Bad Timing???

[The Struggle]

Does anyone know if my timing could be too retard if my initial timing made my truck backfire through the carb (12 degrees initial/ 36 total)under load? I reduced the backfiring ALOT if I kept advancing the timing every time I test drove it and tightened the timing curve springs to one stronger one. I'm now at 18 degrees initial/36 total but have to hold my foot to the floor to start the truck now on a hot start after letting it rest for a bit. It doesn't slow the starter or give any too advanced symptoms when starting just is stubborn and runs rough for about ten seconds. This is in a performance 350, aluminum heads, Holley 650 vacuum secondaries. Cam is SUM-1107 224/234 duration, .466int/.488exh. 114 LSA, Compression ratio is a 9.6:1 with 64cc heads. BTW it backfired WAY more when I retarded it down to 8 degrees initial! Vacuum canister is hooked to ported (above throttle blade source and limited to 11-14 degrees advance on the armature).

I'm lost, should I keep advancing and hope not to ping the engine or go another route? My exhaust is pretty much straights with dumped thrush glass packs 4 feet behind the headers so its right underneath the cab which drones and is loud. To loud to hear any possible pinging I think.
This truck is driving me insane!

 

[1987 GMC Jimmy]

Did this issue start randomly after the truck had been running good, or has it been this way and getting worse since you did the upgrades? What you're saying about having to adjust the timing to make it run correctly tells me that I would look into the timing chain being stretched or missing a tooth. You don't have to pop the timing cover off to see it; you can pop the distributor cap off and turn the motor by hand. If there's a lot of play in the translation between you turning the crank and the rotor turning, there's a problem. A little delay is okay, but if you don't feel comfortable with what you see, I'd go in to look at the timing chain. There's a more precise way of diagnosing it using degrees of arc of play, but mine's the more shade tree explanation because that's the one I'm familiar with.

 

[The Struggle]

I've had the truck running better over time but as soon as one issue seemed fixed like getting it to run reasonably smooth it started back firing. Ive had the light on it many times and never saw the mark jump or come back to idle other than smoothly back in place. I too havent ruled out the timing chain, but its a clyotes double roller though it has seen a few 6,300 rpm passes from when tbe truck woulx run reasonable. I'll probably suck it up and pull the timing cover off and check it at TDC. It's my first SBC build so I won't pass it off as being perfect since its a hotrod built to learn from. I have defenately learned alot from this whole experience.

 

[The Struggle]

You can check my engine oit on my misc picture ip loads under my profile name. I shoukd have two almbums worth of pics.

 

[The Struggle]

You can check my engine out on my misc picture my uploads under my profile name. I should have two almbums worth of pics maybe you can see if you see anything I may have missed on the engine side of things.

Thanks for the reply btw my issues always seem to be complex and few people ever want to reply around here.

 

[Rusty Nail]

Complex sbc timing issue?


Ask @bucket , he's real smart.

Seems to me that timing issues that suddenly arise in 35 year old trucks are the result of user error often fixed by undoing the last change. It ain't rock science and it sure as hell aint complex. Its a small block Chevrolet....

If its meant to be a learning experience, go learn. Start with the last change you made prior to the symptoms of complaint.
Does your '81 have a functional computer in it?

[yt]5s67p3X5HT0[/yt]

 

[The Struggle]

No the truck is still carbureted. Its been completly rebuilt by me. The following list is the build...

1981 K10 stepside
3/4 ton axles 4.10 gears
Th400 transmission , 2400 stall coverter and NP208 (new chain)
35" tires with 4"lift

Engine
350 bored .60 over (360)

Cam bearings
Freeze plugs
.60 over eagle rotating assembly, FM 4 valve relief -7 cc, hyperturetic pistons, cast crank and H-beam rods, wrappend in King's bearings.

Summit Vortec Aluminum 27 degree, 210cc intake port heads, 64cc cambers with .525 max lift springs
2.02/1.90 valves (hardened seats)/.040 head gaskets
Vortec rpm air gap intake with holley 650 vacuum secondaries
Compression ratio is 9.6:1
MSD streetfire distributor with vacuum lock out plate currently limited to 11-14 degrees on the can to keep cruise timing below 51 degrees total.
MSD Pro wiring and loom set.
4 bolt main ARP studs to include ARP head studs.
PRO-Street harmonic dampener
Cam: 224/234 @.050 adv duration.468int/.488exh
With 114 LSA topped with hardened pushrods, regular hydraulic lifters, polylock roller rockers and stud girdled. Also has a Clyotes adjustable true double roller timing set on it.

I rebuilt the engine and bought and designed it all myself with ample amount of research. I just need help sqaushing the bugs I've found is all. I've lived and breathed cars ever since i joined the Navy, but I just don't have as much time or money to be playing so many games every weekend is all. I have too much college training and Wifey and the kids are starting to get jealous.

 

[1987 GMC Jimmy]

You can check my engine out on my misc picture my uploads under my profile name. I should have two almbums worth of pics maybe you can see if you see anything I may have missed on the engine side of things.

Thanks for the reply btw my issues always seem to be complex and few people ever want to reply around here.

You're welcome. I looked through your pictures, and I couldn't see anything out of the ordinary. I do see where the timing was advanced a good amount between two pictures, but that's about it. Besides looking for a timing chain issue, it probably would serve you well to retrace your steps if you can think of any that might have made this worse. The reason I was was asking if you'd had these problems all along was to make sure you didn't stab your distributor in 180* backwards. Have you searched for any vacuum leaks in it by either spraying a combustible like starter fluid around areas that could leak or blowing smoke into the lines where they hook at the intake or carb to see if any leaks out? Intake manifold, carb gasket, brake booster, trans modulator, and whatever little accessories you have like the PCV, but the little things won't affect it that much. Have you also tested your vacuum advance to see if the diaphragm might have broken? You could try swapping to a manifold source and seeing if if the idle changes due to a vacuum leak there, or you could see if it changes using your timing light. That Holley you put on there... I'd look down the primaries to see if you get two good fuel streams when you actuate the throttle. It should start pushing fuel the moment you start levering on the throttle. If its reaction is sloppy, the accelerator pump diaphragm might be bad. Those are some easy things to try playing with. The timing chain would be the hardest to look at, but even though this stuff is new, I wouldn't rule out failure. You'll figure all the bugs out pretty soon.

 

[The Struggle]

I've found a leak with my fel-pro gasket set that had a steel core lining about 3 months ago. The gasket had indentions to assist the seal but it blew out between the intake gap of the 4th and 6th intake runners where a the heat crossover ports are made on the gasket which pushed down into the oil galley. I couldn't even tell the intake was leaking but it just wouldn't tune right and left off idle like it had a huge vacuum leak, which it did. I replaced it with the premium edelbrock thick gaskets with liberal amounts of gaskacinch and looks to have worked. I've sprayed all kinds of combustables and probably a whole can of carb cleaner on every orface I could find. I have a check valve I'm going to replace on the brake booster but seems to be good with the suck and blow on it test the booster and master cylinder are also new. The valve covers have one oil breather and one PCV to the carb but I've recently unplugged it and capped both ends for good measure. The th400 had a corvette adjustable vacuum shift diaphram at one time that hardened and popped off but has recently been fixed as of a month ago with new vacuum line. Things generally moved forward in these fixes but ALL bugs need to be fixed obviously. I have the th400 hook up to the manifold source at the front of the base plate of tbe carb. The dizzy is hooked at the metering block and the brakes on the rear manifold port. I'm bout to drive it to O'riellys to pick up the gaskets and rtv for the timing chain check. I'm going to add some timing before I go to 24 BTDC, keeping the vacuum canister unpluged and see if she backfires anywhere under load.

 

[The Struggle]

Took the truck out it liked being at 24 degees initial. I put it at 28 initial no change it mechanical and even better. It's more of a complete turn around now. I'll keep walking it up two degrees to see where it pings and back off two for final setting tomorrow. Then it'll be off to the vacuum canister. The exhaust droans less now and actually has sounds of back pressure when coming off a high rpm pulls. The carb back firing has also seased and throttled response has become more sensitive. It also pulls alot more vacuum now because it shifts faster at the transmissiin solenoid which now needs a few turns in on the hex screw. I purchased a Holley rebuild kit for fine tuning and a 3.5 and a 5.5 hg power valve for starters on the tuning but i know the 5.5 is stock setting. I believe the timing was so retard it was running very hot on the exhaust end also causing issues with back pressure at the carb from residual exhaust still in the cylinder (just my opinion). I did notice the headers had become hot pretty quick this morning too which to me indicates too retard IIRC. More to come tomorrow...

 

[Rusty Nail]

Do you fill it up with E85?

 

[The Struggle]

I havent found any E85 pumps out here in San Antonio lately but I'm running 87 in tank right now so when it pings it'll do it on the cheaper stuff. I havent really looked for the E85 pumps either. 'm usually putting 93 in it at all costs just to keep detonation away but figuring where I stand on it now fuel octane will be added into the equation once the timing and carb get pretty dialed in.

 

[The Struggle]

To finish this thread up I'll post my test results of what the truck likes after my test and tune.

Ignition:
28 degrees initial
5-8 limited vacuum at the canister
10 mechanical via medium sized fling weights and black springs.

Total timing: 38 degrees on 93 octane all in at 2,800 rpm.

Cruise total timing: 48 degrees

Weights and springs are out of a Mr. Gasket HEI recurve kit off the shelf at Orielly's.

 

[The Struggle]

Second update:

Rebuilt my Holley street 650cfm carb, readjusted floats, added all new gaskets and straightened the main throttle blades as they where slightly twisted enough and keep the throttle blades open and affect the idle circuit. Truck will start and run easily and performs well. Also adjusted the vacuum secondary pump from a holley black spring to the 4160 vacuum secondary kits tallest silver (non-colored) spring.