Bogging Carby When Landing Jump.

[wogtaz ryder]

Gday guys.

Im at a mates because i dont have the internet , and wont for a few weeks... finaly moved..

just a quick one , the molkt Carbies on the 155z Bogs down completely when i land a jump while on the gas....

i have jetted the carb , and it runs very nicely .. but i just have this one problem.

i remember reading about this problem with the molkts on planetminis , and people saying to adjust the Floats...

i dont have the time or money to get a new carb (i want to in the future..)

so was just wondering if anyone would no how to solve this , before the next HRI race..

i took the carb apart , and thought about it but couldnt figure how the float was making this happen....

if the impact from landing closes off the fuel from the flaot , there should still be plenty of fuel for that split second to not bog out?

and the other way around , to much fuel ? dont no how that would affect this either..

well any help would be great!!!!!!!!

i will try and check this in a few days ...


Thanks heeps guys.

 

[Shing]

Maybe its not shutting off the FUEL as such. But causing like a suction, kind of like when you pour a bottle of water out it glugs. But put a hole for air to come in the replace the water that is lost and it will flow smoothly.

Am i making sense?

 

[coolmodee01]

needle position

na dunno mine does it sometime under g-force between jumps but up and over it's fine but i have a feeling it just gets to much fuel and bogs out

 

[mrbrnz]

might want to try bending the tang on the float to make it close at a higher level, might not be related to the float closing/opening on landing but the fuel in the bowl could be sloshing around and starving the jets, you may need to play with the needle clip after adjusting float level though.
great way of finding level I read from cactus is you put your fuel drain line up against the bowl and open the drain screw, it will settle at the level of fuel inside the bowl so you know exactly where you're at. Genius!

 

[Cactus Jack]

It's caused by the fuel sloshing around in the float bowl ... I've talked about it heaps of times before ... Including on Planet Minis ... Maybe that's ONE good thing about genuine Mikuni carbs since they have a main jet shroud which they refer to as a "Jet Ring" ... You can also buy a jet extension kit and a deeper jet change plug for the bottom of the float bowl ... They are essential to use on trials bikes since they can't afford to have the engine hesitate or bog at ANY time whatsover ...

I first came across the problem over 30 years ago when racing XR75's ... We had to make up our own jet shrouds which we called a "Jet Bell" ... Later model Keihin carbs had them fitted as stock on XR's ... but a lot of people remove them and lose them thinking they don't do anything ... so you rarely see them in the carbs on second hand bikes ...

A quick search and I just found this thread ... We have covered most subjects on ... Miniiii Rideeeeeeeerrrrssss !!!!!! ... LOL ...

http://www.miniriders.com.au/tech-talk/16765-carby-dramas-angle-yx150-3.html

 

[TomUKRider]

Its Kinda sad to hear this considering the price of the New Lxr's!

Being im a complete novice to pitty's and the only skills i possess is riding ability, hearing that ur experiencing this problem is a kick in the teeth.

Wudnt minibikes benifit from a baffled fuel cell principle like rally cars use and stuff that stops the fuel sloshin around at G-force and over rough terrain and thus preventing fuel starvation?

why you think that hasnt been thought of..or has it and i just dont know about it yet??
I hope this problem is an easy fix, because i wudnt want my new Lxr hesitating..

Can you imagine..

you'd clear the 1st set of big doubles...the bike wud have a "moment" and you'd hit the second doubles half heartedly...and you'll get wrapped around ya handle bars! lol.

i appreciate to some of you this is probably a trival easy fix, because you all seem clued up and quite tech etc.. but to someone that isnt in the know or wudnt know where to start looking for a solution or a new carb its a nightmare!

Wogtaz when you find a good solution to this problem..make sure you post up bout it mate.. would be good for the rest of us following down the Lxr path!

thanks,
Tom.

 

[Cactus Jack]

The link was inactive so I found it again ...

Beta » Factory parts » Trial

With the carb in the below link (genuine Mikuni VM-26) ..

http://www.motoclubgiaveno.com/image003.jpg

They have a flat brass disc with holes in it between the main jet and the needle jet ... The disc slides into a recess in the bottom of the float bowl to trap the fuel , hold it around the jet and prevent it from sloshing away ... To escape , fuel has to go thru the drilled holes ... I'd say the pilot jet is up inside a recess as well to avoid the symptoms of fuel slosh affecting IT too ...

Trials bikes need to be able to sit still balancing in a wheel stand on the top of a rock ... be hopped around by the rider to point the bike in the right direction ... then be instantly accelerated and jumped to the top of another rock and stopped dead instantly still in the wheel stand position ... WITHOUT the engine missing a beat ... They also can't afford to hesitate or lose power with the bike climbing a near 90 degree rock face or wall ... So they need a really neat fitting jet shroud ...

The KX80 VM-28 Mikuni carb has a similar setup ... but there are sides on the brass disc forming a cup ... There are 4 holes in the top of the cup to allow fuel to get thru ... and the cup sits neatly inside the alloy cap on the bottom of the float bowl ...

There's pics in the below link of a VM-26 Mikuni with a jet shroud mod similar to what I used to do to my 22 mm Keihin carbs ...

New Rev 3 Mikuni Carb? - Trials Central

 

[mrbrnz]

haha, my bike used to die when I wheelied so to set the float I let it idle and pick the front wheel up to above waist level, I adjust the float level higher until it doesn't die when I pick the front wheel up and hold it there and tadaaaaaaa, perfect. Had to drop the clip one notch after that though.

 

[wogtaz ryder]

Cheers heeps Cactus printing that all out , and will read later

Will let you all no when i get it sorted.

 

[PIMP50]

hey damo trashthumpts klx did this we jsut playd with the air fuel screw and that fixd it up

 

[Cactus Jack]

Matified explains it good on Planet Minis ... He says that MOLKT has deepened the float bowl and extended the jets on their latest model carbs (I've got the new ones plus the old ones but haven't checked them out yet) ... And he's dead right about the cause of bogging ... it's a LACK of fuel caused by fuel sloshing around and it can be caused by too low a float level ... and also by poor carb float bowl design ...

Molkt Carb Bogs When Landing Jump. - Planet Minis

 

[wogtaz ryder]

Cheers heeps guys . will sought it out when i get to me bike this weekend.

 

[Cactus Jack]

Damned Pat Farsole ... I hate that guy ... I just went to post the below info ... then found out he'd already beat me to it on Planet Minis !!!!

I just pulled a new model and old model MOLKT apart to compare ... both brand new ...

1) Both the float bowls were exactly the same depth from top to bottom ... = 39.5 mm

2) The pilot jet bosses were both exactly the same depth as each other ...

3) The needles were both exactly the same ... 1.45 mm in diameter at the bottom ... 2.4 mm in the middle of the taper ... and 2.6 mm at the top ...

Compared to a genuine Keihin 4 stroke needle ... they are very thin at the bottom with a totally different taper ...

4) The floats looked exactly the same ...

5) The needle jet bosses were both the exact same length ...

6) The old Molkt had a longer hex main jet ... The new Molkt had a shorter round main jet which doesn't go as low in the bowl ...

Maybe we have an intermediate model MOLKT here in OZ ? ... As they're identical to the old one except that they're heavier , have MOLKT printed on the float bowl , and use a round main ...

The real question IS ... Does Pitster have a newer , later model improved version ? ...

 

[Cactus Jack]

And the answer is ... YES they do have a newer improved version with differences that VERIFY the fact that they've known that MOLKT carbs are a bad design for use in a dirt bike ...

So when you buy a MOLKT carburetor that you intend to use on a dirt bike engine ... buy one that has a DEEP float bowl ...

Here's what we've learnt :

1) The latest model NEW improved MOLKT has a 46 mm deep float bowl ... The problematic earlier models have a 40 mm deep bowl ...

2) The pilot jet and main jet have been extended 6 mm deeper into the bowl to allow the engine to run like it's on a smooth road even when it's being hammered over ruts , landing from jumps , wheel standing etc ...

3) The slide needle in the latest MOLKT is a 4 stroke specific needle out of a PZ-26 carb ... and it's marked "PZ26" ... The older 2 stroke type needle in the faulty carbs is marked "D" ...

You can't safely go of the external appearance of the carb unless you actually measure the overall depth of the float bowl ... According to Mattified ... there's THREE different versions of the "shiny silver" MOLKTS ... which we thought were the "latest" models ... The older model has a dull , sand blasted type look to it ... The shiny ones look neater and cleaner ... but you can buy some of those that are exactly the same internally as the "dull" known to be troublesome carbs ... ie Shiny on the outside ... DULL on the inside ... and sinister enough to have you tearing your hair out with frustration after all your tuning attempts to fine tune them ... FAIL ...

The shallow float bowl models need to be modified to work properly under all conditions ... and Matified has stated that he'll post up info on how to do (DIY) mods to improve the way they work ...

Here's a link with carb tuning info showing the PZ-26 carbs which are used on 4 stroke cart race engines ... It also shows jet sizes , float level specs , and mixture screw settings ...

formula_ipl.pdf - By Nitro PDF Software

 

[mrbrnz]

ahh so you're pat farsole, thinking it was strange to have 2 people know so much about motors
I'm burnzy on there.

and also my new deep bowl one is of a semi "dull" matted finish appearance, it's not as shiny as the shallow bowl one which is more of a gloss finish with a bowl that's zinc plated and different color to the top, the new model is is all the same color, grey alloy whereas the older one has a brown/khaki tinge to top half of the carb.

funny how that pz26 has a 102 main jet, so does the new molkt.

 

[wogtaz ryder]

Very good info in this thread .

This carby is having alot of problems , it varies from day to day!
it still does it no matter how high the float , so im gonna try and put fuel line over the jets like you said cactus when i get time and see what that does.

If not ill be getting a Oko 26mm i reckon.

 

[DvDRip]

yea the oko 26mm flatty has like a shroud around the main jet, is that the "bell" type thing that's meant to keep fuel around it?

 

[sean01]

yeah thats it, an i suspect that thats half of my problem with my carb to(doesnt have one)..as well as the wrong main jet..

 

[DvDRip]

haha saun sounds like you should bin that and get a fresh one?

 

[sean01]

yeah its on the cards, but for experience values im gona give it a crack to sort it out....theres several bits in there that should be changed so ill try that first