New engine assembled, won't start - backfires

Hey guys,

So as some of you know I've only had a running LT1 for 80 miles with this swap project that I've been in. It ran great for 80 miles and then I spun two bearings. :(

I got the motor back from the machine shop machined with cylinder walls 040 over, crank was line bored 20/20. :embarassed:

So round 2 started on Friday and yesterday we had the motor all assembled and installed in the car again. We went to start and it cranks and randomly backfires. No start. Timing light showed that the wires are getting pulses and since it's backfiring we're definitely getting fuel and spark.

My dad thinks the motor is 180 degrees off but that's impossible if the crank and cam gears were 12 o'clock and 6 o'clock respectively, right? We used the opti diagnosing thread on CZ28 to check the ignition module and coil and everything seems to be legit so it seems like it's the opti.

Now, my question is - if the opti was bad would it still be sending pulses at all and/or would I get any DTC error codes? I had DataMaster running while I tried starting the car and I didn't have any DTC error codes.

The only other thing we can think of is that the opti isn't indexed right. It's a 1995 vented optispark. We took the original opti that came with the motor and then took the LT1 camshaft and tried putting it into the wrong slots and it wouldn't go into the wrong slots... only way I could see someone accidentally indexing it wrong would be to exert a lot of force on the opti to jam it in there. Mine slid in with no problems.. just wiggled it back and forth a bit and it popped right in.

Also, when we reinstall the optispark or a new one, as long as the key slot lines up to where ever the cam dowel is it should be right, correct?

If the crank gear is at 12 o'clock - is that #1 TDC or #6 TDC? My understanding is that it could be either. If so, how do we tell the difference between #1 TDC and #6 TDC? :confused:

If you were getting an Opti code, it would be DTC 16. When you get that code, it also shuts of fuel. So that's out. You're right there is no way to put the 95+ Opti on wrong. The index for the cam dowel is larger than the rest of the slots in the back of the Opti. Also, it would have had to have been smashed on and most likely would not have sit flush on the front of the motor.

Easiest way to find TDC is pull the plug and put your finger over the hole. Have someone crank the motor and when you feel a 'woosh' over your finger you'll be at TDC on compression stroke.

Did you check your plug wire routing to make sure the wires are on the correct cylinders?

We checked last night but it's hard to tell with all the accessories in place. When I get home from work I'm going to be yanking the water pump and harmonic balancer off. Should give me a lot more room to work with and see what's going on. I'm hoping it's just a misrouted wire ... hell, I'll even take an opti... I just hope to god I didn't have the crank 180 degrees out. It is possible, right? Dot will be at 12 o'clock for #6 TDC and #1 TDC, right? Man if we put the crank 180 degrees out from the cam that's going to SUCK... :( And what really sucks is being stuck at work having to stew on all this and not being able to go home to work on it. :(


Using a timing light - what would it look like when you're cranking and/or running? Would it be a very constant pattern or would it seem random? It seemed like last night when we had the timing light on it was just blinking randomly, no pattern to it. Don't know if it's because the timing light is older than I am or because I've got 8.5" MSD wires or not. Maybe that's a sign the rotor is bad on the opti?

Any time the crank gear dot is at the 12 o'clock position, the #1 AND #6 pistons will be at TDC.

The difference is what they are doing. What they're doing depends on the camshaft gear dot.

If the camshaft gear dot is also at the 12 o'clock position, the #1 piston will be on the compression stroke and the #6 piston will be on the exhaust stroke.

If the camshaft gear dot is at the 6 o'clock position, the situation is reversed - the #1 piston will be on the exhaust stroke and the #6 piston will be on the intake stroke.

Although I have seen posts saying the Opti can't be installed 180 degrees out in the phasing, one regular, highly respectable poster strongly disagrees, saying he's seen the Opti mis-installed many times.

The first thing I'd do is to verify that all the plug wires are correctly routed. It's really easy to mistakenly swap two of them and cause a back-fire. Been there.

Hope this helps.

Jake

West Point ROCKS!

Another possibility is incorrect settings of lifter preload.

Jake

West Point ROCKS!

Quote:

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Originally Posted by JAKEJR

Any time the crank gear dot is at the 12 o'clock position, the #1 AND #6 pistons will be at TDC.

The difference is what they are doing. Wha they're doing depends on the camshaft gear dot.

If the camshaft gear dot is also at the 12 o'clock position, the #1 piston will be on the compression stroke and the #6 piston will be on the exhaust stroke.

If the camshaft gear dot is at the 6 o'clock position, the situation is reversed - the #1 piston will be on the exhaust stroke and the #6 piston will be on the intake. stroke.

Although I have seen posts saying the Opti can't be installed 180 degrees out in the phasing, one regular, highly respectable poster strongly disagrees, saying he's seen theOpti mis-installed many times.

The first thing I'd do is to verify that all the plug wires are correctly routed. It's really easy to mistakenly swap two of them and cause a back-fire. Been there.

Hope this helps.

Jake

West Point ROCKS!

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Quote:

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Originally Posted by JAKEJR

Another possibility is incorrect settings of lifter preload.

Jake

West Point ROCKS!

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Thank you for your explanation Jake - you've put my mind at ease. I've been getting stewing about "worst case scenarios" thinking the valves and pistons could have made contact if it was timed wrong. Your explanation finally made me realize that if the cam is taken out of the equation TDC is TDC it doesn't determine compression or exhaust.

So, it sounds like there's no chance of it being 180 degrees out because #1 AND #6 are both TDC and the gears were dot to dot. We used the method on Shoebox's website where you adjust the valves according to watching the lifters/rockers go up and down. We used 1/2 turn for valve lash - but when I get home we'll try looking at the wires again, maybe pop the valve covers off and watch the car try to crank, and then possibly take the water pump off to get to the optispark.

sounds like the plug wires to me too, mine did the same thing when i had 2 wires crossed.

Quote:

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Originally Posted by 95formula383lt1

sounds like the plug wires to me too, mine did the same thing when i had 2 wires crossed.

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What im thinking as well.

Eh id think it would still start and run with 2 crossed wire's. Mine ran surprisingly decent with 2 wires crossed after my header install.

Got it to start for a few seconds before it stalled from being flooded. It was definitely the opti - it was in one of the wrong slots. It's amazing that it's even able to go into the wrong slots. Should JBweld the slots so it won't happen again hahah... But atleast it started! Now I just need to get all that fuel from cranking out of the chamber so it won't be flooded. Hopefully letting it sit over night will help.