carb issue?

[poorbiker]

Well I have wasted to much time trying to get it right, maybe I`m too close to it.
Tuning problem with linkert M74. Properly rebuilt. Some engine info.
Stock FL Knuck. (new build)
stock comp. (good comp.)
stock cam
unbaffled exhaust
tappets adjusted properly and then rechecked
Morris H5 magneto
Starts on 2-3 kicks hot or cold
No intake air leaks I`m aware of
idles great
throttles up great at idle (no load)
Champion D16 plugs

I think that should be enough info, low speed screw set correct, set idle low, turn in until rpm becomes unsteady and back out until idle becomes unsteady,
and about back to middle of the two, reset idle.
Problem is the high speed screw is at 3/4 of a turn out. Engine still wants to lug a bit under load, (sputers) got better the futher I turned in the high speed screw.
I hesitate to keep turning it in thinking there is some other reason for it to be in so far. I think about a turn and half out is about right for a starting point
but 3/4 turn out?
Admitedly this is only the second Linkert Ive ever messed with, always shelved them for an s&s carb. but I decided to make a change cause several of my buddies run a linkert with no issues.
So what havent I tried or what can I try. thnx

[Rubone]

M74 high speed needle is an air "trimmer" only. It has a fixed high speed jet. The trimmer can be run completely closed if that is what it takes. Or the jet has been changed or drilled out (common practice in those days). It should be a number 17 jet, but an 18 or 19 can be there or something altogether different.

[poorbiker]

Thnx.
it is a #17, I get needle is used to fine tune jet I put it badly. But that makes sense, put it where it wants to be. Just in my limited experience with linkerts
I have never seen a high speed needle ran that far in. Thats why its best to ask, not to proud. thnx Rubone, will go out tommorrow and continue adjustment. thnx again..

[T. Cotten]

PoorBiker!

Sorry for the late reply, but I just returned from the international Davenport Meet.

If your carb is an M74 and not an M74B, please inspect your "power" or highspeed needle to see if it has the correct stepped tip, like the example in the attachment.
The later B models, of which there are four, are notorious for an insensitive HS needle.

Three-quarters of a turn is what I suggest for a starting point for all models, and an arbitrary setting for when the needle seems to have no effect at all.
But Linkerts are SO forgiving that I would really suspect another issue, than just a needle tip.

Vacuum leaks are insidiously evil, and a much more likely culprit.
The simplest, and most absolute test is merely to bubble-test, as discussed with pictures at http://virtualindian.org/11techleaktest.html
It works with O-ring models as well, and prevents over-tightening of the clamps.

Beware that some Pans have suffered perforation of the pan cover screw over an intake port, and only bubbles will find such a leak, as well as casting porosities and other hidden concerns.

Good luck!

....Cotten
PS: Doh! Its a Knuckle. Damn them M74s

[poorbiker]

Thnx Cotten,
I actually dont know if its the correct needle I will check this morning. I am fairly certain there are no leaks in the intake system, I will however recheck. I am using the newer Peak intake seals. Intake nipples are installed correct and seat perfect with intake.
Are you the same Cotten from Vintage Twin builders forum? read a good artical on linkerts there by you.
thnx.

[poorbiker]

Oh and when I say properly rebuilt I mean I rebuilt it, sooooo I think its good, no grooves, scaring, looks clean and un f-d with though I know there are supposed to be some lettering or numbers on the intake side of carb that is no longer there, I suppose it was resurfaced?

[T. Cotten]

Poorbiker!

Please understand that because I do these things for a living, "properly" is a loaded word.
I constantly find fault with my own work, for like a doctor, I am "practicing". (I shall be be a student of carbs and their History, as long as I am able, but true experts cannot exist. They would be Methusalas.)

The first critical interfaces to inspect, in order of importance, are the idle bleeds for any obstructions (like blasting abrasives or goobers, hairballs, etc.), the float circuit (the float should actually float if it is to be set at book spec, daylight around the outside of the venturi and/or the nozzle spigot, and borewear from the throttledisc into the body bore.

Of course, these concerns can be additive as well.

Most folks can do an adequate clean-and-assemble on their kitchen table.
And that's why I suggest eliminating any possibility of a vacuum leak first.
The carb's manifold flange must be re-ground to flat invariably, and the HD-4 is often gone. The manifold flange needs similar attention. as their is no effective way to test this seam, except perhaps an unlit propane torch while running.

PEEK is miraculous, of course, but the seals must be a very tight fit upon pristine manifold spigots, as the nuts press against the nipple far tighter than down upon the spigots.
Impressions will be found in the used PEEK from the nipple, but are easily sanded away for the next re-use. Blemishes in the spigots however, will reduce the re-useability of the expensive seals dramatically.

It all comes down to what the bubbles tell you; PEEK seals will not cure a leak between nipples and head.
If there are no bubbles, even brass will do.
But without bubble-testing, any stone unturned hides something ready to bite you.

Let us know how it goes.

....Cotten
PS: I am not aware of "Vintage Twin builders forum".
Please link me in a PM!
(I have been barred from, or run off of more old motorcycle forums than I can count.
Seriously: There is no quarantee of Freedom of Speech on the internet.)

[poorbiker]

http://vintagetwin.com/

Cotten

I will take your advice and check those items. I guess none of us know anything about anyone else on this or any forums. I`m a mechanic/tech all beit heavy equip/ fire engines.
Have modified many carbs over the years, though they were all either c/v or s&s some Mikunis
The intake is oem nos, the intake nipples are also new and I tested for leaks. The carb however might or could have an issue with with the intake side, where the corners were slightly bent inward. I did the best i could with a mill file, filing from corner to corner, with emery cloth. Tested for flatness with a mill plate I know to be perfectly flat.
Could see no light/ gaps ect. So if I have an issue at all I believe it could be my carb build. Will double check everything though.
The other forum has an artical about carb issues with alot of pics and some prices for repairs by Im assuming you.
Thnx again for the info.

[poorbiker]

Cotten,

Well I pulled the high speed needle and it is NOT the stepped tip.
The carb IS an M74 not the B.
It was in carb when I got it. So does this mean I have the wrong needle. And even if it is correct I would have to replace it. I didnt catch the fact that the tip is buggered.
There is a little pit just above the point and it looks like someone ground it to boot. One of those things I didnt catch on rebuild. Hate to say I didnt see it, honestly I dont even remember looking. Guess I will buy a new needle and a new jet.

[T. Cotten]

Poorbiker!

If you had any response to adjustments, it "works" enough for normal duty, until you find an original.

For the interim, I suggest just grinding or stoneing your needle "clean" (Got a lathe or valve grinder? Or even just a drill press and red India stone?), as the mis-match probably has minimal effect, if all other things are in order. If there are "new" needles for your model, please let me know that link also!
And don't feel bad about over-looking it: An ebay carb mogul sold one of my customers an idle needle for "new" that was only a partially ground used one.

Did you bubble-test your nipples and assembly at a constant and regulated 15psi? (Folks have fallen into folly with tire pumps, airmattress pumps, and the like.)

Sorry to be so demanding, but happy to help when I can.

....Cotten
PS: I must get on a more modern browser to view the 'Vintage Twin' link, as I have a policy of religiously avoiding posting of prices to a public forum.
(Some forums, such as http://www.sscycle.com/tech/ even forbid it.)
Please beware that anything about Liberty upon that forum was posted without my knowledge, permission, or reply, and subject to their edits and agenda.

[poorbiker]

Cotten,
This is what was posted.
I do have a lathe.
Here is the site where I have ben buying my Linkert parts. http://www.linkertcarbs.com/ called "into the wilderness"
No I have not performed the bubble test yet. I do have a compressor and have done it before on other models and carb intakes tracts. It does work well.



Thorough overhauls of Linkerts are averaging ~$280 for labor, including the DURABLE floats that are machined in-house. Parts costs vary widely of course, but assemblies that arrive complete often fall into the $40 to $60 range.
And some extreme repair operations can up the ante as well, such as when the bore must be honed to for an over sized disc as shown in the attachment; This may be required when the groove caused by the throttle disc extends as far as the idle bleed holes, as shown in the attachment. Only fitting a fresh disc will determine the true extent of the damage, and if it is truly necessary. This would add $60 to the tab to assure that this aspect of the carb is fully "blueprinted".
Naturally, braze repairs, thread repairs, extreme blemish removal, and other concerns pop up regularly, so no two carbs are treated the same. Operations are flat-rated.
I appreciate your interest,
...Cotten ~ liberty@npoint.net
Liberty Motorcycle Specialties, Inc.
118 N.
Washington St.
Lacon, IL 61540 USA

[T. Cotten]

Poorbiker!

Those are indeed my words from my formletter replys for emails,
but I regret any appearance of commercial pandering upon any site.
(It is, however, still current and correct!)

Unfortunately, the quality of parts from the cited supplier that have crossed my benches have not encouraged me to contact them for inventory.
That does not mean that I dismiss them for the future, as we all, including myself, are on a perpetual learning curve.
Or at least I hope we are....!

....Cotten

[poorbiker]

Thanks to Rubone and Cotten, carb problem resolved, running great. thnx.