Carb AFR? advice added exhaust and jet kit - MudInMyBlood Forums
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post #1 of 32 Old 01-27-2019, 12:02 PM Thread Starter
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Carb AFR? advice added exhaust and jet kit

hey guys i just rebuilt both of my carbs on 05 brute force 750.
mods:
HMF 3/4 exhaust is the only mod im aware of.

I added 155 jet front and 165 rear per what i read is recommended from HMF website. im about 1,000 ft above sea level.

Im more of a fuel injection guy but i want to try and get these carbs in the sweet spot without removing carbs and plastics 10 times making adjustments.
I have a test tool wide band 02 from (innovate motorsports LM1) i use when tuning other projects of mine. I havent been able to find what the AFR(air fuel ratio) should be both idle and WOT?never tried using the wide band on a ATV either. but well see what happens.

im at 2.25 turns on mixture screw(I read thats stock). looks like whoever owned it bofore me had it about 3 turns out with factory exhaust. it was a bear to start. but i also found both intake boots carb to head were cracked badly. would really only run decent with choke on. some backfiring and fuel spitting out carb.

does anyone know what idle AFR and wide open throttle AFR should be? this way i can get on it without plastics and log my results.

i havent done much to this 750 since i bought it, performace wise. just exhaust since it was cracked probally from running lean.

in my fuel injection offroad jeeping world, o shoot for 14.7 afr idle and 12.7afr WOT.
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post #2 of 32 Old 01-27-2019, 01:24 PM
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I don't have the AFR numbers on the carbed 750s but if all you put on is a slip-on, not a full system and nothing else..not even a CDI, then all you need is one size up on the mains from OE which was 152/158 so 155F/160R, no change to the pilot so keep it at #38 but go with one .020 shim on each OE needle for now and assuming your float levels are correct and all else is correct..ie fuel/air supply, you should be good. The AF or idle adjustment can be anywhere between 2.25-2.75 turns out from closed.

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post #3 of 32 Old 01-27-2019, 05:57 PM Thread Starter
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Thanks for replying. I threw the carbs back on tonight with the 155/165R jets. it did have a little pop out of the front carb. and some minor backfires mid throttle on idle. i was 2.25 turns out. until i put choke on like 1/4 inch.

it starts rite up and it was about 25F out. which was a first.

so i pulled them back out thinking maybe i didnt seat the choke actuators properlly. ill try again tomorrow after work.

i thought that jet was a bit agressive as well, im attaching HMF recommendations on it..

as for the washer on the needle, i have no clue how to do that or where it goes. my carb expertise is very limited. do you have a pic or something on what to add the shim to?
i really appreaciatethe reply. TY
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post #4 of 32 Old 01-27-2019, 06:22 PM Thread Starter
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That last post reguarding choke was confusing. what i was trying to get at
with the choke open just about 1/4 inch only, it seems to run great, very responsive and easy to start. no road test done though.

so i took the carbs back off to adjust afr some. i cant seem to reach the AFR screw and turn it with the carbs installed. im used to popping out intake = lean and popping out exhaust is rich.

now that i think about it, where is the fuel filter on this thing? ... lol.

the engineers at Kawi didnt put alot of thought into changing that rear spark plug or adjusting the carbs. haha.
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post #5 of 32 Old 01-28-2019, 10:42 AM
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I suggested the correct main jet size so if you choose not to use them it's up to you. There is no fuel filter other then the screen on the end of the intake in the tank. And you can always buy one of these to do the A/Fs like I did. I don't use it any more but it's there if I need it.


https://www.amazon.com/Motion-Pro-08.../dp/B000UKD6YC


The choke is just a plunger that seals-off a port to the bowl. Pulled/unsealed it makes a direct connection between the vacuum side of the butterfly and the fuel pulling fuel directly in. Clearly its lean. Need to find and fix the reason(s).

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post #6 of 32 Old 01-28-2019, 10:06 PM Thread Starter
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Thanks for your replies, i will definatly get that tool.
Tonight in a hurry before the snow storm tomorrow i thought i would adjust valves, tune those carbs and have it to plow snow tomorrow.
So the valves took a little longer than planned and really put me in rush mode.Took the carbs 3 turns out and still running lean. but ok with choke.

Then it dawned on me, hows it pumping fuel? not gravity, i bet testing without air box on is my problem? does it get fuel from vacuum actuated pump? 😞
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post #7 of 32 Old 01-29-2019, 06:57 AM
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You can not run it without the airbox on and the lid on. It will be too lean.

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post #8 of 32 Old 01-31-2019, 08:23 PM Thread Starter
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Just an update. I decided to go your route with just one size up on mains155F/160R, no change to the pilot and added the .20 shim to needles. i like the way it sounds there.
My valves were way tight on front cylinder and think that was contributing to the popping out intake.
compression passed but close at 41PSI front 60psi rear.
tomorrow i will road test it. no load its a just a bit fat off throttle but i have a little room for adjustment yet. thinking it will be perfect under a load.
I also picked up that carb adj tool. it definatly saves time.
it also started cold on 2 crank cycle -2 out. thats always a good sign.
just want to say thanks for helping out!
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post #9 of 32 Old 02-01-2019, 04:05 PM Thread Starter
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update 2
its not running fat, off throttle its running out of fuel. temp connected a little fuel lift pump i use on diesels and it runs perfect. time to add the #42 pump people are talking about. i wish they had better tests in the manual. like ounce when cranking or something. might be due to my marginiable compression (40) on fr ok nt cyl. thats why im just going to go grab the electric. i was never a fan of vacuum actuated pumps.
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post #10 of 32 Old 02-01-2019, 04:48 PM
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Just FYI you don't need an expensive low pressure fuel pump. Just something like this will work just fine. Just don't know how much vacuum it puts out so keep the pulse pump hooked up.


New Gas Diesel fuel pump Inline Low Pressure electric fuel pump 12V | eBay

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post #11 of 32 Old 02-01-2019, 09:02 PM Thread Starter
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i bought the 42 since im in a hurry. the pump i tested with wouldve been a better choice but it belongs to my company. was just on my truck for stock. it will lift at least 3ft and self prime. this #42 does not. i ended up keeping that one vacuum line on there to prime and it does work good. runs great. i just need to see how i want ro wire it. definatly attached to kill switch and might get fancy and do a 10 sec prime only with key on. since its still about 1* F out. tomorrow morning ill go hit the key and it it starts within 2-3 tries ill just make sure it goes on and off with the kill switch.
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post #12 of 32 Old 02-02-2019, 05:23 PM Thread Starter
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update 3 "Trail Test" machine ran great, took a WOT ride no issues. pulled it back in garage and noticed rear cylinder pipe glowing red about 6" back. hooked up the wide band and noticed its running and sounding great but its lean. just off throttle is about 17:1. idle is about 15:1. i put the choke on just a little and it gets into a better range around 12-13 AFR.
wish i didnt put all the plastics on already.
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post #13 of 32 Old 02-02-2019, 06:00 PM Thread Starter
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so my needles were from moose and the numbers didnt match factory. i hear they both use the same needle now. i added 2 washers to the needle and its still lean. but runs good. still have the rear exhaust glowing red. I wonder if its carb synconizarion? i didnt do this step they looked pretty close. this process should be easier since im using external fuel pump... any other ideas let me know? the AFR climbs up as i hit 25% throttle.. starts about 15AFR and goes all the way to 20AFR. i almost want to che k my AFR meter and make sure its accurate. my first carb job isnr going to well. 😞 maybe i shouldve saoked the carbs disembled. but they were clean i was just fixing the jetting. shoot me out some causes please. i even went to 4 turns out on pilot screw. didnt make much of a difference.

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post #14 of 32 Old 02-03-2019, 09:19 AM Thread Starter
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another update. running lean. tried to sync carbs but the vacuum needle is bouncing all over rapidly on both cylinders. maybe a valve problem? i already adjusted them. this thing is killing me. 😞
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post #15 of 32 Old 02-03-2019, 10:35 AM
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The needles are at the end of the slides. If it's just the needles, its because you have DJ needles and need to replace with the OEs and their retainers. If you are looking at the slides while running, they will move erratically especially with the box cover off. And you can't run it above 1/2 throttle with the cover off either otherwise it will be very lean. This all assumes there isn't anything else wrong with the carbs. You sync the carbs with the arm that runs between the two- backing the idle control off until cable is slack, then adjust each so that the butterflys just touch shut then adjusting the arm so that they are exactly together. Then a slight turn on the stops each do the butterflys never stick shut. After that bring the cable adjuster back up to normal idle.

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