Board index Flathead Power-Technical Questions, Answers, and Suggestions Big Twin Flatties Model U engine running problems

Model U engine running problems

Post Sat Aug 29, 2009 11:02 am

Posts: 103
Location: Switzerland

Hi there, My U model flathead always shuts off as soon as I open the throttle just a little bit. She idles well and when I open the throttle fast enough to go over the bad spot the engine runs well at high rpm as well. I checked the manifold for leaks already and thing it's ok now but the problem did not go away. I run a M-41 Linkert, which is not the right model but I believe this should not be the problem? Any idea, comment or suggestion is very welcome.
Thanx, Mac

Post Sat Aug 29, 2009 5:36 pm

A complete M-41 should run with no problems, but are you sure that all the internal parts are correct - especially the main nozzle and venturi?
Have you tried raising the bowl level by 1/32"?
Better with the choke slightly on?
Better hot or cold?
Spark fully advanced at idle?

Post Sat Aug 29, 2009 7:56 pm

Posts: 3159
Location: Central Illinois, USA

Just how did you test for vacuum leaks?

I gotta ask, because if you didn't pressuretest with soapy water, it is not tested.


Post Sun Aug 30, 2009 1:47 am

Posts: 103
Location: Switzerland

Cotten, I did read an older thread about vacuum testing and I have to admit that I did not do it the proper way yet. I will probably do that next.

Post Sun Aug 30, 2009 1:52 am

Posts: 103
Location: Switzerland

Panic, I checked the float level and it's correct. I will try with raising it a bit. With the choke slightly on it's better - almost gone. I have the spark fully advanced after the engine starts (it starts very well by the way). I did not check main nozzle and venturi. What do I need to look at?
Thanks Mac

Post Sun Aug 30, 2009 8:41 am

Posts: 3159
Location: Central Illinois, USA

Please use pressure, not vacuum:

A large rubber stopper takes three strong hands, but works as well as a testplate. And only a rubber stopper will allow you to inspect the cylinder nipples separately.
(Take care to rinse diluted dishsoap thoroughly, as it is very corrosive!)

If your float is of original buoyancy, there should be no need for deviating from book spec.
(Ultra-buoyant floats are only now entering the market.)

Random tweaks can muddy the water and mask problems that should be fixed otherwise.
Please make experimental changes one at a time, or you may chase your tail to change them back.


Post Sun Aug 30, 2009 8:45 am

The M-41 venturi is 1-1/16" ID, and has no extra opening at the leading edge (the wrong one has a small square slot on the bottom).
Compare the main nozzle with Palmer etc. for shape and hole pattern.
Sounds like too lean - your transition slot in the carb body may be obscured by carbon or dirt, but not easy to tell assembled.
It may also be over-advanced - is it better with the spark backed off slightly?
Back off the "power" needle 1/2 turn - better or worse?

Post Mon Aug 31, 2009 5:18 am

Posts: 3159
Location: Central Illinois, USA
A throat-vented venturi in a side-vented body presents no functional problem, although it would be "incorrect" historically;
A side-vented venturi in a throat-vented body, however, would defeat the main nozzle's 'accellerator pump' effect, as there would be no vent at all.

An M41 is a side-vented body.

L&L proved the hard way that the size of the vent need only be unrestricted when they needlessly re-designed it for the M35TP, only to eventually return to the same ol' BIG hole for the pinnacle M74 models.

Of greater concern is that the venturi fits with as little daylight as possible around it; Nearly all have shrank to some degree. And a seal around the nozzle spigot helps too, if the loose venturi has worn into it.

But even a perfect carb won't run right if there is a vacuum leak downstream.


Post Fri Sep 04, 2009 9:25 am

Posts: 103
Location: Switzerland

Hi again,
I did the pressure test with the proper equipment and found no leaks between manifold and cylinders or around the nipples. Still the same problem. What if there is a leak between manifold and carb? hoe to test for this. I'll keep on trying other things.


It's the same whether the engine is cold or hot. Ignition advanced or retard - same thing. With or without the joke slightly on does not change things significantly. I even tried an other carb (M54) - same thing. I checked valve clearance and points.

The engine stars every kick and idles very well. When I start opening the throttle very slowly then the engine revs up just a little before it starts spitting and dies. When I open the throttle fast it just spits once and revs up to high rpm and the runs well again.

Any other ideas?

Post Fri Sep 04, 2009 3:03 pm

Posts: 391
With or without the joke slightly on does not change things significantly. I even tried an other carb (M54) - same thing.

So cut it out with the jokes and try to be more serious with it :mrgreen:

OK, no fooling around, It definitely sounds lean at the transition from idle to mid range operation. Have you done a compression test or leak down test? No cracks in the head? Do the plugs look the same or is one showing significantly richer or leaner than the other? Since you say it does it even with a different carb I would look at another source of an air leak, head gaskets, cracked head or cylinder, etc. And are you running an air filter? What kind of pipes?

Post Sat Sep 05, 2009 2:17 am

Posts: 103
Location: Switzerland

Thanks Rubone, I run stock air filter and pipes. I will have a look at the plugs and do a compression test today.

Post Sat Sep 05, 2009 10:13 am

Posts: 369
Location: Bergen, Norway
Do the leak test!
Not only the intake, but the rest also!
Soap and water will detect any leaking head gasket, spark plug, head bolt, valve cover etc.
If yo want to be sure, crank the engine in a position with open intake valve and closed ex valve.

Post Sat Sep 05, 2009 8:01 pm

Posts: 426
Location: Tucson Arizona

Just try a different carb off a buddy's bike

Post Sat Sep 05, 2009 11:33 pm

Posts: 976
Location: Markt Einersheim, Germany

He did..

I even tried an other carb (M54) - same thing

Post Sun Sep 06, 2009 6:08 am

Posts: 251
Location: Hudson, Florida
I had a 76 Sportster do the same thing, chased it around the carb an the ignition for weeks. I would be running fine an it would just quit running,no popping , spitting , nothing. As soon as I pulled over an kicked it it would re-start an go for another few minutes only to do the same thing. Last time out I re-started it 5 times, started easy an idled , pulled strong an smooth an just turned off like you hit the key. Pulled the plugs an they weren't wet, so back home an off with the heads. I looked real hard at the intake valves, figuring if it was an exhaust it would pop thru carb, which it didn't. All checked out good clearance, but there was a small spot on the intake stems that I redressed an decided to open the guide another loose .0015. The bike worked perfect after that an never stalled again.Another problem area on a castiron head is to have the valves just a little too tight, many Sportsters I've worked on the same problem, When started first time easy, rest of day even slightly warm hard starting an popping. Back off on valve adjustment an it will usually cure it. I'll admit to knowing very little about Harley flatheads, I've only worked on 6 in my life time, but been lucky enough to see many Indians , Sporty's, Troubleheads , Pan's an Knuck's. We allways seem to chase an easy problem longer than something obvious:)
Good luck Neil74

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