78 Mercury 200 20hp electric start , backfires and won't run

stub1

Recruit
Joined
Jun 24, 2020
Messages
5
I have this ( 4952145 )1978 Mercury 200 20hp electric start that I have been trying to get started . I have good spark and 110 lbs. of compression on both cyc. all it does is backfire out the exhaust. Any help would be greatly appreciated. Thanks , stub
 
Last edited:

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Timing being off and some on here report that bad reed valves can cause that problem. Reeds are directly behind the carburetor.
 

flyingscott

Fleet Admiral
Joined
Apr 8, 2014
Messages
7,985
Before you go tearing the motor apart, some simple things to do. Switch the spark plug wires around. Check the flywheel key. The reeds are not behind the carb on that motor they are on the crank.
 

stub1

Recruit
Joined
Jun 24, 2020
Messages
5
Thanks , I have swapped around everything , trigger wires and sparkplug wires and swapped coil wires . Put a timing light on it using the starter with no plugs in motor and the timing mark moves with respect of throttle advance . Yes the reeds are on the crank. Do you think the stator could cause this? I've checked both coils on the stator plate and they ohm out at ( front coil ) 908 ohms and the back coil is 918 ohms . The trigger ohms out at 146.3 . All that's left is the control box and the reeds unless the engine has wrong flywheel. The flywheel has one large magnet.
 

Attachments

  • photo334234.jpg
    photo334234.jpg
    994.8 KB · Views: 2

flyingscott

Fleet Admiral
Joined
Apr 8, 2014
Messages
7,985
If your coils are on the stator plate that is a much older ignition. Anything after 1970 on those 20 hp motors has external coils.
 

stub1

Recruit
Joined
Jun 24, 2020
Messages
5
I may have a put together bunch of motors . I might not call some of the parts by the right name .Here's what is under the hood. The foot is all rusted and froze up. If I can figure out what should be here and what is missing , I can go from that . I really need the electric start now as I can't pull the manual one anymore , shoulders are shot and need replacing . I'd rather replace stuff on motor than me :laugh: . Thanks , Any more help will be greatly appreciated .
 

Attachments

  • photo334314.jpg
    photo334314.jpg
    1 MB · Views: 2
  • photo334315.jpg
    photo334315.jpg
    1,006.5 KB · Views: 2
  • photo334316.jpg
    photo334316.jpg
    1 MB · Views: 2
  • photo334317.jpg
    photo334317.jpg
    1,007 KB · Views: 1
  • photo334318.jpg
    photo334318.jpg
    1,010.1 KB · Views: 1

flyingscott

Fleet Admiral
Joined
Apr 8, 2014
Messages
7,985
That looks.right for a 78 model. I say you have an electrical issue.and it sounds so much like plug wires are crossed.
 

stub1

Recruit
Joined
Jun 24, 2020
Messages
5
Not sure how to set timing but this motor is marked 33 degrees BTDC . I took pic of flywheel with top piston at top dead center. Does gap between pointer on motor and flywheel look close ?
 

Attachments

  • photo334506.jpg
    photo334506.jpg
    1 MB · Views: 4
Last edited:

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Timing is usually checked with plugs out with throttle set to min and max position, battery capable of spinning engine at 200 RPM for a number. Timing will be around zero at the idle position and a double digit number at the WOT throttle position. My loop charged engines of higher hp run around 22* BTDC at WOT and I THINK cross flow (domed piston identifies) run much higher...like your 33*BTDC.
 

stub1

Recruit
Joined
Jun 24, 2020
Messages
5
Timing appears to be close enough to run . Is it possible to get a complete rebuild gasket set for this motor ? If I can I may dig into this engine a whole lot more . Thanks to every one for help and advice. stub
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Timing appears to be close enough to run . Is it possible to get a complete rebuild gasket set for this motor ? If I can I may dig into this engine a whole lot more . Thanks to every one for help and advice. stub

If normal channels fail you, try ebay. Lots of older parts and equipment for sale. I shop them frequently and satisfaction is guaranteed and a very secure site.
 
Top