ZZ502 Engine Backfire

[VAroute66]

I have an interesting problem. I bought a ZZ502 off of EBay, and it came on a test stand so I could run it. It ran fine. I had to change out the oil pan so I stuck in a gear drive. I put it back together, and when I try to start it, it backfires through the carburetor, and will not start. I started by looking at the obvious. Did I install the gear drive wrong? I line up the engine at number one TDC (cam mark at the 0600 position, and crank mark at the 1200), and it lined up fine with large idler gear to the left and small to the right. The position of the crank never changed relative to the cam so I'll dismiss for now plus the last thing I want to do it get it back on the engine stand. The distributor is set to fire at 8 deg. before TDC. I have an electric fuel pump. I do not have a fuel pressure gage on it. I have a Mallory unilite distributor. On the stand I was using a battery that was a little weak. The HOlley carb has an electric choke that was almost fully closed. I definitely had fuel, but maybe weak fuel pressure. To see if I had accidentally set the distributor 180 deg. out, I set it 180 deg. out. The engine fired on several cylinders but did not start. This is what I would have expected being 180 deg. out, so I reset it back. It continued to backfire. I looked at the valves and number 1 had both exhaust and intake closed as I thought it should be. Even if the fuel pressure were low or spark a little weak, it did fire nicely 180 deg. out on the cylinders that did fire. Could the fuel air mixture cause misfiring through the carburetor? I keep coming back to timing where I dread going. Anybody have any ideas before I go back into this engine and "fix it" some more? I installed a gear drive in my small block without a hitch and cards with clothes pins on my bicycle wheels when I was a kid with no problem. THanks for any help or advice.

V/R
VAroute66

 

[JohnZ]

I line up the engine at number one TDC (cam mark at the 0600 position, and crank mark at the 1200), and it lined up fine with large idler gear to the left and small to the right. The position of the crank never changed relative to the cam so I'll dismiss for now plus the last thing I want to do it get it back on the engine stand. The distributor is set to fire at 8 deg. before TDC. VAroute66

With the cam and crank gear at "dot-to-dot", the engine is at TDC on the compression stroke on #6, not on #1.

 

[lars]

That's correct: You need to install the distributor in the #1 firing position with the dots on both gears pointing straight up. You're currently 180 out.

Lars

 

[JohnZ]

Here's the diagram they SHOULD HAVE put in the Service Manuals, but never did; after installing the timing set dot-to-dot, you have to rotate the crankshaft one full revolution (which turns the camshaft one-half a revolution) so both dots are at 12 o'clock in order to be at #1 TDC on the compression stroke. The dot-to-dot arrangement is simply easier to see than if both dots are at 12 o'clock.

 

[VAroute66]

JohnZ, Lars,

Thanks. I'll try again 180 deg. from where I was. All the manuals I have tell you to line up dot to dot with #1 TDC. So if I am set up 8 deg. before that, I thought that's when I would want it to fire. I set up my distributor that way. Thanks again!

V/R
VAroute66

 

[lars]

Dot-to-dot does put #1 at TDC. But it's TDC on the exhaust stroke. The dot-to-dot is for ease of engine assembly - not for distributor installation. It is assumed that the engine will be rotated to the firing position before installing the distributor, but it really doesn't say that anywhere...

Lars

 

[BigJimZ16]

Aw screw it... cut off the round pin under the rotor and force the square peg into the round hole, put the distibutor cap back on and run it till the next time it's apart!

That's how I fixed my first booboo at Englishtown in '75. Ran great too...:L

 

[VAroute66]

Lars,

Thanks. I needed the other TDC. Now I need to figure out why it would'nt start when I did set it 180 deg. out.

V/R
VAroute66