LT-1 Transistor Ignition Dwell

[Ted K.]

The specs call for a 30 degree dwell for a 1971 LT-1. Did all LT-1s come with TI? Does a Transistor Ignition TI have an adjustable dwell? My meter indicates a dwell of 39 and there is a sight miss about mid RPM. If it was a standard points distributor I would guess bad points. How do I adjust the dwell?
Thanks,
Ted

[Warren F.]

Re: LT-1 Transistor Ignition Dwell

Ted,

I'll answer part of your questions. Yes, Magnetic Pulse Transistorized ignition system was standard issue on the '70, '71 LT-1 optioned Corvettes.

[Rich P.]

Re: LT-1 Transistor Ignition Dwell

Ted,

There is no dwell adjustment on TI cars. If you are getting your specs out of the service manual I think what you are seeing is the setting for L-T1 Camaros as they had Points distributors.

Rich

[Timothy B.]

Re: LT-1 Transistor Ignition Dwell

Ted,

I think the dwell is established by the rotating pole and magnetic pick up which controls the switching action signal sent to the amp box mounted on the inner fender.

I really did not know you could check dwell on TI, be careful you don't fry something..

[Ted K.]

Re: LT-1 Transistor Ignition Dwell

The 30 degree dwell info came from the emissions sticker in the engine compartment left firewall, also gives timing etc.
Ted

[Clem Z.]

Re: LT-1 Transistor Ignition Dwell

Ted,

I think the dwell is established by the rotating pole and magnetic pick up which controls the switching action signal sent to the amp box mounted on the inner fender.

I really did not know you could check dwell on TI, be careful you don't fry something..

no way to change the dwell on a TI

[Ted K.]

Re: LT-1 Transistor Ignition Dwell

Good info. Now the reason I asked those questions was I have a slight miss in mid to high rpm, 2000 - 4000+. The timing light indicates there is an inconsistent spark. How does one check out the TI ignition system?
Ted

[Terry M.]

Re: LT-1 Transistor Ignition Dwell

I don't know for sure about 1971, but I think 1970 (really 1969, since there is no Corvette 1970 FSM, but that is another story) has a trouble shooting tree in the FSM. I can't imagine why 1971 wouldn't.

But there is a better source. Many sleeps ago there was a story in The Corvette Restorer. I know ink on paper is old fashioned, but I would hope it made it into The Restorer CD. To aid you in your search the author was Keith Kibbe (That will give the old-timers around here a hint as to how long ago it was).

[Duke W.]

Re: LT-1 Transistor Ignition Dwell

Good info. Now the reason I asked those questions was I have a slight miss in mid to high rpm, 2000 - 4000+. The timing light indicates there is an inconsistent spark. How does one check out the TI ignition system?
Ted

My experience with the TI is that it either works perfectly or it doesn't work at all. My bet is that the problem is in the high voltage distribution system - cap, rotor, spark plugs, or wires.

Check the wires with an ohmmeter. Any that are more than about 5K ohms per foot (OE type RFI suppression wires) should be replaced.

Duke

[Terry M.]

Re: LT-1 Transistor Ignition Dwell

When my 1970 LT1 was my daily driver, I found this engine is very hard on spark plugs -- and mid-range missing was a symptom. With the lower compression ratio of the 1971 I would think that might be less an issue, though. I just pass it on FWIW.

[Ted K.]

Re: LT-1 Transistor Ignition Dwell

This is a fresh motor with 10 miles on it. New plugs, wires, distributor cap, old rotor. I'll check out them all out tomorrow.
Thanks,
Ted

[Clem Z.]

Re: LT-1 Transistor Ignition Dwell

make sure the plug wires are all the way down into the cap. pull the boots back on the wires and make sure the wire ends are bottomed out in the cap. also try running the engine without the ignition shielding to make sure you are not having it shorting out the wires. also make sure you have a clean ground wire to the amp box.

[Jack H.]

Re: LT-1 Transistor Ignition Dwell

Well, there are two answers here regarding 'checking out' the TI system: quick and dirty vs. thorough/professional.

With the quick and dirty approach, you disconnect the coil + and - wires, and connect a dummy load. The filament resistance of an ordinary 1-3 candle power incandescent lamp gets that job done.

Turn the ignition switch ON and the lamp ought to illuminate. When you move the ignition switch to START, the illumination from the lamp should exhibit VERY MINOR fluxuations in brilliance as the individual TI ignition pulses pass through it. That's telling you the TI system is grossly functional.

Now, if you're more savvy electrically, you can connect an oscilloscope across the dummy load of the lamp, adjust the scopes' trigger (level and threshold) appropriately to see a 'stop action' display of the individual ignition pulses.

Then, re-run the engine cranking test and look at the O-scope display for pulse waveform voltage (height on the scope display) for consistency. If there's an inconsistency issue, it's time to delve into the distributor for issues with the magnetic coil pickup and it's relation to the air gap between it and the distributor lobe...

The drawback(s) here are pretty obvious. First, you're only seeing the TI function during engine cranking operation vs. engine run AND you're observing artificially LOW RPM operation under component 'cold' conditions. Second, you need an oscilloscope and the knowledge to properly connect it and set its voltage level and trigger...

The 'thorough/professional' approach is to take the car to a competent auto shop and have it put up on an engine analyzer. There, you'll capture the actual individual spark wave forms with the engine actually running (not just cranking)...

[Terry M.]

Re: LT-1 Transistor Ignition Dwell

Current automotive technology uses a product called a Lab Scope. It is today's version of the oscilloscope Jack talks about. Only it is solid state and flat screen. I have seen them with up to six traces in different colors. Makes the old Techtronics scope look like the dinosaur that it is.

Remember all this whiz bang hi-tech equipment is only as good as the person using it. Good problem solving and analytical skills are needed more so now than years ago.

[Jack H.]

Re: LT-1 Transistor Ignition Dwell

"Remember all this whiz bang hi-tech equipment is only as good as the person using it."

AMEN, brother!!!!

I've seen young engineers and technicians 'prove' just about anything during test analysis all due to their relative skills in setting up and interpreting test equipment readings...