1939 Linkert M51 Throttle lever

[dukekleman]

I'm looking for advice, options and/or opinions on the throttle lever travel on a 1939 HD UL with a M51 Carburetor.

The throttle lever will not travel from from Idle position to 100% open throttle. It will travel from idle to approximately 7/8 throttle opening.

IMG_20260804_141422174.jpg
I believe I've checked and measured everything to be correct?

My thoughts are that I need to change the geometry of this particular throttle lever...I'm thinking about bending it to move the two pivot points (center lines) closer to one another.

Has anyone encountered this before?
Does anyone have any advice to share?
Has anyone bent an arm before?
If so, can you please share your method used to do so successfully?

Thank you,

Duke Kleman

[T. Cotten]

[QUOTE=dukekleman;n350985]I'm looking for advice, options and/or opinions on the throttle lever travel on a 1939 HD UL with a M51 Carburetor.

The throttle lever will not travel from from Idle position to 100% open throttle. It will travel from idle to approximately 7/8 throttle opening.

I believe I've checked and measured everything to be correct?

My thoughts are that I need to change the geometry of this particular throttle lever...I'm thinking about bending it to move the two pivot points (center lines) closer to one another.

Has anyone encountered this before?

Not really, Duke,

Does anyone have any advice to share?
Has anyone bent an arm before?

Only to un-bend them, Duke!
(Chill them; do not heat.)

If so, can you please share your method used to do so successfully?

Please open the installed throttledisc fully, to where it is parallel to the bore;
Then place the lever upon the shaft (with friction spring) such that its stop is against the tower upon the body, before tightening the pinch screw.

The disc should be able to close fully, only to then be adjusted slightly open by the idle adjustment screw.

Let us know if it doesn't...

....Cotten

[dukekleman]

Thank you Cotten.

I use a machinist square to set the throttle disc at WOT as you explained and then tighten the screen when at 90 degrees.

I'm measuring cable travel of 0.875"
The required lever travel currently is 1.200"
My thought was to achieve another 0.325" by changing its ratio...
I'm not 100% sure it's possible?
Perhaps I'm overlooking something yet?

Thank you,
Duke Kleman

[T. Cotten]

Thank you Cotten.

I use a machinist square to set the throttle disc at WOT as you explained and then tighten the screen when at 90 degrees.

I'm measuring cable travel of 0.875"
The required lever travel currently is 1.200"
My thought was to achieve another 0.325" by changing its ratio...
I'm not 100% sure it's possible?
Perhaps I'm overlooking something yet?

Thank you,
Duke Kleman

I wouldn't punish the lever, Duke,

If there is a 'cable' or spiral control issue!

The carb has its own stops, but the 'cable' throw must exceed it both ways to work.

....Cotten

[dukekleman]

I agree Cotten!
I haven't gone after the throttle lever yet because I really don't want too...
This motorcycle was fully restored 500 miles ago by someone else.
Everything I've checked in the throttle system seems to check out, but like I keep thinking, what am I missing???

Thank you,
Duke Kleman

[Rubone]

I start at WOT against the stop and work backwards. I've never seen an OEM spiral that did not supply enough travel. I would look upstream beyond the lever.

[otis71]

I’ve hit this problem several times on Tillotson diaphragm carburetors which seem to require 1” of travel and only getting the .875 out of the spiral, sacrificing high speed for an idle. There’s enough space in the handlebar for the cable to travel further but restricted by the spiral. Maybe the spiral could be machined to spin farther. Please let us know what you find. Thanks.

[T. Cotten]

One issue I encountered, Duke,

Was when the handlebar set screw (often hidden under a switch cover) was not properly engaged within the control sleeve collar;

It loosened to where the collar and coil slipped with the movement of the spiral, and defeated the proper action of the wire.

.....Cotten

[dukekleman]

Ok, the problem has been located and resolved.
Let me start by saying thank to everyone that commented on this post. It is greatly appreciated!

The problem was upstream as Robbie suggested in the throttle control. I went through this originally made changes but missed this small but very important item.

So the photo below shows the zone , mark by the black sharpie lines, that seemed to be problematic.
IMG_20260805_143411383_HDR.jpg
Once I located this spot, I looked closer with my odoscope and saw this on one side of the channel only. See photo below.
IMG_20260805_143821387.jpg
In the upper channel you'll notice a fine edge line to the right of the sharpie line. The edge was enough to stop the sliding guide .
IMG_20260805_144849034.jpg
In the photo above note the upper right corner I put a radius on. The other 3 corners are sharp yet. I did radius all 4 corners of course prior to final installation. These sharp edges caused that sharp edge in the channel.
IMG_20260805_145246607.jpg
I've worked on bikes in the past that eliminated the guide and installed two rollers instead. I believe the guide has significant importance so I choose to radius all 4 corners instead.

I'm happy to report after polishing the handlebar channel smoothly and assembling everything with proper lubricant, it works excellent with plenty of travel.

I hope this helps in the future for others,

Duke Kleman