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Xvs dragstar lacks power: carb setting, big bore or stroker?

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Xvs dragstar lacks power: carb setting, big bore or stroker?

PostAuthor: Telefonmast » Fri Mar 01, 2019 7:53 am

Hi,
I know that there are many experts for those small cc virago Engines around here, since the virago and the dragstar engine are very similar I thought I should ask my questions here.

Well, here we go:
I ride xvs 125 dragstar since almost 9 years now, it was my first bike and I Love everything about it, atleast ALMOST everything.

Problem is that it seems to lack top end power,always has, It never Even did 100kmh, flat out top speed is straight 98, tach gets pinned right there no matter what.
Always thought that this was normal, just small engine and Heavy bike, until I had the chance to ride a friends Xvs.
It did around 115 on the tach top and would easily cruise at 100, and is all stock. He said "well, they should run like that, top Speed in the papers is 107, so It should do about 110 on the tach".

I then checked everything on my bike, but It all seems correct, sprockets, carb, air intake, exhaust and tcu is all unrestricted.
Next i looked around for tuning guides, found the "power up guide" on this forums and some other Guide in another Forum.
Tried both, didn't work. Acceleration and low-mid torque changed depending on configuration, but never the top speed. Really weird Pulls VERY fast until 80, slowly crawls to 98 and thats it.

My current setting is:
-sport air filter in stock box, but rubber nozzle removed from the airbox
-105 main, slow jet from xvs 250, cant remember the number.
-mix screw 3.5 turns out
-needle Clip Full rich with tiny chim
-18t Front sprocket
-bsm exhaust
This seems to be the best between comfort and acceleration, top speed unaffected.

I tried everthing between full stock and this config, all jets from 90 to 110, sprockets from 15t to 18t, exhaust from stock to straigt pipe , airbox from stock to Cone straight on Carb and countless combination from the above.

Now I'd be satisfied if it did 110 top but cruise consistently at 100 and don't drop too much at small hills/little touch of wind.

Im also thinking about doing a big bore kit,but someone told me that they had lots of crankshaft failures on viragos with that, but is seems common to do around these forums...
I read that most people here get stock 250 barrels and pistons, here in Germany most people send in their 125 cylinders to have them rebored, maybe thats the difference or some shops f**k up the bore alignment and the crank damage occurs short after install..
Would it be safe to throw used 250 virago pistons and cylinders in a otherwise stock 125 engine(heads, crankshaft)?

Another possibility would be a xv 250 crankshaft with 125 cylinders to make a "stroker". The 125 and 250 dragstar share the same crankcase part number, the 250 dragstar and 250 virago the same crank part number, so a virago 250 crank should fit a 125 dragstar engine.
Pistons /wrist pins should also fit, since using 250 pistons on a 125 crank assembly for a big bore kit is possible, just don't now at what compression this would turn out...

One other thing, the power up guide says to wind the mix. Screw out with larger jets, but a Yamaha mechanic told me I should turn it in the larger the main jet gets, I found that the screw has something to do with top speed, it only does 80
With 4 turns out and 98 with 3.5, but also only 98 at the stock 2 3/4...haven't tried further in yet..

Sorry for the wall of text,
But thanks in advance for your input!
Last edited by Jake on Fri Mar 01, 2019 8:45 am, edited 2 times in total.
Reason: Moved to Yamaha Star help

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Re: Xvs dragstar lacks power: carb setting, big bore or stro

PostAuthor: Jake » Fri Mar 01, 2019 8:37 am

:VTF:


Please add your location to your profile so no one has to ask in the future. With members all over the globe, some info is geographically specific.

Add your location at this link -----> ucp.php?i=264

Also add the year of your XV125, and if it’s an XVS125, please make that clear in your profile as well, we have different sections of the forum and it helps to put things where they belong. For the time being, I moved this to Yamaha Star help.
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Re: Xvs dragstar lacks power: carb setting, big bore or stro

PostAuthor: bstig60 » Fri Mar 01, 2019 10:13 am

:VTF: The stroke on the 250 is very different from the 125. (47v66mm) Can't see how the crank would be the same part number.... Perhaps the 250 top end would bolt on, but you would be putting a lot of extra strain on the stock crankshaft, so I can see how it would be prone to failure.... I would not recommend making the change to the larger cc jug and top end. What you might want to consider is finding a used 250 engine and installing it in your current frame or perhaps it is time to trade for a larger displacement bike if you want to go faster....
PS: Top speed is dependent on lots of things, i.e. engine getting enough fuel, (float settings; main jet); engine gearing, sprockets, etc. What you are indicating is that the engine is not getting enough fuel at higher speeds; (rpm's). You might want to increase the size of the main jet and see if that improves performance. The pilot screw adjustment provides fuel for starting and low rpm's and very little fuel once the main jet takes hold... I would set the pilot screw back to the factory setting, check that the fuel level in the carb float is within spec. If all this is correct, go with the larger main jet.
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Re: Xvs dragstar lacks power: carb setting, big bore or stro

PostAuthor: Flyingdog » Fri Mar 01, 2019 10:31 am

:VTF:

I'm totally unfamiliar with the 125 dragstars, other than what a mag would say. It's always quite understandable wanting more power. Specially from something that's kinda small in the first place.
Boy, I don't know, but just quickly I pulled up 3 reviews on that bike and they all say top speed 60mph. 13hp A 10k?...that's not much to get ya anywhere near 70+mph. Except maybe...going down hill with a tailwind..maybe a good fart for little extra. Maybe your friend just got a freak version of the bike. You two fairly close in girth? Is this side-by-side and does he/she pull away. Checked your speedos against each other, at speed..side-by-side? Or is that what you meant, already? Wasn't clear if you two actually did it or just talked about it.

And...going up 3 teeth on the counter sprocket didn't gain ground speed? Hmm..sure it'll lower your rpms at a certain speed, but not hinder gaining some of those rpms back, higher up. Unless that size bike is really touchy on being able to make those higher rpms.
How's the tire pressure? Just some wondering thoughts, here. But it really all comes down to, is sinking bucks into a 125 to make it equivilant to a 250 or so, worth it? Don't know, it's totally up to the owner. Some place along the lines, you have to start wondering about the stresses you'd be putting on all the other components. I mean, it was designed/tested as a 125 cruiser style. Who knows how far one can go on something that small. Be interesting.

Hopefully someone, with hands on, will respond to ya,too.
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