Suzuki DT85 Head Scratcher

[1crazyfordguy]

Good morning all,

The last time I posted to the forum was 2008 and I was a senior in high school so color me surprised that my login and password still work now that I'm 34 with two kids of my own!

Long story short, my brother and I bought an '82 Sea Nymph SC-170 Fish Tracker with an '82 Suzuki DT85 from a friend for $300 and fully restored it (I'll post a separate thread on the boat with pics and the long story). The PO didn't know much about the motor but said it ran great however I question that as I've found and replaced far too many missing, incorrect, and failed parts.

The maiden voyage at the lake after the resto, we verified 34mph via GPS with the 13x21 prop that the PO installed but only 4300rpm on the tach. I suspected the new Faria tach was inaccurate so we tested again with a hand-held inductive clamp tach and found the Faria tach was actually correct. The PO gave us the original beat-up 13.2x17 Suzuki prop for a spare so I figured if the boat came with a 17p originally, it was over-propped with a 21p, and I put a new 17p on it and retested. This time we verified 31mph via GPS with the 13.2x17 and still only 4300rpm. Michigan Wheel's prop estimator recommends a 21p prop for this motor and hull size so the low RPM is definitely the problem and correct me if I'm wrong but I feel this boat should be in the 40mph range. It doesn't miss anywhere from idle to WOT, pulls strong out of the hole, and just won't pull past 4300rpm - no breakup, shaking, or noise - totally smooth. It starts instantly on the muffs but is hard to start on the lake with exhaust backpressure and I feel like the idle speed adjustment for the carbs is open farther than necessary leading me to suspect the timing is too far retarded.

I checked timing with a timing light several times and it's set at the recommended 7-deg retarded at idle and 21.5-deg advanced at WOT. I ohmed out the ignition module, pulsers, condenser charge coils, battery charge coils, and ignition coil packs per the genuine Suzuki manual and all check out within spec. I have not measured peak voltages while cranking yet but I have a DVA adapter on order to do that this week. What I did find that is throwing me off is despite the timing showing correct with the timing light, the idle and WOT timing markings on the timer base and lower flywheel cover do not line up and show the timer base as being set very retarded. I suspected a sheared flywheel key but I found TDC of the #1 piston and it lines up with TDC on the flywheel and timing pointer. I'm tempted to set the timing strictly off the timer base markings, ignore the timing light, and retest to see if the scale on the flywheel is wrong somehow. I don't like the idea but I'm getting desperate and wondering if I have a parts mismatch between the timer base and flywheel.

Here's the background info:
-Compression is 145/135/145 - #2 (middle) piston has some four-corner skirt scuffing as viewed through the exhaust port but nevertheless not bad numbers.
-Head gasket and exhaust cover gaskets were replaced after finding two impellers worth of broken blades in the water jackets and witness hole (explains scuffed #2 piston).
-Exhaust is unobstructed from the ports down through the exhaust tube. Cylinder sleeves are all indexed correctly and all intake/exhaust ports line up.
-Verified the control box and throttle cable pulls the throttle blades and timing advance arm fully open to their stops.
-New thermostat, impeller, and backing plate.
-Carbs fully disassembled, ultrasonically cleaned, all passages wired and blown out, new needles/seats, new gaskets, floats adjusted, throttle blade linkages synced and fine-tuned with a carb balancer on the lake. Verified carbs are correct for the model/year by stampings on main bodies.
-All fuel lines are new from tank to carbs, under-cowl filter is a new assembly, tank is a new Moeller 23/gal with 5/8" open vent, no anti-siphon valve, and quick-connects are new and the correct hose barb ID.
-Quicksilver primer bulb is new, stays hard with engine running, doesn't lose prime, and pumping the primer bulb at WOT does not increase the rpm.
-Recirculation hoses and check valves are new, properly installed, and check valves function properly.
-Oil injection functions properly and hoses are new.
-Spark plugs are new NGK B8HS as recommended and all three look identical showing normal AFR and temp coloration. Temp pattern on the ground strap indicates retarded timing.
-Motor is mounted to the transom properly with the bottom of the anti-cavitation plate flush with the keel and skimming the top of the water as verified watching over the transom.
-Hull has been gutted of all foam and meticulously cleaned, all plywood is new marine quality, and bilge is dry after 16hrs on the water. Plywood transom core was replaced with a fabricated aluminum tubular core. Boat is outfitted with the basic stuff you'd expect on a fishing boat of this size and weight seems distributed well.

If you have any suggestions or theories, please send them my way!

 

[1crazyfordguy]

I haven't had any suggestions on this issue but posting an update.

I removed the flywheel to inspect the key, keyways, magnets, stator, pulsers, and timing base and everything looked great. I reinstalled the flywheel and using a degree wheel and piston stop, I was able to verify that TDC on #1 was perfectly aligned with TDC on the flywheel timing scale and proper pointer alignment. I removed the timing advance stop bracket looking for reasons as to why the timer base pointer was not lining up with the stator base. The bracket is slotted for adjustment and I found witness marks where the bracket's mounting bolt washers had been previously installed. When I bolted the bracket down in that position, which was much farther advanced than it had previously been, the timer base and stator base marks lined up.

I filled the barrel, ran the motor, checked timing with the timing light, and to my surprise the timing was perfect! This didn't make sense as timing was perfect with the bracket mounted in it's previous retarded position so I loosened the advance stop bracket bolts with the engine running and timing light in hand, moved it to it's previous position, and the timing dropped degrees, lost rpm, but picked up rpm again and the timing became advanced and back into the correct range.

I believe what I just discovered is that the PO had moved the timing advance stop so far retarded that the pulser coils were picking up the wrong trigger on the flywheel. The inner magnet on the flywheel has two shields installed over it with 1/8" gaps where they meet that allow the magnet to be exposed in two locations that appear to be 120-deg apart and I assume giving the trigger signal to the pulsers. With the timer base retarded far enough, the first pulse would come too late but the second pulse would be closer to the correct timing.

I readjusted and locked down the advance stop where it was correct again and went to the lake for a test run with my dad. After readjusting the carbs, it idled a lot cleaner and was much less prone to stalling when shifted into gear. At WOT it picked up 150-200rpm for a new reading around 4500rpm but no more than that. There was a lot less boat traffic on the lake during this outing and we were able to open it up flat out and hold it there long and steady enough to safely watch the AV plate again and get pics. The plate is definitely not skimming the top of the water as I had previously thought with the plate at least 1" too low. Attempting to trim the motor up for some more rpm resulted in violent porpoising anywhere beyond the full down position, likely a symptom of the low mounting position.

My DVA adapter finally showed up so I'm going to run those tests this week to rule out further electrical and timing related issues then the motor is going up one hole. Due to the location of the splashwell floor in this boat and the new transom setup, I had to drill the motor mounting holes through the center holes in the motor bracket leaving me one hole up and zero down for adjustment. If needs more than one hole up, I'll have to start looking into jack plates. I'm skeptical that the low mounting is what's killing the WOT rpm but more testing will tell the tale.

 

[1crazyfordguy]

This project is teaching me a lot. I'm starting to believe there was nothing major wrong with the motor to begin with.

The mounting holes on this motor are 1" center/center so we raised it 1 hole and ran to the lake for testing. I instantly felt a reduction in steering effort and an improvement in speed while idling in gear. The full-throttle smash felt good with quicker time to plane. We gained 5mph with the same 17p prop (improvement from 31mph to 36mph) at the same 4500rpm with no prop blowout in hard turns trimmed full down. I was never able to trim the engine high enough to gain any RPM due to the low mounting and porpoising but this time I was able to get it high enough to hit 5000rpm. It was trimmed super high though, gave no speed increase, and was still porpoising hard, but now I know the motor will exceed 4500rpm.

The AV plate is still low and still throwing the V-spray out the back so I ordered a set of 2.5" fixed plates to bring the motor up another inch. I don't know how this hull is supposed to behave since it's new to me but I do know that I should be picking up some RPM with just a few up-taps of the trim and not a full four seconds of motor travel. After raising another inch, the AV plate will be almost 3" higher than the lowest point of the keel rib. Seems excessive but if that's what it wants, that's what it gets!

 

[99yam40]

maybe you need to add a foil to you lower unit or transom wedges to eliminate the porpising.
my used 16' aluminum Express came with a tail on the Yamaha C40TLRX motor.
I removed it and had to get my brother to sit on the front of the boat to stop the porpising.
I put the tail back on

 

[1crazyfordguy]

maybe you need to add a foil to you lower unit or transom wedges to eliminate the porpising.
my used 16' aluminum Express came with a tail on the Yamaha C40TLRX motor.
I removed it and had to get my brother to sit on the front of the boat to stop the porpising.
I put the tail back on

I'm not sure if I would need wedges as the trim stop pins are still installed in the lowest engine bracket holes - I could remove them to get some more negative trim if need be.

I'm mostly looking for the missing RPMs at WOT but If the correct engine height doesn't stop the porpoising, I'm not opposed to trying trim tabs or a foil. Great idea though, I should try moving some weight to the bow to see if it helps the porpoising.

 

[1crazyfordguy]

An adjustable jack plate just isn't in my budget right now so I ended up buying a pair of CMC 2.5" setback plates and modifying them to act as a fixed jack plate of sorts. I was able to get another 2" of height adjustment with these and can get more if I drill more holes.



We took it to the lake and found the AV plate totally uncovered and another 200rpm increase (4800rpm) with the motor trimmed down and an extra 200rpm increase (5000rpm) with the motor trimmed to it's sweet spot. Over-trimming resulted in noticeable prop slip and 5400rpm. I was never able to trim to the sweet spot before because the motor was so low but now I'm able to trim it up without any porpoising issues. These changes didn't gain any speed increases - still running at 36mph on the GPS. No prop blowout in turns while trimmed full down but it does blow out in turns if trimmed up at sweet-spot height. Performance out of the hole is the best it's been so far.

This is where I started a few weeks ago with motor mounted at it's lowest on the transom and trimmed full down:



This is where I ended up yesterday after installing the setback plates, also trimmed full down. The v-spray off the deflector plate goes away when it's trimmed to the sweet spot.



Online sources say WOT RPM on this motor should be 5000-5600 but my original Suzuki manual says 4500-5400. The fact that I gained RPM without an increase in speed is leading me to believe that this motor doesn't want to rev any further into the 5k range and I might be on track to start experimenting with different props.