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Hopefully easy Jetting question that hasn't been beat to death

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I'm planning on purchasing a Mikuni Pumper Carb from XRs Only for my 99 XR400 R. My question is more about elevation than anything. I ride in Utah anywhere from 4600 to 8000 feet. XRs only asks for an average elevation that I ride at. I was thinking of telling them 6000 feet but was worried that I might run too lean when riding at 4600 feet. Any thoughts?

The reason I'm asking is that I read that the Mikuni Carb is much less susceptible to elevation changes, that's why I thought I may be able to get away with telling them 6000 as an averag.

I run with K&N filter, White Brothers Exhaust (Includes Header).

Any help would be greatly appreciated.

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I got the carb in position and got the bike started and running. Everything seems like it is working pretty well however it doesn't seem like there is any difference between the mikuni and the stock carb. I can still wack the throttle from idle and bog the engine down so it almost dies. Does anyone have any suggestions as to what this carb is supposed to be able to do and how I can get the carb adjusted so it functions correctly or am I expecting too much from the carb. The throttle response seems no different than with the stock carb and there may be a slight power increase but it is negligible.

Help

Main Jet 130

Pilot Jet 15.0

I havent adjusted the carb since I recieve it from XRs only with the exception of adjusting the air screw (White Knob)

I'd like some advice on adjusting the pump actuator Where to have it start and stop.

Thanks

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Yeah, if i'm putting along real slow it'll bog right down when I really romp on the throttle. I'm gonna play around with the accelerator pump settings tonight and see if it helps. It really only happens when I take the throttle from 0 to full real quick. Guess you'd very seldom do that while actually riding but thought this carb would take care of that problem. It says it's supposed to get rid of the "Flat spot right off the bottom". I thought this engine bog was what they were talking about.

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I'd expect the engine to bog when you whack the throttle from closed to wide open with the engine at, or about at idle. Is that the bog you're trying to tune out? If so, I'm guessing a 2 stroke would fix it.

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There is a couple of fixes, one is a smaller carb, then the bike would be slow but you could whack at it, another would be a CV carb, see responce above (or slow).

My guess is you need to raise the idle speed and learn the disiplin to roll on the throttle at a rate the engine can accept. BTW the guy you bought the carb from knows more about that carb/engine & combo than anyone so talk with him he may have some tuning tweeks but 4 stroke & big carb this is for the most part how it is

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Make sure you have your setting for the pump on correct. Set the pump on screw all the way in. What is most likely happening is that the pump is being triggered too soon and it drowns the engine. I have a pumper carb on my xr400 and i can whack the throttle as hard as i want and it will respond without bogging. if your moving at idle in first and you whack the throttle you should experience no bog same as in 2nd and as a result instant wheelie.

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Thanks, I'll give that a shot and see how it ends up. I appreciate the help, The thumper forum hasn't let me down yet.

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Tried tinkering with the carb last night, screwed in the pump actuator till the spring was seated all the way, didn't make a change. Also discovered this beast is even harder to start than before.

I sent the following e-mail to XRs only.... Hopefully I can get a response from them and maybe some additional help

I just bought the Mikuni from XRs only and I'm having a lot of trouble getting it to run right. It seems that once I get it going it runs ok, (No noticeable improvement over the stock carb, even seems to bog out a bit more and the throttle response may be even worse). The thing I'm having the hardest time with is being able to get the thing started. It will take 15 to 20 strong kicks to get this thing going, the old carb was significantly better in this regard. Even after the bike is warmed up, it seems I can let it sit for 5 min or so and then it takes another 15 to 20 hard kicks to get it going again. It almost seems like it is flooded or something. I know almost nothing about carbs which is one of the reasons I bought this carb from you guys because it was supposed to be set up for me, so I am seeking your wisdom hopefully you can help me out.

I ride an XR400 with an White Brothers E-Series exhaust, K&N filter stock motor, at an average of 5000 feet. I am doing the testing in my garage and around my neighborhood at around 4600 feet. The carb came jetted with a 130 main jet and a 15 pilot jet.

Are the jets interchangeable from the stock carb? Could I use the same jets or same size jets to achieve the same results as my stocker?

Thanks for your input on this. I appreciate any help I can get.

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I haven't gotten a response back from XRs Only yet, any of you have any ideas for me? Do I need to change needle position to help with starting?

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I haven't gotten a response back from XRs Only yet, any of you have any ideas for me? Do I need to change needle position to help with starting?

I don't think the needle position has anything to do with fuel mixture at idle. There is the pilot jet and a fuel mixture screw for fuel/air mixture at idle. That's how my carb works anyway.

After trying to start it for about 5 kicks, pull the spark plug to see if it's wet. If wet, too much gas for the amount of air coming in, or no spark.

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I called XRs only today and spoke with Junior - He suggested that I try a 12.5 Pilot with the fuel screw turned 1.5 times out and a main jet of 127.5

I'll give that a shot and see if that works. I'll be getting that plug wrench out to check the plug too, thanks for the tip.

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I called XRs only today and spoke with Junior - He suggested that I try a 12.5 Pilot with the fuel screw turned 1.5 times out and a main jet of 127.5

Problem Solved - I was able to change out the Pilot jet and I ended up turning the fuel screw out approx, 2 times from being seated hand tight. The Main jet that was suggested didn't come with my carb so XRs only is sending one out free of charge. Long story short, Got the carb back together and the bike started with one kick. The bike pulls through all the gears better now. It was the wow experience I was going for and worth the money. I also had to turn the spray so it hit the needle. I read somewhere that you should do that so I did.

For anyone who is searching Mikuni Carb Setup -Mikuni TM36-50 99 XR400R 5000 feet 12.5 pilot and a 127.5 Main, fuel screw 1.5 to 2 turns out.

Does anyone have a jetting chart for the mikuni and the XR400 that can be used to get the jetting right from the get go? I'm pretty sure XRs only has something like that because the guy rattled off those numbers without blinking an eye. Maybe we can get something like that from them to post on the forum.

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