More Fun with a Rochester Carb
04-02-2010, 12:34 PM
#1
Burning Brakes
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More Fun with a Rochester Carb
I really like these carbs and Lars has inspired me to keep at it. He has been kind enough to trade a few PM replies with me recently and now I think I have something that others may get some value from...
Looking at Lars's carb paper, table 1 shows that my car came with carb#7043201 and 77 jets with 48 rods having the preferred smaller 0.026" rod tips. This gave 2.8471 thoudandths area at cruise and 4.1257 WOT.
I have a JET #36002 carb that uses the newer primary "M" rods with their 0.036" tips. It came out of the box with 73/50M - way too lean! Unfortunately, by quick visual inspection, the M rods are also shorter and the taper is way different than the older rods so a direct swap out for thinner looks like a no go... Anyway, I wanted to start at the as-delivered metering area if I could...
Now I have the 77 jets (the biggest I have) and 48M rods (the smallest I have) in the car. It is really close but still maybe a tad lean at WOT and just a touch of lean surging at cruise (say 40 mph). I plan to adjust my APT - which with the simple allen plug and direct-access phillips screw through the top of the carb is why I went with a later-style carb anyway... - to deal with the cruise lean if possible, but I note that the screw is already pretty high up and the rods don't travel too far down in the carb as is with the current APT screw adjustment in place.
I compute that using any M rod with its 0.036 rod tip I get a WOT "M" rod total area 3.639 - far short of that 4.1257 desired starting point for WOT. Dropping down to a 48M primary rod seems to provide a cruise number close to stock specs - 2.847 at cruise. JET claims to have modified these carbs for performance so they are likely using a different set up in the Venturis, etc. (thanks Lars for that info).
Bottom line - as best as I can tell, the WOT metering may be impossible to reach using M rods - even with #80 jets if I had any... Per Lars, I am using WOT as the first step in carb tuning. I have verified that my float is at 0.375" (12/32" on the paper t-scale provided in the rebuild kits) as he advises us to do.
So.. options? Anybody make M style rods (in taper, length, placement of thickest part of the shank, etc) but with the preferred thinner 0.026" tips of the older rods? Those would be great! Otherwise, my carb may be at its richest possible settings with parts I have on hand... I also recognize that my new top-end and cam will seal the engine up better and draw more air in than before - requiring more fuel as well than before...
Looking at Lars's carb paper, table 1 shows that my car came with carb#7043201 and 77 jets with 48 rods having the preferred smaller 0.026" rod tips. This gave 2.8471 thoudandths area at cruise and 4.1257 WOT.
I have a JET #36002 carb that uses the newer primary "M" rods with their 0.036" tips. It came out of the box with 73/50M - way too lean! Unfortunately, by quick visual inspection, the M rods are also shorter and the taper is way different than the older rods so a direct swap out for thinner looks like a no go... Anyway, I wanted to start at the as-delivered metering area if I could...
Now I have the 77 jets (the biggest I have) and 48M rods (the smallest I have) in the car. It is really close but still maybe a tad lean at WOT and just a touch of lean surging at cruise (say 40 mph). I plan to adjust my APT - which with the simple allen plug and direct-access phillips screw through the top of the carb is why I went with a later-style carb anyway... - to deal with the cruise lean if possible, but I note that the screw is already pretty high up and the rods don't travel too far down in the carb as is with the current APT screw adjustment in place.
I compute that using any M rod with its 0.036 rod tip I get a WOT "M" rod total area 3.639 - far short of that 4.1257 desired starting point for WOT. Dropping down to a 48M primary rod seems to provide a cruise number close to stock specs - 2.847 at cruise. JET claims to have modified these carbs for performance so they are likely using a different set up in the Venturis, etc. (thanks Lars for that info).
Bottom line - as best as I can tell, the WOT metering may be impossible to reach using M rods - even with #80 jets if I had any... Per Lars, I am using WOT as the first step in carb tuning. I have verified that my float is at 0.375" (12/32" on the paper t-scale provided in the rebuild kits) as he advises us to do.
So.. options? Anybody make M style rods (in taper, length, placement of thickest part of the shank, etc) but with the preferred thinner 0.026" tips of the older rods? Those would be great! Otherwise, my carb may be at its richest possible settings with parts I have on hand... I also recognize that my new top-end and cam will seal the engine up better and draw more air in than before - requiring more fuel as well than before...
Last edited by djcwardog; 04-02-2010 at 12:37 PM.
04-02-2010, 11:14 PM
#2
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Take your "M" rods, chuck them up in a drill motor clamped in a vise, and use a little sandpaper on a popsicle stick to turn your .036" "M" tips down to .026" "K" tips. With about 30 minutes worth of work, you can have a pair of "K" rods for your carb so you can do some good WOT tuning. You may have to drop down a few jet sizes, but you'll now have a good "tuning range" to work with.
Lars
Lars
04-03-2010, 07:52 PM
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Take your "M" rods, chuck them up in a drill motor clamped in a vise, and use a little sandpaper on a popsicle stick to turn your .036" "M" tips down to .026" "K" tips. With about 30 minutes worth of work, you can have a pair of "K" rods for your carb so you can do some good WOT tuning. You may have to drop down a few jet sizes, but you'll now have a good "tuning range" to work with.
Lars
Lars
