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So I was rebuilding my carburetor and discovered that one of my metering rods was broken (the power tip snapped off). After a quick email with Lars I learned that I was going to run into the problem that many other late-model quadrajet owners have had, being that nobody makes the rods I need anymore (53K). On his suggestion I got myself a pair of 53M rods and carefully turned one down to have the .026 power tip that I needed. In this process I made an observation that interested me enough to make this post.
Pictured below are two metering rods, the one on the left is the original 53K (with a broken power tip), and the one on the left is the unturned 53M:
The cruise/idle portions of the rods are different, the 53K has no taper, and the 53M does. Measurements as follows (matching the positions indicated int he picture below):
A- .053in (53k rod)
B- .053in (53K rod)
C- .053in (53M rod)
D- .042in (53M rod)
I'm mostly just interested in how this affects the way these rods behave compared to one another, if at all and why they are made this way. I also only started thinking about this different after I installed the new rod, so currently my carburetor has one 53K rod and one 53M rod that has been modified. Should I bother going through the effort of grinding down this other rod to replace the 53K and have both metering rods perfectly match? Would I ever notice a difference?
Yes....the 53M is much smaller at the end because when you are accelerating at WOT.....the rods will pull up and let that much more fuel through the jet because the needle is smaller...thus not plugging the hole in the jet as much......when you are at idle to off idle...the rod is pulled down into the jet.....and the steps that are on the rod are the calibration at that throttle opening/engine speed/load.......at idle or off idle, the vacuum is high which pulls the metering rods down overcoming the spring.....
Hope that helps....
Yes....the 53M is much smaller at the end because when you are accelerating at WOT.....the rods will pull up and let that much more fuel through the jet because the needle is smaller...thus not plugging the hole in the jet as much......when you are at idle to off idle...the rod is pulled down into the jet.....and the steps that are on the rod are the calibration at that throttle opening/engine speed/load.......at idle or off idle, the vacuum is high which pulls the metering rods down overcoming the spring.....
Hope that helps....
Jebby
I more or less understand the idea behind it, I guess my curiosity is more as to why the 53K rod doesn't have this same taper. Where the 53M has this taper, followed by a small step into the power tip, the 53K has no taper, and a bigger step into the power tip.
I more or less understand the idea behind it, I guess my curiosity is more as to why the 53K rod doesn't have this same taper. Where the 53M has this taper, followed by a small step into the power tip, the 53K has no taper, and a bigger step into the power tip.
Hope I'm making sense here
The only thing I can think of is strictly cost......the rod with the taper is going to be a lot more apt to meter as opposed to the stepped deal......I believe since it is up higher on the rod that it is for off idle and part throttle running and with the taper would meter a lot more precisely than just the step....which could in effect do nothing given the stroke of the rod as it goes up and down.....you have to understand that these rods are constantly moving......I would suffice to say that if Lars told you to get these rods that is the reason why. A lot more fuel economy to be had with a tapered rod......ALL motorcycle carb needles are tapered for this reason.
As Jim noted in his link above, the different letter-code rods have different tapers. The original "K" rods have a very sharp taper - almost none. The "M" rods actually have a very nice taper transition to provide very linear fuel transition through the power range, but they have those fat power tips. If you turn the "M" rod tip down to .026, you actually end up with a superior primary rod than the original "K" rod. You probably won't notice any actual difference by running the one "K" rod and the one modified "M" rod, but if I were you, I'd modify one more "M" rod and give your carb a nice taper-rod upgrade.
.a lot of these small changes were made to meet part throttle emissions. I rebuilt Q jets that had damaged or missing primary rods and all I did made sure was the tip was the correct diameter so WOT was correct. there were some olds performance Q jets that had no primary rods
Last edited by PAmotorman; 04-22-2019 at 11:30 AM.
04-21-2019, 03:17 PM
primary metering rods observation and question
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