Can't maintain fuel pressure

[siteunseen]

3 hours ago, jalexquijano said:

Just talk to an experience Corvette Mechanic. He argues that the problem with these cars is the amount of heat coming out of the manifold and heating up the float bowls on both 3 screws rounded SU CARBS. I already have the headshield, and heat shield extensión installed. He says i should also wrap it with the same insulation i used for the Steel fuel rail and other recommendations he wil explain tomorrow when we test the car on heavy traffic. Should i also wrap the exhaust tube that is connected to the manifold? WHat type of insulation provides the best heat dissipation?

I agree with your corvette mechanic.  I have msa's ceramic coated headers in place of the big arse piece of cast iron exhaust manifold.  No where near the heat under the hood now. Never ever never had an issue recranking mine and it gets pretty hot around here too.

I've wondered if there's a way to push the hot fuel through the rail and carb bowls with fresh gas? Some way to purge the fuel through the return line.

[siteunseen]

2 hours ago, Patcon said:

@SteveJ

Sorry for the thread jack, I'm done...

Me too! I thought this was one of many he has going. Sorry Steve.

[gwri8]

18 hours ago, SteveJ said:

Going by the limited image in the parts manual, I would think that the 280Z heat shield might not extend far enough too shield the carburetors from the headers, though I like how it goes up to the head to shield the intake runners. Aside from that, it's a matter of fabricating mounts to go on the SU intake manifold.

Out of curiosity I measured an extra heat shield I have, It comes to 7.5"W X 22.5" L.  Just for future reference in case you ever get spare time on your hands.

[jalexquijano]

On ‎04‎/‎22‎/‎2017 at 6:15 PM, Patcon said:

Yes! Rubber hoses! This is a process of elimination. Try some rubber hoses and bypass the metal fuel rail. If it helps come up with a more elegant permanent solution later! Solve the problem first, then make it pretty...

Well the mechanic i am using now used to repair and manufacture parts for the locomotives that pull the vessels transiting the Panama Canal. He is very Smart and has concluded that the issue with my car is the amount of heat produced by the manifold as when the car is at idle during traffic the heat makes the gas boil and evaropates at the bows. We will be replacing the metal fuel rail with 5 feet long 1/4" rubber hoses to see if it solves the vapor lock or fuel percolation issue. He also recommended pulling the heat shield extensión out and wrapping it in header insulation.

[Diseazd]

What puzzles me is that vapor lock only happens on the 73 and 74 Z's. even when you change to round top (early) carbs, you still have a vapor lock problem with the 73 cars.  WHY???? I've never had a pre 73 Z give me a bit of problem with vapor lock....there must be a reason.....but my 73 gave me fits in the heat.....until I built a 72 motor for it....then, no more problems....why? Must be something peculiar to the 73 Z's.

[siteunseen]

Do the pre '73s have the air tube on the exhaust manifolds? Both my '72s did not but  '73 motor I bought had those nipples on the manifold.

[Mark Maras]

1 hour ago, Diseazd said:

What puzzles me is that vapor lock only happens on the 73 and 74 Z's. even when you change to round top (early) carbs, you still have a vapor lock problem with the 73 cars.  WHY???? I've never had a pre 73 Z give me a bit of problem with vapor lock....there must be a reason.....but my 73 gave me fits in the heat.....until I built a 72 motor for it....then, no more problems....why? Must be something peculiar to the 73 Z's.

 What carbs and manifolds did you use on the 72 motor?

 Cliff brings up another possible piece to the puzzle. 

 I've been wondering if the problem lies in the 4 screw (early) carbs and the later 3 screw carbs. We'll leave flat-tops out for now. It's been reported that blocking the coolant flow to the later carbs makes little to no difference. I've not worked on 3 screw carbs. Is the coolant passage the only major difference between the two?

[Diseazd]

3 screw or 4 screw.....no problem with the pre 73 Z's. 3 screw or 4 screw or flat tops are a problem in the 73's. It must be something else peculiar to the 73 cars. I don't have a clue, but all I know is I never had one case of vapor lock in any of the many 70, 71, or 72 Z's I've owned, but had a problem with the only 73 I've ever owned, even though it had 3 screw round tops. How about it.....anyone ever had vapor lock problems with a 70, 71 or 72 Z? Nissan never added insulation to the fuel lines till 73 which makes me think it wasn't a problem till 73. 

[siteunseen]

I agree that removing the coolant lines had little to no benefits on my '72.  After rebuilding the motor and firing it for the 20 minute run in I had some leaking so I shut it down and plugged all that going through the manifolds.  I can't tell one second of difference between warm up times but I did eliminate future leaky areas so I'll not go backwards.

The only things I've noticed between a '73 & '72 is the rod size and the air pump nipples. 

Dizeased did you use '72 manifolds and balance tube on your '73? I can't think of anything major that is different but then again I DON'T CARE, MINE RUNS GREAT. 

I bet it's those dang front bumper mounts!

That extra weight.

[siteunseen]

1 hour ago, Mark Maras said:

 What carbs and manifolds did you use on the 72 motor?

 Cliff brings up another possible piece to the puzzle. 

 I've been wondering if the problem lies in the 4 screw (early) carbs and the later 3 screw carbs. We'll leave flat-tops out for now. It's been reported that blocking the coolant flow to the later carbs makes little to no difference. I've not worked on 3 screw carbs. Is the coolant passage the only major difference between the two?

The coolant lines and the float ears is all I can say. The 3 screws have longer in front carb. I put two rear carb lids on mine and it finally runs like what I suspected it should, good up hill and everywhere else.  Floats were easier to set, hopefully I'll never have to touch them again.  Whichinever nephew gets it might have to but I'll be worm food.

[Mark Maras]

My early 71 started with a click of the key. If there was ever a problem with heat, the only thing I noticed was it took a couple of revolutions for the engine to start. No problems running at all. That minor change may have been heat soak, to a minor degree.

 

[Diseazd]

Cliff....Yes the 72 intake and balance tube....disconnect manifold water on all my cars. The 73's had a large chamber head....very different from the earlier cars....the only real difference that I can think of.....maybe it gave off more heat. Someone posted somewhere that the fuel pumps were different, but I can't see where that would be a problem.....but something causes these cars to vapor lock.