Compression issue after rebuild

[Emsguy]

Do you know if the cam shaft style would make a difference? My old one is rounded at the end while the new one is slotted. From what I understand, the slotted one is also used for an engine with an oil pump.
New one

View attachment 63040

Do you know if the cam shaft style would make a difference? My old one is rounded at the end while the new one is slotted. From what I understand, the slotted one is also used for an engine with an oil pump.
Old one
View attachment 63040

New one

 

[thesilentone]

Worth a watch.

 

[StarTech]

As long as it is the 793880 superseded to 84005207 no it doesn't. Briggs went the oil pump only version to reduce inventory stocking.

 

[Bertrrr]

Pull the flywheel and see if the key is sheared or even partially sheared , this will throw off the ignition timing ,backfiring will contribute to this

 

[Emsguy]

Pull the flywheel and see if the key is sheared or even partially sheared , this will throw off the ignition timing ,backfiring will contribute to this

Checked it already, it’s fine

 

[Hammermechanicman]

I will throw the old $0.02 worth in.
Make 100% sure the cam to crank gear timing is correct
Make 100% sure the valves are being set properly. One valve open and adjust the other.
After valves are adjusted with spark plugs out slowly rotate engine and see that all valves open the same amount.
Double check valve lash.
Make 100% sure the flywheel key is intact and set coil gap to .010".
Install new spark plugs.
Get a can of starting fluid and with choke off and throttle at max spray about a 1 second shot into the intake and try to start engine. If everything is good engine will fire up for a few seconds. If it doesn't tell what it does.
If it doesn't start the next step would be a leakdown test but most DIY guys don't have one.

 

[Emsguy]

I will throw the old $0.02 worth in.

Make 100% sure the cam to crank gear timing is correct
Make 100% sure the valves are being set properly. One valve open and adjust the other.
After valves are adjusted with spark plugs out slowly rotate engine and see that all valves open the same amount.
Double check valve lash.
Make 100% sure the flywheel key is intact and set coil gap to .010".
Install new spark plugs.
Get a can of starting fluid and with choke off and throttle at max spray about a 1 second shot into the intake and try to start engine. If everything is good engine will fire up for a few seconds. If it doesn't tell what it does.
If it doesn't start the next step would be a leakdown test but most DIY guys don't have one.

welp, I guess I have some positive news. I now have a large back fire from the exhaust with a ton of white smoke when I have the throttle lever on the “choke” position. Other than that still won’t start. Spark gap in the service manual says .30 which is what I have it at. I’ve adjusted the valves now about 7 times. I’m running out of ideas. I’ve checked the cam gear twice for timing and I corrected it the 2nd time I opened it.

 

[bertsmobile1]

In cases like this I resort to a timing tape
In this case it is a piece of masking tape wrapped around the flywheel
Then with the piston at TDC ( or full front if you like ) I run a fine point pen down the side of the magneto laminations
I keep rotating and mark inlet opening & closing + the same for the exhaust + BDC
Then I peel it off & lay it on the bench
The distance between open & closed for each valve should be roughly the same and there must be about 1" of overlap at BDC .
Check that the coils are mounted the right way round ( usually kill wire is underneath )
If you don't know what you are doing then it is best to go by the book
The book says to turn the engine till it is on TDC compression stroke then go 1/4" back down which would be the firing stroke
TDC is when the piston is closest to the spark plug end of the cylinder
There are 2 on them on a 4 stroke
After the inlet has closed = firing stroke
When the exhaust is open = exhaust stroke
So forget what some smart arse says on boob tube.
Do it BY THE BOOK till you know what yo are doing & why .

 

[Emsguy]

In cases like this I resort to a timing tape
In this case it is a piece of masking tape wrapped around the flywheel
Then with the piston at TDC ( or full front if you like ) I run a fine point pen down the side of the magneto laminations
I keep rotating and mark inlet opening & closing + the same for the exhaust + BDC
Then I peel it off & lay it on the bench
The distance between open & closed for each valve should be roughly the same and there must be about 1" of overlap at BDC .
Check that the coils are mounted the right way round ( usually kill wire is underneath )
If you don't know what you are doing then it is best to go by the book
The book says to turn the engine till it is on TDC compression stroke then go 1/4" back down which would be the firing stroke
TDC is when the piston is closest to the spark plug end of the cylinder
There are 2 on them on a 4 stroke
After the inlet has closed = firing stroke
When the exhaust is open = exhaust stroke
So forget what some smart arse says on boob tube.
Do it BY THE BOOK till you know what yo are doing & why .

Thanks for the tip. Does this sound like valve issue to you? With the back firing and white smoke?

 

[thesilentone]

Thanks for the tip. Does this sound like valve issue to you? With the back firing and white smoke?

This must be getting pretty frustrating.

If everything ' mechanical ' is OK, as has been discussed, timing, valve clearance etc, then time to look elsewhere.

You have a spark, so the only thing left is fuel.

How clean in the fuel you are using ?

Pour a little petrol into the inlet (not to much), then start the engine, it should run OK until that fuel has gone.

If it's still back-firing and spluttering, try another new spark plug.

If the problems still exist, you are doing/have done something wrong.

Just a final reminder, the valve your adjusting must be fully CLOSED.