I am try to sort out several issues with a 1971 Chevelle 454 with Q-Jet carburetor. It currently has a service replacement Q-Jet installed and I do not have any vacuum connected to the distributor.

I am having issues in setting the idle correctly and am not getting much change in adjusting the idle screws all the way in to 5 turns out. No noticeable change.

Can anyone steer me in the right direction to trouble shoot? Vacuum hoses have all been replaced but I only get about 10" of vacuum on the carb from the port above the fuel filter on passenger side and much less than that on the port on drivers side.

The reason for no vacuum connection in my mind (I could well be wrong) is becuase of what the solenoid is supposed to do. (My thoughts anyway) and that is to provide vacuum a) 15 seconds upon startup b) when th400 is in 3rd gear, and c) then transmission is in reverse.

Please correct my thinking on this if I am wrong.

Tx,

How is the idle quality now? If it is pretty poor, and you can turn the needles in all the way and the engine still runs, you have a nozzle drip problem. If you verify this, I can walk you through a solution. Do you see fuel dripping out of the boosters while the car is running?

Part of your idle situation is due to the fact the vacuum advance is not hooked up. Usually manifold vacuum works good, but some like ported. IMHO use manifold vacuum if you aren't emission testing.

The vacuum advance should get vacuum anytime the engine load is light, not just the "a,b,c" situations mentioned in the above post.

Mike

Bags.

I would welcome your help in determining whether or not I have a fuel drip.

With regards to the vacuum issues. 1) I have less than 10" of vacuum at idle from any port on the carburetor or anywhere on the intake manifold. This is not normal from what I see in other posts - should have 15 or more.
2) My thoughts for not connecting the vacuum to distributor are a) 71 model had the solenoid limiting when vacuum was directed to distributor b) thinking that at no time was vacuum "on" while in low gear.

At the stop sign the brake has a hard pedal, the cowl induction flutters open, and engine runs like crap at that point. Upon acceleration it takes a while for the engine "to catch" up with the pedal pressed "to get it on".

Maybe I am fighting other problems and welcome the input.

TxHusker,

If you do have nozzle drip, you'll see it when you look down the primaries with the engine running. Look for an occaisonal drip out of the boosters. You'll probably need a flashlight for this.

If it does drip, it may be because you are running either a high idle speed, OR not enough spark advance at idle. Based on that, the first thing to do would be to run a vacuum line to the distributor. Use a manifold vacuum source.

I can appreciate your wanting to help the environment and plumb up your spark advance to work only under certain conditions like the stock 1971 system, but at least for a diagnostic aid run full time manifold vacuum.

If you do find that you have a nozzle drip problem, you can also double check your float level.

Unclepennybags. Thanks again for your help.

This morning I rechecked the timing and dwell (both okay) and then did the hand over the carb trick to determine if too rich or lean. Found that with the hand over the carb engine picked up RPM's so thinking the mixture was too lean I turned the idle screws about 1 turn out on each with some pickup in RPM's. Car ran some better at this point.

I am still having vacuum problems (not enough vacuum) to keep cowl door closed and power brakes
feeling right.

I did hook up manifold vacuum to distributor and the car immediately picked up RPMS (300 -400) or so. However the engine still idled down to 600 or so when I put it into drive - same RPMs as without vacuum connected it seems. The extra RPMS smoothed the idle but they are not "POWER" RPMS and I can't understand why.

Expanding the scope I did compression check and am getting 125 or so on 7 cylinders and 140 on the 8th cylinder. Service manual suggests 160 PSI and I am no where near that amount.

I believe the engine (original owner said so) has the original cam but I am thinking that maybe a change was made and that is the reason for low vacuum at idle.

Is this where my trouble could be?

Boy, short of hooking up a degree wheel to the balancer, a dial indicator on a pushrod and measuring the actual duration at, say .006 lift, you just dont know unless he has a cam card. Doesnt anyone but me keep a cam card in the glove box!? :rolleyes:

If its the case that your vacuum in this low, you are most certainly opening up the QJ power piston ( like a power valve in a Holley) at idle. This the piston, that the primary metering rods are connected to) is sucked down by vacuum. And hence the primary metering rods go down into the jets. They're tapered, so now the restriction in the jet is not only the point, but up onto the tapered shaft. Now, based on the spring under that thing, when the engine sees less vaccum, the spring overcomes it, and up goes the piston and the hung rods, richening the mixture.

Long story short, you might need to go out and by an Edelbrock QJ spring set and put a lighter spring in there, so that 10" of idle vaccum doesnt overcome it. Whoops, I mean so that the 10" of idle vacuum is not overcome by the spring tension. A stock OEM replacement carb would have a heavier spring that would rise at , say 12-15" Hg, and is nearly all the way up at 10". But at 16-20" Hg, it would be overcome by the vacuum levle and keep the metering rods fully in the jets.

Hope this makes sense. And my guess is that once you get the carb metering right at idle, and vacuum advance connected at idle, you 'll see more vaccum at idle, say 13". One thing you *might* want to do is also pick up a set of jets and bump the jet size up 1 or 2. 72-73.

Also, and most importantly, do check for a BIG vacuum leak, like in your brake booster, before you proceed. Always start with the basic checks, hate to go back to them after alot of time and $ fiddle with stuff that isnt broke.

Another thing to lookinto is are you running a perf cam,if so what are the spec's(int & ex lift, dur @ .05, L/S angle? Most perf cams need approx 15-18 deg btdc initial advance (without vacuum or mechanical advance)because if your timing is too retarded it will cause your throttle plates to be positioned past the idle circut in the carb due to having to raise the idle screw farther than normal to get the motor to idle in it's luggy/laggy state making the motor extra soggy on the bottom. This will also cause the idle vacuum to be lower than it should be for your particular cam/motor setup.

This is likely why your idle mixture screws are not affecting your idle when screwed in/out and can also lower the idle vacuum too so check it out.

let us know how you make out.

Scott

Gene and Scott - Thanks for the posts and the information. At this time I am going to hunt hard for possible source of vacuum leak (brake booster and now that I think of it the vacuum module on the tranny which was replaced during all of this) before working the other suggestions.

Partially because of my own limitations and technical know how. You have however spurred me on to keep trying to get it right and for that I thank you.

I will post the results.