Timing & Vibration

[gene8084]

Mercruiser, Ford 302, Motorcraft - Non vacuum advance distributor.
Idle timing set to 10 degrees BTDC per manual.

1. When I set the timing, bolt down nut loose the engine sounds great (idle and throttle advanced).
2. When I tighten the bolt down it sounds rough at idle. (I confirmed the timing is still dead on 10 Degrees BTDC.)
3. When I advance the throttle (1900-2000 RPM) the engine vibrates. Timing appears to advance smoothly well past the timing marks.

Notes:

• I set the point gap to .017 per manual (feeler gage). I will check the dwell with the meter, but didn't think this could be the cause.
• Confirmed wires are connected in the correct firing order.

Any thoughts?

 

[Haut Medoc]

Re: Timing & Vibration

Well, you do need to check the dwell.....
When you say dead on, it does not vary at all?.....
Have you ever checked the compression?....
IE are all of the cylinders within spec?....​

 

[gene8084]

Re: Timing & Vibration

Well, you do need to check the dwell.....

ok, will do. historically I've relied on feeler and done pretty well, but I did pick up a meter, so I'm prepared to check it.

When you say dead on, it does not vary at all?.....

yes, the timing is still good after I tighten the bolt...it's certainly not moving enough to change the performance as much as I'm hearing. I also tweaked in both directions and tried tightening to see if there was a change from slighly off to better after tighening.

[
Have you ever checked the compression?....

Yes. Tested cold. I have 1 lower @ 90 psi, 5 @ 120, 1 @ 124, and 1 @ 140.

I previously had this running smoothly so I don't think the compression is the problem.

So, what's changed?
I replaced plugs, wires, coil, points, condenser, had starter and alternator rebuilt, and rebuilt the carb.

Most significant relevent change was rotating the distributor. The previous owner had it installed 180 degrees from correct.

[

IE are all of the cylinders within spec?....​

Previous posters (a different thread) felt the compression should not be an issue even though one was a bit low and one a bit higher.

 

[a70eliminator]

Re: Timing & Vibration

Gene that sure is a strange one, It's sounds like when you ground the distributor to the block (tighten the bolt) it grounds out a circuit somewhere. If you think about it the distributor has the oring seal that would prevent a good ground without the hold down clamp, even though the gear is touching the cam thats all internal to the ungrounded distributor, I'm at a loss right now but someone may follow up with another idea. You are feeding the distributor with the negative side of the coil right?

 

[Haut Medoc]

Re: Timing & Vibration

ok, will do. historically I've relied on feeler and done pretty well, but I did pick up a meter, so I'm prepared to check it.


yes, the timing is still good after I tighten the bolt...it's certainly not moving enough to change the performance as much as I'm hearing. I also tweaked in both directions and tried tightening to see if there was a change from slighly off to better after tighening.


Yes. Tested cold. I have 1 lower @ 90 psi, 5 @ 120, 1 @ 124, and 1 @ 140.
Well, I think a 50# variation is alot, but I don't think it would cause serious vibration unless under load.....
I previously had this running smoothly so I don't think the compression is the problem.

So, what's changed?
I replaced plugs, wires, coil, points, condenser, had starter and alternator rebuilt, and rebuilt the carb.

Most significant relevent change was rotating the distributor. The previous owner had it installed 180 degrees from correct.


Previous posters (a different thread) felt the compression should not be an issue even though one was a bit low and one a bit higher.

What do you mean it was 180 out?.....
It wouldn't run....
As long as the rotor is pointing to the #1 wire with the #1 cylinder @ the top of the compression stroke, (TDC), the relative position of the distributer should not matter.....
So, I'm confused.....
You are saying that before you turned it, it ran fine & you just rearanged the wires , say #1 was in the back & is now in the front?.....
& it only gets uneven when you tighten down the bolt in the new position?.....

 

[mkast]

Re: Timing & Vibration

Total hours on the distributor?
Bearings in the distributor, cracked distributor housing?
Is there a home made hold down bracket?
Rough idle, as in a cylinder miss firing?
I want to ask you to pull the distributor and check for a bent distributor shaft.

 

[Haut Medoc]

Re: Timing & Vibration

Total hours on the distributor?
Bearings in the distributor, cracked distributor housing?
Is there a home made hold down bracket?
Rough idle, as in a cylinder miss firing?
I want to ask you to pull the distributor and check for a bent distributor shaft.

That's kind of where I was going.......

 

[gene8084]

Re: Timing & Vibration

Gene that sure is a strange one, It's sounds like when you ground the distributor to the block (tighten the bolt) it grounds out a circuit somewhere. If you think about it the distributor has the oring seal that would prevent a good ground without the hold down clamp, even though the gear is touching the cam thats all internal to the ungrounded distributor, I'm at a loss right now but someone may follow up with another idea. You are feeding the distributor with the negative side of the coil right?

Leave it to me to find another strange one. I have to tell you a70eliminator I'm getting an education.

Not sure about this, "...the o-ring seal that would prevent a good ground without the hold down clamp..." where's this o-ring?

If the distributor is grounded by bolting down...how is it running smoothly without being thoroughly grounded before bolting down?

I wonder if since I rotated the distributor 180 degrees and the hold down clamp is in a completely different spot I'm not getting good contact metal to metal for grounding?

...You are feeding the distributor with the negative side of the coil right?

Yes, terminated on the neg (-) coil terminal.

 

[Haut Medoc]

Re: Timing & Vibration

Did it run OK with the dist in the other position?.....​

 

[gene8084]

Re: Timing & Vibration

Total hours on the distributor? It's a 1974 Ford 302. I'm not the original owner. This is/was a stock distributor for the Mercruiser at that time. No proof meter, so I really have no idea on hours.
Bearings in the distributor, cracked distributor housing? I have not rebuilt it so I don't know about the bearings. Didn't see any cracking.
Is there a home made hold down bracket? Looks original.
Rough idle, as in a cylinder miss firing? No, more sluggish, then really obvious vibration of the engine, boat, dash @ advanced RPM.
I want to ask you to pull the distributor and check for a bent distributor shaft. ok, good one. My first fear when I tighten the distributor after rotating it, but I reasoned the shaft rotates so it can't be.

Thanks...

 

[a70eliminator]

Re: Timing & Vibration

Ok there's a big fat o-ring oil seal around the main body of the distributor where it fits into the block, without it oil would more than likely ooze from the base of the disributor, that o-ring is what I'm thinking would prevent a good ground. Now try to follow, what I'm trying to say is that the distributor should be grounded and when you tighten the bolt the distributor does in fact ground, and thats where your rough misfire happens right? So somewhere theres a wire crossed in the ignition circuit to where when the distributor makes ground it also grounds out the circuit in question. I'm probably just overthinking this but I'm still with ya anyhow, It's good you got that distributor oriented correctly if anything your definately getting to know your engine, you'll get it going.

 

[gene8084]

Re: Timing & Vibration

Did it run OK with the dist in the other position?.....​

Yes, I'd say it did...when it ran. It was "ill", so while I had good days on the hose & muffs, the other problems led to the general repairs I've done.

Two obvious problems in the previous state:

1. It was backwards and thus the distributor wire (for points / condenser) was coming out the hole in the front of the housing and it was sharply bent around to avoid the cooling hose running in front of the distributor.
2. A number of posters indicated in it's previous position that it was firing on the exhaust stoke, rather than compression. (not sure about this, but #1 seemed like enough to reverse it) Eventually the wire would crack or melt.

Gene

 

[gene8084]

Re: Timing & Vibration

What do you mean it was 180 out?.....
It wouldn't run....

Picture the small hole the wires are fed through to the coil coming out the front, rather than the back. It ran, the PO id'd TDC to the correct position on the cap. So, my net gain was having the coil to points wire come out where they should, on the back of the distributor housing.

You are saying that before you turned it, it ran fine & you just rearanged the wires , say #1 was in the back & is now in the front?.....
& it only gets uneven when you tighten down the bolt in the new position?.....

Exactly!...Although, in the spirit of full disclosure some 4 weeks passed while I completed other rebuilding activities...and got my self back to a starting & running state.

So the holddown is technically pressing on the distributor shaft/housing 180 degrees from where it was before.

 

[gene8084]

Re: Timing & Vibration

Did it run OK with the dist in the other position?.....​

Yes!

 

[a70eliminator]

Re: Timing & Vibration

Hey I just thought of something! This would go hand in hand with what your saying about the engine cutting out when you tighten the distributor bolt or in other words ground the distributor. The black wire that comes through the distributor housing is frayed right where it comes through the housing within the rubber thimble and is touching the distributor grounding out the points. check that wire real close for any bad spots, it must not touch metal.

 

[gene8084]

Re: Timing & Vibration

Well, you do need to check the dwell.....
​

The DWELL was the problem. With a point gap of .017 the dwell was 8-9, not 26-31.

I adjusted the points to 26-27, checked the timing (lucky it started...timing went from 10 degrees BTDC to 10 degrees ATDC). Set the timing back to 10 BTDC and @ 2800 RPM, timing advances smoothly, no engine skipping, and NO vibration.

Thanks Haut Medoc!​

 

[Haut Medoc]

Re: Timing & Vibration

Well that's good news!.....​