Does anyone where to find factory stock jet & rod sizes on quadrajets? Im rebuilding my carb and would need a starting point, the engine is a 70 buick 350 with a TA 112 cam, otherwise stock.
1970 350 Qjet (7040244) used 69 jets, 45 rods and BS secondary rods in stock configuration. With the TA 212 you would benefit from some modifications to your Qjet, though they are not "necessary" check out Cliff's book and the Doug Roe Book for tons of great info.
So what I understand the differece between the rods & jets is what counts? I have a pair of new .74 Jets, so would it be good if Id buy a pair of .48 Rods? Stock would be .69 jets & .45 rods = difference is 24 so my combo would be .74 Jets & .48 rods = Difference is 26 And that means that my setup would be a little bit richer than stock, I think maybe not too much? The Stock jets size of .69 seems small to me? Any suugestions welcome, I am also considering a lighter spring under the power piston but the TA 112 cam is not vey radical so I dont know if that is needed? THANKS in advance!
BAD MOVE installing much larger jets into one of those carbs. Each carburetor part number was set up with an application specific calibration. The jets and rods used, combined with the specific position of those rods in the jets using the APT system in the baseplate, determines the fuel delivery to the engine over the the entire load/speed range. Many early Buick carburetors used very small main airbleeds, combined with small main jets and metering rods. In any and all cases, base the jet and metering rod selection on the airbleed sizes. In the vast majority of cases, the factory jets and rods will be fine, or very close to perfect. I would use the factory parts first, completely/correctly rebuild the carb with modern ethanol compatable parts, then put it in service and evaluate performance in all areas. Then go back and make metering changes if/as needed. With the early Buick carbs, in most cases we free up the APT system in the baseplate, and fine tune the part throttle A/F ratio with stock or very close to stock jets and metering rods. We end up replacing the stock parts as in many cases the part or worn some from many years of use.....Cliff
Jani, be careful with your math assumptions. You cannot simply subtract rod from jet diameters and get any meaningful info, even for the sake of comparisons. You're dealing with an area (square inches) calculation, not a linear one. Devon
Thanks for the info, tommorrow I will be putting the carb back to stock specs and see what It will do. The carb is a 73model quadrajet and the engine it will be feeding is a freshly rebuilt 70model buick 350. So I will be putting the carb to the same specs as a 1970 qjet feeding a 4barrel buick 350? I bought the carb a year ago (was rebuilt by the company I bought it from) It did have .72 Jets in it last year and maybe .43 rods? The stamp on the rods is so worn out I cant tell for sure. With that combo the engine did run OK, but at that time the engine was not rebuilt and had quite a low compression at three cylinders, had two slighly bent valves and cracked both exhaust manifolds. Maybe at that time the rich mixture was good for it? But now all that has been fixed so the carb is too rich with those jets. Ill get back when I have changed the qjet back to stock.
Beware of carbs "rebuilt" and "remanufactured" by Companies who mass produce them. They use generic calibrations, typically extremely rich and "one size fits all". This minimizes customer complaints, as any lean carb would come right back to them. They also use poor quality components, and quality control is OK to pathetic, with more on the pathetic end than OK. "Jani, be careful with your math assumptions. You cannot simply subtract rod from jet diameters and get any meaningful info, even for the sake of comparisons. You're dealing with an area (square inches) calculation, not a linear one." +1 All of that information is also highly flawed by how the carb is set-up. It is actually quite easy to have a richer fuel curve with .067" jets and .039" rods, than .078" and .044" rods (for example), simply when the carb with the smaller jets has much smaller main airbleeds......Cliff ...Cliff
Right. I put new (stock size) jets & rods in this thing (.69 & .45), And all new gaskets and inspected every part and tuned it to my best knowledge. It seemed to make little or no effect. The Idle is still not good, I get virtually no kickdown (is that the correct term?) And the car does not accelerate much and seems to be lacking in overall power. I am simply so tired with this car, I dont even know anymore if its in the carb but everything has been tested. Everything in the motor is new. Everything in the ignition is new. It does get spark in every plug, but I have no way of telling how "strong" the spark is. I put well over 1500$ in rebuilding the motor and what did I get? I got from about 20mpg to 17mpg Lost about half of "the speed" the car had Lost a s**tload of money and my nerves. My next plan is to try out a friends edelbrock carb and when that fails I will sell the damned car. Id really hope the edelbrock would do the trick.
I understand your frustration, but you started this thread asking for the stock jetting. You made no mention of the performance problems you are having. Was the camshaft degreed in? Are you running points? Have you verified that the mechanical advance is working in the distributor? Have you performed a compression check? What vacuum is the engine pulling at idle fully warmed up?
Yeah sorry about this, its just so annoying Yes the camshaft was degreed At the moment I am running new points, I also have two years old Lectric Limited electronic ignition module, using either one showed no difference. Both Mechanical and vacuum advance are working. Enginge timing is done per the instructions in "power timing you buick" Limited amount of vacuum advance, etc. Total engine timing is at the moment 38degrees. Lighter spring installed in distributor etc. Compression was good when tested, cant remember the numbers but they were within very close tolerances. I have not checked the vacuum yet, I have checked all the hoses, power braker check valve (or whatever that is called) And all vacuum related that I could find. By the way Now that Id need to check the vacuum, from where and how will I check it and what types of tools do I need?
You need a vacuum gauge. They are very inexpensive. You need to hook it to a manifold vacuum source. It can be on the intake or at the carburetor. A manifold vacuum source will hiss when you open it up with the engine at idle. http://www.youtube.com/watch?v=NrnzD7PgoHI http://www.youtube.com/watch?v=9whoWkMqj34
Any larger than stock cam (seat to seat timing, overlap, tighter LSA, more duration at .050", or all of the above) is going to hurt idle quality, and throttle response right off idle. It's been that was since the mid 1970's when I first started into this hobby. Without a compression ratio increase to offset making the engine a less efficient airpump at low rpm's, it's just going to happen. Not a big deal either, but trying to run parts specifically tuned precisely for the original set-up, more times than not, will produce less than desirable results. I'm just talking in general terms here. If you can provide ALL of the engine/drivetrain/vehicle specs, I might be able to provide more specific information, or things to do to help out the situation?.....Cliff
Cliff, It's a TA 112 cam, 210/215/110 LSA. Should produce plenty of vacuum. I want to see if maybe he has a vacuum leak at the intake/valley from fit problems.
So I bought the vacuumgauge and measured it: The readings are mostly 11-15 inHg with slight fluctuation of about 2inHg. The earlier I set the initial timing the lower reading I get. With a initial timing of about 20 degrees btdc I got about 12 inHg of vacuum. It shoudnt be the carb, I replaced all the gasket during rebuild. So I can now figure out it by myself that this isnt normal, I am now suspecting the Intake manifold for leaks? I did check all the vacuum lines that I could find in the engine bay. It saddens me though since I was extra careful during engine rebuild to make sure I would seal everything very well, new gaskets everywhere. Well the intake manifold gasket was not new (the big metal gasket) Since I was under the impression that it could be re-used? I will check the intake manifold but are there any other things to check while I am at it? THANKS!
Well, you have found the problem. That vacuum is low for that cam. It should be at least 15", hell, I'd expect 17-18" with that cam. And the reading should be steady. If the deck and or heads were surfaced, it is possible for the vacuum leak to be in the valley. There is a way to check for that kind of leak. I lifted this from one of WE1 posts: To check for an internal vacuum leak, "Plug all the ports in the manifold (vacuum ports such as power brake, PCV, trans modulator, etc...), plug all vacuum connection ports on carb, plug the block oil dipstick tube or hole, plug any vented valve cover breather. You want to seal up the base of the engine. Now remove the oil fill cap and start the car. Put the palm of your hand over the valve cover oil fill hole. Bring the rpm up slightly off idle. If you feel pressure build up under your hand after several seconds, which is an indication of base pressure, that is good and shows the engine is internally sealed. With enough time it'll try to push your hand off the cover. If when you remove your hand there is no pressure or even a vacuum then you have an internal leak and any base pressure is being relieved, most likely by a leaky intake seal to the cylinder head and being sucked down the intake port, or a cracked intake manifold or head".
Another thought. Did you measure for correct push rod length, or use adjustable push rods when you built the engine? Whenever you use an aftermarket cam , and have an engine rebuilt, it is always a good idea to check push rod length. If the push rods are too long because of decking of the block and/or heads, the valves may not seal correctly. That would also give a lower vacuum reading.
What is the compression ratio of your 350 cid engine? The vacuum reading(s) are pretty low, at least for that cam in a 350 with 10 to 1 or so compression.....Cliff
Yes, at the moment. If I put eg. 10btdc without vacuum advance, the vacuum meter shows very little (about 6-9") of vacuum and the engine cant keep idling. And I think the engine compression ratio should be something like 10:1. All I know for sure is that the engine is a 1970 buick 350 engine 4 barrel. It is not a numbers matching to this car.
