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XR 400 breathing problems

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Fitted a 17.5 pilot jet today....made no difference to bog....even with pumper adjusted to come on as early as possible, if I whack open the throttle when coasting at low revs it bogs and will almost die....the same happened with the 15 pilot jet.....the bike starts ok hot or cold...

Edited by oakframer

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Why do you have the pump set to come on as early as possible? I suggest setting it to come on as late as possible, having the screw all the way in, bottoming its spring.

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+10000000 you need to set it to come on as late as possible.

Doing what you described, going from real low to full opening, will almost always cause a bog. Let the motor build a few rpm's before full opening. I'm talking less than a second to roll the throttle to full open. If the motor is in a high gear and under load it can't just instantly rev to max rpm. If you still have a bog when rolling on like that, then you still need some adjustment.

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Thanks for the good advice. I was playing with pumper come on positions I have re-set to come on as late as possible again now ,screw all the way in .

 

Last year bike was running fine with Honda stock end can baffle removed and tm36 pumper carb  the pumper adjustment screw screwed all the way in .All these problems started after I left bike  in storage over a very cold  winter in the garage , I fitted a new FMF power core 4 pipe in the spring  and then when I tried to tune it up with fuel screw I broke the tip of the  fuel screw off leaving a bit in the carb .it must have seized in there :-( and was  blocking the passage. This was the cause of the popping on decel ,I removed the broken fuel screw tip with a ground tapered steel reamer , (It was a final resort) that was the same size as the tip and the passage is now clear. So, I ordered a new fuel screw and fitted it with the spring washer and o ring . the bike starts fine and will tick over fine too, if I wind the screw in the revs climb if I wind it out they drop. If I do the bog test from low revs in first gear the bike will wheelie no problem it will pull the front up  in second gear also, the intial off idle response is fine. Mid to top end the bike sings and if the revs are up throttle response is great . . The problem occurs just  after 1/4 throttle opening , if I open the throttle very quickly I get a terrible bog , like I said before if I coast down hill then grab a handful of throttle the bike bogs.when I close the throttle it picks up again. Is this a lean or rich condition or is it the result of removing broken fuel screw. It sucks but , I wonder If I damaged the carb, but surely above 1/4 throttle the pilot screw has no affect does it ? I notice the bog if I slow down to go into a corner and open up quickly the bike bogs....I thought the pumper would assist here ? As you can see I am grasping at straws here...LOL ...I dont know if I have damaged the carb or I have another problem going on.....!!  This is a great site and people here have been very helpful thank you.

 

 

2003 XR400 FMF Powercore 4 tail pipe

Mikuni  TM36 pumper carb- Main jet 130 -pilot jet 15 - Needle  9 DZH 5-50- clip in middle position

Edited by oakframer

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Float height has a big effect on your jetting and being able to tune it. Last post I saw said you cleaned the inlet needle passage but left float level as is, which may be wrong. You need to check that. From what I remember, (it's been a while since I set mine), the float height figure given in the .pdf is wrong for our use. The picture shows the float level with the bowl mating surface, but the measurement is wrong. Just set it level as shown.

 

Something else I"ve thought about, since this is a used carb, the pilot air jet most likely is the stock 1.1 and would not have been changed by the PO. But the position of adjuster screw #2 could have been altered. Measure how far above the carb top cover it is sticking out.

 

Did you retune the fuel screw after putting in the 17.5 pilot?

 

Yes, it's possible you damaged the fuel screw bore with the reamer. Hard to say. If you removed any material from the bore while getting the broken tip out, enlarging it, the new fuel screws tip isn't going to meter flow the same as before, it will allow more fuel to pass at the same given position, as it did before the repair, so to get the same amount of metering for a given screw position before the repair, the screw would now need to be turned in more. So what once was a normal adjustment range of 1-3.5 turns out, might have gone to a new normal of 0-2.5 turns out. Don't know. The cross section relationship between the fuel screw tip and it's bore and how it seats in there has a lot to do with how it meters fuel also, and if you altered that............?

 

But let's not worry about that now, try to eliminate a wrong float setting and adjuster screw #2 position for now.

 

I'll say, you sure have me scratching my head.

Edited by Trailryder42
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Thanks very much  Trailryder42.

 

I had another look at the carb today and checked the float height in the diagram they suggest inclining the carb between 10 and 20 degrees and then setting float height to 19mm plus or minus 2mm so I set heigh to 20mm and will see what happens. Incidentally I retro fitted the stock carb Keihin carb to test if it was a problem with the bike rather than the carb. it runs fine, apart from the off idle bog of course , but power everywhere else is smooth. So it is definitely a problem with the Mikuni. Checked adjuster screw 2 it sticks up above the carb top cover by 6mm or 1/4 of an inch...

Edited by oakframer

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The key to inclining the carb while checking/setting the level is to be sure that the weight of the float is not compressing the little spring loaded nub in the end of the inlet needle. You want the float to just seat the needle in its bore but not be compressing that nub. You hold the carb at whatever angle it takes to acheive that, then take your measurement.

 

And that adjuster screw #2 should measure 9mm sticking above the top cap.

Edited by Trailryder42
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 Checked adjuster screw 2 it sticks up above the carb top cover by 6mm or 1/4 of an inch...


Edited by oakframer

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Thanks Trailryder42 I have wound out  the adjuster screw #2 should it now measures 9mm, I have a question , my jet needle   9 DZH 5-50-  has the clip in middle position is it worth moving it ....raising or lowering it one notch..?

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I had lots of problems with my Mikuni carb. The problem was the tip of the fuel screw broke off. I am not the only one to have this problem. You can remove the fuel screw, being careful not to lose the washers and o ring. The fuel screw should have a long thin tip. If not, look in the carb intake throat where the fuel screw goes in below. There should be a pin hole. If it is plugged with brass, your fuel screw broke off. I pushed the broken tip out with a dental pick.

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I went back over this thread from the beginning. No where do I see you mention what air filter you're running. Can't believe I didn't catch it earlier. You mention Gordon Mods done and I guess I assumed you've replaced the stock air filter with aftermarket, like the UNI and that the snorkle is removed from under the seat. Is this right?

 

Was the FMF new when you put it on?

 

Sure, you can try altering needle clip position, but you shouldn't need to. Given the way you say it's acting, I'd go first with raising it one position(richening).

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Hi sorry, yes its a Twinair filter ...all the airbox mods done but  welds on headers not ground. The FMF Powercore 4 pipe is brand new...

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Non restricted cage ....different to stock one 

Edited by oakframer

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Ok, so all aftermarket. But, there has to be some kind of cage that the foam filter covers, so it holds its shape and makes the filter capable of being fastened to the airbox carb inlet without collapsing.

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yes plastic cage but nothing like the stock one.....Thanks for your time Trailryder42.....

Edited by oakframer

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Good news !! with the good advice and help from here I seem to have sorted some of the problem(s) out....

 

These are the following things I did to the carb before I re-fitted to the bike -:

 

1) wound  out and re-set the adjuster screw #2  it now measures 9mm (was set at 6mm)

 

2)Jet needle 9 DZH 5-50- now has the clip in  the forth notch position one below the middle position , raising it (originally in middle position)

 

3)Checked and re-set float level  it is now set at 20mm (was set at 19mm)

 

4) Pumper come on screw wound all the way in, set to activate as late as possible

 

5)Pilot fuel screw replaced with new screw spring ,washer and o ring and set at 2 turns out

 

6) header pipe connection to exhaust manifolds checked , header pipe connection to FMF muffler /tail pipe checked and found to be slightly loose ,FMF make an adaptor which loosely fits over the stock headers and fits their pipe, I smeared some high temperature silicone on this joint before re-assembling it just in case.

 

7) CHecked all breather hoses and pipes on carb, main fuel pipe from petcock to carb found to have slight split in the area where it fits over the metal tap( the ribbed bit) not leaking fuel but replaced with new fuel pipe. Air and fuel ways cleaned /blown out, also pumper plunger and spring remove and checked for spray operation ,spray nozzle aimed at to spray at needle. Main jet 130 and pilot jet 15 fitted.

 

8) Carb fitted on bike connections screwed up and checked for air tight fit on both manifolds , inlet manifold gasket replaced with new one.

 

9) Throttle cables checked for proper operation open and closing of slide and not sticking when bars turned in either direction.

 

10) Fuel tank flushed out and cleaned, filled with premium fuel 98 RON.

 

 

 

Ok so deep breath and started the bike , almost no choke needed ,took a few kicks, let it warm up and rode....well what a difference !!..I did the bog test off idle,. The  front wheel lifts up nicely in first and rises in second gear too , there is a very slight hesitation before the revs pick up but ever so slight  and in real riding conditions not that noticeable . Mid to top end power smooth  all the way no bogging at all from quarter throttle position now like there was, nice power band , no popping on decel either.Bike is quite fierce now ,no flat spots apart from that initial hesitation. I think this is the best I have had it ever....dare I say it...!!

 

So inspecting the bike I found a few things that need sorting (there's always room for improvement)

 

1) Fuel screw adjustment after 'repair' new setting 1 to 2 turns out ,need to play with this more cant find a 'mid point ' revs climb when screwed  in and bike will die when wound out more than 4 turns . Would fitting 17 pilot help this ?

 

2) Bike appears to be running rich, some  black smoke when revved up...but mostly after idling for a while  ,it doesnt stink of fuel when I ride it however ..but seems to use fuel up, perhaps it was the way I was riding it !

 

3)With bike on side stand  ,fuel turned  on and not running,  fuel is leaking from overflow pipe connected to  bottom of carb bowl , I suspect float is too high ?

 

4) After riding there used to be some black soot around the exhaust pipe hole on the muffler ,  now there are less deposits than before

 

5) Bike seems  hot , oil cooler hot , weather temperature is  about 29 celsius 84 fahrenheit, perhaps this is  paranoia .!!

 

6) Checking pumper operation with bike switched off I notice when throttle opened and white plastic lever (59) compresses pump rod (66) it seems to hesitate, move slower than the throttle it doesnt snap like the slide , almost like you open the throttle but the lever moves slower for its last part of pushing down the plunger is this normal ? I seem to remember it always was like this.

Edited by oakframer

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Good news.

 

#1 The Mikuni is notorius for vague, detectable fuel screw adjustment at idle. When adjusting mine, I come out with it like you have til idle starts to slow, then back in slowly til it rises back to the speed that it seems to keep all the way until you're almost closed with it. Which usually ends up being around 3 turns out. Then adjust the fuel screw for best response off idle while actually riding it. That's my one peeve with the Mikuni.

 

#2 Get back to this after clearing up the others

 

#3 Float is too high or there's crud between the needle and seat or the needle tip is worn and is bypassing fuel, even when the float has it shut off. Tell me, how  was the float oriented after setting to 20mm?

 

As I said before, I remember the figure given in the .pdf ( 19mm +/-2mm) to be wrong. The float needs to be parallel with the bowl mating surface. As shown in the .pdf, that position would be 19mm, but from what I remember, it's not. And your setting of 20mm would be 1mm lower, which should preclude the possibility of it being too high for it to overflow, but it's not. So, deductive reasoning suggests the figure in the .pdf is wrong, or one or both of your floats have a hole in them and they are filling with fuel, preventing them from doing their job.

 

#5 Just be sure your oil level is correct. It's an air cooled engine, it's going to seem hot. A hot oil cooler says it's doing it's job. For peace of mind and to know your engine oil temps, find yourself an XRs Only oil temp dipstick. But one way to tell if your oil is overheating is if you get a vapor or burning oil smell out of the crankcase vent line(that big nipple on the top of the crankcase). A full synth 15-20wt oil is an XRs friend.

 

#6 Yes, that's normal. That screw #2 that you adjusted back to the stock 9mm, sets the stop for how far in the throttle range the pumper works. Someone decreased that range. Since the stop for it on that white cam is very close to center of it, even small adjustments in the screw have a big effect on throttle range cutoff. That was initially one speculation of mine as to why it was stumbling at just past 1/4 throttle.

 

Answer #3 and we'll go from there.

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3 Float is too high or there's crud between the needle and seat or the needle tip is worn and is bypassing fuel, even when the float has it shut off. Tell me, how  was the float oriented after setting to 20mm?


 


Hi Trailryder, The float was set by angling the carb between 10 and 20 degrees then I measured 20mm from the carb casting the bowl mating surface  (where gasket sits) to the edge of the float. The float is not  set parallel to this it is inclined , it rises up at one end.....it does not look like the PDF diagram . If yo look at that diagram my float is rising on right hand side of the drawing it is not parallel with the bowl mating surface / casting of the carb,  it was at that point I measured 20mm.


 


Incidentally, when I first took the bowl off the carb the floats where set like this and they measured 19mm from the bowl mating surface ,I assumed this was the factory setting...?  I re-set them at 20 mmyesterday, this afternoon I set them back to 19mm again (they are still inclined not parallel to the bowl mating surface) there is no sign of flooding as yet and bike performs as described in above account...


 


cheers.


Edited by oakframer

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My concern is that the picture in the .pdf showing the carb at 10-20* would normally have the weight of the float compressing the inlet needles spring loaded tip at that angle. In reality, if you look at the picture I've included, the carb would need to be held closer to the angle shown to prevent that. Then with the carb held there, bend the tang that the needle hangs from to acheive a "parallel with the mating surface" level. Throw the measurement and carb angle in the .pdf out the window.

 

 The .pdf was written for an HS40 carb for Harley motorcycles, but we use it because it's the closest thing to our TM36 in referencing its parts and setup. But it's not the gospel.

Mikuni Float.JPG

Float Angle.JPG

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