Which jet size would you recommend?

I've got a near-stock 350 with a Qjet made for a '71 Big-Block, and I'm about to learn how to rebuild the carb

It seems to run rich so I wonder what opinions I can get from the wisemen here, although I don't yet know the size of my current jets: What jets should I be using?

--->oh also I'm not usually heavy on the accelerator, I'd rather save fuel most of the time

 

The 455 carburetors can be set up to work fine on a near stock 350. The changes are not specifically in jets and metering rods, but to the idle and off idle fuel delivery. They also made several different 455 carburetors in 1971, and they are not set up the same. So basically there is no simple answer to the questions, and none of the jet sizes listed really apply to what you are trying to do.....Cliff

 

so I should swap in .083 jets and all will be well?

 

lots od Qjet info in the libraries at the Full Size Buick and Boattail registries
http://fullsizebuickregistries.boattail-riviera-by-buick.com/Library/Common/1960s-70s/documents/index.php?model=Common

http://boattail-riviera-by-buick.com/library/documents/documents.php

I suggest you start here - lots of right way to mod - jet info

Tuning by Lars - Qjet & Engine Tuning Manual Lars Grimsrud sent in by Lars
if you are going to mess with one - (may change your mind most the hate for Qjets comes from one with mixed parts - they work great when left alone and rebuilt with correct parts for the application - correct parts are in the Carb charts starting with 65 on page 379 and covers thru to 76
Parts Catalogue Illustrations May 81 GM Corp sent in by Dana Laiche


here's the 71 Qjet pages

 

For a stock motor, you should begin with the factory rod/jet sizes, and then work in 10% steps up or down. Remember, the fuel metering is accomplished by more than just changing the jet size. There are a lot of other factors at work with the carb, such as the power piston spring, accelerator pump springs/rod position, etc. Is the engine the same year as the carb? Are all of the vacuum lines hooked up correctly? Nothing is leaking? Is your ignition timing set correctly? If your engine has an issue in one or more of these areas, it will make tuning the carb much more difficult. I tried to tune my Q-Jet for months before I realized that my primary throttle shaft was leaking vacuum...making all of my tuning efforts worthless. Check that your carb doesn't have this problem. If you can rattle the primary throttle shaft in the carb, it will need to have the bushings installed before it can be tuned.

If the engine is currently running rich, then you may already have a rod/jet combination which is metering too much fuel. There are books with tables that list all of the metering areas for different rod/jet combinations. The rods will sit at two different positions, depending on the throttle position/vacuum pull on the power piston. At idle, the rods will be seated in the jets to their widest point on the taper...which is the number on the rod, itself. At part throttle, the vacuum to the piston is reduced, and the spring pushes up the piston to lift the rods out of the jets and only have the tips in place...allowing more fuel to flow. The rods have tips which are 0.026".

Increasing the jets while keeping the rods the same will increase your metering area at both positions. I'd start out by adjusting the idle mixture screws at the base of the carb, first. Start at about 2-1/2 turns out for both screws, and with the engine at hot idle, tighten the screws slowly, one at a time, until engine RPM drops by about 50 RPM, then back out 1/8 a turn. If turning the screws has no desired effects, then I'd look into changing other parts.

The stock rods/jets for my 430 were 41 and 71. I've upped my jets to 73s, and the rods as 39s, sometimes 37s. The jet size will have the greatest impact on fuel metering, but it is important to adjust the rod size when changing jets. If you alter the fuel metering area by more than 10%, there will be drastic changes to the engine operation which will make it more difficult to tune.

 

ok so I did find a little info to help (Thank you robs71redriv) I honestly didn't think anybody had so much written down!

While I haven't yet found a listing with the jet size, carbsunlimited verified a part # for rods size 44
GM factory setups for 350ci AutoTrans were at or near jet#74/rod#44 combinations

My carb # is 7041540 which appears to be from a 1971 455ci Auto. Engine is (I believe) correct for my '70 GS
Camshaft is unknown but previous owner told me it was a mild hop-up part
Timing is as good as I can get it but way off the scale
it all runs decently when it does start, and today it's idling very nicely


EDIT: I had time so I cracked it open to check things out
Jets are .075
Primary rods are labeled 45E and 45B, are they labeled differently to indicate left/right?
secondary rods are labeled AU
I had worried there was a problem with the fuel plunger, and so I found its seal with kind of a wrinkle in it

surprisingly though, I don't see any leaking fuel out thebottom that would cause the carb to dry out so quickly

The jet sizes I mentioned at first were just guesses based on random info, I expected to find smaller than .075 in there and I was hoping to get a ballpark idea what is normal to most people. How different would things likely be if I swapped in a pair of .073s? Would I be better off changing to bigger rods instead, or must I change both sizes?

Also I'm not satisfied with the gaskets in the rebuild kit I have. Any recommendations there?

 

There is no right/left numbering...the rods are identical. Both rods are 45B, which indicates that they are dual-taper (1968 or later rods), with a 0.045" outer diameter. Primary rods in Q-Jets which are 1967-issue or earlier (like mine) don't have the B, are single-taper, and are slightly longer. The 'B' was added in order to avoid mixing up the two styles of rods. If the dual-taper rods are used in a 67 or earlier Q-Jet, the metering will be all screwed up since the lengths are shorter, and the rods could even jam during operation.

I usually go to summitracing.com to order rods/jets, the Edelbrocks are good replacements...some are even NOS Rochester parts. For gaskets and all the rest, I go to quadrajetparts.com, which is Gessler's Head Porting Service. They've got everything, including a kit for installing the throttle shaft bushings. If your carb doesn't have them, and the primary shaft rattles in the bores, you'll need to install the bronze bushings. Just about every Q-Jet core has wear in this area, and needs the bushing fix.

 

As Cliff says the 455 carbs have different idle air passages vs the 350 carbs.

I have spent a lot of time using 455 carbs on the 350 and after reading cliffs book a few times, spending 30+ hours modifying and rebuilding Q jets I have found that yes it is possible to make the 455 carbs work well on the 350 but it takes a lot of trial and error. In the end I found that the 800 CFM version of the 350 carb was the best for me requiring the least amount of work. All I had the do was change the jets and hangers and they work great.

It is a good idea to get an LM-1 or LM-2 air fuel meter so you can measure the air fuel ratio in your exhaust, otherwise it is tough to know if you are too rich or too lean while you are driving.

Cliff sells the best rebuild kits:

http://www.cliffshighperformance.com/

 

Thank you very much guys for all the helpful info! I'm gonna do some more window shopping and reading before I make any big changes

While taking the main body apart last night I didn't catch the exact position of the needle and its spring, before dislodging it when the float came out. I don't have any instruction regarding the needle and I can only assume it should hang, since the float pulled it out in the first place.

Is that spring meant to hang the needle from the float bracket, thus moving the needle inversely by the float, or does the spring go under the float bracket to keep the needle down by the weight of the float?

 

+1 on Cliff's book, it has a lot of useful information...especially when it comes to Buick 4MV Q-Jets. Doug Roe's book is also good, but Cliff's is the better of the two. I learned a lot from it, even though I'd been rebuilding Q-Jets for various vehicles for 10 years. There were things I didn't even know could be done with them.

I'm currently working on a 800 CFM Buick 4MV (7042240), gonna see about retro-fitting it for my 430. I need to soak it good, and change out the primary shaft to one that's compatible with my switch-pitch linkage. I also need to rig an alternate fuel-line, since the later carbs are front-inlet instead of side-inlet.

 

I did find Cliff's book online and after a few extra days waiting, it finally came to me. Great resource for sure! As far as my case I'm surprised that I could even get the carb back together and make the engine run (on the first try)!! I really just needed to identify a few parts and look for obvious problems. I'll take some time again soon to replace a few parts with a better rebuild kit than I found at my parts store. After some more reading I'll have plenty figured out and I hope I'll be able to get some more of my old fuel mileage back.

Thank you guys so much for helping me learn what the rods are actually for!!

I've done some searching, and I still haven't found a picture or a text instruction on how exactly to install the needle in its seat. I hung it by the spring-hook, from the end of the float arm. Anybody care to shed some light or point me to a resource??

 

If you're not clear on how the spring retainer is set up, you might be better off leaving the needle loose in the seat. The float pressure will seat it when it neads to. If you've got the spring in wrong, it might bind the float and flood you out.

 

The inlet needle valve is designed to hang from the float arm using the clip (hanger). As the fuel bowl is emptied during operation, the float pivots on the fulcrum raising the needle valve off the seat, thus allowing fuel to refill the bowl. Install the hanger on the float arm parallel to the center line of the float. It should not hang from either hole in the float arm. If the hanger is omitted, you will be relying on fuel pressure alone to raise the needle valve off the seat. This will work under normal driving conditions, but under wide-open-throttle operation, the pressure may be inadequate to raise it. My advise is to use the hanger.

 

Thank you Bigpig and Jim!

It starts, it drives around the neighborhood, and I just found a fuel filter with a check valve to install. I'm crossing my fingers that it makes a noticeable difference in starting after sitting in the garage longer than a couple days.

Now that I've learned a bit more about the choke and the separate idle-speed settings, it all seems to come together more smoothly EXCEPT I'm having trouble getting the choke to hold onto its cold setting. I still have to tease the throttle a little to start up initially, and then the choke drops open and theRPMS drop, which as you might guess makes idle rough until it gets warm. As per my long-standing habit, I don't drive a cold engine until it's run for a minute or two, thus a small problem I can't yet get fixed.

I'm also trying to learn the finer points of ignition timing, but that may end up in a new thread later. I increased the initial advance a bit and life seems better for it. I have a cheap timing light so no exact details here, but I'm still reading on the topic