# ST824 924050 Engine RPM Surging



## badbmwbrad (Jul 30, 2019)

This is a recurring problem on an 8 HP Tecumseh Snow King engine. The engine date is circa 1988. The Machine runs fine for 90 minutes then it starts surging. Fixing it involves removal of the spring-loaded idle mixture (?) adjustment screw which threads into the carburetor body from the side (screw hole axis is perpendicular to venturi axis). I spray inject carburetor choke cleaner into the empty screw hole, stretch the spring to give it more tension and reinstall the screw. 

In addition, the float bowl is removed and the two bowl nut orifices are verified clear (they're always clear). I also spray choke cleaner up through the main emulsion tube in the center of the carb body.

After reassembling everything and especially after adjusting the idle mixture screw, the engine surging goes away.
I suspect the idle mixture screw actually moves during operation because of insufficient spring tension to hold it in one place. 

Does this screw control fuel flow or air flow? Am I correct in understanding this screw to be for idle mixture control?


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## Ian Ariens 924 (Dec 22, 2015)

I would try a new carb. Available from amazon.
It’s the easy way to fix it.


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## ChrisJ (Nov 27, 2014)

Being as I went through this, are you 100% sure the problem won't go away if you just let the engine sit and cool down for the time it takes you to do all of that?


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## ChrisJ (Nov 27, 2014)

The reason I ask is I'd be willing to bet your exhaust valve clearance is way too tight.
Pull the valve cover and check it, chances are it's under 0.006". If you find this is the case look at increasing it to 0.009-0.012.

Also check the intake valve while you're at it, it may be a little on the tight side but probably not too terrible yet.


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## badbmwbrad (Jul 30, 2019)

I agree that tight valve clearance will affect engine operation. Prior to carburetor disassembly, the engine was started up from cold and was immediately surging during choke and subsequent no-choke operation. The engine would readily hot-start with no apparent loss of compression.

It's a simple matter to check valve clearance. Adjusting valve clearance is another matter... I've done it several times. 



ChrisJ said:


> The reason I ask is I'd be willing to bet your exhaust valve clearance is way too tight.
> Pull the valve cover and check it, chances are it's under 0.006". If you find this is the case look at increasing it to 0.009-0.012.


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## TX610 (Jan 15, 2021)

badbmwbrad said:


> This is a recurring problem on an 8 HP Tecumseh Snow King engine. The engine date is circa 1988. The Machine runs fine for 90 minutes then it starts surging. Fixing it involves removal of the spring-loaded idle mixture (?) adjustment screw which threads into the carburetor body from the side (screw hole axis is perpendicular to venturi axis). I spray inject carburetor choke cleaner into the empty screw hole, stretch the spring to give it more tension and reinstall the screw.
> 
> In addition, the float bowl is removed and the two bowl nut orifices are verified clear (they're always clear). I also spray choke cleaner up through the main emulsion tube in the center of the carb body.
> 
> ...


I just repaired one of these. even after putting on new carburetor surging persisted due to EPA regulations leaned out mixture too much. i pulled the plastic pilot jet out and drilled 2 size bigger. issue was resolved. this was a OEM replacement carburetor . As others have mentioned aftermarket carburetors are available that don't have fixed jets. I've used them occasionally with good luck 98% of the time .


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## ChrisJ (Nov 27, 2014)

badbmwbrad said:


> I agree that tight valve clearance will affect engine operation. Prior to carburetor disassembly, the engine was started up from cold and was immediately surging during choke and subsequent no-choke operation. The engine would readily hot-start with no apparent loss of compression.
> 
> It's a simple matter to check valve clearance. Adjusting valve clearance is another matter... I've done it several times.



I don't recall of the idle adjust controls fuel or air, I think it's fuel. From what I recall, counter clockwise is richer.

There's a very tiny vent hole on the side of the carb near that screw. Very, very tiny.
I don't recall it's purpose, but I do know if it's clogged it causes problems.

Here's the location.


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## ChrisJ (Nov 27, 2014)

TX610 said:


> I just repaired one of these. even after putting on new carburetor surging persisted due to EPA regulations leaned out mixture too much. i pulled the plastic pilot jet out and drilled 2 size bigger. issue was resolved. this was a OEM replacement carburetor . As others have mentioned aftermarket carburetors are available that don't have fixed jets. I've used them occasionally with good luck 98% of the time .


I don't think a 1988 would have the fixed jet modification.
The fixed jet also shouldn't have any spring as it's not adjustable.


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## TX610 (Jan 15, 2021)

ChrisJ said:


> I don't think a 1988 would have the fixed jet modification.
> The fixed jet also shouldn't have any spring as it's not adjustable.


I agree 88 wouldn't be fixed. 


ChrisJ said:


> I don't recall of the idle adjust controls fuel or air, I think it's fuel. From what I recall, counter clockwise is richer.
> 
> There's a very tiny vent hole on the side of the carb near that screw. Very, very tiny.
> I don't recall it's purpose, but I do know if it's clogged it causes problems.
> ...


When I repair for others I don't repair old carburetor. To be honest they are so cheap I can't rebuild and make money. IMO.


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## badbmwbrad (Jul 30, 2019)

Thanks for this photograph and for explaining the function of the fuel:air mixture adjustment screw. I was completely unaware of that tiny little hole on the carburetor body and strongly suspect it admits atmospheric pressure into the float bowl. 

My understanding of carburetor operation is airflow through the venturi creates a pressure drop where the brass emulsion tube's upper end is located. Atmospheric pressure inside the bowl acts upon the fuel level; pushing fuel up past the adjustable main jet and through the tiny little slow-speed jet on the brass bowl nut. Most of the fuel exits via the emulsion tube. A small amount of fuel passes by the idle mixture screw and enters the induction air passageway immediately downstream from the throttle plate. 



ChrisJ said:


> I don't recall of the idle adjust controls fuel or air, I think it's fuel. From what I recall, counter clockwise is richer.
> 
> There's a very tiny vent hole on the side of the carb near that screw. Very, very tiny.
> I don't recall it's purpose, but I do know if it's clogged it causes problems.
> ...


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## ChrisJ (Nov 27, 2014)

badbmwbrad said:


> Thanks for this photograph and for explaining the function of the fuel:air mixture adjustment screw. I was completely unaware of that tiny little hole on the carburetor body and strongly suspect it admits atmospheric pressure into the float bowl.
> 
> My understanding of carburetor operation is airflow through the venturi creates a pressure drop where the brass emulsion tube's upper end is located. Atmospheric pressure inside the bowl acts upon the fuel level; pushing fuel up past the adjustable main jet and through the tiny little slow-speed jet on the brass bowl nut. Most of the fuel exits via the emulsion tube. A small amount of fuel passes by the idle mixture screw and enters the induction air passageway immediately downstream from the throttle plate.


I'm not sure but I doubt it's the bowl vent.
The reason I say that is I assumed the bowl vented through the primer pump. They seem to all have a small hole in the front that your finger covers when you pump it.

But, I'm not sure honestly. I just know it's important.


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## badbmwbrad (Jul 30, 2019)

I took an as-found and as-left photograph. A copper wire strand was used to open up the hole and some carburetor choke cleaner was sprayed into the hole. One would have had to already know there was a little hole at this location on the carburetor body.

I spent an hour plowing through some heavy, wet corn snow with that machine and absolutely no engine RPM surging is evident. An extended run during the next big snow storm will determine if the engine surging is truely gone for good. 

Thanks for your advice. I learned something new about Tecumseh carburetors.


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## tadawson (Jan 3, 2018)

I have a 1989 Honda mower that had developed a surge. Multiple cleanings, etc. and no change. Replaced the pilot jet (and only the pilot jet) and it's been surge free close to 10 years. I think the plastic, if old enough, and shrink/deform over time based on fuel exposure. (Tried cleaning the pilot with a fine drill, but that just made the plastic fuzzy . . . )


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## badbmwbrad (Jul 30, 2019)

Plastic - not brass - pilot jet? I've never heard of using plastic for a fuel jet in a carburetor but it sounds very cheap and unreliable.



tadawson said:


> I have a 1989 Honda mower that had developed a surge. Multiple cleanings, etc. and no change. Replaced the pilot jet (and only the pilot jet) and it's been surge free close to 10 years. I think the plastic, if old enough, and shrink/deform over time based on fuel exposure. (Tried cleaning the pilot with a fine drill, but that just made the plastic fuzzy . . . )


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## ChrisJ (Nov 27, 2014)

badbmwbrad said:


> Plastic - not brass - pilot jet? I've never heard of using plastic for a fuel jet in a carburetor but it sounds very cheap and unreliable.


Yes,
As you know Honda small engines are known for being cheap and unreliable. 

Or rather, the complete opposite.


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## TX610 (Jan 15, 2021)

badbmwbrad said:


> Plastic - not brass - pilot jet? I've never heard of using plastic for a fuel jet in a carburetor but it sounds very cheap and unreliable.


 it's under the plastic base idle screw. Most are black. The shell around the base idle screw holds the jet down in place.


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## tadawson (Jan 3, 2018)

badbmwbrad said:


> Plastic - not brass - pilot jet? I've never heard of using plastic for a fuel jet in a carburetor but it sounds very cheap and unreliable.


Yup, on my GX engines. (I was in a GC and the carb was diffeerent, and the jet was brass.)


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## TX610 (Jan 15, 2021)

tadawson said:


> Yup, on my GX engines. (I was in a GC and the carb was diffeerent, and the jet was brass.)


The jet is brass but it's moulded inside that plastic.


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## tadawson (Jan 3, 2018)

TX610 said:


> The jet is brass but it's moulded inside that plastic.


Mine weren't - solid black plastic. (I know since I tried to tweak it with a small drill, and never saw any signs of metal . . . just fuzzfrom the black plastic.) This was on a GXV140, so they may have updated. Replacement circa 2010 was still plastic, fwiw . . . later engines may have changed, but they certainly look the same from the outside.


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## ChrisJ (Nov 27, 2014)

Here's the pilot jet from a no name replacement carb for a Honda GX from Amazon.

Sure looks like brass to me


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## TX610 (Jan 15, 2021)

ChrisJ said:


> Here's the pilot jet from a no name replacement carb for a Honda GX from Amazon.
> 
> Sure looks like brass to me
> 
> View attachment 189421


That's exactly how it looks. Great pic.


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## TX610 (Jan 15, 2021)

TX610 said:


> That's exactly how it looks. Great pic.


Yes I think I used a # 77 hand drill that jet.


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## tadawson (Jan 3, 2018)

ChrisJ said:


> Here's the pilot jet from a no name replacement carb for a Honda GX from Amazon.
> 
> Sure looks like brass to me
> 
> View attachment 189421


Looks absolutely nothing like what I have, or Amazon may have an aftermarket, or Honda has updated (Which would not bother me at all). Mine _ARE_ solid black plastic, (no matter how much you want to argue) and are actual Honda parts.

You did also say this was from a Chinese clone, not OEM Honda, so who knows . . . 

And no, I'm not tearing it down to get you a photo . . . this is the best I can find, since not many show the end view:






Jet Set-Pilot-#35 [99204-ZE0-0350] for Honda Lawn Equipments | eReplacement Parts


Buy a Honda Jet Set-Pilot-#35 [99204-ZE0-0350] for your Honda Lawn Equipment - We have the parts and diagrams to make your repairs easy.




www.ereplacementparts.com





Honda part number is 99204-ZE0-0350


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## ChrisJ (Nov 27, 2014)

I mean I went through the work to get a picture.

@tabora Do Honda pilot jets have a brass insert or were older ones solid plastic?


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## tabora (Mar 1, 2017)

ChrisJ said:


> Here's the pilot jet from a no name replacement carb for a Honda GX from Amazon.
> Sure looks like brass to me





ChrisJ said:


> @tabora Do Honda pilot jets have a brass insert or were older ones solid plastic?


Genuine Honda pilot jets for GX engines are all plastic in my experience. No brass visible. (Disclaimer: I have not had to mess with one in years...)








The non-genuine ones like this Aokus do have brass showing:


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## 1132le (Feb 23, 2017)

ChrisJ said:


> Here's the pilot jet from a no name replacement carb for a Honda GX from Amazon.
> 
> Sure looks like brass to me
> 
> View attachment 189421


of course it is
you knew it was
maybe maybe they upgraded to brass in china
lol


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## TX610 (Jan 15, 2021)

1132le said:


> of course it is
> you knew it was
> maybe maybe they upgraded to brass in china
> lol


With parts anymore nothing would surprise me. LOL.


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## ChrisJ (Nov 27, 2014)

I'm guessing the brass is easier to machine.
So ultimately the brass one could be a down grade though I'm not sure how or why.


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## Toro-8-2-4 (Dec 28, 2013)

My guess is that Honda has better control over their injection molding process and is confident they can hold the orifice diameter to a very tight spec. The Aftermarket poroducers do not want to invest in the process and find it easier (more reliable) to over mold a machined brass insert. But I am just guessing. I would be interested to know what is really going on.


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## TX610 (Jan 15, 2021)

ChrisJ said:


> I'm guessing the brass is easier to machine.
> So ultimately the brass one could be a down grade though I'm not sure how or why.


I would guess the opposite. Gasoline is a solvent so is ethonal so brass would be the desired material as most jets are. Plain plastic is cheaper to manufacture.


Toro-8-2-4 said:


> My guess is that Honda has better control over their injection molding process and is confident they can hold the orifice diameter to a very tight spec. The Aftermarket poroducers do not want to invest in the process and find it easier (more reliable) to over mold a machined brass insert. But I am just guessing. I would be interested to know what is really going on.


I think like everything it's about the bottom line. Get it past the warranty that's good enough for them.


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## ChrisJ (Nov 27, 2014)

Gasoline and ethanol doesn't bother some plastics.


Polypropylene isn't effected by either at all. It certainly doesn't dissolve them. Think about plastic gas tanks and intake manifolds.

The only example of plastic being better than metal I can think of which probably isn't relevant here is in pumps. Plastic pumps will tolerate cavitation much more than metal ones


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## TX610 (Jan 15, 2021)

ChrisJ said:


> Gasoline and ethanol doesn't bother some plastics.
> 
> 
> Polypropylene isn't effected by either at all. It certainly doesn't dissolve them. Think about plastic gas tanks and intake manifolds.
> ...


I was more in line with the abrasive effects of liquid passing through the plastic jet at high velocity.


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## ChrisJ (Nov 27, 2014)

TX610 said:


> I was more in line with the abrasive effects of liquid passing through the plastic jet at high velocity.


That I don't know.
Like I said, plastic pumps tolerate cavitation better than metal ones.

But I honestly have no idea. I have to assume Honda did their engineering and found the plastic worked. Outside of that I really don't know which is better.


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## ChrisJ (Nov 27, 2014)

If I had to guess. I would think you're right.

But I've never heard of a Honda GX running too rich idling.


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## TX610 (Jan 15, 2021)

ChrisJ said:


> That I don't know.
> Like I said, plastic pumps tolerate cavitation better than metal ones.
> 
> But I honestly have no idea. I have to assume Honda did their engineering and found the plastic worked. Outside of that I really don't know which is better.


I agree who knows their reasoning.


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## tadawson (Jan 3, 2018)

Just speculating, but, unlike brass, plastic can't corrode. Wonder if they went plastic for that reason . . .


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## tadawson (Jan 3, 2018)

TX610 said:


> I was more in line with the abrasive effects of liquid passing through the plastic jet at high velocity.


This is pilot jet, not main, and the orifice is tiny, so I doubt there is much flow, but ???. My only experience with these was shrinkage making my engine go lean and surge . . .


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## TX610 (Jan 15, 2021)

tadawson said:


> Just speculating, but, unlike brass, plastic can't corrode. Wonder if they went plastic for that reason . . .


I would say totally economics. Brass has been used for years. Brass is reliable easy to machine. Plastic mouldings is cheap once set up.


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## Spindler (Aug 19, 2019)

ChrisJ said:


> I don't recall of the idle adjust controls fuel or air, I think it's fuel. From what I recall, counter clockwise is richer.
> 
> There's a very tiny vent hole on the side of the carb near that screw. Very, very tiny.
> I don't recall it's purpose, but I do know if it's clogged it causes problems.
> ...


I believe that is a float bowl vent.


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## Toro-8-2-4 (Dec 28, 2013)

Spindler said:


> I believe that is a float bowl vent.


It sure is the bowl vent. You can clean it out with a tiny wire.


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## Johner (Dec 30, 2020)

badbmwbrad said:


> This is a recurring problem on an 8 HP Tecumseh Snow King engine. The engine date is circa 1988. The Machine runs fine for 90 minutes then it starts surging. Fixing it involves removal of the spring-loaded idle mixture (?) adjustment screw which threads into the carburetor body from the side (screw hole axis is perpendicular to venturi axis). I spray inject carburetor choke cleaner into the empty screw hole, stretch the spring to give it more tension and reinstall the screw.
> 
> In addition, the float bowl is removed and the two bowl nut orifices are verified clear (they're always clear). I also spray choke cleaner up through the main emulsion tube in the center of the carb body.
> 
> ...


Might have some moisture some place and freezes in the jet . Try using some muffler heat to keep it warm.


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