Linkert m74 idle control/throttle plate light leak

[CAP'N BOB]

Stoppers arrived today. Set one up and tested both rear (still on the bike) and front intake nipples. No air bubbles anywhere. You were right, Tom, one atmosphere (14+PSI) wants to push the stopper out! :-)

I will test the manifold once I have the front head back on.

As a side note about over-oiling, I reseated the check ball and put in a longer spring (what the restoration manual said belongs), put about 2.5 quarts of oil in, and it does not leak from the open front cylinder as before. When I play with the kick starter to find TDC and 32 degrees, because I wanted to take photos of the Timer, Piston, and Timing Mark in the window, for my own reference, oil does come out of the feed hole in the open front cylinder, so I basically know that is all working. Hope that was the only problem, and I plan to put it all back together, and find out.

This is the stopper and the plate, I made, based on your design, Tom. Thank you, again.
20211102_135831.jpg

[CAP'N BOB]

Ok. Will get to work on it. Thank you. Will let you know how this goes, Tom. I appreciate your help.

[T. Cotten]

I'd try the bottom set in your first pic, Bob...

(The top set is for later "band seals", the two sets in the second pic with quarter-inch bolts are great for exhausts only. I have no idea what the bottom set are for..)

Please note that the manifold flange and carb flange interface, where they are gasketed to a phenolic spacer, cannot be bubble-tested, so both flanges should be arbitrarily ground to flat.

On to the carb, complete disassembly is mandatory.
Beyond clearances of the throttleshaft to bushings, please pay special attention to the idle bleed holes under the large plug on the 'idle' side of the carb. It is not uncommon for particles, usually glass beads, to become lodged in the slot, and hard to see without magnification.

And of course, make sure you have a float that floats.

Reassembly is by the book, but be certain that the lowspeed lift spring collar has the "top hat" upward, and slides easily upon the body

....Cotten

[CAP'N BOB]

I will change and use thin wall hose, Tom. Thank you.

I will remove and inspect the venturi, no matter how much that scares me, never having done that. As I said, this is my first Linkert.

It is clear to me that the throttle plate/disc is not right, and somehow, that must be resolved, even if there are any issues with the nipples and intake manifold when I bubble test them.

These are the clamps I remembered having in a tin where I keep all kinds of old clamps. The split ones are probably what you told me, and maybe the other set in the first photo is chrome.

Second photo is three more sets. One is SS, I think, says 5 Star MFG, Crest Hill, Il, with a 27 and 65M, on it along with 404, leading me to believe that means 404 Stainless. One other set appears to be stainless and says Drag Specialties on the inside. The one with the small screws and covers over the screws says UF4, and 768-36 on the outside, with UF4 C-650-36 on the inside. Not even sure where I got these, to tell you the truth.

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[T. Cotten]

The problem with a fat, heavy hose to the testplate, Bob,..

Is that it can move your manifold while your are messing with it, after you have lined it up to the carb with its support.

I suggest light, flexible vinyl tubing; At one atmosphere, you won't even need clamps.

....Cotten
PS: it is always prudent to completely disassemble, and remove the venturi for inspection.
I lost count of how many have come to me full of blasting media.
When clean and re-installed, there should be no daylight around its outside diameter.

Your extreme daylight around the throttledisc reminds me of a Tedd rebuild, where it was bored oversized, but a standard disc installed.

[T. Cotten]

I use a pool cue, Bob!

Drop the venturi over it 'til it stops, mark the spigot hole, and then put a caliper on it to be exact.

You would be surprised how much a nominal ID can vary,.. but its only the OD that matters.

....Cotten

[CAP'N BOB]

1-5/16", by the thin cardboard test. 1-1/8" was way off, so I cut another at 1-5/16", which was almost exact. Amazing what 3/16" looks like in that small area. Thank you, again. At least, now I know and the knowledge base of these Linkert carburetors keeps growing. That little trick will go into my Linkert book and other trick book! :-)

[CAP'N BOB]

I did not know that, Tom, and would have ordered from you. I made the one for the intake manifold (probably overkill, I used a 3/8' hose) and have ordered a couple of stoppers.

I have no idea if what I made will work, but the pictures of it are attached.

Do you have a site I can go to? How much do you charge for the professionally done ones, instead of my shop made ones (once the stoppers are here) that I have no idea if they work?

Work arounds. That is a great idea, I will do that, ktabel. Thank you.

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[kitabel]

Cut a piece of shirt cardboard (cereal box etc.) with a razor to 1-1/8" wide and poke it in. If it touches...

[T. Cotten]

Great. Bob!

(By the way, I sell all that stuff, although my last regulator went out to Canada, and must re-stock.)

....Cotten

[CAP'N BOB]

Measured the spigot opening and it is about 1-17/32" or about 39MM, or about 1.544", so I ordered 1 #8 and 1 #9 black rubber stopper(s) with a 5/16" hole(s) in the middle. Should be here Monday, so by then, I will have the rear head off and all prepped to check for air leaks around the intake inserts.

May even make a plate that will fit over the intake manifold. Does not have to be pretty, just the right size. Then, I can also check that once I have it all together again.

I will keep you informed and I very much appreciate you taking the time to help me with this.

[T. Cotten]

It certainly can overheat things, Bob!

Extra air requires extra fuel which produces extra heat.

Usually one cylinder is affected more than the other, and the carb is tuned for that cylinder, at the expense of the other.

There may be other issues of course (like a boat anchor float), but now is the time to eliminate the air leak variable.

....Cotten
PS: When its time to reassemble, I suggest Viton O-rings to go with the lighter clamps.

[CAP'N BOB]

I think I get it, we are not testing the manifold, we are testing at head at the the aftermarket replacements for the plumber intake inserts. Thank you for the photo. I can do that, I believe.

Thank you, I have always been afraid that an air leak could lean the bike out so much that it could superheat the heads/pistons. This one was certainly running rich! :-)

[T. Cotten]

Oh, and I thought I'd mention, I have the front head off and plan to remove the rear one tomorrow (chasing the leaking pans or potential for why, anyway), so right now, I cannot check the intake for bubbles. Gives me time to get the test plate or try to make one that is flat. I think I understand what you told me to check and why.

Seems to me, when there is an air leak, bikes run lean, not rich, and this one is running VERY rich, when it is running, black smoke and black soot on the spark plugs. Do I have it wrong about lean vs rich with an air leak?

Seriously Bob,..

When the heads are on the bench is the very best time to test the replacement nipples.

Just use a large stopper with a nipple poked through it, although it takes a strong hand:
NPPLTSTR.jpg

Aside from that...

Air leaks make bikes run rich because otherwise they wouldn't run at all.

....Cotten

[CAP'N BOB]

Oh, and I thought I'd mention, I have the front head off and plan to remove the rear one tomorrow (chasing the leaking pans or potential for why, anyway), so right now, I cannot check the intake for bubbles. Gives me time to get the test plate or try to make one that is flat. I think I understand what you told me to check and why.

Seems to me, when there is an air leak, bikes run lean, not rich, and this one is running VERY rich, when it is running, black smoke and black soot on the spark plugs. Do I have it wrong about lean vs rich with an air leak?