timing gears need changed on 1978 merc 120?

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iron_horse

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1978 gm 120 2.5L merc serial number 501_ _ _ _ something

my problem is at 700 rpms the engine has a miss (it sounds like a little old tractor) cylinder number 1 and 2 are not firing the plugs are wet. cylinder number 3 and 4 are fine those plugs look great.
I have changed the cap, plugs, points, rotor, cond, wires, checked the distributer gears (they look fine), checked the timing, and did a rebuild on the carb. when you add more rpms it does NOT miss. it seems to sound just fine.

do I need to replace the timing gears? any other suggestions?
thanks
 
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Bondo

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Re: timing gears need changed on 1978 merc 120?

Ayuh,... What's the Compression #s,..??
 
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iron_horse

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Re: timing gears need changed on 1978 merc 120?

bond-o
what would you say is causing the miss? i am puzzled as to why it has the miss at low rpms and it runs fine at higher rpms and why the plugs look so wet on 1 and 2.
 
Bondo

Bondo

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Re: timing gears need changed on 1978 merc 120?

Ayuh,... I'd have to Guess 1, 'n 2 ain't firin'...
Is the distributer shaft loose, 'n wobbling,..?? The bushings could be Bad...
 
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iron_horse

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Re: timing gears need changed on 1978 merc 120?

bond-o
i will check later today and get back to you
thanks
 
Bt Doctur

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Re: timing gears need changed on 1978 merc 120?

they break more than they wear.
1 make sure spark comes out of each plug wire
2 check plug firing order 1,3,4,2
3 if points, measure each lobe . a worn dist bushing can do it.
 
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iron_horse

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Re: timing gears need changed on 1978 merc 120?

bond-o
distributer is tight.
what is my next option?
thanks
 
Bondo

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Re: timing gears need changed on 1978 merc 120?

Ayuh,... BD just listed 3 more places to look...
 
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iron_horse

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Re: timing gears need changed on 1978 merc 120?

1. plug wires checked - good spark on all
2. checked firing order again - its correct 1,3,4,2
3. points- i still need to measure the lobes

would i have to buy a whole distributer if the bushing is worn, or just replace the bushing?
thanks
 
45Auto

45Auto

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Re: timing gears need changed on 1978 merc 120?

Make sure you know which way the distributor rotates when checking the firing order. If you put the wires on the distributor cap in the correct order (1,3,4,2) but wrong rotation direction, then 1 and 4 are still correct but 3 and 2 are 180 degrees off and firing on the exhaust stroke. It will do EXACTLY what you're describing - the timing checks perfectly, all the plugs will fire fine, it sounds fine if you rev it a little, but chugs like a tractor idling.

Not going to tell you how I know this! :D
 
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fishrdan

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Re: timing gears need changed on 1978 merc 120?

LOL @ 45 :D (not like I've never done anything like that before)

If the distributor bushings are loose you will need to replace the whole distributor, most likely the shaft and bushing are both worn. To see if the bushing is bad, remove the rotor and try to wiggle the shaft side-side in several directions. If you feel it wiggling around the distributor needs to be replaced.
 
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iron_horse

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Re: timing gears need changed on 1978 merc 120?

i will double check the wires and distributer
thanks guys
 
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iron_horse

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Re: timing gears need changed on 1978 merc 120?

checked the distibuter and it was fine and checked the wires again they are correct.
could i try putting in a used disributer off of a 165 into my 120 and see if that makes a difference or would the gears and shaft be wrong?( the 165 is out of a mid 1970's motor)
 
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daydreamer1252

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Re: timing gears need changed on 1978 merc 120?

From what I understand the gears and shaft would probably fit.......BUT you could only chose one cylinder to fire as there are 6 bumps on the distributor cam and they will not correspond to when your 4 cylinders need fire....could get #1 but not the rest......sorry
 
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