[marty matthews]

I need some help with an idle problem on my 65 GTO. It is a tri-power car. When I first start the car, it starts fine, and the choke keeps it idled up until it warms up. It then runs great and idles fine for about the first 20 minutes or so. Then, when I pull up to a stop sign or light, the engine idle drops, and it wants to die. If I keep my foot on the gas just a little, it idles fine. It almost seems like I have a vaccum leak? I have adjusted the idle on the carb, and it has fairly new points and plugs. The car runs great otherwise. This problem just started this year, and I haven't changed anything recently on the car. Any thoughts on what could be making the idle drop after about 20 minutes of driving?

[pngoat]

You could have a leaking needle & seat in one of the carbs. Also check the throttle return springs, on the front & rear carbs, to see if they are broken or binding thus not allowing the throttle blades to fully close causing a vacuum leak. Also check the linkage for binding. If all of the above are good remove the front & rear carbs. Remove the throttle bodys from them. Hold them up to direct sun light. You shouldn't see any light around the throttle blades. If you see any light - even just a little bit - that will be enough of a vacuum leak to upset the idle. Good luck. Post back to let us know what you found.

[marty matthews]

Thanks, that is what I am thinking- some sort of vaccum leak. It has power brakes as well, so I may check those fittings. Seems like I should be able to hook up a vaccum gauge to it, and that might give me some indication of what is going on.

[Old Man Taylor]

The easiest thing to start with is to block the air entry of the end carburetors. Have someone else keep the engine running, and then cover each of the end carbs so no air can enter. If this changes the idle characteristics significantly then your butterflies are not seated as well as they need to be. If this has no effect, the proceed with the recommendatoins above. I used to have that problem on one of my cars and it turned out that the rear float had problems. I had to assure it had no restrictions, and I set it a little lower than spec. However, I did see external signs of fuel on that carb.

[geeteeohguy]

It sounds too lean. It runs fine with cold enrichment(choke), but when the choke comes off, it stumbles. Try this: with the engine running and warmed up, take off the center carb's air filter and drizzle a little carb cleaner into the carb, or slowly hold your hand over the throat of the carb. If the idle speeds up, it's too lean. You may be able fine tune it by turning the air fuel mixture screws counterclockwise a bit at a time until it smoothes out. You may indeed have an air leak. The outer carbs on these set-ups are pretty bomb proof, in my experience. Check the center carb and mixture settings, and for vacuum leaks (hoses, etc.) first. JMHO.......

[carbking]

Several things come to mind:

(1) I would agree with Jim (Old Man Taylor) about the seal of the throttles on the end carbs. Check this first.

(2) It could also be ethanol fuel, which is becoming more and more prevalent. We suggest opening the idle tubes by 0.002 inch over stock to compensate for the lower caloric content of the ethanol.

(3) Take a close look at the center carburetor. You did not specify in your initial post much about your car. Automatic? 4 Speed? Original carbs? Non-original carbs??? Many enthusiasts seem to feel that any two-barrel will work, or non-original carbs with repro throttle bases will work (and sometimes they will); but GM did NOT make all of those different parts just to reduce the stockholders profits.

(3A) A four speed car should NOT have a center carb with a hot idle compensator. It can cause a poor idle hot.

(3B) An automatic car SHOULD have a hot idle compensator. Lack of the compensator can cause a poor idle hot.

(3C) Take a look at this picture:

http://www.thecarburetorshop.com/Hotidle2.jpg

ALL 1965 center carburetors should have a throttle body gasket similar (not quite exact) to the one on the upper right with the cutouts. USE OF THE GASKET ON THE LOWER RIGHT CAN CAUSE HOT IDLE ISSUES! Many think the upper right gasket will cause a vacuum leak. This thought is totally erroneous, as the cutouts are above the throttle plates. The cutout area is necessary to bleed excessive fuel pressure when hot at idle from the venturi/throttle area on some carburetors, and the 1965 tripower center is one. The end carbs (at least in 1965) should use the sealing gaskets similar to the one on the lower right.

Just a few things to check.

Jon.

[marty matthews]

Thanks for all of the replies. Yes, the car is a 4spd, original carbs, etc. As I mentioned, this problem just started. The car was running great and it idled fine. I did not change anything on the carbs, or rebuild them, or do anything other than drive the car. Based on the above replies, I am almost convinced that it must be some sort of vaccum leak. I will try some of the above when I get a chance to work on the car. I appreciate all of the assistance.

[veet66]

You didn't say if you have A/C. If so cap the vac line at the TEE leaving the back of the center carb. I have had the vac. lines under the dash for the A/C to crack and leak.

I just had a similar problem with my tri power. It ran fine until after I had driven it a while and then I stopped at a light and the car would idle down to a point where it wanted to cut off. It took me awhile to track the problem down but it turned out the the front butterflies did not close tight and was allowing extra air into the manifold.

The reason it was hard to find was that the only time it would stick was when I put the peddle down and caused the front and rear carb to kicked in. While parked I had to rev up the engine to the point that the front and back carbs. kicked in and noticed when I let the throttle go back to normal is how I found that the throttle body butterflies were not all the way closed. Seemed like once the carbs got hot there was some seizing in the shaft of the trottle body.

[61-63]

Check your intake manifold over carefully; it may have developed a hairline crack that opens up as the engine and manifold heat up, creating a more severe leak up to a point, the longer the engine runs.

[marty matthews]

That is what I am now suspecting- some sort of vacuum leak. Car does not have ac, but has power brakes. I am thinking that the line going to the power brake booster might be leaking, or it could be the outer carbs, as others have suggested. I will also check around the intake manifold. The car runs fine otherwise, so I don't think it is anything too major. I will let you know what I find. Thanks again for all of the good suggestions.

[Pecosbill]

Also, check the heat riser on the exhaust manifold. I had a similar problem with one of my cars and found that the heat riser had frozen in the closed position. As long as the car was running on the fast idle cam it was okay. But as soon as the rpm's came down when the car warmed up it wanted to stumble and ran very rough. I freed up the heat riser and it is running as it should again.

[pontiaclady]

Marty - did you ever get your problem resolved? I have exact same problem with my 66 GP Tripower. See thread http://forums.maxperformanceinc.com/...d.php?t=639843 for all the suggestions. It is running better now, but still stalling at lights. I did have a hairline crack in the manifold under the center carb that widened after warm. Kicked fuel mix up, but it smells too rich. Did the spacers help, that might be my issue also. Mine started the same way - ran fine, then one day started stalling and got worse and worse.

[GOOD39]

Marty,
This may sound crazy but I had a similar idle problem. I was told to check the carbs, the intake for a leak. The problem ended up being a new PCV valve that had a weak plunger that did not close allowing the leak that would stall the engine upon idle. It's a long shot but an easy process to check. What I was told to do was to unplug the PCV hose and tape the hose end. When I did that, it allowed the engine to idle fine. I replaced the PCV and the car now idles fine. Like I said it's a long shot but an easy one to try.

[marty matthews]

Thanks for the follow up replies. No, I have not yet figured out what is causing it to want to die after it warms up. I have not had a chance yet to block off the end carbs, but I should have some time to do that in the next month or so. I will let you know when I get it figured out. I am sure it is something simple, as the car runs super as long as you have your foot on the gas pedal. When I put the vaccum gauge on it, it reads steady right at 17 without a bit of flutter, and when I rev it up, it drops to about 5, and then quickly rebounds to the 17-18 range. All of the manuals that I have consulted say that those are normal readings. I will keep looking and let you know what I find.

[Terry M. Hunt]

Just tracked down an idle issue on my '65. Discovered a leak in the "F" fitting that connects the PCV hose to the intake. Evidently, somebody had a problem removing one of the short hose sections from this plastic/nylon fitting in the past. They must have used a knife to split the hose to get it off, but wound up slicing into the fitting itself. I had to clean it thoroughly and look at it with a magnifying glass to see the slice in the plastic. Replaced it with a beat up ugly spare I had until I can get a new one, but the idle issue was solved. There are a number of places a vacuum leak can appear on this PCV hose and its fittings, so check each connection. That is what I was doing when I stumbled on the fitting problem.

[marty matthews]

Thanks Terry, I will check that. I think mine has a metal F pipe connecting the PCV hose. I am also thinking that it might be something with the power brake vacuum connection. I am sure it must be something simple, because of the vacuum gauge readings, and the fact that the car runs great otherwise. Thanks for all of the help.

[Terry M. Hunt]

Marty, I think the metal F fitting is the way to go. I'm ordering a couple of those next week.

[Terry M. Hunt]

Got the metal F fittings today. Seem to be pretty well made. Stronger than the plastic, it appears. They are painted silver, which I glass beaded off and painted Navaho White to mimic the plastic piece. Think they are about $15 each.