1971 LT-1 Idle Problems

[Les G.]

1971 LT-1 car will not idle below 1100 rpm most of the time about 1500 rpm when it idles down eventually after the car is driven for 20-30 minutes it will go down to 700rpm and stall out. Just had the Holley carb rebuilt. Checked the vacuum lines they all look good. Could it be the TI and is there a way to test it. What about the TCS where is that valve located and how can you test that for vacuum leaks? There is a vacuum valve inside the passenger front fender under the windshield washer container what does that valve do? The headlight and wiper vacuum works good. The car also hesitates a little during acceleration like a flat spot.

[Domenic T.]

Re: 1971 LT-1 Idle Problems

Les,
I tried to respond but the system went down.

I am not a holly gut but it sounds like you are on the fast idle cam when you start it and then as it warms up the choke opens and you drop to the idle cam which isn't adjusted.

I do a lot with Q-jets and have to make those adjustments.

A possible vacume leak would seem to (in most cases) keep the same RPM.

The drop makes it sound like your high idle assosiated with the choke.

DOM

[Robert S.]

Re: 1971 LT-1 Idle Problems

To start, you probably need to check all of the basics tune up specs on the engine if you have not already done so. Assuming the engine is unmodified, check the valve lash and ignition timing and set to factory specs if required. Also, make sure that the centrifugal advance is working properly and not sticking. It sounds like you may have already looked for vacuum leaks, but did you check the vacuum advance unit on the distributor?

In 1971 TCS was not used. The system is called CES, which is functionally the same as TCS, but uses a combined idle solenoid and vacuum switch unit which is mounted on the left front of the carb. Over the years these generally get disgarded by previous owners, so it may be missing. If that is the case, then there is probably a direct vacuum line between the carb and the distributor. With or without the CES system in place, the engine should still idle below 1100 rpm.

Did you check the idle screws on the side of the carb (one on each side of the primary metering block)? These need to be adjusted to provide the best idle speed. If you search the TDB you can find several posts on how to do this. On my 71 LT-1, the screws are about 1 counter clockwise turn out from being seated. The engine will not idle very well or not at all if these are turned too far in.

I would not do anything with the TI system at this time until you have verified the basic timing and carb setup.

Note that the mechanical tach in the instrument panel may not be accurate. It's best to read the idle speed with another instrument. For example, my dial back timing light will provide the engine rpm.

The vacuum unit on the right inner fender you are referring to is the relay for the wiper door actuator.

[William P.]

Re: 1971 LT-1 Idle Problems

Hi Les, you can rule out vaccume leaks in the head light doors, wiper door and the brake booster by removing and plugging the fitting on the intake behind the carb . This will not affect engine idol .
I had a lot of leaks in my 71 LT1 and ended up replacing all 3 relays and found a serious leak between the throttle plate and the carb body . I used a piece of glass and sand paper to do the job .
If you find that by isolating the light and booster out it idols then you know that there is leaks in that area .
Mine had leaks everywhere .

Good Luck Bill

[Les W.]

Re: 1971 LT-1 Idle Problems

Hi Les, you can rule out vaccume leaks in the head light doors, wiper door and the brake booster by removing and plugging the fitting on the intake behind the carb . This will not affect engine idol .
I had a lot of leaks in my 71 LT1 and ended up replacing all 3 relays and found a serious leak between the throttle plate and the carb body . I used a piece of glass and sand paper to do the job .
If you find that by isolating the light and booster out it idols then you know that there is leaks in that area .
Mine had leaks everywhere .

Good Luck Bill

Hi Bill, I have a '72 LT-1 with vacuum leaks. I have replaced the two H-light relays, and find that there is still leaks but not as savere as before. I checked the wipers that relay is good. Have not checked the booster. My question to you is by isolating the manifold vacuum supply, how do I check to find leaks at the carb and what was the use of the glass and sand paper? I am a novice at this but want to learn more of the operation of these great cars. This is a survivor car and has a great history. I want to and am working towards a PV for next year. I have a lot to learn. Can you help? Thanks, another Les.

[Bob H.]

Re: 1971 LT-1 Idle Problems

Hi Les:

I used starter fluid to find leaks on my tri-power. if you spray around the base and other suspect areas and there is a leak, you will get a temporary increase in rpm. I believe others can tell you what other sprays will work.

Bob

Hi Bill, I have a '72 LT-1 with vacuum leaks. I have replaced the two H-light relays, and find that there is still leaks but not as savere as before. I checked the wipers that relay is good. Have not checked the booster. My question to you is by isolating the manifold vacuum supply, how do I check to find leaks at the carb and what was the use of the glass and sand paper? I am a novice at this but want to learn more of the operation of these great cars. This is a survivor car and has a great history. I want to and am working towards a PV for next year. I have a lot to learn. Can you help? Thanks, another Les.

[Joe C.]

Re: 1971 LT-1 Idle Problems

Could be a myriad of things, too numerous to cover all of them.

Perhaps one of the most common problems is operation of the distributor advance weights. Sticking and erratic operation will cause this condition. Once you've got them working freely and very lightly lubed on the pivot pins, then you'll have to recheck/recalibrate your static timing as well as your centrifugal advance parameters.

[Timothy B.]

Re: 1971 LT-1 Idle Problems

To add to what Joe mentioned, the power valve could be bad causing raw fuel to be sucked into the engine at idle. Did the engine backfire recently as this can rupture the power valve diaphram, does it run rich and smell heavy with gas.

Disconnect and plug the vacuum supply to the car controls and power brake to see if anything improves. You need to isolate the problem more before jumping into that holley. How much vacuum does the engine draw at idle after warmed.

[Les W.]

Re: 1971 LT-1 Idle Problems

Tim, with all the vacuum supplied to everything and the engine warm, the vacuum reads 12 to 14 hg.

[Timothy B.]

Re: 1971 LT-1 Idle Problems

What idle rpm is the 12-14" vacuum. The best way to isolate is to plug the car vacuum at the manifold so you can eliminate all the vacuum switches.

Start with Joe advice above and check the centrifugal advance to make sure it's moving freely and returning completely, this will only take 1/2 hour to check. After that make sure the dwell and inital timing is correct and go through the idle mixture (emulsion screw) adjustment. If you can't idle the car down and the emulsion screws have no effect on idle then there are carburetor problems.

[Jim T.]

Re: 1971 LT-1 Idle Problems

To add to what Joe mentioned, the power valve could be bad causing raw fuel to be sucked into the engine at idle. Did the engine backfire recently as this can rupture the power valve diaphram, does it run rich and smell heavy with gas.

Disconnect and plug the vacuum supply to the car controls and power brake to see if anything improves. You need to isolate the problem more before jumping into that holley. How much vacuum does the engine draw at idle after warmed.

In checking for a ruputred power valve, the easiest method I have used is to remove the air cleaner lid and place a finger over the vertical vent tube for the front fuel bowl with the engine running. If the engine dies within seconds of putting a finger on the vent tube the power valve needs replacement. If the power valve is good, the engine will continue to run for several seconds with a finger over the fuel bowl vertical vent.