Running problems

Hi there,
I have a Pan with a Linkert M-74B. The engine idles well when the low speed needle is at only 1/2 turn from fully in. This seems very lean but the engine idles perfectly. When I open the throttle the engine starts missing, shoots, backfires and finally dies. No acceleration is possible. When I turn the low speed needle out to about 2 to 3 turns, accelaration is possible (when riding it still misses when I try to accelarate quickly) but at idle it's too rich, smokes a lot and finally dies.

What was already done: carburator cleaned, Float level checked and set to 1/4 (black foam float), new float valve and seat (with the rubber tip), main nozzle changed to the proper (5 air bleed hole) one, all new gaskets at the manifold, ignition checked, new points and condesor, timing adjusted, new spark plugs, different coil tested, valve play adjusted.

The fixed jet in the carburator has the number 108. Is that the propper one?

What else can there be??

Thanks for any advise!
Mac

HDMac!

The first variable to eliminate is the possibility of a vacuum leak.

Please pressuretest your manifold assembly as discussed at http://virtualindian.org/11techleaktest.html
Note that the pan cover screw over each intake port is a possible site for a leak as well, since some have perforated into the port.

(Barnyard tests of spraying stuff while the motor is running only hints at huge leaks, and overlooks tiny ones.)

Your rubber-tipped needle is a liability in modern fuels, but probably isn't the problem, yet.

Before-and-after swelling of "viton" needle

VITON.jpg (21 KiB) Viewed 7428 times

I think your jet is really a #18, and proper, if it hasn't been boogered.

Good luck!

...Cotten

We will do the pressuretest on the weekend. Have to modify the plate to fit the 4 bolt manifold first.

Your rubber-tipped needle is a liability in modern fuels, but probably isn't the problem, yet.

Do you normally use the all brass needles instead of the rubber tip ones?

HDMac!

I endorse valves that are as similar to original design as possible, as the brass alloy used is subject to rapid corrosion.

VITON2.jpg (12.51 KiB) Viewed 7372 times

At this time, the only ones with anything close to a monel metal needle are from Larry Elias of the Netherlands, identified by a octagonal black-oxide steel needle. They, too, are subject to corrosion, but service far better than oriental productions.

Euro fuels are not yet as horribly digestive as USA fuels , so it is unlikely that your soft-tipped needle has swollen enough to change your float level as much as the one shown, but it should be inspected for loosened adhesive to the tip, or even a tip that was ground badly out-of-round.

Once again, I urge you to pressuretest your manifold assembly, before addressing the carburetor itself.

....Cotten

..

Is this it?

Yes thats the nozzle I have in there now. Before there was one with only 4 holes and no square part near the top. But no change in the running pattern

When you say "ignition checked", what advance?

Fully advanced to the line on th flywheel (beginning of the inspection hole)

Thanks for all the tips and tricks. Here is the solution to the problem:

First we did the proper pressure test and found a manifold leak. I thought that this is the problem, fixed it until under pressure no leak could be detected anymore. After all the work, guess what - same thing. The engine did not run any better

I tried some other stuff with the carb. no change

After all that I decided to try another M74 that I got from a friend and - the engine runns perfect!! Somehow ths Linkert I used has a problem. Also with this one the idle needle is right where it should be - about 2 turns out (not like with the first one where the engine only idls well when the idle adjusting needle is at only half a turn out)

Anybody and idea what the problem of the carb is?
Mac

[Pa]

Look for a crack in the carb body or a badly warped mounting flange.

Make sure the low speed needle spring washer isn't on backwards, it makes it hard to idle.

Keep using your buddys carb and send the mis-fit to Cotten for a going over.

When you think it's the carb, it will be electrical! Condensors can give that symptom as well. Ask me how i know!

we checked all electrical before changing the carb. There where problems but non of them solved the running problem of the engine. we changed the condenser too. As said at the end changing the carb solved the problem.

Take a look at the shaft that the butterfly is attached to. When the holes in the carb that this shaft goes through, get wallowed out, it will cause a big vacumn leak that will cause the same symptoms you reported. Those don't show up in the pressure test either. You an repair the carb body by reaming the holes larger and putting flanged bushing in there for the shaft to ride in. Also, check your total advance timing with a timing light.

Take a look at the shaft that the butterfly is attached to. When the holes in the carb that this shaft goes through, get wallowed out, it will cause a big vacumn leak that will cause the same symptoms you reported. Those don't show up in the pressure test either. You an repair the carb body by reaming the holes larger and putting flanged bushing in there for the shaft to ride in. Also, check your total advance timing with a timing light.

Boots99!

Why must you go over-size and use a flanged bushing?

....Cotten