monoshock gt250/yzf

[Gil Gallad]

still prepping the fuel tank at the mo, i've been at it for well over a week. somebody said to me to wait until i've got the high build primer on and then i'll see all the dents - magnified. they weren't wrong. even little tiny dents i couldn't even feel with my fingers are now showing up here there and everywhere. i think i'm over the hill with it now though. as far as i know i've just done the last 2 dents. rub them flat tomorrow and start with the high build again. did i mention i hate prepping for paint? give me an engine and gearbox to build any day of the week 
cheers, gil.
p.s. yes, i am too tight to get it done professionally, and anyway, this is supposed to be a shed build 

[Czakky]

I just did a tank for a different bike (no 2t content). I have a nice solid tank full of filler, and a rusty tank full of holes but no filler. Painted them both, I will never do this again!

My point is I feel your pain! The kicker is are we ever satisfied with the result?

[IR8D8R]

Wow. Buzkill. Just when I was getting brave.

I think I have decided to find a pro and save myself from disappointment.

How come everybody who gets a professional has to have it redone at least once?

I'm already worried about chipping the paint. Maybe it should be left raw and clearcoated. I understand patina now...

IR8D8R

[Gil Gallad]

after getting sick and tired of will she/won't she start, dependent on the weather, the government of the congo or the price of eider duck eggs, i decided i'd had enough. a few years ago i had the same problem with a newly rebuilt rg250 powervalve engine, which turned out to be the chokes not working properly in the flatslide carbs. so i decided to build myself a pair of butterfly type chokes. after i fitted them the rg engine fired and ran first kick every time. got bored with it, took the engine out of the modified gt250 frame and stashed it for another day. butterfly chokes doing nothing, so why not fit them to the gt250 reed engine? what did i have to lose? scratted round for some clips - trust me, new mikalor clips the right size on order - and put some old wide jubilee clips on. normal choke off, butterflies closed and she fires and runs first kick every time. couple of photo's of the butterfly chokes fitted and the inside of one of them. just need a good clean, new mikalor clips fitting and bob's your uncle.
cheers, gil.

butterfly chokes:


choke closed:


choke open:

[pidjones]

I can see how running with a non-stock airbox would almost require such a setup to give sufficient restriction for starting. A good mod for pods users.

[teazer]

That is an interesting solution for sure.  A butterfly choke will reduce airflow by 5-20%, so I hope they are larger than the carb bore.

But the fact that they work and regular enrichment circuits don't is more interesting.  It suggests that the enriching circuits are not doing what they are supposed to do and that suggests a lack of air velocity through the carb.  With that sort of choke, the pressure differential between the intake side of the carb and atmospheric will be substantial and consequently fuel will be dragged through every jet.

Makes me wonder if it needs softer reed petals that open further at very low engine speed.  In theory, two stage reeds would be perfect as long as the small reeds petals are thinner or are "softer" ie they bend with much less force.  I wonder what they came from and if there are more suitable reeds available.

[Gil Gallad]

thanks teazer, informative as ever  a few answers then: carbs are gt250a 28mm bore. i.d. of chokes is 38mm. thought i'd posted this already, but obviously forgot, i fitted some new dual stage reed petals, made over here in england by a reputable company. the top petals are very flexible, the bottom ones are a lot stiffer. after the problems i had with the rg250 engine, which turned out to be the chokes/enricher circuits not working properly and this engine showing the same symptoms, i decided to fit the butterflies, as they worked perfectly on the rg250 engine. and touch wood they are working well on the gt250 reed engine. i fitted the new reeds and tried  them before fitting the butterflies, but it was the same as the standard ypvs reeds i had on originally, i.e sometimes it would start perfectly first kick and other times it just wouldn't start at all, until i took the plugs out, put my hand over the carb inlet, turned the engine over by hand a few times until my hand started getting wet. then replaced the plugs and airfilter and she'd kick up first kick 
cheers, gil.

[Gil Gallad]

this is the dual stage reed petal kit i bought and fitted 
cheers, gil.

petal kit:
https://www.ebay.co.uk/itm/362743199636?hash=item54752df794:g:awwAAOSwIr5dbXjG

[m in sc]

it shouldn't matter. the slide is behind the enricher circuit which should supply plenty of vacuum to pull it up IF the slide is down far enough. Me, id drill out the starter jet a few thou and will work just fine. however, that's a neat solution.

I mean, if i can get a 350 to start on enricheners with v force reeds and 36mm carbs.... yours should work just fine.

[Gil Gallad]

the one problem with this set-up is the quarter turn knob for the butterflies sits inside the left sidepanel. on the rgt it sat outside. anyway, i decided to buy a 12v push-pull type solenoid, get a brass bracket brazed on the butterflies shaft and go from there. i decided to repurpose the headlamp flasher button. press it and the chokes close. let go and the chokes open again. well that's the plan, wish me luck 
cheers, gil.

[Gil Gallad]

unfortunately mark, mine are the old type mikuni vm28's. they only have one line for starting, and that's the pilot jet. i don't have the other line - or bypass hole as some people call it - which sits between the slide and the engine intake manifold, where there is vacuum to suck fuel up and start the engine. the hole from the pilot jet sits inside the slide, towards the front [engine side]. with the bypass hole it would probably start a damned site easier. if you've got mr bell's bible anywhere to hand, look at pictures 5.1 and 5.2 in the carb section, and you'll see what i mean. mine are the one illustrated in 5.2. anyway, thanks for help and suggestions, always appreciated, and i'm still enjoying what i'm doing 
cheers, gil.

[Gil Gallad]

i've got her running well. not as well as i'd like as i think she's still running a bit rich. that could all be sorted after a phone call to ron at fahron who's put me onto a bloke about 20 miles away with a dyno. plus, he only does 2 strokes. so hopefully i'll find out what meagre bhp she's kicking out and he'll sort my jetting out for me as well. i replaced the long bananas on the rear shocker with shorter ones to lift the rear end up, which meant i had to extend both stands. all done now, apart from having the stands powdercoated. anyway, photo is as she sits now - hopefully nearly finished. new project in the pipeline, after buying a 2019 gsx-r125 rear end for a start 
cheers, gil.

reed valve monoshock gt250:

[m in sc]

so cool, i like it a lot. curious to see the numbers on this. 

[dgorms]

Hats off to ya Gil, she's a beaut!

[teazer]

it shouldn't matter. the slide is behind the enricher circuit which should supply plenty of vacuum to pull it up IF the slide is down far enough. Me, id drill out the starter jet a few thou and will work just fine. however, that's a neat solution.

I mean, if i can get a 350 to start on enricheners with v force reeds and 36mm carbs.... yours should work just fine.

AGree.

I kick start our drag bike which has TZ750 cylinders with Blaster reeds and 34mm carbs.  Are you 110% sure that the tiny starter jet in the float bowl is clear? They are only 0.6mm diameter and block easily.