I am trying to sort out the jetting on my 36mm Mikuni pumper carb. When I back off the throttle, I get a fair amount of popping. I upped the pilot jet from 15.0 to 17.5, but now it runs inconsistently in the low end and still pops.
Specs:
XR400, snorkle removed, Uni-filter, Vortip exhaust.

Mikuni:
Main 130
Pilot 15.0
Needle 9EBY1-50
Needle jet P-4

Any help would be appreciated.

Man, I'm going way out on a limb here because its been a couple years since i've looked in my carb, but I'm pretty sure that I'm running a pilot jet that is in the 40's.  A 17.5 sure seems lean which would account for some popping.  Are you reffering to your XR400?  Anyway its 5 in the morning and realy tired so its most likely me.
Good night!

You didn't list other specifics like, elevation, temps, needle clip position, etc., these all have to be taken into acct. Are you sure you have a P-4 needle jet? It should be a P-8, and clip should start in the middle position.

[ April 07, 2002: Message edited by: ochster ]

You are right.  As I logged on, I remembered:

17.5 is pretty big for a Mikuni carb pilot.
17.5 is suitable for a moped for a Keihin carb.
I ran a 60 in the Keihin carb.

Are you sure about the P-8 Pilot Jet?
I know it's a P-4 as I pulled it yesteday.
A P-2 was shipped as a spare.  I would think the needle jet would affect mid-throttle more than low throttle, but I will consider your input.

I did a lot of searching on rec.motorcycles.dirt last night.  Opinions for a "popping sound when chopping the throttle" range from "too lean", "too rich", "check for intake leaks", "check for exhaust leaks".

===============================================
Re-posting with more data below
===============================================

I am trying to sort out the jetting on my 36mm Mikuni pumper carb. When I back off the throttle, I get a fair amount of popping. I upped the pilot jet from 15.0 to 17.5, but now it runs inconsistently in the low end and still pops.

Specs:
XR400, snorkle removed, Uni-filter, headers ground, Vortip exhaust.

Mikuni:
Main 130
Pilot 15.0
Needle 9EBY1-50 (3rd position)
Needle jet P-4
Air jet: ?

Location:  S.F. Bay Area
Elevation: Sea level
Temp:      70 degrees

Any help would be appreciated.

I have the same setup with a bakers super trapp with 12 discs the carb came with a 130 main and a 17.5 pilot it ran fine for a year untill today when i started to get alot of popping when decelerating no bogs though,but the pop and gurgle concern me. my carb came with no manual and i have no carb experence at all. i will soon if you find more out let me know.

Kevin,

For some perspective, I have used the P-2 needle jet with the 36mm Mik pumper and a 9DZH6 needle, on a KLX250 jetted for race fuel. On a pump fuel, sea level XR4, your carb., should have been set-up with the P-8. Of course you could fine tune from there for conditions, ie; elevation, mods, temps, fuel, etc. Beyond that, if your getting a sudden popping, I would check the pilot jet for obstruction. A 17.5 pilot on your machine should be the ticket, considering your conditions.

The truth or better yet accuracy, can sometimes be very difficult to find. A good place to start on such topics is DirtRider.Net, not all websites are created equal.

DaveO,

Have you talked with XRsOnly? Interesting that your bike ran fine, then started to pop when decelerating.  I am wondering if the popping is a symptom of a clogged passage?

DaveO,

I called Sudco, who distributes Mikuni carbs.

They said popping in the exhaust could be a sign of a leak in either the intake or exhaust.  Or possibly a lean condition.

Since your bike suddenly started popping, an air leak might be your trouble.

I'm going to check mine tonight.  My methods:

Exhaust leak
Start the bike.  Take a damp rag and hold it over the exhaust.  Look for puffs of smoke near the headers and header clamps.

Intake leak
Start the bike.  Spray some Chem-Tool carb cleaner around the intake manifold and airbox boot.  If there is a leak, the bike will stumble. My car mechanic freind recommended this rather than WD-40 as it doesn't combust as easily.

FYI:
Stock Sudco jetting
main 127.5
pilot 12.5
Needle 9DZH-50, 3rd clip
Needle jet, P-4
Slow air 0.9 (intake bell mouth)
1 turn on fuel screw

The info you have posted as "stock jetting", is not for the XR400. You are either talking to someone that is not aware of the XR400 specs, or you are getting them out of the Sudco manual, wich was not meant for this application. Those specs are not correct!

Call Sudco back and ask for "Steve".

The air leak is a good place to start. could even be a valve staying open ,,adjustment of the same,,or a clogged low speed passage.  The trick is of course to find out what has changed so drastically.

While you are there check your float needle and setting.

I just called Sudco.  There is no "Steve" there.

I thought "Steve" was the guy to talk to at XR's Only?

Jeremy

That is true, Steve Aviliano is the "go to" guy at XR's Only.

I had talked several times, with the offroad R&D guy about some custom needles. I had wrote Steve down as the guy. But Steve from XR's Only was who refered me, I may have been screwed up with the names. Now that I think about it, it may have been Scott. I will find out tomorrow. Can't believe I'm actually devoting this kind of energy to this place, the dissinfo here is incredible. I just won't be responsible for adding to it.

Thanks Ochster, I for one really appreciate your knowledge and help, and I'm sure many others here do as well.

Jeremy

I called Sudco and asked for Steve as that is what was posted.  Thanks for the correction.
"Dissinfo here"?  I just posted what Sudco quoted me.  I am not making it up.

I for one would appreciate getting the corrected or best available information.  

I took a look at the jetting specs from a TM36-68 carb from a Mikuni sales guide. (Note this might be a generic set up, not specifically for the XR400.)
Main 130
Pilot 12.5
Needle Jet 568 P-8
Needle 9DZH6-50
Throttle valve TM36/137 2.0
Air Jet 1.0

If someone talks to Steve or Scott at XR's Only, I would like to hear the results.  All I want is to jet my XR400 properly.

Pumper Timing
I read an interesting article on the Yamaha YZ250 forum on the "PK mod".  From what I gathered, they are adding the pumper actuation cut off adjustment screw that comes stock on the Mikuni.  The article talked about delaying the start of the stream until the slide lifts out of the way of the gas spray.  (Makes sense).  Then making sure the stream only lasts about 0.3 to 0.4 seconds.  Yet another thing to play with when I pull my carb the next time.

The saga continues...

I called XR's Only today.  Steve no longer works there.  I don't know who I talked to, but he was EXTREMELY friendly and helpful.  He talked with the mechanic, who said the original jetting specs I posted were correct (130 main, 15 pilot, 9EBY1-50 needle, P-4 needle jet).  If anything, it should be on the rich side.  (125 main and 12.5 pilot being better for a stock exhaust) An air leak is suspected, so I will install new intake and exhaust gaskets next. (Under $10.)

YEARS LATER...

The carb still pops a lot when I back off the throttle.
Question:  Could this be due to the float set too high?   I set it to 19mm, but I noticed fuel over flowing as I tipped over the bike slightly to put it away.

There the thread above left off.
So I replaced the intake and exhaust gaskets.  That did not help.  Got angry, put the stock carb back on and left the pumper on the shelf. Recently, my bike died while riding.   The float valve was stuck.  Gas was going out of the petcock, but not out of the over flow vent when tipped over.  As long as I have to pull the carb, might as well put the Mikuni back in.  

Changes to the Mikuni carb, I soaked the carb and cleaned it out.  I set the float height to 19mm.  New 130 main and 17, 5 pilot.  Still pops like crazy.  I put in a 15.0.   No difference.  It seems to be running really hot too.   I noticed gas spilling out as I tilted it to put it away.  Not any more than going onto the kick stand.


Specs:
XR400, snorkle removed, Uni-filter, headers ground, Vortip exhaust.

Mikuni:
Main 130
Pilot 15.0
Needle 9EBY1-50 (3rd position)
Needle jet P-4
Air jet: ?

Location: S.F. Bay Area
Elevation: Sea level
Temp: 70 degrees

Edited by Kev_XR, 19 June 2012 - 05:09 PM.

For the float height, have you tried anything other than 19mm? The spec allows for a +/- of 2mm. I think I would try 21mm.

Looking back over the post, I don't see any mention of checking the fuel screw o-rings condition.

Another place air could enter with a bad o-ring is at the choke knob assembly. Also the funnel gasket #9 and top cover gasket #6.

Make sure the main air jet #39 in the schematic(Mikuni HS40 manual .pdf) is still plugged and hasn't come out.

Just thinking out loud here..... ya know, the stock carb doesn't have a true start enrichment circuit(just a butterfly valve for choke), so has a dedicated air diverter valve to prevent a lean pop when closing the throttle. The Mikuni has a true start enrichment circuit, so no diverter valve. So if I'm not mistaken, there should be drillings in the carb that access the starter circuit for fuel to prevent a lean condition when backing off the throttle.

Run a wire thru all the drillings you can find.

Edited by Trailryder42, 19 June 2012 - 07:46 PM.

Thanks.

It was dribbling gas, so I was going to try a lower float height.  I might have set it with it pushing down the round plunger on top of the float valve, but not pushing down the float valve.  That might have made it set at the wrong height.

I'll pull the carb and check the o-rings for the fuel screw and idle adjustment.
I put in a new float bowl gasket and a new top cover gasket.

Edited by Kev_XR, 19 June 2012 - 09:47 PM.

10 years really?! Mate, get an FCR. I really don't understand why people persist with the Mikuni.