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Old 11-08-2010, 01:32 AM   #1
katumo_jtb OP
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Rebuilding a DellOrto PHM-SD carburetor

DellOrto PHM carbs were used on mid to late 90's KTMs, and probably some other euro bikes as well. Specifically, the PHM38SD was used on the 400 LC4s, and the PHM40SD was used on the 620 LC4s... like mine! Anyway, there's not much online info for these old-tech carbs, so I thought I'd share my experience rebuilding one. I wish I had done this about 15 years ago, then I could have attained Creeper-like status. Anyway, better late than never. I hope this helps somebody out there.

Disclaimer: I am not an expert, and barely even a novice when it comes to carburetors. I understand the basic theory and successfully took mine apart, cleaned it, and reinstalled it. I can do basic diagnosis, but if you're looking for advice on jetting/fine tuning, you should probably check the manual or ask an expert. Also, if anyone has suggestions on ways to improve this guide, please comment!

I'll separate this into 6 parts and link to each from the first post (for those who want to skip ahead). The posts are pretty picture-heavy so I'll add links to the next/previous on each, in case you want to view them as single posts to speed up loading.

1. Required tools/parts
2. External disassembly
3. Inside the float chamber: needle/jet disassembly
4. Setting float height and float chamber installation
5. External installation
6. Summary and general diagnostics

Enjoy!

-John

next post: Required tools/parts

katumo_jtb screwed with this post 11-08-2010 at 01:45 AM
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Old 11-08-2010, 01:33 AM   #2
katumo_jtb OP
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Required tools/parts

previous post: Introduction

First, I assume you have the carb off the bike. You can consult the owner's manual on that, but it's pretty straightforward. Second, you'll need the following tools and materials:

1. Can of carb or brake cleaner
2. Gasket set (KTM part #58031199000)
3. Flat-head screwdriver (one is necessary, 2 sizes is ideal)
4. Wrench (adjustable, or several sizes to fit various pieces)
5. 3mm allen wrench
6. Vice grips (regular pliers might work)
7. Optional: carburetor manual. This is targeted more towards jet and needle selection, but it's handy to have. Here's a link to the PDF: DellOrto factory manual



Now, let's get familiar with your carburetor. Starting from driver's left, we have the hot start button, choke body, and some vents. Note: not all carbs have the bottom float vent; I have one that does and one that doesn't.



On the right side, we have the fuel intake (and behind it the fuel filter), the idle speed adjuster, and the idle mixture screw. There's also another float chamber vent on the right side. Note, the idle mixture screw is a fuel screw... so, screw in to decrease fuel (lean mixture), screw out to increase fuel (rich mixture). Now would be a good time to see how many turns out your idle adjustment screws are. Write this down; you'll want to return them to their original positions during installation.



On top of the carb, we see the throttle cable and the choke cable. The throttle cable has a metal tube surrounding it; I've already removed it in this picture.



Before we begin the rebuild, remember the float drain screw from 2 pictures back? Now would be a good time to loosen it, so you can drain fuel from the carb somewhere other than your work bench.



next post: External disassembly

katumo_jtb screwed with this post 11-08-2010 at 10:38 AM
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Old 11-08-2010, 01:34 AM   #3
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External Disassembly

previous post: Required tools/parts

Removing the top two screws exposes the throttle slide assembly. This consists of the throttle cable, return spring, throttle slide, and the needle. Note: there's a large rubber o-ring under the throttle cap; you'll want to replace this with a new one from your gasket set later.



The throttle cable and needle are retained with a brass screw. Remove it to get them out.



At this time, you can inspect the needle for wear. You might want to note the number on the needle, in case you want to note the baseline specs of your carb setup. The stock needle for PHM40SDs (on the 620) was a K51.



Also note the clip position. There are 4 notches in the needle. You can drop it down (higher notch) to lean out the mixture throughout the throttle range, or raise it up (lower notch) to make the mixture richer. The stock clip position on the PHM40SD was in the 3rd notch from the top.



On the left side of the carb you'll find the choke assembly. On the PHM carburetors, the choke is basically a miniature secondary throttle. There's another cable, return spring, and slide, just like the main throttle. This slide covers the starting jet, adding fuel to the mixture when the choke is engaged.





After removing the choke parts, remove the entire housing from the carburetor.



There's a gasket behind the choke housing. You'll want to scrape off the remnants so you can replace it with a new one later.



The hot-start button is also on the left side of the carb. It screws into the carb body, with a nut locking it in place. A 12mm wrench can be used to loosen the nut and unscrew the whole thing.



On the intake side of the carb, you'll want to remove the intake bell. This is held in place with 2 3mm allen head screws. There's a not-quite-round o-ring behind the intake bell that you'll want to replace later.



Moving to the right side of the carb, you have the fuel intake, idle speed screw, and idle mixture screw (from back to front, in order). Remove them by unscrewing each. Make a note of how far the idle speed and mixture screws are turned out before removing them. You'll want to return them to these positions during reassembly. That way they'll be close to the right place before fine-tuning, not so far out that the bike doesn't start or idle.



Gently pry the fuel filter out with a flat-head screwdriver after removing the fuel intake.



On the idle speed adjuster, note there is a washer and o-ring behind the spring. You may need to pry these out if they don't come out with the screw itself.



Also, there's another washer and o-ring behind the idle adjuster screw. You may need to pry these out as well.



next post: Inside the float chamber: needle/jet disassembly

katumo_jtb screwed with this post 11-08-2010 at 01:48 AM
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Old 11-08-2010, 01:34 AM   #4
katumo_jtb OP
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Inside the float chamber: needle/jet disassembly

previous post: External disassembly

Now let's move on to the float chamber. Start removal by completely unscrewing the drain screw. This has a gasket under it and holds the float chamber to the carb body.



Removing the bottom of the carb exposes the float and all of the jets.



Here are the exposed jets. Note: each jet has its number engraved on the bottom or side. You may want to record them for future reference, in case you plan on changing the mixture later.



Remove the copper housing holding the main jet, along with the little hat (float baffle?), to expose the needle jet and the idle jet.



A note on jets: the starting jet controls how much fuel goes through the choke when engaged. The idle jet controls how much fuel goes through the idle mixture screw. The needle jet is what the needle goes through; the difference between the needle jet and needle cross-sections, along with the needle taper, determine the progression of mixtures from idle to 3/4 throttle. Finally, the main jet determines how much fuel is available at 3/4 to full throttle.



Now you're ready to remove the float. It's held in place by a pin with one grooved end. Grip the grooved end with a pliers or vice-grips, and pull it out to remove the float.



The float is connected to a plunger, which opens and closes the float valve via a spring. I'll talk about how the float operates later on when setting the float height. Basically, inspect the float valve for normal operation and sealing. If the valve sticks or doesn't seal properly, you'll need to replace it. If you do, the new assembly should contain the plunger, float needle, valve, and a copper seat together. You're supposed to replace them as a unit, if at all.



Congratulations, your carburetor is completely disassembled! At this point, the carb body shouldn't have anything else attached to it; no gaskets, o-rings, or copper parts.



Now you can clean the crap out of it with carb or brake cleaner. You should also run some carb cleaner through all the jets, screws, and slides. From now on you want to keep the carb as clean as possible, so none of the jets get clogged with dirt during reassembly.

NOTE: be careful not to get carb or brake cleaner on any plastic or rubber parts. This includes the float valve. Harsh cleaners can damage the viton tip and prevent it from sealing. Also, make sure all cleaners are dry before reassembly; you don't want residual cleaner damaging any of the fresh o-rings you install.

next post: Setting float height and float chamber installation

katumo_jtb screwed with this post 11-08-2010 at 01:49 AM
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Old 11-08-2010, 01:35 AM   #5
katumo_jtb OP
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Setting float height and float chamber installation

previous post: Inside the float chamber: needle/jet disassembly

First, you want to replace the intake bell o-ring and install the cleaned intake bell. This is because setting the float height requires laying the carb flat on the intake side, which is only possible once the bell is installed.



Now, you're ready to set the float height. The height is measured from the bottom surface of the carb to the bottom surface of the float, while the carb is sitting flat and horizontal on the intake. The spec for PHM-series carbs is 17.5 to 18.5mm for this measurement.

At this time, I also temporarily reinstalled the fuel intake and a length of clean fuel hose. I did this to test that the float valve was sealing properly when the float was at the specified height from the bottom of the carb surface. Basically, just blow through the hose while moving the float around to make sure it blocks the flow at the right position. If not, your float valve could need replacing.



Assuming your float valve is functional, you can fine-tune the float height by bending the metal tab that holds the float to the plunger.



Once the float height is set, screw all of the jets back in place. Make sure to use a new o-ring on the starting jet.



After all jets are installed, including the main jet and baffle, you can close up the float chamber. Don't forget the rubber gasket that seals the edge of the float chamber, or the gasket under the drain screw.



next post: External installation

katumo_jtb screwed with this post 11-08-2010 at 01:50 AM
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Old 11-08-2010, 01:36 AM   #6
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External installation

previous post: Setting float height and float chamber installation

This is the easy part. Reinstall the fuel intake, idle speed adjuster, and idle mixture adjuster. Don't forget the (clean) fuel filter under the intake, or the washers and (new) o-rings under the idle speed/mixture adjusters. Set the idle adjusters at their original settings from when you removed them; that way the bike is likely to start and idle right off the bat, before any fine-tuning.



On the left side of the carb, reinstall the choke housing and the hot start button. Don't forget the new gasket under the choke housing, and the o-ring on the hot start button. When you put the throttle back in, you want to set the hot start so it's close to touching the slide, but not quite. If it's too far in, the hot start will set your idle speed instead of the idle adjuster. If it's not far in enough, it will pop out and your hot start won't function. You can always adjust it later on, just make sure it's set properly whenever you change your idle speed.



Slide the needle through the throttle slide, reinstall the throttle cable, and lock them in place with the screw.



Note that the throttle cable needs to go through the metal tube, through the cap (with fresh o-ring), through the return spring, through the slotted screw, and into the slide. You'll need to manually compress the return spring to make it all happen.



If you did everything right, you now have a fully assembled DellOrto carburetor -- just like the one you took off your bike, except a lot cleaner and hopefully better functioning. Next step: put it back on the bike and see if it runs!



next post: Summary and general diagnostics

katumo_jtb screwed with this post 11-08-2010 at 01:50 AM
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Old 11-08-2010, 01:36 AM   #7
katumo_jtb OP
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Summary and general diagnostics

previous post: External installation

Cleaning and reassembling a carburetor isn't rocket surgery. Jetting and diagnostics are a little more complicated, so I'll leave it up to more capable folks to describe in detail. I'll describe the simple stuff that I know about.

Basically, I've run into 3 general issues with mine. One (several times) was when the bike was hard to start and would only run on choke. This was due to a clogged idle jet, the smallest of all the jets (and therefore most likely to clog). Revving the bike a bit on the choke usually cleared it out and fixed the problem, although in extreme cases you might need to remove and flush the idle jet. Another issue was fuel delivery, where the bike would start and idle but gradually die at higher speeds. This was due to a clogged fuel filter. Finally, with extreme altitude changes I've run into performance problems. I've only adjusted idle mixture, but dropping the needle a notch and/or swapping in a smaller main jet would help too.

Finally, if you are planning on dialing in your carb jetting, here's a good place to start: the stock settings for a PHM40SD on a KTM LC4 (620, I assume). The main jet is probably a little lean, but otherwise these settings are not too far from what was in my carb.



That's all, I hope somebody found this helpful! If so, please comment, or if you have any suggestions or words of wisdom, feel free to add those too.

-John

katumo_jtb screwed with this post 11-08-2010 at 01:51 AM
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Old 11-08-2010, 03:34 AM   #8
lowbudget
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That´s a nice rebuild thread! Good work.
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Old 11-08-2010, 05:16 AM   #9
Tseta
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Great thread, thanks Katumo!

The only other relevant Dell'Orto threads so far (that discuss jetting and such) seem to be:

Dell'Orto PHM 40 SD - Stock or tuning

Dell orto on KTM RXC

Cheers,

Tseta
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Old 11-08-2010, 08:02 AM   #10
motomal
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Perfect! I have the LC4-400 with the 38 Dell'Orto and was looking for service info.

Thanks.

(PM sent.)
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Old 11-08-2010, 08:17 AM   #11
NICO
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Great write-up. One of the better ones I've seen here. Good job.

Hey Meatpop, you around?
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Old 11-08-2010, 09:52 AM   #12
laramie LC4
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WOW! nicely done! kinda reminds me of another thread around here.....

i like the way you linked it at the start, that was a good idea.

thanks for the time, effort, and willingness to share with the rest of us. trust me, i know its not easy.

laramie
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Old 11-08-2010, 10:17 AM   #13
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Quote:
Originally Posted by laramie LC4
WOW! nicely done! kinda reminds me of another thread around here.....
Haha, yup. I decided against calling it the PHM-40 bible to avoid too much scrutiny

Thanks for the kind words everyone. This is my first how-to thread and it was more work than I thought.

I've already had a couple requests for the factory manual/tuning guide. Google docs file hosting seems to work. Here's the link (also linked from the second post above):

https://docs.google.com/fileview?id=...thkey=COvypZUK

John

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Old 11-08-2010, 10:52 AM   #14
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Some thoughts/questions:

This Dell'Orto carb is a "slide"-type carburetor, as opposed to the common "constant velocity"-type (like the BST). The slide movement is directly linked to the throttle cable. However, the Dell'Orto doesn't seem to have any type of accelerator pump on it. I thought that the momentary lean condition when opening the throttle was one of the main reasons why CV-carbs were developed, and why slide-type carbs require some kind of accelerator pump.

I noticed that the linked document (thanks for making it publicly available) has some references to a few types of accelerator pumps that are "available" or "optional" on this carb. However, the KTM version seems to have come without it.

Why doesn't this Dell'Orto have an accelerator pump? How does it deal with the lean condition when opening the throttle?

Cheers,

Tseta
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Old 11-08-2010, 01:17 PM   #15
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Quote:
Originally Posted by Tseta
Some thoughts/questions:

This Dell'Orto carb is a "slide"-type carburetor, as opposed to the common "constant velocity"-type (like the BST). The slide movement is directly linked to the throttle cable. However, the Dell'Orto doesn't seem to have any type of accelerator pump on it. I thought that the momentary lean condition when opening the throttle was one of the main reasons why CV-carbs were developed, and why slide-type carbs require some kind of accelerator pump.

I noticed that the linked document (thanks for making it publicly available) has some references to a few types of accelerator pumps that are "available" or "optional" on this carb. However, the KTM version seems to have come without it.

Why doesn't this Dell'Orto have an accelerator pump? How does it deal with the lean condition when opening the throttle?

Cheers,

Tseta
I wondered about this myself. Maybe it just doesn't account for the lean condition? This carb isn't as snappy as a pumper like the FCR. It seems to get pretty decent fuel economy... probably because acceleration is limited by the lack of a pump

There does seem to be a port on the intake side that's covered up. I assume this is where the accelerator pump would go. Has anyone installed one on this carb? I would be interested to hear about it.

John
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