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GSX400F hanging idle issues, losing the plot

  • Thread starter Thread starter Kayak23
  • Start date Start date
Kayak23 said:
It's a 4 cylinder and I hope it has O-rings as I've just ordered some :D

I put it back together again today, checked and double checked all the rubber bits, disconnected the throttle and started it up.

I got it to idle and then went nuts with the carb spray. I found that one of the boots I was definitely getting a slowing of revs when I directed the spray behind the boot where the O-ring would be.

So, I'm hoping that changing the O-rings will cure it. Fingers crossed. I would have ordered rubbers too but they were ?30 each and to be honest I reckon the ones on there are spot on. It's more the O-ring perhaps leaking than the rubber.

Strangely though I've just taken it for about forty miles and it totally behaved with virtually no hanging idle.

Weird ......but anyway, I'm really hoping the O-rings are the cause.
Click to expand...

oops,
is it a 16 valve motor?
 
Nessism said:
Leaking intake boot O-rings are one of the most common of all GS issues. I change them first thing every time I get a new GS as preventative maintenance. Same thing for the carb O-rings. They rot and deteriorate, particularly the O-rings on the fuel transfer tubes. Did you take the carbs fully apart, ungang them? You can't replace the transfer tube O-rings otherwise.
Click to expand...

No, I didn't ungang them, but I guess it's not too late to seeing as I've to take them off again to get to the boots.

Anything else you'd suggest doing while I'm in there poking around again?
:)

is it a 16 valve motor?
Click to expand...

Aye, it is.
 
So the bike generally has been running ok and I don't appear to be having the same hanging idle issue as before I fitted the new intake O-rings.

Along with some other kit to tune it up and get it running better I bought a compression tester.

Testing each cylinder I got 90,40,65,75...

I'm thinking that there is something weird going on as you'd think the bike probably wouldn't even start on those numbers!

Warm engine, throttle wide open.

I put teaspoon of oil in the worst cylinder and it bought it up to about 75, suggesting the rings might be bad.

Thing is, this bike has only done 16k. Surely the rings wouldn't have given up just yet?

Next step is maybe get a 'leakdown' testing kit to further isolate any issues.

The strange thing in all of this is that it actually runs relatively well except an unreliable idle.

Frustrating stuff
 
No idea to be honest. Only had the bike a few weeks.

I've got a leak down kit on order and I'll be checking the valves once I watch enough YouTube tutorials to feel happy tackling it! :D

I reseated the valves on my mini back in the day and it actually worked again so hopefully I can do it right.

Main thing is finding parts for these like gaskets etc, so if anyone can recommend a good source I'd appreciate it.
 
I provided a link to the Newbie Mistakes thread one day after your first post in this thread yet it's obvious that you have not gone through it and/or ignored the advice. Your frustration is at least partly your own doing.
 
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Nessism said:
I provided a link to the Newbie Mistakes thread one day after your first post in this thread yet it's obvious that you have not gone through it and/or ignored the advice. Your frustration is at least partly your own doing.
Click to expand...

That's me told! :D
I've been through your thread and it was very nice I thought. This is a forum though and they are good for garnering lots of opinions generally. I'll go through it again though seeing as you're getting a bit cross with me. :p
 
So, leakdown test results just in.

I hope I don't offend Nessism again with this post but here's what I found.
Test done on stone cold engine.

Cylinder 1
Gauge shows less than 10% leakage but rises when compressor motor kicks in. Under 20% at peak.
Air loss can be heard all around the removed rocker cover and at the oil filler cap.
No air loss detected at airbox or exhaust.

Cylinder 2 (the one that read the lowest in my earlier compression test)
15% on gauge rising to about 22% on compressor motor.
Same air loss heard around rocker and at oil filler cap.
Air loss heard at airbox when throttle opened. No loss at exhaust.

Cylinder 3
Low loss reading on gauge, maybe 10%ish.
No loss heard at exhaust or airbox.

Cylinder 4
Low gauge reading again for loss.
Seems a little quieter around rocker.
Seems a touch louder at oil filler.
Nothing at airbox or exhaust.

So it seems like the intake valve on cylinder 3 isn't seating right? I suppose my next course of action is taking the head off and sorting the valve seating.

I adjusted all the valve clearances while I was in there and it seems to run smoother I'm sure but still not quite firing into life like I hoped.

So ultimately, am I likely to do lots of damage if I ride it as is?

Cheers.:)
 
Not too sure if the high idle is solved. It's certainly not doing it as much but I'd describe the idle as erratic now.
It's very sensitive to idle adjustment and will often be too high, to the point that it feels like I have no engine braking, or the other way so the bike just fizzles out at traffic lights etc.

Strangely it seems to go pretty well. Just got back from a blast and it's generally ok, but still dies if you don't nurse the idle.
 
I have had this same problem on at least 2 of my bikes.
I to be honest can never put my finger on one thing that fixed it but i regiment-ally go through all tuning again and again.

I think i am useless at using my vacuum guages at least on a engine not run for years and only when i bench sync the carbs sync did i start
to get all the parts working right.
re check timing, valve clearances, plugs, ht leads, coils, mixture, airleaks around carbs

think you are getting close - hows the carb sync ?
 
well I synced the carbs yesterday with my gauge set via eBay and got them all close to each other.

I think the thing for me is knowing what order to check things. Check something, rule that out/fix that and then move onto the next.

At the moment because I'm rusty and not so experienced I'm probably chasing my tail a bit.

Having said that though, my strip and rebuild of the carbs, bench sync and general tinkering have defo improved things but it looks as if I'm going to have to take things a bit further if I want to get it all working as best as it can.

I think that's going to involve taking the head off and seating the valves perhaps.

Also, my last bike, some 16 years ago was a lovely 1974 Z900, and I have to remind myself that this is a little 400 and will never pull quite so well... :D
 
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I at this point always go over what's done.
Doesn't hurt and doesn't take long. Need to be exact.

Timing
Plugs
Mixture
( You said valve clearances all ok )?

How are these and how are the plugs after a run
with them all set ?

P.s I agree with everyone else but trying to restart
and check all basics ...
 
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