wrong jets?

[banff]

Hello there,
I recently acquired a 1988 johnson 70hp on an older 15.5' flat transomed fiberglass boat rated 50hp. Engine was bought by previous owner off an ebay seller in Florida. Boat goes 24mph at approx 2/3 to 3/4 throttle. Does not go faster at full throttle, just sounds rougher. 13-1/4 x 17 prop. I was thinking boat should be good for 30-35 mph. Took carbs apart and jets are 58D, although Johnson website shows carbs just like mine and they have 65D jets. Not sure how it got 58d jets in there or if motor ever ran well with them.I'm thinking maybe this is making it lean enough to cause this problem . I drive boat in two different lakes, 2500' and 5000' elevation. was thinking 63D jets may be better than the 'factory' 65D due to elevation. Was thinking of drilling existing jets to 1/16" (.0625) or spraying gas in throttles at high speed to see if performance improves.

Thanks,
Jason

 

[Mntom]

Re: wrong jets?

Before you drill it out try the spray to the carbs to confirm your hunch. Then you would be farther ahead (in my opinion) to get the right jets instead of drilling them.

 

[Chris1956]

Re: wrong jets?

Jason, The boat should pick up speed as you increase throttle. Where do you have the trim set? Perhaps high throttle settings push the boat nose down, causing the boat to plow water.

Does it have good compression?

 

[banff]

Re: wrong jets?

Can't remember the #'s, but I recall the compression tested good last year. Boat seems to go about the same speed at different motor angles and only slightly slower with 4 people in it.
Once when I forgot to unscrew the tank vent screw (mercury style tank) the boat had a serious increase in power for a couple seconds right before stalling. This power level seemed more like what it should have always. I think it jumped up to approx 28mph. Wouldn't that suggest engine was running rich (then power increased as fuel bowl level dropped, causing leaning)? Wondering how the engine could be running too rich if it has 58D jets when it should have come with 65D jets?
Hopefully taking silencer off and spraying extra gas in one carb at a time at Wide Open Throttle using a spray bottle is a good test to see if engine is running lean. If speed doesn't increase is it running too rich?(amazingly with 58D's) I'm also thinking of trying a timing light on each cylinder at WOT to see if maybe the spark is dying at full throttle. The time power surged up while running out of gas it was also quite wavy and boat was bouncing a bit. Wondering if that may have wiggled a bad ignition system wire?

Thanks,
Jason

 

[Daviet]

Re: wrong jets?

I agree with Mntom, install the correct jets and see how it preforms.

 

[ezeke]

Re: wrong jets?

There was a service bulletin that specified the correct orifice sizes for your motor at various elevations. You may find that your service department has a copy; ask for SB #2217. The 1988 70HP listing is on page 11.

The entire SB is now listed as obsolete, which generally means that there is some more current information available, but at least it would provide a starting point.

 

[banff]

Re: wrong jets?

So a funny thing happened today. Pulled the jets out and they are actually 58B not 58D as I originally posted. Not sure what the difference is between 'B' or 'D' series jets. I wanted to enlarge the 58 to more like 65 factory orifice by drilling out to .0625" using a 1/16" drill bit. Turns out those 58B's are already slightly bigger than my 1/16" bit that mic's out at .062"! Surprise!?- Someone drilled them already? Did a cross reference and 58B jets were used in 1970 70hp carbs, which look different than my 1988 carbs. I think I'll go ahead and order some 63D's to get me close to factory, but with a slight drop for elevation.
In the mean time,Curiosity is killing me, I'm thinking of fishing a .010" wire through each jet to lean it out a bit to see if there is more power with a leaner combo....Looking to get that power boost (probably from leaning out) that I had for a few seconds before it ran out of gas once. Wish these carbs had adjustable needle valves for high speed mixture.

 

[tashasdaddy]

Re: wrong jets?

check and make sure carbs are opening all the way, and not past wot. improper adjustment of the carb butterfly, will cause many problems. Have you check compression and spark with testers???

 

[banff]

Re: wrong jets?

Today I got brave and decided to lean my 'drilled' jets that had been installed by some previous owner. Put a short length of .015" wire through each jet. Engine now has noticeably much more power and revs higher. Boat now goes 27mph on Lake Minnewanka (approx 5000')and I think there is still more power to be found.
Strange thing however is that after a Full throttle run, if I slow down and go off plane it will Bog seriously (firing on only 1 cylinder?) when I gun it again. After a while it will slowly clear up if I lower the throttle lever, then I can gun it and all is good again. Could this be that now that the engine is revving higher , a bunch more excess fuel is flowing through carbs high speed circuit and fouling the plugs? Didn't have time to check plugs, but will do that before I dare to put an additional .010" wire in the jets to lean out mixture more.
Also, boat is of flat transom design which is causing boat to plow at this higher speed-even with engine tilted out. Seems like I'm approaching a top speed brick wall for this hull design. I'm ok with that..... boat is only rated for 50hp, extra pulling power of the 70hp will be nice though!There is a noticeable 'hook' in the hull near the back of the bottom which I suspect may be sucking the hull down. At top speed there is spray shooting off the hull ahead of the driver's seat!

 

[jtexas]

Re: wrong jets?

"There is a noticeable 'hook' in the hull..."

And you wonder why it won't speed up? Go ahead, run the motor lean, have a good time while it lasts.

 

[JIMTMCDANIELS]

Paying it forward here, just sharing the pages I have of the bulletin.

I hope this helps someone looking to adjust for high altitude(or verify factory jet sizes).
To get perhaps a slight amount of power back and to reduce excess smoking and fuel use and engine bogging if still set to sea level from the factory.
Here I've attached some of the pages from the high altitude OMC 2217 bulletin people refer to look at. It doesn't seem to readily available...
The only pages I have that someone shared a few years ago for my 1980 CS 140 Evinrude. The intermediate in mine (probably from the factory) is 29M.

My low-idle jets have air passing through them to control the air fuel mixture. This is why larger jet sizes for my low circuit actually leans the fuel mixture.
Some say that the ethanol in our modern gas makes it slightly leaner.
If true, I suppose perhaps this chart's recommended jet sizes should be adjusted by one size for our 10% ethanol gas.
I live at 6,000 ft and use my boat at 5,000 ft. I bought the orifice-jets I needed on e-bay for about $6 each.
Don't go too lean or for the idle circuit, you'll get the idle lean sneezing and for the high circuit you could burn up your pistons-ruin engine.
Oddly, no-one seems to recommend increasing the timing advance by 1/2+ a degree for every 1,000 feet elevation for high altitude like they do for 4 cycle engines. Evidently it's hard to tell with these 2 cycles if they are set too high experiencing knocking till it's too late and the engine is damaged. It kind of scares me that these engines rev so high at full speed. I'm just happy to have rejetted to reduce excess smoking and fumes.
Take care and let's keep our old 2 cycle running respectably!

Jim