Time to finally dial in the circuits on this W72 carb...

[nUcLeArEnVoY]

I've been putting this off for awhile, since my '79 T/A never sees WOT. I'm delicate on the motor, usually shift at 2k, and granny it around town to a point where I hum the theme to Driving Miss Daisy. But, I think some of the car's carb circuits need some fattening up to account for the mods done to it over the years which add leanness little by little.

Car is a 1979 Trans Am T/A 6.6 W72, carb is original to the car 17059263.

Engine is non-matching numbers. Block is a plain jane XA '77 500557 block, while the heads are the original 6X-4, and original 10003395 intake.

I got the car last year, the engine's definitely been gone over but I don't know exactly how, so I can't be precise in telling y'all what's been done to it, but I can estimate:

*Definitely cammed. Based on a 16-16.5" idle vac at 750 RPM, I'm guessing it's a relatively mild cam. Converted to a Chevy-style adjustable valvetrain since the heads have been gone through based on the copper shims under the valve springs, but the rockers are stock-style stamped ones.

*The exhaust manifolds are stock, but the catback is converted to true dual without cats

*EGR deleted

*Heat crossover unblocked and still in use, advised as such by Cliff for driveability, but choke was converted to electric since as usual my hot air choke tubes rusted over and for some reason, Inline Tube just refuses to repro the Pontiac 400 choke tubes

*Clearly this engine build was meant to be close-to-stock, but with one of Cliff's very-much-hated high LSA lope cams, yaaaaay. Pretty sure it's still very low compression, especially with that 500557 block. For ignition, I have a new distributor but I tossed in GM 045 weights and 346 center cam. I have my initial timing set at stock 18 BTDC, vacuum advance regulated through the factory delay valve which permits a precise 9" of manifold vac to the advance at idle until the port vacuum signal exceeds 9", at which point the vacuum advance is on port vac. Advance springs are the heavy ones out of the Moroso kit, since they closest resemble the advance curve as the original heavy stock springs, and I don't want timing creeping in at idle like I had before. With all this, the engine doesn't ping at all and no starter kickback during heatsoak, so again, I'm guessing it's low compression. I fill her with REC-90 ethanol-free since it has a NOS AC fuel pump, so I don't trust ethanol on the diaphragm.

*3.23 gears out back

[nUcLeArEnVoY]

I sent the airhorn to Cliff recently to have it planed to correct a gasket wetting issue (which worked WONDERS, Cliff, thanks!) While he was at it, he opened up access to the ATP, so I think it's finally time I take advantage of that.

Right now, the carb has 73 jets in it that I threw in just in case to account for the higher leanness of no EGR, but still has 42K rods in it that came in the carb when I first opened it up (factory I think is 40K). So what should I do as a starting point to start dialing in my off-idle, part-throttle/cruise circuits? Maybe move up to 74 jets and go with Cliff's 45C rods and then dial-in from there with ATP? As I understand, my carb doesn't have LMABs, so I shouldn't have to mess with the air bleeds, but I'm not sure. Secondary rods are stock DB. Maybe richen that circuit up, too? Y'all are the experts.

For what it's worth despite today's crapola fuel, my spark plugs are pretty damn white these days, maybe a little bit of tan at the very tip near the electrode. Using .045" gap ACDelcos with HEI.

I'm PRETTY SURE my idle circuit is good. I have control with the mixture screws, they're the metric threads. The car will start to stumble and sputter at 4 turns out and dies at 3 turns out, so they're actually a little TOO sensitive. Vac/RPM stops rising at around 7 turns out, which as I understand is  normal for metric mixture screws. Idle tubes are stock .038, DCRs .043, mixture screw holes untampered. She does idle at 750-775 RPM as per the stock spec, no nozzle drip, no port vac signal, tip-in slows down the motor at idle, and pulling a vac line speeds it up, so do you think my idle system is good and doesn't need modifying? Maybe I should open up the DCR's just a little bit to richen up the idle, because I've been noticing this summer the car will temp creep up to 210-215 w/ a 180 degree stat while it's idling, so maybe it's a little lean? Not sure.

Let me know what y'all come up with!

[old cars]

Start by installing an air/fuel gauge. I know some of the old-timers are gonna bark but

[Cliff Ruggles]

Lean doesn't mean it will run hot so forget that line of thinking.  Lean will cause it to hesitate, stumble, bog, surge at light part throttle, lack of power at light throttle openings, etc. 

NEVER read plugs to tune and it's normal to see white to light tan when you aren't using a gas guzzling aftermarket carburetor on one of these engines. 

If you aren't using factory advance springs you need a positive stop in the advance mechanism or it can add timing at high RPM's and destroy your engine.

The DB rods are too fat (lean) at WOT even on a stock engine with this new fuel so it needs some help there.

Do NOT modify any airbleeds in that carburetor they are exactly where they need to be.  If you have full control of idle fuel with the mixture screws and it is smooth right off idle and not lacking anyplace the DCR's don't need to be any larger either.

73 jets are plenty for that unit.  I use the same carb feeding nearly 600hp and only use 73 jets in it with 44 tapered metering rods.  I don't remember the job but hopefully you put one of my rebuild kits in it with the non-windowed high flow N/S assembly and lifetime warranty accl pump.  Those OEM fuel pumps were pretty high pressure so be careful with the float level.  It should be set to factory specs if you are still using to original mechanical fuel pump.

Hopefully the vacuum supply to the choke housing was blocked off when it was converted to electric or you will have a leak there.......Cliff

[nUcLeArEnVoY]

Cliff,

Yeah, I always wondered about that whole "lean=hot" thing, since even people who spout that idea are sort of iffy on it, always pre-ambling the remark with "lean mixtures *may* make it run hot."

For the advance springs, the ones out of the moroso kit I'm using match the tension (are actually a LITTLE heavier) than several stock sets of springs I have. To get max advance out of the weights/center cam, I'd have to go near redline which 1), I never do, and 2), near redline will mean my throttle is open enough to drop vac advance, anyway.

So DB rods are too fat, which would you recommend?

Copy that on the airbleeds, and yeah, I do have full control with the mixture screws. I do get a bit of off idle hesitation if I don't give enough gas, and admittedly, my idle throttle angle is a little higher than I'd like it to be. Even though I have no nozzle drip or port vac signal, I am showing a good 1.5-1.75mm of transfer slot, as opposed to the ideal "square," but I recall you telling me before that's inconsequential as long as she holds an idle with no nozzle drip and port vac signal.

I'll keep the 73 jets in it, no problem. Should I change out the rods? Right now I have those 42Ks in there... what would your recommend? And yes, I have one of your .135 NON-WINDOWED seats and viton needles in there right now, which as I understand is factory spec, anyhow for the T/A 6.6. Also have one of your accelerator pumps in there. Right now, float level is set at stock 17/32. If I set it any higher, even so much as 13/32, I'll get fuel pooling around the accelerator pump piston at the top of the airhorn.

As for the vacuum supply to the choke, well... no, I don't have it plugged off. I did, however, fashion a vacuum fitting for the choke housing so I can still use a vacuum line to connect it to the choke modulator filter in the air cleaner like from the factory, only difference now is that the air it sucks in doesn't get heated by exhaust (doesn't really make a difference in how long the e-choke takes to open). I wanted to keep that vac leak because it serves as the factory bypass air... should I not do this? I've blocked it off before, and it dropped my idle a decent amount - I'd be afraid to try to compensate for it by raising the throttle angle anymore, because then I think I would breach nozzle drip and port vac signal territory...

[Cliff Ruggles]

If your e-choke conversion has one wire and grounds on the housing you MUST block the vacuum supply to the housing.  The two wire variety (rare) allow the gasket to be used and you can block the vacuum supply where the heat supply tube attaches to the housing instead.

Folks associate running lean with running hot, and leak with improved fuel economy. Two myths in this hobby that get regurgitated on nearly every Forum I've been on since the Internet was introduced.......

[nUcLeArEnVoY]

If your e-choke conversion has one wire and grounds on the housing you MUST block the vacuum supply to the housing.  The two wire variety (rare) allow the gasket to be used and you can block the vacuum supply where the heat supply tube attaches to the housing instead.

Folks associate running lean with running hot, and leak with improved fuel economy. Two myths in this hobby that get regurgitated on nearly every Forum I've been on since the Internet was introduced.......

I'm using one of your e-chokes which grounds through the housing, and I don't use a gasket, obviously. But to maintain that bypass air, I have a vacuum tube routed from the choke housing to the little filter in the air cleaner that normally filters the air entering the hot air choke tube. I would think the metal on metal seal of the choke housing and e-choke is enough to prevent contaminated air from getting in, otherwise the air coming in through the vacuum tube is filtered by the modulator in the air cleaner just like from the factory, only it's no longer getting heated. Like I said, doesn't make any difference in the e-choke operation. It works just fine, only now I get to keep the original bypass air design.

So talking rods, what secondary and primary metering rods would you recommend I get, given what's been done to the engine and the carb unit #?

[Cliff Ruggles]

"I would think the metal on metal seal of the choke housing and e-choke is enough to prevent contaminated air from getting in"

It does NOT establish a positive seal so would suck unfiltered air into your engine.  Block the vacuum before the housing.  It may not even miss the lost bypass air and if it did you can simply open up the bypass air holes in the baseplate slightly to bring it right back.

If you want to try different jets and metering rods call the shop.  I have full tapered 45C metering rods that would provide a much wider range of control with the APT that a "K" metering rod.  They taper from .045" all the way to .026"........

[quadrajam]

Newbie 1 finger typer here...
Back to the lean/hot thing. Hot only happens at very high
cylinder pressures like WOT. You can lean it out all you want
at idle or cruise and it will just shut off.