RPM hesitates after 5k

phatman1969

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Newbie here, this is a great forum and I appreciate all the free advice. This is my second xs400, had one a while back and when i saw this one on craigslist I had to buy it. It is 1982 XS400K DOHC. Ok, got the carbs clean today. Bike now runs well up to about 5000 rpm and then starts popping a bit and losing some power. Let out of the gas and when the rpms come down gas it again same problem around 5 and above. I bought it with the pod filters on it, and the pipes cut.No idea what jets are in it. I have been playing around with the screws, thought I had it dialed in.Not sure what to do next. Any help out there?
 
From what I've heard that can be a vibration issue in the boots, or you can check the condenser but it's "never the condenser" according to Dan's MC. (You should go on Dan's MC if you get the chance.

Lastly, float bowl height!

Good luck.
 
Thanks for the reply. The boots are good, snug, no cracks. Will tear em down again and check the float bowl height. Any recommendations on what it should be set at?
 
Sounds like your main jets aren't big enough. Check your jet size (by all means, check your float heights first, as it's easier), but the pull your carbs, and check the little numbers on the main jets. You may need this guide to help identify if this is your first attempt at looking at carbs: http://www.amckayltd.com/carbguide.pdf

Report back with your numbers. If the PO switched to pods, they may not have increased the jets, and once you stop using all that byzantine plastic, you definitely need bigger jets to deal with the bigger airflow.

Oh, the guide says for XS650s, but we use the same carbs (with different jet sizes, but the theory and pictures all work for the years in question. You have BS34 carbs, if they are stock, and they probably are.)
 
Yes, it's electric ignition.

Floats should be set for "3mm +/- 1mm below the carburetor mixing chamber body edge" (from the service manual) which means 3mm +/- 1mm below the top edge of the float bowl.
 
Yes, it's electric ignition.

Floats should be set for "3mm +/- 1mm below the carburetor mixing chamber body edge" (from the service manual) which means 3mm +/- 1mm below the top edge of the float bowl.

thought all our ignitions are electric! :laugh:
 
Hello again everybody. I finally got around to pulling the carbs apart again today. Was a little surprised by what I found. Left carb had 125 main jet, right carb had 115 main jet!? What gives, I thought these things came with 135s. Its a 1982 DOHC. This lead me to a few questions upon tearing them down. Any help is appreciated very much.
1. What parts make up the float spring assembly? You know the springs that hold up the little metal tab on the float.
2. What is the proper float setting for plastic floats?
3. Do I need to change my pilot jets?
4. Big confusion here! Are there supposed to be rubber plugs over the holes that hold the pilot jets. Some say yes, some say no. The carb manual for the 650 that is on this site says yes. I dont have them so not sure if I am supposed to.

Thanks again for all the help.
 
re float bowl height. The deflection should be the same for both and the spec would be in your manual for your year/model bike.
 
Main jets should be 127.5 L and 117.5 R, if you still have the standard intake. Pods go larger.

1. Float spring assembly? There should be a float with a pin that holds it in. A metal tab adjusts the float height. The float valve has a tiny spring in it (the valve looks like a little torpedo), and if that's shot, then it needs to be replaced (inexpensive part.)
2. Hmm... let me look at my previous post. Yep, still the same. Floats set to "3mm +/- 1mm below the carburetor mixing chamber body edge."
3. I don't know - are they broken? Have you inspected them? What do you see?
4. I don't have plugs. I suspect they were drilled out long ago. They are not required.
 
First thanks for the quick reply. I should have been more specific. I have the floats, and the pins, but my float valve is the item in question. It sits down in a hole and when you pull it out there is no rubber tip, screen, or anything else. Just a torpedo thing with a metal point that goes into the opening and then a little tip that contacts the metal float tab. The float valve doesnt have the little thin wire that wraps around the float tab. Do I need that? And is anything else in the hole the float valve sits in?
2. Sorry for the repeat question, thought it said brass floats in the earlier post. My bad.
3. PO installed the pod filters. My pilot jets are fine just didnt know if they require changing using pod filters.
4. Maybe I am confused. I am talking about the pilot jets on the carb opening not the idle air screws. The 650 manual says that the pilot jets have a rubber plug over them that must seal tight. Am I looking at the wrong thing?

thanks for the info on the main jets. The PO did a bunch of weird stuff on this bike and I am still trying to clean it up.
 
No worries.

My float valves don't have the wire. The important thing is that the little nub moves and springs freely. I found out mine didn't when my carbs went into overflow mode and dumped gas all over my leg. Honestly, when I checked them they seemed fine. As I said, they're a cheap part, and an easy replace.

Ooookay. If you're already running pod filters, then the fun begins. Tuning the carbs will take a little bit of trial and error, and will be unique to your bike, altitude, humidity, etc. Tune the main jets first by testing under WOT all the way to red line (try 3rd or 4th on an on ramp). If you pull strong all the way through with no hesitation, then you're good for those (and likely your needles too, which are much tougher to modify.) Pilots are tested really after you have your mains right, and can be diagnosed by how well you start, idle, and take off at a light. If you feel like you have no power down low in the rpm band, then play with the size of the pilots. If you pull from idle, though, then you're probably fine. I don't think pods have much affect on pilot jets.

#4 - a picture maybe? I'm having trouble picturing what you're referring to.
 
Here is what the 650 manual says about the pilot circuit. I just copy and pasted this directly from the manual.The BS38 pilot jet seats inside the float bowl. It is the small brass fitting with a slotted
head. Above the pilot jet in the BS38 there is a recessed area in the float bowl casting,
and above that, a small hole in the carburetor body. This is where the pilot air passage
emerges after running through a tubular casting. On the other side of center, the main
air jet feeds into a similar hole and casting. In BS34 carbs the pilot jet is under a rubber
plug at the end of the smaller vertical tubular casting in the carburetor body; the plug
must seal correctly.


Does that make any sense? I know I dont have any rubber plugs down there.
You made sense with the float valves so I ordered the new ones last night. Going to pull it all out again and make sure its all there and clean.
When I got this bike the pods were on but air box was still in there and half hacked up so I felt no choice but to just take it out. Couldnt use it anyway.
Thanks again.
 
mine has a brass end screwed on the pilot jet.

maybe Mikuni skimped and decided to push a rubber bung in there instead!
 
Whats up everyone. Finally got back in town today and put the new mains in, also installed the new float valve needles and adjusted the floats. Right after putting the floats back in I had gas dumping out of the air filters. I finally got them right. Bike started fine, idles, but cant get full power. Max out at around 4,000 rpm and weak power. Played with the mixture screws and still no real power. Checked carbs again to make sure all clean they looked good. Started it up again and it idles fine, you can throttle it up to around 3500-4000 and then the rpms start bouncing up and down from 3000 to 4000 quickly and wont stop as long as you are holding the throttle. Release it and it idles again. Do you have any ideas what may be happening? I put 140 main jets in it because it is what they had in stock. Any help is appreciated from anyone out there. Oh, yeah it 1982 XS400 DOHC.
 
One more thought. Stupid question. Is it possible that I reversed the spark plug leads when i put the wires back on? It shouldnt run at all with the leads on the wrong plugs, right? I think it would just backfire. Grasping at straws here cause I am going crazy. Another 85 degree day without riding. :(
 
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