More carb questions.

[jgimar]

I have probably worn this subject out, but I am still looking for a miracle I guess.
5twins and a few others have been very helpful and many thanks. still having this one problem.
I have a ,77 650D, following setup:
BS38 carbs, pod filters, 1 1/2 inch open pipes reduced down with a set of torque cones and thumb screw baffle.
jetting is MJ 130, PJ 30, stock needle, stock needle jet, stock float needle, etc. all these are new as is the mix screws and choke plungers.
everything set correctly (valves, cam chain, timing, floats) close as I can tell.
not running to bad but I still have this one problem. I can't get the right side mix screw to respond.
the left one, seems to function. turn it in from 1 1/2 and it starts to cut it out. turn it back out to around 1 1/2 and it smooths out, any further and it starts to pick up rpm and popping. turn the right screw in, no response, turn it out no response.
I have cleaned every single little jet and orifice until I'm blue in the face and still can't get that right mix screw to work.
at a loss, HELP!

 

[650Skull]

The only thing the pilot screw, (mix screw), controls is the amount of mixed fuel/air it lets into the chamber..............must be somewhere between the where the fuel/air comes in to get mixed to where the Pilot screw lets it into the chamber.

 

[grizld1]

When the motor doesn't respond to changes in mixture screw setting, it's often (in my experience usually) a sign of high fuel level caused either by low float level adjustment, bad float, or defective float valve. You might start by submerging the float in very warm water and looking for bubbles. If the float is good, reinstall it and check level: low float level = high fuel level.

 

[jgimar]

When the motor doesn't respond to changes in mixture screw setting, it's often (in my experience usually) a sign of high fuel level caused either by low float level adjustment, bad float, or defective float valve. You might start by submerging the float in very warm water and looking for bubbles. If the float is good, reinstall it and check level: low float level = high fuel level.

Thank you! Keep it coming! Brand new stock size valve and needle. Tear ‘em down again and i’ll check the float and level. Let ya know what happens.

 

[grizld1]

Never assume! If the new valve is genuine Mikuni it's probably fine, but knockoffs have been known to cause trouble. A few quick searches will turn up plenty on the subject of taming an unruly float valve. You might consider fabricating a sight tube to inspect fuel level; a search will turn up some good ideas for that as well.

 

[jgimar]

Never assume! If the new valve is genuine Mikuni it's probably fine, but knockoffs have been known to cause trouble. A few quick searches will turn up plenty on the subject of taming an unruly float valve. You might consider fabricating a sight tube to inspect fuel level; a search will turn up some good ideas for that as well.

 

[5twins]

While many of the BS38 carb sets spec the float setting at 24mm, your '76-'77 set is different. It's spec is 25mm. Also, when setting the floats, you must check them from both sides. They are often twisted on their bracket which makes one side sit higher or lower than the other.

 

[jgimar]

Hoorah! Figure i’d hear out of you sooner or later. I’ll definitely add this to the other wisdom i have received. I’m bound to get this worked out someday. Soon I hope! Thanks for the help as always!

 

[grizld1]

BTW in Mikuni vacuum diaphragm carburetors, the mixture screw controls the amount of engine vacuum admitted to the pilot jet via the pilot nozzle in the spigot; more vacuum=more fuel drawn through the jet, richening the mixture as the screw is turned out. This is the case with all Mikuni and all other carbs that I know of in which the mix screw is located forward of the slide tower. When the mix screw is located to the rear of the slide tower, vacuum remains constant and turning the screw out adds air to the mixture, making it leaner.

 

[jgimar]

BTW in Mikuni vacuum diaphragm carburetors, the mixture screw controls the amount of engine vacuum admitted to the pilot jet via the pilot nozzle in the spigot; more vacuum=more fuel drawn through the jet, richening the mixture as the screw is turned out. This is the case with all Mikuni and all other carbs that I know of in which the mix screw is located forward of the slide tower. When the mix screw is located to the rear of the slide tower, vacuum remains constant and turning the screw out adds air to the mixture, making it leaner.

Thank you. Tinkered wit it a bit today but no changes. Tomorrow I’m starting from scratch, tear it down to the bear bones, clean and blow out everything i can possibly think of, slap it back together and go from there. Let you guys know how it ends up! Appreciate the help.

 

[RustiePyles]

Take the pod filters off and try setting the mixture screw then. If it works with the pod filters off your pods are blocking the air bleed jets in the throat of the carb.

 

[jgimar]

will do.

 

[jgimar]

Okay. I have tore down these carbs about 5 more time now. Starting with the response to the leading thread:
I am assuming what 650skull means check all the air passages. Done that . Started at the bell of the body, made sure air was passing from that point thru the smaller passage inside the body and then from there thru the pilot jet onto the passage to the mix screw. Everything is clear. To address grizld1 's concerns. Replaced both float valves (needle and seat). Both are working properly. 5twins, have checked and set float at 25mm as instructed. Rustiepyles, pulled the pods as suggested.......... and the final result is:

There was no appreciative response or change with the right side and how the mix screw is suppose to preform. the left side, 1 1/2 turns out, and it's pretty much right there. the right side however. Rustiepyles suggestion created the most interest. he said take the pods off, might be blocking the air passage. upon doing this, initially nothing happened so I decided to block the air passage with my thumb, both the left carb and the right carb, one at a time. upon covering the left carb air passage, it killed the bike. upon covering the right carb air passage , I did get a little fluctuation, slowed down some but no noticeable difference as with the left side. so I tore it all down again, really worked over that circuit, slapped it all back together, set the mix screws to 1 1/2 turns, cranked it up, warm up, left side still okay, right side very minimal response. upon turning the screw in I noticed a slight dip but it wasn't even enough to kill the bike. turning out from1 1/2 , there was an increase in rpm, running rough and backfiring. definitely not what one would want it to do.
Guys, I just don't know which way to go. I have tried this with all variations of jets and needle settings combination example:
M-135, P-30 or P-27.5
M-130, P-30 or P-27.5 , in both cases, needle dropped one notch or left in stock location.
From everything you guys have suggested on jets and needle settings, I don't think I would need to go any larger.
About the only thing I haven't done is stick the stock mains and pilots back in it and see what happens.

 

[Jim]

block the air passage with my thumb, both the left carb and the right carb, one at a time. upon covering the left carb air passage, it killed the bike. upon covering the right carb air passage , I did get a little fluctuation, slowed down some but no noticeable difference as with the left side.

You're either badly out of sync with the left carb carrying the majority of the load, or the idle circuit in the right carb is completely plugged/dead.

 

[5twins]

Well, it does seem like there's still some sort of blockage in that right carb's pilot circuit. So, I'll walk you through the pilot circuit cleaning procedure, maybe you missed something ......

The fuel/air mix from the pilot jet is fed into the carb's main body through a small pilot inlet on the bottom of the carb body, shown here .....

This fuel/air mix exits into the carb's main bore through 4 tiny holes. Three are clustered together at the top of the bore about where the butterfly plate closes. The 4th is off to the side and comes from the mix screw .....

The passageway leading to these 4 holes splits, directing some of the fuel/air mix out the 3 holes and some to the mix screw. The amount getting past the mix screw can be adjusted with the screw. The mix sent out the 3 holes can't be adjusted. The only way to change it's strength is to change the pilot jet size, altering the fuel to air ratio in the fuel/air mix. To do a proper idle circuit cleaning, you'll need to remove the mix screw.

OK, to start with, dribble some liquid into the small pilot inlet on the bottom of the carbs. For testing purposes, just to check if the circuit is clear, you can use something mild like WD40. If you find blockage in there, then you can switch to the actual carb cleaner. Cover the hole on the outside of the carb where the mix screw went in with a finger and blow the liquid through from the pilot inlet with compressed air. You should see it exit out of all 4 of the little holes in the main bore. You can get it to come out just some of the holes by alternately blocking others. For instance, block the outside mix screw hole and it's inlet, and the liquid will only come out the 3 holes clustered together. Block the 3 holes and the outside mix screw hole, and the liquid will only come out the mix screw inlet. Put liquid in the mix screw hole and blow it through while blocking the 3 holes and the pilot inlet on the bottom of the carb. The liquid should only come out the mix screw inlet. To back-flush the circuit, blow liquid through from the mix screw hole while blocking all 4 little holes in the main bore. The liquid will come out the pilot inlet on the bottom of the carb body.

By coming at the circuit from alternate directions like this, and alternately blocking the different holes, you should be able to isolate and find which area of the circuit is plugged.

If the circuit is all clear then maybe the mix screw or it's seat in the carb is damaged. To test the screw, you could swap it into the other good working carb and see if it works OK there. I don't think there's much you can do if the mix screw seat is damaged other than replace the carb.

Another possibility might be the float bowl gasket. If it's a replacement, many times they come with the various needed holes scored but not punched out. Make sure they have had those punched out bits removed .....

Also inspect the pilot inlet punch-out. Maybe it's not aligned quite right with the hole in the carb body and partially blocking it.

 

[650Skull]

Just a thought, a bit outside the box...........have you taken the Pilot screw right out and inspected it for damage. just maybe the tip has broken off...........

76/77 Pilot screw

 

[5twins]

You also mentioned experimenting with blocking the air passages in the intake bell, but you didn't specify which one .....

The large oval slot at the top is for creating vacuum to lift the slide. Blocking that at idle probably won't have any effect. The lower two, the small ones, are the actual air jets that feed the carb circuits. The one on the left side feeds the main jet and the needle jet. On your '77 carb, it should have an "M" next to it to identify it as the main air jet. Blocking that one at idle probably won't do much either. The one you would be most concerned with is the one on the right side. It's the pilot air jet. Both of these air jets deliver the air to the jets in the float bowl via the "air feeds" shown in my 1st pic in the post above. It is a simple matter to squirt some liquid through them from the carb bottom side and watch it come out the jets in the bell mouth.

 

[jgimar]

Okay. Just finished doing all as instructed. Every hole that you have pointed out to me has been checked, air blown through, wd40 blown through, more air, everything is clear. The mix screws are brand new. I’ll slap ‘em back together and see what happens. But it may be tomorrow before I report the results. 4th tear down today and I am burnt out! Thank you for the help however.

 

[jgimar]

You also mentioned experimenting with blocking the air passages in the intake bell, but you didn't specify which one .....

The large oval slot at the top is for creating vacuum to lift the slide. Blocking that at idle probably won't have any effect. The lower two, the small ones, are the actual air jets that feed the carb circuits. The one on the left side feeds the main jet and the needle jet. On your '77 carb, it should have an "M" next to it to identify it as the main air jet. Blocking that one at idle probably won't do much either. The one you would be most concerned with is the one on the right side. It's the pilot air jet. Both of these air jets deliver the air to the jets in the float bowl via the "air feeds" shown in my 1st pic in the post above. It is a simple matter to squirt some liquid through them from the carb bottom side and watch it come out the jets in the bell mouth.

Put my thumb over pilot port. Step ahead of ya, picked the one your pic had shown. Obviously opposite the one marked “M”

 

[jgimar]

Okay, put it all back together, drum roll please........ cranked it up, warmed it up....... drats, no appreciative response, a very slight blip on the radar when i turned the screw in a turn but it didn't continue, kept turning , gave no indication of bogging down, etc. turn it back out to the approx. 1 1/2 turn area and nuthin, remained the same. left side does what it's suppose to, turn it in, starts to kill it out, back it out to the smooth spot and it's good to go.
i have allso noticed, can't do the dead cylinder test. pull the right plug, bike keeps runnin, pull the left side, kills the bike everytime. ??????????????