Help: Rebuilt carb didn't cure it

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  #1  
Old 05-13-10, 12:26 PM
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Help: Rebuilt carb didn't cure it

I have a Murray with an 18-hp B&S engine that won't idle without being choked. As I retard the throttle toward idle, the RPMs start fluctuating (surging?) and it dies long before getting to idle RPMs. If I crack the choke about 1/2", it starts and runs fine but still won't idle with the throttle all the way to the idle position.

So I installed a B&S carb rebuild kit, replaced the air filter, the fuel filter and the main jet and adjusted the choke and throttle as per the owner's manual. I replaced the gas cap with a vented cap left the vent wide open.

After all that, the problem with the choke and idling is unchanged.

So what did I miss?

The mower is a 46" Murray Ultra 46901X192A

The motor model is 422707, type 1264 01, code 950 1195a
 
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  #2  
Old 05-13-10, 01:15 PM
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Did the governor adjustment get changed by chance? I know if that is off it won't run worth beans.
 
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Old 05-13-10, 02:40 PM
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If it runs normally (open throttle/no choke) except for the idle position of the throttle, you have a restriction in the idle circuit.

You can take it apart, move all the rubber/neoprene things to one side and soak it in a bath cleaner, then blow it out with compressed air or try adding Sea Foam to the gas. That will clean up a lot of things if it doesn't have a solid plug.

If you go the Sea Foam route keep it in there for a couple of tanks full. Frequently, if the fuel is flowing at all past the restriction you'll see improvement right away.
 
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Old 05-13-10, 02:54 PM
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Originally Posted by Honda 327 View Post
Did the governor adjustment get changed by chance? I know if that is off it won't run worth beans.
I have to admit I didn't even know it had a governor. I just checked and all the manual says about it is not to tamper with the spring or levers in an attempt to hot rod it. Short answer, I didn't but I can't swear some previous owner didn't.

There is an adjustment screw on the carb that the manual calls the "non-adjustable idle mixture jet." I take that to mean that you can twist it all you want and it won't change nuthin'. But I didn't touch it no how.

After about an hour and a half's use since new, the plugs are very, very, very pale tan, almost bone white. And there's what looks like soot on the top of the center electrode and the ring at the end of the threads facing into the chamber.
 
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Old 05-13-10, 03:00 PM
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Originally Posted by marbobj View Post
If it runs normally (open throttle/no choke) except for the idle position of the throttle, you have a restriction in the idle circuit.

You can take it apart, move all the rubber/neoprene things to one side and soak it in a bath cleaner, then blow it out with compressed air or try adding Sea Foam to the gas. That will clean up a lot of things if it doesn't have a solid plug.

If you go the Sea Foam route keep it in there for a couple of tanks full. Frequently, if the fuel is flowing at all past the restriction you'll see improvement right away.
I'm not sure it has good power even at full throttle but, with the choke off, it starts the surging business from about 3/4ths throttle all the way to it just died.

In the course of rebuilding it, I soaked the all the pieces overnight in MEK or PCP or FBI or one of them three-letter solvents. And I blew it out with compressed air as I was reassembling it. And I already put seafoam in the gas.
 
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Old 05-13-10, 04:20 PM
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Your carb is a fixed jet carb. No adjustments.

"the plugs are very, very, very pale tan, almost bone white"
This indicates a lean condition. Your carb is still plugged up. It needs to be soaked & cleaned again. Buy a Gallon of Hydroseal at your local auto parts store.
 
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Old 05-13-10, 04:30 PM
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Originally Posted by Fred_C_Dobbs View Post
I have to admit I didn't even know it had a governor. I just checked and all the manual says about it is not to tamper with the spring or levers in an attempt to hot rod it. Short answer, I didn't but I can't swear some previous owner didn't.

There is an adjustment screw on the carb that the manual calls the "non-adjustable idle mixture jet." I take that to mean that you can twist it all you want and it won't change nuthin'. But I didn't touch it no how.

After about an hour and a half's use since new, the plugs are very, very, very pale tan, almost bone white. And there's what looks like soot on the top of the center electrode and the ring at the end of the threads facing into the chamber.
can you remove that idel jet for cleaning. It just takes one little hole to be plugged. I just has a tecumseh push mower with gummy carb. Took aprt and cleaned with cleaner and air and back together it still only ran on primer gas or sprayed in carb cleaner. Took apart again and found the other little hole in main nut that was still plugged. Cleaned that and it runs great
 
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Old 05-13-10, 06:30 PM
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You have more of a problem than the idle circuit if the surging starts at 3/4 throttle and eventually it dies. You are pretty well plugged up all the way. It is apparently relying on the choke circuit to run at all.

You'll just have to clean it again.
 
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Old 05-13-10, 10:38 PM
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I agree, the carb needs more than just a kit, it needs a proper cleaning. Do not mess with the governor adjustments. It is set from the factory and should never need adjusting over the life of the engine in normal circumstances.
 
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Old 05-14-10, 04:40 AM
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Like everyone mentioned, look for all those tint tiny little holes in the carb and it's parts and make sure they are clean. You can pull a piece of wire of a wire brush and poke it in the tine little holes to make sure they are clean that's how small the holes are.

If you don't think the governor has been tampered with then by all means don't touch it, if it ran fine for you at one point it's you carb that is causing the problems.
 
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Old 05-14-10, 09:13 AM
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Changing out and adjusting what the manual calls the "non-adjustable idle mixture jet" fixed it.

The rebuild kit came with extra gaskets and other parts that my model of carb didn't need. Being such a noob, I pretty much had no idea which were the needed ones and which were the extras (I have rejetted quite a few 2-stroke carbs but that's the full extent of my carbonator expertise). As I was diassembling and cleaning, I just compared what I was taking out to what was in the box and replaced everything I could tell I had a replacement for. So that left me with a number of leftover rebuild kit parts but no way of knowing for certain whether I'd missed anything.

Your answers got me to looking really hard at that "non-adjustable" idle screw and the three plunger needles among my leftover kit parts. The one on the mower was gray in color, like pewter, but the three in the kit were brass. But I found it odd that they'd put a spring on an external screw not intended to be adjusted, so I pulled it out. The needle portion had corrosion buildup, and my micrometer eyeballs told me it was a very close match for one of those three brass plunger needles, so I swapped it out. It tried to start, even with the choke closed and at about half throttle, so I knew I was on to something. After a minute's fiddling with the "non-adjustable" idle mixture screw and the throttle's idle screw, it idles serenely at ~1000 RPMs with the choke completely closed.

I won't mow again for another three or four days but, provided my plug reads improve, I'll pronounce it well and truly fixed.

Thanks for all you're help, fellas!
 
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Old 05-14-10, 01:55 PM
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I bet he really means choke fully open or fully off.
 
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Old 05-14-10, 03:03 PM
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Originally Posted by 38racing View Post
I bet he really means choke fully open or fully off.
Choke operating button completely closed.
Choke butterfly valve fully open.
 
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Old 05-18-10, 08:45 AM
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I mowed again yesterday. After, the plugs were just a little light of the ideal cinnamon brown on one side of the insulator and a very pale tan on the other. I'm not accustomed to seeing a two-tone insulator, even in a side valve engine so I'm not sure what that means.

At any rate, it ran strong until about the last five minutes, when it started stumbling. There was half a tank of fuel remaining, the fuel cap vent was wide open, and I'd cleaned out the fuel tank and replaced the in-line fuel filter before putting it all back together, so I don't know what that's all about but I'll sort that out next time I mow.
 
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Old 05-29-10, 04:06 PM
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A couple of tanks of fuel spiked with SeaFoam have got this thing running like a spring buck. Now it has torque enough to pull up the steepest grade in my lawn where before I had to either run up it sideways across the grade or hit it with a big head of steam.
 
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