Today's mystery question!

See if you can catch what happened. Wanted to set point gap, so put the car in 2nd gear and slowly rolled the car forward and backward, until points were on highest point of cam lobe.

Set points to correct gap.

Started engine, but got bad "klunk" sound immediately on first plug firing.

Checked timing chain alignment with cam bearing and crankshaft. All correct.

But timing light shows plugs now firing 30 degrees advance - not the TDC I had before I reset the point gap.

What probably caused the change to such a big advance?

Mickey - you might be interested in this.

Tom

So, do you already know what it was?

I've gone back over this in my pea-brain several times since we reset the points, and the only thing I can come up with is that the advance mechanism (spring?) shifted somehow.

"So, do you already know what it was?"

I *think* I know ... but not sure until I get my hands dirty.

After setting the points with the feeler gauge, we didn't check the timing. Engine started and ran great - no need to check timing.

But ... next morning on first start (Harriman TN was freezing!), big KLUNK and starter pinion grinding sound. Cranked again, same thing. Third time, engine fired up and ran perfectly all day. But I had a ping on upgrades all the way home. And every morning when cold, starting caused the same "klunk" and starter grind sound. Once started, engine would run fine, except for the ping on load.

At home, I pulled the valve cover to check alignment of timing chain, cam bearing, and crankshaft timing disc/arrow. Pretty much perfect. But use a timing light and see the plugs firing 30 degrees advance. Distributor mechanical and vacuum advance working perfectly. But points open about 30 degrees early.

Remember - we put the car in 2nd gear, then moved it back and forth to get the points on the lobe of the cam.

There's the clue. Are you thinking what I'm thinking?

Tom

NutmegCT said:

Remember - we put the car in 2nd gear, then moved it back and forth to get the points on the lobe of the cam.

There's the clue. Are you thinking what I'm thinking?

Tom

Click to expand...

Not sure. Both the klunking and the pinging under load indicate too much advance. But even though we rocked it to line up the lobe, and it klunked, after pulling the cap and re-checking everything, it fired immediately. :confuse:

I'll be standing by...

I think the timing chain slipped.

The 50+ year old timing chain was slightly stretched. I found this out when I tested for tautness yesterday, pushing with fingers like testing a fan belt. The "power" side of the chain was "soft" and could be pushed in about 1/4".

So ... I think ... when we had the car in 2nd, and rolled it *backwards* a bit, the chain may have popped over a tooth on the sprocket that also runs the distributor and oil pump.

Interesting coincidence: that sprocket has 12 teeth. 360/12= 30 degrees!

No guarantee this is the cause of the 30 degree spark advance; won't know until I pull it apart. But when you line up the chain alignment mark with the cam sprocket alignment mark, and see the crankshaft nearly spot on - the "jumped" tooth at the distributor seems the only explanation for the 30 degrees advance spark. There's no way to rotate that distributor itself 30 degrees - interference with plug #1 and fine adjuster. I think the only way to correct is by loosening a chain sprocket and moving the chain one tooth, or removing distributor and moving the sprocket one tooth.

I think

Could be.

But I wonder why it started so easily after we checked it. And it did that a couple times. Seems to me that if it were 30 degrees out, it would have bogged every time.

~The plot thickens...~

The plot thickens indeed!

The only difference in starting with, and starting without, the "klunk" - is that the klunk only happens after car sits overnight or cools off.

Engine started immediately at your house - but engine was warm already. Started easily that afternoon when i bought gas heading toward Harriman after interstate driving.

But next morning at the motel, I got the klunk several times before it started. Then it ran (and re-started) fine all day. Next day - same thing. Klunk at the first crank - then ok rest of day.

Yesterday I disconnected the vacuum advance, plugged the vacuum line, and it started immediately. Ran fine - but I had the timing light so I checked it and saw the spark was at 30 degrees advance. Fully retarded the distributor (max amount adjustable) and saw the spark move to 20 degrees advance. All measured at idle speed. Still way too early.

I'm betting that when engine is cold, it turns slower at first crank (thick oil). The mechanical advance, and vacuum advance, added to the already way too high distributor advance, make the spark occur at the wrong point (piston down, not up). Once it's going, and/or once it's warm, there's less resistance, and it spins smoothly (despite the still too advanced spark). Note there's now a "ping" in 4th gear, when engine is under load. Never had that before either.

The clouds of doubt begin to rise.

Fine adjustment for dist. timing:

View attachment 29866

When you set the points you had rocked the car car with the wrong engine rotation. The slack in the timing chaine caused the points to be set at the wrong timing setting.

I'm with Dar on this.

Do you have the vacuum line connected to the wrong vacuum port?

Might be getting full vacuum at cranking and advancing the timing improperly.

I believe the problem was resolved - see this thread, post # 10:

https://www.britishcarforum.com/bcf/showthread.php?93559-ballast-resistor

Sorry - there's another thread where I posted the final results. Basically, the cam follower (points) had worn so much, that resetting the gap caused the timing to be way off. I replaced the points with new, then reset the timing (2 degrees BTDC static), then carefully adjusted the timing and points until I have 8 BTDC at idle, 15 at 1500 rpm, 30 at 3000 rpm (give or take) per spec.

Engine now starts immediately even in 0 degree weather (full choke), idles smoothly at 800 rpm when warm, and revs with full power on the road.

Victory!
Thanks.
Tom

Darwin said:

When you set the points you had rocked the car car with the wrong engine rotation. The slack in the timing chaine caused the points to be set at the wrong timing setting.

Click to expand...

He set the gap at maximum lift - it doesn't matter how he got there, backward or forward.

The thing is, after setting the gap, normal best practice is to reset the timing, but he doesn't say he did that, nor how. When you do that, you must be on the compression stroke with correct rotation.

Edit - just read the conclusion which illustrates perfectly why you should check and reset the timing after you adjust the points gap.