# Briggs OHV - Too much compression?



## wdraike (Jul 10, 2010)

I have a Brigg and Stratton OHV engine Model 121312-0145-E1.

I just replace a broken connecting rod and reassembled the unit.

I believe I set up the cam timing correctly but notice the following when I try to start it: With the spark plug out, I can have not trouble pulling the starter cord and see everything is moving well, including the piston. With the spark plug in, it becomes difficult to pull, especially during the compression cycle, and it won't start. I'm guessing this is due to too much compression.

I have tested and see that I have a spark and I also installed a brand new carburetor.

I would appreciate any help on what I should do to address this.
Thanks in advance.


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## Basementgeek (Feb 7, 2005)

Hi and welcome to TSF :wave:

My first guess that the valve timing is off

BG


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## wdraike (Jul 10, 2010)

I wondered about that too. The marking on the drive shaft gear is pretty clear and that gear is keyed. The cam marking is less obvious to me, so I chose what I thought was the right marking (a square mark with an indent) . Should I have used a different mark for the alignment?

Here is what I am seeing with it is aligned this way:
- During first down-stroke of the piston the valve to the carburetor opens
- During the up-stroke all valves are closed (this is where I think the compression is causing me problems)
- During the next down-stroke all valves are closed
- Then during the next upstroke the exhaust valve opens.

The spark is driven off a magnet on the fly wheel, which is keyed to fire on the up-strokes.

This sequence seems right to me, but maybe I'm missing something?

Any help is appreciated.


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## kbowley (Sep 23, 2009)

Did you check the compression release...it is on the exhaust valve cam lobe. during the compression stroke about 1/4 from bdc the exhaust valve should slightly open for 20 degrees or so than close for the completion of the stroke. it is controlled with weights so as the engine reaches about 300 rpms, the weights will contract the lobe for the exhaust valve so the engine runs properly and yet below 300 rpms allows some compression to escape so the engine is easier to start. I would check for proper operation, installation such as the weights and springs...let us know.


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## wdraike (Jul 10, 2010)

I am seeing the slight bump in the exhaust valve during the compression stroke. I have tried some adjustments of the valves to see if I could get it running. I am able to turn the engine more easily now, but still can't get it to fire up.
I'm seeing spark on my plug but starting to wonder if its good-enough. Thoughts?
Thanks in advance.


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## kbowley (Sep 23, 2009)

Check the gap between the flywheel and magneto (ignition module), it should be .008-.012 thousands...its kinda hard to set because the magnets are strong and will pull the magneto into the flywheel as soon as you loosen the mag. A trick many use; get a book of matches, turn flywheel so that the magnets are not aligned with the mag...slightly loosen nuts holding mag just enough to allow it to slide, place the unfolded matchbook between the mag and flywheel, turn flywheel so magnets are aligned with the mag and allow it to be pulled in to the magnets with the cardboard from the matchbook separating the two, tighten mag bolts and turn flywheel to remove the cardboard. Gap should be very close, certainly close enough to run if it was done correctly. try and start.


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