HD ULH + M51 adjustment

[asavolai]

Hi,
Last summer I had a combination -38 ULH (74" cylinders though)+M35TP carb. It ran just fine. I found (was told rebuilt) M51 carb. I also bought a pair of new FHP barrels, now 80". I changed M51 venturi to bigger one, which should be the one used in M51L.
Now I cannot get the bike run properly. I have checked the intake leaks several times, just a few bubbles from throttle shaft.

The bike starts very easily, and idle is very good. Strange thing is that I cannot open too much idle screw?

Problem starts with the higher revs, maybe over 3000rpm. There's dead rpm range when I cannot use full throttle.
With 4th gear it runs well (I suppose the revs are under the critical limit). Max speed is 130 km/h=80mph.

Mostly the problem exists when accelerating with first and second gear (full throttle).

Linkerts have been a mystery to me, there's so much small channels, different venturies, main nozzles etc. so I hope some one could help me out?

Thanks,
BR
Antti

[T. Cotten]

Antti!

Bubble-testing the manifold is performed with the carb removed (http://virtualindian.org/11techleaktest.html), so it confuses me when you say it bubbles at the shaft.
The bushings never seal well enough for such a test..

An M35TP has a four-bolt flange, whereas an M51 is a three-bolt, so we assume you have two manifolds?

Linkerts will always be a mystery, so I would cover all bases upon the manifold first, and dress both manifold and carburetor flanges accordingly.

Hope to help if I can,

....Cotten

[asavolai]

Hi Cotten,
Yes, I have two manifolds. When I changed the cylinders and installed 3 bolt manifold I did the pressure testing with soap water. No bubbles at all.
After the test runs, I wanted to try again the leaks, just to be sure. And then I was lazy, I didn't take carb off, just took air cleaner off and put my pressure test system to carb.
I have rubber "cylinder" with pressure regulator that I press against the carb or manifold. Anyways, there were positive pressure inside of carb and manifold.

Just to mention, I have PEEK seals.

Thanks for the quick answer, I will be looking forward to hear more!
Antti

[T. Cotten]

Antti!

Since the manifold/carburetor attachment is difficult to bubbletest properly, I suggest grinding the manifold flange and lathe-cutting the carburetor flange arbitrarily.

If we assume that all of the carb hardware is properly in place, then issues of wear would not present themselves suddenly, as undesirable as they may be.
Beyond the manifold, the float is the next most common culprit for a sudden change.

Has it soaked itself? Has it changed dimensions? Swelling is predominately a USA fuel problem, but it may know no boundaries!

Please let us know what you observe,

....Cotten

[duffeycycles]

Are the points arching against the mounting screw at higher RPMs...cap off,run in dark and watch for sparks

[asavolai]

Hi,
I have magneto there, but will check these issues. I have rubber ducky (or whatsoever) modern float. Maybe I should try to increase the fuel level a bit?

BR
Antti

[duffeycycles]

I have seen mags that out of time/position[Indian Scout] do the same...drop out the ignition as it advances...It is not always a carb problem.Make sure you have spark in the dead zone with a timing light.Intake leak problems dissapear at 3,000 RPM because the throttle plate is open.There is little vaccuum in the manifold.It is not an intake leak.Or low speed circuit problem........in my opinion

[T. Cotten]

"Intake leak problems dissapear at 3,000 RPM", Dewey (Doh! Sorry!, I mean Duffey!)?

Tell that to all the folks who have burnt pistons or valves on the highway!
No leak is an acceptible leak, particularly if it favors one cylinder over the other.

Annti!

If you are using commercial one-size-fits-all PEEK seals, you will want to snug them tight again often, and hope they stablilize.

....Cotten

[duffeycycles]

Duffey no Dewey...as far as running above 3,000 RPM a small intake leak will not be a total dead spot..but as I've said before telling Cotton anything is a waste of both of our time

[T. Cotten]

Doh!

Sorry Duffey, I got the name wrong again. Gotta blame it on spellcheck!

Comparing information and experience may be a waste of your time, but not for others with an open mind, especially those who have burnt a cylinder at high speeds.

The only two vacuum leaks that behave the same are those that prevent the motor from running at all.

....Cotten

[duffeycycles]

Did you put the venturi in backwards?

[T. Cotten]

Did you put the venturi in backwards?

Just for the record, Folks,...

I get carbs in occasionally with venturi and/or throttledisc in backwards, and often reported to have run that way for some time.
One M-5 was even installed with soldered and shaved shafts, submitted by an AMCA tech 'authority'!

(If Linkerts weren't so forgiving, I wouldn't specialize in them...)

More common problems are missing LS lift spring collars, shrunken venturies, and objects such as glass beads stuck in the idle bleed slot.
(Got one of those in the shop Wednesday...)

....Cotten

[duffeycycles]

Years ago another shop owner/bike builder brought me an M74B he had rebuilt.Asked me what I thought.When I looked at it,I said everything you could have put in backwards or upside down you did!!

[T. Cotten]

Years ago another shop owner/bike builder brought me an M74B he had rebuilt.Asked me what I thought.When I looked at it,I said everything you could have put in backwards or upside down you did!!

It wasn't Tedd Cycle, was it?

....Cotten

[duffeycycles]

He was more painter than mechanic.Had me fix the carb & install on beautiful '64Pan w/stunning Scott Takes Indian head paint job.I test rode it..very nice.He came to get it and I said it rides nice,he freaked,YOU RODE MY BIKE?!!!I said yes,handles well even with no hands!We became very good friends.