Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

Issue about vapor or smoke through the carb.

 

A 7hp Tecumseh L-head went to half speed while clearing and could not be throttled higher.  The engine was in parts when I got it so I can only go by reports about that.  I thought it might be carb related so went through the carb. These was gas in the float so I put the head back on and fired it up. 

 

It still only ran to half throttle. Seems to run somewhat well at half speed and lower but does sound not “usual”.  There seems to be an extra snap sound (non-metal) which I think is coming out of the carb but not sure.  It could be from the muffler. 

 

There is either smoke or vapor out of the carb.  Not a lot and disburses quickly.   I thought it could be timing causing the intake to not be closing fully.  The engine has solid state ignition so I thought it might be a broken flywheel key.  Pulled the flywheel and the key is fine.

 

It seems to be the intake is not seating fully.  What I don’t understand is how can that happen instantly.  The machine has run well through the prior storms and in the middle of the last storm it went to half throttle.  I was told it was spitting gas out of the throat but what I see is vapor or smoke.  It’s not much or a strong flow.  

 

The valves are at .008 intake and .010 exhaust.  The spec is 8 and 12.  The next move is to lap the valves.  If that does not cure it then possibly there’s a crack somewhere in the head under the valve head.  ?? Anyone ever have something like this or suggestions if not seat or wall crack??

If a valve problem is it usual for it to show itself all of a sudden?

Try setting the intake valve at .010 and the exhaust at .012 just to see if it's an issue with clearances.  It's always best to set valves toward the maximum clearance.  That way, if they tighten up a bit, the valves will still close.   If that doesn't work, there's probably a problem with the intake valve or valve seat.

Could just be a sticking valve -check the stem and guide.  Does the valve travel full length freely in the guide when its apart?

Looking through the plug hole they were ok and later with the head off the same.

Springs are ok.

   Check the engine by doing  a leak down test....  see if you have air howling through the closed intake valve..

... The valve could be gummy-----the stem might have bad gas residue dried on it that is slowing its travel ( have seen this on the intake before-lots and it gets worse as the engine gets hot...)

Try spraying a penatrating oil on the intake stem through carb or throught the spark hole with the valve open, let it sit for a while and turn the valve face with a popsicle stick to help wash it around...

Let us know what you find..

Friiy

I had a problem similar to what your describing one time.  Checked everything, adjusted valves, pulled head a couple of times. Opened up the crankcase checked in there.
Went over the carb. numerous times.  And in the end it ended up being a problem with the carb. First time I ever had to replace a carb.  Maybe your problem is something different.
But just thought I would mention it.

Jtrebor,

   The carb was rebuilt a few months ago and got a touchup cleaning yesterday before testing.  It was run very rich and would not produce black smoke.  I figured it was not firing fully with the vapor/smoke and was the valves so took it apart again. 

 

Friiy>>>  Check the engine by doing  a leak down test....  see if you have air howling through the closed intake valve..

   I got it as parts so could not do a leakdown .   Going by the symptoms I was told the carb was gone through and the engine reassembled and run.  It seemed like valves so I opened it again.  It was a mistake at that point to not have done a confirmation leakdown test.  I should be able to lap the valves today and after do a leakdown.

The slap noise maybe the tappet with lots of play - going up to hit the valve that is slow or gummy..

Friiy

friiy wrote:
Or the piston tapping a valve that is hanging open.

CharlesW wrote:

They're on the block , not on the head on this engine..

Friiy

friiy wrote:
My bad.
Totally missed the "L head" in the original post.