carb adjustments

[Paul]

Cliff,
 I will be pulling my Quadrajet apart # 17057266, this month, to drill and tap the well plugs, im assuming they leak because after the car sits for a couple days, the electric fuel pump has to run for a minute to fill the carb, and I have to pump the accelerator a few times to get the engine to start.
 Anyways, I just ordered Your plug kit, and while I got the carb apart, I would like to make other adjustments like,  the A/F screws hardly slow the engine at idle when turned all the way in, what can I do to fix that? Thanks,

Paul

[Cliff Ruggles]

I'm hoping the electric pump only has to run a few seconds to fill the carb, not a minute or so.

Fuel leaves the carb after shut-down in three ways.  It will evaporate, leak at the bottom plugs, and can drain down to the level of the seat if there is an open path (return system).

We can't do anything about evaporation, it just happens, and worse with these new fuels than older blends.

Leaving the clip off the needle will prevent any drain back, and it never presents a problem unless the carb were to sit for many months where the fuel could "glue" the seat on to the needle.  I've never had this issue, but that is why the little clip is there, to lift the needle off the seat as the fuel evaporates away.

If you haven't installed one, get one of our SR kits to upgrade the carb for this new fuel while it's apart......thanks....Cliff

[Paul]

when I rebuilt the carb using Your book, I also bought a rebuild kit from You as well as a  needle seat with larger than average needle hole, that doesn't have the slots on the sides, at that time I tested the plugs and they didn't leak and they looked good so I didn't plug them, I should have. The fuel pump only runs for a few seconds be for filling the carb, the carb never used to drain at first, now if the car sits for about 2 days it will be empty, but thinking back when the carb draining issue started, is about when the very slight hesitation from idle to acceleration started. I'm not sure if there is any relation, but I got to do something. There is a fuel return line, with no one way check valve.
 Since the carbs A/F mixture screws wont kill the engine at idle, or even come close, should I drill the holes in the base plate a little larger? Or what can I do to get better adjustments out of the mixture screws?

Paul

[Cliff Ruggles]

I'd have to know the engine specs and how the carb was set up to make any tuning recommendations.  It's not a big deal if the mixtures screws woln't kill the engine when you turn them in, but they should slow it down some, showing you have some control with them.  Might want to take a gander into the primaries at idle speed to see if there is any fuel coming from the boosters (nozzle drip).  This would indicate not enough idle fuel and/or idle bypass air.....Cliff

[Paul]

Cliff,
 The engine is a 74 Pontiac 455, bored 30 over, forged rods forged pistons, 9.5:1 compression with the Comps XE284 cam, 1970 factory iron intake, #13 heads, TH-400 tranny, A Continental converter with 2800 stall (purchased from You a couple years ago ) its in a 77 Pontiac T/A I rebuilt the carb using Your book, the lower side of recipe #3, Your primary rods with 73 jets, I sent the base plate to You for throttle shaft bushings and idle bypass holes installed, I don't recall what size the idle bypass holes are, the engine idles fine and accelerates hard, its just the slight hesitation on initial acceleration from a stop, there is no nozzle drip at idle. Since the car is sitting through the winter I figure its a good time to make adjustments, and I do plan on driving it, weather permitting, through winter. Thanks,

Paul

[Cliff Ruggles]

For that cam in a 455 with 9.5 to 1 compression, you will need to be on the upper end of recipe #3.  What size idle tubes, idle DCR's, and bypass air are you using now.  What metering rods are teamed up with the 73 main jets?  Which power piston spring?....Cliff

[Paul]

Cliff,
 The idle tubes are .38, the idle down channel is .060, the idle DCR's are .093, ( if You are referring to the idle screw holes in the base of the carb ) idle bypass is .90, upper idle air bleed (main body) is.070, lower idle air bleed .070, accelerator discharge holes is .028, main air bleed (main body) .070, primary jets 73, primary metering rods are Your rods #44, fuel inlet seat is .135 (yours) im using the orange power piston spring ( lightest one ) I purchased one of Your rebuild kits when I did the carb. Thank You,

Paul

[Cliff Ruggles]

I'd try to tune out the slight off idle stumble with the APT, if you have plenty of idle fuel and full control with the mixture screws.....Cliff

[Paul]

Ok thank You. With the above mentioned specs, should I still tune the carb to the high side of recipe 3?

Paul

[Cliff Ruggles]

In any and all cases you have to tune the carb for the engine.  The XE284 cam acts pretty big in a 455 engine at 9.5 to 1 compression.   Due to the very fast opening/closing ramps and tight LSA, they also induce a "quirky" idle for lack of better terminology.  I've tuned at least half a dozen 455's here using that cam, and I do NOT like it.  The cam is big enough for the XE line to make decent power, but why should one have to deal with crappy idle quality.  Those cams actually set the valves down so fast they will bounce on the seats, which is why (IMHO) they idle poorly.

In any case, the exact position of the camshaft (ICL), and ignition timing at idle play a huge role here.  In any and all cases, you have to continue to add idle fuel until the engine gets what it wants.  This may include idle bypass air as well to prevent nozzle drip.

The specs that you provided are close, might want to try .039 or even .040" idle tubes, and .110" idle bypass air to see if it helps out some.  DCR's at .060" should be enough......Cliff

[Paul]

ok thank You! im going to order another set of idle tubes, the last time I worked on the carb, I destroyed the idle tubes while trying to get them out. If they survive, well I will have an extra set   
 Paul