Cascade Crew

[aaron_sK]

So I've been fighting a problem with the timing on the Merc engine. It seems to love a gang of advance at idle. It wont start with less than 20*, and seems to pull the highest idle vacuum at around 40* BTDC. I've checked and rechecked the marks on the balancer, the wires, the firing order, and everything seems perfect.

The problem is when I set it the initial that high it pings like crazy any time you get past part throttle with a load on it. But when I pull timing out the idle goes down the toilet.

I've been running my vacuum advance canister off the ported vacuum on my Holley (like they recommend). I tried switching to manifold vacuum, and pulling out about 8* of mechanical, figuring that the idle vacuum would bump it up to compensate. Sure enough it idled great, had much better throttle response, and hardly pinged even at WOT.

I understand the basic idea behind ported vacuum (doesn't pull till primaries open) but why does Holley recommend so strongly that you only run your advance off of that port? Am I just throwing a band-aid on a more serious problem, or is this a viable fix?

[Twilightoptics]

I like to run my advance off manifold as well.

Ported starts adding the advance when the blades open, but also gets to a point where there is no vacuum just like manifold.

You need to get a piston stop, and use it to verify the balancer marks line up at TDC. To ensure the balancer has not slipped.

Should start just fine with 0-10º timing. With a carb you gotta run the idle circuit screws in conjuction with the timing. If you need that much timing and it's a relatively stock motor at idle, it sounds like you're running kinda rich there at idle.


Give the motor what it wants, not what the book says.

[aaron_sK]

I have checked the balancer mark with a dead-stop made out of a busted spark plug and a pencil, and it checked out. I have a true screw-in stop coming with my Summit order today, and I'll check it again.

I know it is running rich at idle. I kinda figured I would set the timing first, then fine-tune the carb. Maybe I'm thinking about it backwards.

*Edit: BTW Paul, assuming that the balancer checks out, could a timing chain being installed a few notches off, or a cam being degreed (or installed advanced) throw off the timing like that? I didn't build this short block, and don't really trust the shop that did.

[Twilightoptics]

Advancing the cam or retarding the cam a few degrees is only going to make it act like a larger or smaller cam.

It still shouldn't need 40º at idle.

You want about 36-38ºAfter 3k, no vacuum advance. So typically you'd set that with a dial back light or timing tape. Then adjust your vacuum advance according to what the motor likes better. Some dizzys have adjustable vacuum pods that you stick an allan wrench into the vacuum port and can raise or lower the advance it gives the dizzy.

[rjmcgee]

The adjustable vacume cans I have used don't work that way. The little allen screw only changes how fast or slow the can pulls in timing. There is a little ecentric(sp) deal that goes in the distributor and limits how far the arm out of the vacume can will travel. That is how you limit the amount of vacume advance.

Unless you want to modify the amount of mechanical advance in the distributor all you can really do is unhook the vacume advance and set your 36*-40* of timing with the RPM's high enough that all of the mechanical advance is in. Don't worry about base timing. Then hook your vacume back up to manifold vacume, not ported, and you should see around 30*- 40* at idle.

[chevymad]

Sounds like these guys have the right method for getting the timing correct. On your question about what happens if the cam is off a tooth. I did this once in my 396. I believe the cam was 1 tooth retarded, which moves the rpm range up top. The engine would not run at all with less then 20* of advance. Even revved up if I turned the distributor below 20 it just died. It too liked 30-40* at idle. Ran like a raped ape up top.

[GREG DAVIDSON]

i dont clame to know much i may sound dumb but in the built performance engines you need to run premium fuel or else it pings depending on piston type ...??? well thats what i was under the understanding of

[aaron_sK]

Brandon, that sounds like exactly the same issue that I'm having. Except that it doesn't run like any type of ape anywhere in the band.

I put the cylinder stop on it and verified the timing marks on the balancer as correct. I put new timing tape on it and started it up. At 1000rpm (cold idle) the timing was 30* with no vacuum, and 55* with vacuum. When I open the throttle up it goes way off the tape, somewhere in the 65-70* range.

Could this be a timing chain problem? Installed wrong, or jumped a tooth before the engine was broken in or some BS?

[Twilightoptics]

25º is ALOT of advance to be coming from the vacuum.

Do as Rod and I said. Set the total timing to 36º. Then just leave the vacuum off for now and see how it runs.

[Twilightoptics]