I had an off the shelf Keihin 39pwk laying around from an LT500R project that never got off the ground. Over the weekend I decided to attempt install of the carb onto my scrambler. I ended up struggling for about an hour with it but the boot on the airbox side just won't stretch.

I tried boiling it and heating it over the stove but no luck. I've done countless searches and read that it is possible to do but may be difficult on older airboxes where the rubber has hardened up.

My question - is there some type of adapter that I could use, or an aftermarket airbox, or other model airbox that might make it fit?

I read that RCR used to make an adapter but now they machine the intake to fit. Is there another vendor who sells pwk adapters? I could machine the carb but would prefer to explore other options first.

The other idea I've been thinking about is using an exhaust tubing expander and heat gun but would be afraid of wrecking the intake boot.

Any help or feedback would be greatly appreciated, thank you.

 

Cut a sized heavy duty motorcycle tube to the desired width and slide it over the back of the carb and existing air box tube and clamp it in place. May have to rig up a support bracket. Seen several guys do this with modified carbs. Plus had one guy came around ever year to get used tubes or ones in the trash. Made booties for his hunting dogs..

 

Hot Seat used to sell an adapter for the stock airbox with the 38mm & 42mm TMX carbs they used to sell and the last 42mm TMX carb that I bought from Aaen came with an adapter for the airbox. They wouldn't be hard to make like OPT stated or you could run a clamp on filter and jet for that.

I run a wet nitrous kit from Nitrous Oxide Systems on my Polaris and as you can see space is an issue with the nitrous solenoid, fuel solenoid, fuel pump, relay and wiring so I kind of have to run a clamp on filter. If you go with one of those it's a good idea to attach a bracket or something to the carb so that it has a solid mounting point like with the airbox and doesn't bounce around or vibrate too much because this will cause fuel to pour out of the overflow hoses when it is idling.

A good tip when installing a bigger carb is to grease up the rubber boot with a small amount of oil on your finger so that the carb will slide into the boot without too much trouble. I do this with my 42mm TMX and even though it's a tight fit it slides in without too much trouble.

Hot Seat


Aaen


Clamp on filter

 

Great guys, thanks for the replies.

Regarding the use of an inner tube - How well does that rubber hold up to gasoline? I would be concerned about deterioration and quality of seal using that method. Perhaps I misinterpreted the suggestion, do you have a picture that shows how to fit an inner tube over the factory carb boot? Also does this cause a disruption in flow at all? It would seem that going from the 2" factory intake diameter and back up to 2.5" of the PWK would cause a loss in air velocity impacting the carburetors function.

I was able to find some plumbing parts at home depot (fernco 3000-215), which is a neoprene rubber 2 - 2.5" ID pipe adapter. I also used a nylon sink fitting with an inner diameter of 2" to connect the adapter to the airbox boot. Again I am concerned at how the rubber will hold up to fuel vapor/mist from the carb.

I dont intend to ever run through deep mud, so if all else fails I can always go the clamp on route as shown above and fab some brackets to keep vibrations to a minimum.

I'm happy to hear some more ideas and thank you again for the feedback.

 

The ones I saw used heavy duty Moose tubes and didn't seem to be restrictive when overlapping the existing air tube,as far as how they ran. Seems like you've found something that will work as well or better. Probably last as long as anything else without the cost of adapters if you're looking to go cheap.

 

Here's a better picture of the Hot Seat airbox piece. All it is is a rubber boot like you would buy at Lowe's or Home Depot and a small aluminium cylinder that goes inside the rubber boot and is tightened up with a couple of worm clamps. Probably costs around $8-$10 to make and works very well. I ran this setup for several years in some muddy conditions with no problems at all.

Not sure if anyone has talked to you about running the 39mm carb with the airbox or not, but you'll have to open up the lid or modify the lid/airbox to get the full potential of the bigger carb with the factory airbox. I had good results with adding a few spacers under the airbox lid to raise it up a little when I tightened it down with the two wingnuts and in dry conditions for best performance I left the lid off. Make a couple of test runs with the factory airbox and then try it with the lid off or raised and you'll see what I'm talking about.

 

That Hot Seat adapter is very similar to what I made. If I had a camera here I'd upload a picture to share. I think I should be good on the fitment, I'll update on how it works out when I can install later.

On the topic on Airbox - The snorkel has been removed completely and the front oval has been completely carved out, which should let it flow pretty good.

Any suggestions on my jetting? I fired it up last night w/o the intake on it and it seemed to hesitate badly under partial throttle, but came onto the pipe violently at 2/3rd throttle. I've read it may relate to the DGH needle being used causing lean spots in the midrange. As I understand it most people use the EEG Needle, 6.5 Slide, 35-45 Pilot and 162-168 Main.

My Jetting specs are as follows

DGH needle 3rd clip (leaner than EEG)
#6 Slide (richer than #6.5)
170 Main
40 Pilot

 

Sorry, but I don't have any jetting experience with the PWK carbs. I've ran either a 38mm or 42mm TMX on my Polaris since 1999. A couple of my friends used to run them on their Sports/Scramblers several years ago and we used to compare the PWK with the TMX, but that's as close as I've ever came to one. About a week after I bought my Polaris I started modifying it so I haven't touched a factory carb since 1999. All of my carb experience for 2 stroke Polaris ATV's is with the TMX line of carbs.

The Keihin and Mikuni brands of carbs use jets that are numbered differently. A jet for the PWK might be numbered 170 while a similar flowing jet for a TMX might be numbered 320. If you had a TMX I could compare your mods with mine and give you an idea of what my jetting was with similar mods, but trying to compare jetting from a Keihin to a Mikuni is like trying to compare software from a PC to a MAC.

I do know from experience that trying to run without an airfilter and airbox doesn't work very well with a carb jetted for an airbox. Once I had my clamp on filter become loose and fall off while riding and my Polaris would just bog and die when I gave it gas.

 

I finished up the install yesterday and took it around the block for a quick tuning run. Definitely a huge improvement over the stock carb as far as throttle response. It is difficult to tell if it made any additional power because I was spinning so badly in the snow. The DGH needle is way lean even on the last clip, I barely had any color on the insulator and it was dry after a couple high speed runs.

One thing I noticed is that the stock throttle is unable to open the 39mm carb fully, I am only able to get about 2/3rds open out of it. Is there a modification to allow larger carbs? Can I grind the stops on the throttle arm, or is there another ATV that uses a larger carb that the throttle would swap directly?

Thank you!

 

You can always try places like this if you have the money to burn. Aaen Performance - Polaris ATV Two Stroke - Top End Pipes