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Stupid Question On Sticking Valves


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Years ago on another forum as a novice I was told the only stupid question was the one that wasn't asked. I'd like to test that theory. Several years ago after getting my 1917 Maxwell running to my satisfaction I put it into winter storage. When I went to restart it in the spring it wouldn't run worth a darn. Pretty sure my problem then was an almost trashed brass K D carburetor due to ethanol gas. I've never been able to get it to run right since. I'm fairly certain I've managed to cure all my ignition ills in the Atwater Kent distributor after battles with both original AND new parts. I've gone through three different brands of period carburetors used on Maxwell's and have finally returned to another K D carburetor that seemed to solve the problem and run decently for a short period but now I noticed while walking back into the garage some light missing from the running car's exhaust. 

History on this engine: It was totally rebuilt in the 1960's by a cost no object wealthy owner. When I acquired it the motor was the only restored part and had never been run until several years ago.  Here's my stupid question, could my intermittent miss possibly be a sticking valve and how do I pin that down as the problem?   The engine still doesn't have more that 2 hours running time since the 1960's rebuild and this crazy miss is totally random. Any suggestion as to a scope or instrument that would allow me to pin down the cylinder and the  eliminate the possibility that it could still be fuel or ignition related?

 

Howard Dennis

Edited by hddennis (see edit history)
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Thanks TerryB,

but wouldn't this just show a weak cylinder as far as compression goes?  I've worked on this engine enough over the years to know that all cylinders have  decent compression, exact figures I don't know but all four are strong. The valves and springs are all new and the settings are correct.

 

Howard Dennis

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Leak down looks for drop in pressure vs time which might indicate a weak sealing of a valve or head gasket.  Would be a place to start.  The other thing I thought of was a oscilloscope type view of each spark plug firing to see if any unusual patterns show up.  Not sure how to hook up a scope on an engine as old as yours.

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If you suspect a sticking valve, try 2 oz.  of Marvel or two cycle oil per ten gallons of gas.  Take it out for a drive and get it up to full temp.  Then park it and see if it still misses.  Just putting a bit of oil on each valve/spring wouldn't hurt either.

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If you had sticking valves that cylinder or cylinders would not have compression. If you have a miss I would look somewhere else. Use an infra red thermometer on a completely warmed up engine, check the exhaust manifold looking for a cold cylinder. If it's considerably colder it's not firing right. Let the engine idle and pull off one spark plug at a time. If the engine doesn't sound different it's dead. A good way is to find a strong cylinder and leave that wire off, then pull another. It will intensify the miss. Hold the wire to make the spark jump to the plug, it makes the spark stronger. The Marvel Mystery suggestion should be used in ALL old engines, even if you use non ethanol gas. 

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1 hour ago, cahartley said:

If it is skipping beats only at idle and straightens out the second you put a load on it the most likely cause is weak exhaust valve springs.

Wouldn't weak valve springs show more problems at higher rpm's with valve float?

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Hi,

I have an old British van that sits unused for long periods. It occasionally suffers from a stuck valve which makes the engine miss. The misfire is very regular and most importantly you can here the tapping from the extra gap that is in the valve train.

It quiet often will stick at higher revs. and then free itself while idling. Some upper cylinder lubricant  added to the petrol has improved matters.

If your miss is only noticeable at idle and doesn’t have a constant beat I would be looking for the cause of a weak mixture. Carb. adjustment or manifold air leak.

Hope you find it.

John

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First off, Thanks for all the replies and suggestions. I have the manifolds off now to chase a pesky oil leak so until that is done and reassembled I can't apply and test your suggestions. As mentioned during the rebuild the valves and springs were replaced with New Old Stock so I think that isn't the problem. Mixture if anything is a little rich for my taste but hard to address properly until engine is running smoothly. When it's running again I plan to add Marvel Mystery oil and  purchase an infra red thermometer and test that as I think it will at least point me in the right direction. I tried the plug wire search in the past but didn't get a definitive result but I was doing one at a time and will see now if I can find the strongest cylinder as per your instructions. 

 

Howard Dennis

Edited by hddennis (see edit history)
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On 12/2/2020 at 2:52 AM, kgreen said:

Wouldn't weak valve springs show more problems at higher rpm's with valve float?

Not on any of the tractor engines on which I have worked.

Granted these are antiques with a top end of around 1000 rpms but invariably when there has been an irregular idle which goes away with the slightest increase in load it has been weak exhaust springs.

I know it doesn't make sense but I also know it cured the problem every time.

hddennis's reply ruled out this as being a factor.

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