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.

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.

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.

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