Small Engines (Lawn Mowers, etc.)/Follow up 8hp Surging

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QUESTION: Hi
The system wouldn't let me ask another question last time. The model number is 71-52118-1 Serial number is 101991-1296 Craftsman 8hp 25in wide snowblower. I have taken the cylinder head off and have piston at top dead center both valves are open. Intake valve is carboned up and exhaust is white in colour. What is the next step?  Thanks

ANSWER: You can remove the valves and clean them but you really should lap or cut the seats to make sure they are sealing.  I used to just lap before I had a cutter...lapping works just fine but it takes more time.  Once lapped or cut, you will have to re-gap the valves.

There are several You Tube videos on lapping valves.

https://www.youtube.com/watch?v=6BX5ab12-Wk

If the URL does not work just search You Tube for "valve job on tecumseh engine part 4".

What is the clearance on each valve?

Eric



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valves
valves  
top dead center piston
top dead center piston  
QUESTION: Hi
I have attached pictures.I can't get the feeler gauge in between the valve stem. The first picture shows piston at top dead center and both valves open and I can turn them with my fingers

ANSWER: Is the piston at TDC on the compression stroke?  Just turn the engine by hand, follow the strokes, intake (piston goes down, intake valve opens, compression (piston comes up intake closes), power stroke (both valves closed, piston goes down), exhaust (exhaust valve opens piston comes up).

Turn the engine for a couple of cycles following the strokes.  After a couple, stop at TDC on the compression stroke and check the clearance.

BTW, what you are feeling is valve overlap.  Overlap is normal on all 4 cycle engines at one of the TDCs...if there is no overlap on one of the TDCs then the mechanical timing is off and the engine will never run.  

I believe you are on the overlap TDC which is why you can not get the feeler gauge in to measure the clearance.  You should be able to rotate to the next TDC (compression) and both valves should be closed.  One valve may be open slightly, use your fingers to check, as this could be the compression release which keeps one valve slightly open so you can pull the starter to start the engine. Once running the cam is spinning so fast the compression release has little impact on the power or efficiency.

Eric

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top dead center piston
top dead center piston  
QUESTION: ok.....That makes sense I will cycle it through  I downloaded two of the same pictures by mistake here is the original top dead center picture with valves up

Answer
I have not noticed that much valve overlap but I usually do not visually look at the overlap...I just feel with my fingers to see if I can rotate the valves then I know overlap is there.

I'd cycle to the next TDC and see if you have any clearance.  Also, you might try using a small screwdriver and making sure the valve tappets are full resting on the can...just make sure nothing is holding the tappets up which is causing the valve to be open.

Eric

Small Engines (Lawn Mowers, etc.)

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Eric A. Jones

Expertise

Lawnmower Repair . Certified Master Service Technician from B&S. Have 23 years experience on B&S, Lawn Chief, Weed Eater, Echo, Peerless, Wheel Horse, Snapper, Atlas, MTD, McCulloch, Homelite and many other numerous brands. Specialize in electrical repair.

Experience

Born and raised in the midwest. Started tinkering with engines when I was about 14 on my Suzuki RM-80. I began lawn mower repair at a small hardware store. I knew absolutely nothing. I read lots of repair manuals and met an older fellow who taught me many lessons. I continued working on small engines through high school and paid my way through college working on mowers at the same hardware store. Decided to get away from the midwest and mower repair so I joined the Air Force. I repaired air traffic control electronic equipment and ended up in Hawaii where I got a part time job at Small Engine Clinic. I gained a lot of experience from the Small Engine Clinic and had a blast repairing small engines. I then took the Briggs and Stratton Master Service Technician test and earned my MST. I then traveled to Wisconsin where I attended the factory update training seminar and received formal training. Continued working on mowers part time as I completed 20 years of military. Retired from the military on a Friday and continued in the lawn and garden industry the next Monday.

Education/Credentials
MAS Aerospace Operations BA Mathematics AAS Electronic Communications AAS Electronic Technology

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