1971 LT-1 Performance Question
Thread Starter
Burning Brakes
Joined: Aug 2007
Posts: 1,084
Likes: 8
From: Illinois
1971 LT-1 Performance Question
My 71 Lt-1 only seems to accelerate up to about 4500 rpms and then nothing. The motor is original (89,000 miles???). The carb, TI system, etc have all been rebuilt. New plugs, plug wires, coil, entire front wiring harness. The car is 99% original. Just doesn't seem to have the power everyone says a LT-1 should have--I also have a 2002 Z06 and maybe I'm comparing them too much--it never stops pulling. It just doesn't have any pull after 4500 rpms and doesn't seem to want to go to 6000 rpms. Any thoughts. I don't push it that often to 5500-6000 because I don't want to blow up the motor.
Burning Brakes
Joined: May 2008
Posts: 860
Likes: 0
From: Stratford Ontario
Check to make sure that Your secondaries are capable of opening, it could be a distributor or timing issue. I would check the simple things first. Could even be running out of fuel, from weak pump or pluged filter or line.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If the engine does not generate a deep, throaty roar under full acceleration, the carb secondaries are not opening. The linkage/throttle cable could be out of adjustment and/or the secondary 'lockout' feature in the choke mechanism may not be releasing (especially, if the choke isn't working properly). Getting someone else to do this work will be expensive and risky...as most mechanics aren't familiar with Q-Jet functions. Your best solution is to get a copy of the Chevrolet Chassis Service Manual and work through the carb/choke setup procedures. That will make you familiar enough with the system to see what could be wrong. You also need to make sure that the accelerator PEDAL will fully open the lower throttle plats when you 'go to the floor'. To do that, you will have to hold the upper secondary airvalve open with your hands and look down into the carb while someone else depresses the accelerator (engine OFF, of course). Pulling the linkage by hand is NOT the same action.
Thread Starter
Burning Brakes
Joined: Aug 2007
Posts: 1,084
Likes: 8
From: Illinois
The carburator was just rebuilt by Jerry MacNeish--Camaro Hi-Performance. He runs them on a car before he returns them. I can here the secondaries come in when I stomp on the gas. The car has a new coil and the TI system was completely rebuilt by Dave Fielder at TI Specialties.
Drifting
Joined: Dec 1999
Posts: 1,462
Likes: 1
From: Reno nevada
That is what I was going to say. As the proud owner of a 71 LT1 the springs with those heavy rotators are shot not just from the miles, but time!
I got rid of the crappy Lt1 solid cam and went with a Crane PowerMax solid cam, with spring and lifter kit, and with 1.6 roller tip rockers
I got rid of the crappy Lt1 solid cam and went with a Crane PowerMax solid cam, with spring and lifter kit, and with 1.6 roller tip rockers
Race Director
Joined: Dec 1999
Posts: 11,096
Likes: 990
From: Sierra Foothills CA
If the engine does not generate a deep, throaty roar under full acceleration, the carb secondaries are not opening. The linkage/throttle cable could be out of adjustment and/or the secondary 'lockout' feature in the choke mechanism may not be releasing (especially, if the choke isn't working properly). Getting someone else to do this work will be expensive and risky...as most mechanics aren't familiar with Q-Jet functions. Your best solution is to get a copy of the Chevrolet Chassis Service Manual and work through the carb/choke setup procedures. That will make you familiar enough with the system to see what could be wrong. You also need to make sure that the accelerator PEDAL will fully open the lower throttle plats when you 'go to the floor'. To do that, you will have to hold the upper secondary airvalve open with your hands and look down into the carb while someone else depresses the accelerator (engine OFF, of course). Pulling the linkage by hand is NOT the same action.
Last edited by Les; Aug 27, 2010 at 02:02 PM.
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Melting Slicks
Joined: Oct 2001
Posts: 2,065
Likes: 1,395
From: Halifax, Nova Scotia, Canada
You mentioned that the carb was rebuilt, did you replace the Holley filters behind the inlet fittings.
These filters are small and get restricted easily. When dirty/restricted the engine acts exactly as you describe.
I discarded mine years ago and replaced with an inline filter.
Barry
These filters are small and get restricted easily. When dirty/restricted the engine acts exactly as you describe.
I discarded mine years ago and replaced with an inline filter.
Barry
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
Thread Starter
Burning Brakes
Joined: Aug 2007
Posts: 1,084
Likes: 8
From: Illinois
The car hasn't been driven since last October. Just too it out for another run--missing lic crazy--had trouble getting it above 4500 rpms. I will need to chec the timing and then if it is alright--pull the plugs and chec them. As far as I now the motor has the original valve springs. Everything was changed, cleaned, or replaced when the carb was rebuilt--Jerry M. is topnotch. The problem before was--when I set the timing per GM 8 degrees BTDC it didn't run well. Any suggestions appreciated.
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
Let's assume the carb is good and look at the timing side. Here are some thoughts:
1) 8 degrees initial is probably too low. But forget that. Set her to 34-36 degrees at 3000 rpm (with the vacuum advance hose pulled and plugged) and let the initial fall where it falls.
2) Do the mechanical weights in the distributor move feely?
3) Are you still using points/condensor? Did you set the dwell?
4) How fresh are the plug wires?
5) Is the distributor cap cracked or is there any moisture build up?
6) How fresh are the plugs?
Thinking out loud here.
Lastly, if the fuel in the tank has sat a year and has degraded and collected moisture, it won't help you achieve bonzai rpms.
1) 8 degrees initial is probably too low. But forget that. Set her to 34-36 degrees at 3000 rpm (with the vacuum advance hose pulled and plugged) and let the initial fall where it falls.
2) Do the mechanical weights in the distributor move feely?
3) Are you still using points/condensor? Did you set the dwell?
4) How fresh are the plug wires?
5) Is the distributor cap cracked or is there any moisture build up?
6) How fresh are the plugs?
Thinking out loud here.
Lastly, if the fuel in the tank has sat a year and has degraded and collected moisture, it won't help you achieve bonzai rpms.
Last edited by 73, Dark Blue 454; Aug 27, 2010 at 02:37 PM.
Drifting
Joined: Oct 2007
Posts: 1,254
Likes: 5
From: Brown Summit North Carolina
The carburator was just rebuilt by Jerry MacNeish--Camaro Hi-Performance. He runs them on a car before he returns them. I can here the secondaries come in when I stomp on the gas. The car has a new coil and the TI system was completely rebuilt by Dave Fielder at TI Specialties.
You can please a paper clip on the rod shaft of the diaphram plunger set as high up the rod as possible. then driver the car and hold it wide open for a few seconds.
Then check the paper clip to see how far dowm the rod it was pushed.
The manually open the secondaries and compare
I am running a new Holly SA 670 and the cars runs much stronger than with the 780
Last edited by dmayhew; Aug 27, 2010 at 08:26 PM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
LT-1.....Holley carb.....yep, that rings a bell. {senior moment}
But, my point is the same. The secondary throttle plates have to open fully to get full power out of the engine. And, I don't know how you "see" that from the driver's seat. Good luck with your search.
P.S. As complicated as you think the problem is, it is probably something very simple.
But, my point is the same. The secondary throttle plates have to open fully to get full power out of the engine. And, I don't know how you "see" that from the driver's seat. Good luck with your search.
P.S. As complicated as you think the problem is, it is probably something very simple.
