t5 square tail problem

[shipscat]

if riding down hill or throttling off and try accelerating it bogs for a split second (some times it pops back through carb) and then goes like a t5
it does't happen when you wind it out changing up the gears , it pulls strong and doesn't miss a beat

i have changed the stator plate timing is set @ 16 deg
fuel tank is clean carb and jets are clean blown out with compressed air the float valve is a spring loaded type not sure if this is the correct one?
changed both carb base gaskets lapped the carb base
exhaust tight no leaks same for cyl head and barrel

its got me scratching my head
any help will be much appreciated

[kru251]

Carb jets?

[henri]

sounds like its over fuelling/flooding the carb momentarily, i think i'd change the needle valve an check the float/pin for wear,
if that doesnt help its pilot jets n slide maybe , from ya description its in the first 1/4 of the throttle that it occurs ,but it could be excess fuel being cleared from when throttles shut on overun ,
one cure is ,dont shut the throttle ,ride full on all the time , not really practical i guess , H

[vespasco]

If your engine is a totally stock t5?? then check you have the correct jets,especially the idle .

Try readjusting your idle,,,warm engine,,adjust mix screw,, when the mix screw is set, give your idle adjuster a bit more so it ticks over a little faster.

Has it always been like this or has it just started (maybe after you adjusted/changed something)?
Whatexhaust do you have?

[sbwnik]

Wind the mix screw in half a turn, see how that feels - they can vibrate outwards over time, richening the mix, which gives your symptoms, a slight hesitation as you pull away when rolling?

[shipscat]

think i got it sorted after riding/playing trying to repeat the problem it was the spring loaded float valve
the float drops a long way in the carb bowl before opening the valve because of the spring
if ridden hard W.O.T the problem was a lot worse ie the flat/dead spot was a lot longer , ridden lightly no flat/dead spot
lesson learned , note to self if possible don't use spring loaded float valves
its first time iv'e ridden a t5 I WANT 1 !!!

thanks for all your help and suggestions

[vespasco]

Glad to hear you got it sorted in time. Fuel starvation can lead to all sorts of potentially fatal problems!

[sbwnik]

Spring loaded float? What carb is it?

[henri]

as he'd said he'd changed carb gaskets n lapped carb i was thinkin it could only be a si type ,apart from rare jetex copys there the only "slab" carbs on vespas i thought,all others are manifold/rubber mounted arent they ,
prob just a needle from a diff carb thrown in coz it was the only one to hand ,most mikuni's/keihin/an oko on bikes have sprung needle valves ,so do s.u carbs same size as scoots ,but you'll never get the float height right . H

[vespasco]

I use a spring loaded float needle valve in my si24\24e but i do have a cosa float bowl top which uses the spring loaded needles/valve as standard. Im not sure what the T5 uses..ill have a look at my manual when i get home..
Ive never had any problems with mine.
I beleive they were bought in to help combat fuel seeping/leaking issues when, for instance, the fuel tap gets left on overnight.
You do need to use the correct needle for the correct float bowl top thats one thing thats not advisable to mix n match!!
Its weird...i was actually thinking not enough fuel (hence suggesting giving it more revs at idle) and h nailed it with the needle valve check but for the opposite reason!! Between us all, we can change the world!!

[henri]

yes vespaco, but for better or worse , didnt know ya cosa top had a spring valve , my missus's 125 1 doesnt ,just a normal one , but am guessing yours is a 200 1 , i was told the spring was to keep needle in contact with float to cut down wear an be more responsive ,but thats on S.U car carbs ,where gravity draining/flooding overnight aint a prob ,
least i reckon weve sussed where the needle came from if t5's dont have em as standard,haynes just says a 24/24 with different/simplified jetting , but we all know piaggio can knock out "parts bin specials" ,an t5's n cosa's where made at roughly same time/1 following tother, or somebodys done same as you an switched bowl tops at some time , H

[vespasco]

oh no!! i think someone must have nicked/swapped your missus float bowl top and swapped it for a normal one as im 90% confident all cosas carbs (20 H and 24 H) came with the larger flowing top...
personally i hate how the f'ugly cosa looks but inside my engine is as many lovely cosa parts as i can fit, as the cosa parts are the 'most bestest, most cheapest' upgrade/straight swap you can do to a px engine.... eg...Clutch, gear box, float bowl top, centre spark head? (stator?? higher wattage? -cant remember), etc etc.

the only reason i would buy a cosa is to nick, i mean swap said parts. straight swap mostly. but i would never buy a cosa as id probably find someone had already beaten me to it and nicked/swapped all the good bits already!!! probably what happened to your missus's!?

its possible the T5 carb (24G) could have a similar float bowl top to the cosa but im really not sure. it would make sense if they had

[henri]

possible ,but the guy i bought it from was a smallie/ss nut ,movin to canada an not wanting to ship sumting so ugly an it was his ex's old scoot,maybe my memorys wrong on valve ,an it was real ugly til a session with the grinder sorted that ,now its "babys urban assault scoot" in matt black with moulded in zip rear indies ,a cut down vbb horncast on front an a 1-off custom fastback seat in ox-blood leather with a dr martin tag,sip tubeless polished edge black rims n conti-moves.
an even tho its the 125 i'm real impressed with its engine an the brakes are the best on any vesp/scoot ive ever ridden ,more feel than a disc an just as much stopping power .
am so taken with the lump if i can find a 200 1 cheap enuff thats whats going in my vbb street racer project ,an to get the purists spinning i'm toying with the idea of ss90 leggies n floor on it too.just for giggles of course , H