Suzuki DT115 High speed problems

[jimowen]

This saga will be long and verbose, but The entire story will let you see what I've done (to no avail).<br /><br />I just bought the boat (20ft Bayliner Trophy '81) with a Suzuki DT115TCLS (9/23/94 mfg, '95 model). The guy I bought it from had been tinkering with it, and couldn't get it started.<br /><br />It had been "running fine", and then got rough, then died. He spent several days trying to get it running and gave up ($6500 as-is).<br /><br />When I started working on it, I checked every coil, every sensor, every connection. Eventually, I found the Flywheel key sheared (about 60 degrees out of time) I replaced the key, and it started right up. It now idles fine, runs smooth, compression is ~115 each cyl., but will only top end (under load) at about 2800-3000 rpm. In neutral, will hit 6K easily. In neutral, it appears to accelerate normally, and at about 2800 rpm, there is a definite change in the sound of the engine (spark advancing?) and it then accelerates rapidly. (None of this happens, when in gear)<br />All sensor readings appear normal, except the throttle position sensor (which was recently replaced). My measurements show only .1 to .25 volts (should be .45 to .55 v at FCT)<br /><br />Even though Suzuki says "CDIs never go bad", I have swapped several "good" CDI units (albeit from older model engines) to see if it was the CDI (No change in performance)<br /><br />My questions are:<br />1. Does this sound like a purely Throttle sensor problem? (any "tweaks" to verify?)<br />2. Since the voltages being supplied are from the CDI (and they are low) does this tend to mark the CDI as bad? ($1000+ to replace new)<br />3. Are there any specific voltage/resistance checks that will positively confirm the CDI is bad?<br />4. Since the engine spins up to 6K with no load, does that rule out gummy carbs?<br />5. Is there any other relatively simple thing that may be causing a loaded top end of only 3K?<br />6. Are there any remote connections that could be shorted/corroded/open that could have this effect?<br /><br />V1700 prop is installed (same one as always) that previously got 5200-5500rpm.<br /><br />Thanks for any help, I'm stumped without more detailed schematics of the "black box".<br /><br />Jim

 

[DJ]

Re: Suzuki DT115 High speed problems

Jim,<br /><br />You can just about bet it's gummy carbs.<br /><br />Any outboard will rev up to it's full potential while running with no load. And they will sound OK too. A four cylinder will easily rev up running on only two-with no load.<br /><br />I owned a DT-140 of the same year. It's the same engine.<br /><br />I had to clean the carbs. (4-Mikuni's) annually. <br /><br />It would act just like yours is.

 

[TexSkeeter150]

Re: Suzuki DT115 High speed problems

I agree with djohns19. I just sprayed Beremans(sp?) B-12 in the carb at idle and ran two tanks with B-12 in the tank. Had immediate results and runs like a champ now. I would recommend that you change out the fuel filter after doing this also. Good luck

 

[jimowen]

Re: Suzuki DT115 High speed problems

Nope! That didn't fix it. I did succeed in fouling out 3 of the four plugs before I got that one figured out though... The carb cleaner makes lots (breaks loose?) lots of carbon, and those brand new plugs had resistances of about 10K (instead of infinity). Ran 5 gals of new gas with B12, still only turns 3K (under load). I have the throttle position switch ordered, but I still don't believe that will fix it. I also noticed the charging circuit didn't come up to power until the engine was over 3500... that seems a bit fast to first charge the batteries (with battery only at 12.4) does about 12.5 at idle, 12.7 at 2300, and about 13.5 at 3000)<br /><br />The previous owner said he replaced "one of the coils under the flywheel". I've checked the resistances on all of them and they are within spec. Could there be a different "high power" coil required in a newer engine? (Still just guessing...and trying to avoid buying the CDI)<br /><br />Thanks for your help up til now... the carbs are clean, the fuel filters are replaced, the spark plugs are replaced (again). Compression is now 125 in all cyls. I did notice a rough spot on acceleration - at about 2600-2800 (no load) which I assumed was where spark advance kicked in.<br /><br />Another question - since the fuel pump was recently replaced, I presume it is good. Short of sticking an in-line gauge in the fuel line, is there any way to be sure it is providing enough pressure? I think that at high rpm, the same amount of fuel flows whether under load or not (bournoulli's principle?), but am not sure.<br /><br />regards,<br />Jim

 

[jimowen]

Re: Suzuki DT115 High speed problems

FIXED IT!!!! Now that I've rebuilt the carbs, cleaned everything, wiggled everything and it still didn't work... I returned to basics (for the third time)<br /><br />The local tech said to disconnect all the sensors, if that fixed it, that sensor was at fault. Well, I knew the throttle position switch was bad (incorrect voltages), but removing it didn't fix the problem. So, this time I got creative and SHORTED the two wires...Voila! Ran like a scalded ape! Took it out for a quick spin and was registering 46 mph (gps). Thanks for everybody's help! (New part is ordered)

 

[errol_white]

Re: Suzuki DT115 High speed problems

Scalded ape. Good one! Maybe once I get my motor above 3200 RPM...

 

[Suzuki Nut]

Hi Jim, I know I'm a little late replying to your problem but I have a similar situation with my 1989 DT115, lost full throttle rpm and acceleration out of the hole ! You seem to have found the problem with your engine by shorting the two wires on the TPS ? Can you tell me exactly what you did and how you shorted the wires ? Maybe we can talk about it some time if you can contact me thru the AOMCI ?

MOD EDIT: Yes, 20 years late. Please review the forum rules regarding posting to old dormant threads. It is best to start your own thread with the specific information.