advanced carb tuning help needed.

[chefevan88]

So I finished my first build in spring and been riding all summer. After a lot of carb work details to follow my bikes starts and rides smooth until mid to full throttle on each gear. Where it sounds like choking out. Here's what I have done.
77 xs650 stock 38 carbs w new mikes XS rebuild kit including stock jets 135 mj 27.5 PJ and new needle (now at position2)
Chopped stock pipes w baffles
Oversized pods

I did late spring check cam chain adjuster valve lash and check timing regularly on my pamco.

So after I cleaned and installed new jets I synced carb to best of my ability w emgo dial guages.
And did my best at air screw adjustment. Seems OK didn't use dead cylinder kinda scared.

I had a few air leaks in boots I used some gasket seal and am supporting carbs w zip ties to upper frame for weight support. Best it's ever been w heavy pods and rubber manifolds

So the question is what direction do I need to go bike runs good until I wanna blow by a bunch of cars.

I read a few threads and saw a good chart of jet to range performance and I deduce main jet.

Mj to big? Everyone else went up with similar mods

Needle to top just doesn't seem right?

Thanks ahead sorry if I break any post rules I'm a noob.

 

[retiredgentleman]

The carb jets from Mikesxs are the wrong size for your 1977. Those sizes are for 1978/79 carbs.

You need #122.5 for main jet and #25 for pilot jet. Since you have modified air filters and exhaust, you likely need larger that stock. Try using stock, and then try one or two sizes larger.

The #27.5 pilot may be OK, but try #122.5 ,#125, and maybe #127.5 main jets.

Edit: read the "Carb Guide" that is in the "Tech" section

 

[grizld1]

Oh, no, RG, the Carb Guide just has basic guidelines; the man needs ADVANCED instruction! So here it is: DON'T START THE TUNING PROCESS BY FATTENING YOUR MAIN JETS SIX STEPS.

 

[chefevan88]

Ok thanks i do refer to carb guide often. Its a lot to absorb for a chef lol. Part of my issues was trying to get to a base point. That's y I bought the kit from mikes my bike is a little pieced together the frame and engine are 75 but carbs and everything else came off a 77. I'm gonna order those jets. The main can be changed on bike? Also no one mentioned needle jet I'm at 2.

I'm also doing a rebore on a rt 360 right now so im over my head ATM.
But when I'm done I'll have two sweet yamahas.

Thanks the knowledge on this site is great.

 

[grizld1]

As 5T and I explained in the Guide, you jet by the year of the carb set, not the year of the motor. 5T included a year-by-year chart of OE jetting, including needle positions; look again, you'll find it. If you're 6 steps fat on your MJ's it's no wonder you've had to drop the needle, you're getting a helluva fuel dump from that; your stock position is #3. Yeah, you can swap mains without pulling the carbies. Word to the wise: you'll have better luck with genuine Mikuni jets and carb parts.

 

[retiredgentleman]

The main and pilot jets are in the float bowl on BS38 carbs. If you use a short phillips screw driver, you can remove the 4 screws that hold each bowl, and drop the bowl.

Many of us have replaced the phillips with allen head cap screws................makes removal of the bowls much easier.

 

[XSLeo]

On most any carb rebuild kit you get the not only are the jets wrong but the needles are wrong too. Usually too short. Use the needles that came in the carb.
The float bowl gasket works, the float valve may work. That's about all that the kits have that do.
Best to inspect what you have and if found worn replace with correct parts.
That too short needle can be all your trouble.
Leo

 

[chefevan88]

Man I thought I did myself a favor by ordering the mikes kit. Why does he sell the 77 w wrong jets. A
Lot of hands have been on this bike before me so I was hoping to get to stock. I didn't obviously double check carb guide. The needles Dont come in that kit but I did replace with ones from mikes cause originals were rough. I did buy the ones For the 77.

First question is can someone suggest a good source for OEM carb parts. I've only used mikes and eBay. Did a quick search and didn't like bike bandits or a few other top listed. They didn't list jet size easily.
I need parts for my rt1 too.

So I wanted to test drive again before I made changes. This time I reset air screw and set in a Lil lean.
Took bike out for long drive was a nice warm evening. Had to adjust idle up a Lil after choke was off but seems to idle nice. (BTW what is reccomend idle speed ).my adjuster is at lowest barley touches so seemed better than before.
Took off when down 1\2 mile to first light bogged out very Lil in high second fine in third but ran into red light before got high rpm. Took turn then opened it up no problems went one mile changing speeds and rpms in all gears no problems. Stopped at gas station after another mile. Turned off let cool inspected bike did notice oil leak from left side lower jug gasket. Small but by burning a Lil when gets to exhaust. Added a Lil oil drove around Michigan ave for 30 min its a 55 w a few
Lights blvd style. Abosolutly no bogging out. A few crackly pops on coasting but nowhere near the pops it had before rejet.

Did i lean it just right or am I running too lean. Bike finished ride with 247f temp behind plug same as always.

Thanks again

 

[chefevan88]

I did find the rpm in a thread at 1200. I know I was running it low after the carb rejet. It sounds pretty sweet low at lights. Good to know I got room to adjust.

 

[grizld1]

Ya know, I've posted this more times than I can count, but here it is again--owner's manuals for all years and the shop manual for 78-83 XS650's (with specs that apply to your 75 motor) are available as free pdf downloads from www.biker.net . It would make your life much easier to download that stuff, print it off, and read it.