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Discussion Starter · #1 ·
I have a 78 Bronco, when I am driving down the road and get above 40 mph it starts to stumble. If I let off the gas and get under 40 mph itruns fine, but when it's in neutral I can rev it and it won't stumble. I have blown out the fuel lines, replaced the plugs, distributor cap, put heet in the fuel tank, I tried rerouting the fuel line to prevent vapor locking. I ran out of ideas could any one please help me.
 

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Discussion Starter · #4 ·
i have replaced the fuel filters also, and i ran it without the cap to see if it was a venting problem but that wasn't it. i have a spare fuel pump around here somewhere ill give that a try
thanks
 

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If you don't slow down or if you punch it will it gradually accelerate/pull past the problem? or just continue to stumble?

Are you sure its the engine or could it be the trans cutting out?
mine(C6) shifts to 2nd at around 40mph.... the band slipping or the forward(what the manual calls it)clutch starting a chatter as it goes out could feel like the engine cutting out.

(I am assuming its an auto, if its a stick, then nevermind :) )

G.
 

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You will get the same reaction if the "power enrichment valve" is blown out. Engine will flood out as you accelerate which causes it to hesitate and stumble.
 

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Discussion Starter · #9 ·
if i punch it it will continue to stumble and it is defenetly the motor i have checked the choke also and it is ok. were is the power enrichment valve at? oh forgot to tell ya that when it starts to stumble i can smell fuel. thanks for your help it is kicking mya$$
 

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Power enrichment valve or power valve is on the front of the carb, bottom of the bowl between the two jet screws. You will have to pull the carb off to get to it. Look for a square cover with four screws in each corner, if you pull the cover off and there is fuel in it, chances are it's shot. You can buy just the power valve at a good auto parts store, take the old one with you if you don't have the carb tag.
 

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if i punch it it will continue to stumble and it is defenetly the motor i have checked the choke also and it is ok. were is the power enrichment valve at? oh forgot to tell ya that when it starts to stumble i can smell fuel. thanks for your help it is kicking mya$$
My Bronco was doing the samething. I poured a bottle of B-12 in it. It did a good job of eating all the dirt out of the carb. I now have a clear inline fuel filter. My problem now is the filter keeps clogin up with trash and I have to clean it every few days and then it is fine again. The filter I have has very small holes and a round filter inside it, it might clogged up often but atleast it keeps that crap outta my carb and I only have to clean the filter to get her going again. The B-12 helps disolve the dirt and grim in the tank to so I am not cleaning my filter as often now. I took the filter off and placed a fuel line on the pump to a can and then cranked the engine over, the fuel is coming out alot clearer now, before it had lots of dirt and was brown colored, looks more normal now.
 

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Yeah Seafoam works great to. I am at walmart alot and they carry B-12 so I always get it so I dont have to make an extra stop at the parts store to get Seafoam. I bought some B-12 in the aresol can to and I spray it directly in the carb while the engine is running, works great.
 

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I used 2 cans of B-12 to. I could tell the results of the first can very quick. I drove it around awhile then stopped and put alot more gas in before I added the next can, didnt want to get to much in the system at once, it might would cause a problem Im not sure.
 

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FYI, that seafoam is some wicked chit, use it in moderation. I've seen what it can do to gaskets and seals if too much is used. Dont pour straight into carb either, run it through a vacuum line such as the booster vac line.
 
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