loss of power at top end

[mastuhblastuh]

Hello friends, i had a post on here about a week ago and iv done many things since then so i wanted to start over with more info...

for starters, i bought the bike which had a "new" chinese carb on it.. eww. i went riding all day yesterday (saturday) and although it didnt blow up, im still having a problem with top end power. 1st, 2nd, and 3rd gear i can damn near top out without any hesitation... but once i get to forth, it struggles to top out, then 5th i can barely get the rpms to get into 6th. and of course, i cant sustain a cruising speed in 6th... the problem is that when i get over half throttle, it wants to fall on its face...

to me, this screams that its starving of fuel... but i did a plug chop earlier that day and came to the conclusion it was runing just slightly lean. plug actually had some pretty good color to it... so i moved the clip on the needle DOWN, to move the needle UP and richen up the mix. this helped out quite a bit! but the same problem was there just not as bad. so later im going to lower the clip to the lowest position and see what that does.

with all this being said, heres my question... if i take the seat off, im staring at the air filter... does that mean im missing the airbox cover? and if i am missing the cover, does that really mean that much more air that it needs jetted? i bought a .240 jet for my carb from jmp cycle and it would not thread in. im pretty convinced that its because its a cheap ass carb... however, if i drop the clip on the needle to the lowest position will that hold the same effect as installing a 1 size bigger jet?

got a brand new plug, tors delete, oil injection delete, (mixing my own at 32:1 with 93 octane) and although my air filter needs replaced, i cleaned the hell out of it this week for my day yesterday.

you guys have been a huge help and i appreciate your time there may be something i left out but feel free to pick my brain on my setup if it helps you help me! lol thanks in advance

 

[speedyslyder]

upload some up close pics of this beast!
chinese carb??

 

[mastuhblastuh]

yes... chinese carb. i need to upload pics because for the age it is a very clean quad. all the shocks look good, tires are brand new, and this bad boy was KILLING it in the trails saturday! but as of now its only good for half maybe a little bit more throttle and it wants to fall on its face.. to give you an idea however, that alone aloud me to ride all day and keep up with the rest of the pack! but i wanna pop some wheelies... and thats hard to do without all the throttle!! does this sound like lack of fuel to you? i did have the float checked by a pro also... and he verified the carb is chinese. the plan is to get a genuine carb first before anything else but i wanna see if i can make due with what i have for now! thanks for help in advance!

 

[Larry's Shee]

No on moving clip. Needle is only for 1/2- 7/8 throttle. You could try drilling out the main jet, especially if you have a GOOD selection of bits. Or you could use a Presidential Solution, take a bit that fits the hole and "ream" the hole out a little at a time.
No lid adds a lot of air. Depending on mods it's at least one size on the main. List all your modifications that you know of , someone will give a good starting point for jetting a stock carb.

 

[mastuhblastuh]

i was told the top end is new.. but no receipts so we all know how that goes.. it does have very good compression tho. the only mods that i know of are the lack of airbox lid and a silencer that may or may not actually belong on the bike... im supposed to get you guys pics but its been a busy couple weeks lately and every time i get home i wind up getting tied up with stuff. the carb is new also but i feel like i should mention again on top of that that i think the carb is the issue.

*side note* i have never confirmed this my self... but i was told once by a very smart mechanic that some jets have very... slight "swirls" so to speak in the orifice to help atomize the fuel. im skeptical to drill out a jet because of this. can anyone confirm or deny this? reason i ask is because i bought a jet at jmp cycle without taking my carb in and the jet looked identical but the threads wound up being like.. .01" bigger and wouldnt thread in. i dont wanna ruin a jet if i cant get another for it...

 

[best]

i was told the top end is new.. but no receipts so we all know how that goes.. it does have very good compression tho. the only mods that i know of are the lack of airbox lid and a silencer that may or may not actually belong on the bike... im supposed to get you guys pics but its been a busy couple weeks lately and every time i get home i wind up getting tied up with stuff. the carb is new also but i feel like i should mention again on top of that that i think the carb is the issue.

*side note* i have never confirmed this my self... but i was told once by a very smart mechanic that some jets have very... slight "swirls" so to speak in the orifice to help atomize the fuel. im skeptical to drill out a jet because of this. can anyone confirm or deny this? reason i ask is because i bought a jet at jmp cycle without taking my carb in and the jet looked identical but the threads wound up being like.. .01" bigger and wouldnt thread in. i dont wanna ruin a jet if i cant get another for it...

Good to go slow and think things out. Fools rush in where wise men fear to tread.
There are no "swirls" but drilling jets is a very inexact science in the backyard.
Better to buy a jet so you always know where you came from.

That said, Larry's advice is pretty good. We (old farts) have all done it.
I am guessing you are not running the long PWK jets?

Instead your carb uses the short little Mikuni or Dellorto 5mm thread "buttons" like this:

There is a slightly different pitch 5mm thread on the 2 brands.
0.8mm pitch vs 0.75mm They crank in about 3 threads and bind up.
Both look very similar.

Steve

 

[mastuhblastuh]

BEST!!!! this is the information i have been looking for!!! Thank you very very much for doing the research for me!! i would like to think im very mechanically inclined. been working on cars all my life, small engines since i was like 6 and i have a very good understanding of 2 cycle. before i decided to drill the jet i took a trip to jmp cycle to see if i could get a replacement jet just in case i went to far in on my chinese jet. the mikuni jets they have like you say, are just barely different on threads.. so im sad to say, i could not get a jet so i held off on the drilling because im not sure i can figure out how to translate the jet size to a drill bit number... im genuinely pleased you have supplied these cheat sheets on jets because i wanted to learn more about them. that being said, i am good friends with the parts guy at jmp and he just so happens to have an extra carb for a blaster (a genuine mikuni) that i can play with and find jets for!! also i did get some pics last night that i will post momentarily for you guys. you have been so much help and i appreciate all of you!!!

 

[mastuhblastuh]

 

[mastuhblastuh]

 

[mastuhblastuh]

 

[mastuhblastuh]

 

[mastuhblastuh]

since im a noob to the forum i had a bit of trouble figuring out how to put the pics in but here she is!

 

[mastuhblastuh]

Good to go slow and think things out. Fools rush in where wise men fear to tread.
There are no "swirls" but drilling jets is a very inexact science in the backyard.
Better to buy a jet so you always know where you came from.

That said, Larry's advice is pretty good. We (old farts) have all done it.
I am guessing you are not running the long PWK jets?

Instead your carb uses the short little Mikuni or Dellorto 5mm thread "buttons" like this:

There is a slightly different pitch 5mm thread on the 2 brands.
0.8mm pitch vs 0.75mm They crank in about 3 threads and bind up.
Both look very similar.

Steve

best, i also forgot to mention earlier that while i was at jmp cycle, they did get a circle style jet to fit like the bottom picture you posted.. but 2 things worried me... 1, will the needle seat properly in a different style jet? and 2, the size # wasnt even close to the size number the blaster manual calls for... i wanna say when they eyeballed it to get a sort of "base" number, that they found a 120-130 (circle style) when my bike calls for a 230 mikuni (stock) and id like to try a 240 next... is there any way (if the needle assembly works properly with the circle jet) to translate the jet number for the mikuni style into the the circle style? also, if i didnt note before, the jet in the carb looks just like the ones in the top picture. (the hex style) but would not accept an actual mikuni in the carb i have... again and again, thanks so much for your help!! ALWAYS looking to learn!!!

 

[best]

The Dellorto style is more common on scooters, so try those shops.
I've interchanged them when pressed, they don't seat but they will work.
Don't force anything, if the threads leak it goes rich, allow for it.

The numbers on most are flow rate and will interchange to a degree.
Always best to buy a set from the same manufacturer. Chinese quality is not good.
You can compare sizes by using a sewing needle set to see how far the taper goes in.

 

[mastuhblastuh]

I think what im going to do is just get the mikuni from my friend and upsize the jet in it and call it a day. At least then il know what the base is and where to go from there! Would a different silencer cause for rejetting also? It doesnt have the stock silencer and i dont think this one is necessarily designed for a blaster but its on there! I believe one of the pics i posted has a good view of the silencer

 

[best]

I think what im going to do is just get the mikuni from my friend and upsize the jet in it and call it a day. At least then il know what the base is and where to go from there! Would a different silencer cause for rejetting also? It doesnt have the stock silencer and i dont think this one is necessarily designed for a blaster but its on there! I believe one of the pics i posted has a good view of the silencer

Argh!
You have to do a plug chop to know if you have the right jetting. No shortcuts.

Steve

 

[mastuhblastuh]

Lol i agree with you steve!! But i did do that just after i dropped the clip on the needle and although it helped quite a bit, the color on the plug was still a teeny bit to bright for my liking!!

 

[Larry's Shee]

I don't get it. You claim to have an unknown carb but the pic shows a Yammy TORS brick

 

[Larry's Shee]

BTW, I have never drilled/reamed a jet, figured with an unknown carb wouldn't matter. JMHO, your results may and will vary !!

 

[mastuhblastuh]

it is an unknown carb. but the throttle cable remained the same. i just got my authentic mikuni last night so now i can actually buy jets to play with. the mikuni jets wouldnt fit in the chinese carb. the tors brick you speak of wasnt replaced however... next in line is a rear shorty brake cable and a new throttle cable without the tors brick!!