M74B idle running away

Having trouble sorting out issues the m74b on my 51 FL

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panama86
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M74B idle running away

#1

Post by panama86 » Sat Oct 12, 2019 12:43 am

Hey all,
I am having some trouble with my pan running an m74b.

The motor is a fresh rebuild, never ridden just started once when I first put it in the bike with a super B.

I took the super B off as I had the linkert on the shelf and wanted to use the B on my shovel. Anyway, I can get her to fire up but the second it fires it just starts revving out of control and I rush to kill it. I have tried the reccomended 5 turns out on the low and 2 out on the high. Maybe half a turn out if that from seated on the idle adjustment screw on the throttle arm.

I have leak tested the manifold at 15psi, no leaks. Replaced throttle shaft bushings. Checked float height @ 1/4". New low and high speed needles. Fresh manifold to carb gaskets and insulator, the surface of the carb was cleaned up before fitting to make sure mating surface was flat. All other gaskets in the carb were replaced also. The carb was ultrasonically cleaned before putting on the bike. I can't seem to figure out what's going on with this thing. I am running a magneto with an adjustable advance cap. Timing should be correct, front cylinder compression stroke line in the middle of the timing plug hole. I am starting the bike with the timing barely retarded.

On the last start, I attempted I tried winding the low speed right out till it was almost ready to run out of threads (this should richen the mixture right?). It has no effect what so ever. Pipes get hot almost instantly.

This seems like a lean condition and an air leak somewhere but I am out of ideas. The heads have been converted to o-ring which is where I would usually look next. However, the thing ran fine with the super B so that rules out a leak there.

Any ideas guys?



panama86
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Re: M74B idle running away

#2

Post by panama86 » Sat Oct 12, 2019 1:34 am

Sorry, forgot to mention float needle and seat also replaced with steel tip, the old viton tipped needle had fallen apart.

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Re: M74B idle running away

#3

Post by Bone Head » Sat Oct 12, 2019 1:55 am

I'm curious, is your throttle cable adjusted properly and allowing the linkage to rest against the idle speed screw? Simple thing, I know.

panama86
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Re: M74B idle running away

#4

Post by panama86 » Sat Oct 12, 2019 2:08 am

Yep, it's sitting up against the idle screw flush. The revs are so high too that even if it was lifted slightly it wouldn't account for how high its climbing.

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Re: M74B idle running away

#5

Post by Excalibur » Sat Oct 12, 2019 3:41 am

Timing full advance mark:
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Re: M74B idle running away

#6

Post by Excalibur » Sat Oct 12, 2019 3:52 am

Recheck the throttle cable wire. It needs to forcibly pull the throttle arm back to the stop with some conviction.

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Re: M74B idle running away

#7

Post by beets » Sat Oct 12, 2019 12:00 pm

Excalibur wrote:
Sat Oct 12, 2019 3:52 am
Recheck the throttle cable wire. It needs to forcibly pull the throttle arm back to the stop with some conviction.
The way I do this is loosen cable retaining screw at carb then while holding lever against the stop, open throttle slightly. Tighten the retaining screw.

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Re: M74B idle running away

#8

Post by Andygears » Sat Oct 12, 2019 12:18 pm

I would guess that the throttle arm is misplaced on the shaft. Try this, with the bike shut off and the air cleaner off, push the throttle hand grip against the stop. Then with the eraser end of a long pencil, shove it thru the body and see if you can move the throttle plate more closed. The throttle plate, arm clamping, and handlebar grip all have to align at “wide open throttle”. I explained the procedure not long ago.

Re:Control coil cable for throttle bracket. Dated Sept 9


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Re: M74B idle running away

#9

Post by panhead » Sat Oct 12, 2019 1:00 pm


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Re: M74B idle running away

#10

Post by rocket ron » Sun Oct 13, 2019 6:41 am

Did you centre the throttle disc in the carb throat before tightening shaft screws? Just a thought.
Ron

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Re: M74B idle running away

#11

Post by panama86 » Mon Oct 14, 2019 12:28 am

Hey all, bit of an update. It seems like it was a combination of things.

I adjusted my timing so the line was just entering the right side of the viewing window. Then inspected the throttle arm return and you guys were correct, it wasn't completely closing the throttle plate. The cable end stop was contacting the motor mount and preventing it from returning completely.

Interested in that timing mark compoment, it seems information on center or right side of window varies. Is that the final word? Right side.

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Re: M74B idle running away

#12

Post by RooDog » Mon Oct 14, 2019 4:19 am

panama86 wrote:
Mon Oct 14, 2019 12:28 am


Interested in that timing mark compoment, it seems information on center or right side of window varies. Is that the final word? Right side.
What does it say in you Harley Davidson factory Service Manual? You do have one, don't you? and not the evil three, Chilton, Haynes, and Clymer.
Timing may vary depending on compression, load, and fuel quality. It is adjustable for a reason. Like stop signs, and speed limits, the line at the rear of the hole, 35* BTDC, is only a suggestion.
....RooDog....

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Re: M74B idle running away

#13

Post by Sarre » Mon Oct 14, 2019 10:16 am

panama86 wrote:
Mon Oct 14, 2019 12:28 am
Interested in that timing mark compoment, it seems information on center or right side of window varies. Is that the final word? Right side.
RooDog wrote:
Mon Oct 14, 2019 4:19 am
Timing may vary depending on compression, load, and fuel quality. It is adjustable for a reason.
What RooDog writes makes sense to me.
I started out at the right side of the screen as well, which led to hard starting, now I have it centred and figured out a starting procedure without flooding the intake (S&S super e currently). I did this because I was unable to properly (to my satisfaction) retard the engine with the circuit breaker. I followed Pacific Mike's instruction on youtube, who also has the timing mark centered. Will probably adjust it again when really riding the bike and getting to know her better.

If you move the timing mark in the window to the left that means the timing is late, and if you move it more towards the right edge of the window it is early.

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Re: M74B idle running away

#14

Post by RooDog » Mon Oct 14, 2019 12:28 pm

Just as an example:
As in my avatar, a solo 1950 Panny, 8.5 advertised compression, Andrews #1 cam, with a Bendix carb and an aftermarket automatic advance ignition breaker, the timing is set at the rear of the hole with the advanceer forced to full advance. Starts well with a few priming twists of the throttle, and usually no choke, even cold. Very torquie, I love that Andy #1 cam. and have a new respect for that old Bendis/Zenith carby, $27 from eBay....LOL....
....RooDog....

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Re: M74B idle running away

#15

Post by Sarre » Mon Oct 14, 2019 1:21 pm

Interesting, my FL is also '50 but with andrews J cam, [have to recheck compression], stock circuit breaker, and the S&S super E.

Does the bendix have an acceleration pump?

With the S&S I now prime with choke on/ key off 3 times (no twisting the throttle for accel. pump), then choke off/key on and one or two kicks it will start @ full advance.

Good to compare :) but again, I will probably retune my bike again in the future.

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