Technical Forum > Mechanical

06 XL125V performance issues

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tibbi:
Hi All,

I've got a '06 XL 125V, bought second hand with relatively low mileage (cca 17500 km) and struggling with it ever since...
So I've diasassembled, inspected and cleaned the carburettors, found 85 main jet and C12C needle fwd, 92 main jet and B76E needle aft position, according to parts catalogues it's the defauld setting since '04. All the forums I've read and the factory workshop manual (XL 125V1) talks about 82/88 main jets, C12A & C12B needles, though.
My problem is the poor performance and relatively high fuel consumption of my bike, I've expected slightly more power than I actually have most of the times. That's what make this situation weird, since sometimes it works just fine for a short while and goes to creepy again. Meaning that around 9500RM it feels like stalling, no more rev achieved in 4th/5th gears, which in turn also limits my max speed around 95/100 kph. But, it's been better, especially after I've repacked my aftermarket LeoVince SBK exhaust, it has revved easily up to 12k, but only for a very short period, and stalled at 9.5k again the next day.
I've tried to install the 5 hole disk, nice again for a while, and the same crap again at the next ride...
My exhaust muffler is black inside from unburnt fuel, brand new factory specs fwd spark plug looks quite bad, aft is a bit better, bit still smoky. First I've installed a new normal paper air filter, and today a Simota performance air filter, no significant difference noted. Valve clearances are within specs.
Tried to sync carbs, but the manual is quite unclear, "vacuum difference" is 3cm, but in what direction???
If it's lower on the aft, then it means more or less vacuum, using a needle type gauge?
I've talked with people experienced with engines, but it just does not make any sense for nobody why would we want to add more fuel to the cylinder equipped with a smaller diameter exhaust pipe?
Please understand me that I don't want to gain more power for free, just find the healthy settings for my bike.
Thanks for any advice.

Taught2BeCautious:
My understanding is that Honda really struggled to get the power output down to the legal level for the Learner market they were pitching to, and one of several measures they took, was to downgrade the jetting for just the rear cylinder.

The official explanation is that there is a reduced air cooling effect for the rear cylinder, so it was necessary to reduce the power - but it's a water cooled engine, so I'm not buying it!

Bearing in mind that the front exhaust downpipe has an inner and outer tube, the inner tube is not very far off the diameter of the rear exhaust, but to setup the carburation differently for both cylinders is not a good idea (from an engineering point of view) and is one of the things that will harm performance.

The other things include placing a restrictor bung in the airbox intake, and using a filter with a smaller than ideal flow rate.

I believe that on some of the older models, there is also a stepped section inside the intake rubbers, that sit between the carburettors and the cylinder head, which further restricts the airflow.

If you decide to do anything about these 'features' make sure you don't exceed the legal limit that your licence permits :)

tibbi:
I'm so glad you've replied, I had searched the forums before I actually posted mine, therefore I'm familiar with the L theory. But I'm afraid there has always been a misunderstanding around this subject in previous topics, since the rear cylinder's carb is equipped with the bigger jet, not the forward one.
Models from '01 thru '03 have 82 FWD, 88 AFT, and from '04 thru '06 they installed  85 FWD and 92 AFT main jets. So the Honda itself upjetted the bike for some reasons, however there is no difference in the max power (15Hp or 11kW both).
Carb IDs are VPU2A/VPU2C, I'm not quite sure the slash represent position (FWD/AFT) or the model change ('01-'03/'04-'06).
I haven't heard about the double tubes at the FWD cylinder, but I'll remove the whole piping for painting, so I can take a look later.
I've tried to remove the restrictor bung from the filter housing, no difference noted at all, same thing with the high flow filter.
Somehow I believe it's got something to do with the exhaust, since there was a major improvement in performance and top rev after the exhaust got repacked, sadly didn't last long at all. Tried to remove the db killer, max rev got reduced to around 8k, and struggled at all revs. Reinstalled the db killer, went up to 9.5-10k again. Unfortunately I don't have the stock muffler, but a LeoVince SBK is installed, so I can't check out the difference.
Right now in 1st and 2nd gears can rev up to 12k, but 3rd, 4th and 5th 10k max, and if I keep it at the max rev I can feel it wants to go up higher, but it gives a "pulsing" feeling, and can't go up to 11k rpm, so it never achieves the max rated power.
And to repeat myself, it's been better a few times, worked as it was expected, but never lasted long. I don't know why.
The exhaust is black from unburnt fuel, I'm still waiting for the idle screw adjusting tool, it's quite hard to adjust without it.
When I removed the cylinder head covers, the PAIR reed check valves also got cleaned, they seem to be closed, no gap noted, and the PAIR control valve seems to be working fine, too (closed when vacuum applied).

If you decide to do anything about these 'features' make sure you don't exceed the legal limit that your licence permits

I just wish I could regain the normal, factory rated performance...

Update:

So, I removed and attempted to clean as much as I can  the muffler by using pressurized air, blowing out a lot of carbon deposit. Having it installed, works much better again (rev up to 12k in 3/4th gear, around 11k in 5th), as it already had happened before. Now i know for sure the exhaust plays a key role in this issue, and the  too rich mixture makes it bad somehow, even though the cenral pipe is obviously not clogged.
Google translated a french forum, it says:
Sorry to contradict you but the wealth setting changes from 2004 models (pollution standard requires).
Boisseaux (???) are of different sizes to compensate for the retention of exhaust gases from the catalyzed pot.
The above original setting changes:
New setting for 2004 motorcycles (check that the tenth character of the serial number is a 4)
2 1/2 turns
This value is valid for both carbons
This information has been confirmed by the site Bike-parts.fr

Taught2BeCautious:
I can't think of any scientific reason why the front and rear carbs should be jetted differently.  In all other multi-cylinder engines that I know of, great care is taken to ensure the carbs are jetted and balanced to be the same.

I'm always a little wary of the claims made by those that sell 'Performance Exhausts' because the source of power is the actual combustion process that takes place in a closed cylinder.

The exhaust valve opens as the piston is on it's way back up the cylinder on the exhaust stroke, so unless the spent gasses are used to drive a turbocharger, there's no way of squeezing more power out of the combustion stroke, from exhaust gasses that are now locked-out of the process when the exhaust valve shuts.

Obviously, there is some advantage in retaining a small amount of the hot spent gasses in the cylinder, but not to a point where the fresh incoming charge is restricted - and yes, I know about the different requirements in a 2-stroke engine.

The effects of larger bore exhausts are limited by the size of the exhaust valve (and port design), so there is a point where a smaller diameter would be detrimental to performance, and a larger diameter would have no noticeable effect at all, until it reaches a point where performance is actually reduced.

Apart from the diameter of the pipe, the actual design of the exhaust can create pressure waves that also have an effect.

To see examples of this, have a look at the differences in exhaust design between Dragsters and F1 Racing cars.  If the experts that design these systems could get just 1bhp more out of the engine, it would be worth them spending thousands to do it - literally 'for the win'.

Even if you could get a genuine 20% increase in performance for the Vara125, you are still only looking at 2 or 3 bhp max.

To see how a pressure wave can be as effective as a solid valve, google 'Pulse Jet Engine'.

Sorry for the nerdy reply, but it was my pet subject at college - and I got to try out many different designs and check the results live, on a brake dynamometer.

Oh, and the closest you can get to an optimum exhaust tube diameter from standard stainless stock tube, is 32mm.

black rider:
the transalp 600 has different jets front and back I read on a forum . Honda know their shit I just wouldn't question it . as far as I am aware there isn't much can be done to get more power or speed out of a transalp 600 . I put a  stock Suzuki gsxr alloy end can on my transalp after reading  on transalp.org that it makes the bike breath easier run stronger and get better mpg . I have to say I did notice an improvement my own exaust had blow a hole in it . it sounds much better too and looks better than the goofy stock pipe  . I know a dominator can be easily tuned for a decent increase in power by jetting and baffle removal I think .

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