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A couple carb questions.


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I just installed a brand new TM33 pumper on my 7K mile DR250 hoping to get similar throttle response as my DR350 (TM33 carb also) which performs fantastic!

The DR250..... so far.... not so fantastic ๐Ÿ‘

Question, What part of the throttle circuit does the pilot jet control? Idle only? Idle/part throttle?

Heres the story...I get a bog right off idle until about 1/4- 1/3 throttle then its fine up until about 3/4 throttle where it drops off pretty bad for a split second. After that it's all good right up to WOT when it hits the rev limiter.

The idle is not great. It's best at about three turns out on the fuel screw. I'm not sure if I need to go up one size on the pilot or if raising the needle will take care of both problems.

I'm sure its running lean because it burned off my Wal Mart 1,200* BBQ grill paint off of the header pipe which has been on the pipe for years๐Ÿคฃ I'm getting some bad decell pops as well.

For anybody that may be able to help the specs are: TM33 pumper on a DR250.

37.5 pilot

Needle clip on slot 3 (middle)

135 main jet. (seems fine)

Modified 3"X3" airbox opening.

Thanks for any help.

Bob

Edited by Bob Onit
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I just installed a brand new TM33 pumper on my 7K mile DR250 hoping to get similar throttle response as my DR350 (TM33 carb also) which performs fantastic!

The DR250..... so far.... not so fantastic ๐Ÿ™‚

Question, What part of the throttle circuit does the pilot jet control? Idle only? Idle/part throttle?

Heres the story...I get a bog right off idle until about 1/4- 1/3 throttle then its fine up until about 3/4 throttle where it drops off pretty bad for a split second. After that it's all good right up to WOT when it hits the rev limiter.

The idle is not great. It's best at about three turns out on the fuel screw. I'm not sure if I need to go up one size on the pilot or if raising the needle take care of both problems.

I'm sure its running lean because it burned off my Wal Mart 1,200* BBQ grill paint off of the header pipe which has been on the pipe for years :applause: I'm getting some bad decell pops as well.

For anybody that may be able to help the specs are: TM33 pumper on a DR250.

37.5 pilot

Needle clip on slot 3 (middle)

135 main jet. (seems fine)

Modified 3"X3" airbox opening.

Thanks for any help.

Bob

The pilot jet controls idle and, on a tapering off degree, up to nearly 1/3 throttle. The next tuning segment is actually your slide cut-away. That has a huge affect between 3/16 and 1/3 throttle. Of course the needle overlaps all of these to some degree.๐Ÿคฃ The best way to attack jetting is backwards. Start with WFO main jet and work your way to idle. ๐Ÿ‘

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The idle is not great. It's best at about three turns out on the fuel screw.

If best idle is at 3 turns out it's telling you it wants a bigger pilot.

Raising the needle won't have much effect below about 1/4 throttle.

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The next tuning segment is actually your slide cut-away. That has a huge affect between 3/16 and 1/3 throttle.
Rob, I've never heard anybody include the term "slide cut-away" in a tuning procedure so please forgive me but, could you please explain what can be done with the slide cut-away as far as tuning goes?

That's definitely one area (3/16 thru 1/3 throttle) where I'm getting a bog.

*Edit* Rob, I did a little research on the slide "cut-away"

Since there's no option for altering the cut-away I imagine it's the needle that needs to be moved/modified.๐Ÿคฃ:

If best idle is at 3 turns out it's telling you it wants a bigger pilot.

Raising the needle won't have much effect below about 1/4 throttle.

I'll go up to a 40 on the pilot Jeff.

It's kinda strange that the DR250 wants more fuel than the bigger DR350. ๐Ÿ™‚

Thanks very much Rob/Jeff.

I'll change the pilot and raise the needle one slot and report back. ๐Ÿ‘

Edited by Bob Onit
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It's kinda strange that the DR250 wants more fuel than the bigger DR350.

It is a common misconception that a bigger jet automatically equals more fuel.

The 250 motor moves less air than the 350 motor. This means it is creating less air velocity through the carb. It's the velocity that creates the 'suction' to pull fuel through the jets. If you have less suction you need a bigger jet to supply the same amount of fuel. I'd say it's pretty likely you will have to jet up from the DR350 setup to make the 250 motor happy.

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It is a common misconception that a bigger jet automatically equals more fuel.

The 250 motor moves less air than the 350 motor. This means it is creating less air velocity through the carb. It's the velocity that creates the 'suction' to pull fuel through the jets. If you have less suction you need a bigger jet to supply the same amount of fuel. I'd say it's pretty likely you will have to jet up from the DR350 setup to make the 250 motor happy.

I'll buy that theory Jeff.

The DR250 is also one point higher on the compression ratio compared to the DR350.

I just hope I didn't do any damage to the internals.... the header pipe definitely got extremely hot.

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Well, I just spent an hour+ in the garage removing/re-tuning the new TM33.

It's too late to mount and test it tonight so I'll give the report tomorrow..

I changed the pilot to a .040 and dropped the clip one groove richer. I'll mount the carb tomorrow and hope for the best.

On a side note, I found this diagram showing the stages of operation of a Mikuni carb.

I thought it was pretty interesting and figured I'd post it for the heck of it.

effects3.gif

Link to full carb page

http://www.iwt.com.au/mikunicarb.htm

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It is a common misconception that a bigger jet automatically equals more fuel.

The 250 motor moves less air than the 350 motor. This means it is creating less air velocity through the carb. It's the velocity that creates the 'suction' to pull fuel through the jets. If you have less suction you need a bigger jet to supply the same amount of fuel. I'd say it's pretty likely you will have to jet up from the DR350 setup to make the 250 motor happy.

+1 Case in point... my big bore 42mm FCR-MX needs a 170+ main jet and the next richer jet needle compared to the 39mm FCR-MX's 155 main to get the same AFR. Less velocity=less fuel draw.

And building on that, Bob, the slide-cut-away is responsible for controlling velocity over the needle jet at the lower throttle positions. The smaller the cut-away (you'll notice it's centered in the venturi and inline with the needle jet) the higher the velocity that passes directly over the jet so more fuel is drawn for the same amount of air. The last mod I did on my FCR was to increase the cut-away size to reduce velocity and lean out a cruise throttle AFR I had no luck controlling with different needles. Just to clarify-some folks look at this from the vacuum perspective... IE: It's engine vacuum that is drawing fuel up out of the needle jet so opening the size of the slide cut-away reduces vacuum in the center of the slide in the lower throttle range. Six of one half a dozen of the other....๐Ÿ‘

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Hi Bob - how did the tuning go??

I think you may need to go larger on the pilot, the #40 may be too small. I think you compounded the problem by going to a larger carb (the DR250 should come with a TM31).

Here is another tuning chart from Mikuni:

mikunituning.jpg

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The last mod I did on my FCR was to increase the cut-away size to reduce velocity and lean out a cruise throttle AFR I had no luck controlling with different needles.
You have much more patience than I do Rob๐Ÿคฃ:bonk:

Thanks for the cut-away mod explanation ๐Ÿ‘

Hi Bob - how did the tuning go??

I think you may need to go larger on the pilot, the #40 may be too small. I think you compounded the problem by going to a larger carb (the DR250 should come with a TM31).

Thanks for the diagram John.

I now have the carb on the bench set up with a 40 pilot and I raised the needle one groove but now I'm sitting here second guessing the changes.

The stock carb is indeed a BST31 so I'm thinking about trying a 42.5 pilot rather than the 40. I'm also wondering if I should move the needle clip down one more groove.

Changing jets isn't bad but changing clip positions takes a while since I have to remove the carb each time. ๐Ÿ™‚

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The only thing you can do is keep trying different settings until you get it right. With the pumper carb, one pilot jet size bigger = about 1 turn on the mixture screw. If you are close to three turns out now with the 37.5, you should be around two turns out with the #40.

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Just got back from a 15 mile ride.

The pilot issue appears to be solved. I went with the .40 and raised the needle one notch.

The mid-range transition is fine but there is a flat spot at 7/8 throttle and possibly up to WOT, kind of hard to tell with the rev limiter kicking in.

I'm going to try a larger main jet. Running a 135 now so I'll go 137.5

I re-painted the head pipe last night and after the test ride today it's all still there so it seems the lean condition is cured for the most part.

I can definitely say that from my experience, there is a huge difference between the DR250 and the DR350.

They are listed at almost the same HP (250=29 hp/ 350=30 hp).

I have no idea what the real hp numbers are but there is no way in hell that a DR250 is anywhere close to a DR350 in real world power.

I'm quite disappointed in the performance since the pumper swap. With my DR350 the change was like night and day. The 250.... still a slug. Just a little more responsive slug.

I'll keep tweaking a bit more and see how it goes but I'm not holding any high hopes.

I honestly only keep the 250 around for friends to ride and for the occasional, very handy spare part for my 350 ๐Ÿ™‚

I'm probably going to sell the 250 while the market is good but not with the pumper. Theres no way I'll get the $$$ that I paid for the pumper setup, so I'll put the CV carb back on and sell the complete pumper setup separately.

I'm sure I'll take a little beating on the pumper setup but not as much as if I let it go with the bike.

Frustrating.....๐Ÿคฃ

Thanks to all for your help getting the carb pretty much straightened out so far. As always...you guys are the best! ๐Ÿ‘

Edited by Bob Onit
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Yeah, I can count on less than one hand how many times a bigger carb actually improved any real world riding portion of a basically stock engine. In rare cases where the mfg actually used the carb size as a performance restriction is about the only exceptions. Until you do something more than just exhaust and air filter breathing mods (like compression, cam timing and port/valve size) "going big" is a much bigger loss over a wider part of the powerband than what little you might gain up top. ๐Ÿ‘

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I don't think you are seeing full power at WOT. Main jet could be too small. Keep increasing the size until you get an intermittent skip developing at WOT, upper rpm range.

I heard the DR250's made ~19hp compared to the DR350's ~27hp.

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I don't think you are seeing full power at WOT. Main jet could be too small. Keep increasing the size until you get an intermittent skip developing at WOT, upper rpm range.

I heard the DR250's made ~19hp compared to the DR350's ~27hp.

Full throttle is damn near what it should be (I'll try a 137.5 or a 140 if needed) but this thing wont even lift the front wheel with the throttle and its got 14/53 gearing!

My XL200r supposedly makes 17 HP and the DR250 will kill it under any conditions but the DR250 doesn't have half the guts that my DR350 does.

It' a mystery ๐Ÿ‘

I'm trying to find the specs for the 250 vs 350 but I'm not having any success.

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Yeah, I can count on less than one hand how many times a bigger carb actually improved any real world riding

I understand Rob.

What I didn't know was that the DR250 came with a 31mm BST

I thought it was a BST33.... that's why I bought the TM33

Oh well..... anybody looking for a deal on a brand new complete DR350 TM33 pumper setup ๐Ÿ‘๐Ÿคฃ

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The mid-range transition is fine but there is a flat spot at 7/8 throttle and possibly up to WOT, kind of hard to tell with the rev limiter kicking in.

I'm going to try a larger pilot. Running a 135 now so I'll go 137.5

I'll bet you aren't feeling the rev limiter but rather a misfire due to lean jetting.

Going one jet size at a time is doing it the hard way. one or two sizes either way from the ideal jet size is usually something you can't feel but need a dyno to see the difference. I would make main jet changes in 3 size increments (127,135,142,150, etc) until you know you are close and then fine tune from there.

This idea applies to the pilot and needle also. Make big changes so you can know with certainty what is too rich and what is too lean. Once you know what the unacceptable extremes are you can narrow it down to the best settings.

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this thing wont even lift the front wheel with the throttle and its got 14/53 gearing!

It should definitely have the power to loft the front. I suspect the jetting needs quite a bit time and effort to get it fully dialed in. Jetting can be more fun if you think of it as an excuse to spend a whole day (or more) riding and wrenching.๐Ÿ‘

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I'll bet you aren't feeling the rev limiter but rather a misfire due to lean jetting.

Going one jet size at a time is doing it the hard way. one or two sizes either way from the ideal jet size is usually something you can't feel but need a dyno to see the difference. I would make main jet changes in 3 size increments (127,135,142,150, etc) until you know you are close and then fine tune from there.

Thanks for the advice Jeff. I haven't given up yet but I have to wait until Tuesday to get some more jets local. The largest I have on hand is a 140. ๐Ÿ‘
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