Ok, another nightmare is upon me...

Trying to tune (and therefore time) my '72 454. Now, I didn't do the work to this car, so I don't know what parts are correct or not.

The car is a '72, original 454/4-spd. I had a local Corvette shop install a GMPP crate motor (425hp, 465ftlbs, HO, mechanical fuel pump boss, rectangle port heads, etc) and I don't know if they used the damper that came on the motor or my original damper. I guess I could look at the stock/factory motor next time I'm at my mom's house (storing it there) and see if it has the damper on it still.

So, there seems to be two pointers on this engine... One at the bottom, cast into the front cover (can tell it is a "late model" engine, as there is a boss for a crank sensor) and one pointer up top (not sure if it is built into the front cover or not...) between the water pump pulley and the PS pulley.

Well, I had marked the large line (figured 0*) with a white pencil and we were going off of that. This GMPP motor spec sheet calls for 4*BTDC and we were dialing that in. Took a closer look at the damper and it is at 6* where the large mark is. First mistake.

So, we keep timing it and did the screwdriver in #1 on compression stroke to feel the piston and marked the damper where it was lined up with the timing mark.

Set the timing. But now the motor starts to kick backwards, which leads me (with my limited carb/dist knowledge) to think that it is way too advanced, as the ignition must be trying to ignite before the piston is all the way up, trying to run backwards.

Also, when we did get it running and idling "ok" when we shut it off, it would diesel really bad and sometimes cough out the carb.

The guy helping me tune seems to really know what he is doing and used to campaign a few 8 and 9 second drag cars with carbs/dist and nitrous. He is baffled with this situation... And I am lost as usual now that I am working on this thing.

I wish I had done all the work to this car from the time I had it, but I didn't have the mechanical experience that I have now...

 

You can't get a TDC indication accurate enough with the screwdriver. Get a positive TDC stop/check tool or build one. Then, get TDC set up and verified within 1 degree and make sure your readings are repeatable. Once you have TDC accurately verified, you need to set up total timing to 36 degrees and verify your centrifuigal advance curve. The engine should not be kicking back at any initial timing below 20 degrees, and a good initial timing target is in the mid-teens. But you have to get that TDC verified accurately in order to do any timing checks.

 

My personal experience with my motor build is that the recommended initial 4-6* BTDC is crap. It is better, IMHO, to over advance it by 6-8* initially, for an initial between 10-12*, and then bring it down after break in than to try the other way. As Lars already stated, get the full in set and then work back from there on your advance curve. The man knows his business.

 

The only reason we used the thumb over the spark plug hole and the screwdriver method is because I don't have a dial indicator. When I get exhaust on the car and take it over to a friends house, we are going to remove the intake and replace the gaskets (in case of a vacumn leak, and he says they have cast iron manifold gaskets on the alluminum manifold anyways...) and use his dial indicator to find true TDC and then mark that.

The engine is "broken in" already... Has about 500-800 miles on it probably by now. But who knows if the timing was ever set correctly with this hodge podge of parts...

Thanks for the help.

 

How are you going to get the dial indicatoer on top of the piston. You need to get a piston stop tool to do this with the heads on the motor. The dial indicater will not work with the heads on.

 

Exactly.

 

i have seen shaft extensions for dial indicators.

they can even be fabricated fairly easily.

wouldn't a dial indicator work fine then?

how does this special tool you are recomending do the job?

just curious.........

 

Most dial indicaters are designed to be used at a 90 degree angle to get a proper reading. The angle you have to go into the hole is so severe you would not get a good reading. A piston stop is just that. It does not allow the piston to come all the way up. You bring the piston up to where it stops and mark the balancer. Rotate the motor the other way until it stops again. Mark the balancer. Exactly 1/2 between the two marks is going to be your top dead center.

 

very cool. thanks, i did not know that.....

engines are something i am still learning about.

 

You can't find TDC through the valvetrain?

I do have a piston stop tool, but it is the wrong thread pitch...

And turning this motor over by hand is a MAJOR undertaking. I guess I could remove all the spark plugs to make that easier though.

 

No. You need to do it with the pistons.

 

This is crude but effective way to find TDC. I use a 18 gauge piece of wire and shove it in the spark plug hole. I then turnthe engine over until I can no longer easily pull the wire in and out of the hole. Mark on the balancer. Then keep turning until you can pull the wire. Mark the balancer again. Half way in between is TDC. Cheap method too, wire costs like $0.01.