# RPM Oscillating with New Carb



## Booleu (Oct 26, 2016)

Hello. I just installed a new carburetor on an older Snapper with a Tecumseh HS40-55552J and the engine RMP is now oscillating wildly. It mellows with the choke on high. What may be causing this? Here's a video... youtu(dot)be/EP2hgX1R1jY Any help would be appreciated. Thank you.


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## db9938 (Nov 17, 2013)

Well, your video link did not come through. What engine is on your machine?


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## Booleu (Oct 26, 2016)

@db9938
Thanks for the reply. You have to copy the website link: youtu(dot)be/EP2hgX1R1jY and paste in into your browser's Address bar, then replace the "(dot)" with a "." - I'm new to the forum so I can't actually insert a link. The engine is a Tecumseh HS40-55552J.


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## stromr (Jul 20, 2016)

*Welcome Aboard!*

Sounds too lean...


:white^_^arial^_^0^_


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## dbert (Aug 25, 2013)

linked for you. I wish we could lighten up a bit on the newb restrictions.


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## stromr (Jul 20, 2016)

*I second the motion Mr. Chairman...*



dbert said:


> linked for you. I wish we could lighten up a bit on the newb restrictions.
> https://www.youtube.com/watch?v=EP2hgX1R1jY




I second the motion Mr. Chairman...


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## russkat (Feb 25, 2015)

Surprising it runs much at all with full choke, all my small engines barely run on full choke.
Honda's, Tec's, and Kohler's.

Is the new carb a factory Tecumseh or chinese copy ?
Did it run the same with the old carb ?


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## Booleu (Oct 26, 2016)

Thanks dbert. Yeah, most forums are like this, need to have tenure before you can link.


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## Booleu (Oct 26, 2016)

russkat said:


> Surprising it runs much at all with full choke, all my small engines barely run on full choke.
> Honda's, Tec's, and Kohler's.
> 
> Is the new carb a factory Tecumseh or chinese copy ?
> Did it run the same with the old carb ?


I believe it's a Tecumseh. I got it off Amazon, said it was Tecumseh, but who knows these days. It didn't really run at all with the old carb. Which is why I got this one. At least it's running now, albeit poorly. So given that it runs (actually better) with the choke on, would that seem to indicate an air issue, like it's not getting enough if it? I've just never seen a carb oscillate like that. But then again, I'm a computer geek, not a petrol head.

If it is an air-related problem, is there an adjustment for that on this carb?


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## Car78412 (Oct 26, 2016)

I have the same issues with my Craftsman 9 hp 2 stage snow thrower. It is 14 years old. The oscillation is so severe, it stalls. Sears repair said it was the carb. It still oscillates after they changed the carb twice. Strange thing is that when the engine is plowing snow, the oscillation stops! I am so frustrated by this that I am getting a new machine.


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## classiccat (Mar 1, 2014)

Car78412 said:


> I have the same issues with my Craftsman 9 hp 2 stage snow thrower. It is 14 years old. The oscillation is so severe, it stalls. Sears repair said it was the carb. It still oscillates after they changed the carb twice. Strange thing is that when the engine is plowing snow, the oscillation stops! I am so frustrated by this that I am getting a new machine.


That's the classic sign of lean condition caused by a restricted idle circuit...that fact that it goes away once the throttle opens-up is a clue.

You bought an adjustable carb (PreEPA) so that's good.

Do you see that screw on the side of the carb body? That's the low-speed adjustment...turn that guy out in 1/8th turn increments waiting 15s between each turn until the surging goes away.

If you refer to the tecumseh L-head service manual, it contains the starting needle settings (how far to turn it out from being* lightly *seated):

for series-1 carbs like you have: 
Low-speed: 1 turn out from *lightly* seated.
High-speed (bottom of the float bowl): 1 1/2 turns out from *lightly*-seated


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## bad69cat (Nov 30, 2015)

If it's running with the choke closed and warmed you have an air leak of some kind. check that the mating surface on the new carb is flat/true. make sure the gasket is actually working....spray a little carb cleaner around the mating surface while it's running and you should not see it stumble act up. If it does you got a leaker...... then after you have verified no leaks try to richen it up (if it's adjustable) and also check that your governor is adjusted properly. See Donny boy's vids on youtube if you need pointers...... he covers all of this stuff quite thoroughly.


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## Infantryman (Oct 3, 2016)

It was oscillating like that on my Honda lawn mower and stopped once cutting the grass. Result: The engine was surging. It could be old gasoline, gummed carb/jet or even a governor spring. In my case, I cleaned up the carb with cleaner then solved my problem. In your case, it is a brand new carb so it is hard to tell....


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## Car78412 (Oct 26, 2016)

Thanks for the response. The sears tech installed a non adjustable carb. He said the new emission laws doesn't allow him to install adjustable carbs.


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## stromr (Jul 20, 2016)

*Thank you EPA!*



Car78412 said:


> Thanks for the response. The sears tech installed a non adjustable carb. He said the new emission laws doesn't allow him to install adjustable carbs.



Another good reason to do your own repairs! The fact that the bureaucrats in Washington DC think pollution from small engines is a problem shows how stupid our government is.


:banghead:


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## topher5150 (Nov 5, 2014)

I saw someone mention the governor. For most of last winter I was chasing wild revving problems and I narrowed it down to the governor. In my Briggs & Stratton the gov arm linkage is attached to the gov shaft with a small screw. That screw had a majority of it's threads stripped so it wasn't able to clamp down to the gov shaft. The guys working on my blower didn't even notice. Not sure how yours is set up but might be worth checking out.


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## LS6 Tommy (Oct 20, 2016)

classiccat said:


> That's the classic sign of lean condition caused by a restricted idle circuit...that fact that it goes away once the throttle opens-up is a clue.
> 
> You bought an adjustable carb (PreEPA) so that's good.
> 
> ...




Classiccat nailed it. Snow Kings are prone to surging and are VERY sensitive to carb adjustment. I've seen more than one run poorly right from the factory.


Take your time, make small adjustments, and only adjust one thing at a time.


Tommy


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## Ariens hydro pro (Jan 24, 2014)

It's running lean. Either the jets are plugged or it's out of adjustment. Also spray some starting fluid around the gasket area of the carb, perhaps the gasket is sucking air which will cause it to run lean.

While the engine is running, spray the gasket and if the engine changes tone, your sucking air so repair that gasket.


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## renegade:32 (Jan 8, 2018)

I’ve had the same oscillaing / hunting problem with a few of the eBay knockoff carb. In ALL of my cases, it was air leaking in through too much play in the throttle shaft. 
As pointed out above, spraying your aresol of choice on it should pinpoint the problem. 
A polite email to eBay sellers always resulted in a quick refund (with no return necessary) or a replacement. 
A dab of grease around where the throttle shaft meets the carb casting has worked for about 10 minutes- just to prove the problem. 
Good luck solving it!


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## leonz (Dec 12, 2014)

Car78412 said:


> I have the same issues with my Craftsman 9 hp 2 stage snow thrower. It is 14 years old. The oscillation is so severe, it stalls. Sears repair said it was the carb. It still oscillates after they changed the carb twice. Strange thing is that when the engine is plowing snow, the oscillation stops! I am so frustrated by this that I am getting a new machine.


==================================================

Hello Car78412,

if you have rubber fuel lines I would buy new ones and add a fuel filter or change the one that is on it if so equipped before you drain your wallet. 

The other usual culprit is debris in the tank choking off the fuel inlet screen in the gas tank.

With Sears telling you it was the carb that just tells me that they did not do much with it anyway.

You probably need to run sea foam through the fuel system too clean the fuel system.
They should have soaked the carburator in alcohol or peroxide or put it in an ultrasonic parts cleaner with the fuel bowl and jets cleaned separately.


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## orangputeh (Nov 24, 2016)

renegade:32 said:


> I’ve had the same oscillaing / hunting problem with a few of the eBay knockoff carb. In ALL of my cases, it was air leaking in through too much play in the throttle shaft.
> As pointed out above, spraying your aresol of choice on it should pinpoint the problem.
> A polite email to eBay sellers always resulted in a quick refund (with no return necessary) or a replacement.
> A dab of grease around where the throttle shaft meets the carb casting has worked for about 10 minutes- just to prove the problem.
> Good luck solving it!


wow. thanks for this tip. have had this problem with some carbs from ebay for my honda's.


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## Kiss4aFrog (Nov 3, 2013)

I checked with the admin and they say there isn't a minimum post count you need to be able to link videos so it might just be the OP wasn't attaching it correctly. That or it's changed to zero required posts since this thread started in oct 16.

Either way a brand new member can post photos and links in his very first post currently.

.


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