85 HP Johnson producing less power than expected.

[SparkieBoat]

I have a 1979 85 HP johnson. It runs great. has 110-120 on all cylinders. the left bank is 120 and the right bank is 110. the coils all test good, carbs have been rebuilt and seem to function fine. The motor runs much smoother than I expected but seems to be lacking some power. I put the motor on a 14.5' tri hull. I expected maybe 40 MPH or close..only getting about 32 or so. I have not yet checked the pitch of the prop..it is an aluminum prop. I also have not hooked up a tach.
But the motor just seems to not wind out like you would expect..I am almost sure the prop is not some real high pitch, probably a 15 or 17. it just sounds like it is running at about what I would expect to be about 3/4 throttle. I know a prop can make a big difference..but is there anything else I should check??
Throttle cable seems to be adjusted right.
It is very smooth with no surging or anything that would indicate fuel issues.
Thanks Sparkie

 

[Daviet]

Re: 85 HP Johnson producing less power than expected.

Need to start with RPM.

 

[dieselcat]

Re: 85 HP Johnson producing less power than expected.

my 88hp rude did the same thing and it ended up being that the timing was way off.after i took it to the shop and got the timing done,now it runs great!

 

[SparkieBoat]

Re: 85 HP Johnson producing less power than expected.

yes ..I was thinking that was the issue..it acts like the timing is off. Just got my manual in today..so that will be my next check.

 

[emdsapmgr]

Re: 85 HP Johnson producing less power than expected.

Do you have strong spark on all 4 plug wires? When running at idle, pull one plug wire off the spark plug and see if the engine slows down. Repeat on the other 3 wires.

 

[SparkieBoat]

Re: 85 HP Johnson producing less power than expected.

i have a spark tester and just replaced 2 coils..fire seems good. I am about to go check the timing now.

 

[jonesg]

Re: 85 HP Johnson producing less power than expected.

IT NEEDS A TACH, you're running blind if you can't put a real number to the rpm.

 

[tashasdaddy]

Re: 85 HP Johnson producing less power than expected.

have you ever considered you may have a waterlogged hull? timing, http://forums.iboats.com/showthread.php?t=299680

also link n sinc, are the carb butterflies horizonal when you put control at WOT.

 

[SparkieBoat]

Re: 85 HP Johnson producing less power than expected.

hull is not water logged..it is an early 80s tri hull. Thunder Craft brand. It is kind of heavy but seems normal for this type of boat. And yes the butterflies are synced. I had a 14X13 aluminum prop on it..changed to a unknown pitch SS prop. I suspect it is a 13 or 15 pitch. It appears almost identical to my aluminum prop as far as angle of the blades.
boat did preform better, with cowl off and carb cover off I hit 5k RPM at 3/4 throttle and reved it up to 5600 WOT, after I put the cowl back on and the carb cover 3/4 throttle was about 4700 and WOT trimmed out was 5200. I did not have a gps with me today but will wednesday..but guessing at top speeds, without cowl and carb cover maybe 40 and with maybe 35.
I am considering putting some 115 carbs on the engine to boost the HP. It does jump out of the hole. Some one said that I also need to change the exhaust cover. I have a friend with a couple of more props that I will try out. I would like to see what a 19 or 21 prop would do.
I believe the motor is doing what it is supposed to do, I did adjust the timing from 17 BTC at 3/4 throttle to 26. the manual said max is 28. made the motor run a little smoother and rev up faster..not boggy at all now.
Did not expect such a difference with the cover off but it made a very noticeable difference.
Thanks for all of your help guys.

 

[kenmyfam]

Re: 85 HP Johnson producing less power than expected.

Set up is also huge. Motor height trim, loading etc.
I agree that you really need to know your RPM at WOT though.

 

[SparkieBoat]

Re: 85 HP Johnson producing less power than expected.

By the Way..that is a GREAT Timing post you linked me to Tashasdaddy..thanks for all of your great advice.