Briggs flathead cam question

[sparkwizard]

This is a minor detail that I would like to know about before I even take this one apart. I am preparing to tear into a 1981 5 horse flathead. Minor performance mods for a homemade yard kart project with my 9 year old son. We plan to keep it as stock looking as possible. Pulsa jet carb, 3 qt tank.
I have been doing some reading and I have a 4 horse that we are taking apart and he is learning what all the parts do. We are planning to shave the eyebrows, clean up the ports a little bit, do a 3 angle valve job and a thinner head gasket.
The 4 horse engine holds the intake valve open a few thousandths of an inch for about half of the compression stroke. I assume this is a compression release for easier starting. I also assume it stays that way after the engine is running. I suspect that might not be good for performance.
My question is this: If we decide to defeat this function, would we be better to grind the end of the valve stem to increase lash and close the valve, or gently grind the cam down to close the valve on compression stroke?
It seems to me that grinding the valve stem would reduce valve lift on intake stroke and decrease flow.
Please someone, tell me not to worry about it, or tell me what you folks have done to get around this. Thanks in advance.

Dave

 

[massacre]

Pull starting the engine is tricky with no compression release.

 

[sparkwizard]

I understand that. Does it make much difference in performance after it is running?
Thanks for the reply.

 

[sparkwizard]

I have seen the chokes on old Briggs engines snapping in and out when the engines were running. I see now that there is air blowing back out through the intake valve. OHV engines have self-defeating compression releases.
This just seems like an easy way to effectively raise cylinder pressure.

 

[ole4]

If it is a centrifugal release it disengages when the engine starts, if it is a compression bump on the exhaust and you remove it you would be hard pressed to feel it. With no compression release on my hs50 ( I removed the bump). I could not tell any difference but now trying to start it required pulling on compression to TDC which takes about 5 seconds each pull. If I forget it either yanks the handle out of my hand or breaks the rope. I also have a 5hp briggs with an aftermarket racing kart cam with no compressor release that I do not have to get it to TDC I can still pull it through but it’s harder to pull than a stocker. Probably does not seal as good as my Tec.

 

[sparkwizard]

Thank you.
It seems to me that the decrease in air leakage from the intake valve being open an extra 90 degrees would produce a noticeable increase in torque. I think I will grind one of the cams and try it.

 

[Minimichael]

I am very 5hp naive, but I thought compression release opened the exhaust valve, not the intake...?

 

[sparkwizard]

This intake stays open. On GX clones, the exhaust is held open until they start, then the flyweight moves and the valve operates normally. This old flathead seems to have a cam lobe that holds the intake valve open until the piston is about halfway up on the compression stroke, then it closes the valve. I do not see any moving parts on the camshaft.

 

[massacre]

I have seen the chokes on old Briggs engines snapping in and out when the engines were running. I see now that there is air blowing back out through the intake valve. OHV engines have self-defeating compression releases.
This just seems like an easy way to effectively raise cylinder pressure.

On my Briggs flatheads, they are all-out race engines and i use a remote electric starter for those.
On my Tecumseh Hs-50 5hp engines with race cams (no compression bump) it can be tricky to start.
I don't think the compression bump is a big problem for performance. It barely cracks the valve open for a very short time. If you look at the cam, the compression bump is very small. If you were building a race engine then you would not want the compression bump.
Having an engine that is easy to pull start might be preferred over the almost imperceptible increase in performance.
If it's for you and your son to learn on.
After you guys get familiar with the bike, you could always remove the bump later. But once you remove it, it's very hard to put back.
You may be able to find a cam for your engine that has a centrifugal compression release like was mentioned by ole.
That would be the best of both worlds, but it may require some modifications to your engine. I'm not familiar with those for a Briggs, i'm sure someone here will know more.

 

[sparkwizard]

Having an engine that is easy to pull start might be preferred over the almost imperceptible increase in performance.
If it's for you and your son to learn on.
.

I think you just answered my question and satisfied my curiosity. Thank you all for your input!

 

[massacre]

I think you just answered my question and satisfied my curiosity. Thank you all for your input!

No problem!
You can always cam it later on. Pull starting a motor with no compression release isn't hard but it can be a pain. Especially in the beginning after a new build, before you get it running good.

 

[Minimichael]

Yes thanks for the interesting thread. Now I'll be interested to see my own 5 horse's operation... Once I end up opening it.

 

[sparkwizard]

remove the head, turn the flywheel slowly and watch the intake valve. Mine (a 5 and a 4 hp) both hold the intake valve open until the piston is about halfway up on the compression stroke. Seems downright unnatural!

 

[Minimichael]

I might even suggest the duration could be shortened a tad, without grinding down the entire decompression affect, ya know?

I mean for the sake of performance and all...

 

[sparkwizard]

Dremel is on the bench, I do have two stock cams. We may sand a little bit, install it and see what it looks like. There are no pushrods to mess with, no lash to adjust.

 

[VeritableFox]

Dremel is on the bench, I do have two stock cams. We may sand a little bit, install it and see what it looks like. There are no pushrods to mess with, no lash to adjust.

You still want valve lash. After you get about 1/4 turn past TDC the intake should be .005-.007 and .007-.009 on the exhaust when running stock springs. If using an aftermarket cam it should be set to the spec sheet. As the engine heats up having zero lash when cold could actually stop the valve from seating when the engine is hot.

 

[Thepaetsguy]

Removing the compression release bump will do nothing for on throttle performance.. it will make it harder to pull and it will absolutely make it idle a ton better.. leave the compression release alone and throw in some more duration with stock lift and you can make a really really hard revving 5hp.. I’ve been experimenting with bigger and bigger cams and results have blown away expectations.. the second picture is my latest cam. It’s going in a junk 5 for fun. Real table topper hope it hits harrrrrrd.

 

[sparkwizard]

You are not adding any height to the "PEAK", are you? Looks like you are adding weld metal to the ramps to increase duration?

 

[Thepaetsguy]

You are not adding any height to the "PEAK", are you? Looks like you are adding weld metal to the ramps to increase duration?

Yes actually four cams so far I’ve welded on and had awesome performance with the three I have ran. More duration a tiny bit of lift. As for the question at hand leave the compression bump alone. The improved idle isn’t worth the harder starting. Go to the Briggs Facebook forums and ask for a hotstock 93ss I wouldn’t doubt theres a member on there who has one and would give you an ok deal. No modifications needed to run it either.

 

[sparkwizard]

Nothing too scientific here, right? Mostly trial and error based on common sense and test drives?
I appreciate the advice in the compression release.