Confirm poor idle and low rpm conditions?

[Gary S.]

The subject car is a 1972 LT-1 with 39000 miles and 100% original drivetrain and engine compartment. These last 6-8 years have kept me from driving more than about 500 miles a year (in a good year) with my job being out of town. Anyhow, when warmed the car runs at full throttle like a scalded ape! I love it. At mid to high rpms, there isn't anything wrong with the way this car drives. At low rpm, I get hesitation, chugging and an occasional backfire at very low rpms when starting out. It is worse when cold but then again, I have failed at adjusting the "doohickey" that keeps it on the idle cam when cold - that is another story. When idling and warm, the rpm drops down to 600+ wavering around that number. Blip it and it returns to normal of 850+/- for a few seconds. Yesterday I was on the interstate and we had a bottleneck slowdown. I was in and out of first gear and you would have thought I have a wild cam in there or something like that.

About 8 or 10 years ago Jerry MacNeish (good reputation, btw) worked over my distributor and carb, both original. I told him "do your magic. All I want is a good running, reliable car and I will never have it on the strip or even at high speed. Make it driveable and reliable." For the most part it is.

Here are my suspicions: Vaccuum leak, either at the base plate or one of the vacuum lines going into the carb - they are all original; float that has a hole in it (stretching on this one); or ...?

Today, I am going to try some starter fluid, sprayed around the various spots of the carb to see if rpm will increase. In addition, I am going to throw my vacuum gauge on one the carb lines to see what I get. My out of town job kept me so busy that often the only way for me to drive my LT-1 was to get home from work (I commuted via the airlines), throw off my uniform and toss on jeans and shirt to go driving. When finished, I would wash it, put the cover on it and wait for the next time.

Ideas? Today is a rainy day and I told my wife that it is Corvette day.

Gary

[William P.]

Re: Confirm poor idle and low rpm conditions?

Start with the easy things first . Check that the vac advance is working by unhooking it , the idol rpm should drop off , I assume it is hooked to a full time vac source .
Pull and plug the vac source line for the accesories at the rear of the intake . this will eliminate any leaks from the headlight and wiper door actuators .
Pull and plug the brake booster line for a possible leak in the booster .

Bill

[Patrick H.]

Re: Confirm poor idle and low rpm conditions?

Also verify that your idle solenoid on the left front of the carb is actually working, and not slowly allowing your idle to stumble down.

[Gary S.]

Re: Confirm poor idle and low rpm conditions?

I am working on taxes now but will look at the points you two mentioned shortly.

Thank you,

Gary

[Timothy B.]

Re: Confirm poor idle and low rpm conditions?

Gary,

Take a look at the accelerator pump to make sure the fuel shot is instant when you move the throttle and that it's adjusted properly, do this with the engine off. When you open the throttle and the vacuum drops, the fuel drops out of suspension and the engine goes lean which is why you need that instant fuel shot from the accelerator pump.

Many times spraying around the carburetor does not reveal a vacuum leak because it's internal to the carburetor. The base and or metering block gasket could have dried and leaking internally.

Check carefully the spark plug wires, the low rpm lean mixtures are hard to light and if the wires are leaking voltage that's where it can show up. Get a ohm meter to check them, good wires should ohm out at approx 3000 ohm/ft., the wire lengths are in the archives somewhere.

Double check the dwell and timing because retarded timing can make it run sluggish at low rpm and your Lt-1 should have points.

[Terry M.]

Re: Confirm poor idle and low rpm conditions?

Years ago I had similar problems with my 1970. Spark plugs had only a few thousand miles on them. At the insistance of one of my chapter mates (he had had a 1970 Z28) I changed the spark plugs, even though I doubted it with that few miles. All good after that.

[Duke W.]

Re: Confirm poor idle and low rpm conditions?

Start with the easy things first . Check that the vac advance is working by unhooking it , the idol rpm should drop off , I assume it is hooked to a full time vac source .
Pull and plug the vac source line for the accesories at the rear of the intake . this will eliminate any leaks from the headlight and wiper door actuators .
Pull and plug the brake booster line for a possible leak in the booster .

Bill

No, it's an emission controlled engine, so vacuum advance is ported unless it's been modified previously. I am not aware of any '68-up engines that have full time vacuum advance as built by Chevrolet.

What what done to the distributor? Understanding the current configuration is necessary to understand where you need to go.

I do recommend converting LT-1s to full time vacuum advance but this MUST BE ACCOMPANIED by swapping the OE 201 VAC for a current B28 replacement.

Full time vacuum advance with the 201 VAC will cause idle instability and stalling because it doesn't meet the Two-Inch Rule.

Duke

[Gary S.]

Re: Confirm poor idle and low rpm conditions?

Well, taxes are finished but the hot water heater decided it didn't like life.

Responses are great but this engine is bone stock with the TCS system installed (no full time vacuum advance), the distributor was set to stock (more or less by MacNeish for nice & reliable running and I don't know how he curved it)

[Gary S.]

Re: Confirm poor idle and low rpm conditions?

Ok, water heater issues are gone and I had a few minutes to look at the car.

1. vacuum advance is direct with the hose from the TCS plugged. Rpm increases when the hose is pulled off of the can. No, I didn't replace the vacuum advance and it is original to the best of my knowledge.
2. I checked the vacuum lines quickly and all are intact with no discernible leaking (yeah I know that doesn't say much). I am going to hook up a vacuum gauge tomorrow to see exactly.
3. the carb plate screws were snug (more or less) but I gave them a quarter turn to ensure that the carb is snug against the intake.
4. the engine rpm was a tad low at around 675-700 based on the tach and I set it up to 850. It idles smoother.
5. accelerator pump gives shot from each nozzle immediately as the accelerator cable is pulled.

Here are some odd things - I have a can of starter fluid left over from trying to help my son in law get his snow blower started last fall. As a test, I gave the carb a quick shot of it while the engine was idling and the engine slowed and almost stalled. Never having used starter fluid as it was intended, I thought it was highly volatile and the engine should have increased rpm. Correct?

Last night I came in from getting new tires, as mentioned in a previous thread, and I shut off my car. I popped the headlights up after the engine had shut down. Then I lowered them. Only one came down indicating a minor vacuum leak right? How long should the vacuum hold on a car with original hoses, reservoir etc? Longer than 10 seconds and one headlight extension/retraction? Fwiw, the headlights pop up immediately and simultaneously with the engine idling.

More testing tomorrow.

Thank you again for your responses.

Gary

[Duke W.]

Re: Confirm poor idle and low rpm conditions?

Ok, water heater issues are gone and I had a few minutes to look at the car.

1. vacuum advance is direct with the hose from the TCS plugged. Rpm increases when the hose is pulled off of the can. No, I didn't replace the vacuum advance and it is original to the best of my knowledge.
Gary

Are you saying the vacuum advance line is at full manifold vacuum at idle? If so, the the OE VAC is not properly matched to the engine's idle vacuum characteristics. Please reread post #7.. I believe the OE VAC is 201 15. What's the stampded data on yours?

Duke

[Gary S.]

Re: Confirm poor idle and low rpm conditions?

Duke,
You are right, this is manifold vacuum directly off of the carb base. A few years ago, I pulled the TCS lines off and plugged the one to the TCS and plugged the distributor directly to the base of the carb. I can't get at the vacuum can right now, but I do recall that is is original when I pulled it off a few years ago to have the distributor rebuilt. I am certain that it can't be matched to its current condition.

[Terry M.]

Re: Confirm poor idle and low rpm conditions?

Ok, water heater issues are gone and I had a few minutes to look at the car.

1. vacuum advance is direct with the hose from the TCS plugged. Rpm increases when the hose is pulled off of the can. No, I didn't replace the vacuum advance and it is original to the best of my knowledge.
2. I checked the vacuum lines quickly and all are intact with no discernible leaking (yeah I know that doesn't say much). I am going to hook up a vacuum gauge tomorrow to see exactly.
3. the carb plate screws were snug (more or less) but I gave them a quarter turn to ensure that the carb is snug against the intake.
4. the engine rpm was a tad low at around 675-700 based on the tach and I set it up to 850. It idles smoother.
5. accelerator pump gives shot from each nozzle immediately as the accelerator cable is pulled.

Here are some odd things - I have a can of starter fluid left over from trying to help my son in law get his snow blower started last fall. As a test, I gave the carb a quick shot of it while the engine was idling and the engine slowed and almost stalled. Never having used starter fluid as it was intended, I thought it was highly volatile and the engine should have increased rpm. Correct?

Last night I came in from getting new tires, as mentioned in a previous thread, and I shut off my car. I popped the headlights up after the engine had shut down. Then I lowered them. Only one came down indicating a minor vacuum leak right? How long should the vacuum hold on a car with original hoses, reservoir etc? Longer than 10 seconds and one headlight extension/retraction? Fwiw, the headlights pop up immediately and simultaneously with the engine idling.

More testing tomorrow.

Thank you again for your responses.

Gary

Your vacuum system is under performing. Remove the vacuum line that goes from the intake port to the vacuum filter from the intake port, and plug (cap) the intake port. Then see how your engine runs. You may have to readjust your basic carburetor settings. After that you can chase your vacuum leak. I don't have the PV vacuum standards handy, but I know they require a longer wait than you report.

[Bill L.]

Re: Confirm poor idle and low rpm conditions?

I had a very similar issue albeit with a 70 L46. We did as an earlier response suggested an plugged up the vacuum for the wiper door and headlights at the source. This solved the problem. Then we had to trace the vacuum leak(s).

Ultimately the vacuum leaks were traced to the three vacuum valves with the worst of the three being one of the valves for the headlights. I suggest this may be the issue on your LT-1 given the way the headlight was lazy. Once fixed, the headlights easily closed with the engine shut off. Pretty neat to see them working as intended.

We did not bypass the TCS and did not have an issue with the vacuum advance.

Bill

[Patrick H.]

Re: Confirm poor idle and low rpm conditions?

Ultimately the vacuum leaks were traced to the three vacuum valves with the worst of the three being one of the valves for the headlights. I suggest this may be the issue on your LT-1 given the way the headlight was lazy. Once fixed, the headlights easily closed with the engine shut off. Pretty neat to see them working as intended.

Bill

Ah, I forgot about the valve when we talked earlier today Gary. Another good item to check out.

Patrick

[Jim T.]

Re: Confirm poor idle and low rpm conditions?

Gary have you checked out the operation of your Holley's power valve. All it takes is a slight disruption to cause the Holley's power valve to rupture and not hold vacuum and operate as designed. A quick check I have used for many years on my Holley's is place a finger over the Holley's front vertical carb vent on a running engine. If the engine dies immediately, the power valve needs replacing. If holding the finger on the vertical vent and engine does not die immediately, it will soon die, this indicates the power valve is ok and is functioning from engine vacuum.
My TCS has not been hooked up since SEP 1970. Better gas mileage in town/city and cooler running engine with hot temperature days. Have used full time vacuum to the vacuum advance 100%.