# brute force bogging at WOT and backfire on let off



## Langbanger (Dec 5, 2016)

Hello. Im new to the forum and so far it has helped me out a lot in learning my new to me 2010 brute force 750i. first off, here is what i know about the mods. it has 30" silver backs, HMF slip on exhaust, 2" snorkles and an unknown clutch kit for the big tires. the problem i am having is a bog down at WOT. it also backfires when i punch it and let off. after all the reading i have done i figured the fuel pump may be going bad. i ordered a new fuel pump from amazon (not the whole assembly) with a new filter sock. it has not arrived yet. after looking through the bike some more i found a crack in the exhaust header. I have read that this can cause the backfire issue, but will it also cause the bogging at WOT? in any case, it needs to be changed. i didnt really plan on forking over money for a new full exhaust system because i'm getting ready to send my clutch to VFJohn, but i'm having trouble finding just the headers. any leads up here in Canada? Thanks in advance for your advice!


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## dman66 (Feb 20, 2011)

Weld the crack in the header for now. Been there- worked well.


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## johnny ukelele (Oct 12, 2016)

dman66 said:


> Weld the crack in the header for now. Been there- worked well.


I see in your signature that you have a 3" intake snorkel....
With restriction...

My question is....
What is the restriction and why?
And what elevation are you at?


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## Langbanger (Dec 5, 2016)

So after reading this site all night I have come up with a list of what I'm going to try (so much knowledge here)
1 new exhaust header
2 new uni filter 
3 new plugs
4 fuse box delete mod, it looks 200 years old and the cover is broke
5 New fuel pump and sock filter
6 msd controller with VFJ custom mapping
7 have the dealer set the valves not sure I'm comfortable tackling this one
hopefully this gets the brute running top notch for the VFJ stage 3 clutch.
If I am missing anything I should be trying, please let me know. Thanks.


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## johnny ukelele (Oct 12, 2016)

Just get that crack fixed...exhaust leaks cause a lot of problems....
Not to mention, could burn a valve if not taken care of....


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## Langbanger (Dec 5, 2016)

You bet, that's first on my list before I ride again, all the rest are my winter projects anyhow, except the fuel pump, if it's not needed it can go on the shelf until it is. I hope its just the crack.


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## johnny ukelele (Oct 12, 2016)

What happens with a crack in the header, is that it sucks in cold air and that is why I think it is backfiring on you...do you have snorkels?
Is it popping out of the exhaust or thu the airbox?


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## Langbanger (Dec 5, 2016)

It is popping out the exhaust. Yes it does have 2" snorkels as well. I'm confident the leaky exhaust is the culprit for the backfires but I'm just not sure about the bogging at WOT. Hopefully I'll find out after this weekend if I get the leaks all taken care of. I'll post my results and go from there. Thanks for all the replies.


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## dman66 (Feb 20, 2011)

johnny ukelele said:


> I see in your signature that you have a 3" intake snorkel....
> With restriction...
> 
> My question is....
> ...


Johnny,
I have a 05 750 with carburetors. I ride anywhere from 400 - 900 feet in elevation.I installed an intake snorkel way back when I bought the machine with basically no engine or exhaust mods. I tried installing 2" intake snorkel with no good luck of tuning and re jetting carbs. Fast forward, and I wound up installing a 3" intake snorkel. I found that in order for a wide open 3" intake, I needed to over size the pilot and main jets. What I then tried is installing a sewer grate in the 3" intake fitting. This kept my jetting simplified and kept the jets close to stock even with a full Muzzy Pro exhaust. I tuned and jetted with an O2 set up. There should be attachments in the snorkel how to thread. Link to snorkel thread for pics page 14 , post 202 - http://www.mudinmyblood.net/forum/66-snorkeling-jetting/4256-3-center-snorkel-brute-650i-750-a.html


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## dman66 (Feb 20, 2011)

the problem i am having is a bog down at WOT. it also backfires when i punch it and let off. after all the reading i have done i figured the fuel pump may be going bad. i ordered a new fuel pump from amazon (not the whole assembly) with a new filter sock. it has not arrived yet. 

You may want to try and unscrew the gas cap about to turns or so to allow air into the gas tank - it may be that the gas tank check valve is not allowing the gas tank to vent properly. 
It may also be the 2" intake snorkel is too restrictive - You can disconnect the snorkel under the left fender at the air box and see if it clears up.


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## johnny ukelele (Oct 12, 2016)

dman66 said:


> Johnny,
> I have a 05 750 with carburetors. I ride anywhere from 400 - 900 feet in elevation.I installed an intake snorkel way back when I bought the machine with basically no engine or exhaust mods. I tried installing 2" intake snorkel with no good luck of tuning and re jetting carbs. Fast forward, and I wound up installing a 3" intake snorkel. I found that in order for a wide open 3" intake, I needed to over size the pilot and main jets. What I then tried is installing a sewer grate in the 3" intake fitting. This kept my jetting simplified and kept the jets close to stock even with a full Muzzy Pro exhaust. I tuned and jetted with an O2 set up. There should be attachments in the snorkel how to thread. Link to snorkel thread for pics page 14 , post 202 -  3" center snorkel Brute 650i/750 - MudInMyBlood Forums


Thanx for the info brother...


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## Langbanger (Dec 5, 2016)

Cool, I'll try loosening off the gas tank when it's not quite so darn cold out here. I thought that having 2" snorkel intake would increase airflow from stock? There are quite a few different leaks in the exhaust so I'm scrapping it and undecided on a big guns evo or trinity full exhaust setup along with a vfj mapped msd.


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## dman66 (Feb 20, 2011)

Langbanger said:


> Cool, I'll try loosening off the gas tank when it's not quite so darn cold out here. I thought that having 2" snorkel intake would increase airflow from stock? There are quite a few different leaks in the exhaust so I'm scrapping it and undecided on a big guns evo or trinity full exhaust setup along with a vfj mapped msd.


The stock air box intake is more like 2.25" of intake space - So a 2.5" diameter intake snorkel would be ideal. But not many places have 2.5" pipe. So we install a 3" intake. I used thin wall pipe for my intake snorkel, and schedule 40 pipe for my belt intake / exhaust.
Being that you have a 750 with fuel injection, you can purchase an aftermarket programmer that you can load different fuel maps into it to accommodate your set up. 
You will need to look at your altitude / elevation to determine air / fuel ratios. The higher above sea level you are, the more air will be needed. If you are at sea level, you could get away with a 2" intake snorkel, but lets say 400 foot or more above sea level, you will need more air into the air box.


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