Merc 90 backfiring

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Gatorkiller67

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Just put a new powerhead- a97 on my 90 2 stroke. Installed electrical. Starts but backfires. Checked all wires, there on correctly. Swapped power packs Same problem. Almost like firing order is off. Any ideas?
 
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Maxz695

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Re: Merc 90 backfiring

double tripple check switchbox wiring and the plug wire order. also check carburetor air mixture adjustment.
 
Texasmark

Texasmark

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Re: Merc 90 backfiring

I'll go for plug wiring order....wrong CDM connected to a particular plug or you got the wrong input wire to the CDM. Otherwise, you may not have your timing set correctly. The idle timing is 2 BTDC whereas the high speed, if set with the plugs out and engine spun by the starter is 22 BTDC.

CDM1 which I assume is the upper most, connects to the top plug and input wiring color codes are: Blk, blk/yel, PPL (purple), wht/grn.

CDM 2 to second plug and input: Blk, blk/yel, brn, grn/wht.

CDM3 to bottom plug and input: Blk, blk/yel, wht, grn/wht.

Right out of the manual.

Mark
 
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Gatorkiller67

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Re: Merc 90 backfiring

Ok does the black or red flywheel for these 90's have a difference? A mechanic seems to think so with the firing order
 
Faztbullet

Faztbullet

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Re: Merc 90 backfiring

Cranks are either 1-2-3 or 1-3-2 firing order, and CDM flywheel will not work on a ADI ignition and vice versa
 
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Gatorkiller67

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Re: Merc 90 backfiring

I'm using the older style power pack on this new powerhead. So red flywheel?
 
Texasmark

Texasmark

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Re: Merc 90 backfiring

Gatorkiller67 said:
I'm using the older style power pack on this new powerhead. So red flywheel?
Click to expand...

I have an '02 90 with CDM ignition and a red stator which signifies 16 amp alternator. I understand the design goes back into the 90's but don't know how far.....just looked up how far.

Per my Serv manual, as far as it goes back since it is dated Dec. '97, '94 and '95 was 132. Later models starting in '96 was 123. I think Fazt nailed it. You have a 132 crankshaft with a 123 stator which makes for firing out of time and causing your problem.

Mark
 
D

Dave1027

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Re: Merc 90 backfiring

Texasmark said:
Per my Serv manual, as far as it goes back since it is dated Dec. '97, '94 and '95 was 132. Later models starting in '96 was 123. I think Fazt nailed it. You have a 132 crankshaft with a 123 stator which makes for firing out of time and causing your problem.
Click to expand...
If that's the case then can't the OP simply swap the wires to coils 2 and 3?
 
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Maxz695

Guest
Re: Merc 90 backfiring

Check the pistons starting with 1. Take out the plugs, turn to see if the next piston to hit the top is 2 or 3. If 3 hits next it,s the newer crank. The old switchbox would have to swap the wire for the 2 and 3 coil to get the right firing order Thats waht I did.
 
Texasmark

Texasmark

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Re: Merc 90 backfiring

Dave1027 said:
If that's the case then can't the OP simply swap the wires to coils 2 and 3?
Click to expand...

No reason that wouldn't work and the plug wires will reach. Or you could swap the input wires to the CDMsand leave the plug wires where they are. Same thing either way.

Mark
 
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Dave1027

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Re: Merc 90 backfiring

Texasmark said:
No reason that wouldn't work and the plug wires will reach. Or you could swap the input wires to the CDMsand leave the plug wires where they are. Same thing either way.
Click to expand...
Actually I thought the cleanest and simplest way to do it was to swap the wires that go from the switchbox to the coils. That way you don't have to cross the plug wires and worry about one stretching that far.
 
M

Maxz695

Guest
Re: Merc 90 backfiring

That is correct but you may have to litterally switch both ends of the wires as 1 is shorter than the other from the switch box to the coil leave the plug wires alone
 
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