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Can't get engine to run rich, main adustment screw comes out first.


squireone's Avatar
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12-30-07, 06:52 PM   #1  
Can't get engine to run rich, main adustment screw comes out first.

I have an older Ariens snowthrower with an Tecumseh 5hp engine with an adjustable carb. When I loosen the main adjustment screw with the throttle in Fast position and choke fully open, the screw will actually unscrew completely (and fall out if I let it), without the engine running rich or smoking.

Technical details:
Ariens 922020 Body, 922024 Thrower
Tecumseh HS50 67008B engine
Series 1 carburetor code 568 6D21, part number 631914

I tried rebuilding the carb with kit 31840, spraying carb cleaner through the passageways, and replacing the float (it is adjusted correctly). The emolltion tube is not stuck as it rattles when you shake the carb. I did not pry off the welch plug by the idle adjustment screw, as I don't have an appropriate tool. The idle does allow you to adjust it until it runs rich (smokes), so it appears the idle is working correctly though.

The issue is that the engine surges at higher rpms. If I run the snow thrower at low rpm by loosening the high speed rpm adjustment screw, the engine does not surge, and it is possible to clear snow with it (heavy wet snow, putting it under load). However, it does not throw the snow very far due to the low rpm. If I try to speed up the RPM by adjusting the high rpm screw tighter, the engine surges. As the screw is tightened and speed increases the surges become more and more pronounced. I believe this is a symptom of the engine running too lean.

Why doesn't the main adjustment screw allow me to get it to run rich? What is wrong?

Thanks!

 
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12-30-07, 07:07 PM   #2  
When you talk about the RPM adjust screw, are you adjusting the idle speed or the govenor spring preload?? Did you soak the carb in a bath & blow out with a spray cleaner & compressed air?? I sounds like something is still plugged. Have a closer look "cus surging is usually a sign of a passage still blocked.Let us know what you find, Roger

 
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12-30-07, 09:38 PM   #3  
Posted By: hopkinsr2 When you talk about the RPM adjust screw, are you adjusting the idle speed or the govenor spring preload?? Did you soak the carb in a bath & blow out with a spray cleaner & compressed air?? I sounds like something is still plugged. Have a closer look "cus surging is usually a sign of a passage still blocked.Let us know what you find, Roger
High speed adjustment screw is the one that adjusts the governor spring.

I blew out everything with spray cleaner and compressed air. I didn't soak it overnight though.

Do you think it is one of the jets under the welch plug by the idle adjustment screw? I can get the idle to run rich, so it doesn't appear the idle has problems.

 
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12-30-07, 09:51 PM   #4  
what is the right tool to remove the welch plugs?

 
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12-31-07, 05:20 AM   #5  
Posted By: squireone High speed adjustment screw is the one that adjusts the governor spring.

I blew out everything with spray cleaner and compressed air. I didn't soak it overnight though.

Do you think it is one of the jets under the welch plug by the idle adjustment screw? I can get the idle to run rich, so it doesn't appear the idle has problems.
adjusting the tension on the governer spring does not effect mixture nor does the idle adjustment that contacts the throttle shaft. drop the bowl and see if there is a blocked removable main jet in the carb.

 
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12-31-07, 05:26 AM   #6  
If you're taking the idle mixture screw out when the engine is at full RPM you wont get it to smoke or run rich because that circuit is bypassed by the high speed circuit. There are three screws that can be adjusted, two of which change fuel mixture. The one at the bottom of the float bowl is the high speed screw and the one in the front of the carb (as you're looking straight at it) is the idle mixture screw with the (usually) idle speed being 90* to the left on the very top of the carb. No other screws are to be adjusted without major problem. The one misconception is that spray carb cleaner cleans everything. In fact, it can cause more problems than fix. The two welch plugs dont need to come out. Soak that carb (remove ALL parts you installed with the carb kit including the rubber needle seat if so equipped) in Napa #6402 or Gunk Carb & Parts cleaner for an hour at least and rinse with h2o. blow with compressed air then and re-assemble. The High speed screw should be out about 2 turns and idle screw about 1 1/2 turns. I know I probably babbled a lot but this should get you down the right path.

 
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12-31-07, 06:31 AM   #7  
No, I mean the high speed carb adjustment screw, referred to as the "main adjustment screw" in the tecumseh manual, located on the bottom of the carb bowl.

Flapshot: My theory is that as the speed of the engine increases, the flow of fuel does not increase due to a blockage somewhere, so as air flow increases, but fuel flow stays the same making the mixture become more and more lean at the higher rpms (and higher suction). Is this theory plausible?

I do believe something is blocked. I'll soak the carb, and see if that helps.

By the way, what tool is normally used for removing those welch plugs anyway?

Thanks for the replies.

 
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12-31-07, 07:09 AM   #8  
Posted By: squireone By the way, what tool is normally used for removing those welch plugs anyway?
I usually use a small standard screwdriver to remove them. Although I have yet to see a reason to replace them (there are no jets or moving parts behind them) I now just leave them in place.

 
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