I have absolutely no vacuum to my distributor diaphram at any rpm. I am running a '70 Rochester carb for a 396 w/auto trans on a '69 396 w/manual trans (that's what was on it when I bought it). The hose comes out of the port on the LH front of the carb. I don't hear any vacuum leaks, but the car just doesn't have the power I think it should. I have a vacuum gage, but don't know what/where to check with it. Has anybody had this carb problem and fixed it or heard of a similar problem? Should I just plug the hose and run mechanical advance? If yes, how much for initial & how much in dist? Thanks.

CC,

If you face the carb the vacuum advance port on your QJ should be located on the right side front, about mid way between the top and bottom of the carb.

Note that at idle this port will not have much vacuum at all. Easy to verify, just attach your vacuum gauge to this port. At idle you may see something like 2, 3 or so inches but if you slowly rev the engine vacuum should climb quite a bit.

If you have no vacuum at that port under any circumstances then someone has somehow sealed it off. If you do have vacuum there, but not at your advance can, then the nipple on the can has either been sealed or the can has a leak.

Dan

Dan, I have no vacuum from that front RH port at idle or as I increase rpm. I am wondering if I should pull the carb and see if something on the bottom is plugged with sealer. Where does that vacuum port connect to? Thanks, Chuck

Oh, another thing I noticed is I am not able to blow air through the vacuum port into the carb, either. And I have plenty of vacuum to my choke unload diaphram. This carb has a red and white "Quality Remanufactured" sticker on the side. The carb # is 7040206IF 0370 and the distributor is an old Accel dual point w/vacuum diaphram (I hope it doesn't retard with vacuum). Does the vacuum come from the closest venturi above the throttle butterfly? Chuck

I took the carb off and removed the base of the carb and the port for the distributor vacuum goes through the thick base gasket into a dead end rectangular hole in the carb base. Now I am wondering if I either have the wrong base "sandwich" gasket or wrong base on this carb. If I slot the gasket into the hole next to the vacuum hole I would get vacuum from below the butterfly. Wouldn't that mean that as rpm increases vacuum decreases? Maybe there is a different distributor port on this carb #? Any ideas? Thanks, Chuck S.

Right you are Chuck. That vacuum port does go to the venturi on that side of the carb. That's why it won't have hardly any vacuum at idle because it's above the throttle blades. As soon as you open the throttle the resulting air flow in the venturi passing that port draws the vacuum.

It's completely possible that the carb has the wrong middle gasket on it, or even the wrong throttle plate. Since you know the carb number you could order a rebuild kit for it - which will come with the correct middle gasket so you can verify but you'd stil be outta luck if the throttle plate has been swapped. Worth a shot though.

Or. A lot of folks, me included, run their vac advance right off a "manifold" source. In this case the vacuum advance will see full manifold vacuum at idle but as soon as you crack the blades manifold vac will drop, your mechanical will ramp up and as soon as vacuum starts building again it will start to pull in more vacuum advance. It works very well.

Most folks here dial in anywhere from 14 to 18 (and some even more) degrees of initial advance, have their mechanical advance curve set up to give 34 or a bit more degrees by 3000 RPM's, and limit the vacuum advance to something like 12 degrees. Natually, the numbers will vary a bit depending on how the engine is set up.

Dan

Thanks, Dan. Looks like you're right about the throttle body. It is not a match to the carb because the carb has the dist vacuum port in front (like it,s supposed to) and the throttle body is for a carb that supplies vacuum from someplace else. I'll find some other vacuum until I can get the correct throttle body. It just doesn't run like the 396s I remember from the "days". Thanks, Chuck S

Thanks, Dan. I plugged in to a tube right above that white nylon choke unload diaphram and it's starting to run more like a 396 should. Still have to get the rest of the timing in line. This is a stock SS396/350 w/M20 and 3:55 posi rear. My vacuum gage says 15 at 800RPM at that port. Do I need more advance? What should vacuum be at what RPMs when timing is right? I may need to run the Accel dual point on a Sun machine to find out what it is doing. Thanks, Chuck S.

Progress is sweet, no?

That's good news Chuck, glad you're getting it sorted.

Barring vacuum leaks, camshaft specs will largely determine what your vacuum will be. If the 396 is truly running a stock 350HP cam I'd guess it should be pulling something closer to 18 inches of vac at idle. Are you certain on the cam? To really zero in on this you may want to consider disconnecting every vac line to the carb and manifold and plug all the ports, leaving only your vacuum gauge connected to a port that sees full manifold vacuum, like the one it sounds like you're tapped into now.

Probably would be a good idea to have the advance curve in your dual point set up on a distributor machine. But unless the distributor is a mess it will probably be good enough to at least get your idle dialed in correctly - before any of the advance kicks in.

warm it up, set your idle at 800, dial in 14 degrees of timing at idle, re-set the idle speed. Use your vac gauge to set the idle mixture - obtain the highest vac reading on the gauge possible with the mixture screws. Turn the right side in till the engine starts to stumble then slowly back it out while watching the vac gauge. Then do the left side. If the idle speed changed re-set it and run the screws again.

If the idle mixture screw seem to have little or no effect there's a different problem. Once you think you've got 'em set idle the engine, grab a flash light, and look into the primary's on the QJ. If you see fuel being discharged from the main circuit you've got a different problem (don't ask me know I know).

Dan

Do I dial in the 14 degrees at 800rpm with the distributor vacuum connected or plugged?

Chuck,

Disconnect and plug the vacuum advance port on the carb, then set the idle, then adjust the timing. Your idle speed WILL change when you set the timing. Once you're at 14 just reset the idle.

When you reconnect your VA the idle will go UP, just adjust it back to where it was with the VA disconnected.

By the way, consider the 14 degrees of initial advance just a ball park. Some set ups like more, some less but from what I've read here, and what I'm running on my own 396, 14 should be close. When all is said and done you want to end up with as much initial as the engine can stand without casuing detonation and without causing starting problems.

I don't check here much during the weekend. If you get in a bind feel free to phone. 703-675-3410 is my cell.

Dan

I didn't think a 396 would take 14 initial, I thought 8 max, that's why I had to ask about the vacuum hose. That seems high, but the engine still doesn't feel as powerful as it should, but maybe this is why. I would think my max total should be 34? Maybe 14 initial, 8 vacuum and 12 mechanical? I haven't had a real muscle car since sold my '69 Z28 in '83, but I am thinking I should be able to break the tires loose from a 20mph roll in 1st by just putting the pedal down in a 396/350, but maybe I'm mistaken. Thanks again, and thanks for the weekend help offer. I'll see how it goes. Chuck S.

Chuck,

The fellow who worked over my distributor set it up with 22* of mechanical advance and the 22* is all in by 3k. Add the 14 degrees initial and it totals 36* by 3k during WOT.

The vacuum advance can is an adjustible unit and is set at about 14*. So under light load/steady cruise it might see 50* tops (14 initial + 22 mechanical + 14 vacuum), but probably a little less. Not an issue under light load.

The only time it pings running 93 octane is if I set the initial timing at 16*.

Yes, it probably should break the tires loose at a 20 MPH roll in 1st with the 355's you have in the rear, unless you've got some serious meats back there.

Did you happen to note what size jets and rods are in the primary side of the QJ? How about the secondary rods?

How long have you had this car? Do you know exactly what's inside the engine?

I bought the car in November from the 2nd owner who sold it in the 70s and bought it back in the early 90s and was told the engine was a stock 396/350 and it sounds like it. And the valve covers, wire looms and everything else around it iis original parts. The numbers match the protecto plate and say it is a 396/350. The car is mostly original, but it has a worn out Accel dual point and a 7040206 '70 396 remanufactured (by Champion) carb, which has the wrong throttle plate. I got it with the distributor and starter in the trunk, so I don't know how it was set before. I have a new HEI that came with it, but I am trying to put it back to stock for now because I remember how good these ran from the factory and just like the idea of driving it the way it was from the factory. I'm looking for a better Rochester and a 1111498, 1111445 or 1111999 distributor that is reasonably priced. So I haven't looked onto the carb yet, but suspect some improvements are needed there, too. I also have a vibration in the shifter handle above 4000 rpm which may keep the engine from revving up as fast as it should, may be a re-man clutch that's not balanced. So I probably have to go through one component at a time and make it right, but I know I am running 12* initial advance and it seems like it can take more, have no idea how the distributor vac/mech advance breaks down yet. Thanks for the timing specs, I might try bumping it up a bit more this week. Is yours a stock 396/350? Thanks, CS

Chuck,

Certainly sounds stock but with a car that old it's hard to tell if anyone has been inside the engine. Big block Chevelles, Vettes, Camaros were usually bought by folks that wanted performance and it wouldn't be unusual at all for someone to have swapped a cam. One thing you can do to help determine what's in it is do a compression check. With the engine warm, ignition disconnected, all plugs removed, and the carb open, if you see cranking compression numbers in the neighborhood of 180-200 whatever cam is in it is probably stock or close to stock. You may also want to check the head casting numbers to see if anyone put different heads on it to drop compression.

Not sure about 69's but I do know - that in '66, the difference between a 396/325 and a 396/350 was the camshaft (and possibly the carb). Same heads, compression, exhaust.

Mine is a '66 SS Impala convertible, 396/325, 4-speed and QJ. Basically stock with the exception of a pretty mild solid lifter cam and better flowing exhaust (but no headers). Static compression is right around 9.5:1. Like you, I kind of like the idea of motoring around with the old technology so I also run the stock points distributor. Even with the low tech points doing their mechanical thing it pulls hard to at least 5500. It might run a little better with electronics but probably not enough to make me wanna change it.

Each engine set up is a little different. The 14, 36@3k WOT, and 50 max that seems to work pretty well on mine may not on your's but these #'s should get you in the ball park. Get your distributor set up or find a correct one to have set up (have you checked the member classified's here?)

Thanks for the follow up. That's alot of good info. I have a line on a stock distributor and I'm looking for the correct carb. I have put an ad in the wanted section a few times, but I can't believe how much $ people are asking for these old junk parts we used to throw in the trash! I still need to finish trying all the suggestions you gave me as I am updating to correct parts. I'm sure the correct carb and distributor (curve & advance) will make a difference. I think I have a '70 396 truck carb on it now that has the wrong throttle body and probably wrong needles, etc. Thanks, Chuck S.

Noooooooo problem Chuck, happy to help where I can. Good luck rounding up all the parts you need. I've got an excellent source for rebuilding and setting up the curve for your distributor if you don't have someone local - let me know if you need that resource.

Note of caution on the carb. Since the "correct" QJ for your car is so old it isn't at all uncommon that the air horns and throttle plates get swapped. So just because the main body has the correct numbers doesn't mean the rest will be right (as I found out when I found the number correct QJ for my 66). Differences in the choke pull off's, the secondary lock-out's, throttle shaft arm (to which the gas pedal rod/cable connects), etc-etc. If you can find a picture of the correct carb - possibly in the service or overhaul manuals like mine had, you'll be able to verify that the carb you get from whatever source is correct.

One last thought on the carb. If your intention is to compete in shows at a very high level (points), then the correct carb will be a must. I spent litterally hundreds of dollars on my numbers matching QJ but not a single judge has ever asked that I take the a/c off so he/she can inspect the carb. There were so many variations of these things, some with only subtle differences, that it would take a very keen eye to spot them.

Best-o-luck, drop an email if ya need soem help.

Dan

I have posted in the "wanted" section for the distributor, but I'm not willing to pay $250.00 and up. I'll find one eventually. Do you know anywhere I can find out what rods I should have in my carb? Or how I should have it set up? Thanks, Chuck S.

Chuck,

A listing of engines and what those engines used for jets and rods does exist someplace but darned if I can remember where. If you really want to understand QJ's may I recommend that you purchase the "Rochester" book by Doug Roe. Incredibly informative.

http://www.amazon.com/exec/obidos/tg/detail/-/0895863014/qid=1088689407/sr=1-1/ref=sr_1_1/103-6875211-4318232?v=glance&s=books

It's probably also available locally.

Setting up a carb can be fun but can also be time consuming. In a QJ you'll have the primary jets, the primary rods, and the power piston spring to tweak. Once you get the primary side dialed in you'll have to then experiment with the secondary rods and possibly the seconday rod hanger, and also adjust the opening rate of the secondary air valve (the trash can lid sized butterflys).

I know I'm not telling you what you need for your engine. I'll give you a ball park for the primary jets/rods. Start with #73 jets and 43B rods. Rods can probably range from 41B's to 45B's with the 73 jets. If you use #72 jets follow the same logic with the rods.

You'll need a baseline first though. Get all the rest of the stuff we've talked about (ignition, idle quality, timing) dialed in first. Then install a new set of plugs and drive it for 200 or 300 hundred miles - trying to avoid opening the secondary's. Then pull the plugs and have a look at them. The plugs will give you an indication of which way ya need to go with the jets/rods.

Dan

Dan, thank you for all the great info. Guess I know what I'll be doing this weekend! It's gonna be pretty hard to drive it 200 miles under 3000rpm, though. Chuck S.

Ya don't need the secondary's to spin it to 4 or 5k Chuck! Just a careful foot.

Good luck, I've got some tuning to try on mine this weekend too.

Dan