Thread: Very confused-chronic timing event

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Good Morning,

Sorry for the length of the post. Thanks for everyone's help thus far. It's been a slow process as I tend to travel too much and the wife has a list of "honey-do's" a mile long every weekend. I'm sure most of you understand the lack of wrench time.

I've been building an SBC that consisted of a couple of odd parts that were discussed here:

http://www.clubhotrod.com/forums/showthread.php?t=26594

I'm still struggling with the timing and was hoping someone could shed some light on this. The backfiring, through the exhaust, has continued so i dumped the fuel injection and went to a carb. Backfiring continued. Next, I went after the ignition. Everything was already new: MSD 6A, Blaster 2 and a billet distributor. Swapped the reluctor, pick-up, rotor, cap and plugs. seperated wires, isolated power and ground aalong with trigger wires, etc... No joy.

Pulled the engine and went through it checking to make sure the cam didn't flat-lobe, heads/valves were ok, etc... Everything checked out and re-assembled (lined up crank and cam dots and rotated to #1 top dead center)and put it back in. Still backfiring. Pulled plug wires hoping to isolate a specific cylinder with no success. It seems as though there is raw fuel dumping into the exhaust, sporadically, and lighting up when it hits air.

I went over to the salvage yard where I got the block/tranny and was talking to the guys. I'm there a lot and they shed some light on the origin of the engine. It was in a service truck owned by the local power utility and was being run on propane. I can't find where I wrote down the casting number off the block and I can't get to it installed in the Jeep.

A few questions I have:

-Was propane a factory option on a Gen 1 Block ?
-If so, was the firing order set up differently ?
-Given the wierd nature of the heads and intake, could it be a 18246537
firing order as per '96 and up (taken from a Haynes Manual I found) ?
-Is there a way to determine if the crank is set-up for a different firing order
short of pulling the block ?
-Will engine damage occur if I change the firing order on the distributor to
18246537 and start the engine ?

I'm really stumped on this one. Specs are as follows:

-Stock bore and stroke
-Heads-casting number 10147898
-Valves 1.72" Intake, 1.50" Exhaust 76 CC Chambers
-Cam-Hyd, flat tappet, Intake .420, Exhaust .443. Duration 270, 280 I/E
-Compression Ratio (If my math is correct) 9.013
-Edelbrock Performer RPM Manifold
-Edelbrock 650 Carb
-Fuel pressure at idle 6lbs. Mixture seems good
-SM465 Tranny to Dana 300 Transfer Case
-Initial timing at 8*(although it is bouncing all over the place) with an additional 24* in by 3000 RPM. (32*total)
-Mallory adjustable fuel pressure regulator with return line (3/8" in and out)
-

This has to be something very simple as it is, clearly, a timing event. I feel like the cam and crank are out of synch meaning, valve timinmg and crank stroke aren't where they're supposed to be, when they're supposed to be there. I'm out of ideas. What am I missing ??

Any and all input is gratefully accepted. I'm willing to test/check just about anything at this point. Thanks in advance for any help.

All the best,

Mike

Originally Posted by mhn1961

Good Morning,


-Initial timing at 8*(although it is bouncing all over the place) Mike

This doesn't seem right. May be related to the problem.

As far as firing order. You should be able to pull the plugs and rotate the engine by hand and feel each cylinder come up on compression at the plug hole.
Get a helper to rotate it and check each cylinder in the fireing sequence as he turns it over.

I would run a compression and a cylinder leak down test as it sounds like an exhaust valve is not seating. What about valve lash ? whats it set at ?

Cylinders are all between 160-165 psi. Valve lash is set at 0 lash (rolling the pushrods between fingers) plus 1/2 turn.

Pull the valve covers and turn it by hand,as each cylinder comes on to split overlap, put that wire on the cap across from where the rotor points. That way, you don't have to remember what order they come up in. I see no reason why a propane engine would be any different from any other.

Will it backfire hot or cold or both, I have seen machinist get the valve guides to tight and hang the valve open when she gets up to temp.

the last time i suggested this i got very irate responses and left this site for 6 months or so .. ( so very quietly i say ) .. let a mechanic look at it !!

R Pope, Great advice. I'll try it tonight and let you know how it comes out.

Hworrell. It backfires whether it's hot or cold.

Hoss. You're not going to hurt my feelings. I'm going to exhaust all possibilities before I take it in. At this point, it's a matter of principle. Can't let a mechanical device whip me with putting up a fight.

I'm open to any and all ideas. Keep 'em coming. I've got a few things to change tonight. I'll let everyone know how it goes. Thanks for all the help.

Mike

That sure isnt the firing order for a small block Chevy!! Never was, never will be!!

Wire it like your firing order 18246537 and it is certain to pop. Haynes manuals are not a good source of info but even they should have a Chevy firing order correct.

Yes every gen 1 chevy I know of fires 18436572 unless its had the 4-7 cam swap wich would make it 18736542 same as the Gen 3....
And don't forget cyl. 1357 (front to rear)is the driver side bank and 2468 is the passenger side bank unlike the Fords your use to.....

Robot,

Believe it or not, they reference 1996 and later SFI V8 engines as having a 18246537 firing order. I was hoping maybe they used that firing order for a propane/NG engine. I appreciate the help. Mike

perhaps i`m confused .. your problem is you have a backfiring condition ?? .. when does the offending action take place .. at idle .. on de-acceleration .. full throttle .. .. my V6 pinto will backfire on de-acceleration when one of my exhaust pipe gaskets loosen up as they do from time to time .. tighten up the pipe and it goes away ..

Originally Posted by mhn1961

Cylinders are all between 160-165 psi. Valve lash is set at 0 lash (rolling the pushrods between fingers) plus 1/2 turn.

Rolling the pushrods can result in too tight lash. Hold the tip of the rocker down against the valve tip. Grasp the pushrod with your thumb and forefinger and jiggle it up and down until all play is removed, then tighten 1/2 to 3/4 turn.

Fuel pressure is on the edge for an Edelbrock carb. Reduce it to 5 lbs. max.

Hoss, Yes, it's backfiring through the exhaust. It occurs at idle all the way through the RPM range, assuming I can get it to go off idle. If I feather the throttle I can bring the RPM's up but it's raising hell all the way. The good news is, it's consistent. If it weren't and that thing backfired just every once in a while, I'm at the age where I'd have to worry about a heart attack :-)

techinspector, I'll re-lash them using your method. That makes more sense to me than rolling them. Is it best to adjust them while they're hot and pumped up or after they've bled down ?? I can dial the fuel pressure down to 5 lbs. I'm pretty sure I can adjust down to 3. WOuld that be better at idle ?

Late model small block engines (not LS engines) are the same as all other 283-307-350-400 engines. Propane or not, does not matter. That's one thing (of many) that Chevy did right...they dont change stuff like some of the other brands.......the other guys found their problem and circled it....on their emblem.