# 06 brute force 750 stumble and backfire



## fightermatt (May 9, 2017)

Don't mean to sound short but there is a lot of info and I'm going to try and keep this short. Bought an 06 BF 750 that had a stubble and backfire on hard take offs. Only way it would run decent was with the a/f screws 4.5 turns out. Installed mimb snorkel, dynatek cdi and a stage 1 dynojet kit. The dj needles fixed it, it ran like a scalded dog. 

Took it down a creek one day and after being stuck running in the water for a little while it started to want to bog down when I gave it the gas. Took it home cleaned the carbs and decided to try and turn the a/f screws back in a little since the dj needles fixed the stumble. Put screws at 2.5 reassembled and tested it, ran great snatching wheelies and all. Noticed fuel leaking where the line connects to carbs. Went to auto parts store got a fuel line and replaced the line and clamps, tested it, all good, put plastics on asks fueled it up. As soon as I took it for a ride it was stumbling again, no big deal, I'll just turn the screws back to 4.5 and go up in pilot jet size right. No, put it all back to the way it was and it's still stumbling and backfiring. Idles fine, runs wot fine, stubble on hard acceleration and if I rev the engine it doesn't want to come back down all the way to idle sometimes. Carb setting are dj 144 front 150 rear (around sea level) pilots are 40, dj needles 4th notch with one washer that came with the kit on them. I am lost, any suggestions are appreciated, I feel like changing stuff in the carbs might not be the answer because it ran well before on these settings. Isn't 40 pilots usually the fix for stumbling, do I need to go higher?


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## NMKawierider (Apr 11, 2009)

Not always is the #40s the fix for stumbling but it's a common need with the Dynatek CDIs. I would look at a couple of other things like..float levels, cracks on a carb boot under the head-side clamps, tight valves, low fuel volume or pressure, stuck tank vent check valve, low coil voltage.


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## fightermatt (May 9, 2017)

Thanks for the response. I know the boots are good, and I just adjusted valves. I replaced fuel line but it is a tiny tiny tiny bit smaller than factory, the bike runs great up top so I didn't think it was lacking fuel from that, could that be the problem? Where is the tank check valve? I don't know how to check the regulator or check or adjust float levels but I might could find a YouTube video. 42 pilots?


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## Polaris425 (Dec 16, 2004)

Tank check valve will be above the tank, tucked into the frame rail. Most have pulled it out, and put a longer tube on it, and run it up with their other vent lines.

If it wasn't a returning issue, I'd say something electrical just got wet under the seat. My 07 had a similar issue when coming out of water, it would stumble and spit for the next few minutes until everything dried out. A little DE grease on all electrical connections under the seat and problem solved...

But since your's didnt just happen after being stuck in the water, I don't know that that's the problem.


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## fightermatt (May 9, 2017)

The only thing different on the bike is the fuel line, like I said, it's a touch smaller in diameter than oem. Even though it still runs hard on top I guess that could be the problem, I'll try a bigger one, and see if that helps. I've had for delivery problems on my motorcycle before and it presented when I tried to go fast. I'll look into the check valve as well. Thanks guys I'll keep you posted.


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## NMKawierider (Apr 11, 2009)

Grab the service manual and go for it.


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## fightermatt (May 9, 2017)

Ok, I read through the mimb jetting guide and it seems I'm lean on the slow circuit (hesitation stumble and a little rolling idle). I don't think it's the check valve because the bike runs the same with the fuel cap off. Even when it ran good it still had a little stumble and pop at a slow pace and I was 4.5 turns out on the a/f screws, so I ordered 42 pilots. However, my plugs are jet black, I think I'm rich on top. I've heard you shouldn't change more than one thing at a time but I can change the mains down a size when I install the 42 pilots right, I fell like the 2 circuits are doing opposite things, and I don't want to have to take the carbs off again if I don't have to, I've done it 5 times now and it's getting old?


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## dman66 (Feb 20, 2011)

Actually, if you dug deeper into the MIMB snorkel thread, you would have noticed that dialing in the 2" intake snorkel sucks ***. A lot of us went to a 3" intake snorkel which can allow for easier carb tuning. I personally did the 3" intake and installed a restriction on the intake so I could jet the carbs closer to stock jetting rather than installing bigger jets. I think if you are at sea level, then the 2" intake is OK, but once you get higher in elevation - you will struggle to properly tune those carbs with the 2" intake. The belt intake and exhaust are good with the 2" pipe. If you have a newer quad with fuel injection, then there are fuel programmers that seem to allow people to successfully tune the fuel system with the 2" intake.


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## fightermatt (May 9, 2017)

I think mine is 2.5 with 3 at the air box.


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## dman66 (Feb 20, 2011)

Sounds like ur pilot jets need to be cleaned.Ore the carbs need re cleaning.


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## fightermatt (May 9, 2017)

I installed keihin 42 pilots in and turned the a/f screws down to 2.25 out. No change, a/f to 3 turns out, got a little better; so I moved the needles to the 5th notch to richen up the mix, it is much better now but it will still stumble every once in awhile. Because the plugs were so black I put in 142 and 148 mains. I do think the float levels should be adjusted but I don't have a manual or special tools, so that will have to wait. Should I turn the a/f screw out to 3.5 to clean up the hesitation and backfire that happens every once in awhile on hard take offs, or take the needle to the 6th notch and go in on the screw? Maybe leave it alone until I can get to the floats.


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## dman66 (Feb 20, 2011)

It sounds like your carbs pilots need cleaning to me. You should not be adjusting pilot screws out past 3 turns. I don't know of anyone with a stock 750 engine - with snorkels running # 42 pilots - too rich in fuel. Lets see a pic of that snorkel set up.

After re reading this thread, it sounds like the you are experiencing the common 2" snorkel intake problems.


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## NMKawierider (Apr 11, 2009)

To me this indicates either a blockage or low float levels. No one goes to #42 pilots like Dman says..way to rich for any 750...and the extra A/F opening indicates a lean condition as well. Check the float levels per the manual and check for carb boot cracks under the head side clamps. Also check fuel supply and the tank vent check.


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## fightermatt (May 9, 2017)

First let me say thanks to you guys for your help thus far, I know it gets old hearing the same problems from newbies like me. As of right now it has 42 pilots, a/f 1 turn out 142 front and 148 rear dynojet, dj needles on 5th notch. It is much better than before but I think I should have left the 144 and 150s in, right when I get to 60 mph it starts backfiring and dying, then backfires on deceleration. After it gets back to idle it will stumble and hesitate if I give it any gas for a bit then it cleans back up and goes back to normal. Sorry for the misinformation, it is 2 inch snorkel with 3 inch into the air box. I'm moving right soon so haven't had a lot of time to dig into it too much. I don't know how it's so lean, I know no-one is running 42s especially with the needles on the 5th notch, I'm going to put the 144/150 mains back in when I get off work Friday, after than everything else will have to wait until I finish moving. Can I get a manual from mimb?


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## dman66 (Feb 20, 2011)

A Dynojet 142 in front and a 144 or 146 in the rear carb should be good. You stated she gets up and boogies at WOT. So I would doubt that the slides are a problem, or the float levels either. So, I recommended cleaning you pilot jets, because it sounds like they are blocked with crap. I don't think you went and did this yet, but tried other crap. While your in there cleaning the pilot jets, check you float levels - I think the MacGyver way is to measure the bottom of the float to the middle flat side of the carb body where the bowl meets the carb body - I think it is like 3 to 4mm - This is spelled out in the service manual which you can google search for a free brute force 750 service manual pdf. Reinstall the #40 Keihin pilot jets and set the pilot screws at 2 1/2 turns out. Reinstall the OEM needles if you have them, or set the Dynojet washers back to mid range on the needle - you can richen her up later if needed. Disconnect the 2" intake and your machine should flip over backwards as long as your battery is good, valves were set properly, timing is proper, and other stuff needs to be running properly as well - You claimed it runs great from mid to WOT.


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## Dustinc (May 13, 2017)

fightermatt said:


> First let me say thanks to you guys for your help thus far, I know it gets old hearing the same problems from newbies like me. As of right now it has 42 pilots, a/f 1 turn out 142 front and 148 rear dynojet, dj needles on 5th notch. It is much better than before but I think I should have left the 144 and 150s in, right when I get to 60 mph it starts backfiring and dying, then backfires on deceleration. After it gets back to idle it will stumble and hesitate if I give it any gas for a bit then it cleans back up and goes back to normal. Sorry for the misinformation, it is 2 inch snorkel with 3 inch into the air box. I'm moving right soon so haven't had a lot of time to dig into it too much. I don't know how it's so lean, I know no-one is running 42s especially with the needles on the 5th notch, I'm going to put the 144/150 mains back in when I get off work Friday, after than everything else will have to wait until I finish moving. Can I get a manual from mimb?


I have 07 brute 750 Im running 165 front 170 rear with the dyana tech cdi also running #42 with my A/F set at 2.25 turns out and i cant seam to keep all 4 tires on the ground....also the tank gets its relive air from inside the frame there is another vent tube up front that gos from the frame to up under the handle bars make sure both of these are free and clear dirt dobbers love them....hope this helps


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## fightermatt (May 9, 2017)

Dman, I've changed the carbs and all the jets a couple times and changed the pilots up a size, but I think I have figured out the problem, maybe. I cleaned the carbs Again and the bowls were full of gel, I don't think the shop I took it to when I bought it cleaned the fuel tank, I assumed they would when I told them it had been sitting for a year or so with ethanol fuel in it. Anyway,I installed an in line fuel filter after reassembly and fired it up, it was fine until I rode it up the street and back, symptoms returned, looked at the filter and the chamber was almost empty, that's why it's acting lean, outs not getting enough fuel to the carbs. I'm pulling the tank and cleaning it out myself and replacing the fuel pump, I'll report back when I'm done, I'll probably have to jet back down too.


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## dman66 (Feb 20, 2011)

Sweet. I hate ETHANOL !!!!!!!!!


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## fightermatt (May 9, 2017)

Alright guys, so I pulled the tank and the fuel tap was super corroded and cracked and just worthless. Picked up a new tap, washed the inside of the tank, switched out the pulse pump for an electric one; I left all the jets as they were and I'm glad I did, for some reason this bike really wants to run lean. Right now I'm 3.25 turns out on the a/f screws, #42 keihin pilots, 144f and 150 rear dynjet mains, dj needles on the 4th notch and its running good. I don't know how a healthy one is supposed to run because when I bought these, they were not running, but it'll do wheelies and run fast, haven't checked the top speed because my new place doesn't have the room, but the low and midrange feels good. Thanks for everyone's help, I'll check the plugs and see if I should go back down on the mains later but after fighting this effing thing for like 2 months I'm just gonna enjoy the win for a bit. Now to fix the one my wife redecorated a tree with.


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