Linkert m74 idle control/throttle plate light leak

[CAP'N BOB]

Good afternoon:

I am working on a 1948 FL for a customer. He brought the bike to me in multiple baskets, and I have been working to put it together for him. I have had it running and cannot control the idle on this M74 Linkert. I read a post from 2001 that discussed checking the throttle plate for light leaks if there was an idle control issue. The first attached photo is the throttle plate on this Linkert, and it certainly does leak light. I'm guessing that is the air leak but would like some help from someone who knows Linkert carburetors. Before this bike I have NO experience with Linkert carburetors.
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If that is the probable source of the problem with this Linkert M74, is the fix as simple as a new plate or do I need to have someone who knows what they are doing rebuild this carburetor? My guess is that it would be prudent to send this to someone who rebuilds Linkert carburetors and have it done right. This bike needs to run, properly.

A second issue is oil leak. When I had the bike running, oil leaked from the rocker boxes and even from the rear exhaust lifter lower tube. I read in the Restoration Manual that the 1948s were very bad for oil leaks, to the extent that Harley changed the oil pumps to fix the issue. So, I am going to put the oil leak issue out as a separate string. Just mention it here because until I had the bike running, I did not know there WAS an oil leak! :-)

Bob

[T. Cotten]

That's certainly enough daylight to be an issue, Cap'n!

Since it is at both top and bottom, I would suspect an 'eyebrow' of wear into the body like this:
WEARFIX.jpg

The remedy on the right is complicated, but frankly, there are many other things that can cause such grief, such as the manifold (which was unique for '48-'49).

Until you are certain that the manifold assembly is sound (tested as discussed at http://virtualindian.org/11techleaktest.html), no carburetor will perform properly.

.....Cotten
PS: Note also that your flange really needs a grind, and undoubtedly the manifold does as well.

[CAP'N BOB]

The intake is a complicating factor, Cotten, the heads have been modified, I believe. I have a 1947 FL, and this 1948 should have the same type of intake (I think they are called Plumber?). It does not, as you can see. I think someone, somewhere along the line "updated" these heads, and expect that is something the guy is going to have to live with, and I have to live with getting this bike on the road for him.

I do not have one of those leak testers. This being the first Linkert for me, I have never had the need to purchase those type of tools. I am not opposed to purchasing one, but wonder it if would be better to have a Linkert expert set this up, including the intake manifold, so that it is all proper.

Is there anyone out there you would recommend to me to get this carburetor and manifold rebuilt and set up? Do you do that? I do not really want to start grinding on it, not knowing much about the Linkert carbs. On my Knuck and Pan, I run S&S carbs, and understand them, but those are my bikes, neither of which is any way a restoration. He wants this to be more or less original, (a "rustoration").

Thank you for your help.

Bob

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[T. Cotten]

A '47 would be a Knuck and a different animal, Bob,..

So let us assume you have ~'48 Pan heads where the "plumbers-style" inlet nipples have been replaced with probably COLONY O-ring conversion nipples. Thus you also have a '55 and later O-ring-style aftermarket manifold. It would be prudent to grind its flange, and replace the exhaust clamps with similar that have smaller #10 screw fasteners.

But please bubble-test the inlet replacements separately first, as they are notorious.

If that sounds scary, it should.

....Cotten
PS: Please also test thoroughly the welds, and pan cover screws over each port. And Good Luck!

[kitabel]

Hard to tell from the picture, but that venturi looks small to me. There are only 2 that fit that body: 1-1/8" and 1-5/16". An M-74B should have the larger; the smaller is M-36A, M-61 for EL and FLE, not recommended for the M-74B main nozzle.
It's not related to your problem, but I'd find out what you have before you're done.

[CAP'N BOB]

Quite honestly, Tom, it all sounds scary to me, not being familiar with Linkert carburetors. Where do I get those intake check plates for the Linkert?

This one is an M74B, 1-1/2 Mod M, 2-418. Short of taking the venturi out and measuring it, I have no idea how to tell you, kitabel, if this is a 1-1/8" or a 1-5/16" venturi. Any recommendations for me on how to tell, would be appreciated. You said the venturi size does not relate to my problem, but it seems to me that it could since the venturi controls the air flow/pressure, right? If I cannot get it to idle properly, the throttle plate leaking air and the venturi being wrong for this carb (if it is) seems to me could/would all be connected.

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[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?

[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]

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]

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]

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]

Great. Bob!

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

....Cotten

[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...

[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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[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! :-)