Whaler Central - Boston Whaler Boat Information and Photos

Posted by Jacksmorto3747 on 07/05/20 - 9:19 AM

I have an old 1987 or 1988 90 hp Johnson that gets my 1987 Montauk [to] 31 mph.

At 25 mph I’m 8.5 gph. This seems very off for this engine.

Is there something I can fix?

[edit for typo, model year]

Edited by Phil T on 07/05/20 - 9:25 AM

Posted by Phil T on 07/05/20 - 9:27 AM

Jack -

Is 31 mph wide-open-throttle (WOT) or just a cruising speed?

How are you measuring speed?
At what RPM are you at when you reach the stated 25 mph?

How are you measuring GPH?

Posted by Jacksmorto3747 on 07/05/20 - 9:40 AM

31 wot
25 is an arbitrary curing speed.

Gps is my method.

Gph is calculated with gas usage

Tack is broken.
I installed a new tack that popped fuses.

While it was working it maxed out at 4200-4500

Posted by Phil T on 07/05/20 - 1:12 PM

It reads as you have several issues, the least of which is GPH.

Do a compression check on the engine. The engine is running 1000 rpm's less than it should at WOT.

A typical 90hp 2 stroke on a classic Montauk 17, mounted a few holes up, decent stainless steel prop with a light load should hit 42-46.

Did it ever run 5500 WOT?

Install a new tach and re-test.

Report back.

Edited by Phil T on 07/05/20 - 1:23 PM

Posted by Jacksmorto3747 on 07/05/20 - 8:00 PM

Can’t fix tac for a while. Compression was tested recently.
Could this be a build up of some sort or a bad plug. I want to start off with easier forms of trouble shooting.
Thanks for all you help.

Posted by Phil T on 07/06/20 - 7:24 AM

Detailed information is needed to provide you with specific items to check or proceedures to do.

What were the compression numbers for each cylinder?

Has this engine been run much this season, if so, how many hours.
Was it winterized last winter.
Is the fuel new/fresh or stabilized from last year?
Do you have a water separator on the boat?
Are you mixing 50:1 gas to oil or using the auto oiling feature?
Spark plugs less than 5 years old?
Thermostats changed in the last 5 years?

Does the boat have bottom paint?
What is the make/model and size of the prop?

At what height is the engine mounted on the transom? If you look at the top bolt through the engine bracket how many, if any empty bolt holes are above the bolt?

What is the typical load of people, fuel and gear on the boat to reach 36 in lbs.

Posted by Jacksmorto3747 on 07/06/20 - 7:55 AM

Prop should be stock

Spark plugs way less than 5y

A mechanic took care of it past 2 years

Yes I have bottom paint

Engine position unknown will measure

Compression was only confirmed by mechanic.

Weight was 300 lb passengers 55 lb battery
10 gal gas.

60 lb ice
485lb

I Believe the hull is1350 pounds I don’t know if that includes The rps bench seat

Posted by Phil T on 07/06/20 - 9:50 AM

#11

Two stroke engines are not as efficient as 4 stroke motors. Idle and slow speed are especially poor.

Without knowing the precise amount of minutes at each RPM, there is no way to know if the gallons-per-hour is outside of the expected.

Many owners look at the total distance traveled and how many gallons used and use that average. If you have long no-wake zones, idling while fishing that will increase your average.

The sweet spot for speed/gph for most 17-19' boats is somewhere around ~4k rpms. Running averages for most 2 stroke owners with a Montauk or 16' is in the 8-10gph range but that includes periods of time running at 35+mph.

Posted by Jacksmorto3747 on 07/06/20 - 10:52 AM

#12

Il look into everything You said.
Im looking to replace all the wiring so I can get my tac going ect.
Do you have any suggestions or the original schematic. I see you own the same boat.
Thx

Posted by biggiefl on 07/06/20 - 11:51 AM

#13

Many times with OMC engines, the tach will not work if the voltage rectifier went bad. You will know this as the old one would work sporatically and then finally quit and the charging rate is very high like 16v+ That engine running well willl burn about 6gph at cruise. and around 9 at WOT. Not possible to be burning 8.5 at part throttle.

Posted by acassidy on 07/06/20 - 10:17 PM

#16

I owned a 1978 Montauk with a 1990 90 Evinrude and that motor was a beast on that boat. Top speed was 45 mph easily and cruise in the 30s easily. Prop was a 17" Stainless. Something is wrong with your motor.

Things that could the culprit. Wrong prop. What prop are you spinning. Loss of compression in motor. Do a compression check. Replace plugs if you have not done so. Clogged fuel filters. Carburetors not set up correctly. Carb leakage not set up correctly. Throttle cable not setup correctly. WOT stop adjusted incorrectly. Not firing on all cylinders. Run a sparks tester. Missing while running would explain high fuel use while running low speeds. Bad spark plug boots. Bad spark plug wirings.

These are all simple things you can do and check to get that motor running like it should. Beware that running that motor with lean fuel in a cylinder means lean oil too and will burn the motor up. Weak spark will foul plug and make it miss even more. How do your plugs look. Is there one that look different and looks oily.
Good luck
Archie