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

[Challenger84]

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 750 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

Update: I fixed the problem and got the boat to go 42mph on W.O.T.
Thanks to everyone that help.

 

[SparkieBoat]

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

do you have an RPM gauge?? do a compression check and test coils with a spark gap tester set a 7/16 gap. does pumping the bulb in the fuel line while running help any??

 

[Challenger84]

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

I don't have an RPM guage.
I will do a compression test.( Is it the same as an car? All spark plugs out and crank it?Test one by one)
pumping the bulb doesn't help.
Thanks for the quick response.
I appreciate it!

 

[sk]

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

clean the carbs. That helped me.

 

[reelfishin]

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

I've got a 1985 Johnson 70hp on a 16' Starcraft aluminum boat, a 1974 SS, which I figure is near 1,500 lbs in the water with two guys and that motor carries me faster than I care to go in that boat. I've yet to be able to let it max out at top speed on the river due to no wake zones and obstructions, but I'm sure it would pass the 40mph mark on the GPS. It sprints right up to 35 mph in a big hurry, it actually runs better than a buddies same model boat with an 85 hp Johnson V4 on it.

I figured the weight of my boat this way, the boat weighs 618 on a scale, the motor weighs in at 273 with controls and cables included, both me and my fishing partner are just over 300lbs, for about 625 total, plus tackle, ice, and a fishing cooler. Plus 18 gallons of fuel at 115 pounds, (Full tank weighed in at 116.77lbs). That puts the boat at full fishing weight with two guys at just over 1600 lbs, ad another 250 if a third guy comes along.
So those 70hp triples can really get the job done when their right. It's buy far my favorite motor. They are definitely the most bang for the buck when it comes to hp vs. weight.

I'd start with checking compression, to be sure you have a healthy motor to work with, check the spark on all three cylinders, and rebuild the carbs if needed.
You really need a tach to fine tune the prop situation, you want to see proper RPM at wide open throttle.
Yes, check the compression like you would a car, open the throttle plates and pull all plugs. Also, turn off the ignition when doing so to prevent ignition damage. I also like to keep the cylinders lubed a bit when doing a compression check. I just checked the compression on mine and got 145 on all three, with a Snap On motorcycle compression gauge set. I typically check the compression in my motors each season, and toss in a fresh set of plugs at the same time. My 70hp is a non VRO motor, which came sans tilt and trim which I added from a V4 I junked, and it's had it's carbs done twice in 5 years now. I've owned it about 7 years now. I've got two more triples just waiting for a boat to use them on.

 

[Challenger84]

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

I had the carbs done already and my head replaced due to low compression on one cylinder.
I've read somewhere on here that it might be a "spun hub " or something like that. Not sure what that means and how to fix but if someone show me the right link that would be great!
My boat capacity is 900 lbs. so I am well under the limit with 4 guys that are about 150lbs.

Sorry for the car metaphor but can it be something like a "clutch" slipping?

The engine sounds great when on muffs. Idles very smooth and when I give it a little gas she picks up right away.

I just bought a tube and gear for a BBQ at the lake that my family will be having soon and now I feel like I can't even pull my own weight let a lone a tuber and a spotter.

Maybe the boat is water logged?
When the boat is in the water it does not "feel heavy".
In the water it sits up pretty high and doesn't "look" weighed down.
example: when in the water the water line comes up 3-4 inches under the black paint of my boat.
When the four of us hoped on board the water line got to about where the black paint starts.
I have included picture to show you what I mean.
Thanks again for all the help!

 

[cougar1985]

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

have you tried the 15 prop ?its very possible that the 17 does have a spun hub.you can check this by scribing a mark on the inside of the hub and on the rubber part also.test drive ,if after they dont line up anymore you have a spun hub which is sort of like a clutch slipping ,yes.you really need a tac also so you know where your running in wot rpm,s.try the 15 ,if you notice a vast improvement your 17 is spun .also without the motor running verify that the carbs open completly when the throttle is advanced.do not do this with the motor on muffs and running as you will most likely blow up your engine.you should never run on muffs at anything more than fast idle as you may get a runaway motor due to the lack of load and there goes your nice 70,pow!for tubing you might want the 15p anyways.

 

[abarron]

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

Man that's slow! are you sure that your trodle is working at it max!

 

[Challenger84]

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

I have a fast idle lever on my control and when I engaged it the OB screams really loud. (this is on the muffs).. I don't use the fast idle.
Should I?

I'll swap out my prop with the extra one that came with the boat and see what happens. It's in better condition anyways.

When I do the compression test what kind of numbers would I be expecting?
145psi?
Sorry for all these question. I'm a Noob when it comes to boats!
I'll try these things out in the AM and update you guy.
Thanks again for all the help!

 

[N Wolf]

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

By "fast idle" he means about 1500rpm. The fast idle on your throtle lever will probably put you up to 5g or more, not good. I dont know what kind of experience you have so please dont be offended, are you sure that you have the motor trimmed all the way down before you take off? Does the bow point real high when you try to take off?

 

[Chinewalker]

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

If the prop hub is slipping RPM would go up while speed goes down. You state at the start that it doesn't sound like it's revving up underway. Revving on the muffs tells you nothing with no load on the motor.

SOunds to me like you're running on fewer than 3-cylinders. Verify spark in all three cylinders. Verify compression is at least 120psi in all three cylinders with no more than 5% variance between high and low readings. Verify all three carbs are getting and feeding fuel. Fuel + compression + spark = running motor.

 

[Challenger84]

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

By "fast idle" he means about 1500rpm. The fast idle on your throtle lever will probably put you up to 5g or more, not good. I dont know what kind of experience you have so please dont be offended, are you sure that you have the motor trimmed all the way down before you take off? Does the bow point real high when you try to take off?

No offense taken.
Yes the front does shoot up kinda high.
Well I have it trimmed down as far as I can go without scraping it on the ramp going down to launch. It's a manual trim and I'm on the second bolt hole.
I don't know how to lower it any further without damage to the prop.
Is my trailer too low?
How does everyone else trim the OB all the way down with a manual trim?

If the prop hub is slipping RPM would go up while speed goes down. You state at the start that it doesn't sound like it's revving up underway. Revving on the muffs tells you nothing with no load on the motor.

SOunds to me like you're running on fewer than 3-cylinders. Verify spark in all three cylinders. Verify compression is at least 120psi in all three cylinders with no more than 5% variance between high and low readings. Verify all three carbs are getting and feeding fuel. Fuel + compression + spark = running motor.

I will check compression in the morning and get back with the result.

Thank you everyone for their help.

 

[cougar1985]

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

being a manual trim means you got to do the work manually friend.with the boat out of the water go to the back and lift her up(motor)by the lowerunit and there is a lever on the side that will lock it in for trailering and launching.when you,ve launched walk back to the motor and pull on the cowling and release the lever and the motor will go down on its own.reverse for when you get back to the dock.

 

[SparkieBoat]

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

I have seen trim bog a motor out...yes compression test is done same as a car..hope for 120-130 PSI but most importantly all cylinders need to be with in %10 of each other. do a spark check with a spark gap tester..need strong spark at 7/16 gap...motor will idle ok and seem fine in driveway on 2 cylinders.

 

[Andy in NY]

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

No offense taken.
Yes the front does shoot up kinda high.
Well I have it trimmed down as far as I can go without scraping it on the ramp going down to launch. It's a manual trim and I'm on the second bolt hole.
I don't know how to lower it any further without damage to the prop.
Is my trailer too low?
How does everyone else trim the OB all the way down with a manual trim?



I will check compression in the morning and get back with the result.

Thank you everyone for their help.

watch this video, at the 2 min mark you will see how it goes down... you cant see the lever, but its on the port side of the motor just under the cowling.

 

[mike1245]

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

i trim mine down by putting a seat post in there to hold engine wile i back it down the ramp then lift motor and remove seat post and lep motor go all the way down thats the one man way without getting wet my pin stays in the first hole

 

[Silvertip]

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

An engine that "idles nicely and revs quickly" while on muffs or in neutral means absolutley nothing other than the engine is actually running. There are a number of things to check before you start throwing parts at this engine. Start by just listening to the enigne. Does it actually sound like all three cylinders are firing? A spark tester or inductive timing light clipped to each plug wire will tell you that in an instant. If one or two cylinders are not firing you have ignition problems. Next, verify that the butterflies in each carb are opening fully (fully horizontal in the throttle bore). If not, the engine needs a Link & Sync procedure. NO -- this will not be provided here (at least by me) since it is very long and you should have a service manual for your engine if you attempt this.

You do not have a spun hub which means the rubber that separates the blade area on the prop has lost its bond with the hub thus causing the hub to spin inside the prop. That cannot be the issue because that condition causes the engine to rev high but the boat doesn't move.

You have one head and heads are not typically replaced due to low compression. A head gasket however is replaced for that reason. I would be concerned about that since overheating can blow a head gasket and cause internal engine damage. NOW is the time to check compression.

Yes - a boat full of water soaked foam can turn a boat into an absolute slug so investigate that.

 

[cajuncook1]

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

I was wondering if you check for spark on all cylinders? The power pack could be giving you problems. Had a 55hp Evinrude that would start up and idle fine, but run at half speed/power. The powerpack can give out completely or partially without notice, but usually partially. When I discovered that not all the cylinders were firing and excluded the coils, then I replaced the powerpack and I was back in business!!

Just a consideration to keep in mind.

Good luck with your boat!

cajuncook1

 

[Challenger84]

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

Here's a little update, some good news I guess.
I did the compression test and here are theresults
Cylinder 1= 130psi
Cylinder 2= 125psi
Cylinder 2= 125psi

So it looks like a have good compression on all cylinders.
I will be doing the spark test next and keep everyone updated.

Thanks everyone for all the help and advice.

 

[bktheking]

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

WOT @ 15mph=down a cylinder, spark or fuel