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I have a B&S engine (10T502-0457-B1) that when I pull it a couples of times it seems ok but then when I go to pull it again it will violently snatch the pull rope back. I checked the flywheel key and it seems to be fine. What else would possibly cause this reaction. Any help would be appreciated, If I get it to start it will run but it will pop every now and then. Hope you guys can help. Thanks.

could be valves out of adjustment, flat head engine so grinding the end of the valves a little at time usually opens the gap back up....
Intake 005 to 007, exhaust 007-009, thousandths of an inch.



the flywheel key should make a perfect a square in the key slot... if any doubt, remove the flywheel and l o o k.

Slight change to Scrub's posted valve specs. The following is per Briggs Power Portal

Also camshaft that has mechanical ACR that can fail. So look for the bump of one of the valves (I believe it is the exhaust on this model) just TDC compression stroke.
Also has plastic on steel cam lobes that slip out of place.

Scrubcadet10 said:

could be valves out of adjustment, flat head engine so grinding the end of the valves a little at time usually opens the gap back up....

10A000 Thru 10M000

L-Head Aluminum/Cast Sleeve Single Cylinder

.005 / .007

.007 / .009

Intake 005 to 007, exhaust 007-009, thousandths of an inch.



the flywheel key should make a perfect a square in the key slot... if any doubt, remove the flywheel and l o o k.

Click to expand...

Thanks for the info, i will check that.

StarTech said:

Slight change to Scrub's posted valve specs. The following is per Briggs Power Portal

Intake Valve	.004-.006 in (.10-.15 mm)
Exhaust Valve	.007-.009 in (.18-.23 mm)

Also camshaft that has mechanical ACR that can fail. So look for the bump of one of the valves (I believe it is the exhaust on this model) just TDC compression stroke.

Also has plastic on steel cam lobes that slip out of place.

Click to expand...

Thanks, I will check that.

Sounds like you are using an engine from a push mower. If so, you will have to find another flywheel for it, as the pushmowers have a much lighter flywheel.

I assume you have the engine on a push mower with a blade on it. Things that can cause kick back are a partial sheard key and a loose blade. Valves in a briggs L head engine vary rarely need the lash adjusted unless valves are worked on. If the engine does not have a blade on it will start and run but can kick back trying to start and sometimes have a rough idle. The blade augments the lighter flywheel on mush mowers.

It is a two wheel string trimmer. And that engine could have the light flywheel; although, I would think the OEM would selected the heavy version. Easy to tell if light weight weight as it will aluminum vs cast iron.

I just reread the original post. I missed the string trimmer thing. It probably does have a heavy flywheel. I built my own string trimmer using a push mower engine with the aluminum flywheel. Works ok but a heavier flywheel would help.

Fish said:

Sounds like you are using an engine from a push mower. If so, you will have to find another flywheel for it, as the pushmowers have a much lighter flywheel.

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It is the Original engine that came on the Poulan Weedeater WT4000C.

I would also pull the head and decarbon, excess carbon will raise the compression ratio a lot.

Update: I pulled the head off and took the cover off the valves. I checked the valve clearance with the piston 1/4 past TDC on the combustion stroke. the Intake clearance was .005 and the Exhaust was .007 and neither vale would turn or move on head. So i think they are good. The top of the piston had a lot of carbon buildup as well as the inside of the head. So i cleaned both real good and am about to put it back together unless some else has an idea.

Fish said:

I would also pull the head and decarbon, excess carbon will raise the compression ratio a lot.

Click to expand...

Did that and there was a lot of buildup. checked valve lash both were within specs. Intake 5 and Exhaust 7, (thousands)

Built this string trimmer out of a push mower about 20 years ago. Has the lightweight aluminum flywheel.

Hammermechanicman said:

View attachment 52983
Built this string trimmer out of a push mower about 20 years ago. Has the lightweight aluminum flywheel.

Click to expand...

looks like it will do the job.

No, pop the head back on and see what happens.

Was the filter real dirty?

Can you take a pic of the valves and cylinder wall?

Was the carbon powdery/chalky, or black and oily?

Hammermechanicman said:

I assume you have the engine on a push mower with a blade on it. Things that can cause kick back are a partial sheard key and a loose blade. Valves in a briggs L head engine vary rarely need the lash adjusted unless valves are worked on. If the engine does not have a blade on it will start and run but can kick back trying to start and sometimes have a rough idle. The blade augments the lighter flywheel on mush mowers.

Click to expand...

Sorry it has taken so long to get back with you. Once I put the adapter on the crankshaft and installed the new belt after checking everything else the motor started and there was no bad recoil. thanks for your help.

Fish said:

Can you take a pic of the valves and cylinder wall?

Was the carbon powdery/chalky, or black and oily?

Click to expand...

Hi, it seems the engine was fine, the problem was that without the adapter on the crankshaft and the operating belt installed it would recoil bad. Once those where installed it didn't recoil badly. Thanks for your help. Sorry for taking so long to get back with you.