Choke a Matic Crab Problem


Old 10-02-04, 07:21 AM
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Choke a Matic Carb Problem

Thanks in advance for any help or advice! I have a 11 HP Briggs & Stratton ,model 252707. Mounted on a Murray Lawn Tractor. Poblem croped up over time and got worse with carb adjustments.

Engine (cylinder block, head, intake and exhaust valves) are in great shape however, can not get the "choke a matic" carb adjusted properly. Have cleaned and rebuilt with new main jet, emulsion tube, needle valve, gaskets and mixture screws. Throttle and choke plates installed and work correctly.

Gas backs up in the throat and floods but not to the point of not running. Sputters and surges slightly at full throttle. Does tend to do slightly better when up to temp and under load. Idles good for a while but eventually gas will build in the throat and flood it out but only at idle.

I do know the proper procedure for mixture screw adustments but no matter where the main jet mixture screw is set it still will not smooth and even out.
I have confirmed that the gravity feed needle valve is closing however, not sure of the proper gas level. I assume that as long as the fuel level is over the pick up holes that the mixture screw should compensate the flow.

Thanks again for any help with a tough one!

Last edited by dave237; 10-02-04 at 03:38 PM.
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Old 10-02-04, 11:34 AM
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Make sure the choke is adjusted properly and is opening completly. If so see if using partial choke makes any difference.
Sounds like you may have an intake leak somewhere allowing un-metered air into the cylinder causing lean condition and may also explain why the mixture screw has no effect.
Two reasons it may run better at temp, as parts heat up they expand possibly sealing or lessining a leak, also at temp, the engine requires a leaner mixture, which you would already have if indeed there is an intake leak.
Old 10-02-04, 03:26 PM
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Thank you very much for your reply. I did consider the intake valve. I removed the head and cleaned. Both valves looked in very good condition considering it's age. Valves and seats looked smooth and even to the eye. No pits or burned spots. However, your reply makes me think....!

Maybe the valve spring could have become weak and does not apply enough tension on the valve to close it properly. Is this possible?

You can see gas blowing backwards through the throat especially when cold. This does deminish as the temp rises as you suggested in your reply and would explain why the conditon got worse with valve/seat cleaning. I will follow this idea with a valve job and new springs!

Yes, choke functions properly. Closed when choked and wide open at full throtle. Yes, with particular settings of the main mixture screw I can smooth the engine somewhat more by giving about 2/3 choke.

Thank you very very much for your help and consideration. Please. reply with any additionl thoughts or suggestions.

Old 10-03-04, 02:38 AM
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I agree with the valve possibility. Check the valve stem clearance. I bet you'll find that there is NO clearance, and the stem will need to be ground shorter.
Old 10-06-04, 01:42 PM
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Solution ,Problem solved

Azis and Cheese,

I thank you guys very much for your insight and most helpful suggestions and adivice. The problem I was having was an intake valve that was in steady contact with the tappet with the piston 1/4" past TDC.

I removed the intake valve and with a machined flat surface and 320 grit sand paper, removed enough of the valve stem to obtain .006 clearence between stem and tappet. I assume the tappet had worn outwards through the years.

Afterwards, all I needed was a carb adjustment and now she puuuurs like a kitten.

Thank you guy's again and if can be of any help please, feel free to ask.

Old 10-06-04, 03:09 PM
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Originally Posted by dave237

Afterwards, all I needed was a carb adjustment and now she puuuurs like a kitten.
does she purr like a kitten or roar like a new engine. great news that you have it running right now and keep er purring.

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