[70gtorag]

Fresh build and just broke in. Only 5" vacuum and have the curb idle screw set high to keep the car at 800rpm idle, runs really rich. Not sure where to start looking, assuming the timing is not right. Have 12deg initial and about 34 total timing. Turned dist to get 16deg and idle changes slightly, no change in vacuum. Have not had it run on dyno and tuned yet, just trying to do basic garage tune to get it to run right. Relatively newb, go easy!
Did we do something wrong with the distributor install? Before initial fire, engine at TDC, dropped in distributor and with cap on, the rotor points to just before #1 plug. Car started instantly and ran great, still runs really good just trying to sort out the low vacuum and rich condition.


1970 400 .030 over with Keith Black pistons, eagle rods, stock heads-reworked, edelbrock intake.
Cam is Comp XE284H-10, 240/246 @ .05, 110 LSA
MSD pro billet dist and 6AL box
Jet stage 2 quadrajet carb

[dmac]

Did you adjust the idle mixture screws to get max vacuum yet? You'll need to go back and forth between adjusting idle speed screw, timing, and idle mixture screws. Plugging vacuum line to distributor, and getting carb running best at idle eliminates vacuum advance as an issue in idle setup.

[Will]

Originally Posted by dmac

...Plugging vacuum line to distributor, and getting carb running best at idle eliminates vacuum advance as an issue in idle setup.

Definitely make sure you're not checking your idle and total timing with the vac advance hooked up!

I dont' know how much vac you're going to get with that cam but 5" does seem a bit low.

[STEELCITYFIREBIRD]

Are you picking up manifold or ported vacuum with your gauge placement? Would seem to be late timing.
That's a big cam in a 400,
what is the compression ratio?
that cam would require a relatively high ratio.
Any possible vacuum leaks, PCV, brake booster, correct carb fit to manifold?

[70gtorag]

Its MSD8563 no vacuum advance, just mechanical. I have tried to adjust the idle mixture screws but makes no difference whether at seated or at 5 turns out.

I believe the CR should be 10:1.

[dmac]

Screw in idle mix screws as far as possible until rpms drop., then out 1/4 turn. Then turn dist to see if it makes any difference. Go back and forth to give best vacuum., adjusting idle speed if it goes too high.Make sure all vacuum sources are plugged. In this case, I would even disconnect power brake source, too. You can spray something like carb cleaner around carb base and intake manifold to make sure you don't have any leaks. Plus, try measuring at a different port. You may be using a ported source which would give you false readings, especially if your throttle is open a little wide.

You might end up with low vacuum, but if you have power brakes, you may need more. A carb expert should chime in soon to mention something about a 'slot' on the throttle blades that could be an issue too.

[skullbucket]

Whats that one trick you can do with a big cam and a Quadrajet where you change out a spring ( primary metering rods spring, power piston) ? with one out of a ball point pen to make the carb idle better, I've done it with some improvement, but I haven't really ever seen a quadrajet run rich more like super lean. I suspect timing is a large part of the problem.

[moontower69]

Did you degree the cam? Either that's an issue or you might have a vacuum leak in that qjet or carb gasket which explains that low 4-5 vacuum reading at idle. Obviously power brakes aren't gonna work well with that level of vacuum.

But that's just too big of a cam for a street car w/ 400 cubes IMHO. Maybe ok for a 455? I'd say even dialed in properly, you're not looking at more than 8 to 10 vacuum max on a 400 w/ that large of a cam.

[steve25]

That's way too big of a Cam for the stock power piston spring in the Carb if that's what they rebuilt it with?

[DOC]

Had the same cam in a 462. installed straight up. Pulled almost 10" of Vacuum at 850 rpms / Idle .

[TedRamAirII]

You should be able to manually depress the power piston and see if idle improves........but the primary should not even be in the mix, unless the idle screw is opened too much, and its dribbling fuel down the barrels.

[Formulas]

Look for nozzle drip at idle as a indication of high float, bad needle seat or excessive throttle blade angle .. power piston is not in play for idle same as power valve >value< in a holley. .. transfer slot and idle fuel are metered through the orrifice at the bottom of the down tube then further regulated by idle mixture screws for idle at the carb base.

[70gtorag]

https://www.dropbox.com/s/5gn1u0f94h..._1367.MOV?dl=0

Yes I think the idle screw is opened too much. I have pasted a link above that is a short video looking into the carb with engine running, lots of fuel dumping in. Does this mean that because of low vacuum the power piston cant overcome the spring pressure, keeping the primary rods out of the jets causing extra fuel dumping in, will a softer power piston spring help to lean out the mix or can the apt screw be adjusted down to allow the rods to come down further?
The carb is a Jet Performance Stage 2 qjet, anybody know what power piston spring comes with that rebuild? I would assume a medium spring and that changing to a softer spring would be better for the low vacuum cam I have installed.

[HWYSTR455]

Wow, that is just dumping fuel in there.

.

[johnta1]

I doubt it's ignition timing related.
Probably something else, like vacuum leak, carb or cam timing?

What does it do when the idle is turned up some?

[grivera]

Make sure you check your oil as that much fuel may dilute it and potentially wash out your rings.

[leeklm]

This won't help your immediate issue, but since it sounds like you like to get your hands dirty, I would highly suggest buying a copy of Cliffs qjet book. After doing a few qjets now, I would not trust any mass produced qjet without taking apart and going through some of the basics according to Cliff's book. Something as simple as idle bypass air could get you going in the right direction.

Try to find a known good running carb from a performance engine to test so at least you know you are chasing the right thing.

Sent from my XT1585 using Tapatalk

[STEELCITYFIREBIRD]

Originally Posted by 70gtorag

https://www.dropbox.com/s/5gn1u0f94h..._1367.MOV?dl=0

Yes I think the idle screw is opened too much. I have pasted a link above that is a short video looking into the carb with engine running, lots of fuel dumping in. Does this mean that because of low vacuum the power piston cant overcome the spring pressure, keeping the primary rods out of the jets causing extra fuel dumping in, will a softer power piston spring help to lean out the mix or can the apt screw be adjusted down to allow the rods to come down further?
The carb is a Jet Performance Stage 2 qjet, anybody know what power piston spring comes with that rebuild? I would assume a medium spring and that changing to a softer spring would be better for the low vacuum cam I have installed.

Push the piston down and see if it cleans up, then lower curb idle, advance the timing to ~22, adjust idle mixture screws.
Pen springs are calibrated for pens, carb springs are calibrated for carbs.
EXAMPLE:
https://cliffshighperformance.com/Qu...piston-springs

............Call JET for help!!!

[johnta1]

70gtorag, just looked at your pic, where is the gauge hooked to?
The carb is a chevy style qjet. Possibly the base gasket is wrong?
Also, the brake booster looks like the hose may not be connected?
Is the carb fitting on rear of carb blocked off?

Know of anyone with a good carb that you could borrow?

[skullbucket]

Originally Posted by STEELCITYFIREBIRD

Pen springs are calibrated for pens, carb springs are calibrated for carbs.

Oh since someone could fix their car without sending off for parts, paying money and waiting a week is wrong.
Iv'e never seen a spring with a description engraved on it for its use, so your wrong there.
Just remember that screw driver you use to clean you ear with is to take that carb apart when the parts arrive in a week. wink back at cha.