I decided to remove and check the carb on my new-to-me Evo 300 4T. I wasn't totally happy with the way it ran on my first practice day so I went diving in to see if I could find anything wrong. Just understanding the thing is the main problem.

I've got the table of settings and the factory parts list, but I'm struggling to understand which part is which in a few cases. First off, the tables gives the start jet as a 60 and the slow jet as 17.5 or 27.5. However the parts list calls the jets that are 17.5 - 30.0 the starter jet! I'm hoping that's a mis-translationas the Italian version calls it a getto minimo (as opposed to the main jet which is getto massimo).

Anyway, the photo shows my carb viewed from underneath. The jet on the right appears to be the main jet. With a touch of optimism I think I can read it as 120, so that would be standard. I think the one to the left of it is the start jet (as described in the table) and I think it is a 60. Am I right so far? Between and above those two is a third jet so I assume that is the slow jet (according to the table). I can't make out any number on it but it has a small emulsion tube built in to it, which looks a bit like the starter jet (getto minimo) in the parts list. When I took it out and looked through it, the light at the end of the tunnel was not circular so I surmised that it was partially blocked. Blowing through wouldn't clear it so I tried poking it with a fine piece of copper wire. Instead of moving the dirt I appear to have jammed it in and now there is no light getting through at all. Eventually I gave up with the copper wire and decided to get brutal with a small drill bit. I twiddled it by hand from both ends. I can feel the obstruction but I can't clear it. Am I right in identifying this jet? And am I right that I should be able to see clear through it? If so, it seems like I'll have to get a new one.

Finally, just to make sure I'm not doing something else wrong: is that screw on the far left the air screw (that should be 2 turns out from closed)?

Thanks for your patience, and please be gentle with me.

 I would say that the 250 figures are right with the 127.5 main jet. Just buy a new pilot jet as you need one. And yes that is the air screw. I believe Jitse makes a longer screw that is easily adjusted.

Hi, you could have used carb cleaner to dissolve the blockage in jet and after that use compressed air. I wouldn't try drill bits to clean as they could change size of jet.

You can easily get pilot jets for beta evo 4t from fleabay, just search under name:

 EBC Mikuni Carburetor Pilot Jets - N224.103 - Pilot Jets Slow Running Jet .

And for main jet (has to be small 6mm round jet, not 8mm large round main jet for mikuni) :

Brass Main Jet - Round Slot Head Type - Mikuni Carburettors Carb range of sizes 

 When I got my Beta 4t evo 300 with stock exhaust at first I set it up to: 2+3/4 turn on air screw - raised the needle (clip all way down)and it was better than original settings.

After getting Arrow exhaust i have changed: Mains jet:130(from120) and air mixture screw to 2+1/2 turns. Works nice and has improved top end dramatically.  

Edited September 15, 20178 yr by klonheadd

Your best doing a bit of research, that carb looks bog standard stuff, they haven't changed much...if at all since I started messing with carbs. Not sure where I found these, might help. 

When I was doing research on carburretor, when setting up Beta 4t evo Mikuni BSR, I have found that website very helpful:

Mikuni motorcycle carburetor operation and tuning - Ian Williams Tuning http://www.iwt.com.au/mikunicarb.htm

Definitely worth reading and opened my eyes to what I am actualy doing with carb settings.

10 hours ago, klonheadd said:

Hi, you could have used carb cleaner to dissolve the blockage in jet and after that use compressed air. I wouldn't try drill bits to clean as they could change size of jet.

You can easily get pilot jets for beta evo 4t from fleabay, just search under name:

 EBC Mikuni Carburetor Pilot Jets - N224.103 - Pilot Jets Slow Running Jet .

And for main jet (has to be small 6mm round jet, not 8mm large round main jet for mikuni) :

Brass Main Jet - Round Slot Head Type - Mikuni Carburettors Carb range of sizes 

 When I got my Beta 4t evo 300 with stock exhaust at first I set it up to: 2+3/4 turn on air screw - raised the needle (clip all way down)and it was better than original settings.

After getting Arrow exhaust i have changed: Mains jet:130(from120) and air mixture screw to 2+1/2 turns. Works nice and has improved top end dramatically.  

I don't think carb cleaner would have done much as the blockage was rock-hard. The attack with a drill was the final attempt before throwing it away so there was nothing left to lose. As it is, I now have to throw it away: the drill did nothing to the object causing the blockage but did start spitting out brass swarf. 

I tried eBay but couldn't find what I needed. Lampkins put me on to Allens Performance though so the part is now on order. They had 99 in stock!

8 hours ago, klonheadd said:

When I was doing research on carburretor, when setting up Beta 4t evo Mikuni BSR, I have found that website very helpful:

Mikuni motorcycle carburetor operation and tuning - Ian Williams Tuning http://www.iwt.com.au/mikunicarb.htm

Definitely worth reading and opened my eyes to what I am actualy doing with carb settings.

Interesting reading. For a wobbler who hardly cracks the throttle off idle, it clearly shows the bits I need to concentrate on, and the bits I hardly need to worry about.

I'm hoping the jet might turn up in the post tomorrow, in which case I should get a run out on Sunday to test it all. I'm not holding my breath though.

Its too late now for your plugged pilot jet, but :  (not a sales pitch here) honda sells a spray can of carb/combustion cleaner.....I was given a can by a friend to try. It really works well, have sprayed into a little containers and soaked Very stubborn bike jets with perfect results, even soaked my oil furnace jet overnight and the crapola that dissolved out of it was amazing...and  worked perfectly afterwords. Good stuff, could be cheaper than buying new jets or reaming them with a drill bit.... 

The jet did arrive in the post on Saturday (thanks, Allens Performance) so I went out for a practice today (Sunday). As far as I can tell, everything is exactly to Beta recommendations (120 main jet, 27.5 slow jet, needle in position 3, 2 turns on air screw, no idea about the spacer because I don't know what it is). I had also fitted an Amal slow throttle since last time. The slow throttle was definitely a help, though I still find the power comes in very sharply when going from closed to just-a-tiny-bit. The idle speed is set very low so I'll try upping that a bit next time. I still have the popping on the overrun. I'm not sure that it's a problem but it would be nice if it didn't do it.

Try a 127.5 main jet, clip on middle position. My 2016 Factory 300 would stall on full throttle going up a sharp incline with the stock 120 main jet

.

I stayed with 120 main jet but took the air screw (which isn't an air screw, it's a fuel screw) out to 2.5 turns. That stopped the popping and pushed the idle speed up quite a lot, so I adjusted the idle back down on the normal idle adjuster. It ran well today.

The spacer is either under or over the needle clip and gives another "1/2 clip" adjustment.

Also remember that the bike ships from the factory set to pass emissions testing. To do this the mid range is very lean.

You can dramatically change the 4T bike performance by raising the clip to the bottom (likely too far) or next to bottom position. The mid range is way fuller, smoother, and more powerful.

Jitse make a fantastic finger/wheel screw adjuster for the idle mix

Edited October 10, 20178 yr by nzralphy

Here we go again! After rather more than a few hours in the workshop and the inestimable advice of some Trialscentral forumites, my carburettor woes are reduced to a small fuel leak. Not a bad leak but an annoying one as the garage and car both smell of petrol.

I've traced the leak to the screw in the bottom of the float chamber. It's the one that's readily visible and accessible on the lefthand side (that's a first!) after removing the exhaust. It doesn't appear to perform any useful function, so I have to assume it's just a drain screw. (Though why anyone would want to drain the contents of their float chamber over the top of their gearbox is beyond me, and if you've taken the carb off you can empty it any way without needing a special drain screw.) Am I right? Or am I missing something?

The reason for the leak now becomes obvious. The screw has a tapered end that is just rammed into a hole in the float chamber casting as the screw is tightened. Sealing petrol with a metal-to-metal joint? I don't think so! Should the screw have an O ring on it? If so, what size? Or could I get away with aralditing the screw into place? Or maybe Loctite thread sealant?

People like to drain the carb without removing it to prevent problems with modern petrol left for too long.  What it hits is of no concern.

16 hours ago, 2stroke4stroke said:

People like to drain the carb without removing it to prevent problems with modern petrol left for too long.  What it hits is of no concern.

Thanks. Confirmation that it's a drain screw. Now I just have to find a way of sealing it. O ring? Thread sealant?

On 10/30/2017 at 9:51 AM, trapezeartist said:

Thanks. Confirmation that it's a drain screw. Now I just have to find a way of sealing it. O ring? Thread sealant?

Hey, Blue locktite(med strengh)  is petrol resistant (red high strengh isn't). If it comes to o-ring it depends on material they're made from. https://www.sterlitech.com/oring-compatibility-chart. I would suggest using  white plumbers PTFE tape (which is teflon) and it should be well resistant to fuel - or maybe you can find ptfe orings somewhere.