Idle problems, vacuum leak?

[berend2712]

Hello fellow xs400 enthousiasts,

I had a thread earlier with some issues I wasn't able to resolve, and unfortunately they still haunt me. It seems to be a fairly common problem: when the bike is warm, it won't idle properly, which means I have to keep it revved at about 2-2.5k rpm, taking a lot of attention when I ride. The problem seems to get a lot worse if the bike gets warm, after about 45 minutes of riding or so.

Since the last time I posted, I managed to fix all the holes in the exhaust. There was quite a big gap in one of the mufflers, so I figured that would be a large part of the problem, but alas, it seems to have gotten even worse now.

As I understand all the posts here, I'm probably dealing with a lean condition in the idle circuit, due to a vacuum leak or maybe incorrect idle screws. The fuel setup on the bike is all stock as far as I can tell. Problem is, one of the idle screws is stuck and stripped and I will take them out at some point, but I'm a bit apprehensive to, because I'm afraid I'll do more damage than I'm fixing. So before I go in, I would like to rule out other possibilities. The one that wasn't stuck was set at 1.5 turns out, so I assume the stuck one is as well.

I have extensively checked for air leaks, and I'm quite certain the boots, throttle shaft seals, and vacuum ports on the exhaust and the air intake aren't leaking (wd40 spray test). I did find some pinholes in the carb diaphragms, but the slides do stay up if I put my finger on the intake port. I think the pinholes might be the cause of the leak, but as far as I can tell from the diagrams, the vacuum is not connected to the idle circuit, so it couldn't be the cause of the leak.

Also at higher revs the bike performs fine, which makes me think that the problem is in the idle circuit only. Carbs are clean (although I don't know what hides under the stuck screw). Haven't synced the carbs yet, but the exhaust pressure seems very equal and the fine test is something you do after all is fixed anyway right?

Sorry for the long read, lot of information, but I can't decide what the important bit is

Do any of you have suggestions for what I should try to fix first? Diaphragms or mixture screws? Any other things I should check?

Cheers

Berend

 

[bcware]

Synch carbs
Adjust valves
Adjust timing if applicable
Verify coils are good
Replace/fix exhaust
Remove/fix mixture screws replace o-rings if applicable
Fix/replace diaphragms
Trim/replace ignition cables
Replace spark plug boots
Replace/verify battery is good

There is probably more than this, but if you haven't addressed all of this yet there are a lot of loose ends to tie up before anyone can say anything for sure.

The best way to diagnose problems is to knock off all of these standard tune up items one by one. Approach things systematically.

 

[berend2712]

Thanks for the list .

Anything specific on that list that would cause a poor idle but otherwise OK performance?

Cheers,
Berend

 

[bcware]

All of them are possible to be honest.

If I had to pick things to check first it would be carb synch, carb cleanliness, idle mixture screws (settings and o-ring leaks if applicable), timing, and valve clearances. Valve adjustments should be done before syncing the carbs or fiddling too much with mixture settings.

1.5 turns out at idle is fairly conservative and likely lean.

 

[berend2712]

I adjusted my valves (with shims) about 2 months ago, so they should be good. Timing is electronic, so I can't change that.

I'll start by looking into synching the carbs and trying to extract the stuck mix screw, maybe there's an air leak in one of the o-rings.

Thanks for the tips

 

[bcware]

Yes, an air leak would do it and synching the carbs makes a big difference if they are out of spec; it is the only way to get a nice, steady low idle speed.

 

[berend2712]

Have been travelling for a while so no real time to work on it much, but I started thinking it might be actually be something related to the ignition like bcware suggested. Checked it yesterday, and it seems that if it gets hot the right cylinder hardly makes a decent spark. I could get it to change a lot by pushing the spark plug cap in a certain way, so I think I'll just change all of that out and see what happens. Funny thing is that with the spark plug cap held in that way, it started climbing in rpm through the idle again (the archetypal vacuum leak signal if I understand it correctly), so that gets me back to my stuck mixture screws and diaphragms.

Has anyone ever used an extractor on these idle screws with succes? I am a bit hesitant to try because if I mess up I can go look for a new set of bodies. Do I have to use a left handed drill bit to prevent the screw from tightening more and breaking off the tip?

Thanks for all the help guys, greatly appreciated!

 

[berend2712]

So I decided to give it a go with the easy out. Someone at my work promised to help me, it started out great, we used the drill press so the hole was straight and centered, started turning the easy out, and the screw started to move. The joy was unfortunately soon followed by the snapping sound of hardened steel, meaning I now had a small plug of easy out stuck in my idle screw.

A quick search on the internet revealed that the best thing to do was "cry, go buy a new set of carbs".

So after a lot of drilling and trying to get it out (unsuccessfully), we went to the machine shop at my work and soon after, all the people working at the machine shop were trying to help me mill out the hardened steel out of the screw. A carbide bit later, we finally got it out, at which point we tried a bigger easy out, it didn't move. At that point I decided to ask the machine shop to mill out the rest of the screw so the bottom part would come off, and there was only a hollow brass cylinder left in the body (see pictures).

Then yesterday I took a really small file and with about 2 hours of filing managed to carefully make two small slots so I could lift out the screw and it came out .

All this drilling, milling and filing does mean that the threads are in quite a rough shape but hopefully a tap will clean that right up.

So now on the hunt for an M6x0.5 tap in Vancouver, BC. Anyone living in vancouver that can recommend me a tool shop where they might stock specialty tools like that? I can order it, but it'll probably take a long time to arrive .

Anyway, good day. I think the takeaway lesson is that you don't give up and keep trying .

Cheers

 

[BBS360]

Damn, I'm on the ferry to Vancouver right now. I have a couple different M6 taps.

KMS tools might have it, though they don't list it online.
The local SnapOn truck could be an option.
Acklands-Grainger may have them in stock.
A fastener store might be a good bet.

 

[berend2712]

Thanks for the advice, I will make a couple of calls tomorrow and see if I'm in luck. The machine shop at my work might have it as well, but nobody was in today unfortunately.

Any chance I could use your taps (if you have the right size) in exchange for some beers?

Cheers

 

[BBS360]

Sorry, the damn was because the taps are back home in Victoria. Just came over for the Abbotsford air show.

 

[berend2712]

Ahh that's a shame. KMS don't have them unfortunately and the rest is not open on the weekends, so I'll ask the guys at the machine shop on Monday and if they don't have it I can try the others. Thanks for the advice and enjoy that air show!

 

[berend2712]

So I just tried to put one of the old screws in, and it screws in and out all the way without any resistance. Would it be a really bad idea to skip the tap and just put the new screws in there?

Cheers

 

[berend2712]

So I finished another session of carb cleaning, checked the float level and then put everything back together and did some adjusting when it was idling in the yard, seemed to run really well also when it got warmer. Idle screws are now at 4 turns out. Haven't synced the carbs properly yet (this is next on my list), but trying to find a sweet spot by just listening to the idle made a big difference. Had it run for 3 times 10 minutes with pauses to not get the engine too hot, and checked the plugs, color looked good to me.

Went for a testride, and then found that it still hangs at 3k sometimes, and at other times it will just die. At some point I was at a traffic light, it was slowing down to below 1k, so I gave it some gas to not let it die and it took a good 3 seconds before anything happened (this was new to me?).

It feels like sometimes it runs on one cylinder (when it dies), and then sometimes the other one wakes up and it starts revving to 2-3k. It almost feels like the spark plug cable is a bit dodgy or something (like broken earphones that come on every now and then), but it's very irregular, making it hard to check. I'll try if I can find what's wrong there and then I'll do some adjusting and see what that does. If anyone has any comments on my reasoning I would be glad to hear it.

Last thing: I ordered a set of diaphragms from jbindustries after reading lots of good things about them here, but holding them next to my current ones it seems that they don't allow the gas slides to go up as high as the OEM ones, dit anyone find the same? I'm now in doubt wether to change only my pinholed diaphragm, or both (like I planned to).

 

[xschris]

Still look a bit rich. Do both diaphragms at the same time. I have never used aftermarket ones so I can add any info on how they work. Make sure to sync the carbs with a manometer before you do other tuning. A balanced set will act different than a unbalanced set will.

 

[berend2712]

OK, will do. For now, I checked and confirmed my right spark is waaaay weaker than the left one, so I'll try and find what's wrong there. I think the whole idle thing gets very complicated because the right cylinder just doesn't (or hardly) fires at idle. The idle is basically the same if I have the right one unplugged.

Any comments on the difference between OEM diaphragms and jbindustries ones would still be appreciated if anyone knows .

I will let you all know how it turns out.

 

[berend2712]

Well I found the problem, right hand spark plug cap was connected to the cable through about 3 mm of corroded gunk. I'm amazed I was even able to ride so well with this (cap plus cable measured inf on my multimeter). Attached a picture of the plug in question.

Also, I thought the DOHC had one dual ignition coil, mine clearly has two . Any idea what these should measure? Both are primary 2.7 ohm, and secondary 9 kohm. Because they're consistent with each other (and the left one sparks well) I'm going to assume for now that the coils are OK, but if anyone has some info on it I'd be glad to hear it.

Cheers .

 

[xschris]

Get some new copper core wires and caps

 

[berend2712]

It worked! And for some reason it has also seemed to fix my hanging idle problem. It doesn't die anymore at traffic lights. I'm riding with confidence again, luckily there's some summer left .

Now I can move on to synching the carbs and some fine tuning, but this was definitely a major fix.

Thanks for the help everyone