Do i need to retune Carby ? 125 to 140cc

[Cactus Jack]

Well basic carbs aren't really designed one way or the other, some more suitable for one or the other, buy design, but basically they all have a choke size, deliver a metered ratio of fuel throughout the throttle range and have a mounting for manifold and filter. Carbs have been getting swapped and adapted from engine to engine through trial and error since time eternal.
From my experience 28mm IS or can be problematic to tune on engines which are not flowing well. Hence I have recommended the average joe stick to 26mm and below. The trial of the above carb on a stock engine I would not normally even attempt but as it turned out I did cause I had nothing else at the time on hand. Also I still wouldn't recommend it unless you use a really good flowing pipe which most don't have. Trial and error as mentioned, is probably the main system used by every engine builder in the world, and this is where you should report your results isn't it?

Yes , we recommend a slightly smaller carb (which is good sound advice) because they give better bottom end throttle response and better lugging power than a bigger carb and the power difference up top is minimal anyway ! Aside from that ~ who rides a stroker motor with the engine at peak revs all the time - the extra few mm of a 28 is only used for a few seconds just before you shift gears and ONLY if you peak rev the engine . In actual fact - the engine can't possibly use anything over 26 mm anyway with a 26 mm manifold and intake port ! IF we tell someone to go for the bigger carb and it bogs and carries on or is too finicky and hard to tune - we're the worst mongrels in the world !!

People can ask ANYONE in the US why they don't use a 28 Mikuni and they'll tell you that they bog and lose power over the VM26's !

Here's an interesting NO BS link on comparisons between vertical 122 to 145 bored Honda engines with different strokes and carb sizes (complete with dyno test and track test results) that some people will find interesting :

http://www.oldrice.com/Powroll_vs_Yosh_125_Shootout_Pop_Cycling.jpg

http://www.oldrice.com/Powroll_vs_Yosh_125_Shootout_Pop_Cycling_2.jpg

http://www.oldrice.com/Powroll_vs_Yosh_125_Shootout_Pop_Cycling_3.jpg

http://www.oldrice.com/Powroll_vs_Yosh_125_Shootout_Pop_Cycling_4.jpg

http://www.oldrice.com/Powroll_vs_Yosh_125_Shootout_Pop_Cycling_5-6.jpg

http://www.oldrice.com/Powroll_vs_Yosh_125_Shootout_Swiss_Opinion.jpg

 

[Mack]

The biggest problems people are having IMO is they are not even remotely following a methodical and thorough tuning procedure. Also there are cutaway changes, needle tapers and carb porting which all can be neccesary sometimes to get dialled in.

 

[themountain]

Here's an interesting NO BS link on comparisons between vertical 122 to 145 bored Honda engines with different strokes and carb sizes (complete with dyno test and track test results) that some people will find interesting :
...niice Link...were do you find all this stuff??

 

[themountain]

The biggest problems people are having IMO is they are not even remotely following a methodical and thorough tuning procedure. Also there are cutaway changes, needle tapers and carb porting which all can be neccesary sometimes to get dialled in.

I dunno...they are from this new consumer generation...walk in a store ,spend a buck and everything is great...here in GER its all the same!!

 

[Cactus Jack]

Small bikes BIG power !

...niice Link...were do you find all this stuff??[/QUOTE]

I've known all about that stuff for ages and followed Powroll and Yoshimura - so when I decided to get into pit bikes I went looking for it and thought other people could get some ideas from it ! I've actually got a new old stock SL/XL125 MCM pipe like the Powroll pipe and it's 29 mm I/D out of the head . I can't understand why people only run puny pipes on their pit bike engines these days ! My XR75 DG snake pipe is 27 mm I/D out of the head , stepping up to 30 then going into a 12 inch long reverse cone megaphone muffler ! Those pipes make big torque and HP and they make big carbs work a stack better ! Most people have found that carbs off bigger Hondas work perfectly on their smaller bore engines without jet changes !

You can get those old bikes hauling ass without spending megabucks :

This is Chris Corum doing what he does best, which is dominating big bikes on his highly modified XR75. Check out the cool DMC down pipe. He got 5th in this race.

http://www.socalxrs.com/pics/corum1a.JPG

http://www.vintagefactory.com/GigerXR75.jpg

 

[kwikaz]

no no

no no dont put me on the consumer generation big call when you dont know me?? check out what this ebay seller says about vm mikuni,s ...

Hi

If its a VM we have never got one right that why we don't sell them, they
are for 2 stroke engines.

Regards Adrian and Helena

Mortimer's of Canterbury

Website: OORacing - monkey bike and pit bike specialists

Mobile: 07989 300109
Office: 01304 814360

VAT no 784289772


i responded with why do uk monkey bike parts sell 4 different type,s of vm,s then???

 

[themountain]

...niice Link...were do you find all this stuff??

This is Chris Corum doing what he does best, which is dominating big bikes on his highly modified XR75. Check out the cool DMC down pipe. He got 5th in this race.

http://www.socalxrs.com/pics/corum1a.JPG[/QUOTE]

...HAHAHAHA...gnarly little thing!! Wicked!!

 

[themountain]

?

......who gives a **** about what OO-Racing says ...they sell 500 bucks roadracing engines and nobody knows them even in UK !!

I had the VM on a 2K+ engine and it hauls a**!...don´t tell me nothing about engines , Fool!!

 

[kwikaz]

chill out

chill out mountain why be such A hole im only trying to talk about this stuff, i just some help m8 chill out will ya, but you can see how hard it is for som1 with no expeirience with these carbs to dicide if its right for my motor or not when you got guys like this oo racing ******** telling me its got massive flat spots??? i just need some first hand advice regarding this carb thats all not really asking for much i just wish ppl would shoot striaght.. thanks for the reassurance about the carb any way mountain...

 

[Mack]

So leroy, ya thread got jacked a bit...hows it going with the jetting? You guys also realise the carb sold as a 30mm on ebay has the 100 main jet (I think, by memory) if you want to trial it out of a mates or something before buying one.

 

[Cactus Jack]

Chinese mathematics - 30 = 26 / 25 = 22

Yes - if you buy the "30" Mikuni carbs in a kit (I bought one) - they have a 100 main jet ! But if you check carbs that come supplied on bikes - they all have different sized jets - most likely because each bike is individually tuned to "Chinese" export or EPA import standards or simply to run right over in China at the factory before they get boxed and shipped out ! I checked the "30" Mikuni off our 138 DHZ bike and it had a 96.5 main in it !

Anyone who still thinks those things are "30" Mikunis - do yourself and everyone else a favour - take your carb off your bike then take it to a bike wreckers and ask if you can compare it to a REAL 28 mm carb off an 80 cc 2 stroke . Then you'll realise you've been played like a fool by ebay and pit bike seller advertising ! Even a VM26 is a bigger flowing carb than those "30'' carbs . I've got a 1987 model KX80 VM28 and 2 ebay "30"s and I crack up laughing everytime I compare them .

Chinese mathematics :

Mikuni carbs ~ 30 = 26 / 25 = 22

Lifan engines ~ 138 = 127 / 125 = 120

All verified by checking and accurately measuring !

 

[cass38a]

Chineese and australian reseller maths are always dodgy.

100 km per hour depending on rider weight and conditions = 80 km with a light weight rider and a strong tail wind.

 

[kwikaz]

dont know bout that

i think thats abit harsh m8!!