I searched through the venerable q-jet forum for what the numbers and letters stand for and I could not find any real answers for what the letters mean. I know they denote a difference of taper at the bottom. I've got 2 sets of late model q-jet primary rods, one set of 51P and one set of 51M. What is the real difference between these 2 rods. I know they denote different tapers and I did read somewhere in Doug Roe's book that the "M" denotes a truck rod taper with a different tip size. but is there really that much difference between them. How much difference should there really be from main jet size numer to rod size number. I've seen alot of people mention the 0.030 difference being the optimum for the difference between the main jet and the primary rod. I have so many questions regarding the proper set up for a q-jet on a buick 455 that it is not funny but I do not have much knowledge on the buick 455. If any one could help I would greatly appreciate it.
Garth, You'll Need A Caliper To Measure The Taper Points And Check For Different Lengths Of These Rods. Also Install Them Into The Carb And See Where The Needle Sits With The Piston Depressed. This Simulates Full Vacumn Or Part Throttle/cruise Conditions. Very Small Changes Can Lead To Big Fuel Mileage Savings By Experimenting With Different Rod/jet Combinations. Since The Idle/transfer Circuit Is Fed Through Tubes And Other Restrictions The Metering Rod Doesn't Affect It. Full Throttle Power Is Determined By The Rod Tip. Using The Tables In Roe's Book You Can Determine The Area Of The Restriction By Subtracting The Metering Rod Tip Area From The Area Of The Jet. This Is To Establish A Base For Tuning. The Best Results I Have Gotten From A Q-jet Are The Idle/transfer Circuit Modifications. There Is An Easier Q-jet Book To Follow By Cliff Ruggles Which Recently Came Out With Better Pictures. I Keep The Roe Book Around Because It Has Good References. Good Luck
Don't want to hyjack... but I have a carb off a 1973 LeSabre. Has 73 jets, 49C pri. rods.. with CT secondary rods. Looked at the stuff from Doug Roe's Book posted earlier.. What's up with the Oldsmobile 'C' Rods? Should I be going with a different taper? I have a '73 short block, '69 430 heads, alum intake, full length headers w/ 3" exhaust... 3.42 gears in a gbody. I've got new 75 & 76 jets to play with, but not too sure about the rods.. I know I have to change them to fix cruise a/f ratio..... Any help is appreciated... Thanks Devon
I know it's lean, I'm just trying to get an idea of where to go. With a 75 jet and my 49 rod, I'll be running about the same open area @mid throttle as you Nicholas. Obviously richer at WOT though... Talked to a carb guru around my area. He said he wouldn't change the rods. Others say you have to go up 1 size to get the a/f ratio correct at cruise speed when you jump jet sizes. Other than fuel mileage and those dreaded emissions, what can happen? Define 'very mild'? Anyone else have input?? When does a buick 455 want more fuel? Devon
General rule of thumb is to go up one rod size for every jump of three on jets. Of course, that assumes you have the right ratio of rod to jet in the first place...
