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Antique Gas Engine Discussion

5 hp. Style Z Model B

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Old 08-22-2005, 06:43:16 PM
Byron Roberts's Avatar
Byron Roberts Byron Roberts is offline
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Default 5 hp. Style Z Model B

Just a Question about a T/G Fairbanks that thinks It's a Hit-Miss.... Check Ball was ok.. Pretty much either starving for fuel or blowing out the butterfly..No happy Medieum.. .... Put a test light on the mag has a steady pulse... Would'nt the valve be not seating or maybe not at the right time... For it to blowout of the butterfly.... One of the guys at the show said a burnt valve.. No black smoke or gray ... Ran all weekend but it thinks it a H/M.... ..Byron
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Old 08-22-2005, 08:38:06 PM
KidDynamo KidDynamo is offline
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Default Re: : 5 hp. Style Z Model B :

Not to be too big of a stickler for terms but you are the second guy in a month to call out a "butterfly valve" when you appear to mean the valve that is not a butterfly valve. This confuses things for old gearheads!

Butterfly valve is a general term for an adjustable valve used for flow throttling. Your engine does have a valve that many would call the butterfly and it is the brass disc mounted to the throttle shaft which the governor controls. That is the carb's butterfly! If you don't believe me, do an internet search and you'll see many companies use the term as do I.

According to Hit and Miss Enterprises' catalog, the air inlet valve is one term for the spring "loaded" disc that opens by suction to allow airflow into the Fairbanks Morse carburetor. Others call it a air inlet compensating valve and there are other terms too.

I know language evolves but let's leave this term as-is. Thanks for your indulgence!

In regard to your problem, without pressure inside of the mixer (carburetor) how can anything get pushed out the front? Pretty much not possible, I wouldn't think. This suggests to me that either your valve timing is off or that the intake valve is leaking for some reason.

Checking the valve timing would be "by the book". If the valve timing is correct, I'd check the intake valve for carbon junk causing a leak, a sloppy valve guide preventing proper sealing, lots of wear on the valve and/or seat causing a leaky valve, or other similar type things.

Plenty of these old engines have had valves that are not properly seated for years and years. Inspection can show valves that have been ground so much that they are sunken into the seat and wobble around, preventing proper sealing. The engine will still run becasue they are low performance anyway, but the combustion pressure can creep around the leak and then it can act to further unseat the valve, and there you go- combustion pressure blowing gas charges out the air inlet valve.

If this is what you have, maybe the valve can be lapped one more time to get it to seat enough. Sometimes the gas blowing out of the front of the carb can have a flaming component. Combine this with a field of dry grass or a too close fuel can and the pucker factor goes up exponentially. Yikes !!

Radically incorrect ignition timing can also cause a blast of fuel out of the mixer inlet but usually, the engine won't run. I would still check this too. Good luck.....
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Old 08-22-2005, 09:57:27 PM
Andrew Mackey Andrew Mackey is offline
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Exclamation Re: : 5 hp. Style Z Model B :

Your problem is typical for unloaded F-M engines. These engines like a load! Check the following: Intake valve guide - any more than a couple thousandths is too much. Intake valve spring tension - improper tension will not seat the intake valve. Exhaust valve spring - same as above!. Compensation plate - there should be only a small relief hole in the plate, and the guide hole should be a close fit. If threr is too much clearance, the carb will perform erraticly - rich and lean. Compensation plate spring - proper tension is needed, otherwise the valve will flutter. Fuel level and adjustment - F-M engines need constant care as to fuel adjustment at idle - a 1/2" fuel level drop will change fuel draw charactoristics! Good luck
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Old 08-23-2005, 09:56:09 AM
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Byron Roberts Byron Roberts is offline
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Default Re: : 5 hp. Style Z Model B :

Not to be a stickler here but if I typed butterfly..Thats what I ment...on front of mixer... Thanks for the Replys... Will check spring tensions and like Andrew said probably needs to be loaded...Byron
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Old 08-23-2005, 11:18:49 AM
ErikG ErikG is offline
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Default Re: : 5 hp. Style Z Model B :

Hi Byron,
I ran my Z Style B at a show on Sat. and mine did the same thing! It runs good for a while and then stalls out and recovers blowing black smoke. Ive changed spark plugs, gas and checked point setting. I will now try lapping the valves and a new intake valve spring. I agree with Mr. Mackey that they run better under load. I have found all timing marks and it is timed correctly. It sure starts nice with that impulse mag. My dad found this engine many years ago on top of a scrap iron pile. Paid $5 scrap iron for it. All that was mising was the exhaust push rod. He made a new one. I will also check my compensater disk. I believe the guide hole is worn.

Good Luck,

Erik Grund
Bloomington, MN
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Old 08-23-2005, 12:39:00 PM
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Elden DuRand Elden DuRand is offline
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Cool Re: : 5 hp. Style Z Model B :

Originally Posted by ErikG
I will also check my compensater disk. I believe the guide hole is worn.

I've found that there are only about three things that'll cause a F-M Z engine to run poorly at idle.

One: Valve springs. You can check them while the engine is running. When it's sitting there doing it's hit-n-miss thing, gently hold back on the intake valve spring. If it runs better, you need a new intake valve spring. A temporary fix is to take it off and stretch it a little.

When that's done (of if the intake valve spring isn't the culprit), do the same thing with the exhaust valve but hold back harder. In some cases, you can see the exhaust valve move a little on the intake stroke. This causes the engine to suck some exhaust back into the cylinder, diluting the charge.

Two: The choke plate (on the air inlet of the mixer) can cause the problem. Either the hole in the plate is worn a lot or the spring is weak. What happens is that there is not enough suction at idle to draw fuel up to the needle valve. When the engine tries to slow down and the throttle closes, there isn't enough suction to pull up fuel and the engine dies until the governor opens the throttle enough to make enough suction to pull enough fuel for the engine to fire a time or two.

You can take the choke plate off the mixer and take a peek at the spring. If it's rusty and doesn't hold the choke closed, you can get a new one from Hit-n-Miss Enterprises. You could also look through your junk box and find one that's close.

Three: Leaky or stuck check valve or tank located too low for fuel to draw properly.

Hope this helps.

Take care - Elden

Last edited by Elden DuRand; 08-23-2005 at 12:40:49 PM. Reason: Left somethin' out
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