Won’t idle

[Otis]

The carburetor is a M51L. When I bought the bike it had sat for years but did run but never properly. The engine had been rebuilt with very little use. The carburetor looked fresh also. Compression was fairly equal in both cylinders at 75psi. From my experience doing a test on a big twin with a kick starter is significantly lower than actual. Timing was checked static.
I cleaned and assembled a spare M51 today to try for a comparison. My understanding is the only difference is the venturi size. I did notice there’s a small passage that connects the low speed needle passage to the idle circuit passage. When I went through the 51L I don’t recall checking that passage. When I pull the 51L to try the 51 I’ll check it. I really think I have a carburetor problem and not ignition or compression. I just can’t understand why I can’t get the A/F right but persistence always prevails. Thanks for the help and any future suggestions.

[Otis]

I’m going to answer in several short answers because the site has twice deleted my response as I post it. The carburetor is a M51L. The compression is fairly similar in both cylinders at 75psi. From a experience a kick start test on a big twin reads significantly lower than actual.

[Ferrous_Head]

Yes, getting the correct numbers on bigger engines by kicking will always be "problematical" Especially as you get older.
I use electric rollers to do my tests. They are consistent that way plus, it doesn't put me in the hospital. I'm old.

Having said that you need to ask an expert on these Flatties. I'm an Ironhead man and used to bigger numbers. But having said that 76PSI doesn't really sound high enough to me. I can start Ironheads running 90 psi, but often, only on rollers. And no, they don't run well. Pretty sure you need to be between 80 and 90 psi depending on whether these are 5.5 to 1 heads or 6 to 1 heads. Others on here should know what your numbers should be.

It would also be good if you could put a timing light on it. That can sometimes point to a problem. Don't know if these things have fixed advanced but if not possibly the advance mechanism isn't working properly ?
And sometimes the strobe will indicate a "wandering" ignition point. Loose are pitted points, bad condenser. bad earth, faulty coil.

[Otis]

I’ve tested ohv big twins on rollers and get between 110 and 125 when only showing around 75 with the kicker. I did the same with a 69 xlch that showed 90 with the kicker and 145 with the rollers. Unfortunately the tanks have to be pulled to access the front cylinder to test compression on a flatty. Currently I can’t get it to run consistent enough to time with a light. For now I’m locked in on the fuel system.

[Ferrous_Head]

Fair enough.

Good luck with the carb swap. At least you'll know if the problem is with the carb that way.

[Otis]

Good news! The M51 I put together has the bike running properly and better than it has since I’ve owned it. I cleaned and assembled it the same as I have with the original M51L. So my initial question is still out there: why is there no low speed operation? I can’t find a thing wrong with it. I’m wondering if something has been altered that I don’t notice. The idle circuit passages all seem to be clear.

[wz507]

Glad to hear your carb swap solved the lost idle circuit issue. That said it still sounds like you’d like to resolve the issue with the non-idling carb.

You’ve indicated that the idle circuit passages all appear clear, i.e., either air or fluid transfers through all of them. However, the results of having no idle circuit suggest otherwise – either the idle circuit has a huge air leak and you can’t supply enough fuel via the metering needle to make it run, or there simply is no fuel entering the idle circuit. I am not familiar with the specifics of your particular Linkert, but they are all similar enough that the following thoughts might help you with the next stage of diagnosis.

Using just the carb body it’s pretty straight forward to troubleshoot the idle circuit flow. The images below are of a model M53A1, but should be close enough to your model that you can adapt accordingly. I like to trace the idle circuit by applying a gentle stream of air to it, and monitor the outflow by submersing the carb stem or entire body in weak soap water. To perform this test you need to plug (with clay) all the drillings in the main stem except the bottom outlet. The power needle and/or main jet need to be closed/plugged to shut off those outlets. If the carb has a direct connection of the air correction circuit to the main well this likewise needs to be closed off. The hole at the top of the main well where the main nozzle enters the carb body also has to be closed off with clay. Once these openings are all sealed you simply apply light air pressure somewhere on the idle circuit and watch where the air escapes.

The internal idle circuit connections you are especially interested in probing are the drilling from the main well to the idle mixture metering seat (primary supply of fuel to the idle circuit) and from the idle mixture seat to the greater idle mixture galley, all near the bottom of the stem. Hopefully the image below (courtesy of Tom Cotton) will allow you to visualize the foregoing description.

Sectioned stem.jpg (69.95 KiB) Viewed 823 times

The image below shows how you might test for airflow through the idle circuit. For example, when the idle circuit is lightly pressurized, and no idle mixture needle is present, you should see air flowing freely out the bottom of the stem, which would indicate that the fuel supply from the main stem is wide open. If you now introduce the idle mixture adjusting screw and begin tightening it down, the bubble rate at the stem bottom should begin slowing and totally cease when the needle is fully seated. When all the orifices are allowing air leakage, covering any one of them (the idle slot at the butterfly, the idle mixture needle hole, the bottom of the stem, etc) increased leakage at the other orifices should be observed.

carb with upper main well sealed for testing.jpg (59.89 KiB) Viewed 823 times

It might be most diagnostic to perform such tests on both the functioning and the problematic carb bodies to observe if different behaviors exist.

Interested to hear the rest of the story.

[Otis]

I think you are right that the passage leading to the needle is clogged. Do you know the thread pitch of the idle passage plug for the hole you showed in the picture? The screwdriver slot is nonexistent and I have to drill it out.

[wz507]

To the best of my knowledge all the Linkert/Schebler drill plugs were 10-32 thread.

However, you can perform the suggested diagnostic without removing the subject screw you referred to and instead simply remove the idle passage plug up by the mixing chamber where the idle slot is, or alternatively remove the drill plug situated further along the same galley and accessed from the manifold mount surface as shown in the attached photo, and introduce a gentle stream of air at either of these locales (but one or the other should be plugged).

Idle passage plugs.jpg (71.75 KiB) Viewed 801 times

[Otis]

When you say all the drillings do you mean all the plugs needle holes etc. but not the main inlet where the air cleaner bolts to and outlet for intake manifold or do they need to be capped also?