1976 115 Evinrude with 135 carbs and bubble back exhaust WOT timing

[spirtS niggeB]

The old man that owned my most recent motor before me apparently tried to add HP to my '76 115 by adding larger carbs and exhaust backing. With the motor on the back of my 17 foot Checkmate I am only going 43 mph at around 4300 rpm with a 1/4 tank gas and my 130 lb girlfriend aboard. The prop is 13x19. I feel that I should still be pushing at least 5500 rpm with that load.

Real question is about the timing. The 115 for 1976 should have a spark advance of 24*. The 135 should advance 20*. Would these larger carbs on the 115 block need to be adjusted for the 135 hp engine?

Any input is highly appreciated, my Predictor needs to be pushing greater than 43 mph..

 

[flyingscott]

Compression and spark test first.

 

[spirtS niggeB]

150 on 1 and 3, 135 on 2, 130 on 4 - firing on all cylinders

 

[spirtS niggeB]

About 14% difference on compression on that last cylinder. I hear between 10 - 15 % difference shouldn't matter too much. Do you think 5 psi difference would account for the low RPMs?

Gonna clean plugs tonight before my run. Will the adjustable spark checkers differentiate between a weak and strong spark? I usually just pull the boot while running and look for strong spark, but most recently used my $3 checker.

 

[emdsapmgr]

Marine spark is made to jump a wide gap. Something like 7/16". So set your special marine spark checker to max gap and see if the spark will jump it. Those 135's had some of the highest compression heads ever made by OMC. Over 135 compression. Make sure you set the timing at 20, and then I'd be running premium fuel to make sure it will run forever.

 

[racerone]

20 psi difference indicates that the motor needs work !

 

[boobie]

Give the motor a good de-carb job and compression may improve.

 

[Faztbullet]

If the exhaust deflectors wasn't installed the cylinders can suffer from exhaust reversion, it takes more that bolting on parts to get extra hp