Worth while to lap valves?

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Doodle
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Worth while to lap valves?

Postby Doodle » Tue Jan 27, 2015 6:06 pm

Checking and repairing things on my newly acquired Cub. Dry compression check 1-90, 3-86, 4-85, 2-86. Plugs all looked dry, maybe even a little lean. I thought good enough for now and moved on to removing radiator for a good flushing. Did that and thinking I may as well adjust the valves. Then I start thinking to hard "dangerous" and think maybe I out to pull the head and inspect valves. Maybe pull them and lap them if I don't see any major problems.
To far? Leave it alone? Overkill and a waste of time? I have not ran this Cub much and have not used Seafoam. All early indications seemed like a carb rebuild and a little governor work would do wonders. And of course tune up.

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Re: Worth while to lap valves?

Postby Bigdog » Tue Jan 27, 2015 7:24 pm

I wouldn't call it overkill or a waste of time. Maybe not absolutely necessary but if you have the means to do it, now is the time.
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Re: Worth while to lap valves?

Postby Jim Becker » Tue Jan 27, 2015 7:29 pm

I wouldn't be lapping the valves unless I had some other reason to have the head off. Maybe if you are already removing the head for a leaking valve or another problem, but I don't read that into your original post.

I'd run a compression test to look for problems, not jump in assuming there are problems.

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Re: Worth while to lap valves?

Postby bob in CT » Tue Jan 27, 2015 8:22 pm

You should do a wet compression test in addition to the dry numbers posted. If the wet numbers come up, rings. If they don't change, you may be able to bring those values up with valve work. I would adjust the valves first and then do both dry and wet compression tests.

Once you get the head off you run some risk of breaking a head bolt, you will want to grind the valves and maybe you need new exhaust seats, and on and on.

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Re: Worth while to lap valves?

Postby SONNY » Thu Jan 29, 2015 3:13 pm

If its running / runnable then run it.--you can always do the dirty deed later IF needed! thanks; sonny
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Re: Worth while to lap valves?

Postby BigBill » Thu Jan 29, 2015 4:04 pm

If you lap the valves make sure you decarbonize the ports too.
Personally I would leave it alone. And add seafoam to the gas tank and crankcase. Run a tank of gas through it the recheck the compression. Make sure you run one tank of gas. The rpm will pick up. Seafoam lubes the sticky valve guides, frees stuck rings, cleans gum from the carb and repeats the rings. From sitting things dry out and get sticky/stuck.

Sorry it's not a motor rebuild in a can but it can bring back to life a good motor.

Trust me it works.
I'm technically misunderstood at times i guess its been this way my whole life so why should it change now.

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Re: Worth while to lap valves?

Postby SONNY » Thu Jan 29, 2015 5:46 pm

use diesel in the gas mix---NOT the bio crap, but the real stuff for a while! I use a little diesel in all my gassers to help lube the upper end, may stink for a while but under load its ok! thanks; sonny
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Re: Worth while to lap valves?

Postby Doodle » Thu Jan 29, 2015 6:19 pm

Not the bio crap,

What do you have against soybeans? Just kidding, I understand what you mean.
Actually a good point. I have been used to unleaded gas so long I sometimes forget these old engines need some upper lube.

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Re: Worth while to lap valves?

Postby SONNY » Thu Jan 29, 2015 7:20 pm

What do I have against soy??--its wax and not compatable with most fuel filters!--have a F-350 truck sitting here now dead because of using bio in it! Gasahol is corn alcohol and is very destructive on these old motors no mater what color they are! thanks; sonny
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Re: Worth while to lap valves?

Postby C-Pop » Sun Feb 01, 2015 9:34 am

I agree with Bob in CT, compression tests dry and wet are good test and can tell you a lot about what is going on inside the combustion area. A compression test is a good way to check the engine's ability to create pressure. Another test I like to do is a cylinder leak down test. Instead of indicating how much pressure the engine can create. A leak down test can show you how much and where the pressure is escaping, IE; intake valve, exhaust valve, piston rings, head gasket, cracked cylinder head. No engine can create a total seal and some leakage is expected. I'm not certain on the accepted loss, but up to ten percent loss is OK for me, providing the readings are consistent across all cylinders. A cylinder leak down tester doesn't cost a fortune, and like most tools should last you a lifetime. (I like the twin-gauge style tester). This test may help you decide if it's worth your time to go any further.

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Re: Worth while to lap valves?

Postby Doodle » Sun Feb 01, 2015 11:37 am

C-Pop, I like what you are describing. Do you hand crank to each stroke of the engine then to use this method? Or how do you do it?

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Re: Worth while to lap valves?

Postby C-Pop » Sun Feb 01, 2015 1:11 pm

Hi Doodle, this procedure is done with a leak down test kit (not a lot of money). With the engine at or near operating temperature. Bring the cylinder that you want to check on top dead center of the compression stroke.The kit has a hose that screws into the spark plug hole. Air is added via an air compressor, through a regulator with a gauge and a second gauge, then into the cylinder. The second gauge will tell you your percentage loss. You can tell where any problems are by finding out where the air escapes. 1) air coming out of your exhaust, exhaust valve 2) air coming out of the carburetor with the throttle blade open, intake valve 3) air coming out of the tube where the dip stick is, rings 4) bubbles in the radiator, head gasket. I've been informed that 20% is acceptable by many? Youtube has many demonstrations of an engine leak down test. These videos can do a much better job of describing this procedure than I can.

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Re: Worth while to lap valves?

Postby Doodle » Sun Feb 01, 2015 2:20 pm

C-Pop, Thanks for the info. I will look up one of these test kits.

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Re: Worth while to lap valves?

Postby Doodle » Sun Feb 01, 2015 2:30 pm

I ordered a leak down kit. Another toy for the tool chest. Looking forward to trying it out. :D


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