Please fill me in on the airbox insert. I seemed to have missed that thread. I searched but no luck.

 

Do you honestly believe this is the first time that anyone has tried huge carbs on a small engine? If so, you must be a neophyte in the world of high performance engine modifications.

This has been done back in the early 70s when Skunk Works made a 38mm carb kit for 125cc motocrosser when prevailing knowledge and testing found 32mm to maximize performance. It's been done with singles, twins, fours, sixes, and V-8s during my life time. It's happening a lot less now, since there already has been a paradigm shift - EFI.

Even Honda has struggled with big carbs on high performance CRF250R engines, resolving the issue finally by going to EFI. The 40mm carbed Hondas were noted for bogging and having starting issues that didn't happen with the 37mm carbed bikes.

More power to Richard if he can get good results for his work.

Still, this is not virgin territory. I'm pretty sure Pro Circuit, Muzzi, Ron Woods, CR Axtel, Yoshimura, every flat tracker and drag racer, along with dozens of other high performance builders have worked over the engines from the tip of the air filter to the end of the exhaust in search of that ever elusive peak performance. They have the dynos and flow benches as well as the time, cash, and motivation to have tried.

The tuners' futures will be or already are fiddling with computers and black boxes to tune their EFI. Heck, the muscle car world has self teaching/learning EFI to replace the venerable 4 barrel carb. You drive it, it makes adjustments and remembers. Imagine that for your KLX!

I find it interesting reading about Richard's tests, observations and gains. He did show less low end power in his acceleration test if that is the yard stick being used to measure. It may not be much on the street, but it can easily be too much for off road (but I don't think that's much concern for his use). That's pretty much what happened when we put bigger carbs on our bikes and the reason trials bikes run ridiculously small carbs. Until I decide what I want to do with whatever I will be reading everything I can on whatever interests me to aid in decision making. Gotta say the 351 is pulling ahead.

Why doesn't someone take the information and do dyno work? Take the lead, you have a 250, and one of three carbs for sure, get the other two and have at it. Personally I don't have the cash to do the carbs and dyno time.

 

Glad you got that off your chest.... Now what about this airbox insert?

 

You can click on his name at the top of the post then find the link to all his posts in his profile. Now you learned something new.


Just saying this isn't God's gift to motorcycling, it's sticking huge carb on a small engine and trying to make it outperform the size that seems consistently the best size in practical use unless other modifications are performed. Been done before and will be done again, probably with a 50mm EFI body next time.

 

klx678, you have a lot to offer us as well - with the Dial-a-Jet - if only you can re-direct your energy toward providing a quality post on the installation and AFR ranging capability of the Dial-a-Jet. You have more to offer than posts like yours above - I'm sure of it.


2veedubs, Richard has created an insert to install inside the airbox that should smooth the transition of the airflow inside the airbox to the carb boot. I have one as well and will test it when I can..

 

Thanks Klxster...looking forward to the results.

 

Thank you for your support one and all

For those with bigger bores who want max HP, and some do, this testing in the dark may provide some revelations. Not to say this 40mm is a max hp/hi-rpm only set up because it just isn't, surprising even myself a bit. Though I felt the 36mm had so much low end with this gearing I knew there was some to sacrifice if need be and not give up ease of riding it. There are plenty of owners with SF type bikes who are on the street most of the time btw.

Here' the air box insert info as you scroll down the page

Just saying this isn't God's gift to motorcycling, it's sticking huge carb on a small engine and trying to make it outperform the size that seems consistently the best size in practical use unless other modifications are performed. Been done before and will be done again, probably with a 50mm EFI body next time.

It hasn't been done on this engine AFIK.


The 36mm's only consistently the best because for the most part no one seems to have tried this bigger carb; especially on a bigger bore. 37mm FCRs have been tried, but they're a nightmare to get jetted well and to fit into the available space.

Along with testing the air box insert, I am also going to see if I can rig up the CRF250 dual exhaust I have here up long enough to make a sprint now and then and see if the bike's any quicker with duals. I won't be able to use the current mid pipe that has the O2 sensor bung in it though-so I'll just have to go with the jetting that's in it. I'll probably either have to shorten the head pipe than have that section welded back on, OR shorten the CRF's Y pipe (preferred). This SS can be a pain to cut or drill though.



I don't know where it'll end up, but there's only one to find out. Plus there are ways to boost the 40mm velocity if need be; including this Thunder Products' VIP I have on hand that I trimmed to size yesterday.


I see I may run into a snag trying to video km/h off the speedo to view test results instead of a stop watch because the when it's zipping through the 80+ km/h range it's adding speed fast and I can't stop the vid at each 1 km/h increment-it jumps 2-3 km/h sometimes. I just tried looking at the vids I did the last few days and there are some +80 km/h increments that just don't show up no matter how quickly I double tap the space bar.

 

The "why" is because the cam mod is free and taking a little time to experiment with the 40 both at 250 and 331cc might shed some light as to why the cam mod doesn't work with the larger bores. Supposedly due to ports or valves that won't flow more HP, but the parts are all here so might as well give it a go!

 

Uh, we know why it doesn't work with the larger bores, just go read the MCM thread .

"Edit : Due to the shape and especially the size of the intake duct this mod will yeld the best results on a KLX250 , only moderate results on a KLX300 and it provides little to no benefit on a 330 or a 350 . The 330 and 350 allready makes more lows and mids so the cam mod is also pointless on these displacements ."

the data is deep in the thread somewhere, no time to dig through sorry

My thinking is that if the mcm increases low end, then the cams are being timed to create more of the power at lower rpm. The big bore already have low end grunt, but needs better cams for high end pull (hence the 101 grind cams), so with the mcm, the big bore probably has awful top end.

 

Right, I read that months ago.

I guess I'm not entirely buying Marcelino's guess that the shape and size of the intake duct is the limiting factor. Just because it's mostly a guess and wasn't backed up by flow bench data or experimentation with a bigger carb, drilled air box, etc or knowing what kind of exhaust was on the big bores that tried MCM and got no benefit.


If the 351 makes more max power, then isn't the intake tract already showing it is able to flow more CFM as it adequately supplies the 351cc bore? If it couldn't you'd have a large bore engine that made more low end power, but couldn't develop more max HP. Like some sort of industrial engine.


Why would the cam mod be pointless on a bigger bore that's making more torque already? That's like saying, "Well I've got too much torque now and don't want to pursue any more" Even more torque would be good and the cam mod places the torque peak around 8500 IIRC.


The fact that the 40 and the 36 are within a split second of each other at 250cc, could mean the port just won't flow any more CFM, or it could mean the 250cc engine just doesn't demand more CFM than the 36 can flow.

That the 40 is working so well at all is a potential advance in KLX modding; it just remains to be seen if the larger carb makes a difference when matched with a larger bore or change in cam timing one way or the other.


Anyway, bottom line is I have an extra carb to play with for awhile, and the cam mod is free, so I'm going to see what happens