xs650 judders when off throttle. newby needs advice

[spitfire]

hi,

i recently bought a 79 xs650 (sf) with boyer electronic ignition and i'm having a bit of a problem with it that i would rather fix myself, but cant work out what to do as i'm a bit of a novice.

the bike starts nicely now that i put the proper spark plugs in and accelerates very well. the problem is that when i am off the throttle the bike judders. it feels a bit like its running out of fuel. it sounds as if its the engine that is doing it. it doesnt run nice and smoothly at tick over either. sounds a bit lumpy if you know what i mean. but when i open the throttle its fine, its just this kind of judder that comes and goes every second or so when i am off the throttle.

any idea's ?

thanks

nik

 

[Ironsled77]

Have you cleaned to carbs? That's where I would start. I'm no expert myself, that being said your in the right place cause there is guys who are. I'm thinking its someshing in you jets or idle circuit, or vacuum leaks

 

[inxs]

- before messing with your carbs do a few simple things ... cam chain tension, tappets and timing, a compression test can give some indication of cylinder and valve condition, air filters, look for spark and battery condition

- check also things like engine mounts...while youre at it quickly check swingarm bushes, wheel bearings and steeringhead bearing

- all of this wont take long and although some may not have a direct link to your problem, cummulatively every little bit helps

- no improvement, then have a look at the carbs

 

[jay760]

you could just check your brakes aren't binding at the same time as as you do the items in inxs's reply,

 

[fongbros]

I'd bet almost anything the idle circuit is obstructed. You need to get the little brass plug out (it's not a screw, it's a plug on the upper front of each carb. Using a thin screwdriver, tighten the screw (that is being protected by the plug) gently counting the number of turns to stop. Remove the serew and spring and blast some carb cleaner down the hole. Put the screw back in the same number of turns and try it out. It's a temporary fix vs cleaning the entire carb, but I but it works.

 

[grizld1]

BS34's, with vertical mixture screws forward of the slide towers recessed in capped castings, weren't fitted prior to the G-model. The BS38's on the man's F-model have exposed horizontal mixture screws on the sides of the carbs.

 

[spitfire]

thanks for all the advice everyone. the first thing i will try is take the air mix screws out and spray something into them. can i use wd 40 as i have some in the shed or do i need carb cleaner ?

i was a bit confused over which carb i have as the haynes manual says my bike (79 SF) has a preset pilot screw yet i can see the little screws and they turn in and out. preset implies cant change them. not only that but it doesnt say how many turns from seated is the correct amount. i have done it at 1 and 3 quarters but am only guessing. does anyone know the correct setting ?

oddly enough when i did screw them in it didnt make any difference to rev's thats why i think it maybe blocked.

 

[grizld1]

Originally your pilot screws had plastic caps that restricted their range of motion, and to that extent they were "preset." For more information on cleaning and adjustment, go to www.amckayltd.com/carbguide.pdf

 

[weekendrider]

grizld1, That was going to be my response also. That the mixture screws were in the sides and have a plastic limiting cap on them. But Sport Parts& Service shows the 78 specials as having the first 34's. Throw in the fact they could have been changed by a PO and who knows huh.

 

[weekendrider]

F model carbs with the (1) cap intact and (1) removed.

 

[grizld1]

There's many a flaky application list out there, and many a flake as well; caveat lector!

 

[weekendrider]

strewth

 

[spitfire]

yes, mine are 38's.

sprayed carb cleaner in the air mix screw holes and it was even worse. the air was blue, and not only with exhaust fumes. still, the more i rode it the better it got and now i think its definately improved. so thanks everyone for the advise.

 

[pumps]

- before messing with your carbs do a few simple things ... cam chain tension, tappets and timing, a compression test can give some indication of cylinder and valve condition, air filters, look for spark and battery condition

- check also things like engine mounts...while youre at it quickly check swingarm bushes, wheel bearings and steeringhead bearing

- all of this wont take long and although some may not have a direct link to your problem, cummulatively every little bit helps

- no improvement, then have a look at the carbs

Please still do try this stuff.

 

[gggGary]

Please still do try this stuff.

But all that stuff takes time and it runs (sorta) now.
Sorry OP but this goes on so often you just want to scream some days.
They are old bikes, they HAVE a long list of maintenance that hasn't been done in ages, and it is going to strand you or be unsafe in ways you don't even know about yet.
But you just want to go ride, not deal with that "stuff".
OK rant off.

 

[spitfire]

it did seem a bit better then i ran it out of fuel and all the rubbish from the bottom of the tank went into the carbs somehow and then it wouldnt start again. got sick of it and gave it to a bike mechanic to sort. its been away more than a month, if only he could fix bikes as well as give me excuses why he hasnt sorted it. anyway eventually they cleaned the carbs and it worked in the workshop for a bit then packed up. he thought it was the boyer ignition which was faulty. he said they are the worst in the world. still, it got sent back to boyer uk and for £10 they will check it. it appears its faulty and because its less than five years old i get a replacement free of charge which is nice. so all i have to do is wait for the new unit to be returned, fitted, carbs set up and then it will be autumn and too cold to ride. still, always next year.