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Post '75 primary rods

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scottyp99:
Cliff, I just received your book on Q-jets a few days ago, and I have not finished reading it yet. I gave the whole thing a quick skim, tho, and I believe I have some idea what you are talking about. Once I have read through your book more thoroughly, and made a few measurements, I'll be back in touch with you. For now, could you very briefly explain why the carb worked on the 305 Malibu that it came off of? My 350 isn't that much different, is it? I understand that you are a busy guy, so be brief, and I promise not to bother you again until I am a little smarter about all this. Thanks,

Scott

EDIT-- Just wanted to make sure you know that I understand what you mean when you say your rods are not K rods. I keep thinking of any rod that is the right length and has a .026 powertip as a K rod. I have a couple of 50M rods, is the taper on your 44 rod similar?

Cliff Ruggles:
"K" rods are for the later carbs.  They are "stepped" rods, having a .005" taper on the upper section, and .026" tips.

They also made "P" rods for those, which have a .010" tapered upper section, also "stepped" down to .026" tips.

"J" and "L" rods are not stepped, and taper gently at first, then more quickly down to .026" tips.

"M" rods are stepped, with .010" taper on the upper section and .026" tips.

Most early carbs used "B" serier metering rods, with are very similir to "L" rods, but longer for the early carbs.

They also made rods with no markings at all, and I've seen some stepped rods that are similiar to the "K" rods used on later units, but they are quite rare.....Cliff

scottyp99:
OK, now I'm confused again. I have a set of 40K rods and a set of 50M rods. The k rods have a very slight taper, and then a sudden drop off right at the powertip, but the M rods start out at .050" and taper all the way down to the powertip, which is .036". (I assume it's .036", it's noticably thicker than the k rods I have) I don't really have enough experience with metering rods to have a preference, but I thought I was starting to get the terminology, and now I get this curveball. What am I missing here? Is it just that I am misunderstanding what "stepped" means? I take it to mean a sudden reduction in metering rod diameter, as opposed to "tapered" which I take to mean a gradual reduction in diameter.

I got into this in the first place because I thought I had a handle on what I was doing after studying several papers written by Lars Grimsrud, and studying a book on Rochester carbs by Doug Roe, along with numerous other sites on the internet. I figured all you need to do is get the jet metering area right for WOT, and then adjust the cruise metering area using the metering rod. Now I have to deal with this air bleed stuff. And nobody seems to want to make metering rods for my carb. Oh, well, in for a penny, in for a buck, as my Grandpappy used to say. I'm reading the book, I hope it will make things clear to me.


Scott

Cliff Ruggles:
"M" series rods are also "stepped", it's just not as pronounced as the "K" and "P" series rods.

A 50M metering rod tapers from .050" to .040" on the upper section, then steps to .036"

The "M" series metering rods and made for the later single main airbleed carburetors, as they have a stronger signal to the jets and less emulsion air coming in as the earlier styles with two pairs of main airbleeds.

The SMAB carbs started showing up in 1977, on Chevy trucks, but were still the early divorced choke units.

In 1979 they started showing up on Chevy applications in the later APT style carbs, and were in place across the board around 1980 (side inlet castings only).

When tuning SMAB carbs, refer to the last Chapter in my book on the Edelbrock 1910 units......Cliff

scottyp99:
OK, Cliff, I have finished reading your book, and I have come to several conclusions:

1: It looks like I am going to have to perform some modifications to this carb, so I'm going to have to take it off and replace it for now with another carb that I have. That way I can do a proper job, and not half-*ss it

2: The other carb that I have is a Summit Racing rebuilt Q-jet that ran ok on the car before I got this new carb. When I was taking it apart, I kinda tweaked the arms on the power piston that hold the primary metering rods. I tried to get them even, but I would like to know if you could tell me how far down into the jets the rods should go when the power piston is seated. Right now, the part of the rod where the main shaft ends and the machined "cruise area" part of the rod starts, is even with the bottom of the screwdriver slot in the jet when the power piston is seated. Should it be higher or lower than this? I suppose I could disassemble the '79 carb and remove the power piston and check it, but that seems like an awful hassle if you can just tell me. If not, I guess I'll just have to do it. I may just do it anyway, depends on whether or not I can get another one.

OK, thanks for the help so far, I'll be in touch when I am ready to make my order.


Scott

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