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Discussion Starter · #1 ·
I've just did an engine swap in my 78 c10 from a broken down 350 to a rebuilt '68 307.

I had to swap the water pump, fuel pump, intake, distributor, and carburetor over from the 350 to the 307 and did NOT make any adjustments whatsoever to any of the above parts.

after having to replace the coil, I finally got the engine running again, but every time it starts, it jumps straight into a high rpm, and the carburetor is just DUMPING an excessive amount of fuel into the engine. everything worked when it came off the 350, and as I said before, NOTHING has been adjusted or messed with.

can anyone tell me what the cause(s) of such issues can be?

I've already tried adjusting the needle and seat on the carb, and rotating the distributor in both directions, neither worked)
 

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What kind of carb??

What fuel pump was used??

Choke linkage and choke high idle cam been checked for releasing or being jammed??

Throttle cable/bracket combination have enough slack to fit to carb easily??
 

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Discussion Starter · #4 ·
i have a rochester quadrajet 4bbl carb, with the factory spread bore intake, both from the factory with the '78 350 engine.

fuel pump is also the stock small block pump. all choke components have been inspected, and work the same as when removed from the 350, which wile the engine itself was in bad shape, still ran.

throttle linkage, cable, and parts are all working fine, and there was plenty of slack to install them without tension. transmission kickdown cable is the same. the only thing that has been changed is a half inch carb spacer instead of the factory quarter inch spacer.

and all valves have been checked, and open and close completely without any issues, but for some reason the carb is still putting out too much fuel.

(when all my parts were pulled from the 350, the engine ran fairly smoothly, and at a low idle. the only issues was a few bad rings, a blown head gasket, and a missing thermostat when i got the truck. everything worked, and nothing was adjusted during the swap,)
 

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Did the carb sit any length of time dry, or for long enough for fuel to evaporate out of it??

Carb get turned upside down at all while off??

Fuel filter present in carb inlet??
 

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Discussion Starter · #6 ·
carburetor sat on the intake still installed to the 350 for about a year, never upside down, filter is still installed, the only parts that had been disconnected were the distributor vacuum advance line, the PCV/valve cover breather hose, and the vacuum line to the brake booster.

nothing else was removed until the day i pulled the engine out to put the new one in
 

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I'm guessing vacuum leak, from bad hose or the spacer change, or lack of fuel control caused by Viton rubber tip on inlet needle drying up.

Check any hose that is old for splits and loose connections.

Check around the spacer for vacuum leaks.
 

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Discussion Starter · #8 ·
all rubber lines (vacuum, heater, radiator, coolant over flow, P/S, PCV, brake booster, fuel lines, and washer lines) are brand new. as are the carb spacer and mounting brackets.

the engine runs very smoothly, the issue is that it will not drop below 5000rpm and the carb is getting too much fuel.

im not sure what pressure the pump is running at, and do not have the tools to test that.
 

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Needle and seat is the next logical place to look, if you don't have a hung open throttle situation in some way.

Just something to consider, an engine won't run 5000 rpm on just fuel, it has to also be getting that much air to go with the fuel, so check for vacuum leaks and throttle blades cracked open front and rear too far for any odd reason(jammed linkage, bent linkage, hung on base gasket, etc..

Plug all vacuum lines at carb to eliminate leaks from brake booster or transmission modulator and PCV valve.

Fuel pressure could be an issue if pump is new, may be making too much pressure and blowing fuel through the carb. Without a pressure check, it is just a guess. A low cost pressure/vacuum gauge is a good tool to have around, might want to consider getting one.
 

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Either the throttle plates are hanging on something perhaps the new spacer or you have one heck of a vacuum leak. That is your 1st problem. After you solve that one see if it is still getting too much fuel. Check to see if the needle dried out while the carb was setting by replacing the needle and seat. You may have to get a full carb kit if you can't source just the needle and seat.
 

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If this thing is not pouring black smoke, I would think it is likely either the idle adjustment or the high speed adjustment for the choke has an issue. This 307 is going to idle faster on the same amount of fuel/air than a worn out 350.

John
 

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won't drop below 5000 rpms! The throttle blades would have to be pretty close to fully open to get to that rpms. You definitely have a huge vacuum leak somewhere. Did you use silicone or the cork gaskets on the end rails for sealing the intake to block connection?
 

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A gap at the china walls won't cause a vacuum leak, just an oily mess with leaks, unless the intake isn't sitting flat on the head. Was the intake or heads machined for some reason?

This is so bizarre, I have to ask, you put a new intake gasket on, right?

How are you checking for vacuum leaks? Have you disconnected and plugged every port on the base of the carb and run it? Like others have said, you have to have a massive amount of intake air for this to happen.
 

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thanks for the correction on the end rails. Never really thought about it until now that it won't leak vacuum from there.

Just a thought, does your tachometer drop to zero when the engine is shut off? Do you have a timing light or multimeter that will read engine rpms to rule out a tachometer issue? 5000 rpms is very high rpms for a small vacuum leak......did you install intake gaskets?
 

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[QUOTE- the only thing that has been changed is a half inch carb spacer instead of the factory quarter inch spacer.
)[/QUOTE]

There's the root of your problem.
The factory spacer (your term) is a gasket. You replaced it with some sort of spacer. Your throttle plates are making contact, your linkage is making contact or you have a vacuum leak from poor gaskets on your new spacer.
It's the only thing you changed.
 

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The throttle blades are not the issue. If the engine won't drop below five thousand rpms you wouldn't mistake the throttle blades being open to far for proper idle.
 

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The throttle blades are not the issue. If the engine won't drop below five thousand rpms you wouldn't mistake the throttle blades being open to far for proper idle.
Would only need an opening of about 1/16"-3/64" to get a 4-5000 rpm situation in neutral. Could be rear throttle blades too.
 

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Phenomenon

Raymond, You've got a float issue in the carburetor. Either the needle is hung up in the open position or the float level is too high or both.

I have witnessed this phenomenon twice with the quadrajet my 454 motorhome. It did it one time while driving on the highway, had to get on the brakes in heavy traffic, foot off the gas and on the brakes. Engine was still screaming about 4000rpm, put it in neutral, got it to the side of the road and shut it down. Got in the engine compartment, checked the throttle linkage found nothing wrong. Cranked it up and it idled perfect and never had a problem with it for about 7-8 years.

Second time I witnessed it was in the driveway at home, it was fast idling on the choke ~1200rpm, suddenly the engine speed climbed to about 3500 +. I kept tapping the gas pedal thinking it was hung up on the fast idle cam, it climbed to ~4000rpm and I shut it down.

When I checked the carburetor the top gasket was wet at the seam. Pulled the top off the carburetor and the bowl was FULL. I checked the needle and seat for trash, none. When I checked the float level by eye it appeared good. I bent the tang a tad to drop the float level about 1/8-3/16" because it was obvious the bowl was running over. Apparently if the float level is at or near the maximum height the needle can hang in the seat. When the bowl fills completely to the top it floods over and supplies raw fuel straight to the butterflies. With engine vacuum sucking it in, the engine will run at uncontrolled speed with the butterflies in the idle position.

By the way, I know that the float level was set with a float scale at the recommended chart height because I rebuilt the carburetor myself when I first bought the motor home. The chart was either wrong or possibly the repacement needle and seat was not quite the correct height or depth to keep it from hanging up at that float height. Nolan
 
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