Timing Issues, I Think Help!

abcjm4 · October 26, 2007

I think my 1985 C30 454 is having timing issues. I was having overheating problems and found out my radiator was plugged and was too small for the 454. During the overheating problems I checked the timing and adjusted it to 4 degrees with vac adv disconnected. After that setting my truck is now dieseling and when I floor it, the truck stumbles. It sounds like a intake backfire when it stumbles. Is this 4 deg the correct setting? And if it is how should I trouble shoot from here?

Nytemare · October 27, 2007

did you adjust it to 4* BTDC or ATDC??

abcjm4 · October 29, 2007

did you adjust it to 4* BTDC or ATDC??

4* ATDC

Nytemare · October 29, 2007

I'd say bring it back to 0 ...Sounds like it was working before.

I Believe that these engines worked better with a 2* advance....ie: 2*BTDC

But If you do a lot of highway...1*ATDC is what I would do....

4 is too much by the sound of it. If you think about it...4* of crank rotation is a lot...you have 8 pistons...each one out by 45*

now you take that 45*, that has a 4"stroke so it travels 1 "/11.25*

you removed 4* so that means you made the plug fire when the piston is already .356" down the cylinder. That negates the compression stroke, its actually on the power stroke with no power.

Do this for all 8 cylinders, and thats something noticeable. You've basically taken away 2.84" of stroke....so you have a 413CI engine.

Colorado Kid · October 30, 2007

Nothing personal Nytemare, but you certainly use some "new math".

First of all the crank turns 720 degrees to fire all 8 cylinders.

Second, it has to turn 180 degrees to take any individual cylinder from TDC to BDC.

Third, the relationship between the degrees of crank rotation and the displacement of the pistson from TDC is not linear, it's sinusoidal. The piston doesn't travel 1 inch per 11.25 degrees, it does travel 8 inches every 360 degrees but during any given 36 degrees it may move more or less than 0.8 inches. Around TDC and BDC it's moving much less than .08 inches per 3.6 degrees. Infact, without a precision measuring device you'd be hard pressed to detect ANY downward motion of the piston between TDC and 4 degrees ATDC.

Then there's the vacuum advance . . .timing is set with it disconnected, but the engine runs with it connected, so even with the timing set at 4 degrees after top dead center the plug is firing sometime before TDC.

I don't know what spec is for an '85 454, but I'd set the timing to specs using a timing light, and then I'd fudge it a couple degrees ahead and listen for pinging.

As a general rule retarded timing results is smooth running, low power, poor fuel mileage, slow throttle response and loads of heat, especially on the exhaust valve and cooling system. Over advance results in pinging, hard cranking, backfiring (intake side) and broken pistons. Strangely enough either one can cause dieseling . . . retarded causes carbon and the exhaust valve to glow, sufficient to ignite the charge and advanced overheats the plug, with the same result. Sometimes over advanced will cause the engine to diesel backwards. (Of course the engine quickly runs out of burnable charge when it's turning the wrong way, but this can start an impressive carb fire if you have the air cleaner removed.)

If you have been experiencing intake backfires you probably won't get it to run right until you rebuild the carb . . . parts have probably been bent, especially the "power" (enrichment) valve.

Nytemare · October 30, 2007

Yup...I'm an idiot....Thought about it after I shut the computer down last night...and I just turned it on today...LOL

Either way...4* ATDC is way too much retardation.

I'd go with 0* or 1-2 BTCD

abcjm4 · October 31, 2007

Thanks guys for your help!!! If I have time this weekend I'll try the timing and if that doesn't work I'll try the carb. And hopefully one of the two will work! This is my first non-computer controlled big block, I just don't want make any "stupid" mistakes or miss the obvious.

Colorado Kid: I did have a near miss with the intake back fire. It blew fuel back through carb but with no fire!! I guess lady luck was on my side that day....

Nytemare · October 31, 2007

I'd say man....I had a carb fire once...had no eyebrows for 2 weeks...LOL My fault though, I was pouring fuel in the carb to get an old engine going...

Could easily lose sight in your eyes or leave 2nd degree burns to your face though, I've seen this stuff happen.

abcjm4 · January 5, 2008

To eliminate any hidden issues with the ignition I replaced wires, plugs, and HEI. Re-timed it and started and ran good. It still falls on its face when you step on it, but I am assuming it needs a carb rebuild and hopefully that will go away.

It stopped dieseling when you turn off the ignition, now it only does it when the engine gets hot. What could be causing this issue?

Sorry it took so long to reply I had an unplanned surgery and just got back to where I can turn a wrench!!!