EVO 4 Stroke Carb Spring

[mcman56]

Has anyone experimented with these?  Are the springs intended for higher elevations weaker?  Can you get quicker response at sea level or do you just end up with pop and stall? 

 

https://betausa.com/evo-4-stroke-carb-spring/

 

 

[konrad]

I'll preface these remarks by stating that I have never touched a Beta 4T carb.  But I do have some experience tuning constant velocity (CV) carbs.  

The thing about CV carbs is that no matter what rate the rider opens the butterfly throttle, the vacuum-operated slide only opens in response to engine demand.  This is great because the manufacturer can fit a large throttle bore, and the bike is still easily driveable by a ham-fisted rider.

If you make the spring that controls the diaphragm weaker, the throttle connection will become more direct.  Look at the design equations for springs to see the variables you have to work with.  The simplest solution is just to replicate the standard spring with a greater number of turns (but you have to be careful about coil bind so the slide can still open fully).  Alternatively, you can wind the spring from a thinner diameter wire.   

I have bought spring steel wire from McMaster-Carr  https://www.mcmaster.com/products/spring-wire/steel-1~/  but I expect there are other fine sources.

[lemur]

CV carburetors operate off intake vacuum pressure that is literally robbed from the engines intake stroke.  Key to making a Beta 4T respond as well as a Montesa 4RT would be to install PGM-Fi instead of a carburetor and the vacuum operated fuel tap.  PGM-Fi self adjusts to any altitude and air temperature changes and the engine intake vacuum pressure is 100% dedicated to engine performance, by comparison carburetors basically suck.

[mcman56]

My question was really related to an XT225 (Serow). The carb is a mikuni and looks similar to the Beta 4t carb so I was wondering if the Beta springs would work in the XT225 to improve response.  I have made springs before so that is an option I had not thought about.  

Interesting comment on the 4RT.   I have a 2016 Beta 4t standard model and recently got a 2018 300 4RT but find it harder to ride due to the sharp throttle response.  It is difficult to get just a little bit of power and feels like it has about 1/3 of the throttle resolution compared to the Beta .  Can the 4RT be tuned to respond more like the Beta?  It does have the black throttle tube.  I sort of remember options for early models that allowed the owners to tune or maybe load different maps.  I'm not finding anything online so is anything like that still available.  I know......the solution is to improve clutch control but with arthritic hands and the beginning of trigger finger in my clutch finger that is unlikely.

[konrad]

On 5/11/2025 at 10:22 AM, mcman56 said:

Can the 4RT be tuned to respond more like the Beta?  It does have the black throttle tube.  I sort of remember options for early models that allowed the owners to tune or maybe load different maps.  I'm not finding anything online so is anything like that still available?

A friend loaned me an old Honda "Setting Tool" so I could learn about it. Here is a bit from my notes:

The computer interface requires an RS-232 port. Newer versions work with a USB port. The ECU interface uses the K-Line protocol, which is primitive automotive standard from the early days of OBD2.

The Setting Tool software requires a 32-bit OS. I tried enabling 32-bit apps under Windows 10, but this did not allow the program to run.

The actual EFI map is not accessible, only modifications to it. The modification matrix has 8 default throttle positions (0, 5, 10, 25, 50, 60, 70, and 100%) by 12 RPM positions (1000, 1400, 1800, 2200, 3500, 4500, 5000, 6000, 7000, 8000, 10000, and 12000 RPM) for both fueling and ignition timing. Each of the default throttle and RPM positions may also be changed slightly, for example 1400 rpm has a range of 1100 to 1700. You are not permitted to move the value into an adjacent range bin.

I was also given a half dozen example modification files. Remember they are not actually maps themselves. This is really nice because they could be applied to any year or displacement Montesa trials bike. The only "incompatibility" seems to be if the file is for a single or dual map throttle body. But that changed in 2007, and all bikes going forward seem to have dual map capability. (Well not really, as Montesa made some bikes homologated for road use that don't allow any reprogramming at all.)

Fueling: +/- 30% change in 1% increments. A total of 61 values.

Ignition Timing: +4 degrees to -12 degrees change in 1 degree increments. A total of 17 values.

You can specify whether changes apply to "mode 1" or "mode 2" (map switch), but the default is both modes.

The modification files have an .ep2 extension. They are written in Intel Hex format and are quite compact (~750 bytes). Each file has an area at the end to write comments in ASCII text.

[mcman56]

Is the tool still available?  I don't see anything in the 2018 300RR parts list or service manual.  Searching online, I get no clear answer.  

Although, I see one for a CFR250 is pretty expensive so hard to justify

https://www.rockymountainatvmc.com/parts/honda-pgm-fi-setting-tool-p?srsltid=AfmBOoriVL_t6J6WD2_uqMxRoEZDE-ZnbgTGCmcIl9nImw-E11jsmHkL