1860 Jon boat/ 35 hp Johnson / 2260 rpm

[Oldstech]

Trying to get the best setup for the boat for what it is. Due to lake restrictions I have this 1989 25 hp long shaft and a 9.9hp long shaft. I have a bare bone Tracker 1860. Being I know the boat is under powered and trying to get rpms up on the 25, I put a 35 hp carb. At WOT with the 35 hp carb, I’m turning 2260 rpm with a 3 blade, cupped, 10 3/8 x 11 pitch prop at 19 mph. I’m been talking to a prop shop, he suggested removing my hydrofoil. Which I did and started cavitating the prop. So I installed a 5 inch jack plate. This stop the cavitating, rpms still in the 2000’s. He wants to decup the prop next. But feel there is some miscommunication with the prop shop and me.

Being the engine is under power for the boat, am I stuck. With a low rpm grunt, or could I really get a prop to put the engine in the right zone. BTW I’m really happy with 19 mph. Didn’t except to go any faster with the setup. I feel like I am killing the engine with this much load at a low rpm.

 

[WesNewell]

I'd be happy too if it were true since that's almost double the max possible speed of a 11 pitch prop at 2260 rpm. So either your tach or speedo is off quite a bit.

 

[Oldstech]

Thank-you for the response. Speed is off of GPS on a Garmon 93 I recent purchased. I feel that is fairly accurate. I will chase a rpm issue. The idle has been spot on with a timing light. I haven’t played with the timing light rpm and tach rpm at different rpm seeing if they matched.

i will say this, which is another reason i posted. The sound of the engine sounds like 2000s. When I had the hydrofoil still, I set the engine tilt to max, and had a buddy (250ish lbs) sit on the bow of boat. I was able to see 4000 rpms that day ( still 18-19 mph), and the prop actually grab, what felt like a slight second gear in a way. Being way I think the 2000 rpm is close. I have played with the tilt in the 3rd mid hole and 4th hole aft. Very slight change in rpm other the bow height.

i almost wonder if the prop is labeled wrong. But I have verified, the prop itself says the pitch is 11.

 

[jimmbo]

Being that a 1989 25 hp was prop shaft rated and there no prop shaft rated 35hp of the era, the 35 carb is from a pre 1985 engine. In reality you are looking at about 5 hp difference, which is about 1 - 1 1/2 mph
First of all, is the engine in Good Mechanical Shape? Compression numbers? Spark that can jump a 3/8” gap? Fuel System suppling enough fuel?
When you changed carbs... Did you verify that the Sync between the Spark Timing and Carb was correct? BTW , checking timing at idle, is a waste of Time, as there is no Idle Timing spec. On a 2 Stroke, retardation of the spark timing is how you set the idle speed, which will vary from engine to engine and it’s load. There is however a timing spec for what is called the primary pickup. That is covered under the link and sync procedure.
I know of no one who has Tachometer Ears that are accurate above 240 - 300 rpm, except a few Musicians. Get a Tach.
If an engine is tilted to ‘Max’ it usually means Trimmed Out, which will cause Bow Rise. Minimum Trim(as close to the Transom as possible) will keep the bow down.
When you removed the Hydrofoil, you said it started Ventilating, but lifting it with a 5 inch Jackplate stopped the Venting?

 

[WesNewell]

Physically impossible to get 19 mph with an 11 pitch prop at 2260 rpm unless you have a gear ratio of 1.25. However 19 KPH is ~11 mph. Perhaps that's what you were reading. The lowest gr I could find was for a 25hp NIssan/Tohatsu at 1.92 and it calcs out to a max of 12 mph at 2260 rpm.

 

[QBhoy]

Think your tachometer is in question here. She wouldn’t be anywhere near planing at 2000 rpm. Not a chance. If it’s a tachometer like the digital tiny tach that reads off the spark plug ht lead...then make sure it’s on the right setting for a 2 cylinder and a 2 stroke. If it’s not...it might be your issue.
Even if it’s a dash analogue tachometer, have a look to see if there are settings on it

 

[Truckguy701]

Have to agree with the tach is off. If that lil 25Hp is really pushing you above 20 mph it isn’t running at 2k

 

[Oldstech]

Ok guys, y’all were dead on with the tach. It did match at idle, but above idle it was way off. I took my timing light to the lake with me. The cupped 10 3/8 x 11 prop max rpm was 4200 rpm at 18-19 mph

 

[QBhoy]

More reasonable numbers there...if a little lower than the likely rpm range (at a guess)
at 19 mph, 4200 rpm and 11” pitch...estimating a reasonable 15% slip...suggests you have close to a 2-1 ratio...won’t be far off with those numbers I think.

 

[jimmbo]

Those engines had a 13:28 gear Ratio. WOT rpm range for the 25 was 4500 - 5500
A 30 or 35, would be 5000 - 6000. However, without going thru the Parts Lists and doing a Part by Part comparison, I can't say you have converted a Prop rated 25 to a Prop Rated 30, or to Crank Rated 35hp, by just swapping a carb, there were other differences, such as Intake, possibly Port Timing(different Block or Pistons), Cylinder Head, as well as Exhaust tuning.

 

[Oldstech]

Those engines had a 13:28 gear Ratio. WOT rpm range for the 25 was 4500 - 5500
A 30 or 35, would be 5000 - 6000. However, without going thru the Parts Lists and doing a Part by Part comparison, I can't say you have converted a Prop rated 25 to a Prop Rated 30, or to Crank Rated 35hp, by just swapping a carb, there were other differences, such as Intake, possibly Port Timing(different Block or Pistons), Cylinder Head, as well as Exhaust tuning.

Well this digs into more detail. I took a 1981 35 hp long shaft core ( bolt broke under the flywheel, I purchased this for $100 with controls) and a 1989 25 hp short shaft (115 psi- ran great) . I needed a long shaft, a good engine, so I freakin stein the two. The only thing that is original to the 25 hp is the power head. Exhaust, intake, carb, lower unit, all 81 35 hp. A lot lot lot of research and learning on differences in the two units. So yeah, should be the prop rated mythical 30 hp. That is so debated about. Thank-you for all the help.

So new news, I took a friends prop that is a very used Michigan PG 96 10 1/2 x 11 with the cup removed. I was able to go 5200 rpm at 20.5 mph. My goal is 5500 rpm.

 

[Sea Rider]

Before playing with props and their pitches, check that the conversion was done right upgrading a 25 HP to a 35 HP one, check if both models shares same CC powerhead same reed valves and runs on same or different timing advance. If timing advance is way less than it should motor will not run right, that's too much fuel intake while runnining with less timing advance.. Check specs, adjust if needed, then can play with pitches all you want...

Happy Boating

 

[Faztbullet]

The 1981 35hp was 30°...the 1989 25/30hp was 30° so timing isn't a problem if it has not been dinked with....