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?

from what u described l would bet money thats its a bad valve spring, especially if it wont increase rpm.

  When backfiring through the throat it will only go to half throttle which is reasonable.

   When throttled up slowly to full speed it can run there for several minutes and not go into backfiring.  The throttle can be snapped to full and the response excellent.  If it's chewing a load but running even it runs fine.  It's stumbling that causes it to go into backfiring.  If it's given a bit of choke that will kick it into backfiring.  When the choke is backed off it will stay in backfire mode.  The engine has to be throttled down which causes it to clear and run fine.  The throttle can be run low to high several times and it will not go into backfiring. 

   Tecumseh carbs are not the best for smoothness and have put often.  I think when the engne is running well but gets a put it triggers the backfiring.

My bet is its the ign system.Have you wet the coil wire with water? The problem could be there or the ign ssi or coil or what have you is defective or weak.I see more people over look thiese type of problems in cars or what have you.A coil does not have to be dead to be no good.When the coil was on the way out on my 6hp i had several seasons of poor cold starting and intermitent performance issues.Than no start and no spark.Replaced the points and condenser which were not that old and still same condition.Put in a replacement coil and its 1 pull starts and runs great. Now i could have played with this engine endlessly before the coil died.Like its a1968 engine and original maybe the compression is low thats why it does not start right up.Maybe may valves are sticking or what have you when it started to miss under load intermitently.Or gee the carb may be screwed up even thow i went threw it and rebuilt it.  If you get my drift !   You have made all the mechanical and fuel system checks !  unless something was missed. there is only one other way the finger points !

   Lapped the valves - no change.

   Installed a new intake spring - no change.

   Installed a new exhaust spring - no change.

   Installed a new exhaust valve - no change.  (There was a slight wiggle when the stem was in the seat.  The stem was about .001 - .0015 thinner than a new stem).

   The engine did make unusual noise, not loud but different and it seemed internal.  I thought it might be due to the backfiring as it cleared when the engine ran smoothly which it did occasionally. 

   Today sitting on a stool beside it and running the throttle up and down slowly to make it go into smooth mode I noticed something moving in back of the muffler holes.  The baffle was loose so could float around inside the muffler.  On this muffler the baffle could completely cover the exhaust holes.  Changed the muffler and the problem cleared.   The muffler was the smaller horizontal type as opposed to L types or vertical fat ones.  I had a number of engines with broken baffles but never had a problem like this before so this one was a first.  Spent oodles of time on this engine. 

Ouch!

Nothing like putting tons of effort into something to find it was a five minute fix.   Hate when that happens.

Good info nonetheless.

All's well that ends well. 

In retrospect it makes sense now.  It wanted to fart but could only burp!   ;-)
Glad you found the problem.

good to see u figured it out:)..l've seen it happend once before but my engine wasnt intermittent it was block all the time. it happens all the time with the 2 strokes. ya l realized that after l posted it being a bad valve spring, that wasnt likely because of it being intermittent. anyhow good u got er solved.

I bet it runs good with that mini-overhaul.... We've all been there / done that before. Friiy