Telling them apart

[james ackerman]

I have some early 70s tecumseh 3.5 and 4 4hp engines and I'm curious as to how you tell them apart when they have no shroud? What are the differences between models and engines from late 60s to early 70s.

 

[capguncowboy]

On the H25, H30, H35, the exhaust is on the left side. On HS40, the exhaust is on the right side.

 

[markus]

pre 1970 H25-35 blocks will have a steel cylinder sleeve and little hole on the outer edge of the exhaust valve seat (usually wont see it as they get carboned over) for compression release-which also means the cam will not have any extra humps to raise the exhaust valve, Most of them (3.5hp and under) were also 5/8 output on the crankshaft before 1970 as well. in 1970 they started to phase out the steel sleeve, and cams started getting compression release humps instead of the hole.

Far as HS40's go the blocks changed a few times 1968-1970 which I outlined in a thread awhile back: https://www.oldminibikes.com/forum/tecumseh/138509-hs40-revisions.html

You really wont see much, if anything noticeable in long block changes 1970 to roughly 1974 for either engine version H or HS40 till they started phase out the smaller aluminum flywheel, all the bolt on stuff though starts to vary.