1979 Johnson 70 slow to rev

[91kuhndog]

Thatnks for reading.... I'm hoping for some experienced help. I have a 1979 Johnson 70 that sat for 9 years. Here's what I have.... I have 130# compression all three cylinders, good spark, rebuilt carbs, linked and sync'd. Carbs and L&S done by local marina. Had them inspect timing too. New H2O pump, thermostats, I installed a new stator and lubed and cleaned everything under the cowling! The motor will fire imediately, idle flawlessly, and run great above half throttle. It'll push my McKee 14' 49mph.....FUN! But, from a dead stop, if I jam the throttle down, it takes the motor about 10 seconds to spool up....it's real slow. Once it gets to about 2500RPM, it takes off and runs great. Once back to idle,....and sitting for a minute or two, hit the gas again, and it bogg's and slowly takes off. I'm stumped.....Fuel pump? I took the high speed jets out and cleaned them this afternoon, they look good. HISTORY, fresh 93 fuel, running premix AMSOIL SABRE synthetic oil, new plugs, cylinder head temps running 116*-128* No scoring in Cylinders...boroscoped those. I'm an aircraft mechanic and not a boat mechanic......but I know a wrench and all YEAH,...I have 3 manuals. And another thing....I read threads for 4 hours looking for someone having the same issues before I asked for directions (it's a guy thing) Any help is appreciated!

 

[jtexas]

Re: 1979 Johnson 70 slow to rev

What RPM are you hitting, to get 49 MPH?

You must have about a 20 or 22 pitch prop on there...with that prop, I wouldn't expect any kind of hole shot at all. It's like taking off uphill in 4th gear. While pulling a load.

That's pretty awesome though.

 

[91kuhndog]

Re: 1979 Johnson 70 slow to rev

I'm running a 13"x19* stainless. It's funny you mention that...I just ran over to a friends house and borrowed a 12.5"x15*. I'm going to head to the water this evening to try it out. She chines pretty good at 48-49mph on flat water. The concern I have and thereason I'm searching for something else in this motor. I took a 60hp Evinrude off this boat and with that 13/19 prop, I could run 46-47 on the GPS.

As for the tach,....I don't have one on this motor/boat. I'm getting ready to add one and have to do some research on a tach for a 3 cylinder. thanks for the input.

Brad

 

[d.boat]

Re: 1979 Johnson 70 slow to rev

I'm running a 13"x19* stainless. It's funny you mention that...I just ran over to a friends house and borrowed a 12.5"x15*. I'm going to head to the water this evening to try it out. She chines pretty good at 48-49mph on flat water. The concern I have and thereason I'm searching for something else in this motor. I took a 60hp Evinrude off this boat and with that 13/19 prop, I could run 46-47 on the GPS.

As for the tach,....I don't have one on this motor/boat. I'm getting ready to add one and have to do some research on a tach for a 3 cylinder. thanks for the input.

Brad

I think most commonly available tachs simply have to be set to 3 cylinders. You can check the mfgr. website for tech and installation instructions which will tell you for sure. But I think any tach will work.

 

[jtexas]

Re: 1979 Johnson 70 slow to rev

what year was the 60hp? what was the hole shot like, with the 19p prop?

just make sure the tach says "marine" or "outboard", "2-stroke", and has a "6p" setting.

max recommended RPM on that motor is 5000. I run mine at 5200-5300. gear ratio is, I forget exactly, close to 2.5:1

Any idea how heavy the boat is? It's like a flats boat, right? Does it have a tunnel or stepped transom?
Do you have power trim and if not, what hole is it in?

 

[91kuhndog]

Re: 1979 Johnson 70 slow to rev

I got to digging around and found the 19P in the garage. The prop on the boat is a 22P (that suprised me), not the 19. I've installed a 17 and headed to the water in the morning. The 60 that came off of the boat was a 93 Johnson....so it was measured at the prop whereas the 79' I'm running is measured at 70, at the crank! I also found a OMC tach on ebay...it's on the way.

The weight is supose to be 665# but these boats are motorious for waterlogging and this hull could be holding a little water. It's a encapsulated hull and has foam injected between the two layers of the hull. It doesn't seem to be sitting low in the water but......

I'm running tilt & trim......

I intend to get it running good so I can tear it apart this fall and paint it.... the hull has some serious battle scars and some jelcoat repairs are in store. I'm converting it from a side console to center console and adding a Baystar hydraulic steering setup. Looking forward to that project! I have to get this motor properly tuned before it drives me nuts.....

 

[Challenger84]

Re: 1979 Johnson 70 slow to rev

wow 49mph!!
That's fast!
I think your hole shoot is effected by your prop.

Do you know what year they started measuring the Hp ratings from the prop and not the crank?

 

[jtexas]

Re: 1979 Johnson 70 slow to rev

yeah, I going out on a limb here & say you're getting all the hole shot you're gonna get with a 22" prop (didn't somebody estimate a 22p earlier? ). You might cheat a little on the pickup timing...just don't advance WOT timing beyond spec.

I have the same motor, only in blue & white.

 

[91kuhndog]

Re: 1979 Johnson 70 slow to rev

Hummm.... I put a 15P on it this morning and it's still luging out of the hole. It acts like it's dropping a cylinder when I nail the gas....as it comes up on the RPM, the third cylinder is catching and it's taking off. Even though the carbs have been gone through 2 times since October, I'm going to take it back to the marina and have the ndo it again. To me, I think it's down to a needle/seat issue. I've installed all new fuel hose, new fuel pump, replaced tank and hose assembly, installed new plugs threw a different set of coils on it thinking that might eliminate a weak coil.

It idles great, in fact it'll start cold without choking.... once up to speed, it's running great. I hooked my timing lite up to it this morning and ran cross the flats, it's running 5600 RPM withthe 15P. (max is supose to be 5000). Also, at no point does my timing drop off....I checked it all three coils and monitored it through the RPM range. I was hoping that one of the 3 cylinders would reflect a drop in RPM when it's lugging to indicate an ignition problem.

Challenger 84.... I'm not 100% certain, but I think around 1993, they started measuring prop hp versus crank hp.

 

[Challenger84]

Re: 1979 Johnson 70 slow to rev

^thanks for the info..
What kinda power difference would one expect from the different measurement?
example:
crank prop
120hp ?
100hp ?
80hp ?

 

[91kuhndog]

Re: 1979 Johnson 70 slow to rev

Allright guys....back off vacation and I took it out today and this is what I figured out....BTW,...I put the 19P prop back on it. It's lugging real bad out of the hole.... If I take the engine cowling off, it flies, jumps out of the hole and I get all the performance I could ask out of it. I went as far as driving home and grabbing another hood to try that....no difference. So, with the hood on, runs like crap, with the hood off, runs great. I speculate I'm getting exhaust fumes up into the cowling somehow amd that could be causing this condition. This is wierd, any of you old OMC tech's ever seen this. I do remember (a long time ago) something about the exhaust tube rusting (rotting) off and causing problems but don't recall the problems it caused....HELP!

 

[jtexas]

Re: 1979 Johnson 70 slow to rev

ok, here's a long shot but it'll be a pretty easy fix if this is it -- go round to the aft end of the motor, and take a look at the lower exhaust cover (see pic). Right above the exhaust relief ports. Gasket failure there will only affect low-speed operation.

Challenger, conventional wisdom says you lose 10-15%, but I have no scientific evidence to support that.

 

[91kuhndog]

Re: 1979 Johnson 70 slow to rev

Wow J....when i put hte powerhead back on last fall, I left that out. I saw it and knew where it went but for some reason, didn't reinstall it. I'm going ot put it in and head for the water. I'll keep you all posted on what I find. Sounds like a great explanation because exhause fumes appear to be the low speed culprit. Thanks and be back soon.

Brad

 

[91kuhndog]

Re: 1979 Johnson 70 slow to rev

Challenger & Texas, the difference between crank and prop hp is exactly as Texas said, you lose about 15% hp through the foot. I've seen some updated numbers that support the OMC's from 94 and newer actually supply more power than advertised...i.e. the 60hp 3cylinder actually puts out about 72hp at the crank, and 63-64 at the prop. The ole motor I put on the McKee is a 79' powerhead sitting on a 89 motor; I figure I lost a little power but when you have 70hp hanging on the back of a 600# boat....going 48-50 mph....WHO CARES... As the bumper sticker on my boat reads "Sit Down, Shut up & HANG ON!"

 

[jtexas]

Re: 1979 Johnson 70 slow to rev

Wow J....when i put hte powerhead back on last fall, I left that out. I saw it and knew where it went but for some reason, didn't reinstall it. ...
Brad

to get it back on in one piece, I believe the powerhead has to come off. Were it mine, I'd just cut the dang thing.

I said I had no scientific evidence for the 15%, then I remembered, this spreadsheet model I got from another forum. Based on the work of Daniel Savitsky and other experts in hydrodynamics. Using known parameters from my own boat, including 10% prop slip and 85% gearbox power loss, the model predicted my top speed within 3%. Probably related to ethanol-blended gasoline.

A sample size of one...let's call it psuedo-science.

You can download the spreadsheet from page six of this thread: http://www.boatdesign.net/forums/design-software/savitsky-power-prediction-2187.html

All except the input cells are protected and most of the verbiage is in Dutch (or maybe Afrikaans) so it's hard to follow the logic.

 

[91kuhndog]

Re: 1979 Johnson 70 slow to rev

Nice, thanks Texas. I pulled the side covers, and low and bohold, their was a brand new seal on there!. SO... I added 8 extra quarts of oil to the gas tank (smoke, lots of smioke) and went to the harbor. I ran the guts out of the thing with the hood on and off....what I found is I have a lot of exhaust coming up around the main shaft. I need to pull a manual to see where it can come from but I was able to confirm that it was in fact coming up through the cowl and messing with the air/fuel mixture. I put the 21P back on it to try it out and it ran 49.6 on GPS (on smooth water)....DONT ANYONE TRY THAT! The boat really chimes and walks around above 45mph. That run was done with the hood removed. SO my temp fix so my son can go fishing in the creeks.... I took an old hood off a early 80's motor I have....and drilled holes in the front of it to allow air in. I put some hull vent protectors over them to keep water from going straight into the cowl. At this rate, they can run the rest of the season and this winter I'll pull the powerhead again and investigate the exhaust issue. Thanks again Texas

 

[jtexas]

Re: 1979 Johnson 70 slow to rev

did you think of oiling up the gas like that? v-e-r-y clever.

 

[Dabbler_E]

Re: 1979 Johnson 70 slow to rev

I've heard of "smoke testing" but I think that's the first time I've heard of one where the smoke was on purpose!

 

[91kuhndog]

Re: 1979 Johnson 70 slow to rev

LOL... yeah, troubleshooting hard to find problems is easy....just add more oil! No really, it only made good sence to me and it worked. And we ran it again last night with the hull vent in the front of the hood and it runs great. I'm now in the market for a new inner exhaust tube for a 1979 Johnson 70......

 

[91kuhndog]

Re: 1979 Johnson 70 slow to rev

hey guys....sorry to throw this thread back up. As a note to anyone out there reading this forum and experiencing the same issue; I pulled the powerhead during the off-season and replaced the base gasket between the powerhead and mid-section. The gasket was fine (prior ot removal), the exhaust tube was fine....not rusted or rotted out, and the case seals were fine too. I learned this....the large donut gasket on the bottom of the exhaust tube is no longer available through OMC.... and hardens over time. As I put everything back together,...I heavily siliconed (100% silicone rubber) the seal surfaces and reinstalled the lower unit. I ran the motor in the harbor last weekend and the motor ran fine with the orignal cowling on..... As a note, my mother's mechanic in Ohio rebuilt or replaced a powerhead on a Johnson 60 this winter for the same issue, stating worn rings caused the motor to smoke excessively : ) and filling the cowling with exhaust fumes. She was eager to get her toon ready for the season..... and $2700 later, she's back onthe water! Troubleshooting note here......if your motor's not running as it should, remove the cowling and try it....sometimes, the smallest changes can contribute to an obvious issue... Good Luck!