78 xs, vm34's carb tuning ?s

[my94probese]

ive got a 78 xs650, bobbed, peanut tanks, n some goodies. ive got short open pipes, mikuni 34s, with pod filters. just recently put a pamco on it, as well as boyer blue box ignition. having a little tuning trouble and questions. ive opened and cleaned the carbs numerous times. but i keep gettin the right cylinder to run rich. carbs are correctly synced.

inside the carbs, the needle jet is set on bottom clip, 150 main jet, i believe stock pilot... i live in iowa (1100ft) and i was curious as to some of your guys ideas on what to run for pilot, main, etc

left cylinder is .75 screws out

right cylinder is about 2? and runs a bit rich

both cylinders show 120 psi compressions with choke closed

 

[rybeatty]

I live in Indiana. I don't think our climates are much different. I run 34's,pamco, pods and open exhaust. I have I believe a 30 pilot 190 main and needle clip is in the middle and runs great. With the jet clip position I've heard more than one person say if it's to the extreme of all the way up or down something isn't right

 

[Brian902]

I have a set of VM34 I just took off my bike, they look like this;

Main jet 180
Pilot jet 22.5
Air screw 1 turn
Float level 23 mm
Needle clip 3 rd (middle)
Needle jet 159 P-6
Jet Needle 6F9
Slide 2.5
Air bleed screw removed
Uni-pod UP-4229 air filters
Sudco throttle cable

Running at sea level.

Had to clean the pilot circuit 3 times to get them clean after my son left them for 2 years with fuel in the bowls. Pine Sol worked well but left parts stained slightly, but not ill effects. Bike runs like a top. However, I think it still runs slightly rich and could be improved.

Good luck.

PS: When performing a compression check, throttle wide open is the preferred method for a truer reading. I did mine when the carbs were off the bike and got 155 both cylinders. 10% difference is acceptable down to about 120 psi and then it's rebuild time.

 

[DIRTY DOG]

ive got a 78 xs650, bobbed, peanut tanks, n some goodies. ive got short open pipes, mikuni 34s, with pod filters. just recently put a pamco on it, as well as boyer blue box ignition. having a little tuning trouble and questions. ive opened and cleaned the carbs numerous times. but i keep gettin the right cylinder to run rich. carbs are correctly synced.

inside the carbs, the needle jet is set on bottom clip, 150 main jet, i believe stock pilot... i live in iowa (1100ft) and i was curious as to some of your guys ideas on what to run for pilot, main, etc

left cylinder is .75 screws out

right cylinder is about 2? and runs a bit rich

both cylinders show 120 psi compressions with choke closed

because ther is no stock pilot jet for vm34's if it the stock bs34's the main is is too big
I'd drop it to 135 middle on the clip and 45 on the mail jet if it's vm 34's then I would go with brian902's it the same set up I run here in Ohio. hope this helps
are you running bs34' (stock carb) or vm34's after market carb.)

 

[racerdave]

He said vm 34's in the header title, but 150 mains are way lean

 

[grizld1]

OP, go to the Tech section, click the link, and read. I'm not going to play "Write it again just for me."

 

[DIRTY DOG]

He said vm 34's in the header title, but 150 mains are way lean

yes he did but I think he ment bs34 when he said stock pilots and yes 150 way too lean for vm34's

 

[my94probese]

haha, yes vm34s... i guess by stock, i meant whatever comes with the vm 34s from the factory... when i saw the 150 mains, i bout shit myself... my quad has a bigger main than that...

but with the setup its at now, i cannot figure out why the right cylinder is sooting the plug pretty bad.. not enough to foul it though.

 

[grizld1]

Those carbs don't come from Mikuni with 150 mains, so GOK what kind of a mess you have for the rest of the brass. If the vendor represented those carbs as baselined for the XS650, that claim is a bald faced lie; my guess is that the vendor cheaped out by leaving the generic Q-2 needle jets in place and did half-assed compensation with the lean mains. Check everything against the baseline Brian posted above and follow the setup instructions in the VM Carb Guide. If you cut corners you're going to regret it.

 

[my94probese]

well i tore it apart and it now has 25 pilots, with 175 mains, and needle clip is at 3rd position... fires up n idles pretty decent. couldnt ride due to rain... but i was curious about backfire through the exhaust... my pipes end about 18 inches from the head, and are open.. when i rev it pretty decent now n let go right away, i get some pretty decent fire. sound about normal for my carb setup n pipe setup? or no. i also just ordered some uni filters to get rid of the k&ns

 

[grizld1]

Short open pipes are power killers. Straight pipes make power over a narrow rpm range, and pipes as short as yours will put your power peak at an rpm that the engine will never turn. All you're doing is putting your noise in other peoples' faces at the expense of performance.

One more time: Nobody can give you a recipe for tuning an aftermarket carburetor. Read, try, and learn, or sell.

 

[DIRTY DOG]

yeah the fire is normal with pipes that short ride at night and you will really see it.

 

[my94probese]

i hear tons of different stories about pipe lengths and "muths" of power loss or torque loss etc etc.. ive always hated loud bikes, harleys, etc... but i love the way this thing sounds full throttle. this winter i may build em as a 2-1 dump at the bottom of the motor

 

[grizld1]

So you think exhaust tuning is all stories and myths, do you? Good luck, rookie.

 

[my94probese]

hence why i quoted "myths"... people go back n forth about it, saying not needing back pressure if you can tune the carbs right, but main worry is riding with short pipes in the cold

 

[Jack]

hence why i quoted "myths"... people go back n forth about it, saying not needing back pressure if you can tune the carbs right, but main worry is riding with short pipes in the cold

You're always going to have a certain percentage of back pressure,the trick is finding the balance factor where the engine responds and performs the best without the engine falling flat on it's face. You've gotten solid advice on your problem on how straight pipes suck and how they are more problematic when it comes to tuning. It might do yourself some good in researching the benefits of a true functional exhaust system.

 

[grizld1]

The term "back pressure" is sloppy because it confuses two things. If you relieve RESTRICTION the motor will pump more air and make more power, and you'll need to compensate by rejetting to richen the mixture. This is a different matter from a tuned exhaust in which resonant pulses aid scavenging in the exhaust stroke and fuel retention (back pulse) during the intake stroke in the rpm range in which you want to optimize power. You can't tune the exhaust by playing with the carbies, and what you do to the exhaust if you want performance has to be consistent with the camshaft, etc. that you're using if you want to make power.