Hi All.We have a '39 Mercury engine which is 276 ci and is running triple 94 carbs on an Edelbrock US 359 manifold.Trouble is we have a major flatspot between 1800 and 2800 revs under load and we don't run it much over 4000 revs.The carbs have the power valves blocked off,no.50 jets and correct float level.It has been suggested that for our use we block the middle one off and it shouild run better on two.If we did that would we run power valves or should we leave the three with power valves fitted or just the middle one.Advice appreciated.THANKYOU.
advice on carbs
- Thread starter GEOFFNZ
- Start date
ken ct
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The .050 jets sound ok in the middle carb but run a 5.5 PV in center one and .046 MJ's in the 2 outer ones with 3.5 PV's in the outer 2 also. Idle mix screws closed in outer ones also. Jetting is a crap shoot anyhow. I assume you are running progressive linkage on there. This is OMO others might disagree on it. ken ct. usa.
When PV are blocked off the main jets need to be enlarged about 3 sizes to avoid the lean conditions. This will give an over-rich, but smooth, operation at some load/speed settings while avoiding the lean, flat spots.
I prefer to use low vac. PV in all the carbs if running straight linkage. If using progressive linkage the outer carbs can have blocked PV and larger jets than the sizes needed for carbs with operational PV when those carbs are used only for WOT(wide open throttle).
I prefer to use low vac. PV in all the carbs if running straight linkage. If using progressive linkage the outer carbs can have blocked PV and larger jets than the sizes needed for carbs with operational PV when those carbs are used only for WOT(wide open throttle).
Last edited:
Charlie ny
BANNED
Geoff,
You'll receive many good answers on your ?????. I like the
50 or 51 main jets with a 5.5 pv in the middle but keep the pv's in the ends
blocked with 50 jets using prog linkage. I resist the use of pv's in the end
carbs for practical reasons ,potential for leaks down the road. This poten-
tial for leaks can drive you nuts attempting to unearth the source of
black exhaust emanating from your pipes. I've heard gas is not cheap
in your part of the world. My suggestion as a source for pv's is genuine
Holley. The Holley "hg is right on the money....however a bit of additional
clearance is required in the relief in the bowl to clear the brass ring on
the stem. A Dremel tool with a small burr or an end mill will do the job.
As a Christmas gift from all us guys up here I'll send you a 5.5
Holley pv and correct gasket if you forward your address to me off
site. My email is cas5845@yahoo.com.
Merry Christmas and a Happy New Year !
Charlie ny
You'll receive many good answers on your ?????. I like the
50 or 51 main jets with a 5.5 pv in the middle but keep the pv's in the ends
blocked with 50 jets using prog linkage. I resist the use of pv's in the end
carbs for practical reasons ,potential for leaks down the road. This poten-
tial for leaks can drive you nuts attempting to unearth the source of
black exhaust emanating from your pipes. I've heard gas is not cheap
in your part of the world. My suggestion as a source for pv's is genuine
Holley. The Holley "hg is right on the money....however a bit of additional
clearance is required in the relief in the bowl to clear the brass ring on
the stem. A Dremel tool with a small burr or an end mill will do the job.
As a Christmas gift from all us guys up here I'll send you a 5.5
Holley pv and correct gasket if you forward your address to me off
site. My email is cas5845@yahoo.com.
Merry Christmas and a Happy New Year !
Charlie ny
Thank you all for your responses ,especially Charlie.A member recently found me the correct g/box input gear as well so your experienced advice and generosity are much appreciated out here on the other side of the world.The trouble with a 50 yr old race car is that most parts have been taken to the limit and or much modified.And, yes gasoline is very expensive here at about NZ $8.40 for a US gallon so nothing is going to wear out in a hurry.
All the best to you all for Xmas and the new year whatever it may bring.
Geoff.
All the best to you all for Xmas and the new year whatever it may bring.
Geoff.