Tecumseh 3.5 compression issues

raypennachio

raypennachio

Member
#1
#1
Hello all,
We have recently acquired a manco thunderbird that had been sitting for several years.
I started it and it ran for three minutes before it wouldnt start again.
It had about 40psi of compression.
We took the head off, cleaned the valves piston and head, tried to start it again and had no success.
I did not take a compression rating after we had done this, but it didnt sound any different.
What should we do next? piston and rings? Change the valves?
What should the compression test results be? somewhere near 80-100?
Any advice would be great. Thanks!
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MB165

MB165

Active Member
#3
#3
40 psi is not much, ive seen em run at 60. It will be hard to get a accurate reading if it uses a compression release. there's alot of oil on the cylinder wall, maybe worn rings, but the bore doesn't look all that bad. before ya tear into it check the valve clearances. i have seen alot of older engines suffer from lack of proper gap from valve face and seat wear, couple that with a compression release or in your case possibly a small lump on the cam which nudges the valve open to bleed off for easier starting could be all it takes.....the best way to determine whats going on is with a leakdown tester and a good ear, but those are a waste of money unless your doing alot of repairs. another way is to rig your air compressor hose at around 40 psi, into the spark plug hole and listen for a hiss, intake/ exhaust ports or oil fill hole. the flywheel would need to be chocked at TDC.
 
bikebudy

bikebudy

Banned - Must pay $500
#4
#4
^^^:thumbsup: Also look for a sheered flywheel key ? Never know...

PSI, all these run perfectly, h30=60psi, hs40=60psi, hs40=80psi, hs50=60psi, h60 x 3 =60psi
 
raypennachio

raypennachio

Member
#5
#5
bikebudy said:
I'm going to assume you still have spark and that you put in a new head gasket?.

What is your compression now?
Click to expand...
Im at college away from the bike so i cant give you a number. We never tested it afterwards. And we used rtv/high temp silicone to seal the head to the motor. And it still had spark
 
raypennachio

raypennachio

Member
#7
#7
MB165 said:
40 psi is not much, ive seen em run at 60. It will be hard to get a accurate reading if it uses a compression release. there's alot of oil on the cylinder wall, maybe worn rings, but the bore doesn't look all that bad. before ya tear into it check the valve clearances. i have seen alot of older engines suffer from lack of proper gap from valve face and seat wear, couple that with a compression release or in your case possibly a small lump on the cam which nudges the valve open to bleed off for easier starting could be all it takes.....the best way to determine whats going on is with a leakdown tester and a good ear, but those are a waste of money unless your doing alot of repairs. another way is to rig your air compressor hose at around 40 psi, into the spark plug hole and listen for a hiss, intake/ exhaust ports or oil fill hole. the flywheel would need to be chocked at TDC.
Click to expand...
How can i check to see if my motor has a compression release? Ive been turning it over with a drill as i dont have a pull start. The oil was from us, we tried to add some to the top of the piston to see if it needed rings. We didnt add enough but it increased the psi by about 5psi.
 
raypennachio

raypennachio

Member
#11
#11
bikebudy

bikebudy

Banned - Must pay $500
#12
#12
Oh crap, yes, it could have sheered the flywheel key. Nut should have never come off from starting it.

Can you take a pic with the nut & cup off ? Of the keyway and the flywheel? I know you can't now but,
when you get home.
 
Last edited:
raypennachio

raypennachio

Member
#14
#14
bikebudy said:
Oh crap, yes, it could have sheered the flywheel key. Nut should have never come off from starting it.

Can you take a pic with the nut & cup off ? Of the keyway and the flywheel? I know you can't now but,
when you get home.
Click to expand...
Ill see if i can get one. I figured it wasnt good but i didnt think it could do damage. It never ran with that nut off, it only slowly stopped.
 
raypennachio

raypennachio

Member
#16
#16
Not that I recall. That would mess with the timing right? It has sounded the same since we got the bike and now, with the nut falling off and such. still has good spark. I'll get a picture for you when i can, thanks!
Do you think i should go ahead and do the rings as well as a new head gasket?
 
bikebudy

bikebudy

Banned - Must pay $500
#17
#17
At this point, just the head gasket, it could be all your problems. An get a recoil,

starting with a drill or impact is not good.

It would still have spark if the key is sheered, just at the wrong time.
 
Last edited:
bikebudy

bikebudy

Banned - Must pay $500
#19
#19
I would not, for only one reason, doing the rings is more than just the head being off.

You will need to pull the side cover, you should have a gasket there. You would need to unlock and pull the bolts
to the rod, then you can get to the rings once you pull the piston out of the cyl.

then you need the right type of torque wrench for the rod bolts. I don't know the spec for the torque.

A lot more involved than just changing the rings.

I think you'll find out, all will be fine once you do the head gasket.

Just my gut feeling.
 
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