1978 DT175 - help with rough running

Hi folks! Just found this community, hoping to get some expert guidance on my goal of getting an old 1978 DT175 running right. Backstory: I bought this bike over the summer, and it ran well enough for me to at least ride it and make sure it shifts, brakes, etc. I've slowly been working on getting it to a decent condition, but I'm having a hard time with the engine. It's a strange enough problem that I haven't found a forum post that exactly matches my issue, and I'm not experienced enough with 2-stroke engines to know exactly what to search for. I haven't really ridden it at all since I'm worried about damaging it further.


Description of problem: Bike can be started alright, takes a couple kicks, and idles alright (haven't fully dialed in idle since I'm having bigger issues). It runs, but very poorly: power is a little jerky, and sluggish at low RPM, but feels like full power with a good throttle pull. Coming to a stop, it stalls every time, then takes a few kicks to start again. Bike runs worse off choke, but also is fairly smoky (though nothing horrendous). Sounds terrible to me, but I'm not 100% sure how much is standard 2-stroke noises vs. my specific issues. I have a short video of a 50 yard ride herehere if it's helpful.

What I've done so far: cleaned the carb (twice), taking care to clean all the jets. I'm fairly confident I did this right (not my first gummed up carb). Float isn't quite right (slight fuel drip out of float bowl overfloat), but I don't think it's related.
Cleaned the tank and serviced the petcock, so fuel delivery shouldn't be an issue. Fresh gas too.
Checked that the autoluber was working.
Replaced the spark plug (gapped to spec), and checked spark.
Cursory compression test (thumb on spark plug hole) - haven't properly tested compression though.

Things I'm wondering about:
Timing issue or intermittent CDI failure? lack of primary compression? Something with the reed valve? Leaky head gasket? Fuel mixture problem?

I'm just not very familiar with 2-strokes, any guidance would be very much appreciated! Can take a video and post it if that would help

You have covered a lot of the elementary ideas with the new spark plug, SP gap, and cleaning the carb twice. Are you sure that the pilot jet and its attending passageway are fully open? It just seems starved for gas on the low end when you close the throttle, thereby shutting down the main and needle jets. Have the idle screw and the air screw set to stock specs? (Can't remember: had the pollution controls disallowed setting the air screw by this year? I'm more familiar with the bikes through the '74 model year.)
Have you checked the pipe for any sort of blockage? Anything from excess carbon to a real live mouse nest? Don't laugh; it's happened before.

Thanks for the help! I'll go ahead and check the pilot jet again (took a while to get it out, was totally varnished into place) when I do the float adjustment, but I don't think it was that. I did install an inline fuel filter as well since there was a little bit of crud in the tank that I couldn't quite get out, so the jets should stay clear.

Idle and air screw were set to spec (1978 was the last year with an adjustable air mix screw, lucky me). Haven't checked the pipe actually, though I've had nothing flying out the exhaust at all, rodent or otherwise, so I think it's clear, but definitely worth a check :P

Did the video work for you? Would love some opinions on whether that bike sounds wrong or whether that's just 2-strokes doing 2-stroke things.

Something else to check might be the rate of fuel flow, since you say there was crud in the tank, and you have fit a fuel filter in the line. Maybe the filter has trapped enough stuff by now to slow the rate of flow into the float bowl?

You need to address the fuel dripping out the overflow, your whole issue could be an overfull bowl. Replacement needle & seats are available if the seat is still set with debris. You can use very fine sandpaper (1000 grit or higher), twisted to insert in the seat and polish the bore in a pinch. Float level setting can be found in the Tech library above, but, as MarkT always recommends the top of the float should be dead even with the bowl flange when the needle "seats," without compressing the needle's spring.

MarkT wrote: Maybe just me, but I don't see a video?

(For video you need to upload to YouTube and link here... or another video hosting site but YouTube link will imbed video here automatically.)

He posted with a Go Pro.com link, those won't work to my knowledge anyway.

I saw it. It was here for a while and then now, no go the Go-Pro.

I'd like to see a video of you running it as fast as it can go through the gears.....to see if fuel starvation stalls it, and to make sure it's not "loaded up" and needs clearing out.

There is a spec for the float level / needle/ seat, setting the level of fuel in the bowl. A small piece of rust or debris could be holding the needle/seat open and overflowing fuel.

Ah, I apologize for the lack of working video. On youtube now, here: .

It actually runs best when running up through the gears - Video only goes to 3rd, but I can go higher/faster without it misbehaving much.

I'll redo the float and needle first thing then, and see how much that helps. Previous bikes have had issues with float levels, and the the way they were affected seemed different - I've had fuel starvation before (on my 4-stroke street bikes). I also would have thought that if the issue is overfilling from the float setting, it would behave better after running it around a couple minutes, not worse. I'm clearly misunderstanding something though, hopefully the video is helpful.

Thank you all for the quick responses! Feels great to get some input after struggling by myself for a while.