Tecumseh decompression BCR or valve?

[Carlos]

Anyone tried installing a decompression valve in a Tecumseh flat head for easier starting after filing or grinding the BCR down on the exhaust cam lobe?

I know guy's have installed decompression valves into 2 stoke Bultaco motors to ease starting and not break a leg from the kick back ... and theirs talk on the www of some "Harley D" high compression engine builds having decompression valves installed in the heads for the same reasons.

Anyone here tried it on a Tecumseh?

(I probably should have asked here first as I've already sent the head to be machined for a Stihl chainsaw valve to be installed!)

 

[MB165]

That comp release is what I was thinking of as I read your post, it should work pretty good..I had one put into a CR500 head. where is it going to be located? are they going to spot face off a area large enough to get the realease and a socket?

 

[Carlos]

The comp release valve is M10x1.0 so a 3/4" spot face will leave ample socket clearance.

There's a small segment in the head close to the combustion chamber wall on the exhaust valve side that's clear of the piston and the exhaust valve. The spot face obviously means losing a small area of fins 3/4" dia. so it's almost between one of the studs and the spark plug. I doubt the fins in this area are very effective because the air would have to flow over the spark plug boss, drop down between the fins and then up and over the stud boss. The head is about 8mm thick, so after spot facing it should leave 7mm+ of material which I'm OK with.

I'll post pics when it's done.

Did the release valve make the CR500 easier to start, or kick over?
Did it shut automatically with combustion?

 

[oldfatguy]

A stock flathead doesn't make enough compression to cause any problems pull starting it. In fact I find them easier to start without the compression release on the cam.

 

[ATKRIDER]

A stock flathead doesn't make enough compression to cause any problems pull starting it. In fact I find them easier to start without the compression release on the cam.

I agree 100 percent. IF your HS50 or 40 is kicking back on you it has some serious timing issues OR the cylinder is full of fuel and the thing is hydraulic locked.

Some manufactures will or did use them on certain things like LARGE displacement saws and such but down the road the valve carbons up and doesn't close-guess what happens than? Seizure. It causes a nice vacuum leak and the engine goes lean. Some companies like Partner or now Husqvarna use a 2 stage coil like start and run.

 

[markus]

A stock flathead doesn't make enough compression to cause any problems pull starting it. In fact I find them easier to start without the compression release on the cam.

Are you grinding them off or do you have the mechanical cams and pulling off the flyweight assy? :shrug:

 

[Carlos]

Thanks for the replies and input, I appreciate all feedback and thoughts, but I probably didn't give enough info.

There's no kick back issue and this is not a stock engine. The head has been refaced -.020", the BCR removed plus a few other mods.

This is for my snow thrower, it's not going to see the same hours as my minibike.

I'm not too worried about carbon build up over time, the valve is a serviceable item, every other season I can either clean or replace it no problem.

There's quite a few reasons I want to install one of these comp release valves.

1. Whenever I've modified a single cylinder engine they have always been harder to pull or kick start

2. I have damaged cartilage in my shoulder joints so engines that can be a pain to pull start are literally a pain to pull start.

3. I'm one of those people who can't stop playing with engines! Even if it's my snow thrower!

4. This last winter was brutal in Detroit! I do not want to freeze my nuts while trying to start this engine.

5. My only source of expertise so far was speaking to one guy who runs a Rupp minibike store who said that to start the modified Tecumseh engines with raised CR and no BCR he first turns his engines to TDC before pull starting, which sounds logical, but is not something I want to do in the middle of winter wearing thick gloves if I run out of gas on the sidewalk.

I even considered changing the cam to the mechanism that lifts the cam follower with a rod and flyweight as per Markus's post above, but don't have enough knowledge of Tecumseh's interchangeability to choose the right part # or if this type of cam was even used in any of the Horizontal small frames.

I doubt I'm the first person to experiment with installing a compression release valve to replace the BCR on an HS or HSSK engine, so I am really interested in anyone else's results, good or bad.

 

[markus]

3. I'm one of those people who can't stop playing with engines! Even if it's my snow thrower!

:laugh: Love that!

I found one of those mechanical ones in an engine I had. They may have run them up to about 1970 but I am not 100% on that :shrug: been awhile since I had redone that engine but I think it was the lifters that were different sizes due to the larger lobe. Engine ran good, I yanked the gov and did a little match porting and cleaning up of the intake and exhaust ports.

Only thing that bothered me with it was more moving parts inside, but I figured I'd try it as I tried a thin head gasket, which ended up leaking pretty bad, so I just went back to a stock gasket and said I'd mess with it later. That never happened it was an easy starting, great runner as it was and was more fun to ride than work on :thumbsup:

Heres a pic of the one that it had in it. I have 3 more 1970 and older HS40's 2 of which originally snowblowers though and one is a Rupp roadster engine, I have not cracked any of them open yet to see what cam is actually in there..

Looking at your CT, I would imagine your going to have one bad ass snow thrower :thumbsup:

 

[Carlos]

Looking at the size of that cam lobe, the lifter/tappet would have to be shorter.

I wonder if the duration is different for those cams.
It would be interesting to find out.

 

[oldfatguy]

in theory, that mechanical compression release should allow the valve to close all the way when the engine is running. I ripped all of it out, I took the counter weight off the cam too.
the lifters are different lengths but it doesn't matter. On the cams with the compression release on the cam lobe I cut them off in a small lathe by turning the chuck back and forth by hand. You can keep the cam lobe perfectly round and it's easy to see what metal has to be removed. It's a lot more accurate than filing or grinding the cam lobe. Removing the compression release makes a difference in the low end power and torque. Plus it lets the engine idle slower and smoother without stalling all the time. The older engines were built without the compression release. It's when we came into the age of wimps that they were needed so that some weakling could pull start his lawn mower. They work well on engines that run at a constant speed like a pump or a lawn mower but not on a mini bike that changes from idle to revved up speed. So my advise is take them off because the engine will run better.