79 carbs on an 80 , match float height to year of bike ?

[TDodge7]

i went ahead and decided to use my 79 carbs on my 80sg . with the 80sg carbs i couldn't get cylinder 4 to idle , i tried everything and even ultrasonically cleaned them , they're still clogged with magic . i threw the 79 carbs on my bike uncleaned just to see what was up and luckily i had all 4 cylinders running but cylinder 4 was super lean due to a broken off mixture screw (fixed in another thread) since then i've ultrasonically cleaned the rack of 79 carbs , and adjusted them to 79 spec . but once i put everything back on the bike and synced the carbs i noticed that now i'm running pretty rich , the bike basically falls flat on it's face as soon as it hits midrange . all i can think of that's changed is the float height , it's set to 79 specs with the brass floats (never ended up needing to change them ) if i remember right thats 24.7mm or 25.5mm and i think the 80sg carbs call for 23mm . did yamaha change something on the engine that required them to lower the floats for 1980 ? if so i wonder if thats why i'm so rich now . wonder if i should be running the 80's float height on these 79 carbs .

[tarzan]

You need to run the carb year float heights.So for 79 carbs they should be 25.7mm.You may have to adjust the floats a little leaner than you have them.

You said at midrange its bad?
So from idle to how far is it good,what rpm?
Did you clean this set of carbs too?And check all the circuits?
At WOL what does it do?
Do you have the right jets in these carbs?
Are you running cheap pods?

[DGXSER]

Hey Dodge,

Like Tarzan stated, tune the carb to the carb year.

It will help to know the RPM range your seeing issues with. Remind us what Mods you have, I know it was THOROUGHLY hashed ou in a prior thread that we do nto need to recreate, but would save some searchin to repeat that info here.

[TDodge7]

ok it's a 80sg , kerker 4 into 1 , k&n filter in stock air box .


the 79 carbs i have on there now are set to 79 spec , all jets are the correct stock jets for the year , 25.7mm float height . mixture screws set 1 1/2 turns out .

thing is when i just slapped them on the bike without cleaning them the bike actually ran pretty well , throttle responce was nice , and it seemed to rev up thru mid range no problem . then i took them apart , now after an ultrasonic cleaning , setting the floats to where the manual says they should be , turning the mixture screws 1 1/2 turns out from lightly seated (now that there's no broken off tips) , and re-synching the carbs i'm running rich . the plugs are sooty . and i may have another problem , i noticed that the bottom of the boot for carb 3 is slightly wet with gas between the boot and the head . and my petcock seems to be leaking thru the fuel hose in the off position . so i guess there's a bad seal on that carb boot (tho it never leaked before when i was testing for air leaks) and the needle valve in carb 3 could be sticking . it's running on all 4 , but it seems to foul it's self out at idle and won't even attempt going into midrange under load , it just stutters and boggs most of the time . i'm going to need to pick up some new plugs and do some more testing i guess . i really suck at tuning , but at least i'm getting close .

[tarzan]

OK,your symptom sounds at least one stuck float or that is set too high.
You also have a boot leaking too.
How much were the floats off when you re-set them?
Maybe put them back to the way they were and try that.

[devilschld]

i may be wrong on float setting. u do not set them turned upside down. they are set just like they were on the bike.

[cywelchjr]

i may be wrong on float setting. u do not set them turned upside down. they are set just like they were on the bike.

Umm, no, the floats are set with the carbs upside down, and making sure the needle valve is closed without it compressing the spring in the valve at all.

[TDodge7]

yeah , thats how i set the floats . the tank is a little rusty tho , some junk could have got stuck in the needle valve . i have no filters on right now because i keep having to take the carbs off . i'm surprised that the boot is leaking , i couldn't get the bike to rev up at all with propane around all the carbs .

[TDodge7]

on closer inspection it seems that all the boots have a slight ammount of leakage , but it appears to only be outward leaking (not sucking in air) there's old dried up gas around the back of the boots where they meet the head , i'm guessing it's bad o rings . still doesn't explain why i'm running so rich . it's raining now so we'll have to see another day .

[Crazcnuk]

Also try turning the mixture screws in. The 1 1/2 setting is just a starting point. You have to adjust them fromt here, in or out to where the engine idles the smoothest.

[TDodge7]

... ok , so i just tried something . i took the bottom of my air box off and all of a sudden the bike runs almost 100% . the rubber snorkel isn't crimped up or blocked in anyway . there's about a table spoon of oil in the bottom of the air box . this is with a k&n filter . could the fact that i have a 4 into 1 kerker mean that i'm not getting enough air flow ? it's like night and day ,with the bottom of the air box off it revs all the way up to 8 and sounds mean .

[TDodge7]

my friend rode it too and he said it seemed really rich with the air filter and air box bottom on , tho you couldn't see it belching out any smoke . it would get to 5500 or 6000rpm and it would sputter so hard it falls on it's face . 7 or 8k is impossible to reach with the bottom of the air box on , without the bottom it's super easy .

[TDodge7]

i should also mention that it's just as bad with the bottom of the air box on and the filter removed . so it's not just a clogged up filter . the bike also has no power and idles much worse with the bottom of the air box on . so the question is where do i go from here ?

i would have put all this in one post but after 10 minutes it won't let you edit them .

[bikerphil]

Don't know if you mentioned anything about the settings on the slide needles. The stock position is in the center position (#3). if your setting is different, the bike might run rich at mid to high RPM's.

[TDodge7]

just to state specs again
main 137.5
pilot 42.5
float height 25.7
kerker 4 into 1
k&n filter in stock undrilled air box
airbox snorkel not blocked
carbs synched
i forget what position the clip on the needles is on but if i remember right it was the middle , i didn't change them , just looked at them when i had everything apart and forgot to write it down .


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air box bottom ON
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idle seems strained and there is almost no low end power , no visible smoke , prolonged idling results in sooty plugs (still haven't set mixture tho so this means nothing)
mid range seems ok up until 4-5k where it breaks up and falls flat on it's face
wide open is non existant

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air box bottom OFF
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idle still seems rich , but it's closer , again , the mixture needs to be set ,but way more power ,much nicer sounding idle
midrange seems exactly like it should
wide open seems to be a hair too lean , the plugs come out a little whiter than i'd like after some wide open passes


the biggest question is why it supposedly ran great for the previous owner on the devil 80 carbs that i can't get clean no matter how many times i clean them . the devil carbs have stock jetting in them and the PO says the bike ran awesome , even with the 4 into 1 and the k&n filter . now that i have 79 carbs on it seems i need to re jet ?