I had my 408 rebuilt by my bro in law, he has done tons of engines but this was his first "ls". I have everything for it to start, fuel, spark and compression. The c/r is above 13 and I have my battery moved to the back of the car and the stock starter turns it over sort of slowly so I thought that was the issue. Well even with all of the plugs out of the engine it stills turns over slowly so I guess that's just how it's going to be. I did some research and guys with higher compression engines are using the stock starter like I am (I tried two different high torque mini starters and neither fit).

The thing that made me scratch my head is that it backfires through the intake. Being that it is a fresh build I checked the compression last and wow what a surprise. It varied from 70 to 205 and that's dry, no oil in the cylinders.

I asked him about it and he says it because the cylinders are being washed down with fuel and at a different rate which is why they are so different. But the plugs aren't soaked when they came out and although I know I'm getting fuel there is not an overwhelming odor.

Seems to me it's a timing or valve adjustment issue. What do the gurus here think?

 

I would ask him if he measured PTV and pushrod length. Sounds crazy to ask of an engine builder, but the engine shouldn't turn over slow with the plugs out unless you have an issue with the battery or voltage.

 

I know he checked those because I have pictures of him doing it plus he told me which pushrods to buy. I actually had two batteries hooked up when I was cranking too. Maybe the starter is going out? I thought they either worked or didn't.

 

Is everything plugged in? Crank sensor, Cam reluctor, MAF etc?...
The backfiring and slow cranking is suspect. Does it vary in speed when turning over?

Something's off. Too tight of tolerances or electronics?

Doh,
I re read your post again about the CR being way off between cylinders. Did you change lifters, Cam, heads springs etc...? Do you know how much preload you have on each cylinder?
Even if you're off on this, the slow turning of the engine is still suspect, too.

 

I don't know about the preload per cylinder. Same lifters, springs, heads and cam as before. Only thing different in the engine is of course new bearings throughout and new pistons/rings. Could it be a tune thing? i mean I went up about two full points in compression. The engine is turning over at 90 rpm, maybe that is normal, I don't know. The thing that really bugs me is the backfiring through the intake and now the crazy compression numbers.

I did screw up and cut some wires inside the car coming from the BCM but I thought I had them all squared away since I have all of the elements to start. Even checked the pulse on the injectors, I have it all. It wouldn't surprise me if there is a wire that still isn't reattached but then there is the compression thing...

 

Timing off a few teeth? That might account for the backfiring and low compression...

Something I've seen on big blocks that wouldn't apply to an LS motor was the rotation direction on the distributor not being accounted for. Person installing engine had only done small blocks and was used to clockwise rotation, big block was counter clockwise. So they lined up #1 on the cap and rotor and then wired in 1-8-4-3-6-5-7-2...in the wrong direction. Lol.

 

This is along the lines of what I'm thinking.

 

My LS1 would back fire through the intake and would not start when I had a broken ground wire at the back of the driver’s side head. Engine should spin over fast and easy with the plugs out. Take your starter to AutoZone and have them test it.

 

I hope you were referring to the cam timing gear, not the distributor gear.

You mentioned that the battery was moved to the back of the car. Did you run all new battery cables? The slow turn of the starter indicates that it may not getting the full power from the battery; or the ground/negative cables are not properly installed.

What about the coil harness? Check to see if they're not backwards (which can lead to back firing)

 

Good point about the ground, I'll double check but I even added an extra so I think I'm good there.

Ya all new battery cables, I need to run a negative all the way back to the front though because I have a shorter one right now that is grounded in the back near the battery. Coil harness...I'm 99% sure I have that correct judging by the length of the wires and whatnot. I can't see how it would go a different way. I'll have another look though, anything helps.

Thanks for the replies/ideas, please keep them coming as I'm about to throw a match on this bitch!

 

We built a 6.0 a couple months ago , its has a carb with a 6010 box. It wouldn't spin fast enough to get the box to fire. he bought a high torque starter off Ebay (like $70.00), and we used my jumper box and got it started.. it fired up better after running a little. It has around 12.5 to 1

 

I tried the msd and xs power mini starters and neither fit with my 1 7/8th long tubes.

 

You've got 70psi on one or more cylinders? That is a problem in itself. Whether it cranks or not, you should do a leak down test on those low cylinders.

 

Yep. I really feel like its a timing thing but I'll be digging back into this car in a few weeks. I'll update the post when I figure it out.

 

Did he mess with the reluctor wheel on the crank at all? If that isn't correct it will never start. With no plugs that thing should turn over faster.

 

I'll be looking into it in a few weeks. Since my Mustang is in the driveway I'm going to work on that until it's too cold then mess with the Trans Am over the winter since it's inside.