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Old 11-24-2009, 01:13 AM
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427 tri carb runs poorly after warmup

Hello,
My friend owns an original 69 Vette with a 427 tri carb set up and 4 spd. It's a total restoration to original specs. It's a beautiful car.
The engine starts great and runs fantastic until it reaches operating temp, about 185, then it acts like it is suffering from vapor lock. He lets is cool down and it starts back up with no problem and runs like a scald rabbit until, again, it reaches operating temp.
So far he has insulated the fuel lines, replaced the spark plugs, plug wires, rebuild the carbs, checked the timing, checked for vacuum leaks, replaced a power valve..... you get the picture.
I don't know what distribtor he's running at this time, I'll find out.
Does anybody have any suggestions? Is there any other information I have left out?

Macky

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Old 11-24-2009, 05:33 AM
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Check the fuel line and tank sock for restriction,any restriction causes vacuum in the line to go higher allowing the fuel to flash to vapor.Assuming all was well and problem just came up?
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Old 11-24-2009, 06:43 AM
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You might have him try popping the latch on the cap and see if that helps. The cap's vent might be closed off causing the tank to form a vacuum.
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Old 11-24-2009, 07:18 AM
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Are we talking about 3 rochester 2 bbl's? What kind of fuel pump?
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Old 11-24-2009, 11:55 AM
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My dear old dad had a similar problem on a Hemi once, his problem was that only one carburettor was actually delivering fuel, so when the choke came off it leaned way out.
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Old 11-24-2009, 12:32 PM
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Quote:
Originally Posted by sqzbox
Are we talking about 3 rochester 2 bbl's? What kind of fuel pump?
I don't think so. 427 vette engine came with 3 Holley carbs with 2 vacuum secondaries. It sound to me more the center carb is leaning out. He may have to reduce the exhaust crossover from heating the center carb. It can also may have an internal vacuum leak inside the carb. Could be a warp throttle plate, too.

Last edited by lg1969; 11-24-2009 at 12:48 PM.
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Old 12-01-2009, 12:03 AM
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Quote:
Originally Posted by lg1969
I don't think so. 427 vette engine came with 3 Holley carbs with 2 vacuum secondaries. It sound to me more the center carb is leaning out. He may have to reduce the exhaust crossover from heating the center carb. It can also may have an internal vacuum leak inside the carb. Could be a warp throttle plate, too.

Thank you everyone for responding to this puzzling problem. The carbs are definitly Holley, the engine has been run with the fuel cap loose and fuel lines checked for fuel flow restrictions.
The next thing, I am assuming he hasn't checked this, is to block off the heat riser port, and reinspect the carbs again. I'll let you know what he finds out.
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Old 12-01-2009, 08:53 AM
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Do you happen to know the jet size on the center primary carb?

Vince
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Old 12-01-2009, 05:31 PM
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You may want to put a fuel pressure gauge on it and make sure the pressure isnt going away
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Old 12-01-2009, 09:22 PM
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it sounds like a vaccum leak , what are the carb base gaskets like, try some new base gaskets, or it could be a cracked intake manifold, ( on the lifter side of the intake) or bad intake gaskets.
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Old 12-01-2009, 09:40 PM
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" vapor lock" ......To me when you say this it means will not start and that usually happens after the engine is shut down . The fuel boils out of the line/carb and the engine is starved for fuel until it has be replace as a result of restarting the motor. Just trying to understand what you are trying to describe .

Next you say "cool down and it starts back up with no problem, runs like a scald rabbit"

"scald rabbit" ? driving it putting your foot in it, tri-power open ? that kind of scald rabbit ?

or are you saying it ideals smooth and is overall drivable ? if the later,
I would check the choke adjustment make sure it is open at operating temp.

How far of a cool down? cold ambient/room temp or just anywhere below 185 ..

I believe that tri-power would have progressive linkage, having trouble visualizing vacuum actuated outboard carbs.
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Old 12-02-2009, 08:37 AM
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The early tri power Vette's had vacuum linkage on the outer carbs from the factory. A popular performance upgrade at the time was a mechanical linkage kit. I happened to be working at the local Chevy dealership in 1969 and I saw numerous kits installed on New Vettes. It did not affect the warranty.

Vince
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Old 12-02-2009, 08:45 AM
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I used to have a 68 435. I ended up with a 500 center as the stock 350 cfm was not very responsive. Fuel pressure at idle is going to be 1 thing,also make sure the float level is right.Blocking off the heat riser is a normal deal.How many times do we need to preheat for those cold snowy days? lol After it warms up,try putting your hand over the center throat and see if it dies. You will know if its lean or not or has a vacuum leak. Also make sure you have the .110 needle and seat in the center carb.The stock is .097. Get a temp reading on the exhaust manifolds. Temp guns are cheap.Take a reading on the intake below the center carb.Jets wont effect idle as they are not in the picture, a high power valve will make it idle rich,just make sure it is a number lower than the idle vacuum. Its kinda hard to take a pressure reading with the stock fuel line,just disconnect at the pump and hook up a pressure gauge,it will run long enogh to get a reading.
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