70 LT1 Transistorized Ignition

[James H.]

I recently put my re-built LT1 in my Corvette. I was able to get it started for the initial break in and I did get it started, but it was not immediate. I ran it for 1/2 hour per the engine shop. When it was running, it was running pretty smooth.

I am trying to run it again and I can't get it started this time. I got one blast of back-fire through the carb, so I know it has some spark. As a simple test, pretty much in the dark so i could see it, I separated the coil wire to the cap to allow the spark to make the jump from the wire to the cap. The spark seemed very weak and possibly it was not firing on a regular basis. I do have weak spark.

Are there any troubleshooting steps any one can help with?

Would the coil be the problem? Is there any way to check the coil for proper output?

I know the T.I. module could be bad. Is there any way to test that before deciding to send that out for repair?

Any other troubleshooting ideas would be appreciated.

Thanks

Jim

[Richard M.]

Re: 70 LT1 Transistorized Ignition

Jim there are many things involved that could be the issue.

Are you sure there is fuel? I know you need fuel to get a backfire but maybe it went dry afterwards. If fuel.....

A backfire through the carb usually indicates a timing issue....

Check to be sure your distributor hasn't rotated, maybe a loose clamp bolt or the tach drive gear jammed and rotated it. Happened to me once. Verify rotor is at #1 when the #1 piston is at/near TDC. Verify static timing visually at the balancer & timing tab when the rotor is at cap position #1. If so......

For the TI system basics.....
Check grounds. Things may have rattled loose when you did the engine break-in. The coil - is directly wired to chassis/batttery ground. Check with a ohm meter or add a jumper to be sure. The TI Module case and circuit board gets it's ground via the core support to one of the mount bolts at the module case. Check that too. Make sure your module case has clean metal where the ground connects to the mount bolt(1 of the 3 that secure the module case). I always use a external toothed lock washer there even though the AIM doesn't have one. Good "Ground Insurance".

Also check the 3-wire plug connection on the module TI harness. It may have loosened or a internal terminal in either male or female plug may have a loose connection.

Be aware that it is not advisable to pull more than ONE plug wire to check spark on a original Delco TI module if that's what you have. If you're getting a spark, albeit the appearance of a weak one, the TI system is at least triggering. A ignition coil can fail hot so if you tried restart after your engine ran up to temperature a marginal coil could have failed.

The only real way to test a TI Ignition coil(or any coil) is to see/get spark or just swap out with another coil to verify. For simple test purposes on a TI system you can use a points coil, eg NAPA IC12. It just won't output the peak voltages that the TI coil does. I ran a points coil in a '67 435 L71 for a year before replacing it with a NON-REPRODUCTION NOS Delco 207 TI coil. If you're using a REPRO TI coil get that thing outa' there as it'll fail when it gets hot and leave you stranded. Trust me! I have 2 in the garage display case as a reminder of lessons learned.

I have a slew of TI posts on here I can refer you to that have all kinds of troubleshooting charts and diagrams, etc, but try the above diagnosis steps first, and if you still have troubles I'll point you to them but it'll take me some time to find them all.

Rich

[James H.]

Re: 70 LT1 Transistorized Ignition

Thanks Rich for all of the detail.

Before I saw your post, I tightened the AIM ground on the radiator frame, and I was able to get it started. and it ran pretty good. I do have fuel,wires on correctly, etc and on the 2nd run I set the timing, so that should be all set for my 3rd break-in run.

I will go through the rest of your suggestions also to make sure I have all the bases covered. I will definitely try the new coil. I was not sure if I absolutely had to get a specific T.I. coil or not. Are you saying that no one out there makes a decent T.I. coil replacement and I have to hold out for finding a N.O.S. one?

Thanks

Jim

[Richard M.]

Re: 70 LT1 Transistorized Ignition

Jim Glad you got it running. Grounds are the first thing to check so you did it right.

I'm unsure how many reproduction TI coil manufacturers there are, but many of us have had negative experiences with them.

You don't need a NOS TI coil...... I just checked is it a 1115272 ? A good used original should be fine. For proper top end operation and proper matching to the TI module if original Delco, yes you should use the TI coil. In the L71 I spoke about I was running the repro K&B TI module circuit board which is not as sensitive as the older Delco modules. The K&B part is a great product and has a lifetime warranty from the manufacturer. You can get it directly from them or many of the vendors.

Rich