1985 DT40 mid-throttle miss and popping

[Mark_VTfisherman]

I have been running a 1985 Suzuki DT40 since 2003ish. Never a problem, haven't even changed plugs that often as it has run so good. Love this motor.

Recently having a problem where it will idle (troll) all day long, but it won't run between off-idle and full throttle without popping, missing, backfiring or whatever unless it is at full throttle or idle.

Since full throttle is 28mph even with all my fishing gear it is not runnable at part throttle in a little bit of seas (22mph has traditionally been good) or in higher seas 8 or 9mph which keeps the bow up, not planing. Basically it misses, jumps, bucks, and pops violently enough it makes me think I'm going to blow it up.

Removed and cleaned the carb twice, replacing the float needle. It was remarkably clean to start with but I removed every part, tube, jet possible and sprayed carb cleaner through all the passages and cleaned the metering holes etc. with a wire. This improved it but didn't fix it for sure.

I suspect that it is perhaps a lean condition causing preignition (maybe?) and I wouldn't be so strong on that if it didn't idle and run at full throttle so well... It does not appear to be spark, and reeds seem OK although i didn't pull the manifold to check them

Anyone have some thoughts on this issue? Similar experience? Sound advice?
I can usually fix most things but two strokes have always been my nemesis and I don't have the funds avail to just yanks and replace it with a newer motor.

Any help is appreciated.

Thank

 

[99yam40]

Check Compression, spark, and timing
Also check on fuel pump

Timing light or an inductive tach might show if spark it dropping on a cylinder if hooking it to the different plug wires

But with a motor that old you may have seals on top or bottom of crank letting in air to lean out the top and/or bottom cylinders.
What are the plugs looking like

I guess the seals between 1 & 2 or 2 & 3 on crank could be a problem also

 

[Mark_VTfisherman]

Thank you for the suggestions.

Good compression. Spark doesn't seem to be an issue. Runs fine at full advance full throttle. And idle, off-idle.
Plugs look great as they always have. Almost amazingly clean for a two stroke, but because it is oil injected I guess that should be expected.
This is a 2 cylinder with single carb and common crankcase.
Crankcase should be pressurized as it is a two stroke so air wouldn't get in I don't think- but air/fuel charge could get out out. Because it runs so good at idle and WOT it points to the carb as the issue.
If above items did not check out I would address them but I don't like throwing parts at stuff cuz that almost never works out.

 

[99yam40]

does it kind of sneeze ?
or what is this pop you speak of.

Kind of sounds like a lean sneeze

Never seen a multi cylinder 2 stroke that had a common crank case.
each cylinder has to draw its fuel and air charge while the other is not, reeds seal the other from the carb and there are seal at the top and bottom or crank along with labyrinth seal between the cylinders
test what is easy to test then and only then eliminate that
The case is not pressurized all the time as it has to suck the air and fuel in through the carb

Also I believe there are check valves and lines coming out of the crank cases that can go bad

 

[Mark_VTfisherman]

Thank you for info. The "sneeze" is sortof a squeal. What I am getting is a power-robbing irregular miss at mid RPMs. It will idle /troll all day long, and runs strong at WOT, but from say 1700 rpm until full throttle it misses like it's running out of fuel. And while it's rpm observeable, it appears to be throttle position sensitive in action.
Also, when I pull the cover it is louder enough that it seems like it is backfiring through the carb- a boat tech that won't work on Suzuki but did give it a lesson suggested "lean condition preignition."
If it were a spark miss I can't see why it would only be midRPM and not be prevalent at WOT. I thought it was a knock of some sort till I pulled the cover and heard the engine without the sound suppression of the cowling.

What also bugs me is that I have pulled the carb three times now and blown out passages and this improved things but didn't eliminate them. The only replaceable item in the "rebuild kit" for the carb besides gaskets is the float valve.

Thanks for the help,
Mark

 

[Mark_VTfisherman]

So I removed and cleaned the carb again today...everything looked good. .???.
I am starting to be sad as I can't imagine it is a carb problem. But why would it run fine WOT then?
Also pulled to intake manifold and reeds looked good.
Plug on bottom cylinder was oily; top plug was less oily and tan in center insulator. Switched them around and hope to test run tomorrow. It starts and idles GREAT.

Looks like I need to buy a new compression tester as the buddy that borrowed it lost it. Permanently. Apparently. And isn't offering to buy a new one. One way street? Maybe...

 

[cneicen]

I'm having the same exact problems with mine. 1985 dt40. I was curious if you had a resolution?

 

[GA_Boater]

I'm having the same exact problems with mine. 1985 dt40. I was curious if you had a resolution?

Mark hasn't been here in over a year.

Read the Help Tip on top of the page and start a new thread.

Closed.

 

[Mark_VTfisherman]

I'm having the same exact problems with mine. 1985 dt40. I was curious if you had a resolution?

Actually learned you started your own thread, I'll look that up.

In essence, I need to get it down to a boat shop an hour away. He's the first guy I talked to who didn't want to merely throw parts at it to "try and see" and was immediately familiar with my Suzuki motor. He thought he could test it for $50/$60 so I'm going to do that but I've been busy with work, a Starcraft project, two other boats of friends, and a jeep restoration...

I've determined that it is 90% likely it is the $550 CDI box breaking down once it gets warm. It's not repairable. However, the old guy at the boat shop has an analog oscilloscope and an electronic tester he's rigged up specifically for this kind of symptom. He thinks from dealing with a lot of small to mid size suzukis that it's 50/50 the CDI or the stator.

My suggestion is to find an old school boat repair shop with a long-term good reputation that can (or will!) test the stator to eliminate that as a possibility. I haven't looked into the parts for stator but they gotta be less than $550 I hope! There's just a nice feeling knowing there someone competent left in this world who isn't just going to throw parts at stuff and wants to work for the customer's benefit!