner burn and possibly scorching a holenin the piston.
Am I wrong in My thinking that running the needle screw, clockwise, and closing down the jet will lean out the fuel mixture ?
Is there a work around on getting the engine to gain more revs without seeming to run out of gas ?
I probably can not get a video of the motorcycle in motion but maybe I can somehow get a sound recording of what it sounds like. Maybe the wifey can get a recording of it when I ride past Her. Be later today though.

[pumpguy]

Have you pulled the spark plug to see what the insulator looks like?

Sooty and wet......... too rich

Light to medium brown........ pretty good.

Grey or white .................. too lean.

[MrMarty51]

I did pull the plug yesterday morning, while the engine was cold. It did have the sooty brown appearance.
I will however take it for a drive and run it into the RPM range where it is cutting up, immediately pull over and pop out the plug and see then what it looks like.
I’ll report back.

[MrMarty51]

About a third of a mile, slight up hill pull. Throttle wide open, third gear. Maybe 25 MPH.
I am sure the plug would have turned frosty white had I continued on up the hill, that just crossed the Yellowstone river bridge. From there on the hill gets much steeper and I am sure I would have had to shift down a gear to get to the top, about a mile more distant.
That run across the bridge was wide open and the bike was not gaining speed at all, but it was not loosing any too.

[MrMarty51]

Dismantled the carb and give it a thorough cleaning.
It appeared to be very clean with nothing interfering in the passages or jets.
Reassembled the carb, installed the main jet needle and screwed it all the way in, backed it out 1/8th of a turn and took it for a drive.
Engine runs better and revs easier but to My thinking screwing the main jet in farther would be leaning the fuel mixture.
Is this correct ?

[Mutt]

Yes, lean. But the lower screw (high speed) should start at 1 full turn out

[MrMarty51]

Yes, lean. But the lower screw (high speed) should start at 1 full turn out

Yes. The lower high speed screw is turned in all but about 1/8th of a turn, maybe even less.
The picture of the spark plug was not so good. The electrode appears darker than it really was.
It was more of the desired chocolaty brown, after that short run upwards, across the Yellowstone Bridge, pulled immediately over and pulled the plug. That chocolate brown was beginning to burn off and the plug was going to the more lean condition.
Looking through some of the components I have on hand, I come across this perforated disk screen device.
Is this something that is required to bring the air/fuel mix on the high speed jet to a more rich condition ?
I tried to downsize the pic and even cut off 1/2 of the disk in the picture but I could not seem to get the file size down small enough on my iphone.
I’ll try to send it to my email but that internet provider is wonky and most messages wont make it to the email from my phone.

[MrMarty51]

A pic of the disk.
Wondering if this needs to go under the retaining ring that holds the screen and the Brillo pad in place.
I believe that something like this would enrichen the fuel mixture.

disk2.jpg (447.74 KiB) Viewed 34736 times

[Mutt]

No, that disc doesnt go in the air cleaner. We need to find out why your high speed needle is only 1/8 turn out. It should be 1 full turn out.

[MrMarty51]

When I had the carb off and apart yesterday, I looked through the main jet passage bore and it was wide open. About the same, or the same, size as the bore coming in from the float bowl side.
Carb was clean throughout.