T/A dies when put into gear

[JZ]

Hi,
  New to forum. I run a small auto repair shop and a new customer brought in a 79 Trans Am with a 403. HIs complaint was that the car took along time to start and didn't run right when it did. Short story is that after getting a second carb from a salvage yard, I've finally got it to the point where the idle is down to 800 rpm but when you put it in gear, the rpm drops off to about 400 or 500 rpm  and idles poorly - it won't die but its close. Info on engine is that it was rebuilt quite some time ago but has under 1000 miles on it. .030 over 9:1 pistons, "OE Grind" camshaft, stock intake and exhaust, carb #17058253 (both carbs had same part #) 71 primary jets. The work that has been done to the carb is an overhaul with a Hy-Grade kit where I've run a wire down the idle tubes and sprayed carb clean through all passages to make sure they were open (after soaking carb in cleaner), installed bushings in primary and secondary shafts (Cliff's kits), installed a 1/16" thick gasket (Cliff's) because the airhorn was slightly warped and set the timing at 23 degrees at 1,100 rpm (vac advance plugged, manifold vacuum), mixture screws are out 9 and 10 turns No pickup in rpm now when closing choke valve slightly as there was before. Also now that the idle is under 1300 rpm, the nozzle drip is gone. (no surprise) Other then the idle issue, this thing runs really good - no hesitation, flat spots, etc. Any help to get this to idle as it should would be greatly appreciated.
Thanks for your time!

[blazer74]

Could be a timing issue. If the mechanical advance is in too soon and when placed in gear the mech advance drops out could contribute too your drop and low idle.
Quick check before focusing on carb which I take is the second one having the same trouble is simple enough.
If you have no rpm increase with the choke slightly closed with your finger at approximately 2000 rpm you are on the rich side.

[Cliff Ruggles]

No exactly sure what the issues are, but a couple of things to mention.

71 jets are incorrect for that carb number.

23 degrees timing at 1100rpms, how much timing at 700-800rpm's?

Does it idle smooth or have some "lope".  If it isn't idling smooth I suspect the cam was changed.  That engine with the compression raised to 9 to 1 and a stock cam should be making about 20" vacuum at 1000-1100rpm's, and at least 16-18" at "normal" idle speed with about 16-18 degrees initial timing in it......Cliff

[JZ]

Thanks for replying - hope everyone had a good Thanksgiving. I checked the timing and it was 38 degrees @ 800 rpm with the vac line plugged into the "B" port on the front of carb which is the port that the tune up decal on the radiator support shows. Unplugged, it drops down to 12 degrees @ 500 rpm - all the timing is in apparently. At 2000 rpm with choke valve tip in, rpm stays the same. Vacuum @ 1100 rpm is 10-18" with the needle fluctuating rapidly. Engine does seem to have some "lope" to it but not much - it doesn't sound like a factory cam to me but I haven't really been around many hi performance cars for about 35 years. When I gat a chance I'll try jacking up the initial timing to 16-18 degrees as Cliff suggested but do I need to find a different ported vacuum source for the advance? Also, what should be the correct jet size for this carb and application?
Thanks for your help!
 

[Cliff Ruggles]

Going from memory (and I've built a LOT of those units) they used ported vacuum to the advance.  The source location is in the main casting just to the right of the fuel filter housing (as viewed from the front).  It may not go directly to the distributor depending on how many emission devices are being used and if they are still in place and operational.

Correct part number for the 1979 T/A with the 403 is also 17059253.  The 17058253 is close, but the mixture screws are 10-32 instead of metric, so they usually provide plenty of fuel 3.5-4 turns out from seated.  The metric screws may require 5-7 turns to do the same thing.

38 degrees timing at idle is a LOT, and may account for high idle speed and not a lot of sensitivity with the mixture screws.

The timing at idle speed should be rock solid and those engine idled dead smooth at 700-750rpms with a LOT of vacuum.

If you continue to have idle issues I suspect a larger than stock cam was installed at some point.  This assumes no vacuum leaks and everything associated with engine tuning working as it should.

I'm wondering about the original carburetor and what happened to it?......Cliff

[77cruiser]

If the vacuum is moving around that much 10-18 something else is wrong I suspect. Leaking valves or flat cam.

[Jeffs68]

X2 Jim, if JZ has a shop, I'd suggest doing a cylinder contribution test. No sense in trying to dial in a carburetor on a mechanically faulty motor. If it does only have 1000 miles on it, but was built some time ago, I'd add a can of Seafoam to the crankcase and run it for 15-20 minutes varying the RPMs. Change the oil and filter and see what the vacuum is doing then. May have freed up the sticking valves.

[JZ]

It took awhile, but I'm back on the T/A. The timing adjustment (18 degrees base) and relocating the vac line to the distributor to the port right of the fuel inlet worked to keep it running in gear although I think the 500 rpm it idles at is too low for this engine/carb setup. If I raise the idle in park to 950, I start to get nozzle drip but in gear the 650 it idles at is pretty good. However, I've noticed some other issues. The air horn gasket is appears to be saturated and there is fuel seeping up around the accelerator pump and leaking from the nozzles after shutdown The mixture screws are out nine turns, float level is at 13/32" while running (I have a gauge from OTC that fits into the hole behind the choke valve) and I pulled a vacuum on the needle and seat when I had it apart last time following Cliff's book and it held vacuum fine. The air horn is warped but I didn't think it was too bad because I tightened the air horn down without the gasket in it and put a bend-a-light down it and couldn't see any light showing through (to the outside anyway) so I  used a 1/16" thick gasket from Cliff's. Also, after this car sits for about a week, the float level has dropped to nothing (as per my gauge). The bottom plugs were sealed with J-B quick and since this is a '78 carb could this be a problem or am I loosing my fuel through the top somehow? And would any of this have anything to do with the nozzle drip I'm getting at the higher idle rpm?
To answer Cliff's question, I have no idea where the original carb is and neither does the current owner so I guess its a mystery of the universe.
Thanks again for your help!

[JZ]

Sorry, I forgot something else I noticed. The power piston isn't pulled down at idle. The way I understand it, it should be so is this merely a problem with too heavy of a spring or does it have anything to do with the air horn being warped?
Thanks!

[Cliff Ruggles]

Wrong spring, insufficient vacuum at idle, or the hole is blocked and not supplying vacuum to it.

JB Weld is USELESS for sealing up bottom plugs.  100 percent of the carbs that come in here with JB Weld dabbed over the plugs leaks right thru them.....Cliff