# New Carb installed now we have idle issues, need advice.



## Gunslinger (Jan 5, 2010)

Hey Guys, I don't often tap into the pool of vast auto knowledge that is the GTO forum, but today Im a bit stumped. I have a SBC 400, mild cam, 2.02 valve heads, perfomer intake, headers, MSD 6a, etc. I just swapped the old 600 cfm Holley for a 750 cfm 0-03310c (4160) vac seconds/man choker. At first everything was fine, after I fixed the mess the previous owner had made of the vacuum lines, and it was running smooth and very responsive. Now, the next day the idle is surging, or rather bogging in a very rythmic way, and it wants to stall when in gear (th350). I'm thinking vacuum leak or something like that. When you tap the gas with it in park it now acts like it wants to hesitate and possibly cough back through the carb. power brakes are connected to the port at rear of carb, pcv is connected to maniold which is shared with the trannies vacuum line and the Vacuum advance for the dizzy is in its proper passenger side port. I just put a new grommet and pcv valve in and that didnt help. Fuel pressure is good( between 5-9 psi needles a little shakey). Off idle it seems ok, but I swear it ran better last night before the surging started. The surges go from say 900-1200 at idle in park and dont get better until it comes off idle even if you turn the idle screww in to speed up idle. Any ideas? Happy Holidays


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## geeteeohguy (Feb 2, 2008)

Drift a little carb cleaner into the carb at idle and see if it smooths out. Or hold your hand over the carb throat, with some gaps between your fingers. My bet is lean mixture. Surging/bucking/backfire/hesitation is usually too lean. If you have black smoke and fumes at the tailpipe, it's too rich. My bet is on a vacuum leak---at the base gasket, in the carb, or elsewhere. What about the mixture screws? Did you try to adjust them? Holley mixture screws as you know are back-asswards from other carbs: clockwise to richen and counter clockwise to lean out.


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## Gunslinger (Jan 5, 2010)

I didnt start fiddling with the mixture screws, only the idle screw and everything was running sweet right out of the box. i'm thinking your on to something and the carb might be sucking air at the base, I will recheck the carb/manifold connection and see, that would make sense and cause the idle surging that seems to be getting worse and worse.


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## Gunslinger (Jan 5, 2010)

Carbs on tight, gaskets good, hoses are all connected and seem fine. I took off all the vacuum hoses one by one and pluged them with my thumb, none of them being plugged really had an effect on the idle surging nor did unhooking them seem to do much. maybe tomorrow I will take the carb off and make sure none of the carb vacuum ports sucked any debris into them during the install.


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## 68greengoat (Sep 15, 2005)

Merry Christmas everyone..............:seeya:


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## dimitri (Feb 13, 2009)

With the carb off, I would take a staight edge to the bottom of the carb and see if you can put a feeler gauge under it in various places. Do the same to the intake. If everything looks good get a new good quality base gaset, like Felpro, and toque carb down to 120 in/lbs. Plug and leave plugged every vacuum souce on your carb. Lightly Bottom both mixture screws and go 1 1/2 turns out on both, this is a good starting point. Put your distributer vacuum directly to a manifold vacuum source. Hook up a tach in the engine compartment somewhere you can see it and if you have a vacuum gauge hook that up too. Prime your carb and start the engine. Run your engine until thermostat opens, top hose gets real hot and choke is fully open. With your hand push thottle a couple of times and then let is settle. While you make these adjments do not touch the throttle again. Set idle to 650 - 700 rpm. Let it settle for a minute. Turn both mixture screws in a 1/2 a turn while watching your rpm's. Let it settle for a minute. If rpm's went up, lower it back to 650 and do it again until rpm's stop going up. Make sure to wait a minute after each adj. When you find the point where the rpms no longer increase, back both screws out a 1/4 turn. If when you first turned the screws in idle decreases or engine dies, turn them out 1/2 turn. Keep doing that until you find the rpm's not going up when you turn the screws. Never go more than 2 1/2 turns out. Hook everything back up including air filter. Take it for a drive. When you get back hook up your tach again have someone start the car, put it in Reverse and put the idle to 650 - 700 and try adjusting idle mixtures again while in gear and air cleaner on.


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## Eric Animal (Oct 28, 2007)

Check the adjustments on the secondaries...if they are not shut properly at idle they could also cause a surge...would going up 150cfm in size cause this??.....Also check float levels.


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## xconcepts (Jan 4, 2009)

I'm working on this issue with my tripower I just had rebuilt. I can't even get an idle below 1k without stalling.


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## Eric Animal (Oct 28, 2007)

Trips can be a PITA....check for vacuum leaks and play/leaks around throttle shafts.


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## Gunslinger (Jan 5, 2010)

Running sweet now, thanks guys. Had to do basically everything you recommended and replaced the entire vacuum system (hoses, pcv, etc.) Whoever put this thing together half-assed the all the hoses, fuel lines, vacuum lines, and most of the electrical wiring. Spent most of my New Years time off work redoing and fixing all the bonehead rigging the previous owner had done. I feel kinda bad because the guy sold it to me thinking the brake system was shot, the electrical was shot, and the motor barely ran. Two days of being all up in it and it runs perfect, brakes are actually nearly perfect(after I already bought a master and booster yet to be installed), and all the light and gauges work perfect.


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## Eric Animal (Oct 28, 2007)

Excellent.......any luck with the trips X?


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## xcmac26 (Dec 1, 2008)

resuscitating this thread. having 4160 issues myself. here's the scoop.

Fuel booster pump wasn't working, ruptured diaphragm. So I go about my business ordering a rebuild kit, getting everything put together and...voila, shiny clean carburator. Bolt everything up, lines back in place, fuel flowing through nice and easy. However...can't idle the thing. Backfires at startup unless I hold the throttle wide open. exhaust stinks like gas. I know I'm running rich, but even with the idle screws a half turn out, it's still not running properly. any hints, tips or tricks beyond what the internet has available will be most helpful. Hell, I'll even take suggestions on a shop to send it to in the Houston area so I can get this thing working right again.


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## chuckha62 (Apr 5, 2010)

Sounds like the float adjustment s off. It could be any number of things, but I'd start there. If they're set correctly, then possible needle and seat issue, or fuel pressure.


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