18HP saga continues. Can't get past 14mph and 3,900 RPM.

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stephenspann27
Posts: 88
Joined: Thu Oct 27, 2016 2:16 am

Re: 18HP saga continues. Can't get past 14mph and 3,900 RPM.

Postby stephenspann27 » Sat Oct 07, 2017 1:14 pm

BillW wrote:Yeah, revving in neutral doesn't really prove anything. It takes very little effort to make it WAY past 4500 when no-loaded. It would probably do it with one plug wire off. Very bad form. ;)

As for the adjustable jet thing, I was thinking that even if it didn't fit in the lower pan, you could cut one short, file a slot in the end and tune it with a screw driver, or something of that nature, just to prove or disprove a point.

By the way, 18s do use a ton of fuel. Even good ones. Maybe it's the relatively small carb bore of the earlier ones. (Pre-'69) I don't know why; but they do. I don't want to start a crap storm but Mark 25s are way better on fuel, in my experience.


yea I knew I would get flack over reving it in neutral. I just wanted to show that it can do it with no load, which even though it doesn't prove anything, it does show that the motor can spin that fast. Also ignore the high idle it had in that clip, I adjusted it back down to 800 later. I was messing with the low speed mixture, and it throws the idle off everytime you do that.

You comment about the 25hp motor: I actually have a 1989 Mariner 25hp motor sitting in the garage. It's got a water intrusion problem I'm trying to solve (already replaced the exhaust plate gaskets and there is no head on these motors) I'm sure once I fix the water intrusion on the 25hp motor and run it on my boat I'll forget all about the 18hp. I just don't want this 18hp motor to beat me. I keep reading about how great they are, and it is about 30lbs lighter than the 25hp motor. Since I stripped the 18hp motor all the way down, sandblasted and painted it, it looks nice and I have at ton of time in it. I've replaced quite a few more parts on the 18hp motor than I've mentioned, so in additional to time, I have quite a bit of money into this thing, so I'm reluctant to give up.

FrankR
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Location: Florida

Re: 18HP saga continues. Can't get past 14mph and 3,900 RPM.

Postby FrankR » Sat Oct 07, 2017 2:51 pm

I have a reading problem---don't like to do it. So, with out going through this long saga, maybe I'm asking something already brought up.
Fuel pump diaphragm ruptured?? That'll dump extra gas into the crankcase, making it run rich.

BillW
Posts: 679
Joined: Sat Apr 11, 2015 6:35 pm
Location: Cape Cod MA

Re: 18HP saga continues. Can't get past 14mph and 3,900 RPM.

Postby BillW » Sat Oct 07, 2017 5:48 pm

The Mariner may be faster but the 18 will idle better. The Mariners (and their Merc cousins) idle like a muffled dirt bike. I am a self-proclaimed "Merc guy," but I never liked those 25s.

fleetwin
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Location: Rhode Island

Re: 18HP saga continues. Can't get past 14mph and 3,900 RPM.

Postby fleetwin » Sat Oct 07, 2017 6:22 pm

OK, so your boat performed nicely with a newer OMC 15hp on it, and your Dad's rig performed sluggishly with your engine. This would seem to rule out hull problems. The 9" pitch prop is the smallest available, so the RPMs should be up there....Just review with us what tach you are using to determine you are only getting 3900-4000RPM. All in all, I think the carb swap is the best next thing to try....

stephenspann27
Posts: 88
Joined: Thu Oct 27, 2016 2:16 am

Re: 18HP saga continues. Can't get past 14mph and 3,900 RPM.

Postby stephenspann27 » Sat Oct 07, 2017 10:59 pm

fleetwin wrote:OK, so your boat performed nicely with a newer OMC 15hp on it, and your Dad's rig performed sluggishly with your engine. This would seem to rule out hull problems. The 9" pitch prop is the smallest available, so the RPMs should be up there....Just review with us what tach you are using to determine you are only getting 3900-4000RPM. All in all, I think the carb swap is the best next thing to try....


I'm using a "tiny tach" you can see it in the video links. There is a 9.25x7 prop out there... but I don't think my boat is near heavy enough to warrant that pitch, the 9x9 should be more than enough. I've ordered the dual knob carb..

fleetwin
Posts: 1943
Joined: Mon Mar 09, 2015 4:27 pm
Location: Rhode Island

Re: 18HP saga continues. Can't get past 14mph and 3,900 RPM.

Postby fleetwin » Sun Oct 08, 2017 9:19 am

Yes, I agree, having to under prop with a 7" prop is just hiding a big problem with a band aid. OK, you were using a "tiny tach", have you tried it on any other engines? I am assssuming you used a GPS to get the speed info. Certainly seems like something is holding the old girl back, hopefully the different carb will answer some questions with positive results....

stephenspann27
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Joined: Thu Oct 27, 2016 2:16 am

Re: 18HP saga continues. Can't get past 14mph and 3,900 RPM.

Postby stephenspann27 » Sun Oct 08, 2017 10:20 am

Haven't tried the tiny tach on other engines but it seems dead accurate to me. And for speed I"m using GPS.

stephenspann27
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Joined: Thu Oct 27, 2016 2:16 am

Re: 18HP saga continues. Can't get past 14mph and 3,900 RPM.

Postby stephenspann27 » Thu Oct 12, 2017 10:40 pm

I've got a carb with the adjustable high speed circuit. Hope to test tomorrow.

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BillW
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Re: 18HP saga continues. Can't get past 14mph and 3,900 RPM.

Postby BillW » Fri Oct 13, 2017 5:57 am

I count four carbs total so my hat is off to you. You are really going after this thing!

As a word of encouragement, I will tell the story of a Mark 5 Mercury I have. Of course, it has a high speed mixture adjustment. One day, I put it on a 12 ft aluminum boat and dialed it in nicely, planing that boat off. Awhile later, that same day, I put the same motor on an 8' Tiny Titan homemade hydro. It would barely plane the hydro off, it was so rich for that boat change. Leaning it out more for that boat made a HUGE difference. It planed off and went like an entirely different machine. My point here is that fixed jets are great but cannot possibly be right for every boat. Good luck.

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Garry in Tampa
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Re: 18HP saga continues. Can't get past 14mph and 3,900 RPM.

Postby Garry in Tampa » Fri Oct 13, 2017 3:41 pm

The fixed jets were supposed to keep the constantly fiddling ignorant motor owner from constantly mis-adjusting his motor. I believe they introduced almost as many problems as they fixed. You could no longer adjust for humidity, boat load or altitude. I have put an adjustable needle valve in every fixed jet motor I intended to run. A tiny bit of crud can lean out a fixed jet enough to cause damage under load. Likewise overly aggressive cleaning (using a small file to clean out an oriface) can cause a loss of power and an increase in smoke and fuel use. Then again, that's just my personal opinion . . . :lol:
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