General Category > Quadrajet Carb Talk and Tips
Float level and other Qs
Macadoo:
Man, this is like herding cats, lol. Thank you, Cliff, for your patience as i get this ironed out.
I am running an MSD spring and weight kit, as you say; aftermarket. One light spring and one medium. Although I can't see my tach, it sounds to me like the curve isn't all the way in until around 3000 - 3500. I'll see if I can get the wife to sit in the car while I test this, this weekend.
Do you think I need to open up the idle tubes and DCRs a little more? Currently they are at:
Idle tubes are at .038
Down channels .055
Idle bypass air is at .120
Mixture screw holes; I can't remember exactly (I'm at work), but on the high side of recommendation, maybe a little over. One coil clipped from springs.
Jets: Cliff's 73
Metering Rods: Cliff's 44
Again, I can still pull the AC can vac hose from the intake and the idle speed increases maybe 200 rpm. This is why I took the idle bypass air to .120. Is this to be expected?
Cliff Ruggles:
You will get some idle speed increase from pulling a vacuum hose unless there is a serious lack of idle fuel available, then the engine will act worse and/or RPM drop off.
I use that method more to see if it needs more idle bypass air than any other reason. Just yesterday was testing a Marine carb, they do not have idle bypass available.
To simulate a smoother idling Marine engine and to get rid of the nozzle drip we were seeing I move the vacuum advance to manifold vacuum and pulled off the vacuum supply to the choke pull-off. This allowed me to lower the throttle angle and stop the nozzle drip we were seeing and get control with the mixture screws.
Before you can do any idle tuning you have to verify that no timing is being added at idle speed, it has to be rock solid below about 900 rpm's. If it falls out any at all it makes idle tuning difficult.
To this day I don't know why folks think you have to have the mechanical advance "all in" early in the RPM range. That's more of a "crutch" fix for engines that don't have enough compression for the cam choice, really tight LSA camshafts, or just plain poor choices in general for the combination of parts.
The distributor in my engine is pretty much untouched since the mid 1980's when I first placed it in service. It's been in 4 different engines and even dyno tested against an MSD billet with a 6AL box. The dyno runs were nearly blueprints of each other with a very slight advantage to the HEI. I suspect this is from how precise the tips of the pointers are on the pick-up vs the broad/wide ends on the MSD unit.
I just had it out a few weeks ago to check the status of the bronze gear and cleaned things up. It's a Pontiac HEI with stock weights, stock springs, stock center cam, stock 990 module, and stock coil. The only modification done to it is adding a positive stop so it can't continue to add timing at really high RPMs (common problem with factory HEI's and compounded when you put lighter springs on them).
It starts adding timing right off idle/900rpms and smooth/steady timing curve all in around 3000 rpm's. I add another 14 degrees from the vacuum unit via ported vacuum. Base timing set at 10 degrees, 30 total.
Macadoo:
Excellent info, once again. I'll report back when I get this stuff nailed down.
Cliff Ruggles:
Sounds great.
I will add that at least 50 percent if not more of the problems we see here with tuning is the distributor and not just the carburetor.
It's ALWAYS the same thing, some "goofy" offshore spring weight kit added to the factory distributor at some point and the excellent factory center cam, weights and springs are long gone.
Those kits not only use less spring tension they change the geometry of the parts and allow the weights additional leverage on the center cam to bring in timing early and easy. Problem is that it is often difficult, if not impossible to keep some of the curve out at idle speed with those parts.
Nearly ALL of the aftermarket HEI's we've seen in recent years have the same problem. I'd also add that I reject most that show up here due to low quality of the internals and lessons in humility trying to get a decent spark curve with them.
I use and prefer stock distributors and stock components.
Case in point. Recently we had a Buick 455 Stage I car brought here for custom tuning. Many hands had been on it, and it was NOT making the grade anyplace. Biggest complaint lacking in power (wouldn't even spin the tires).
As soon as we removed the air cleaner the "lacking in power" was clearly evident as the carb "builder" used the wrong choke pull-off (2bbl) and it wasn't allowing the secondary flaps to open.
We went thru the carb and it was a little "hacked" so fixed all the issues with it and recalibrated it for the application. We installed the carb and the owner took it for a drive, came back smiling ear to ear, so power was restored. The idle quality was a little "funky" and I noticed it "bucked" the starter slightly on a hot re-start I we put a timing light on it. At idle timing was over 1" off the scale and hardly any advance when you revved it up.
Removed the cap/rotor and low and behold a "goofy" spring and weight set-up allowing nearly all the advance it an idle speed.
We installed factory weights and springs and took a look at the curve with the timing light and re-timed the engine. Another quick test drive and the owner says that it's just a "touch" down on power right off idle.
I removed the factory springs, opened up the "eyes" just a tad, one more test drive and good to go.
With that simple modification we saw NO advance at idle speed, and the timing started advancing at 900rpm's and all in around 2800-3000rpm's. No more "bucking" the starter on hot restarts and idle settled now nicely every single time you let off the throttle, just like it should be doing.......Cliff
Macadoo:
Okay Cliff, did some checking this morning.
There is no timing difference between 800 and 1100 RPM so it's not advancing at that high idle, probably due to the stiffer springs. However, when revving, there is a quick jump that starts at about 1300 RPM up to 28 degrees, and then slowly advances after that up to 37 degrees at about 4000 RPM (although this is still by ear. I need to find a helper to read the tach). Certainly NOT a smooth timing curve.
As far as the timing "settling nicely" when letting off the throttle; I have little experience with what is to be expected but it seemed to settle a little slowly.
I did save the stock weights, center plate, and springs. However, this is not a stock distributor. [Full discloser]: it's one of the eBay cheap aftermarkets.
If I look for an original GM distributor, it would need to be Oldsmobile, correct? Since they turn counter-clockwise. May be a little hard to find.
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