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Old 05-07-2012, 09:37 PM   #1
Stretch67 OP
Bent, But Not Broken
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Joined: Mar 2006
Location: My Mad Scientist Laboratory
Oddometer: 6,953
Converting a Dual-Carb XL250R to Single Carb

Converting a 1987 dual-carb XL250R to single carb.

Original Dual-Carb setup. Not my photo.

I bought the bike new at South Seas Honda in Honolulu when I was a 20-year-old Lance Corporal in the U.S. Marines. For the first fifteen years or so, it fired up and ran really well. But about ten years ago it started having cold-start issues... in that it wouldn’t.

I figured the Primary Carb’s Enrichment Circuit needed some work, but Honda’s supply of factory carb rebuild kits had dried up a long time ago. So I just cleaned the carbs as best I could and made do. To cold-start, I used ether for a while, until I discovered that I could prime the primary carb by laying the bike down on its left side for eight seconds, then hold the throttle open while kicking through the start cycle. Once warmed up, the bike would start and run perfectly all day.

Until it wouldn’t. Two weeks ago I got the bike running, but it really ran poorly, with an off-idle stumble that wouldn’t adjust out. After having had the carbs out (a chore all by itself) and apart a half-dozen times over the past few years, I had finally had enough of the demon that is a twin-carbed thumper.

I don’t easily give up on beloved items I’ve had for a long time (isn't that right, Honey? Sweetie? Hello? Is this thing on? ), so I set out to fix (rather than replace) the bike. I’ve seen the New Old Stock dual-carb rubber intake boots as high as $130, and an allegedly serviceable dual-carb assembly for $450.

Um... no. Then I found a single universal-replacement 30mm Mikuni on eBay for $36 bucks shipped. Ah-HA!

I then performed an act which I have been dreaming of for ten years... I removed the dual-carb assembly and pitched it down the driveway as hard as I could.
Demon be gone!

During The Final Removal, I heard the original 25-year-old rubber Intake Boot crack a couple times. It had dried out to the point that it came off with the carbs, while it’s aluminum mounting flange was still bolted to the engine. This undoubtedly contributed to the lean condition, as air got past the gap between the loose boot and the flange.

A new intake was in order that would match the new carb to the dual-carb head...

Mad Scientist Time!

First, I removed the right-hand airbox-to-carb snorkel and sealed off the opening with a piece of aluminum sheet and black RTV silicone, bolted into the original bolt holes. The new 30mm single carb will be mounted to the left-hand airbox snorkel, in the location of the original Primary Carb.

I also had to adapt the throttle cable from the original push-pull setup operating butterfly valves to a single cable pulling a slide... longer cable housing, longer cable, different ferrule on the carb end, doctored up by Yours Truly. No photos of the cable.

Using the perfectly functional mounting flange as a foundation, I cut a Carb Mounting Flange from 1/4-inch 6061 Aluminum...

Instead of having two Intake Runners, one leading from the Carb Flange to each of the Intake ports in the Head, I decided to build it as a Plenum... a large, open area fed by the carb so that the fuel/air mixture will be drawn evenly into each of the pair of the Head’s Intake Ports. So I cut away the inboard rims of the Ports and smoothed and rounded the area between the Ports...

With the Intake Flange mounted to the engine, and the new Carb Flange bolted to the new Carb (which was in turn mounted to the airbox), I fitted and tack-welded two pieces of 1-1/4-inch 6061 aluminum tubing which had been cut lengthwise...

Then I filled in the triangular sections between the two pieces of half-tube...

Then lots of welding and grinding and sanding. Then more welding and grinding and sanding. Repeat ad nauseam, then repeat one more time.



Here’s the new flanged carb, with a proper butterfly choke. Note the Idle Speed Screw...

Knurled knob machined from 1/2-inch 6061 round aluminum rod, pressed onto 3/16-inch steel rod, in turn pressed into the (center-drilled) Mikuni Idle Speed Adjustment Screw. All joints JB-Welded.

Cool, minty single-carb ADV goodness.


IT... IS... ALIVE!!!

I rode it around a bit the first afternoon. It ran nice and strong, but was still a bit hard to start, and the idle needed to be set high to keep the bike running... a bit lean on the pilot jetting.

I got an assortment of Pilot and Main jets from Jets R Us and swapped in a #22.5 for the factory-installed #20.

It cold-starts on the first kick now.

It runs through the gears really well, so I believe the Main is nearly right. I'll check the spark plug after some longer rides and see how it's running.

She's back!
IBA # 33113
Blue Ridge Parkway & Outer Banks
XR400 Index

Stretch67 screwed with this post 05-14-2012 at 01:08 PM
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Old 05-07-2012, 11:26 PM   #2
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That turned out really nice in the end!
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Old 05-08-2012, 12:56 AM   #3
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Nice work, Stretch.


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Old 05-08-2012, 01:57 AM   #4
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Old 05-08-2012, 02:43 AM   #5
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Old 05-08-2012, 04:18 AM   #6
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Location: Toukley NSW Australia
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Cool. I had an XL350R with worn carbs. Actually found a new pair of carbs in Wales, but hesitated and missed out on them. Ended up selling the bike, wish I still had it.
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Old 01-21-2013, 05:59 PM   #7
RR mech
Joined: Jan 2012
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carb updates ?

I have a couple xl250r's and was wondering how your conversion has been .Does it still have the power off throttle.Is it still easy to crank. Thanks
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Old 01-23-2013, 05:22 AM   #8
Stretch67 OP
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Joined: Mar 2006
Location: My Mad Scientist Laboratory
Oddometer: 6,953
It starts very easily. Gas on, full choke, ignition off, kick through 6-8 times to get everything primed. Then Ignition on, usually starts first kick. Half-choke for a couple minutes until she's warm, and we're off.

With the single smaller carb, it doesn't have the off-idle snap that the duals had when new, but the bike runs well now... the old carbs didn't run at all.

One problem with my design though... when the engine gets hot, the heat transfers straight through the all-aluminum manifold and soaks into the carb, boiling the fuel in the float bowl. It's not a problem as long as the engine's running. But once the engine dies and the airflow through the carb ceases, the carb float won't float on the surface of the boiling fuel, so the inlet valve stays open, flooding the carb.

I have to be VERY careful to close the petcock a minute or so before I kill the engine, or the fuel in the line and filter will run into the hot carb and flood it.

So I need to insulate the carb from the intake manifold. I'll probably build an inch-thick insulator out of plastic or rubber to keep the manifold heat from soaking into the carb. Doing so will set the carb to the rear another inch, requiring modifications to the airbox snorkle.

But it's perfectly rideable as it is, one just has to know the bike's quirks in order to not have trouble with it on the trail. It took me a couple rides (with lots of kicking and cursing), but I got it figured out.

I have since retired the XL from regular trail-riding duty, and run an XR400 in its place. I'll soon take the XL to bits and restore it, adding the carb insulator at that time.
IBA # 33113
Blue Ridge Parkway & Outer Banks
XR400 Index
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