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?

   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.

I would think it would be a valve type issue also.But is it possible its some type of failure to the mechanical compression release the engine has?  Seems to me i saw a thread some where last year that had a 6hp tecumseh with a like type problem.And it had something broken to do with the mechanical compression release system.

  The problem seems to be with the intake where smoke or mist was comming out.  Like most, this one has MCR is on the exhaust which seems to be lifting the exhaust valve at the right time so I think is fine.  Got the valves done and hopefully will button it up this afternoon. 

   Buttoned up, fired up and it's about the same.

   When I ran it before I did not muck with it much.  It was run, had smoke out of the carb so I stopped and opened it up to get the valves.

   The stems seemed fine, no gum or buildup. 

    On firing it up after lapping there was no smoke and it ran well.  While adjusting the carb it broke into the smoke mode.  It's like a backfire out of the carb throat with white smoke which is from the bore.  The backfire is regular and always there each cycle.  If the engine is throttled down the backfiring will stop and it will idle fine.  It can be snapped to full speed and the response is very quick.  At other times snapping the throttle will make it go into the rapid backfire mode.  Going low throttle clears it. 

   While running at full speed well it will break into backfiring when adjusting the high or low limits of the high speed jet.  The sputter from running too rich or too lean is just enough to make it go into backfire mode sometimes. 

   If it's running fine and some choke applied to make it run rough that also will cause it go into backfire mode.  Going off choke and reducing the speed will clear the backfiring.   Going to high speed quickly wlll cause it to go to backfire mode about 50% of the time.  Going to full speed slowly usually does not cause it to go to backfiring.  It can go along at full throttle for quite a while clearing a heavy load with minimal engine sag.  It runs at times like the engine is very strong.  Occasionally it will break into backfiring though but clear quickly with lowering the throttle.

   Causing the engine to run rough makes it go into backfire mode.  One time when backfiring out of the throat I put my and up to the exhaust stream.  The stream was not the usual puffs but a steady stream of air.  The exhaust valve must have been stuck partly open for that. ?? On the intake side the backfiring is always puffs and never a steady stream of smoke.  It's a steady regular series of puffs. 

Later:

    Checked the exhaust when it's getting white smoke out of the carb.  The exhaust stream is not steady but in puffs as it should be.

Added Later:

    The engine is not stuck at half throttle as mentioned in prior posts.  It can run in the backfire mode at idle or close to full throttle.  When it's in backfire mode speed can be changed and the backfiring continue during the change.