Acceleration Rattle

[KenB]

I know sound problems are tough to diagnose if you can't hear them yourself, but maybe someone can point me in the right direction. I have a sort of high pitched rattling sound when I accelerate. This started after I had the exhaust system rebuilt. (Removed cat, increased pipe size to 1 7/8, and put on a hi flow muffler.) There is no rattle except when accelerating under load. I suspected it might be a timing problem, but adjustments there don't seem to have any effect. Timing now at 6 btdc, and advance diaphragm is adjusted full CCW. The timing stays dead on the mark where it is set. Now I'm wondering if it may be something in the distributor itself or if the lower exhaust back pressure has something to do with it?
81 CJ7 , AMC 258 i6 / 4.2l I6, MC2100 carb, no id on distributor, new spark plugs, coil and wires
I don't mind throwing some $$ to fix it, but I don't want to throw them away.

 

[Old Dog]

The first thing I would try is a higher octane gas, just to see what happens.
The timing thing is kinda throwing me though, even without the vac. adv.
it should still show some advance through the mechanical advance.

 

[MDJEEPER]

Is it a "raddle" -- like the exhaust equipment is vibrating up against something as the engine changes position under acceleration...or is it a like a pre-detonation "ping"? Does it sound like it's coming only from the engine, or is there a resonance??

 

[KenB]

On the timing, what I meant to say was there's no drift. It does advance when accelerated, with or without vacuum. It doesn't sound like a rattle from vibrating parts. The sound seems to coming be from the exhaust at the rear, not from the engine compartment. Sitting still, no load, it's not there at all, and when 'coasting', like downhill with no acceleration, it's quiet. But give it a little gas, even at a cruising speeds, and it comes back. The harder you push it, the louder it gets.

 

[Buuushman]

I'd get under it when it's cool and start bangin' on stuff- should be easy to find if it is body or frame interference- could also be a pipe sliver or something that got into the muffler or pipe during assembly- worth a try.

 

[Petescj]

Timing spec for your Jeep is 8 btdc just so you know.

 

[priceg]

On the timing, what I meant to say was there's no drift. It does advance when accelerated, with or without vacuum. It doesn't sound like a rattle from vibrating parts. The sound seems to coming be from the exhaust at the rear, not from the engine compartment. Sitting still, no load, it's not there at all, and when 'coasting', like downhill with no acceleration, it's quiet. But give it a little gas, even at a cruising speeds, and it comes back. The harder you push it, the louder it gets.

All of the advance is at start up and high manifold vacuum. As load increases and manifold vacuum drops, so does the timing. Base is 8 degrees, total is 35 degrees at 1500rpm.

 

[KenB]

Well, I got rid of almost all of the rattle by replacing the muffler. Seems the one I had installed at the local shop was some kind of knock off hi-flow that set up some kind of wierd resonations. I say almost all, because there is still a little bit of it when I accelerate.
On the timing thing. I did set it back to 8*btdc just like the sticker says, and put the vac advance diaphragm back to its original setting.
priceg, you've got me a little confused now. I thought the advance was supposed to be to ported vacuum, which increases with acceleration, instead of manifold vacuum which decreases. My little bitty mind thinks the more acceleration, the faster the engine turns, the more advance the ignition needs to keep up with it.

 

[mustangduane]

High RPMs=retard timing

 

[KenB]

OK, I need some education here.
1. Ported or manifold vacuum? All the stuff I have on the MC2100 says ported.
2. With the engine at idle, vac advance hose off and plugged, the timing mark is steady at 8*btdc. If I accelerate, the timing mark moves to a higher point (mechanical advance). With the vac advance hose connected, accelerate, and it moves even higher on the scale.

Maybe I'm the retard here, but I just can't seem to wrap my brain around it.

 

[priceg]

Your not doing anything wrong, you can't check total timing with a regular timing light. Set the base timing at 8 degrees at idle with the vacuum line off the distributor. Anther thing you can check without special tools, is your timing chain deflection. Pick a timing mark on your timing cover and with the timing mark on the balancer rotate the balancer by hand both directions. You are measuring how many degrees the crank turns before the slack is taken up and the cam starts to turn. The rule of thumb is around 6 degrees of deflection means time to replace.

 

[MDJEEPER]

Your not doing anything wrong, you can't check total timing with a regular timing light. Set the base timing at 8 degrees at idle with the vacuum line off the distributor. Anther thing you can check without special tools, is your timing chain deflection. Pick a timing mark on your timing cover and with the timing mark on the balancer rotate the balancer by hand both directions. You are measuring how many degrees the crank turns before the slack is taken up and the cam starts to turn. The rule of thumb is around 6 degrees of deflection means time to replace.

Maybe it's 'cuz I worked too long this week, or maybe I am just missing something, but could you please describe this timing test with a little more detail? Any pictures??

 

[KenB]

Well, it took a while, but I did get rid of the ignition knock. Finally saw that there was play in the connection between the vac advance arm and the piece its supposed to move. The whole top assembly in the distributor seemed a little wobbly. I'm guessing it was the original distributor, so after 29 years, there had to be some wear on those moving parts. I think the warranty probably expired. Anyway, a new distributor took care of of the ignition knock.
Still have a little 'tick' noise under load, and I'm thinking something in the drive train? No noise when sitting still. Stepped hard on the brake in drive - no noise. Shift to neutral on the road - no noise. Anybody volunteer to hang under this thing while I drive down the road to figure where it's coming from?