Timing at 25 degrees advanced - harmonic balancer slipped?

[Loww]

Hello,
I have a '79 GMC K2500 with a stock 350 SBC engine. The engine has been running ok since we bought the truck 5 years ago. However out of interest I checked the ignition timing yesterday with a light gun and found, that it was approx 25 degrees (!) in advance on idle with the vacuum of the distributor removed and plugged. The marks on the cover only go up to 12, but the mark on the harmonic balancer was way before that 12 mark under the light gun.
So I retarded the timing to 12 degrees and did a testdrive with my little son. We both noticed that the enging was running subjectively smoother and with less noise than before. I did not notice a reduction in engine power.

Do you think that the engine really ran with 25 degrees advanced ignition on idle (for a couple of years now) without getting damaged. Or is a possible, that the mark on the balancer has somehow slipped and the timing was set correctly - which would mean, my adjustment took the timing after TDC now?

Unfortunately I cannot find TDC of Cylinder 1, because I do not have the tools (piston stop) and they are hard to get here in Germany.

What would you do? Leave it as it is now or go back to indicated 25 degrees before?

Thank you very much!

Dominic

 

[PrairieDrifter]

Hello,
I have a '79 GMC K2500 with a stock 350 SBC engine. The engine has been running ok since we bought the truck 5 years ago. However out of interest I checked the ignition timing yesterday with a light gun and found, that it was approx 25 degrees (!) in advance on idle with the vacuum of the distributor removed and plugged. The marks on the cover only go up to 12, but the mark on the harmonic balancer was way before that 12 mark under the light gun.
So I retarded the timing to 12 degrees and did a testdrive with my little son. We both noticed that the enging was running subjectively smoother and with less noise than before. I did not notice a reduction in engine power.

Do you think that the engine really ran with 25 degrees advanced ignition on idle (for a couple of years now) without getting damaged. Or is a possible, that the mark on the balancer has somehow slipped and the timing was set correctly - which would mean, my adjustment took the timing after TDC now?

Unfortunately I cannot find TDC of Cylinder 1, because I do not have the tools (piston stop) and they are hard to get here in Germany.

What would you do? Leave it as it is now or go back to indicated 25 degrees before?

Thank you very much!

Dominic

I had a bad timing light once, had really really high timing. Small block Chebby didn't give a ****. It started pinging once it got cold that's why I looked into it but I ran it for a GOOD while like that. It also never was angry on the starter, I attribute that to being wore out most likely.

Find top dead center on the compression stroke of #1 and take a marker and mark zero on the balancer where ever it lines up with the tab. That's the mark you should time with. If the balancer is truly slipped I would replace it, don't want it coming apart.

 

[Loww]

Here is a photo of the balancer. Compared to photos of other original balancers on the internet the mark seems to be correctly aligned with the inner holes...

 

[75gmck25]

It’s going to be hard if you can’t verify TDC. There are are at least two different timing pointers used on the SBC (12 o’clock and 2 o’clock), and the original pointer is tack welded to the timing cover. Someone may have swapped the timing cover or the balancer and now they don’t match.

I’ve never tried this, but several articles explain how to make a piston stop from an old spark plug. You break off the porcelain top and drill out the center. Then thread it inside for a long bolt. From there, you can screw the bolt in and out and it serves as a piston stop.

Another simple check is to set the idle very low, and watch the timing as you rev up the engine. It should go to its lowest point at about 500-600 rpm, and then add about 18-20 degrees as the rpm increases. It should max at about 3000 rpm. The mechanism inside the distributor may be stuck or the spring may be allowing it to advance too quickly.

 

[PrairieDrifter]

Here is a photo of the balancer. Compared to photos of other original balancers on the internet the mark seems to be correctly aligned with the inner holes...

Find top dead center. Mark the balancer.

Doesn't matter what timing tab, doesn't matter if the balancer has slipped. Top dead center, mark it, time it.

It's not rocket appliances. Precision is unnecessary.

 

[fast 99]

Here is a photo of the balancer. Compared to photos of other original balancers on the internet the mark seems to be correctly aligned with the inner holes...

That doesn't tell you anything. Need to lay 2 dampers on top of each other with key way aligned.

Yes the engine will run at 25 degrees although at reduced power and increased fuel use.

There are 2 advances vacuum which you checked and mechanical. To check mechanical see if the distributor advances with vacuum disconnected or remove the cap and try twisting the rotor. it should move free.

To get close can remove either #1 or #6 plug and insert a long small screwdriver, rotate engine by hand, bring piston to TDC.

A base total advance starting point should be in the area of 38-40 degrees. Adjust the base timing to achieve those numbers.

 

[Ricko1966]

Rotate engine clockwise only to line-up TDC mark,do not rotate counter clockwise at all to line up. With the marks lined up remove the distributor cap. Now rotate counter clockwise until the rotor just starts to move. Look at your timing marks. How many degrees did it move,that is slack in the timing chain, more than 8 degrees,that's what I'd be concerned about. You've been given a bunch of good advice. As for total timing as @fast 99 said that's the best way to set timing IMHO I use different number and method but we are close. If I'm not going to do any testing afterwards I typically use 34-36 degrees. If I'm being picky I start at 30* and check trap speed between two land marks bumping timing 2 degrees at a time until highest trap speed is reached,it will peak,plateau and then start to drop. You want the lowest setting that gives the highest trap speed. MPH not ET. Pick out 2 land marks that will let you get to about 60. Best trap speed is what the engine wants for timing.

 

[fast 99]

bumping timing 2 degrees at a time until highest trap speed is reached

I generally use the SB for towing not to embarrass myself with a 75 mph trap speed.
Lets race motorhomes next. lol

 

[Ricko1966]

I generally use the SB for towing not to embarrass myself with a 75 mph trap speed.
Lets race motorhomes next. lol

I'll bite,mines 454 TBI whatcho got? Lol.

 

[fast 99]

I'll bite,mines 454 TBI whatcho got? Lol.

Ok, really want to go there?

 

[fast 99]

You win

 

[Ricko1966]

You win

Mines a 36 foot Fleetwood. You ve got the power to weight in this deal. I how ever have better traction .

 

[YakkoWarner]

You can find TDC pretty close even without a piston stop - I do it all the time using nothing more than a wrench/socket that fits the crank pulley and a high-tech chopstick from my favorite Chinese eatery. All you are trying to do is feel when the piston stops coming up and starts going down. If you're trying to verify your balancer mark having absolute precision isn't as big a deal, and it won't matter whether you are on compression or exhaust stroke.

I use a chopstick because it is soft bamboo, and won't scratch the cylinder wall like a steel screwdriver might if it slips. Just pull the #1 plug, stick the chopstick in until you feel the piston and rotate the engine in the same direction as it would normally run. Find where the piston stops at the top and you've got TDC (could possible be a degree or 2 off but close enough to see if your timing mark is near 0 on the marks or way off to the side).

 

[ByrdDog]

You can get pretty close holding your thumb over the plug hole while somebody rotates the engine by hand & just feel for when the air stops pushing out. Just helped a guy time a 383 with no timing pointer doing this.

 

[Camar068]

on this subject, are there differences in timing covers/marks? Or could it be balancer? When I was a young pup I did the same as he did and it screwed things up. The fix was to have an old school mechanic tune it by ear with brake applied.

Sorry to hijack a bit, but going by the timing light and the book screwed me. Was stock 350 as far as know.