# rough idle in gear



## gjones (Oct 8, 2012)

Got the '71 455 in and running.('66 Lemans). Finally got crossmember mounts in and welded. Can finally drive her. The fool thing starts and runs great in park, EXCEPT when you put it in drive, it idles like a POS! Also, when in park, if you punch it to WOT, it'll either pop and stall, OR, it will catch and sound like a damned racecar! I've changed the bloody accelerator pump twice. Shooting a GOOD stream in there. Running a Carter 9635sa competition carb.(625 cfm, I know a little under sized, but PO had a 600 cfm on it and it run great). Rebuilt carb (twice). Running a Mallory Unilite ignition. New wires, etc... Plugs gapped at .045. Even going to half throttle, she burns rubber. Drive it normally, and it's fantastic. if you lift it just above idle, then stomp on it, clear the road-it's gone! Just that #^@>%&# crap idle in gear, and that stinking WOT bs that has me about ripping my hair out! Could it be that stankin' Chinese knock off of a Carter, or is it just me? ( I've had Carters on my '56 New Yorker, '63 Cadillac, etc... 17mpg & 24mpg respectively,and all the power at your disposal-NEVER had any problems like this). 30 over pistons. 4x heads. Edelbrock Performer Pontiac intake.New timing chain & gears. New upper gaskets, etc...rest is stock. Also, going up the road, I had light valve clatter under moderate load.(went away when I disconnected pcv & plugged it). I am about at the end of my rope with this thing. Any assistance rendered would be greatly appreciated. Thanks.


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## gjones (Oct 8, 2012)

*rough idle*

No reason for rough idle when you put it in gear?


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## geeteeohguy (Feb 2, 2008)

First off, re-connect the PCV, unless you want to blow all the gaskets and the rear main seal. Second, you need to do some basic tune-up stuff here. What's the vacuum at idle? Is it steady? Where's the timing at? What's the mixture set to? Vacuum leaks?That kind of thing. And gap the plugs at .035, while you're at it. A rattling PCV is a sign of a vacuum problem, such as a bad intake valve, vacuum leak, etc. It's a symptom, not a cause. If you're not a tune-up guy, take it to a shop to dial it in the rest of the way. It sounds like you're 95% there!!! Good luck.


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## gjones (Oct 8, 2012)

*rough idle*

Thanks for a response geeteeohguy. It's been a long time since I used vacuum gauges. I'll try it out. I actually did reconnect the pcv last night, and it seemed o.k. (haven't taken it back out on the road yet though). One thing I forgot to mention was the air jet (on drivers' side of carb)- when I screw it in slowly, it barely changes rpm- all the way in, doesn't really affect idle at all. (seemed to adjust on 326 o.k.) Isn't that the air jet? I Also thought electronic ignition plugs needed a wider gap? Also, with my carb. rebuild kit, they gave several base gaskets which fit. Used the thick one last year on the 326 (brush tacked it-just to make sure) so, couldn't re-use that one. Running a thin gasket now- seems to be sealing o.k.-or is too thin not good? Also, initially I set the timing @ 12 btdc- ran great, but started hard when hot, so bumped it back to around 8btdc- starts great. Idling around 800 rpm in park w/advance connected. I can knock it back to 600 (or less) runs beautiful-until you put it in gear. Brought it up to 1000- doesn't make much difference in gear. Thanks


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## gjones (Oct 8, 2012)

*rough idle*

P.S. Idle mixture screws opened: 2 1/4 (passenger side) - 2 1/2 (driver's side)


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## jmt455 (Mar 26, 2012)

It might be vacuum advance issue. Do you have the vacuum advance connected to manifold or ported vacuum? If it's currently seeing full manifold vacuum, I'd try it on a ported vacuum source and see if that helps. Also make sure the advance canister isn't leaking.

But it sounds like a vacuum leak to me.

Check vacuum at idle and coming off-idle. At idle, the gauge indicator should be steady as a rock, unless you have a cam with huge overlap.

I'd check everything thoroughly;intake manifold gaskets, all vacuum hoses and fittings, carb base gasket, power brake booster and lines, etc.

Also, if this is a TH400 trans, make sure your vacuum modulator is connected and there are no leaks in the lines or the modulator itself.


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## gjones (Oct 8, 2012)

*rough idle*

One last thing I failed to mention was the way the vacuum setup is configured. On the rear of the carburetor, there are two blocks attached back to back. First one has power brake line attached. Second block has a nipple for vacuum advance AND has the vacuum modulator line connected there. This seemed kind of odd, and I'm not sure if this is correct or not. Thanks for mentioning the modulator. This seemed ok when I drove it, but I thought there was a spot where it malfunction briefly. There's no fluid in the line, but these can leak vacuum internally if they're bad, yes? Thanks again.


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## BearGFR (Aug 25, 2008)

"How much" cam does it have... stock or rowdy?

That the idle mixture screws don't have much effect is a clue. A common cause of this is if it's got a rowdier than stock cam, you've got to set the idle speed screw "up" so much to keep it running that the throttle plates are open far enough to expose the transfer slots in the carb --- it's already beginning to run on the main or "off idle" circuits in the carb thus taking the idle circuits out of the picture, so to speak. Having a carb that's "too small" for the engine can contribute to this as well - i.e. a carb that ran fine on a 326 can't pass enough air to keep a 455 alive on just the idle circuits alone.

I see you've got 4x cast iron heads. Even on a zero-decked .060 over 455 with flat-tops, it would still be at 9.3:1 compression with 98 cc 4x's (unless they've been milled a ton), so I'm thinking that detonation due to too much compression is unlikely. Take that with the other symptoms and my suspicion is that the carb is way too lean and that's what caused it to rattle. If that same carb came off a 326 and hasn't been touched, in fact I'll _guarantee_ it's too lean and you're taking a big risk until that get's corrected. I'd address that first, then make sure the total ignition timing was right (I'd start out at 33-34 degrees and work from there). I'm not that familiar with those carbs so I don't know for sure if the sizing on the main metering circuit also affects the idle circuit. I know it does on QJets, and if it also does on those then that too could be contributing to your idle problem. Sounds to me like your engine is starving for fuel "all over the place" and that will definitely make it idle rough.

Bear


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## gjones (Oct 8, 2012)

*rough idle*

You may be on to something there. To me, it's got that distinctive "lean smell"-(my brother-in-law says it's running rich. True, the smell is quite acrid, like it's rich. And the plugs are dark, like it is running rich). BUT- if it was running lean, then wouldn't the plugs indicate a white stripped look to the electrodes? The PO did have an Edelbrock 600cfm on it. Could that have been built to accommodate the 455? Or is that totally not possible? Also, what would be a good candidate for a carb, that won't break the bank? Thousands have been spent bringing this car back from the grave- almost tapped out, and still haven't put 200 miles on it yet! All I want to do is go out for a good cruise. If I stick a couple of hundred dollar carb on it and it does the same crap, i'm going to wring my hands of the whole affair and call it quits. (I've got hundreds of photos over the past 14 months, documenting what I've done, but now am starting to get fatigued and discouraged). Any suggestions on a carb? TNX.


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## gjones (Oct 8, 2012)

*rough idle*

Yes bear- I believe you're correct that it is running above the idle settings. The fuel jet makes a difference when I adjust it. Not the air jet,though. Messed with all settings today, timing,etc... when I brought idle down to 550-600 in gear, then tried to set it back up, it kind of surged.(advance plugged, 10btdc)-once it hit 650- the rpm's wouldn't go any higher when I set the actual throttle plate idle screw. Pretty odd. I'm almost convinced that the carburetor is at fault. Cam is (according to PO) stock. But who really knows? She likes 10btdc& 800rpm.


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## gjones (Oct 8, 2012)

*rough idle*

P.S. In park, I can bring idle down to 500rpm, and it is still smooth as silk!


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## BearGFR (Aug 25, 2008)

Your byline says 'Texas' - where at? I'm in the Dallas area...

You could fatten up the carb you have just by putting in larger main jets - you shouldn't have to swap carbs. CFM capacity will limit how much total air the engine can breathe, but even if it's pulling a significant vacuum against the carb at WOT it should still be possible to get the mixture right. Surging is another indication of a lean mixture...

"Fuel jet" - "air jet" --- unless that's a carb unlike any I've ever seen, there ain't no such thing. Both the screws on the front of the carb are idle fuel mixture - they control how much fuel the idle passages flow when the idle circuit is in operation. There are two of them because there's one for each of the two front "barrels" - they both control fuel flow and should be set close to the same on both sides. How much air the carb passes at idle is controlled by a combination of the carb's internal passages (some Qjets and others have what is called "idle bypass air") and the idle speed screw that is adjusted to hold the throttle plates very slightly open. Above idle, fuel mixture is controlled by main metering jets inside the carb - they're calibrated orifices inside the carb. You have to take it apart to change them. Once the carb is above idle, the two screws on the front don't affect anything.

Why is it smooth in Park and not smooth in Drive? Simple - in park there's no load on the engine.

Bear


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## gjones (Oct 8, 2012)

*rough idle*

Right! I termed "fuel" jet as the one on the fuel inlet side-the other as "air" side incorrectly (trying to clarify only made matters worse)- When I adjusted the passenger side mixing screw, it changed rpms. Not so on the drivers' side. No change at all. What would this indicate? Also, if I were to change the metering jets(and rods), how would I determine which size is best? I can have this carb pulled apart in a matter of minutes, but surely with my luck, it couldn't be that simple. I'm very comfortable working on a carb, and follow the instructions to the letter, although I feel like I'm in a nail biting suspense movie, with malevolent twists and turns designed to intrigue and perplex the helpless victim. I can't help but feel the durn new metering components would cost as much as a new carb. All this stuff seems to be insanely over inflated, for the cheesy junk they actually are. ANYhoo, we're located in Texas City, just a sight down the road, as the crow flies from y'all.


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## gjones (Oct 8, 2012)

*rough idle*

This just occurred to me-(I'm rustier than an Edsel in a swamp)-that when I had this carb apart, one of the fuel inlet needles seemed to be sticking a bit.(Has 2 floats) As I blew through the fuel inlet and lightly held one float shut, the other needle would sometimes not open unless I blew extra hard, and sometimes I'd have to manually dislodge it. The opposite needle would open and close with ease, never binding at all. This was a brand new kit I put in it-all new needles & seats,etc... Could never find the cause of that. I just pulled the plugs again- 4 looked a little black(not terrible), 4 looked slick as a whistle. Seems like that would explain why one of the idle mixture screws is not functioning. Or is the blow test not valid? Or am I way off base? Thanks again.


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## BearGFR (Aug 25, 2008)

I don't think you're off base ---- I'd find and fix the cause of that sticking problem before moving on to anything else.

Bear


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## geeteeohguy (Feb 2, 2008)

X2: always fix KNOWN problems first. Both idle screws need to be functional!


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## KingJacobo (Jul 6, 2011)

geeteeohguy said:


> X2: always fix KNOWN problems first. Both idle screws need to be functional!


This!! I can't shed any insight on your issue, being a carb newbie myself. I installed a Holley from hell and still have a vacuum leak to plug..fix what you know then deal with the next issue (if it's still there).


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## Fairlane514 (Nov 10, 2011)

Do you know your camshaft specs? You may need a higher stall torque convertor.


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## gjones (Oct 8, 2012)

*rough idle*

I believe the PO said the cam was stock. Not too sure about the converter either. All I know is the PO said he could really "smoke 'em." AND even as bad as she's runnin', I don't have to even push the pedal 1" quick to chirp 'em. I moved the vacuum advance line to the ported fitting and brother, she really LOVES that! I only mashed it to the floor once (very briefly- I don't want to hurt those new tires quite yet-the power is there).The hesitation it had at WOT is completely gone! I tore the carb back down again (#3)& re cleaned. The left side idle mixture screw is finally starting to show signs of life.Still a problem there though. It will affect rpms only at the last half turn of the screw to the closed position, though. I've cleaned those passages till I'm blue in the face-they seem clear. I believe it might be those vacuum blocks at the back of the carb-When I ran the carb this time, she surged about 8 times, then was idling better. I played with the lines at the backside, then it loped and the idle went down to about 600 rpm-(always had a little tick-thought it was lifter noise-but that went away also-sounded better than ever-got in the car and put it in gear and for 9 glorious seconds, it was sweet and smooth in gear. Then it loped. Then it surged. Then it ticked. It's gotta be a vacuum thing. Is there a good sealer for those metal threads? while I'm at it, I'm running 7147 metering rods on this Carter 9635sa. Whats the best way to determine the best sized jets, rods, springs,vacuum pistons, for this 455? THANKS.


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## BearGFR (Aug 25, 2008)

For sealer on threads, I always use teflon tape. Have you fixed that one sticky float problem yet?

As far as tuning, I recommend the tuning paper by Lars. It's for QJet's not Carters, but the same principles apply. In broad terms, you start with the idle mixture, then part throttle/cruise, and finally WOT/max power.

Bear


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## gjones (Oct 8, 2012)

*rough idle*

Yes- I mixed and matched needles & seats- kept new needles, changed needle springs. Neither needle sticks- even after about 100 blow-throughs. Raised the floats a hair- were at 9/64- put 'em at 5/64- didn't look like enough gas was getting into the accelerator pump chamber. I fully intend to speak to Lars- I just need to break down and purchase vac. gauges, dial back timing light, etc... another couple a hundred for a one time use only deal. I realize it's necessary,but this is the last vehicle I'll ever work on.After this one, it's over. ..... but out of the 40 some auto's I've had, I've only experienced this tuning plague 3 times-Twice with this car (326, 455), once w/ an old Chevy in 1990- had a plugged catalytic converter. Course, never had a Pontiac before, either. The thrill is all but gone.


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## jmt455 (Mar 26, 2012)

One more thing to check....

Many carbs don't handle fuel pressures above about 5 psi. Some of the mechanical fuel pumps will put out over 6 psi and that might be enough to overcome the needle seat pressure, dumping too much fuel into the float bowls.

If your fuel pressure is too high, you'll need to either change the pump or add a fuel pressure regulator.


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## gjones (Oct 8, 2012)

*rough idle*

Thanks, jmt455- I'll check that deal, also- new pump, but I pulled it from 326. Don't know if there's any difference to speak of.


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