327/340 Off Idle Problem - I think

[Timothy B.]

Re: 327/340 Off Idle Problem - I think

Les,

A long time ago I had the same problem on a 1974 Nova and the hesitation was from idle to off idle. Turned out the problem was a vacuum leak at the rubber hose connection to the vacuum can on the distributor (egg shaped hose end). It's funny how that little vacuum leak made that car hesitate and it took me forever to figure it out. There is also vacuum at the choke piston for the auto choke, your leak could be there as a small and constant leak and therefore making the car act as Duke says.

I think another good suggestion is the accelerator pump as mentioned in a earlier post. Make sure you check the linkage to be sure your getting a good shot early and every time because if your not there is a small ball valve that may not be allowing fuel into the pump cavity.

[Les Jacobs]

Re: 327/340 Off Idle Problem - I think

Tim Thanks for the suggestions. I'll double check the vacuum lines and the can. (all new). The choke assy has been reworked and tested. (I'll probably pull the carb again, so I'll check it again). FYI The distributor advance is close to 24 degrees. I checked out the assy per your suggeestion, and measured the actual angular advance mechanism (and also ensured it wasn't sticking somehow-it wasn't.) I tried to measure TDC, but gave up after trying to gerryrig something. So I'll go back when I get the right tool. I think its ok though, since the car sarts very easily, and in general accelerates fine. (I previously ensured the accelerator pump was ok). The roughness only apperas when cruising at low speed (constant throttle), or when decelerating. If I take my foot completely off the accelrator @ 2500, it will start jerking @ 1500-1700 and get worse as the car slows down. Les

[Duke W.]

Re: 327/340 Off Idle Problem - I think

Sounds like a correct carb gasket. What you descibe is a "starting stumble",which is what I experienced with the heat riser passage blocked. Since I'm pulling a 3.08 axle, my car is somewhat tough to get launched under the best of conditions, but with the heat riser blocked it was terrible.

Something that gets overlooked on AFBs is the choke housing O-ring - a very small O-ring that seals the choke housing air passage to the main carb. body. It should be included in zip kits. An old or no O-ring can cause a vacuum leak.

Most manual trans cars exhibit a little jerking at zero throttle and low revs, but mine was within the range that I considered normal.

Something else that might be the culprit is the hot idle compensator. It's the screwed in metal tab between the tops of the the secondary throttle bore openings at the top of the main body. It's suppose to open at high temp to admit additional air to reduce an overly rich mixture when things get very hot and create some percolation. I think the bimetallic spring goes out of calibration with cycling. When cold (room temp) it should be closed and open when it gets hot, but I've never seen a spec for the opening temperature.

My suggestion is to remove the hot idle compensator and make sure it's closed at room temperature. Then put it in a pan of water on the stove, and slowly heat up the water while watching to see when it opens and how far it opens when the water is boiling. Also check the hot idle compensator air passage in the main body to be sure it's open.

Duke

[Les Jacobs]

Re: 327/340 Off Idle Problem - I think

Duke Its not the choke housing "O" ring. But I have some new data points. With the car hot, I disconnected rubber choke hose at the carb, and put my finger over the end. Two things: the engine slowed and there was noticeable air flow into the hose. (I was expecting minimal flow, since the choke piston "should' be restricting the air flow-or at least so I thought. So I blocked the hose, readjusted the idle screws and took her out. Mixed results: the off the line stumble was greatly improved-in fact almost gone, but the off idle cruise roughness remained. After that I also disconnected the PCV hose, and blocked the PCV valve. Took her for a spin... no noticeable improvement. The propane test found nothing. Looks like part of the problem is the lean carb..agree? Les

[Duke W.]

Re: 327/340 Off Idle Problem - I think

I believe there should be slight air flow through the choke clean air system at all times. Maybe you have too much. Plug it and look for leaks.

The exhaust manifold internal tube rusts out, and this will usually gum up the choke system fairly fast. It could also overheat the carb and cause percolation. The choke clean air system should be well sealed from the air horm inlet hose - on. Check that all the lines will hold pressure or vacuum except for very slight leakage between the chrome outlet tube and manifold tube.

If you pull the hose off the inlet end of the 594C PCV valve and block flow, what happens to the idle speed? It should drop noticeably.

Your symptoms sound like a lean off idle condition, but at this point I wouldn't rule anyting out.

Duke

[Les Jacobs]

Re: 327/340 Off Idle Problem - I think

I had to replace the choke assy, and Im thinking there is too much clearance in the choke piston cyclinder. Either that or the carb passageway has been bored out. (Choke was a problem when I got the car). Internal tube is good, as are the hose and choke lines. When the PCV line was disconnected and the PCV blocked, the idle speed did drop. What I don't get is, if the problem is a lean condition, why isn't it more noticeable while the car is cold? When I took it out today in 50 degree weather, it ran great until after about 20 minutes. Then the off idle problem reappeared. FYI, I checked the hot air compensator (while the car was hot), and it seemed to be closed. (I opened it, and the engine speed increased-although it ran roughly. When I released it, the speed returned to normal. I put a scope on the low voltage side of the ignition coil(solid state), and it looked very stable. I had to inductively couple to the high voltage side, but there was nothing remarkable there except the peaks of the voltage spike seemed to be fluctuating. Im still stumped, but another carb rebuild kit is on the way.

[Duke W.]

Re: 327/340 Off Idle Problem - I think

Whatever it is, it appears to be themally related. Something is causing a vacuum leak once the engine is fully warmed up(?).

Old coils are known to get flaky when they are hot. Usually this will create a misfire at any engine speed or load, but if you have another 12V coil you can swap in, it might be a good idea.

Like I said, at this point it could be anything.

Once found we will probably scratch our heads and say: Now why didn't we think of that up front?

Duke

[Timothy B.]

Re: 327/340 Off Idle Problem - I think

Les,

Check to make sure the bottom of the carburetor is flat and does not have any pits. The drilled hole for the calibrated vacuum draw at the choke piston looks like it's maybe 1/8" in diameter. I think it is supposed to draw vacuum even when the choke is open, but a small vacuum leak in this area may cause the engine to pull air not fuel as the transfer slot is very close. If you take the carbutetor apart again I would make sure to soak the primary verturis in carburetor cleaner and blow out the air bleeds very well.

If you have the NCRS restorer CD there is a article on the AFB and it explains the different systems of the carburetor. After reading it about five times I have a much better understanding of the operation of the Carter. There are also lead plugs in the bottom of the carburetor that are next to where the heat groove would be on lower HP cars and I wonder if they have been problematic with other owners. I have heard J B weld seals them very well.

[Les Jacobs]

Re: 327/340 Off Idle Problem - I think

Hi Tim Thanks for the info. I don't have the particular Restorer you mentioned. Do you have a reference? I do have a copy of Emanuel's "Carter Carburetor...", and "thought" I understood the different systems fairly well. Obviously not well enough. After putting on a new coil yesterday, I observed significant improvement in the launch (as Duke would say). It was smooth, and peppy, although the off idle jerking at cruise was still noticeable. But I thought I was making progress. Today, I took it out, and it was terrible from the start. No launch, and plenty of stumble, even while cold (normally it is better before it gets hot). So Im thinking the car doesn't need a mechanic (I flatter myself), it needs an exorcist. Looking on the bright side, maybe whatever is causing the problem has worked itself into a position where it can be found. I'll probably be taking the carb apart later in the week. I'll let you know what I've found. Thanks again

[Les Jacobs]

Re: 327/340 Off Idle Problem - I think

Tim Well I thought you nailed it. Took the carb apart(again) , and found a partial restriction in each of the venturi idle jets. Cleaned it out, blew out the jets and air bleeds and reassembled. (also checked base for flatness-its ok), choke circuit is ok too. Reassembled, and SOS. I noticed that there are 4 air bleeds in the venturi, and one of them (main bleed for the venturi I think) is quite large. It would seem like these could lean the idle and off/idle mixture. (Now Im paranoid and thinking the previous owner drilled out all the passages) Not sure the best way to tell if its a carb problem or a vacuum leak (say a crack in the underside of the manifold), or even an ignition problem. (New coil, checked the timing map, but it does have a Mallory breakerless ignition. Think I should swap it out with points?) Still stumped. Any suggestions? Thanks Les

[Wayne P.]

Re: 327/340 Off Idle Problem - I think

Considering everything else you've done, I would try swapping back to points or borrow someone's distribtor to try. I, like Duke, have not had good results with the electronic conversions.

[Joe R.]

Re: 327/340 Off Idle Problem - I think

Les,

I believe you have been afflicted with the virus. "Too many Chefs Spoil The Broth".

There is nothing wrong with your car other than the carb is 40 years old and needs an expert carb man to help you out.

Ask around and find one in your area that is 60 plus years old and he's seen this problem on hundreds of AFB's. Sorry, but my carb man in Memphis has retired because of failing eyesight.

All the help is great to a point but when it comes to fixing a problem you need a man with knowledge on site.

Regards,

JR