On a 68 435 horse rear carb I'm haveing problems. When you shut engine off it will leak gas into the intake. I have replace needle and seat twice already. Float set correct. When running, I'ts fine, just when you sut it off! Any ideas? Tommy

 

The metering block on the carb is probably warped.
We were able to get new ones for both outer carbs from Holly through Summit. I think it could cause serious damage to your engine if you don't repair the leak (hydraulic lock or even fire).

 

 

Outboard carbs don't have metering blocks. Maybe you're referring to the plates.

 

Where's it leaking gas from???? Can you see it coming through the boosters once you turn the engine off?
If so remove the needle and seat and check it for debris.
If your just finding gas in the intake and wetting the plugs...could be the power valve passing (not sure if the Holley two barrels have them)
The center one of my BG six shooter does.

Nick

 

after you set the float and run it do you go back and recheck the float height? this is where the clear sight plugs come in handy.

just curious as i had a similar problem on my 69 coronet with 6 pack last summer.went through needles and seats, floats. drove me crazy for a little while.
my problem ended up being the new carter hi pressure fuel pump i had installed. went back to a stock flow/pressure pump and all the problems stopped.
it was weird too because it only affected the rear carb, it just built too much pressure at the back and was forcing fuel past the seat and affecting the float height.

maybe some carb experts can explain what exactly was happening but switching back to the stock pump was the only thng that solved my problem.

 

Fuel pressure may be the problem. I've herd oldtimers talk about this setup would have been great if they had regulators back in the day. It's a stock pump but they have a lot of pressure I think! I might switch front to rear and see if anything changes! Thanks all for the ideas! Tommy

 

Once engine is turned off, fuel pump shuts down and has no bearing on the gas dripping.

I'm curious, how did you determine that you have gas leaking into the intake when motor is turned off?

 

that is why a lot of 68/69 435 HP corvette do not have the orignal engines. the oil got deluded by the gasoline and cause bearing problems. we fixed the problem by using a gas cap that vented the pressure to to outside because there was pressure build up in the gas tank that overcome the needle/seat. the end carbs use the nytrophil floats that have a shorter fulcrum arm that does not excert as much closing pressure on the needle/seat.

 

Sorry Tommy, I should have said that we replaced metering plates.
Before discovering that the warped plates were the problem on my brother's 69 L71 we replaced the fuel cap with the correct vented cap, next we set the fuel levels on the carbs using clear sight screws, next we replaced the fuel pump. None of these worked. We contacted the Forum but none of the suggestions worked. We phoned Holley Tech.; no idea. The previous owner had fitted a fuel tap which at least limited the leak to the contents of the fuel bowl. The leak just kept the butterflys and lower carb body damp (engine off). It was difficult to believe that it could cause a cylinder to fill in two days( hydraulic lock) if the fuel tap had not been switched off. In desperation we phoned Holley Tech.again. This time they suggested warped metering plates. That was the problem in our case. When we put them up to a straight edge they were very badly warped. I hope you can solve your problem more easily. I think to remove the plate on the rear carb you will have to remove it from the manifold. I hope I am wrong. We felt that we had been very very lucky not to have caused serious damage to the engine.

 

If the needle valve & seat are in good shape & set properly, then I vote with the above. If it were high fuel pressure, the other two carbs would be having similar problems. The metering plates do warp, and they also "dissolve" when left to sit in sour gas for any length of time. This happens when the TriPower setup (or any Holley carb) is removed with gas left inside, and left to sit for a long period of time. I can provide pics of metering plates, metering bodies, and carbs that have been damaged like this.

You do have to remove the rear carb to get at the fuel bowls screws.

There are no "squirters" or power valves in GM BB Tripower secondary carbs (FYI).

The kits are cheap..... if you're sure the float is working properly, pull the carb and examine the metering plate (not metering block). The center carb has the "block". I have straightened the plate on occasion, and it worked, but you can buy a new plate.

Also, in certain circles, I have heard people recommend removing (and not re-installing) the secondary metering body plate (Holleys own misnomer)... the thin plate between the metering plate & the carb. Don't do it. It is required.

If/when you separate the metering plate & the metering body plate from the carb, soak the carb in carb cleaner until the gaskets soften.... don't try to pry it apart until it is loosened by soaking... this will bend it, and it will leak. Hint hint.

You will also need an inch-pound torque wrench to properly re-assemble the carb & prevent damage & leaks.

The screws at this location are clutch head screws... Sears sells the correct screw drivers for these, but you will need a clutch head bit for the torque wrench instead.

Edit:
I don't lube metering plate clutch head screws.... they tend to work themselves loose over time. Petroleum jelly on the rest.

Location, thread pitch, dry torque, lubed torque.

Torque, inch pounds: Thread Dry Lubricated (-20%)
Fuel Bowls 12-24 25-30 20-24
Throttle Body to Main Body 12-24 30-50 24-40
Primary (Main) Jets 1/4-32 30-40 24-32
Power Valve, several small holes 1/2-28 40-50 32-40
Power Valve, two large holes 1/2-28 100 80
Discharge Nozzle (squirter) 48-52 38-42
Choke Cover 8-32 12 10
Secondary Metering Plate 8-32 12-18 10-14
Float Hinges etc 6-32 7-10 6-8
Carb Mounting 5/16-24 60-80 50-60
Fuel Inlet 9/16-24 175-200 140-160
Fuel Inlet 7/8-20 200-250 160-200
Accelerator Pump Cover 8-32 12-18 10-14
Note: Lubricated torque figures can be 10 to 25% lower than dry torque figures.

Source:
http://www.holley.com/Store/product.asp?ID=36-71

 

this is a problem with older carbs but the ones i referred to were brand new corvettes. the older carbs the plates,metering bodies and the carb main body warp and i would think if you had your carbs redone by someone who knew what they were doing they would have corrected these problems

 

PROBLEM SOLVED!!!!!!
It was the metering "PLATE" that was warped inside the carb!
Thanks to all you for the advice and help.
Tommy