1967 427 tri power idling issue

[Mark F.]

I have a 1967 427 tri power, all original and in excellent condition. I am having an idling problem. it's starts fine, and if the engine is cold, it idles high (1,500 rpm) on the choke as it should. After about 3-5 min as it warms up the idle starts coming down as the choke starts to open up, will hang at 900 or so, for maybe 10 seconds but then continues a slow downward rpm trend and stalls out. I have disconnected both front and rear carb linkages and the results are the same. I have also checked for vacuum leaks around all hoses, and at the base of the carbs while running, and nothing. Right now my focus has only been on the carbs. I should mention, that it runs fine when I drive it except for the idling issue.

I have looked at all the posts here that have to do with 427 tri power idle issues but none are relative to what I am experiencing.

Any comments would be appreciated.

[Timothy B.]

Re: 1967 427 tri power idling issue

Does the engine seem to get to rich to run or does it seem to run out of fuel?

Try to manually close the choke blade and see it it stays running at idle.

When the choke is open and the engine is warm do the side emulsion screws have any effect on the idle speed. If so, try adjusting them after warm up with the choke full open.

[Duke W.]

Re: 1967 427 tri power idling issue

A common cause of idle stability that can lead to the engine stalling is an incorrect or malfunctioning vacuum advance, like not passing the Two-Inch Rule.

But in order to proceed with testing and diagnosis you need to provide more information.

Is it a 400 or 435 HP 427?

Does it have an OE equivalent camshaft?

If a 435 HP has the vacuum advance been modified to full time from the OE ported.

What is the normal hot idle speed and what is the inlet manifold vacuum at that idle speed?

What is the ID number on the VAC bracket and what are the start and stop points in inches Hg?

Duke

[Danny P.]

Re: 1967 427 tri power idling issue

I have a 1967 427 tri power, all original and in excellent condition. I am having an idling problem. it's starts fine, and if the engine is cold, it idles high (1,500 rpm) on the choke as it should. After about 3-5 min as it warms up the idle starts coming down as the choke starts to open up, will hang at 900 or so, for maybe 10 seconds but then continues a slow downward rpm trend and stalls out. I have disconnected both front and rear carb linkages and the results are the same. I have also checked for vacuum leaks around all hoses, and at the base of the carbs while running, and nothing. Right now my focus has only been on the carbs. I should mention, that it runs fine when I drive it except for the idling issue.

I have looked at all the posts here that have to do with 427 tri power idle issues but none are relative to what I am experiencing.

Any comments would be appreciated.

Mark, check for fuel pressure first, if that ok pull the needle seat check valve out make sure there no dirt or just replace it , then pull side plug and then set float level until fuel just comes out when rocking the car, or may need a center carb. rebuild hard to tell.

[Mark F.]

Re: 1967 427 tri power idling issue

Thanks for the replies guys. Here is what I have done so far, all to no effect. I just got the car. All stock, multiple Regional Top Flights, One National Top Flight, Duntov Mark of Excellence Award, 2 Bloomington Golds (One Triple Crown), Chevyfest Gold Spinner, Chevyfest Best of Show Restored, etc... So extremely original car with no mods. So here is what I've done: Started with fresh fuel-110 racing fuel. Checked all fuel filters. Removed center carb, took it apart, clean out all ports with carb cleaner then air. Replaced needle and seat, new accelerator pump diagram. Checked for vacuum leaks while running could not detect any. Fuel flow from the fuel pump appears to be fine. The car engine starts fine, and runs fine when driving it. It just wont idle. It will idle when cold and I first start it. It will idle around 1,800 RPM then slowly come down as the choke opens. From time to time it will hang in the 1,100 range for a minute or so, but once it starts going down there's no stopping it from stalling other than keeping it running by opening the throttle a little. Everything I looked at thus far has been fuel related. I am good with cars, but I am not familar with the issues common to these tri power 427's. Maybe it's vacuum related??

[Danny P.]

Re: 1967 427 tri power idling issue

Mark, remove front and rear linkage and make sure both throttle plates are closed are not sticking open , still think its your metering plate is warp and may have the wrong metering plate gasket, try backing off idle screw off and check for sticky center carb. throttle plate by hand, or try another center carb. your sure the center base gasket is good.

[Mark F.]

Re: 1967 427 tri power idling issue

Ok. I finally found out what the problem is. I noticed a small leaking of coolant in the intake manifold bolt galleys 1 on each side of the engine. When i looked closer I noticed it was the manifold gasket leaking. This was the first time I have seen this. So I checked for a vacuum leak while it was running where the coolant leaks where and one side had a significant vacuum leak and the other side had a minor one.

Here's my question, where is the best place to get an intake manifold gasket kit for this? Is it possible to still get them at a regular parts store or should I go online? New to the car and building sources so any help would be greatly appreciated. Also, if any one had the GM part number for this gasket kit that would be great.

[Danny P.]

Re: 1967 427 tri power idling issue

Mark , try Central Corvette part # 352064 is the proper gasket set for judging