Ignition timing troubles.

[xsdg]

Mike: There is an '86.5 FSM, and that one jumps to the Z24i.  The '86 FSM is mostly identical to the '85, and covers the Z20, Z24, and SD25.

 

As for the timing difference between the service manuals and the motor specs, I have no idea.

[datzenmike]

Anything is possible. Not trying to say right or wrong here. News to me is all. Around the late '60s carburetors were converted from intake vacuum to run on ported vacuum advance for the distributors. Effectively the retarded timing at idle (about 12 degrees on a L series) reduced emissions. Above idle the vacuum is restored for better economy and power.

 

Engines crave more advance with less cylinder filling like at idle. The L series would like 15 degrees or more at idle but is artificially lowered to static timing levels.

 

True, this is a NAPS low pollution motor so maybe they do run some slight vacuum advance? I'm researching this. Interesting though.

[Robert]

So. The lash was correct. The timing is correct at 3°. I'm getting spark. Dizzy looks fine. Did a compression test...

1- 125

2- 0

3-132

4-120

Looks like I have a bent valve. Only way I can think of this happening was if it was mashed when I torqued the head down. Being that the head went straight on without rocking or any odd contact kinda confuses me. Everything was in the right place.

 

Or set on a flat surface while the camshaft was installed. I have seen many heads get bent vavles this way.

 

if engine was set at TDC beofer putting on the head you can bend valves in cyl 1 and 4 only. IF you put the head on then searched for TDC, you will bend valves in cyl 2 and 3.

 

 

So, if you hade the engine and camshaft at TDC before installing then the valve was probably bent in transpoprt. If you put the head on and then turned over the engine OR camshaft OR both then you bent the valve then.

[Robert]

Yes but he said an '86 manual. I believe all 720 Z24i motors were throttle body after April '85 making all '86s TBI. (except the Z20 and diesel)

 

 

Z24i was only an option in 86

 

Z24 ran to model end.

 

As for the spark advance, the Z24i uses a crank sensor in the distributor (like the KA24DE) and had no vacuum advance on the distributo....it was applied by the ECU and could not run right without a vacuum signal. Timing was then done at 12 degrees without disconnecting any vacuum lines.

 

With a carburetted model, the vacuum advance was done mechanically and located on the distributor. The line could be disconnected and timed without disrupting the fuel delivery like in the Z24i engines. BOTH used 3 degrees while under load. You just cannot simulate a "load" situation with a EFI vehicle without it also adding more fuel.

[Robert]

Oh, BTW, you CAN bend a valve back while the head is still on and nothing taken apart. It is a very simple procedure unless the valve is seriously bent. A slight bend while installing is ok. If a timing chain breaks, then head removal is a must. If you fail to bend the valve back, then you have to remove the head any way and no time is lost.

 

 

here is how I have done it. Find out which valve is bent using an air compressor through the spark plug hole attached to a compression testing hose. (Usually always the intake valve). Remove the valve cover, turn the valve exactly 180 degrees in the head, put that cylinder at TDC, then losen the cam sprocket and slip the gear off the pin. Back the crank off about 45 degrees, turn the camshaft until that valve is compressed, the carefully and controllably turn the crank shaft to bend the valve back.

 

Skill and experience tells you how much to do it. Do 1-2 degrees and then check leakage. Keep doing until its sealed.

 

Of course this is not found in the chiltons. :)

 

Do at you own risk.

 

No, it doesnt ruin the piston. If it happends while driving, you will damage them.