Small Tecumseh mystery

[johnl_]

I apologize this is long, trying to cover everything that was checked and tried to figure this out, still can't get it to run right, don't know what is wrong with it...


Tecumseh 3.5hp TVS90-43504H serial 1248J on a Craftsman Eager-1 push mower trying to fix it for a friend, will run OK for short periods then starts to miss and stumble. Owner tried to clean the carb with no improvement. When I got it the bowl gasket was out of place and the governor rod was disconnected, but carb was completely spotless inside. I belive they refer to this carb as the dual series, has the large unvented primer bulb and only 1 hole in the bowl nut. Has the two speed throttle selector directly on carb. I ran spray cleaner through all passages, ran wire through them as well, replaced needle and seat, bowl gasket, primer bulb. Set float height, and checked float for leaks, none found. Still has steady misfires. Tried three different spark plugs, including a new NGK, all ran the same. Pulled the flywheel, key was slightly sheared, replaced. No change. Sealed carb to block with gasket sealer on gasket, let cure, no change. Tried spraying with water bottle to detect any change in rpm looking for vacuum leaks while running, none found. Tried partially blocking intake with thumb while running to force it rich, was able to stall it out but never found a point that got rid of the miss. Took off tank with fuel line attached and gas cap on, fuel flowed freely out of it for entire content of tank into gas can. Tried running without gas cap, no change. Tried running with spark tester, could not see any falter in the spark while misfiring. Checked valve clearances, intake was 0.006", exhaust was 0.008, seem to move freely as seen from tappet cover and ports, not gummy or scored. Checked compression, is low but my tester reads a little low, 81 psi cold, 86 hot. This is with pulling it over by the rope. The compression release does raise the exhaust valve on the compression stroke slightly seems normal. Disconnected the kill wire from the coil to eliminate possibility it may be shorting to ground somewhere, no change. Using same gas as in many other engines that are running fine. I have not removed the head, and I have not opened the crankcase to check cam timing. One possible clue is that it seems to run best when first started cold, runs fine for a few minutes before the misfires start slowly, then increase in frequency. RPM falters a little while it does this.

Now, I have a very similar engine that's been sitting unused outside for quite a few years, it is a TVS105 from about the same year, appears to share mostly all the same engine components. I decided to try and get it running to use as a source of "known good" parts if possible. Did the carburetor, oil change, and got it running very well. Compression 76psi cold pulled over by rope. Intake 0.006", exhaust 0.011". Old champion plug from the 90's. I took everything fuel and ignition related except the flywheel from good running engine and swapped it onto the problem engine, NO CHANGE, still sputters and misses.

I am ready to give up unless there is something else to look at?

thanks john

[KE4AVB]

Sounds you might have valve train problem. Like a sticky valve or one one not sealing good.

Down the 3-11 hp Tecumseh L head service manual thing to check.

Tecumseh 3HP-TO-11HP-4-CYCLE-L-HEAD Service Manual

[38racing]

I had a friend's cub cadet with old kohler. It would run and just stop. I checked and spark was staying when it shut. It had good fuel flow. Fuel cap off still stopped. I dosed it with 50-50 seafoam and ran a bunch of runs/sits and the problem went away. That was early last and no further problems. maybe coincidence.

[dennis]

Did you try the ignition coil from the old mower on the TVS 90?

[johnl_]

Dennis, yes, and another used one in addition to that one, in total all 3 coils ran exactly the same on it, one of which ran fine on a different engine. I did the Seafoam treatment just for kicks, no effect. I bought one of those Ebay universal Tecumseh chinese carbs that has the adjustable main jet to try and rule out A/F mix, haven't tried that yet. The last thing to try might be to take out the valves, if it has a weak spring I wouldn't know how to check, all I could do would be to swap in a different set. Valves look fine when engine turned over by hand, clearances OK.

[johnl_]

Chinese Ebay Carburetor fixed it, apparently it was running too rich. Explains why is ran best cold. I can make it run exactly like it did before by turning out the a/f adjustment too rich on the new adjustable carb. Set just right it runs with no missfires at all. I don't get it but if it works I'll take it.

[KE4AVB]

Chinese Ebay Carburetor fixed it, apparently it was running too rich. Explains why is ran best cold. I can make it run exactly like it did before by turning out the a/f adjustment too rich on the new adjustable carb. Set just right it runs with no missfires at all. I don't get it but if it works I'll take it.

It sounds like the strange problem I had earlier on the C4 engine I was working on where it ran better nearly fully choke but yet it was running extremely rich as when I tuned the carburetor leaner the engine ran better with the choke off so I finished leaning it for best speed. Just defies logic but I not arguing with the results either. I had fought with the problem for over a month changing out worn parts which each time made the engine run a little better.