DT40 tops out at barely 1000 rpm

btruong · Jul 18, 2010

i recently got a tachometer to gauge my work-in-progress 1994 Suzuki DT40. ive been recently trying to get the motor running good and all, but i cant seem to get anywhere near the recommended WOT rpm (5,000-6,000). i barely get 1000 rpm out of it at the wide open position. the motor was rebuilt 6 years ago and left to sit; now im trying to use it again.

boat: 16' Cobia center console
motor: 1994 Suzuki DT40
compression test: 100 top cyl, 120 bottom
recently replaced fuel pump
changed lower unit oil
15p prop (13p factory)
spark plugs look good
fresh gas/clean filters

what could be limiting my rpms? thanks a lot, because im getting kind of frustrated with this motor.

99yam40 · Jul 19, 2010

Re: DT40 tops out at barely 1000 rpm

Is the timing advancing like it should?

I would think it would be the carbs after sitting up for 6 years, take apart and clean

btruong · Jul 19, 2010

Re: DT40 tops out at barely 1000 rpm

how do i check the timing? does it require a test harness?

i forgot to mention that the carb was cleaned 2-3 times before and once when the fuel pump was replaced. i cleaned it following the guide on this forum so im positive its not clogged or anything. i believe i should also mention there is fuel stabilizer in the gas, if that helps.

thanks for the quick reply!

99yam40 · Jul 19, 2010

Re: DT40 tops out at barely 1000 rpm

a timing light just like a car is used to check timing.
when/why were the carbs cleaned and how?
Did it do any good when it was done?

btruong · Jul 19, 2010

Re: DT40 tops out at barely 1000 rpm

oh, alright. i have one of those, luckily!

the carb was cleaned the first time because the motor would die out at anything higher than idle (~600 rpm). the float bowl and jets were dirty. it worked fine after that, but still no high WOT. so we ordered a fuel pump and it took a while and i was afraid the fuel mightve clogged the carb again; so it was cleaned out. the new fuel pump was put in, along with new fuel lines from tank to carb.

the only good it done was i could increase the throttle and not have it die.

i should also mention that it if i push the throttle lever to about halfway down (halfway from idle to the throttle stop) it reaches about 3,000 rpm, in neutral gear. im guessing that means at WOT (in neutral) it should reach 5,000-6,000. in forward gear, it barely does 1000 at WOT.

also, could the weight of the boat affect this? its been sitting outside for 7 years and im afraid the foam or wood might be wet/rotted.

99yam40 · Jul 20, 2010

Re: DT40 tops out at barely 1000 rpm

Check the timing and make sure it is not loosing spark.

But in my opinion you still have carb problems

btruong · Jul 20, 2010

Re: DT40 tops out at barely 1000 rpm

alright, ill get to checking the timing.

if it was the carb, do i need to rebuild it with a kit or something? or just clean it out again?

99yam40 · Jul 20, 2010

Re: DT40 tops out at barely 1000 rpm

The best way is to disassemble completely and soak in carb cleaner, blow out all passages and jets with compressed air, and then reassemble. Make sure to adjust all linkage, sync, and adjust the carbs as the manual states. Not sure what all comes in the kit for DT40 and if you need those parts replaced.

btruong · Jul 20, 2010

Re: DT40 tops out at barely 1000 rpm

ill try that this weekend. one thing i ran into when cleaning the carb was that the main nozzle (#5 in the included diagram) wont come out. its supposed to be screwed out with a 10 mm wrench but the nut is nearly stripped now. does this have to be removed, and if so, how?

on the note of the timing check: i called up my friend who works on engines (mostly cars sometimes boats) and he said that the timing could only be checked by a pro with special equipment. referring to the shop manual it says i can do it with a tach and timing light (both i have). the problem is, it says check at idle and at high speed, which is 5,000 rpm in FWD gear. if i cant get to 1000 rpm, how would i test the high speed timing?

btw, thank you so much for all the help. i really appreciate it. im kind of new to the whole outboard thing. much different than cars.

99yam40 · Jul 20, 2010

Re: DT40 tops out at barely 1000 rpm

A 6 point socket is my only guess on the nozzle.

Check the timing at idle to see if it is what is called for in manual and with some help driving and checking the timing see it is advancing and to what point it is going to.
Or better yet, I have read where some have pulled the plug wires, hooked them to a spark board, hooked up the timing light , and then opened the throttle and turned the motor over to see where the timing is at with out running the motor.

I am not sure how your motor advance works,if it is controlled electronically by CDI or mechanical linkage. You need to look at manual and see if there is a way to move the linkage to where it should be at WOT and see what the light tells you

btruong · Jul 20, 2010

Re: DT40 tops out at barely 1000 rpm

ive used a socket but it was a 12-point. ill try the 6 point.

ill try to check the timing while driving. im a little confused on the spark board thing...?

i believe the timing is controlled by the CDI but there is a section in the manual (which i havent read yet) about "static timing adjustment".

DJ · Jul 20, 2010

Re: DT40 tops out at barely 1000 rpm

Once owned a 1995 DT 140.

The carb's (Mikuni's) are super sensitve.

Clean them and go on.

btruong · Jul 20, 2010

Re: DT40 tops out at barely 1000 rpm

"once owned"? any reason you got rid of it? haha. im thinking about selling mine once it runs in decent condition.

any suggestions/tips on cleaning the carb?

thank you guys and this forum!

99yam40 · Jul 20, 2010

Re: DT40 tops out at barely 1000 rpm

btruong said:
ive used a socket but it was a 12-point. ill try the 6 point.

ill try to check the timing while driving. im a little confused on the spark board thing...?

i believe the timing is controlled by the CDI but there is a section in the manual (which i havent read yet) about "static timing adjustment".

The spark board was just to keep motor from running, but still sparking the wires so the timing light will work and you can see how well the spark is going on all cylinders while doing that also.

On my C40 yam I popped off the link to the TPS that tells the CDI position of the throttle and just turned it to the position it would be in at WOT and spun the motor and checked the timing with the light. Main thing is to see if it is advancing properly. You can not build much power if the motor timing is not advancing.On the Yamaha that is how they reduce the RPMs in safe mode.Not sure on the Dts.
Still need to work on carbs and one cylinder is a little low , but I just thought timing is something easy to check to make sure of.

James R · Jul 20, 2010

Re: DT40 tops out at barely 1000 rpm

Although it could still be a carb problem it sounds like only one cylinder is firing.
Your timing light can check for power to both plugs but I would not rely on the plugs looking good to determine their serviceability.

btruong · Jul 20, 2010

Re: DT40 tops out at barely 1000 rpm

thank you guys!

so i have planned:

spark test with an inline spark tester i have
timing check
carb rebuild

is there anything else i should check?

DJ · Jul 21, 2010

Re: DT40 tops out at barely 1000 rpm

DJ said:
Once owned a 1995 DT 140.

The carb's (Mikuni's) were/are super sensitve.

Clean them and go on.

Yes, The DT 140 was far too maintenance intensive for me. The carb's needed cleaning at least once/year. If not every six months. No matter what I did, they clogged.

A Maintenance nightmare.

"Perception is reality." I would not put a (2-stroke) Suzy on a boat without some solid proof of performance. 4 Stroke-a bit different story.

James R · Jul 21, 2010

Re: DT40 tops out at barely 1000 rpm

Sorry to be opposite DJ.
I have run a Suzuki DT140 for some years now and have always, without fail, used Stabil in the fuel and a water trap type filter. My oil of choice is Pensoil Synthetic blend. I recommend these products to all especially those who have work done by me or get a used motor from me. I also have two DT 115s out there and running without carb problems for three or more years, perhaps because the owners have taken my advise.
The Suzuki carbs are easy to strip, clean and rebuild. I plan to rebuild my carbs at the end of the season after 5 years.
I have rebuilt several Suzuki's and frankly I much prefer their engineering and reliability over all others.
I will add however that even though the Suzuki four stroke is possibly the best four stroke, they, like all the rest, are using Automotive technology and I doubt the longevity of these.
I also run three other boats, two with my rebuild Johnson Faststrike 150s and one with a 15HP Yamaha 4 Stroke. Even using a filter and Stabil I still have the occasional sticking float on the Yamaha.

btruong · Jul 21, 2010

Re: DT40 tops out at barely 1000 rpm

ive been talking to my friends and one of them said that my linkage adjustments could be causing this. i highly doubt it. what do you guys think?

btruong · Jul 22, 2010

Re: DT40 tops out at barely 1000 rpm

i just read one of the other topics on this forum about reed valves, did a lil research, and now im questioning whether my reed valves could be the issue? the adjustment or anything.

i forgot to mention that the outboard was rebuilt by a mechanic who i now know is really sketchy and received a lot of thumbs downs. could he have screwed up my reeds? (he messed up the remote control throttle linkage adjustments too)

DT40 tops out at barely 1000 rpm

Seaman

Fleet Admiral

Seaman

Fleet Admiral

Seaman

Fleet Admiral

Seaman

Fleet Admiral

Seaman

Fleet Admiral

Seaman

DJ

Guest

Seaman

Fleet Admiral

Commander

Seaman

DJ

Guest

Commander

Seaman

Seaman