1968 Pontiac Bonneville Stalls on idle

LEEBERT

MEMBER
1968 PONTIAC BONNEVILLE
V8
2WD
AUTOMATIC
120,000 MILES

I'm just about at wit's end.

The motor stalls on idle. It doesn't seem to matter where I tune the idle mixture screws on the carb base, still stalls. After revving it in the driveway and the motor decreases to idle it wants to stall unless I partially close the choke (increasing vacuum pressure?). It gets worse after the motors warm, even worse when its hot. I can pull out of the driveway OK, but once out on the street, even coasting in neutral, it stalls.

I've done quite a bit so far: I've partially cleaned & rebuilt the 2-barrel vintage Rochester carburetor (set the float twice), rebuilt the ignition (still waiting on the correct contact points assembly) & checked all the vacuum lines. There's no EGR valve (1968 Catalina actually, but very much like the more-optioned '68 Ventura, Bonneville or Executive).

I say "partially" on the carb rebuild: When I rebuilt the carb I just cleaned out the circuits using a straw nozzle from a spray can with some help from a nylon pipe cleaner. I didn't soak the carb however.

I wish I could just adjust the maximum choke intake opening, that'd keep the vacuum pressure up during idle. Should I go ahead & pull the carb again & really soak the base in a bucket of carb cleaner?

What else should I be looking for? I really appreciate any advice you have to offer. This is a beautiful car but this mechanical problem is becoming an obstacle in getting on with the rest of the restoration work.

Thanks in advance,
/lee

4 Replies

KEN L

MASTER CERTIFIED MECHANIC
43,629 POSTS

If the carb idle circuits are plugged it will never idle.

Double check for vacuum leaks under the carb, make sure the carb base gasket is the right one and seals correctly. Also if you have power brakes plug the main vacuum feed line and check the idle condition. Please get back to me.

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Tuesday, April 22nd, 2008 AT 1:07 PM

LEEBERT

MEMBER
3 POSTS

Hi Ken,

I think the bottom carb gasket is good, but how would I check it for seal? It look like a good mate & I used gasket-stuff to help it. If I spray carb cleaner on it while running hot & it speeds up I've got a leak then, eh?

Hmm. Options.

I tried a rebuild but its filter valve overflowed the float bowl & its filter thread were shot & I've been told 2:3 rebuilds are dogs & man they cost a pretty penny for the experience.

Any thoughts what my chances are w/ the carb exchange? A shop down near Houston says he'll rebuild mine & motor-test it live for $150. He says he can get the base idle circuit channels cleaned out right.

Another thing to note is that the transition between idle & high speed is sudden, not smooth. More of the same I suppose.

Oh, and FWIW, I looked at the main jets & didn't think they needed replacing, they looked just a shade lacquered. My kit didn't have any new ones.

I will check idle with the power brake booster line disconnected & the port plugged and get back to you.

If my vacuum lines are a bit too long, will that make things worse? I've didn't trim back the extra hose, thinking it'd help to keep stuff off the hot motor.

I'm a total newbie at this, but still having fun (short a few gray hairs).

Thanks again,

/lee

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Tuesday, April 22nd, 2008 AT 1:47 PM

KEN L

MASTER CERTIFIED MECHANIC
43,629 POSTS

Sometimes you can get a bad carb, save the box and receipt and return it if needed. The lenth of the vacuum hose wont matter.

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-1

Wednesday, April 23rd, 2008 AT 12:54 PM

LEEBERT

MEMBER
3 POSTS

Hi Ken,

I put a new carb in, much better on idle, but was backfiring.

I got a timing light, found the timing was wrong, too advanced. The tick mark was a groove in the harmonic balancer (timing marks go from 0 to 12, right to left).

Now I have the timing at zero advance on idle.

I backed off the dwell b/c she wouldn't start at first w/ the new points. New points made her advance quite a bit, turned timing back to zero.

There's a little lag warmed up, but runs well warm.

Out on the highway when I floor it there's a dead spot and doesn't downshift.

Coming off the highway stalls out with a backfire. Ran like it had no power for about a minute, straining. Not sure it downshifted.

Back on the streets, cooling off, runs fair. Surged a couple of times, then ran smoother, kind of OK.

All the vacuum hoses check out. Carb plate OK.

I did replace the negative coil wire with a somewhat heavier gauge wire, 14 ga., the old bad wire looks like it's 16 ga. Mounted the coil on the firewall b/c it would be a horror below the distributor.

What did I do wrong this time?? :-) I'm stumped!

I guess I'm almost ready to take it to a mech. who has worked w/ these old cars, but they're rare around here. Lots of mechs don't have timing lights any more, only do electronic ignition. Yes, I'm thinking of putting in an electronic ig. retrofit, but you know, mental inertia, I'd like to see this work.