# Carb AFR? advice added exhaust and jet kit



## jeff919 (Jan 12, 2019)

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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## NMKawierider (Apr 11, 2009)

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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## jeff919 (Jan 12, 2019)

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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## jeff919 (Jan 12, 2019)

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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## NMKawierider (Apr 11, 2009)

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-08-0119-Pilot-Adjusting/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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## jeff919 (Jan 12, 2019)

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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## NMKawierider (Apr 11, 2009)

You can not run it without the airbox on and the lid on. It will be too lean.


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## jeff919 (Jan 12, 2019)

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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## jeff919 (Jan 12, 2019)

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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## NMKawierider (Apr 11, 2009)

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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## jeff919 (Jan 12, 2019)

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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## jeff919 (Jan 12, 2019)

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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## jeff919 (Jan 12, 2019)

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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## jeff919 (Jan 12, 2019)

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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## NMKawierider (Apr 11, 2009)

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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## jeff919 (Jan 12, 2019)

thanks for replying, i pulled everything back apart again. i bought a shindy rebuild kit when i did this rebuild from e bay.
today i pulled the pilot jet and matched it with the stock old one. The old pilot says 38S and the new one just says 38. visually the holes are a different size,(old one bigger holes) so i used a sewing needle to guage check and they are quite different. im thinking about putting the stock 38's in there and see if it improves. I thought the front and rear carbs took the same pilot jet?


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## NMKawierider (Apr 11, 2009)

jeff919 said:


> thanks for replying, i pulled everything back apart again. i bought a shindy rebuild kit when i did this rebuild from e bay.
> today i pulled the pilot jet and matched it with the stock old one. The old pilot says 38S and the new one just says 38. visually the holes are a different size,(old one bigger holes) so i used a sewing needle to guage check and they are quite different. im thinking about putting the stock 38's in there and see if it improves. I thought the front and rear carbs took the same pilot jet?


 Yes, both take the same pilot jets but not the same main jets or CV needles. Pilot jets only provide fuel to the pilot or slow circuit and one of several ports on that circuit has an adjustment- that the AF screw(s) which are idle fuel and are set between 2.25 - 2.75 turns out depending on need.


All carbed 750s came with #38 pilots but needed #40s if an aftermarket CDI was used. But no larger.


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## jeff919 (Jan 12, 2019)

thanks, well i put the factory pilot jet and factory needles back in and it improved. but the afr is about 15afr at idle goes up to 22 until i break 1/4 throttle and it starts dropping AFR to where it should be. 
i took the 2 washers out when i put factory needles and #38 's back in. those factory 38s pilot jets i had definatly have bigger holes than what came in the kit. Is it ok to use washers on the factory needles? cause i was about to order #40 's. i mean it runs great but somethings off. maybe a passage or something is blocked on rear carb. the AFR screw doesnt seem to do much. theres a bike shop close and said he has a sonic cleaner. the insides looked pretty clean but maybe im missing something. 
i did see a you tube video guy using same exhaust same issue ended up needing 40's. but i agree with you. it must be masking some other issue.


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## jeff919 (Jan 12, 2019)

slow pilot pic difference


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## jeff919 (Jan 12, 2019)

no chance that its cause im using 87 octane rite? i wouldnt think so but who knows. lol.


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## NMKawierider (Apr 11, 2009)

Don't go to #40s unless you added initial spark advance with a CDI. Stay with the #38s . If one system isn't working right it is common for the pilot/slow circuits to bet blocked. Sonic cleaning is sometimes the only way but some have had luck with carb cleaner spray being forced through the A/F port with the screw out. About the 1/4 throttle position is where the mans start running and we call that transition. Its where most often it becomes too lean and makes a flat spot but yours it seems becomes richer up until that point. A sign a choke plunger is not sealing. Any way you can close the AF screws down some to lean that out but leave enough for it to idle right on. Maybe 2-1/8 turns out.


The washers or shims on the stock needles-we use then to increase the fuel mixture off the mains as much as 1/2 of a jet size. No more then .040. Not something you probably need to worry about right now.


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## jeff919 (Jan 12, 2019)

i got a hardware store near me he says $30 a carb for sonic cleaning. i see harbor frieght sells one for $70-80. Ever hear of guys having good luck with it on here? i will try the sonic first... idle is 14AFR climbs to 22AFR until i get past 1/4 throttle.


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## mrvmax (Dec 3, 2016)

jeff919 said:


> i got a hardware store near me he says $30 a carb for sonic cleaning. i see harbor frieght sells one for $70-80. Ever hear of guys having good luck with it on here? i will try the sonic first... idle is 14AFR climbs to 22AFR until i get past 1/4 throttle.


 I bought the Yamaha carb cleaner off Amazon and it works well in an ultrasonic. One quart was enough for both carbs. You’ll need a large Ultrasonic to get the entire carb under liquid level. https://www.shopyamaha.com/product/...-dip?b=Search&d=33|33&ls=yamaha-motor-company


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## NMKawierider (Apr 11, 2009)

You should confirm weather there is a passage problem or not first plus other things like float levels and such before buying something. Also the carb spray works wonders and it's cheap.


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## jeff919 (Jan 12, 2019)

alright, maybe ill attempt those float levels. i checked the 3mm above mating surface on floats and thats it.. i didnt do the clear hose from drain part. 
i have a pretty powerfull cleaner called shop kleen. maybe ill spray some in there and let it sit. the ultrasonic cleaner wouldnt hurt since i prob have 10+ carbs on different pieces of equipment. i thought for sure putting that factory pilot jet back in wouldve done the trick. all the holes on it are bigger. ?


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## jeff919 (Jan 12, 2019)

Im basically starting over and first order is cleaning carbs. they were pretty bad to say the least but mostly outside stuck on grime. having them off 12 times though just keeps loosening up more dirt and i really just want to put it to bed this time "as much as i love working on carbs" i had enough. i posted a before and after using the ultrasonic cleaner. then i sprayed them out with solvent and air again. hopefully this clears up the just off idle to 25% throttle lean condition. im also ditching most of that shindy rebuild kit. the holes are smaller for sure in the low speed pilot. 
i also bought some clear tubing to check float levels. only got one carb clean tonight( the rear) the one i think is giving me problems. used 50% simple green and 50% distilled water. rinsed and sprayed with wd40.


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## NMKawierider (Apr 11, 2009)

The only thing that's going to dissolve the dried fuel shellac in the passages is a good actual carb cleaner spray like Gumout or Gunk brand, a carb vat like Barryman's Chem-dip, or a sonic cleaner with the proper solvent. Anything else... is a waste of time.


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## jeff919 (Jan 12, 2019)

Ive been out of town on EM call. But this issue is solved. thanks all for help! it runs better now than the day i bought it. i threw out most of that shindy carb rebuild kit i bought. still used the moose jets and sonic cleaned everything 3 times and runs like a champ. i did end up going 155F 165 R and have 13.1 WOT under load. i do recommend the sonic cleaner. The bike even sounds completely different. now i might start working on some of my other carbs.


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## NMKawierider (Apr 11, 2009)

Good..time to hit the trails


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## jeff919 (Jan 12, 2019)

some day bud. more like look the trailer up to it and get cutting fire wood. but really thanks alot. enjoy your weekend !!


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## NMKawierider (Apr 11, 2009)

Well, they are a sport-utility. They do "work" very well. People sometimes forget how capable they are.


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## jeff919 (Jan 12, 2019)

i love it. this is my first 4x4. never had one with winch and plow. we have about 5 acres and even a mud pit and offroad course. i play some in between. 
i did learn a hard lesson putting it back togeather. i always use this soft greese to get gaskets to seat it desolves in fuel and its usually fine. when i first put it back togeather it woulnt go over 20mph. saw the diafram's wernt moving on both carbs. had to take it apart and reseat both of them. guess they shrunk in the cold whike i was away. i know you help everyone out and probally new that. but when both slides dont move and someone just had ut apart. could be the problem. i really learned the hard way on this whole carb project. but thanks to people like you ans the others, im alot more confident.


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