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Loose Head Causing Rough Idle?


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#1 TerryD OFFLINE  

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Posted July 04, 2013 - 11:07 PM

Picked up a 1971 Massey Ferguson 10 a couple weeks ago. I can't seem to get the idle circuit adjusted properly. PO said it had been sitting for awhile. It has a Walbro LME 37 carb and turning the idle mixture screw has no effect. I suspect a clogged nozzle even though I've had the carb apart once for cleaning. I am hesitant to take out the nozzle without a service nozzle on hand.

Today I wanted to take the head off and decarbon it. Found a wet ring around the head where the gasket sits. It doesn't appear the head was on tight after the PO had the engine rebuilt some years ago.

My question is could a loose head affect the idle circuit and prevent the engine from drawing gas into the combustion chamber causing it to speed up, then drop back down as if it were hunting?

Thanks, Terry
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#2 HowardsMF155 OFFLINE  

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Posted July 04, 2013 - 11:15 PM

I think you are right to suspect the idle circuit in the carb.  The idle circuit in the carbs was a tiny hole drilled through the main jet well and through the needle seat.  If the PO did a rebuild on the engine and didn't put the correct service seat back in ( it has a groove cut all the way around so gas can still enter through the tiny hole)  then the idle circuit isn't working properly.  Even if the carb wasn't touched, those holes clog easily since they are so tiny.  I've not had to fix one myself, so I can't tell you the best way to find the hole and clean it.


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#3 TerryD OFFLINE  

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Posted July 04, 2013 - 11:22 PM

I'm thinking of squirting acetone through the nozzle followed by an air blast from my compressor. Wearing safety goggles of course.

#4 chris m ONLINE  

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Posted July 04, 2013 - 11:35 PM

It could be either of the two causing the problem. Before putting the head on make sure it is flat. Another issue I just had was the mounting flange on the LME 37 carb I was rebuilding was severely warped, and it took quiet a bit to get it flat again.

 

Any type of air leak will cause problems like this, even a worn throttle shaft.

 

And like Howard said, you just never know what a PO might have done if they tried to rebuild the carb.I would just spray every little hole you can see in the carb and make sure it is coming out were it is supposed to.


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#5 JDBrian OFFLINE  

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Posted July 05, 2013 - 05:55 AM

A leaking Head gasket will cause all sorts of running problems. If the HG leaks vacuum on the intake stroke you won't pull fuel into the cylinder properly. I would also check your ignition. IME carb problems often get blamed for other issues. I've heard many times how a carb has been cleaned 4 or 5 times before another issue is identified as the problem. Simply cleaning a carb multiple times is likely to result in another problem with the carb or a gasket so I am always looking for another problem if a careful cleaning or rebuild doesn't solve the issue. 


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#6 NUTNDUN OFFLINE  

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Posted July 05, 2013 - 05:56 AM

If the head wasn't tight and leaking it would affect the whole rpm range and not just idle. Like Chris said check the throttle shaft and there could be air getting by either at the throttle shaft or at a gasket. I would put money on a service nozzle and a rebuild / gasket set for the carb.


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#7 robert_p43 OFFLINE  

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Posted July 05, 2013 - 09:10 AM

When I first got my sears suburban 10, I was going to rebuild the carb.  It's a 1o hp Tecumseh with a Walbro LME carb, just a different number I think, 34 maybe?  Can't remember right now.  Anyway, I went into a local power equipment shop to get the kit and they did have one in stock.  I had to get the carb into the shop to prove the numbers as he wanted to search all over it to look for other numbers, I know there is a "12" on one side, a "34" or "37" or something on the other.  Knowing this guy rebuilds 5 or 600 carbs per year, I decided to just have him do it.  When I picked it up, he mentioned that he didn't replace the jet as he didn't have one and it doesn't come in the kit.  At that time, I didn't know anything about the service jet thing, and I have no idea if he removed the main jet to clean it or not.   Well, my tractor won't idle either and I need to keep it partially choked at all times.  It hunts and surges some.  It was sucking air at the intake, and I did flatten the warped ears and replaced the gaskets.  That made a big difference.  It still leaks around the throttle shaft, if I spray a little carb cleaner on the top of the shaft, it smooths right out for a few seconds.  I can set my finger on the top of the shaft and feel it wiggle a little.  I think for best results, I probably need a service jet, and new throttle shaft and probably bushings tooIf it was wet all around the head gasket, when putting it back together, make sure to torque it correctly.  I had my head off and it leaked so I tightened all the bolts,  Since then, I replaced the head gasket and was surprised how much I had over torqued it to keep the old gasket from leaking.  The new gasket was under $5 at the same local shop so it's a small price to eliminate problems coming from there.

I saw a post here somewhere where they mentioned filing a small grove in the threads where the hole goes through their jet, thereby turning it into a service jet. It may have been in the sears forum someplace and if I spot it, I will post the link back in this thread for you.


Edited by robert_p43, July 05, 2013 - 09:15 AM.

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#8 robert_p43 OFFLINE  

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Posted July 05, 2013 - 09:33 AM

here it is

http://gardentractor...-ss12-findings/

 


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#9 TerryD OFFLINE  

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Posted July 05, 2013 - 09:51 PM

Thanks everyone for the responses. I did a 2 hr acetone soak this morning followed by a blowout from the compressor. Still not much better. We leave in the morning on vacation, so I won't be able to try these suggestions until I get back next week. Maybe a leaking HG or carb gasket, but I'm ready to rule out a blockage in the carb.

Have a safe week everyone.

Terry
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#10 HDWildBill OFFLINE  

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Posted July 06, 2013 - 10:18 AM

When you cleaned the carburetor did you remove the welch plugs and clean behind them?


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#11 TerryD OFFLINE  

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Posted July 16, 2013 - 07:01 PM

I checked the flange and someone had really snugged it down at some point. The flange was a good .030 off from being flat. I filed that almost flush then using a piece of glass for a lapping surface, I worked it the rest of the way down with 400 and 800 grit wet/dry paper. Since I was having a problem with idle I took the main nozzle out and discovered it had a service nozzle with the small holes around the perimeter still plugged. An old toothbrush and brake cleaner took care of that. Guess I'll put the rest of the rebuild kit in tonight and back on the tractor tomorrow night.

It was suppose to be a new carburator, but guess someone got heavy handed with it, and it had set for a number of years unused.

Lesson learned. Thanks to all for your advice. I'm optimistic that tomorrow night it will fire right up.

#12 TerryD OFFLINE  

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Posted July 16, 2013 - 07:24 PM

When you cleaned the carburetor did you remove the welch plugs and clean behind them?


No welch plugs in this carb, but good thought. Most do.

#13 chris m ONLINE  

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Posted July 16, 2013 - 07:55 PM

I checked the flange and someone had really snugged it down at some point. The flange was a good .030 off from being flat. I filed that almost flush then using a piece of glass for a lapping surface, I worked it the rest of the way down with 400 and 800 grit wet/dry paper. Since I was having a problem with idle I took the main nozzle out and discovered it had a service nozzle with the small holes around the perimeter still plugged. An old toothbrush and brake cleaner took care of that. Guess I'll put the rest of the rebuild kit in tonight and back on the tractor tomorrow night.

It was suppose to be a new carburator, but guess someone got heavy handed with it, and it had set for a number of years unused.

Lesson learned. Thanks to all for your advice. I'm optimistic that tomorrow night it will fire right up.

Terry I have lost count at how many times I have found warped surfaces on different engine items lately! I just had a Member on here send me his fuel pump from a Kohler, no matter what he did it wouldn't pump. The problem wound up being the pump body (where the big diaphragm is) was warped a good .020-.025, after I resurfaced it It pumped fuel like crazy!

 

It sounds like you have found all the issues on your carb. I bet she'll run like a champ now! :dancingbanana:


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#14 TerryD OFFLINE  

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Posted July 19, 2013 - 09:55 PM

I hate Walbro carburators. No air leaks at the gaskets anymore. Runs great from 2200 to 3600. Still a bit rough at idle though. Turning the idle screw has no effect on idle mixture. Been reading about folks who add acetone to their fuel to clean carbs and occasionally injectors. Tonight I added a splash to the half full tank of gas and mowed. Seems strong enough at 3600, but it did before too. I really need to idle this thing around the yard for awhile and let the idle circuit work harder at getting clean, problem is it tends to surge throwing me back and forth. Stupid carburator.

Okay, I feel better now....




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