Shdggsdv

I can get the idle set even as crazy high as 1300 RPM in park/neutral, but the moment I shift it to gear she drops to 450 RPM. I can't seem to get the in-gear idle to change. Any ideas what this might be caused by?

NeverTooOld

What year and which carburetor?

Dynra Rockets

vacuum leak?

Shdggsdv

1975 Corvette with a recently rebuilt 1991 Chevy K1500 engine, Comp XE275 cam, Promaxx 64cc aluminum heads, etc

Carb was a 600cfm Edelbrock 1406 that I am in the process of switching to a Holley 670 Street Avenger

I've checked all over for vacuum leaks and I can't find any. Sprayed carb cleaner all along intake and carb base. Plugged all hoses. All that,

HeadsU.P.

Just curious. What advance springs do you have in the dizzy? Sometimes they are too lite / weak and let the wts fly out too soon.

drwet

It sounds like you might have your vacuum advance connected to ported vacuum. Try connecting it to manifold vacuum.

C3 Stroker

Check timing.....block off vacuum advance and check total timing and initial timing.

cardo0

What did you do to the transmission? Rebuild? New torque converter?

Bikespace

silver74vette

Have you measured manifold pressure at idle, what are you running? This also sounds like it could be a cam timing issue.

lakerider57

I also didn't know this and thought the vacuum advance should be connected to ported vacuum. So I went googling for more answers and here is an excerpt from Super Chevy magazine:

"Plugging your vacuum advance into a direct source will allow it to engage at idle, which is good for a number of reasons. Much like cruise conditions, engines run leaner at idle than they do under load. Again, this means the mixture burns slower and needs an earlier spark to optimize the burn. Ensuring that the mixture has a complete burn before leaving through the exhaust port also helps the engine to run cooler at idle. All carbureted cars were set up with direct vacuum to the distributor before more stringent emissions requirements reared their heads.
Ported vacuum sources are a result of emissions laws and manufacturers doing whatever they could to get big V8 engines to pass smog before the incorporation of the catalytic converter. The idea was that by using little to no spark advance at idle, the exhaust gas would leave the cylinder still-on-fire and help maximize the efficiency of antiquated air injection systems. Engines from this era often ran very, very hot, were prone to warped exhaust valves, cracked cylinder heads and all other manner of issues. Using a ported spark advance will still allow the vacuum advance to do its job at steady cruising, but all of the benefits of idle cooling will be lost.
Here’s a quick experiment you can try on your car. Plug the vacuum advance into a ported vacuum source and check the idle rpm. Now, switch the vacuum advance to a direct source of vacuum and again check the idle rpm? We’ll bet money the RPM increased. Why? Because the additional ignition timing provided by the vacuum canister and full manifold vacuum source allowed the engine to more effectively burn the air/fuel mixture. It therefore produces more power (even at idle) and rpm rises as a result."

Richard

MelWff

have you adjusted the idle mixture screws in drive?

gkull

The people that use full vacuum are the ones that aren't smart enough to recurve/limit the dizzy mechanical advance so you can end up with high initial advance of 18-22 degrees. Which is not jumping around like hooking it up to full vacuum to crutch the timing up

it is very sad that super chevy printed that

lakerider57

I haven't setup my vacuum advance on my distributor yet since I got the engine off the dyno.

I have the MSD Billet 8365 distributor and the literature says: "The vacuum advance will advance the timing up to 10° during partial throttle driving (with 15 lbs of vacuum). The vacuum line should be routed to a ported vacuum outlet above the throttle plates."

In readings the specs, it seems that setting 12 deg initial timing plus using the "blue" light springs and blue 21 degree stop bushings will give 12 + 21 = 33 degrees mechanical advance around 3000rpm .

Richard

Shdggsdv

Nothing new for the transmission. Fixed a vacuum leak on the vacuum line that goes to the trans. That's about all

Shdggsdv

Okay so I've got 10" at idle and I should be getting 11" according to Comp Cams (XE274), but that's because I haven't thoroughly tuned the idle mixture due to this issue.

So I did put in lighter springs on my dizzy as recommended by a mechanic. It's possible that was wrong and I'll try heavier springs and see if there's a difference.

As well, my dizzy was connected to ported vacuum I believe as that's what the Edelbrock manual recommended. The passenger side vacuum port is where I had it connected. If the springs don't work, I'll try using manifold vacuum instead.

gkull

it is actually inches of vacuum, but anyway I don't know your head type or altitude . So I can't comment on your correct total degrees

presently I have 14 degrees of mechanical advance. So 14 + 19 = 33

I've written several posts about how to set up the idle mixture screws. I will tell you that adjusting them while in drive is not the preferred idea

you get the dizzy dialed in with high initial, hook a vacuum gauge. Plug off vacuum advance , set the idle speed screw to about 1000. Or wherever you get a steady needle

adjust your mixture for the highest vacuum. You might have to turn the idle down as you get them set

new motors are tight and have less vacuum.

Shdggsdv

You can determine the correct total based just off the heads and altitude??

gkull

you can also add in variables like how well the fuel is atomized entering the chamber. Compression ratio, quench. Exhaust gas dilution levels and exhaust gas temperature.

Modern fast burn chevy chamber designs are from 28 to under 34 degrees. Older stock iron heads with poor flame travel required higher totals

higher altitude requires additional. Timing

this is all online or in books

cardo0

Well if you're using a ported vac port I suggest you disconnect the vac line (and plug it) then just turn the distributor for the best idle/sweet spot. Then reconnect the vac line. If this works then you have to set up the vacuum advance with the right canister for your engine vacuum. Well you need the correct cannister regardless but it can be significantly different than when using manifold vacuum.

Manifold vacuum many use the 2" rule where you want full vacuum advance 2" lower than engine idle vacuum (max vacuum). Ported vacuum should take something different and you have to drive it to find what's going to work.

One more thing is I'm pretty sure the stock Q-jet had a idle solenoid for a auto trans when placed in gear. I don't have time right now but will try and look this up for you. Placing the auto in gear adds a load to the engine - large enough to drop RPM.

Hope this helps.