vacuum petcock converted to gravity

H

hoganJr

hoganJr
Messages
25
Reaction score
0
Points
1
Location
Greenfield, Indiana
Hey folks, I have this '79 XS400 that you all helped me sort out a non-charging issue with. It turned out to be bad and incorrect bulbs in the brake/tail light that allowed both bulbs to burn bright all the time. Fixed that, and charging good!
I continue to have a problem with the engine racing up from idle and slow return or not returning at all to idle speed. I am confident that my carbs are close to sync, less confident that I don't have an air leak. The PO converted the petcock to gravity feed and plugged both vacuum ports. Can this be causing my idle speed issues?

Thank you all

HoganJr
 
If the vacuum tubes are not being used,you should go to an auto parts store and buy some plugs that fit tightly,so that they don't leak.You can put a little bit of silicon on the nipples of the vacuum inlet. You can check for vacuum leaks with W-D 40 or something similar.I use a propane torch[unlit of course] barely open,and when you find a leak,you'll know as it will immediately race.Be careful!!!!!!! lha
 
Yep- if the vacuum ports are plugged well, that's not your problem. What are your idle mixture screws set at?
 
thanks for responding with your wisdom and experience, Ihaolpa and HoughMade. I just finished the ABATE safety course with my son and this is gonna be his ride so I gotta make it run right.
HoughMade, I was in Valpo a couple weeks ago for a class at the University...beautiful campus, great town. We used to drive past the Cathedral when I was a kid on the way to Chicago or Wisconsin and I finally got a chance to actually go inside it.
I am confident that the vacuum ports are plugged well, but I will doble check that again. I have not turned the idle mixture screws so I do not know where they are set.
I must admit that I do not know what model the bike is. Judging from the wiring diagram I have been assuming it is a 400F/2F (maybe the title says what model it is). My manual says turn it out 1-1/4 turns from seated. Then it shows a table that indicates the pilot screws are not adjustable .
I will check it out tonight.

HoganJr
 
Excellent- I am a proud graduate of the Valparaiso University School of Law.

Anyhoo, the "hanging idle" as it is known, is a classic lean symptom. You can check for air leaks, but I solved it by turning the screws to 3-3.5 turns out. The factory setting were intended to get the bike through emissions regs and intended for seventies era gasoline, so a richening is in order.
 
Hey XS400 people, I am determined to figure this out and still appreciate any advice you may have for me. The Pilot screws were set at 1-1/4 turns; I turned them out to 3-1/4 and had excellent results...in the garage. I repeatedly reved up to 4 or 5000 and released the throttle. It snapped back and returned to idle...not crisply, but satisfactorily.
So I took her out on a 3-4 mile run, shifted up to 4th or 5th and on the way back, downshifting and slowing down realized that the tach wanted to stay at 3500- 4000 rpms. If you downshift and brake the rpms will return to idle, but then surge back up again. I got it home and sat in the driveway with it idling. after a minute or two it surged back up spontaneously. It will idle fine and stay running at 12 or 1300 rpms but if you give it some throttle it will not idle back down again.
It seems like if I had a float or needle valve problem the carb(s) would just flood and foul the plug and die, right?
The slide must actually be lifting...right?
I almost pulled the carbs last night, but...

Any advice is appreciated.

HoganJr
 
Well, I think that shows you are on the right track. There could be an air leak, but my money would be on an at least partially clogged idle circuit. Looks like pulling the carbs off for a cleaning....cleaner than you think is necessary, is in order.
 
well I bartered for set of used carbs for my XS400. Had to buy a pair of new pilot screws. Used the best parts from both sets of carbs to come up with a completely rebuilt set of carbs. Turned the screws out to 3.5 and installed. Same darn thing...once the engine warms up a little if you give it some throttle it will stick at 4000 rpms or so. Adjusting the idle down lowers the point at which the rpms stick.
I figured out that one of my spark plugs is installed with a time sert or some such thing. (not a helicoil, but similar) and it is a little loose and leaking.
I am gonna pursue that situation as the cause of my problem.
Any comments greatly appreciated.

hoganJr
 
I had a similar thing what happened was the throttle cable was unseating from the holder on the carb and getting jammed. will only take a sec to have a look.
 
My '81 model's got the over-revving problem too - sometimes but not always!
Starts and warms up in the drive till choke isn't needed and it settles into 1200RPM. But drive it five or six miles and it starts to idle at 3400. Brake and gear down and it idles properly again - til you touch the throttle - then it runs back up to 3400.
I've got two pairs of carbs and they both do this but one pair much worse than the other.
Maybe related; I have at times found gas in the air filter boxes. Scary!
Installed an inline cutoff and filter and that seems to have helped the gas in the filter thing but the over-rev remains.
If the vacuum hose comes of the petcock the machine backfires and idles rough and wants to die but as soon as the line is restored it's "normal" again.
 
Last edited:
The petcock could have a slow,gradual leak and when it sits would leak and overflow to the filters.Sounds like you need to clean and check your carbs,especially the diaphragms. lha
 
lhaolpa said:
The petcock could have a slow,gradual leak and when it sits would leak and overflow to the filters.Sounds like you need to clean and check your carbs,especially the diaphragms. lha
Click to expand...

I am sorry I have not been following up very well. I asked for help and you all have been helpful.

I did a low-tech, no-cost troubleshooting repair that solved my out-of-control revving problem; I coated the carb-to-head boots with aviation sealant and then slipped a section of bicycle inner tube over each one. Problem solved.

I did not think they were leaking; they fit real tight to the carbs, I sealed them to the heads; they did not appear to be cracked. But sealing them as I described cured the problem completely. One of these days I may even buy new ones.

hoganJr
 
That's right,as long as it's a rider and not a show bike,nobody will notice.I sealed mine with a couple of coats of rubber cement,and you can't tell any difference except that they don't leak. lha
 
. . . I coated the carb-to-head boots with aviation sealant and then slipped a section of bicycle inner tube over each one. Problem solved.

hoganJr[/QUOTE]
-------------------------------
"Aviation Sealant" - is that like Form-a-gasket? What color is it when it dries?
 

Attachments

  • form gask.jpg
    form gask.jpg
    8.2 KB · Views: 272
WulfgartheBlack said:
. . . I coated the carb-to-head boots with aviation sealant and then slipped a section of bicycle inner tube over each one. Problem solved.

hoganJr
Click to expand...
-------------------------------
"Aviation Sealant" - is that like Form-a-gasket? What color is it when it dries?[/QUOTE]

that looks like the stuff I have...it dries black
 
hanging idle can be caused by improperly set float levels, bike will high idle as it runs out of gas, idle drops when bowls refill
 

Similar threads

jvdavidson
Carbs or Coils...or something else???
Replies
0
Views
738
jvdavidson
jvdavidson
Daniel Moulton
New XS500 member, I hope you'll accept me. Starting frustration
Replies
9
Views
2K
Daniel Moulton
Daniel Moulton
tstidham
Re-Ring, Dodge VR and Starter Clutch Springs
Replies
12
Views
2K
tstidham
tstidham
1
Help Request: Good ol' Non-running Issue! Hopefully NOT the usual suspects...
Replies
8
Views
935
10mm_is_bestie
1
N
Hanging Revs Issue! Revs decrease slowly. Help!
Replies
13
Views
4K
amc49
A
Back
Top