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?

   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.

Well  keep in mind the spark plug  fires on compression and exhaust strokes in these motors.The valves were out so they must look good not bent or anything crazy.The valve springs must be good.It has good compression i must think? you did a leak down test?  The mechanical compression release is fine.The carb is clean and good.The only thing left is the points, condenser and ign coil and timeing

trouts2 - Just curious. Is the engine running insanely rich?  Do you get smoke out of the exhaust that is fuel rich when it's in backfire mode?

relics,

     No leakdown yet but it runs strong, very little sag under load.  It snapes from idle to full throttle unless it goes into backfire mode.  I did not measure the springs but they felt fine.  Have lots here to replace with but felt it unnecessary.  When turned by hand MCR lifts the exhaust at the right spot.   No points, SSI.  No broken key - timing is fine.

jrtrebor,

    Runs fine, no black smoke.  Idles well and snaps to full throttle crisply i.e., zero hesitation or stumble.  <<-- (that is when it's not backfiring through the carb). The backfire is out of the intake and white smoke (no backfiring or black smoke from muffler).   When it flips into backfire mode (out of the carb throat) the muffler outputs black smoke, the carb white smoke.   The original carb was gone through a few times but I put a different carb on today just for yucks and everyting mentioned is the same.  The engine runs very well when not in backfire mode.

Chances are you have already checked this, but..........
Could it be such a thing as bad gas?
Maybe water in it or an excessive amount of stabilizer.

If you have one i would try another ign module ( SSI ) in it. Also does that carb have the long intake tube on it (Like my 8hp does) Fuel will pool in the bottom of that.And if  i idle for a bit  under 1800 rpm.It will stumble and blow some black smoke(rich) snaped up to full throttle.But after that  it snaps up to 3600 cleanly.Seems to me the only thing that it can be is a intake valve staying open at the wrong time  or the spark fireing to soon. Weak coils can do funny things to

Not sure what the original gas was on the first run but the gas in the bowl was clear, clean and tossed along with a bit run from the tank which was clean. A filter was put under the tank. The mechanic who checked out the engine took off the carb line with pliers and split the line at the carb inlet. After cutting out the split the line was too short to reach the carb. A line filter was installed between the carb and cowling which made enough extra too connect to the carb. The engine was run and tested for quite a while and run out of gas. It was then filled with recently purchased gas. So it was run the second the with fresh gas, two filters, drained bowl and it acted the same as the owner said and I experienced twice. The engine is headed for the bone yard.

Did you check your valve springs? If you are getting exhaust in your carb even after a valve job your intake valve may have a weak or broken spring causing it to float at mid throttle. Also check your timing gears close.

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.