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RPMs drop then revs

Fuel Starved?

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ol' poke

07-02-2003 08:54:45

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An intersting observation from a newbie! I just bought a '50 8N for field work (bush-hogging). After running a while, the RPMs slowly drop to almost stalling, then rev back to normal. Sometimes it runs fine for 5-10 minutes, then slows again, sometimes it happens more freqently - not consistent. I posted this question earlier thinking it only happens when the engine is hot, and got lots of feedback regarding an overheated coil and/or ignition switch (thanks to everyone for the info!), but my recent observations show that this happens when the engine is cold as well - does this emiminate the electrical system? Yesterday while mowing, I found that when the RPMs are slowing, if I pull the choke out slightly, the RPMs return. Also, by "feathering" the choke, the RPMs stay consistent and steady. I am now thinking the problem is more fuel supply related - rather that electrical. On the right track here? I also noticed a small air bubble in the glass-bowl sediment cup. My questions are:
1) Does this sound like I am on the right track?
2) What filters/screens should I check and what is the recommended procedure?
3) Where did that darn air bubble come from and is this even a concern? A possible gasket leak at the sediment cup that sucked it in - but I have no fuel leak? The fuel tank has been over half full at all times, so I don't think it came from the tank.
4) How many fuel filters/screens should I check and where are they located? Sorry so wordy, but you guys are the experts.

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Steve W

07-02-2003 14:37:35

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Re: RPMs drop then revs - Fuel Starved? in reply to ol' poke, 07-02-2003 08:54:45

I think your Intake manifold is leaking. If all those wacky air into sediment bowl theories don't work out, check your manifold gasket on both the carb and the engine.

Take Care
Steve

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pat

07-02-2003 10:49:46

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Re: RPMs drop then revs - Fuel Starved? in reply to ol' poke, 07-02-2003 08:54:45

If air can get into the sediment bowl shouln't gas be able to leak out. This may sound weird but could the bubble be in the glass bowl itself. I had a 50 8n years ago and the glass has a defect in it. it had what looked like a bubble in it. Just a thought.

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Evil Steve

07-02-2003 12:04:53

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Re: Re: RPMs drop then revs - Fuel Starved? in reply to pat, 07-02-2003 10:49:46

Not really because the suction caused by the engine creates vacuum that would prevent gas from leaking.

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Rob

07-02-2003 09:32:11

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Re: RPMs drop then revs - Fuel Starved? in reply to ol' poke, 07-02-2003 08:54:45

Probably getting more air then fuel through the fuel line. Tighten the sediment bowl til it hurts your fingers and the bubble goes away. If the bubble will not go away you will need a new gasket and screen kit from your local farm/ranch store or F/NH dealer or I bet the MFH dealer has em too. They have a tank valve people say is interchangable but it only fits; it's not the same and I cuss the guy that put one on my 2N regularly. I suspect the difference is in the draw tubes (their's has one too many and it leaves a LOT of gas in the tank you can't get too) and tank screen (their's doesn't have one at all) but the bowl, screen and gasket are likely the same.

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souNdguy

07-02-2003 12:35:07

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Re: Re: RPMs drop then revs - Fuel Starved? in reply to Rob, 07-02-2003 09:32:11

I think ( I've heard... ) that napa carries a neophrene gasket that works too.

I had my stock cork one dry rot while i had the tank off for some steering work a while back.. I tried another TSC cork one.. no good.. I cut 3 more from a cork sheet.. no good... I finally cut one from a medium thickness paper gasket sheet... had my doubts .. but it doesn't leak a drop, and hasn't for months. You do look like you are gonna run out of adjustment on the thumb screw.. but there will be enough.

Soundguy

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Lee

07-02-2003 12:48:00

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Re: Re: Re: RPMs drop then revs - Fuel Starved? in reply to souNdguy, 07-02-2003 12:35:07

Napa does indeed carry a neoprene gasket for the sediment bowl. Much better seal than the cork ones that I've cussed so many times and you don't have to tighten the bowl until your hand hurts. NAPA 730-9506 will get ya a good'n. Later, Lee

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souNdguy

07-02-2003 13:25:38

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Re: Re: Re: Re: RPMs drop then revs - Fuel Starved in reply to Lee, 07-02-2003 12:48:00

Thanks.. I'll keep the number, in case I ever need to replace it.

Soundguy

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Evil Steve

07-02-2003 09:25:01

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Re: RPMs drop then revs - Fuel Starved? in reply to ol' poke, 07-02-2003 08:54:45

1) Yep, sounds like it could be running a little lean. Feathering the choke is enrichening the fuel/air mixture. Either a carb adjustment, clogged screen, crud in carb.

2) Three screens: one in gas tank mounted right above your fuel bowl assembly. It's a cylindrical screens that sticks up about 1 1/2 inches into tank. Remove fuel bowl assembly from tank to check. Second screen is flat screen in the bowl itself. Third is in the elbow that goes into carb.

3) The bubble is most likely a bad gasket seal on your bowl assembly. Some air is getting sucked into the bowl around the gasket. Replace gasket.

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Oh, and... Evil Steve

07-02-2003 09:37:20

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Re: Re: RPMs drop then revs - Fuel Starved? in reply to Evil Steve, 07-02-2003 09:25:01

... air can be sucked into the bowl if your gas tank vent is plugged.

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ol' poke

07-02-2003 12:50:43

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Re: Re: Re: RPMs drop then revs - Fuel Starved? in reply to Oh, and... Evil Steve, 07-02-2003 09:37:20

Thanks E.S., while was trudgin' along, I loosened up the gas tank filler cap - no better. I don't think the vent is plugged.

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Rob

07-02-2003 13:44:10

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Re: Re: Re: Re: RPMs drop then revs - Fuel Starved in reply to ol' poke, 07-02-2003 12:50:43

The fuel to the carb comes off the top of the sediment bowl assy and air rises to the top of the fuel in there so guess what enters the fuel line first when the carb float asks for more fuel.
Gahead guess.

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