70hp Johnson WOT 15mph? What is wrong? pls help...

[LongLine]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

You have a cylinder missing. If prop spun, your engine would rev real high. Simple (cheap) stuff first - make sure your wires are ok and properly connected.

Tom B.
(LongLine)

 

[Challenger84]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

GREAT NEWS!!!
So after all the diagnostics
I checked compression on all cylinders and they were good.
Checked spark, all cylinders firing!
What I did was a cylinder balance test.( Like on a car )
Pulled one spark plug at a time to find the bad cylinder.
As I pulled each spark plug I listen to the engine to see if it makes a difference at idle.
Found cylinder one was bad.
Checked the fuel and saw fuel going to the cylinder but not enough.
I adjusted the carbs to allow more fuel to enter.
I then checked for a fuel filter and found that it was all gunked up!!
Cleaned that out!
Took the boat on the water and at WOT I got her up to 42mph!!
I also check to see if I had a spun hub by marking a straight line on both the hub and prop.
When I took it out the water the lines still matched up.

42 MPH!!!!
I'm sure it can go faster with it fine tuned but 42 mph is just a HUGH difference.

I'm so happy with the results!!!
I would like to THANK everyone that gave me help and advice.
I can't wait to take the boat on the lake with the family and make lasting memories!!

THIS IS THE BEST SITE FOR ANYTHING TO DO WITH BOATS! *fact*
The members make this site great!!

 

[cajuncook1]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

Sweeeeeeeeeeeeeeeeeeeeetttttttttttttttttttttttt!!!!!!!!!

Be safe and make sure all the kids and even the parental kids have there life jackets on!!!

 

[Andy in NY]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

glad to hear it worked out!

i had a simmilar issue last suimmer and idnt even think to mention it... wot was a dog... turned out i had a clogged high speed jet.

 

[Challenger84]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

Yeah definitely sweeeeet!
I've put a lot of money and time into the boat to find out that one cylinder was out.
I was so bummed.
When I got her up and going you can't imagine the joy I felt!!
As I slowly punched the throttle forward the motor started screaming!
The boat go on plane really fast and the ride was incredible!
Just wow!
Thanks again!
When ever you guys are in Northern California I will take you out for some cold ones or a cruise around the Bay!!

 

[Andy in NY]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

im hoping for the same kind of showing form my new to me 85 hp... should scream compared to the 50 i had. but ive only run it on muffs, and it was in pretty rough shape when i got it.

 

[Challenger84]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

^yes best of luck to you!
You should also do a cylinder balnce test as well.
Start it up on the muffs and pull the spark plugs one at a time to see if it makes a difference in the idle.
If it does then that's a good sign!
Make sure you put the wires on the spark plugs a little loose so you can pull them off easier.
Careful not to get shocked.
Use some pliers with a rubber handle or rubber gloves!


Quick question.
As I pulled off the cover to get to the carbs I didn't see an "air filter" like they have in cars and trucks.
Should I have one?

 

[SparkieBoat]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

no air filter

 

[Challenger84]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

Thanks Sparkieboat!
Still a noob to boats, but if anyone needs help with their trucks that tow thier boats I'm here to help!

 

[Andy in NY]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

Thanks Sparkieboat!
Still a noob to boats, but if anyone needs help with their trucks that tow thier boats I'm here to help!

dont feel bad i was the same way. i actually had to ask how to mix the gas and oil when i got my first boat!

 

[Challenger84]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

So yesterday the weather was around 80* so I took the boat out with some friends.
I had 5 adults and full gear. 11 galoons of fuel, ice chest,fishing gear ect.
I could only get the boat to go 28mph. With my 42mph run it was just me and 3 gallons of fuel.
Whata diference the weight makes.
The bow was in the air really high so I told a few people to sit up front which got the bow down a little bit.
I'm still playing with a few trim settings to get her just right.
But I'm very pleased with how well the OB preforms.
These little 70 Johnson's really get the job done!
I want to try out the 15 prop because it's in great shape and I'm curious what the difference it would make.
What kind of performance gains or no gains should I be expecting?

 

[mikesea]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

you will get up on plane faster ,but will lose top end speed,if your going to cruise with your friends as you did,you may be better off with the 15,that dragging to get up on plane ,isnt the best for your engine,it wants to run,but also ,with the 15 ,be sure you dont go over the 5500? rpm max rpm too often an long

 

[Challenger84]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

I will be adding a Tach soon!
Doing some reasearch on how to rig it up.

 

[torbjorn]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

I just got this boat
It's a 84' Charger 1600 w/ 83' 70hp Johnson
I've read somewhere on here that the boat weighs about 850 lbs.
So I change all my oil and spark plugs today.
Sea foamed the fuel line and cylinders, Started it up on the muffs and it started up rite away.
Let it run til the smoke cleared and it idles nicely.

I took the boat out to test it and was very disappointed to say the least.
I had a total of 4 people on board w/ no gear and about 5 gallons of fuel.
At WOT the boat clocked in about 10-15mph on my gps!!
The prop I have on it now is a 13-1/4 x 17
I have an extra prop 13-3/4 x 15
The OB doesn't "scream" like other OB @ WOT.
I don't have a clue what to do can someone please help me?
Thanks for looking

Let's assume that the boat's capable of 30 mph with 70 hp. that might be an overestimate (speed goes like the square root of hp at the propshaft divided by total rig weight). With a 12:29 gear ratio, 17" pitch prop, and assuming 20%'slip' the motor'd turn 5600 RPM at 30 mph. The 70/75 is 'rated' at about 5500 RPM, 5000-6000 RPM (and will turn a lot higher-this is the same powerhead as on the much higher geared short shaft 75 racing model but with lower port timing, smaller carburetors, and lower compression). You can safely drop to 15" pitch. At 30 mph under the same assumptions the 15" pitch prop should turn 6400 RPM. The only extra advice is to run 37:1 gas to oil (use only Evinrude, Mercury, Castrol or Yamaha TC-W3 oil) from 6000-6500 RPM. And, get rid of the surface gap plugs, replace them by Champion L77JC4s. Surface gap plugs will foul if you idle too long.

Good compression on that motor is 130psi, 100 psi is way too low.
Use 93 octane gas, not 87.

OMC Mechanic

 

[torbjorn]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

you will get up on plane faster ,but will lose top end speed,if your going to cruise with your friends as you did,you may be better off with the 15,that dragging to get up on plane ,isnt the best for your engine,it wants to run,but also ,with the 15 ,be sure you dont go over the 5500? rpm max rpm too often an long

If the motor's in good condition (bearings, rings, cylinders) then 6000 RPM is no problem with the 49.9 c.i.with 50:1 Evinrude oil. If you use XD-100 Synthetic you can turn the motor 8000 RPM at 50:1. Otherwise, use 37:1 6000-6500 RPM, 25:1 with TC-W3 oil over 6500 RPM. The OMC engineer who was responsible for the motors in that era recommends 25:1 Evinrude, Mercury, Yamaha oil over 6000 RPM. I found I could run a little less up to 6500.

I raced the 3 cylinder 75 for 9 years, on V-bottoms we turned it 6500-6800, on tunnels the motors turned 7200-8000 RPM. Just don't use cheap oil, and use enough oil.

OMC Mechanic

 

[SparkieBoat]

Re: 70hp Johnson WOT 15mph? What is wrong? pls help...

when figuring an estimated top speed, not only do you take into account weight, horse power and hull design, but I have also found that a rough bottom hull can really make a big difference, I was skeptical at first but after much experimenting, I found that a 17.5 McKee craft that I own that had some barnacles on the bottom before I owned it, ( i scraped the few left off) ran way slower than my 17.5 aqua sport about same weight but had slick bottom with same exact motor and prop. the aqua sport got around 10 MPH and 800 RPMs more. so each boat can see big differences even though the same weight. the hull is not that rough in my opinion but it seems that it needs to be sanded and repainted.