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I have a 14 year old mower and that been actually fairly reliable up till this year. Started to pop thru the carb earlier this spring but seemed to clear up as I mowed. I thought it was bad gas from storing it over the winter. Decided I needed to do some maintenance so replaced the fuel lines, filter, changed oil, plugs, and rebuilt the carb. Started and ran reasonably well for one mowing. Last week went to start it and the engine was flooded with gas in the oil. Replaced the oil and checked the carb float again. Used a heat gun to slightly adjust the float so it would seat a bit better. Looks like it's sealing well now but since that episode it's been hard to start and pops out of the carb even while at idle.

Checked the flywheel key by removing the entire flywheel to verify the key was not sheared. Verified that removing individual kill wires from the coils or both of them did not help. Set the valve clearance to .005 (was not all that far off to begin with). I don't have a working compression gauge but will have one in a day or so. Feels like I have good compression on both sides. I verified the pump is pumping fuel. I'm about to pull my hair out as I have checked about everything I can think of. I still need to verify the valve lift to see if the cam lobes are worn. Popping through the carb would seem like the valves may not be seating or something odd. I understand that the guides can move. How to test for that?

Again my compression gauge will be here in a day so I can check it then. Anything else I may have overlooked?

The valve guides can be inspected visually. They will move out toward the rocker arm. Look down inside the valve springs to see if the top and bottom look like they are about the same location from the valve keeper. Also if you removed the flywheel to inspect and didn't get the flywheel properly torqued upon reassembly it will shear the flywheel key on shutdown.

You can also do a dirty test for worm cam lobes by just removing the valve covers, and spark plugs,and turning the engine over by and and watch the valve rocker arms, they should lift the same amount. Normally when the cam lobes go bad it is evident. One valve will open like 1/2 of a inch and the other will open 1/8 of an inch.

Well I inspected the rocker arm movement with a dial indicator and got 0.312"/0.308" at the valve on the left side and only 0.280/0.282 on the right. I was careful to make sure the valves were properly adjusted prior with a .005" feeler. The motor spins over very tight like there is no compression release at all. My battery is slightly weak but If I turn the motor over manually to avoid the high compression section it will have enough speed to turn the motor over freely.

I just did a compression test and both cylinder measure about 165PSI which is fairly good for a 14 year old twin. It will spin over and pop thru the carb but just not start. I tried different gas and no change. I re-did the test where I removed the kill wires from the coils and no change. Spits and pops but will not take off. I've never seen a motor be so Jekyll and Hyde!

The valve movement may indicate that the cam really is wearing. I didn't look inside to see the valve guides but the compression check seems to indicate they are OK. If the cam wears... can it affect the compression release so it's not working?

Any Other ideas? Short of tearing it down anything else I can check?

BTW - I did torque the main flywheel nut down properly and it acted the same before and after. No damage to the key.

that 165 seems high considering that engine should only put out about 110-120 max due to its compression ratio. 70-90 would be more realistic for a compression release engine. You said you adjusted the float height. Is the float sitting level with the carb body when the carb is upside down, or if it has the spring loaded float valve level with carb body when needle is seated when the carb is turned sideways.

Also if the new plugs where sitting in gas due to the leaking carb, the plugs could be fuel shorted and could be now faulty. So that may be something to consider.

ILENGINE said:

that 165 seems high considering that engine should only put out about 110-120 max due to its compression ratio. 70-90 would be more realistic for a compression release engine. You said you adjusted the float height. Is the float sitting level with the carb body when the carb is upside down, or if it has the spring loaded float valve level with carb body when needle is seated when the carb is turned sideways.

Also if the new plugs where sitting in gas due to the leaking carb, the plugs could be fuel shorted and could be now faulty. So that may be something to consider.

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The float height when flipped upside down was nearly level to slightly toward the carb body. I read you can use a heat gun to slightly tweak the float needle guide to engage it a bit sooner. I did that modification but I still saw gas exiting the large venturi's in the Mikuni carb after it was sitting for a few minutes. It's odd since the solenoid on the bottom of the carb bowl seems to be working fine (tested for both open and close). I don't think that solenoid will help if the float is not sealing well. I removed the carb last night again and inspected the seat. Looks OK but I did rotate the needle in the seat bore to see if that will help lap the tip of the needle to seal better. I may need to replace the seat as well? Difficult to replace that seat?

Good tip on the plugs. They were soaked at several points and perhaps that is contributing now. I can take them out and hit them with a torch to burn off any contaminates. They were new this spring when I did the carb tuneup. I sounds like I'm a regular hack with all the troubles this thing is giving me but honest I have a number of years of working on small engines under my belt. I put myself through school as a mechanic for a small engine shop many moons ago. Frustrating since it seems there are several compounding issues going on at the same time. Just need to whittle down the list and get the thing running again.

the newer plugs have porous ceramic material that will short out after being soaked with gas. Even though you heat the surface to burn off the fuel it just doesn't get the stuff inside the ceramic electrode.

ILENGINE said:

the newer plugs have porous ceramic material that will short out after being soaked with gas. Even though you heat the surface to burn off the fuel it just doesn't get the stuff inside the ceramic electrode.

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Double ditto on that one.
To get them hot enough to burn off the contamination you also burn off the plating on the threads so the plug will tend to bind in the plug hole.
Once to convince a customer it was his plug we heated it up with the oxy and got a yellow flame at the tip. it burned for over 10 minutes.
Reheated and it went another 5
Pulled a brand new plug out of the box, heated it and got nothing.
He left shaking his head having just bought $ 300 worth of new parts to replace is perfectly good parts when all the time it was a $ 7 spark plug.

Way back in my teens when I really did not understand what I was doing I bought a Colortune.
For the next 49 years it became the backbone of my tool kit till I had attained the knowledge to "hear" what is wrong ( or right ).
It still comes out when I have a problem that has me beat.

I had a 11 hp Briggs/Stratton engine engine doing the same thing and all it would do is pop back through the carb . I did everything and then I decided to take a plug out of a working engine . It fired right up . This took me 2 years before I tried this . Take it from me I checked the plug on the outside and it would fire but when I put it back in it would just pop and that was it . I Will never make this mistake again . Replace the plug.