Oversize exhaust valve--Tecumseh

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ryan68bug

Member
#1
#1
Anyone ever have to do this procedure on an H series Tecumseh? I have an '87 H35 on my bike that's fully rebuilt and I suspect the exhaust valve has a bit excessive slop in the guide and am considering doing a 1/32" oversize valve install, just for the sake of ensuring proper specs.

What are the signs of a leaking valve? Haven't pulled the head or even breather off yet to look inside. I've only run the engine without a load thus far and it seems to accelerate fine, though a bit of white smoke upon initial acceleration out the exhaust does occur, particularly when cold.

I'm intent on keeping this engine due to its "correctness" for my bike, a late '60s Bird "Duck."
 
minidragbike

minidragbike

Supporting Speed Nut!
#2
#2
If you have a worn out exhaust valve guide you will normally have oil passing the valve and into the exhaust pipe causing smoke from the engine.

You can go over size valve if you like, but it doesn't have anything to do with the valve guide. It's best to just replace the guide itself.
 
Ruppster81

Ruppster81

Member
#3
#3
The old tecumseh engines dont have replacable valve guides, They only offer a oversize valve with a larger diameter stem and you have to buy the special valve guide reamer tool as well. I have been dying to ask about anybody making or can make replacable valve guides and cast iron cylinder sleeves for these motors or if its even possible, I don't think there is much "meat" in the block to have some made?

ryan68bug - I think the white smoke would probably be most likely the valve just needs to be ground or lapped or its steam from the cold. If the guide was worn you would be seing blue smoke.
 
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minidragbike

minidragbike

Supporting Speed Nut!
#4
#4
If there is a factory valve guide it most likely can be replaced. All you have to do is measure the valve stem size. Then it's just finding a guide that has the same I.D.
 
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ryan68bug

Member
#5
#5
Ruppster81 said:
The old tecumseh engines dont have replacable valve guides, They only offer a oversize valve with a larger diameter stem and you have to buy the special valve guide reamer tool as well. I have been dying to ask about anybody making or can make replacable valve guides and cast iron cylinder sleeves for these motors or if its even possible, I don't think there is much "meat" in the block to have some made?

ryan68bug - I think the white smoke would probably be most likely the valve just needs to be ground or lapped or its steam from the cold. If the guide was worn you would be seing blue smoke.
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I sure hope it's not because the valves were ground/lapped in, as I paid a local shop to do it. Really crappy thing is the guy recently sold or moved out of the machine shop. Smoke is definitely white, but I'm thinking I may fix that loose exhaust valve "just because." When I build things, I like the specs to be all kosher.
 
Yellowhand

Yellowhand

#6
#6
As far as I know, it's impossible to press a cast iron sleeve into an existing aluminum block...these sleeves are always inserted into the mold before casting, and the alloy comprising the rest of the component is cast onto the sleeve.

As for valve guides...I've seen cast iron blocks get worn guides welded closed, then redrilled and reamed to exact size...I use nickel rod, some guys use stainless...if the block isn't preheated first, it usually cracks.

Same for aluminum blocks...the new TIG welders and rods make it much easier.

I don't think manufacturers would sell replacement guides, but they seem to love to sell replacement short blocks.
 
Ruppster81

Ruppster81

Member
#7
#7
Yellowhand said:
Same for aluminum blocks...the new TIG welders and rods make it much easier.

I don't think manufacturers would sell replacement guides, but they seem to love to sell replacement short blocks.
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You can fix aluminum block valve guides by TIG welding and ream them out to original size? Good info. And if cylinder sleeves arent feasable how about Nikasil plating the cylinder like snowmobile engines use, That stuff is HARD. and would make the engine more durable.
 
Yellowhand

Yellowhand

#8
#8
There's a weird combination of cheapskate and novice innovator in every kart racer...

It's hard to find a block at a kart race that hasn't been welded somewhere...

Once they start fill-welding and shaping the interior of the block, they really don't want to give it up...

Unfortunately, most of these guys won't share any weld-shaping tips with others...

I personally think cast iron cylinder sleeves are too old-tech to mess with, and the coating and plating materials available today not only wear as well, but they reduce friction while providing a good compression-enhancing surface for the rings to make contact with...

The question is... how much is someone willing to spend on an engine?

There are numerous companies out there who can coat or plate (with the latest technology) whatever you want...for a price...

Eventually, the price drops as more and more people adopt the use...look at titanium nitride 15 years ago versus today...
 
pro69nova

pro69nova

New Member
#9
#9
If you have a shop that knows what they are doing you can install a replacement valve guide, i make all the guides and valves for most of the engines including Tec's, Briggs, yamaha's, Honda. Valves and guides for most are available Thru Charles at Ice House Perfomance. i also make parts for ARC, NR, JR Racecars, Dyno Cams, Uncle franks. just to name a few. what ever you need call Charles at Ice house Performance.
 
Butts

Butts

New Member
#10
#10
I would bore out the old valve guide holes and press in new bronze guides. If you have a lathe this will be easy. If you don't a machine shop should be able to do it for cheap. I've seen this done on a small block chevy. The flat solid lifters mushroomed over and opened the lifter bores. If it works for this it should work for you. Good luck.

J.
 
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ryan68bug

Member
#11
#11
Quick question: the engine, when cold, takes anywhere from six to a dozen pulls on the cord with the choke on before it fires. Once warm, it will start right up on one pull of the rope. Does this sound like a vacumm/valve sealing issue? Doesn't appear the plug is fouling, but it's a consistent issue.
 
minidragbike

minidragbike

Supporting Speed Nut!
#12
#12
That could be a number of things. But it sounds like your motor just maybe be to cold. Either cold oil(to thick), cold rings(weather), fouled spark plug, or even to cold of a spark plug.

I had the same problem, but my was because my rings were not seated to the cylinder because the motor was new.
 
N-gin

N-gin

New Member
#13
#13
Replacing vavles and and guides and drilling and this and that. I dont think its worth it. After you do all that your probobly looking at over $100 EASY.
Why waste your time just get a new Honda clone 6.5 for $129 even comes with a tank....
 
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ryan68bug

Member
#14
#14
N-gin said:
Replacing vavles and and guides and drilling and this and that. I dont think its worth it. After you do all that your probobly looking at over $100 EASY.
Why waste your time just get a new Honda clone 6.5 for $129 even comes with a tank....
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Well, I gurantee you I'm well over that in the engine and several times over that in the restoration project at this point... ;-) The reason I'm committing myself to this engine is because I'm restoring this bike to AS NEW, and want it to be 100 percent (or as physically close to it as possible) correct. I'm pretty much already there, though I need to have my handlebars chromed to have it as it truly was from the factory.

As for the hard-starting tips, thanks guys, I'm thinking it may be a combination of new parts and what not as I fully rebuilt this engine. It hasn't even had a load put on it yet, just free-running. Here soon I'll change out the oil and try to put a little more time into it in terms of tuning and taking it for a test drive. I'm thinking the smoke off idle might be rings that haven't seated yet, as well. It diminishes as the engine warms up.

Other details I neglected to leave out is that it is getting pretty dang cold here in Lake County, Oregon, and I"m living at 4800 feet elevation-plus some change.
 
N-gin

N-gin

New Member
#15
#15
Well I guess if you are restoring then thats is a really good reason for rebuilding that old engine:thumbsup: Hope everything works out for you.

About the white smoke though, you think it might be water in the gas or just condensation in the exhaust. maybe just lean. Have you considered richening up the mixture you said you had trouble starting when cold. Thats when you need the richest mixture. You should pull it three times and it should be running. First pull with the chock on. Second it should sputter with the chock on. For the third pull the chock should be at half and engine should be high idling.:cool:
 
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ryan68bug

Member
#17
#17
Another yet question... I checked and my plug is definitely getting fuel fouled. Raw gas on the electrode, etc. I also noticed gas leaking on top of the cylinder head, at about nine o'cloc, if you're looking from straight above. Is this a sign of a cracked head, possibly? Just wondering. Haven't torn it down yet.
 
N-gin

N-gin

New Member
#18
#18
Did you happen to try a separate tank and run a line to the carb and draining the float bowl. Possibly you have water in the gas.
 
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ryan68bug

Member
#19
#19
Not sure about that, yet... the bike/engine are stored in a dry garage and the gas is very recent. Don't have a separate tank to try, unfortunately.

I did, however, pull the head and breather off last night, and made the following observations. The combustion chamber looked pretty clean, for one, but there was definite raw gas on the valve heads and spark plug, as mentioned earlier. The intake valve head looked a smidge pitted which I didn't notice before (not huge craters, but small pin pricks on top of the valve face).

I checked to see if the valves were sealing by squirting a little carb cleaner around the seating area and looking into the intake and exhaust ports with the valves closed and piston at TDC. They appeared to seal just fine.

On valve clearance, however, the exhaust valve was fine at the bottom of the acceptable Tecumseh-mandated tolerance of .004-.010", but the intake valve was wayyyy too high in gap at .016."

Also, the exhaust port was soaked with raw gas.

My plan at this point is to install that oversize exhaust valve (I already know the guide is way too sloppy) and also install a new intake valve with the proper valve-to-lifter clearance. It's a matter of obtaining parts and getting the valves ground at this point... back to the salt mines...
 
Ruppster81

Ruppster81

Member
#20
#20
Sounds like the valves arent at fault to me. I would perform a leakdown test, with all that raw gas you might have a weak spark (not igniting all the fuel in the cylinder) or a out of tune carb. My rupp did the same thing. I cleaned the points and flywheel put a new plug in and rebuilt the carb and it all went away... It starts on a 1/2 a pull on the rope cold now and the plug is burning a tan color.
 
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