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Carb tuning

  • Luke
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Carb tuning was created by Luke

'77 DT250. Finally got back to finishing my leak down test after every other winter project got in the way. The soap water revealed the valve on the test rig was leaking slightly. After fixing that, it holds pressure overnight, so it seems like my worry over crank seals is unfounded.

Still chasing carb issues though. It has a strong tendency to 4 stroke through the bottom third of the throttle. Under load or heavy throttle, it feels pretty good. The air bypass screw has almost zero impact at idle. I've torn the carb apart a handful of times, although not opposed to trying it again. Would changing the groove position on the main needle have any impact? Seems like my issues are below the point where the main is involved. Short of any ideas, I guess the carb will back on the bench again.

Edit: I also checked the pipe for clogging as I've seen this mentioned many times. My unscientific method was to set the shop vac to "blow" and connect the hose to the pipe flange. Didn't seem to slow the vac down much and had a healthy blast of air coming out the back.

Also will pass on a weird spark plug issue I'd never run into before. It went from running normal, to rough, then just quit after a few seconds. Had spark, had gas, wouldn't even begin to fire after repeated kicks. Had the plug in and out, checking spark again. On one crank with the plug out, I saw the faintest spark between the center electrode and the insulator, rather than jumping the gap. Put a new plug in, fired first kick. Must have had a broke electrode and wouldn't spark at all when threaded in. It was an odd one.
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Last edit: 18 May 2020 09:47 by Luke.
18 May 2020 09:31 #1

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Replied by RT325 on topic Carb tuning

Is the pilot jet seated fully home in the carb body. Is the choke closing off as have had the plastic cap sitting up off the grove stopping it closing fully. Is the mixture screw out a couple of turns & not right in. Is the slide needle clip sitting on top of the plate. Is the needle jet worn oval. Is the air bleed from the carb mouth to the needle jet clogged with crumbling air filter bits. Has an oddball slide been fitted with the guide slot on the wrong side so cutaway is forward. Sorry--lots of Is's. Just things i've come across over the years. I'm sure those don't have the small Oring of the early carbs so no worries there.
Last edit: 18 May 2020 20:00 by RT325.
18 May 2020 20:00 #2

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Replied by Luke on topic Carb tuning

All good stuff to check. Hadn't thought of the starter circuit not fully closed. I'll check that out. Everything else I've been through multiple times and can't find any passages blocked or visibly worn jets. Everything seems correct for the year and Mikuni marked parts, so I don't think its been messed with. I'm about ready just to buy all new internals if I can figure out the parts. The main jet is #150 and pilot is #50. In the book, this corresponds to the specs in the back, but in the exploded carb diagram it lists the main as #140. Maybe just a typo? The needle jet and the needle I can't find markings on. The needle #5DP33 listed in the book I can't find on any of the Mikuni parts sites.

Between the plugs and the 4 stroking, it has all appearances of running rich. It idles great. Revs fine. WOT feels good. Its really that lower end of the throttle that stinks. I keep going back to that pilot and air bypass because of how the air screw acts. I screw it all the way in and it will kill it (so it feels like its not an air passage blocked) but anywhere from 1/2 turn out to fully unscrewed has no impact on idle speed or the low end performance.

I want to drop the needle to the top groove and try it, but I'm worried that its the wrong fix for the bottom end and might lean it out too much up top.
19 May 2020 10:18 #3

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Replied by RT360Fan on topic Carb tuning

Float level?
19 May 2020 10:39 #4

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Replied by Luke on topic Carb tuning

The first time I rebuilt the carb I was extremely careful about the float height. And the second time I sort of cared but not really. And by the 10th time of pulling it apart I don't check anymore. If it all turns out to be float level I'm not sure I'll even admit it! :lol:

I went ahead and ordered a new pilot and needle jet from Niche. I would have gotten an air screw also if I could figure out which one, or if its even offered anymore.

Here's another question...the specs in the picture list the "air jet" as 2.5mm. Is that an actual jet?? Or is it referring to the passage size drilled in the carb body?
19 May 2020 11:59 #5

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Replied by LongStride on topic Carb tuning

Try playing with the needle position, try removing the air filter, try tape (cover1/2 the hole) the intake on the air box , these simple test will tell you if you are rich or lean, to fully warmed up test drive noting running issues at each throttle increment, do one test at a time, as mentioned the needle jet orrifice may be worn, this can sometimes be compensated by lowering the needle, (raising clip) , this could solve your pilot screw issue, other wise may need a one size smaller jet, good luck and keep posted of results
19 May 2020 12:16 #6

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Replied by MarkT on topic Carb tuning

No air jet, yes that's the drill size.

My guess is someone installed a "carb kit" needle.. Original Mikuni will have numbers and the "square in the corner of a square" Mikuni marking.

If you can't find a 5DP33 there are several 5DP variants out there... maybe Sudco or someone can help finding one that's close? Here's a chart that might help.








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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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19 May 2020 13:28 #7

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Replied by Luke on topic Carb tuning

Made some progress and learned a few things. First, I did find the Mikuni number on the needle and it is correct, 5DP33. Its painful to admit that without a magnifying glass, some things just aren't visible anymore. The clip was on #2 position from the top, as I'd dropped the needle once before when testing, so I moved it to the very top position. No real change. Next I removed the air filter. This did seem to have some small effect, as I got a little response out of the air bypass screw.

Next I put the filter back on and pulled the silencer. This was a pretty big change. The air screw started acting fairly normal and the 4 stroking was greatly diminished. Pulled the spark arrestor out and it was really, really bad. I got the rosebud out and cooked the rest of the silencer til the smoke was mostly gone. The screen/packing on the spark arrestor had to be scrapped. Put it all back together, along with moving the needle back to the original center position (I know, too many changes at one time). It was better, but not nearly as good as with the silencer removed. A quick ride just to smell the castor and I was done for the evening.

I did think the silencer cleaning would give me a few hundred more horsepower B) on the top end, but I really couldn't notice a difference there. Definitely some more pep in its step at mid range though.

At least feels like I've got something to work with now. I'll try moving the needle back down incrementally. The silencer was much worse than I expected and I hadn't noticed that before when just pushing air through it. Wondering if I should look for a replacement, or fab up something with an aftermarket part.
20 May 2020 08:31 #8

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Replied by Zonathan on topic Carb tuning

Hi. 
I'm having the same issue with my 1977 dt250. I have my carb disassembled and the needle is a 5dp33 but it seem to be for the 400 and the 250 should have a 6F9 needle. I will order a 6F9 and I'm pretty sure it will fix the issue.
16 Oct 2021 10:27 #9

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Replied by RT325 on topic Carb tuning

If you think the needle wear is the issue there's usually wear in the jet it slides in. I have aX100 yamaha & won't respond to the idle mixture as it should & being rotary valve i can look straight in the carb & watch it spilling up past the needle at idle which of course loads it up when you give it a handful of throttle after a few minutes idling. All std jets & float level so will eyeball the needle jet & closely inspect the needle but expect the jet to be oval shaped if not just worn all around. Not entirely relevant but at work with suzuki quads back in the LTF250-300 days i'd probably replace the jet--"emulsion tube"--& the needle on a quad every week but we had a lot of farm customers with these. Eventually the replacement jet came in stainless rather than brass so fixed all that problem. Wear was 'very' visible on the metering part near the top of the jet & needle too. Possibly a lot of slow running & 4 stroke engine pulses push pulling on the inlet & slide needle--i don't know--just the way it was.
16 Oct 2021 20:58 #10

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