1998 DT-40 40hp 2 Stroke - Rough Reverse

trashcanslam

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Nov 14, 2019
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Hello all,

Background: I upgraded from a 9.9 johnson on a gheenoe to a 40hp 2 stroke suzuki on a stumpknocker. The boat was cheap, but I took a risk on it considering the seller could not start the boat for me, nor demonstrate the tilt and trim working. Fortunately, I was able to get the tilt and trim and motor to run like a top right now, except for a few issues I describe below.

What I've done so far:

Replaced the carburetor. The old one had gas jelly inside the bowl and adjustment screws broken off. Bought a new one off amazon, and had to retrofit it a bit so the throttle linkage could be hooked up on both sides as it was before. Frankly, this was a surprising to me, since the carburetors were identical except for the linkage on the sides. I have not yet calibrated the carburetor.

New Impeller : and water pump housing. The seller said he changed them, but I did not trust his work considering the engine did not pee on muffs. The old impeller was fine, except the arms (not sure of the correct nomenclature) were bent in different directions. The problem, however, was blockage caused by dirt-like debris deep inside the water passages. I managed to get that all cleared out, but not before I had to remove the powerhead to get deep enough. The motor pees great now. Changed the thermostat which was corroded to heck just for fun.

Problem: In doing all this, I disconnected the shift rod from the lower unit. I goofed, and took the barrel nut completely off. When I put it all back together I realized the F-N-R positions were all off. Neutral was Forward, Reverse was Neutral, and Reverse proper was not there. So I put it in forward, disconnected everything, pushed the shift rod all the way down, and reattached everything. On muffs it shifts fine, and in the water the motor runs great EXCEPT, reverse. When I put it in reverse, it grinds and somewhat works, although not smoothly. When I put it all the way in reverse, it lurches and the rpms rise rapidly.

What I am wondering is whether the linkage on the shift rod is still off, can it be adjusted, OR is this a sign of the clutch dog slipping? I don't know whether this problem existed before because the motor did not run before I got to work on it. Your help is very appreciated.

Finally, one problem I did encounter yesterday is that the motor will shut off if I slow down from +20mph to about 8mph or neutral. I am thinking this could be an unadjusted carb, as I have not adjusted it. Do I simply adjust the mixing screw, or does this model have a low idle screw I need to adjust as well?


Thank you for your time in reading this, and thanks ahead of time for your answers! Links to posts and readings on this are very appreciated. I had Leroy's ramblings to go off of with the johnson. I wish there was something comparable with suzukis.
 

racerone

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Dec 28, 2013
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Need to fine tune the barrel nut.----If reverse is a DOWN motion adjust so that rod goes down say 1/8" more.------Then test run.
 

trashcanslam

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Nov 14, 2019
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Need to fine tune the barrel nut.----If reverse is a DOWN motion adjust so that rod goes down say 1/8" more.------Then test run.

Thanks racerone, I suspected this as well. The shift rod comes UP for reverse and pushes DOWN for forward. Based on this, do you suggest going UP by 1/8"?

Thank you.
 

racerone

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Dec 28, 2013
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Agreed------ make it pull the lower shift rod up say an 1/8" or so.
 

Sea Rider

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Sep 20, 2008
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With the motor sufficiently warmed up, say first morning start, does the motor shuts off when geared forward ? If so need to adjust the mixture screw and then the idle screw while the combo is floating in water. With rspect to the final problem encountered, Is the fuel pump in good working condition, are the fuel filters, fuel lines immaculate clean ? was the tank's air valve opened when the motor failed ?

Those carbs has fixed jets which starts working once idle rpm is passed, there's nothing to adjust. It that motor were mine would squirt half a can of power tuner through the carb's throat while geared, install a new set of spark plugs well gapped to specs afterwords the decarbon process., that 22 year oldie should idle well and run good throughout the entire rpm range.

Happy Boating
 

trashcanslam

Seaman Apprentice
Joined
Nov 14, 2019
Messages
38
With the motor sufficiently warmed up, say first morning start, does the motor shuts off when geared forward ? If so need to adjust the mixture screw and then the idle screw while the combo is floating in water. With rspect to the final problem encountered, Is the fuel pump in good working condition, are the fuel filters, fuel lines immaculate clean ? was the tank's air valve opened when the motor failed ?

Those carbs has fixed jets which starts working once idle rpm is passed, there's nothing to adjust. It that motor were mine would squirt half a can of power tuner through the carb's throat while geared, install a new set of spark plugs well gapped to specs afterwords the decarbon process., that 22 year oldie should idle well and run good throughout the entire rpm range.

Happy Boating


Hey Sea Rider,

It does not shut off when geared forward. It won't shut off under any circumstance except for when it is slowing down from going 3/4 WOT to just above idle or geared into neutral. The fuel pump looks new, and considering the above being the only instance of shutting off, I wouldn't suspect it as the culprit. I can idle all day without it stalling, its just when it slows down significantly.

Can't attest to the cleanliness of the lines, although it would seem as though they flow the gas well. I'm going to change them out as a precautionary measure. I'm also going to change the spark plugs today.

I do have a question for you: are you saying I don't need to adjust the mixture screw at all? The carb came with the mixture screw seated. I did not adjust it because the motor seemed to run fine as is. I do know, however, with my johnson you would need to adjust it by turning 1 1/2 counterclockwise and adjust from there.

Also, is there a point in squirting power tuner into the carb if its brand new?

Let me know, thank you!

EDIT: Come to think of it, the carb was so filthy and gunked, I wouldn't be surprised to find the spark plugs are fowled to heck. Incorrect air-fuel mixture could have fowled the plugs, even if they are relatively new. Can't wait to replace them out today.
 
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Sea Rider

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Sep 20, 2008
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Some issues : That's a 2 stroke 2 or 3 cylinder motor, it's a pre mix or autolube model?

Your post states being a 1998 motor, if brand new no need to perform a decarbon. Air fuel mixture screw won't foul plugs, only good when adjusting idle/ralenti rpm, when at higer rpm the fixed jets comes into action. If motor runs fine don't need to play with the selaed mixture screw, unless has moved from the original place when adjusted at the factory, which isn't known.

Adjust the idle screw a bit CW to raise a tad the idle rpm, check if keeps doing that.

Happy Boating
 
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trashcanslam

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Hello,

Posting an update on the issues.

Racerone - the 1/8th adjustment on the shift linkage did the trick! No more grinding in reverse and it reverses nicely now!!

As for the stalling issue, I took it out again on Sunday. I turned the carb screw above the carb throat 1.5 turns out. I changed the plugs out before I hit the water and will be posting pictures of what they looked like before. Definitely fouled. Did a nice 16 mile trip there and back on the river. It stalled out once when I was coming off of plane and approaching the no wake zone. This is how the issue happened in the past. It ran for a bit in idle and turned off. There was a click, click, click, click sound when it turned off, as before. 4 or 5 clicks in rapid succession. It started back up immediately.

I realize the amount of possible issues this could be is enormous. I was hoping someone here could help me narrow down the list of suspects. I'm going to post a pictures of what the old plugs looked like. The carb is new, although I think it needs to be adjusted. I think it helps knowing the issue only occurs when coming off of planing speeds. It will not happen otherwise.

Please let me know if I can provide more information to narrow down the list of possibilities.

Thank you!
 
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trashcanslam

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Sea Rider

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Sep 20, 2008
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Is that a pre mix or autolube 2 stroke 40 HP motor ? Are those the plugs and heat range recommended by the factory, or someone recommended them to you, first time seeing those plugs ? The click, click sound is normal, happens when any motor shuts istself down when has been running geared forward, it's a gear box/prop issue.

Happy Boating
 

trashcanslam

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Nov 14, 2019
Messages
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Is that a pre mix or autolube 2 stroke 40 HP motor ? Are those the plugs and heat range recommended by the factory, or someone recommended them to you, first time seeing those plugs ?

Happy Boating

Hey Sea Rider,

It's oil injected. Those are supposed to be some fancy plugs I've never used before either. Keep in mind I just bought this boat and motor so it was the prior owner who used them. I put manufacturer recommended NGK's in there. Didn't solve the issue though.
 

Sea Rider

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Sep 20, 2008
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To have such fouled plugs something must be wrong with the auto lube system, try to get the Service Manual, on some autolube motors you can adjust the oil delivery following a precise procedure. To test the autolube sytem if being faulty run a small portion of pre mix 50:1 fuel/oil ratio and check if the spark plugs keeps fouling badly. Will need to disconnect the auto lube if possible or remove all the oil contents from the plastic container so the motor only runs on the pre mix ratio.

Happy Boating
 

Faztbullet

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Mar 2, 2008
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15,589
Get rid of them E3 plug as they are more designed for 4 strokes...stick the original NGK"s back in
 

racerone

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Dec 28, 2013
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Plugs are black like that because the plug is too cold.-----Reasons could be as follows.----#1 too much fuel.-----#2 thermostat stuck open or missing.---#3 low compression.---------Fact the sparkplug needs heat to stay clean.-------I have trolled for hrs with a 1963 model 10 HP Johnson at 24:1 no fouling and motor runs awesome.-----8 year old can start that motor by himself. As stated put the factory plugs in in check the facts.
 

Sea Rider

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Hey folks, the OP has stated on post 11 : put manufacturer recommended NGK's in there. The OP does not troll, likes running at high rpm. Plugs tend to foul themselves when the operating temp is under 500 deg C. That's the non self cleaning temperature..

Happy Boating
 

trashcanslam

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The plugs I put in there are regular no-frills NGK's as recommended by the manufacturer's sticker inside the powerhead. The fouled plugs are the ones prior owner had in the motor.

Who knows what could have caused such bad fouling while he had the boat? Considering what the inside of the carburetor looked like, I bet he did all sorts of things wrong. I am not a professional, and I am learning how to work on my motors because most of my local mechanics won't give me the time of day to look at a 22 year old motor (I was refused when I needed help with the 1980 johnson haha), but I try to keep things by the book. Prior owner, however, very clearly did not have the first clue about maintenance or proper operation, so as soon as it wouldn't start he tried to sell it.

The plugs are not fouling right now, but I will keep inspecting them.

I don't want the plug issue to be a red herring here, as I am trying to narrow down the reasons for why the boat runs great all day and stalls only when coming off of planing speeds.

Thank you.
 
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