Discussion VT250F carbs

I recently came across a VT250F owner in Queensland who has removed the standard 2 x carb setup on his VT250F and replaced it with a single VM32 on a custom-made manifold. I have attached some pics of his work. He said he made this change because he was having so much trouble with the standard carbs flooding and the bike not starting etc.

My question(s) to the group...

As a non-mechanic, I was under the impression that the length of the runner (inlet boot) had a direct affect on fueling. The custom manifold he uses is much longer than the standard intake where the carb is very close to the inlet. Is there a 'rule' regarding this e.g. how far way from the inlet valve(s) the carb can/should be?

Seeing this custom carb job, I got to thinking if there was any other 'options' for replacing the standard CV carbs on the VT250F? These carbs are already rare and parts for them are NLA. Most of the carbs I have worked on have different needle jets to what is stated in the manual

 

I'm not concerned with inlet trumpets; my query relates to the length(s) of the inlet manifold. The Carb is at least 150mm away from the front cylinder inlet and at least 250mm away from the back cylinder inlet. everything about this setup screams 'it shouldn't work' to me. Replacing 2 carbs that sit on top of the inlet boots with one carb that is far away from the inlets and having unequal lengths to each cylinder just seems to be 'not right' to me.

I can't see how the emulsified fuel/air mixture remains gaseous given the distance it has to travel to get to the cylinders. I think that the carb he is running is not a CV carb, so does that have any affect on the this setup?

If bringing the carb closer to the inlet valve moves the power/torque peak higher up the rev range, then these extremely long inlet must make zero power as the revs rise. I'd love to take the bike for a ride, but that's not possible. In the end, I am trying to determine if this a workable solution for replacing the standard carbs with something more tuneable and offering better or similar performance to the standard carbs.

 

That's what I thought, but after reading @gregt post I now understand how this odd configuration might actually work. I think Greg's idea/suggestion of a HD 'T' connection makes the most sense to me. I understand that the 2-stroke carb pod filter might be an issue, but I was told non-CV carbs running pods are more tuneable than CV carbs with pods (i.e. NOT tuneable)...correct me if I'm mistaken.

@ruckusman is right about the amount of space I can play with in the carb (under tank) area, but I think that will be a problem to worry about way later. The standard carbs and airbox take up a lot of space, so it might not be a big problem especially if I go down the 1 carb feeding both cylinders path.

"What about two CB125 slide carbs on original manifolds?" That's the other route that could be taken. The Keihin VD6 carbs on the stock VT250F are 32mm units, so finding something that has the equivalent flow and fuel supply rates might be a good idea. it's not a problem I need to solve right now, but it is good for me to hear what experienced people on here have to say; plus I like the idea of making something unusual (that works) to replace the stock VT250F carbs.

 

Yes, they are pretty cheap to buy. I might buy a 32mm and have a play.

My problem isn't the carb per se, it's my lack of skills (not being able to weld).

What should I make the 'T' piece from? Should it be a hard 'T' piece or should I try and build some rounding into the shape to help move/flow the air/fuel mixture better? More like a 'Y' shape.

 

I thought we were looking at one of those carbs mounted to a 'Y' piece and either sitting the carb outside the frame or in the area normally occupied by the standard carbs/airbox. I didn't realise we were discussing mounting a pair of them. I'm not keen on the 'pair' idea as it adds much more work e.g. syncing dual throttles, dual chokes, fuel supply etc etc. I don't think an FCR carb needs an airbox, so having one carb and removing all of those other 'bits' is actually a big bonus of this 'HD setup' idea.

@gregt mikuniOZ sell a VM32 for $220. I might get that cheap aliexpress one that @Murdo linked to play around and get the real thing if needed. Does anyone have any experience with the aliexpress units? Do they work or are they rubbish?

 

All good.

 

Yes, and they're a PITA to work on. A single FCR carb is a very attractive option on many levels.

I'll get an OKO knockoff to start with so I can get all the connections, throttle, fuel etc sorted and then I'll cough up for the real thing. I have an OKO PWK34 flatslide, but I'll get a 32mm unless someone thinks the 34mm won't be overkill.

 

When I was at Broadford recently, I asked about carbs with a few racers and they basically said exactly that about Chinese carbs. Some work fine and others are impossible to tune. They're so cheap, they throw the bad ones away. My friend from TAS said they bought 4 carbs from the same chinese company for the bucket engines...2 worked fine and 2 didn't...exact same carbs

 

So, in the case of the uneven length inlet runners on this VT250F, will it be difficult to tune?