SputterIng off and on

[Ccrider]

Stock 1975. Just overhauled carbs, new pod filters, new plugs, tank cleaned. Fresh gas
Carbs had 127.5 mains, kit came with 130's. I put the 130's in? Stock pipes but Im pretty sure the baffles are gone.
Took it down the road and I get off and on sputter. Any ideas?

 

[Ccrider]

I get this sputtering through all ranges of throttle. But Idle seems good.
Would this be issues with ignition/points?

 

[shotgunjoe]

Could be failing intake boots. Bouncing down the road... ya know?

 

[Ccrider]

Points checked fine. Intake boots ? Is this common? When I put carbs back in they seemed to be nice and tight. Also what is normal throttle response on 650's

 

[nezzer]

I recently read an intake troubleshooting technique that sounds like the right mixture of danger versus usefulness. To check for air leaks in the intake, get a propane torch. Get the flame to about 1 inch long then blow out the flame. Next put the torch, gas still flowing but flame out, around all the areas of your intake. If there is a leak, then the rpms will rise when you hit the leak.

I like this method versus the usual WD-40 on the intake boots because there is no sticky residue to deal with. I haven't killed myself with it yet so I give two thumbs up.

It does sound as if you have an air leak somewhere on the air intake. That is quick to check. It could also be the float bowls are way way low.

 

[shotgunjoe]

Cracked intake boots is very common, especially when running without the stock airbox. The part that cracks is not the part you can visibly see, its the parts that are hidden under the metal reinforcements. Ethanol + age = cracked rubber intake boots.

 

[Ccrider]

Im going to check them out, hope its something easy like this. Also going to check compression.
If I have low compression would it idle good and Sputter or hesitate as throttle advances?
If compression sucks are there any other checks before I start look for new rings ?

 

[5twins]

With pods and larger mains, you may need to lean your needles a step. Stock setting for your '75 carbs is the #4 slot (4th from the top). Try the #3 slot, maybe it will clear up the sputtering .....

 

[Ccrider]

Moved the needle clip , better results in upper throttle ranges. Low , mid range better but not ideal. Still hesitates but no backfire. Plugs looking brown.

 

[5twins]

If your rebuild kits came with new needles, you probably shouldn't use them. They're usually a generic "World" needle, a Y-22 whatever that is, and are not correct for your carb set. If you have original needles still in there, next I would try one size up on the pilots.

 

[Ccrider]

Right you are, the new needles were obviously off - I put my originals back in when I put them back together. New pilots are correct to spec. I'll have to see if I can find some pilots and intake boots ?

 

[Ccrider]

The 1975 bs38's Spec pilot jets are 45. I just pulled my carbs out and I have 42.5's. Is this a significant difference"? Enough to cause my hesitation/sputtering problems??

 

[shotgunjoe]

Right you are, the new needles were obviously off - I put my originals back in when I put them back together. New pilots are correct to spec. I'll have to see if I can find some pilots and intake boots ?

Check yours before you buy of course... but.
JBM Industries has the boots... ethanol approved.

Click

 

[5twins]

Well yes, one size on the pilots can make a real difference if you need bigger ones. And, when you lean the needles a step, you usually do need larger pilots. The 3 systems in your carbs overlap. Make a change to one and it affects the one or ones next to it a little bit. Change the midrange and you influence both the (lower) main and (upper) pilot circuit slightly. Try the original 45s but you may even need 47.5s.

 

[Ccrider]

Thanks for the diagram..I am Almost dialed in. Now with the needle setting from above, 130 mains and 45 pilots the bike responds well. But after 5-10 minutes riding my upper mid-top end sucks. Any ideas? Should I keep chasing the pilots

 

[5twins]

No, now you work with the mains and needle position. "Sucks ..." doesn't really describe the problem very well. Does it stumble (rich) or hesitate (lean)? Since the problem occurs after it gets warm, you could be too rich with the 130s. A bike needs more fuel when cold, that's why there's a choke for cold starts and maybe why it runs OK until warm. Try your original 127.5s if you still have them. Or your needle position could be too rich if it's still in the #4 slot with the larger 130 mains.

 

[Ccrider]

Your right "sucks" doesn't describe much. As stated "the problem occurs after it gets warm". Now I'm cruzing at 30mph and goto accelerate is where it's really a pooch. The bike is slow to wind up not hesitant, lazy or stumble is better description.
Also the plugs were dry flat black in color. I do have the needle @ #3 - and I still have the original (127.5) mains laying on the bench. I can go back to them and see what I get. Thanks

 

[5twins]

Yes, put the 127.5 mains back in. That should tell you which way you need to work - richer or leaner. Your plugs being black are telling you you're too rich but that reading can depend on how you took it. If, for instance, you start the bike using the choke then shut it off and read the plugs, they will show black because of that choke use. Ideally, you want to ride the bike down the road for a few miles in the RPM range you're trying to check. Then pull over quickly, shut the motor off (don't let it idle for a long time), and check the plugs. When race tuners do "plug chops", they kill the motor and coast to a stop but you don't need to do that. Like I said, just don't let the motor run long at other RPMs than the area you're trying to check. That will start changing the plug color you're trying to read.

 

[Ccrider]

Carb setup is back to stock spec for the 75.with one exception I am using k&n oval pods.
New jbm boots. I've been doin a little ridin and still have intermitent sputter (like there's no gas and all of a sudden you have gas again)?? No backfires and good looking plugs.
I have stock ignition, which is all set correct. Could my problem be old ignition system, or the k&n's..

 

[5twins]

Take the filters off and go for a test ride. If the sputter goes away, you've found the problem. K&N pods work but are not ideal for CV carbs. Do you have the RC type with the chrome end caps? I ask because CV carbs also do not like a filter with much of a taper and the RC type do taper quite a bit. A better choice in the K&Ns is the RU rubber end cap type as they have very little taper. Better still are UNI foam filters.

If a test without the filters doesn't change anything, I would start checking ignition components. You may have something going bad like a plug, plug cap, condenser, or points set.