Suzuki DT40 fails at high RPM

[BoatAddicti0n]

Any input will be really appreciated!

Not being really experienced with outboards, but not (so) stupid, I have been trying to make my engine perform well at high RMP, but I have failed.
Happens that my Suzuki DT40 (1990), which is (was) in quite good shape fires 100% successfully, runs perfectly at low RPM, and idles forever.
However, it stalls/fails after some point when I push(open) the throttle, recovering if I release the throttle. It actually runs at high RPM, but fails after some time, lets say 10-15 seconds. Seems like missing fuel or something like that. Note here in Sweden the water is today ranging close to 42 F.

WHAT I HAVE DONE:
I have ensured new/clean gasoline, new fuel hose connector toward engine(was broken from last year), new external filter, cleaned the "internal" filter, new plugs, connected a straight fuel line(no filter or bulb), cleaned the carb, but unfortunately did not change the behavior. There was some resin in the bottom of the carb float chamber, and cleaning the carb seems to have improved the performance (not the bugging behavior). I have been playing with the carburetor's pilot air screw, which of course changes the performance, but did not change the behavior.

WHAT I HAVE NOT NOT NOT DONE:
- Did not change the pump
- Did not change the fuel hose connector toward the tank (that seems OK)
- Did not change the fuel line.

QUESTIONS:
- Should I change the PUMP, clean it, or anything?
- Any other suggestion?

THANKS A LOT!

 

[99yam40]

Re: Suzuki DT40 fails at high RPM

Test to see what problem is

http://forums.iboats.com/showthread.php?t=283363

 

[BoatAddicti0n]

Re: Suzuki DT40 fails at high RPM

THANKS FOR THE INPUT!

I took the action to double-check the spark plugs and they are brand new original NGK BR8HS (0.9mm gap), as required.

I did not manage to find a vacuum gauge yet, but following the Admiral's "HOW TO: Testing your boats fuel system" recommendations, I have done everything I could: The test for airbound/leakge. The result was OK, no bubbles, whatsoever. But still stalling at WOT.

In addition, as I had them at home, I have replaced tank for a brand new plastic one, and brand new fuel line also. Fail again at WOT.

For a quick test, I have connected the fuel line directly to the fuel filter/pump, bypassing the fuel connector. This, together with new tank/line, ensure me there is no restriction in the boat side. The result was the same: stall after aprox 10 seconds of WOT. I have bypassed the filter, no chages in the behavior.

I could not find a vacuum gauge, as I said, but I did find a diaphragm kit for the fuel pump. I have replaced it, as I wanted to ensure its function. The result was the same, fails at WOT.

My boat, Joe the Boater's, has no tach, so I am running blind. It is a Flipper 470S (240Kg) and is reaching aprx 23knots.

The engine, has been extremely reliable, fires at first spark, runs low rmp like a clock.

Now I am wondering what else could cause this bug:
- carburetor? ( I have checked the "float valve" which seems ok, opens and close, but the float itself could be stuck in the "up" position, closing the fuel, right? but this is unlikely as the bug occurrs only at WOT)
- Electrical bugs?
- Would be possible that I am actualling crossing the limit of the engine/pump, so that it is actually not a bug?

 

[99yam40]

Re: Suzuki DT40 fails at high RPM

what do you mean by it stalls/ fails?
Shut off? start missing? reduce RPM? Are you dropping 1 or more cylinders?

You could measure the voltages to the plug coils while the problem is happening With a Special peak reading meter or a DVA with a regular meter or use a inductive timing light on the different plug wires to see if it is dropping spark.A inductive tach hooked to the different plug wires will also show if spark is dropping on different plugs
If you are not loosing spark and the timing is doing its proper thing then you would have to look for a restriction in the fuel either into the motor or at/in the carbs.
Sounds like you may not getting enough fuel while at WOT.

IS this motor oil injected or premixed?

Does your alarm system work properly? could be motor goes into safe mode from oil or water problem.

 

[BoatAddicti0n]

Re: Suzuki DT40 fails at high RPM

I meant the engine reduces RPM at WOT.

The motor is Suzi DT40 '90, oil injected. The alarm system seems to work (a loud noise warned when I disconnected oil reservoir for maintenance), and I will double-check it. The water system is pumping out with good presure all the time.

It might be tricky to setup the elctrical checks, but I will look probe points (I can probably have a Fluke with peak reading, but inductive timing light seems more for my knwledge - thanks!)

In the meanwhile, will double-check for fuel restriction, now with focus on after-pump and carb (as I have included brand new tank, fuel line, and fuel pump diaphragm in the setup).

Thanks a lot for helping me with this bug, it is really hard to find professionals available in the Swedish spring, not mentioning their prices!

 

[99yam40]

Re: Suzuki DT40 fails at high RPM

A fluke with the peak reading mode will not work unless it is one made for these outboard ignitions. I know I have tried.
You can buy a DVA adapter or build one yourself that will work OK with your normal meter. Just do a forum search for Making one.

I have found a piece of trash hung up in a needle seat before not letting full fuel flow.
Not sure about your motor but some carbs had kind of a filter or restriction on the inlet of the carb that will plug

 

[BoatAddicti0n]

Re: Suzuki DT40 fails at high RPM

DVA test was not performed, but I did similar: when the problem occurred, I have removed the spark cable for one plug, no change in RPM. Did the same to the other plug, no change in RPM. This at least rules out problem in one coil. (might have in both and I will setup a DVA soon).

I have inspected the electrical wiring and realized there was no voltage coming to the white wire in the control box (there was a blown 20A fuse inside the "electrical parts holder": fixed!). I have replaced the key switch for an original one. Straight fwd, same colors, same connectors. It chokes, starts, and kills perfectly.

NOW IT ALARMS on half way to WOT !! ( I actually did not check if it still looses RPM at WOT... have of course released and back to the dock due to the alarm)

It pees water steady/strongly.
It does not seem overheated at all.
The oil gauge display/led itself has never worked since I bought the boat.

QUESTION:
1) Which sensors could alarm my Suzi DT40 1990? (Oil flow, oil level, thermostat, water flow ???)

"Old machines have typically more than one problem"
I might be just one more of its problems...

 

[deejaycee_2000]

Re: Suzuki DT40 fails at high RPM

Oil flow sensor/ filter under the oil reservoir or the oil level switch inside the oil reservoir, take it out and give good clean ......

 

[BoatAddicti0n]

Re: Suzuki DT40 fails at high RPM

Thanks a lot, Deejaycee2000!! Cleaning the oil filter cleared the alarm.

Now I am back on the original problem: the suzi reduces RPM at WOT...reproduced the symptom again a few minutes ago.

I have pretty much ruled out air in the fuel system (visual check using vinyl line, for test only), and fuel restriction (used new tank and line I had in the pocket).
Have new original spark plugs, rebuilt the fuel pump, cleaned fuel filter.
I have tested, when the problem happens, to remove one cable to the spark plug, then the other. I could not notice difference, in any case (of course it reduced even more when I removed the cable, but the same in for both cases).

So, what to do next?
Plug a DVA adapter and actualy measure the voltage?

Thanks again!

 

[qwackhead]

Re: Suzuki DT40 fails at high RPM

are you sure you are not hitting the rev limiter this sounds like what a dt25 dose when it hits the rev limiter the limiter on the dt25 is a little diff than most when you bump the limiter it goes into a kind of limp mode and has to be shut down then restarted to reset it, once you hit it it will pull down and not run well till reset. shat happens if you just run it like 1/2 or 3/4 throttle. dont know but this could be a possibility

 

[BoatAddicti0n]

Re: Suzuki DT40 reduces RPM at WOT

Re: Suzuki DT40 reduces RPM at WOT

Q:
What would be the rev limit in RPM?
Should the alarm sound if rev limit is crossed? (I only have sound alarm, no gauges, and that is not alarming)

Note I do not need to reset (turnoff/on the engine) to recover speed. I just need to release the throtle, go low RPM (the engine is reacts perfectly fine) open it again, and get up to 23 knots, before it reduces RPM again.



Thx,

 

[mphelle8vld]

Re: Suzuki DT40 fails at high RPM

How are you checking your RPM at WOT? My Suzuki 55hp has a rev limiter of 5900 built in to the cdi, it sounds like you have something similar so your motor is doing what it's supposed to do, it lowers the rpm and then resets when you reduce rpm, there is no audible alarm for this. You are either under-propped, cavitating, or your prop hub has failed allowing the motor to over-rev. Find a way to check the rpm at wot.

 

[deejaycee_2000]

Re: Suzuki DT40 fails at high RPM

Did you clean the oil level switch aswell? Rev limiter will kick in at 5500rpm .... this is not an electrical issue these CDi's are bulletproof .... you will hear if it hits the rev limiter, it sounds like a yamaha R1 superbike when you open up the throttle ....

 

[BoatAddicti0n]

Re: Suzuki DT40 fails at high RPM

I have checked in the brownpoint.com and the propeller I had before, S1200, is not the standard for my engine. I have replaced the propeller, now using a brand new Propulse I had at home: THE SYMPTOM DISAPPEARED!!! In addition, the boat is about 6 knots faster, reaching 28knots!! I assume this is what I can get from a (nice) 40HP in a 240kg boat.

Did you clean the oil level switch as well?

Not actually, I forgot it. I have removed and checked the resistance: really high resistance, as I recall, when the float is in the bottom, and open when it is on the top. I will clean it this weekend to see it changes, if the oil meter starts to work.

.... you will hear if it hits the rev limiter, it sounds like a yamaha R1 superbike when you open up the throttle ....

I did not hear that kind of sound, it actually just reduced the RPM/power...but it is working fine now!

THANK YOU ALL for the very good input, I have learned a lot in this troubleshooting!!


boatAddict0n
-------------
Integration&Verification Engineer
#### Complex problems may have simple solutions ####

 

[mphelle8vld]

Re: Suzuki DT40 fails at high RPM

Thanks for following up on your post, nothing better than fixing a problem and finding more speed than you had before.