40mm in situ (including air box boot)




It looks monstrous, but it's the same external dimensions as the TM36 (give or take a mm here or there)




This was just a test fitting to make sure the dimensions I'd seen printed online were accurate and that it fit in as nicely as the little 36mm. I was tempted to try and fire it up, but I have a lot of other mods to do with the 36mm in place before I move up to the 40mm, and I didn't see any reason to introduce gas to it just to hear it idle, then take it off and let it sit. Plus I am going to make some mods to the carb itslelf, so...

The stock rubber intake manifold from eBay is stretching nicely after a few days of having the 40mm jammed into it. After I installed the stretched intake on the engine, the carb slid in easily with a thin film of WD40 wiped onto its' spigot.

I still have to at least port out that intake a little. Just need to grind a taper on the inside where the end of the 36 or any carb butts against the ridge inside the intake. I have an after market rubber manifold that's made for the 40mm more or less, but it's flange is not angle cut like the OEM manifold, and its bolt hole spread is a little wider. I think I can elongate the manifold's bolt holes enough should I get serious about using it, but that intake's straight flange may present a challenge.

There are lots of ways to perk up the velocity if it's too much carb for a 250 ; Plus, I have a BB 331 cylinder and piston that will step up to the challenge in a few months.

Regardless of the jabs and derision from the Illuminati here (they haven't tried it have they?) I'm intent on seeing where it leads 40mm carb sets are sometimes used on 4 cylinder 1200cc bikes, and - FCR39 carbs are used now and then in Japan on 250 D Trackers of unknown displacement.

I have a selection of needles, air jets and main jets to choose from; and an a/f meter to simplify testing once the monster carb is on to stay. Which should prove to be a major improvement in tuning ease.

Adding the 40mm will probably be one of the last mods I do; there are so many other ideas to sort out first. But I may give in to temptation and put it on to see how it runs ahead of schedule..

All this is eventually leading up to 331cc combined with the cam mod and 40mm carb + breathing better through a modified (drilled) air box. Then - I'll see if the standard air box is what's affecting the cam mod's ability to produce gains when going over 300cc, or whether it's the IN valve/port.




- A/F measurements with the 36mm in place will be begin soon! I just got back from the big city where I had the exhaust bung welded in place in the mid pipe. I'm going to take this time to clean up the steel (not ss) header and ss mid pipe, then shoot them both with a can of clear satin heat paint.

Then the testing will begin, including documenting the change in a/f ratio after doing the cam mod. I don't think anyone's pinpointed exactly what the MCM does to otherwise suitable jetting. I only remember seeing guesses about going x richer on the needle and main.

 

Best of luck. I think you won't find much of an improvement if any, but I'd gladly be proven wrong.

 

Go ahead and take a big cheap shot at my comments from before, ignore the comments and suggestions if you wish. I haven't run my bike without the head pipe either, but am fairly certain it won't work well.

They were comments, not derision, made based on knowledge and experience, plus what history has shown from the work done by others. That research included Bill Blue's choice for his 351, what riders have been fitting here in the forum, and some research on the carbs used on KLX300s. Part of my reasoning was an FCR39 from a parted out MXer is far less costly than a modified 34 or the Mikuni 36. But if it won't work it's a waste of money.

You see, I learned a few decades ago by doing what you are doing - seat of the pants. I over carbureted my flat tracker - a bike that thrives on being wide open - and bad pipe design on my old motocrosser. I also saw the results others had from over carbureting or poor exhaust design. Then I learned to do research first, acting second. It was a lot harder though, only print media, no internet, but most all of it has been backed up by internet research when working with my bikes now.

I learned from articles and books I calculated carb size and pipe dimensions for my little MB5 I played with when they were around - 20mm carb was max, the pipe dimensions made the MB rev like crazy (formulas found in the Two Stroke Tuners Guide), but the ignition limited the potential. But it seems there are formulas and calculators to figure exhaust dimensions per application. Used them when doing the SR500 for the best carb size and pipe dimensions.

What I didn't learn or find in motorcycles I found from other sources that also use internal combustion engines - aka street rods and drag racers. There's a lot that can be found out there. Including header pipe design. A four stroke cylinder is a four stroke cylinder. While working in industry as an engineer I had to learn what I didn't know. Find resources first, learn, then look at the situation and do the work necessary, nature of the beast. I don't shoot from the hip. I try to take aim and hit right the first time.

So, yes, I have done a bit and have learned a lot. Feel free to take more cheap shots if you like though. But have the courage to actually put up your cheap shot image for others to see. I can deal with it better than you can deal with comments and suggestions.

FWIW Marcelino documented his results with dyno testing using a baseline and comparing. Sheer performance, regardless of anything else.

Good luck with your work.

 

LOL!...well, yeah, he did come off a little snarky, didn't he.

I especially like the comment, "illuminati". That's some high brow snarkiness...LOL! I don't know...I think the beauty of forums like this is the differing observations, experiences, and opinions about different aspects of a variety of discussions. It's how one comes off in the discussion that can turn things sour. Over in "Adventure Rider", that thread about buying a WRR and regreting it is probably over-the-top in some of those comments and downright impolite in some others. It can be a fine line between humor and impoliteness.

Regardless, when one asks a question or makes comments on a forum like this, one shouldn't be too thin skinned as long as the responses are on topic and delivered with an acceptable level of decorum.

Richard, I hope you have good luck with your project. The general assumption can be made that the engine would be overcarburated with a 40 based on past history and other similar engine designs...but...nothing wrong with trying it. Sometimes it's how you find limits. When I was a teenager I had a '57 Chevy with a stock 283. I installed dual exhausts and replaced the WCFB cheesy stock 4-bbl. Someone had a 750cfm double-pumper Holley for me to try. I jetted it down as much as possible to fit the engine, but obviously the setup was heavily over-carbed. It was a fun experiment, and I definitely learned a few things with it, even outside the realm of the carb tuning. Nothing wrong with that.

 

I am watching with great interest.

As for the "cheap shot" - sure, you can take it like that. I like the fact that he has acknowledged your opinion but is willing to go out and get some actual first hand experience on the KLX to feed back to this forum for those of us that would like to know for certain what our options are. If he comes back and basically proves you right, then go ahead and beat your chest and say "I told you so". Right now you come across as if your ego has been bruised because someone dares question your opinion. How is your ego going to fair if he comes back with success?

If no-one else on this forum shows interest Richard, then please can you at least PM your results and findings as I am actually interested in what you're trying out.

 

How dare you Richard, the earth is FLAT ! ... PERIOD!

LOL, More power to ya.. Keep the faith.. We are with you and waiting on your observations.

 

It was a far shot from "acknowledged". I think the proper term would have been more like what a child does when they can't go play in traffic because someone who knows what can happen tells them it is not a good idea. Just short of a tantrum, because I pointed out what has been true for IC engines and is still being pointed out in any and every publication dealing with performance building. Unfortunately the motorcycle media no longer does any performance work, so one has to source knowledge from the automotive world.

Try a 50 injector off a Ducati for all I honestly care. The whole process without paying attention to what has been done with similar equipment in the past is like tearing down and rebuilding an engine ignoring the shop manual. Why retrace paths that have been found to lead to a dead end,much less trod aimlessly through the wilderness and fall off a cliff? But if trace you must, press on regardless. Then when you've found the drowning point for the 250 let me know... my 650 can actually use a 40mm.

As for the flat earth concept, seems those who sailed away not buying the concept did research and learned before sailing off. They certainly didn't wake up one day and say "I think I'll go sail off to the horizon and see if I fall off the edge." They looked at the actual scientific facts, ignoring the myths, then made a choice. I presented sources of scientific facts as well as some past experience.

I have not taken any cheap shots that I know of, so I guess it's my turn to lob a volley - no links though:



Bon voyage, more power to ya, Richard...

Much like the air box/filter thing I will have no comment on the carburetion subject any further since it is neither understood nor accepted with any grace.

 

The xr650l is often upgraded to a 40mm pumper from a 40mm cv....makes you wonder why they dont use a 50mm pumper...


Overcarburation is a real possibility

 

Over carb is likely, but I appreciate you pushing the envelope as this helps us all learn what works best. Keep us posted as to what you find?
Thanks!

 

Once the current mini typhoon passes I should be ready to do some a/f testing with the NGK AFX meter