Strange engine vibration at idle

[GeoMetron]

I have a '91 Geo Metro convertible that is in excellent physical shape. My old Geo Metro was so rusted that the front suspension started to become dangerously flexible. I got the new-to-me Geo from a dealer in Little Rock for a steal! I'm almost embarassed to say how little it cost me.

Anyway, the vehicle does have some mechanical problems. But hey, I've swapped engines in Suzuki Samurais, converted 2-wheel to 4 wheel drive, installed TBI fuel injection on a carb'd 1.3, and I figured that there "weren't nuthin' wrong wid it dat I cain't fix."

Well, I'm stumped.

I've replaced the engine mounts, rented a timing light to properly time the thing (though I had guess it awful close just by listening). It's had a complete tune up with the plugs, wires, roters, timing belt, and coil. The old plugs were nice and clean when I pulled them out but there was a little oil residue under the air filter (maybe a clogged PCV).

I'm getting no codes from the ECM but occassionally the engine will bog down and backfire before running properly. Above 1,000 rpm the engine doesn't vibrate as bad but it's no where near as smooth as my old Geo.

Any suggestions?

GeoMetron

[geozukigti]

Sounds like the timing belt is loose. I have heard cases where the bolt that holds the timing belt gear to the crank isn't tight enough, and it causes vibration there. Over time, it wears out the keyway, and the timing belt gear will start to wander, and vary your valve timing. Might be worth a shot to take the timing belt off, replace it, and check out the timing gear. You also should do a compression test and see if your valves are leaking.