Q-Jet Issues?

[pajeff02]

Our 1974 Corvette runs a .060 over 350 with Vortec heads, Performer intake, mild performance cam, headers, and a stock tach drive distributor with an Accel dual point set-up. In order for the stock air cleaner assembly to fit properly, I had to go with a Q-jet carburetor. At the time of restoration, I found that a prior owner had installed a Holley Q-jet replacement, so I ordered a Jet Stage II Q-jet.

Our problem is that the idle is doing weird things. Some days it idles just fine, and on others it will either fall on its face, or idle high. To make matters more confusing, it can change during a short drive, going from one to the other. The carb is equipped with an electric choke and I have full disengagement once the engine is up to temperature. I have adjusted on everything and even checked to make sure my distributor was not hanging advanced.

Only other symptom is it appears to have a lean idle and will occasionally pop out the exhaust or upon deceleration. I have also disconnected and plugged off all vacuum accessories to isolate any possible vacuum leak, replaced the carb base gasket, and even pulled the intake thinking it may have an internal vacuum leak. Since it does not do it all the time, I do not think it is a vacuum issue -- on a gauge it pulls somewhere between 15 and 18 inches of steady vacuum at idle.

Anyone?

[Firebirdjones]

I'm not sure what power piston spring Jet uses in their stage II setups. But if you have at least 15 inches of vacuum at idle, I assume that is without load in park.
What is it in gear at idle??? Are you using the factory vacuum advance in the distributor??
What do you have initial and total timing set at with the vacuum advance plugged???

[pajeff02]

I'm not sure what power piston spring Jet uses in their stage II setups. But if you have at least 15 inches of vacuum at idle, I assume that is without load in park.
What is it in gear at idle??? Are you using the factory vacuum advance in the distributor??
What do you have initial and total timing set at with the vacuum advance plugged???

Car is equipped with a manual transmission. The distributor utilizes an Accel adjustable vacuum advance unit. Initial timing is set at 10 degrees with the vacuum line disconnected and the port plugged. I will have to go check my notes (if I can find them) as to the mechanical timing and total timing with vacuum advance.

We tested the distributor and adjusted both the mechanical and vacuum advance on a Sun Distributor Bench -- I have a friend who is really into the old time diagnostic tools. This was then later checked and re-verified on the car after installation with an advanceable timing light.

[Firebirdjones]

Car is equipped with a manual transmission. The distributor utilizes an Accel adjustable vacuum advance unit. Initial timing is set at 10 degrees with the vacuum line disconnected and the port plugged. I will have to go check my notes (if I can find them) as to the mechanical timing and total timing with vacuum advance.

We tested the distributor and adjusted both the mechanical and vacuum advance on a Sun Distributor Bench -- I have a friend who is really into the old time diagnostic tools. This was then later checked and re-verified on the car after installation with an advanceable timing light.

I still use a Sun Distributor machine as well. Good to hear, sounds like you are on the right track.
Do you run the vacuum advance on ported vacuum, or manifold vacuum??? I know people give instruction to run ported vacuum, but you'll find better part throttle performance as well as better idle and more idle vacuum if you use a manifold vacuum source. Q-jets have ports for either configuration.

Using manifold vacuum will pull on the vacuum advance at idle,,,,to what degree depends on how sensative you have the adjustable vacuum advance can set. But these extra few degrees of timing at idle will help to stabalize idle, and increase idle vacuum,,,,both of which are what is needed on any engine running a larger than stock camshaft. It will also run cooler while idling around, and increase gas mileage.
It won't have an effect on total advance when at WOT because there is no manifold vacuum present, so vacuum advance becomes inop at that point. It's only purpose is for part throttle cruising. Which is why it should be hooked to manifold vacuum.

Hooking it to a ported vacuum source only works when the throttle blades are open, what's really bad is it keeps working at WOT,,,not a good thing. This is why people complain of detonation and plug off the vacuum advance entirely. Then they miss the benefits.
Hooking to ported vacuum also does not allow the vacuum advance to operate at idle conditions. Engines tend to run warmer this way.

Sorry for the explaination, I just want to make sure we are on the same page. Once all that is set properly it's much easier to diagnose and tune the carb. The vacuum source for the vacuum advance makes a huge difference on carb tuning.

[pajeff02]

The advance is attached to the ported vacuum -- verified at the time of install with a vacuum gauge. I assume that upon attaching it to manifoild vacuum I will need to reset my initial timing -- had never heard of it explained in this manner as gospel has always been to use ported vacuum.

I found my initial spec - the vacuum advance begins to pull right at 7 inches.

[Firebirdjones]

You can leave the initial at 10 degrees. The vacuum will add a few when plugged into manifold vacuum, but it depends on how sensative the can is adjusted. Probably add 5-8 degrees, but will be fine, and won't affect hot starts as vacuum only works when the engine is running. It will help to smooth the idle and the engine will make more vacuum as a result, which may help to cure some of the erratic idle you are having. A low vacuum situation may be pulling the power piston open on the carb,,,,which is why I wanted to make sure the timing is straight first,,as more vacuum at idle will cure that, and possibly not require a power piston spring change.

If it start pulling at 7 inches, then it sounds like you have some tension on the adjusting screw/spring inside the can where the hose attaches. You can easily hook a hand held vacuum pump to the vacuum advance and pump it up slowly while shooting the timing light on the balancer and see how much the can is capable of adding. (the same can be done on the distributor machine) but since it's in the car at this point.....
If the engine has say,,,16 inches of vacuum at idle,,,pump it to that point and see how much timing it adds. At cruise speeds the engine vacuum can be as high as 20 inches with light loads. If the can is adding anywhere from 10-12 degrees of timing it's about right. I would adjust until you reach that amount.

So if you have a total timing of say,,,,36 degrees in the distributor (without vacuum) then the added 10-12 degrees of vacuum advance at light loads while cruising will give you roughly 46-48 degrees while cruising. That is fine. It may have to be tailored slightly depending on how sensative the engine is to pump gas, what octane you use,,,etc...
But that's a good starting point for a mild engine. As you apply more throttle the vacuum signal drops,,,and the vacuum advance is reduced as a result while at the same time more engine load is being applied,,,IF it's plugged into manifold vacuum.
When you go full throttle, there is no manifold vacuum, so the can is shut off and taken out of the equation and the engine will run on the total you set in the distributor, such as the 36 degrees I used as an example. That's how it should work,,,you want the vacuum advance reduced as more load is being applied, keeps the engine out of detonation, yet gives you a boost in timing at light loads to promote fuel economy while cruising.

If you use ported vacuum it works just the opposite. I know that seems to be the holy grail but it just doesn't work properly that way and causes other issues. At full throttle,,,ported vacuum is still applied,,,so all that vacuum advance is still added at full throttle along with the mechanical advance you have in the distributor.
Using the same 36 degrees as an example,,,and 10 degrees in the can,,,,you could have as much as 46 degrees at full throttle. The engine won't be too happy there to say the least. You only want the vacuum advance to be working at idle and part throttle conditions. Which is why manifold vacuum works best here.

I've even seen some magazines talk about the subject and refer to a ported vacuum source for the distributor. Cracks me up,,,can't believe they would print that for millions to see. I can't tell you how many cars I've had come in with the distributor hooked to ported vacuum, then complain the car is rattling to death at full throttle.

When the cars were new, most were hooked to ported vacuum, some were through a vacuum control switch wired to the transmission starting in 1971 that only turned on when the car was in high gear. The biggest reason it worked on the cars when new,,,was because the distributors were setup very conservative and only had 25-28 degrees of total timing when new, and the added vacuum advance at full throttle didn't hurt anything. This was done for emmisions reasons.
But it gave poor gas mileage as a result. Your vette being a 74 had a setup just like this when new. My wifes 71 454 vette was the same way. Got a miserable 12 mpg at best. Once I recurved the distributor for 38 degrees total, and bypassed the vacuum control switch and went straight to manifold vacuum, for full vacuum advance at idle and part throttle cruising,,,,the car got 17 mpg, and picked up 4 tenths and 3 mph in the quarter, idled smoother, ran cooler, etc...The dragstrip improvement was more of a result of the distributor recurve more than anything, as the stock curve sucked,,,but the gas mileage and cooler temps were a result of the added vacuum advance at idle and cruise.
Sorry for the long post,,,,if you want to get more involved or have a problem with anything I mentioned here, shoot me a pm and I'll give ya my cell number. Hope this helps.

[pajeff02]

Thanks FBJ! I should have some time this week to switch things around and see what happens. I will take new readings and post up the results. I had to move the car around today to install a new cabinet in the garage. As soon as it came off high idle it wanted to die. Even allowing it to warm up a bit more it did not want to idle when I let off the pedal.

[Firebirdjones]

Not a problem.

What are the specs of the camshaft?? I wouldn't think it's very large if you have 16 inches of vacuum at idle in park. But even with a mild camshaft you could get away with more than 10 degrees initial. Depending on how the distributor was setup and the amount of mechanical advance in it, you could dial in more initial.
Say if there is 20 degrees of mechanical advance and it's done,,,,and you only have 10 initial,,,then you have only 30 total. Not alot when it comes to this old stuff, unless you are running a decent head with an efficient combustion chamber,,,I'll bet the car will run better with more ignition lead, somewhere in the 34-36 range. To get that you can do one of two things,,,,

Bump the initial up to 16,,,,the 20 degrees of mechanical advance will get you 36 total. This is what I do to most cars, especially anything with a camshaft,,,sometimes as much as 18 initial, then I limit the mechanical to stop at 34-36 or so,,,depending on the car.

Or,,,,if you want to keep the 10 initial, you'll have to elongate the mechanical advance slot and/or install a smaller bushing.

Again it really depends on how the mechanical advance is setup and the total timing you have. I would shoot for 12-14 initial though just to help the idle out a bit, and then dial in the mechanical advance to the total number you need. Hope this isn't confusing you. Just let me know what you find and we'll go from there.