Author Topic: Bleed rate of IC valves?  (Read 1608 times)

Offline 90LX_Notch

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Bleed rate of IC valves?
« on: March 09, 2017, 03:06:50 AM »
What is a good bleed rate for the valves for a model Internal Combustion four stroke?

I built a test fixture for my "Tiny"'s head and it bleeds off vacuum at about one inch of mercury per second from 20 to around 15 then slows.  In all, it takes about 40 seconds to bleed from 20 to 0.

Would the engine run with the valves bleeding off at this rate?

I still have a long ways to go with the build, but this would be a good milestone to pass.

-Bob
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Offline 10KPete

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Re: Bleed rate of IC valves?
« Reply #1 on: March 09, 2017, 04:12:33 AM »
At 500 rpm a valve in a 4 cycle is moving ever 250 rpm so each valve cycle is like .24 seconds.

That's how long the valve has to seal.

Pete
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Offline Zephyrin

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Re: Bleed rate of IC valves?
« Reply #2 on: March 09, 2017, 09:16:10 AM »
 0.24 seconds is a maximum figure since valve closes for about half the cycle only !
a milestone to pass, yes it is !
I never did valves so small, and neither did measures on the valve's sealing...
I simply put a piece of tubing in the cylinder port with its valve (without spring) and with a good breathing in with the mouth by the other end of the tube, the tip of the tongue feels easily the depression, which must be hold for more than about 5 sec, and its done.

Offline 90LX_Notch

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Re: Bleed rate of IC valves?
« Reply #3 on: March 09, 2017, 10:25:32 AM »
Thanks Pete and Zephyrin.

I checked with the head assembled and pulling vacuum through the spark plug hole.  I'll remove the springs and check from the individual ports.

-Bob

Update:  I checked through the ports with springs removed and have about the same rate.  I think the valves and seats are good.  Only time will tell.
« Last Edit: March 09, 2017, 12:50:07 PM by 90LX_Notch »
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Offline philjoe5

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Re: Bleed rate of IC valves?
« Reply #4 on: March 09, 2017, 04:08:47 PM »
Bob,
If the seal isn't optimum but the engine will run, then after a few minutes of run time they tend to form a better seal.  Good luck in your progress

Cheers,
Phil
If you pick up a starving dog and make him prosperous, he will not bite you. This is the principal difference between a dog and a man.  - Mark Twain

Offline 10KPete

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Re: Bleed rate of IC valves?
« Reply #5 on: March 09, 2017, 04:27:15 PM »
If you are going to lap your valves to the seats, use only a very fine abrasive. Auto store stuff is way too coarse for little valves. I've found that Brasso polish is very good especially for light or final lapping. I have some 1200 mesh silicon carbide powder that works really well for first cut. Always do a final with something like Brasso or Timesaver or the like to 'clean out' the silicon carbide. Otherwise it will continue to cut some.

Pete
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Retired, finally!
SB 10K lathe, Benchmaster mill. And stuff.

Offline 90LX_Notch

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Re: Bleed rate of IC valves?
« Reply #6 on: March 09, 2017, 07:03:06 PM »
Thanks for the heads up Phil.

Thanks for the info Pete.  I used some 500 grit Clover that I have. 

-Bob
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