I am tuning my q-jet and learning a lot. Very fun stuff actually. Anyhow, I am looking for a chart to tell which primary metering rods I should be running with which jets? For instance I jumped from a 68 - 72 jet, which metering rod should I be using? My carb is a 73 Buick 750 CFM. Trying to decide if I need more jet, or smaller rods? Thanks, Jon
I made an excel spreadsheet years ago to help me tune exactly as you are suggesting. Unfortunately I can't locate it at home; probably is located on my work computer. If I find it I'll share. I made a few of these sheets. The sheet for secondary rods also has a histogram showing a visual relationship of fuel flow based on the profile of the rod. Visually you can follow how rich or lean the rod is along its length at various depths. Regardless, and in general, you would be wise to maintain stock part-throttle relationships between primary jets and rods (you can find these relationships in the service manual). I catalogued a number of GM cars with carbs I dissected in junkyards. If you jet up two steps to account for a set of headers move up two steps with rods so that the amount of part throttle jet opening remains similar to stock. Only when the rods are completely out of the jet (at WOT) will you see the richer fuel flow. From a seat of the pants dyno perspective you can actually feel, through throttle response, if your part-throttle mixture is lean or fat.
I would be very interested in seeing that Excel sheet. My engine is not stock and has quite a cam in it, so stock doesn't apply here, but I think what your saying is to keep the stock relationships. I am running lean on the primaries, so are you saying I should jet up and also go up in the primary rods as well? Thanks, Jon
I found my spreadsheet. Too bad we can't attach spreadsheets. So I dropped the thing into a Word document. It only contains the combos I've found or know of (actually this is my entire tuning collection). Interesting to see the difference between the standard 455 and the Stage 1. Only boosted the primary jet and rod one step for an engine package that has been reported as hot as 420hp. Note the relationship between jet and rod remains the same. Next time I open my carb (it's been 15 years now) I'm probably going leaner than what you see as "Buick Now." It has always felt a bit too rich, especially on my rare trip to altitude. On the secondary rods, the thinner profile is the richer rod.
Also, it helps to physically check the diameter of the rod with precision calipers. The numbers stamped in the rod can be inaccurate: my original 41 rods actually measured 0.0385" when checked with my digital calipers. Those couple-thousandths do make a difference in fuel flow. I've got 39 rods in 73 jets now...gonna try 41 rods in 74 jets next.
Take note Buick fans; * 1970 and older carbs are 750 cfm....jet range 68-71 is common * 1971 and up carbs are 800.....jet range 73-76 is common * Secondary rods and hangers are the same for all Q-jets. Be sure you match the size of your carb to the sample carb you are copying. 750 primary jets in an 800 carb could be a detonation disaster. Great info contributed here. Very usefull. Thanks guys.
Several things to "touch on" here. Impossible to compare early 750cfm Buick units to 71-74 800cfm units based on jet and rod usage. The larger units had a completely different main air bleed set-up, so they will use different jets and metering rods to supply the correct A/F ratio across the load speed range. Using the larger .073" main jets in an early small MAB unit is a disaster in the making, same deal using the smaller jets in the later units. Although relative here but complicated, the idle airbleed and idle fuel set-up effects the main fuel delivery thru transition and at light throttle, so it becomes a "player" in calculating the final fuel delivery the engine see's in part of the "normal" driving range. Buick q-jets also used "B" series primary metering rods (and yes, I know they also used a fully tapered upper section rod on some models). They will have some taper to the upper section, then gently taper to .026" tips. The factory used an APT system in the baseplate to precisely locate these rods in the jets for fine control of the part throttle A/F ratio. Swapping in different jets, or different metering rods, depending on the type of rod specifically, and not freeing up the APT system to locate them in the jets, is another recipe for disaster. I continue to see folks installing HUGE jets into the 71-74 Buick carbs, simply because they exhibit lean symptoms at light/part throttle on this new fuel, when all that was needed was to free up the APT system to raise the jets to a slightly smaller diameter area on the metering rods. Another comment, and take this FWIW. There is a TON of precise information in print on calculating fuel area past jets and metering rods. Nice to know stuff, but pretty much USELESS in determining the fuel curve that is delivered to the engine because it does NOT take into account the emulsion air that enters the incoming fuel stream from any particular carburetors main airbleed system. Case in point. Some early Chevy units used 64 main jets and 36B primary rods. The same basic castings 10 years later used 76 jets and 44B rods. The early carburetors delivery a lower A/F ratio (richer fuel curve) clear across the load/speed range that the later units, simply because they used a completely different MAB sizes at every specific point. I hope this post doesn't come across as wrong, but there is so much miss-information out there these days, folks need to know the real deal when making specific changes to these units.:beer .......Cliff
This is great information and I thank everyone for their contributions. I hope to get soem time this weekend to pull the carb off and start measuring etc. Thanks again, Jon
I have another 7027241 4MV Q-jet core on the bench now, and just need to seal off the secondary wells...the plugs are aluminum with rubber o-rings. Strange, never seen plugs like these before, but the o-rings in there now are shot...I was able to pull the plugs right out with my fingertips. They'll leak like hell if I put it back together as is. Gotta get new o-rings, then assemble & epoxy them shut. One important tip is to not stray more than 10% away from your present setting when swapping out rods & jets. Go through the table & calculate your current idle & WOT metering areas, then choose a combination which is within 10% higher or lower of those areas.
"I have another 7027241 4MV Q-jet core on the bench now, and just need to seal off the secondary wells...the plugs are aluminum with rubber o-rings." Non factory parts added at some point to replace the factory brass cup plugs. Just about useless, every single one I've ever pressure tested leaked, usually like Niagra Falls! If the casting is sound, screw in plugs coated with Marine Tex is the only repair I know of that will hold up in long term service......Cliff
