Advise on carb setup. Running rich I think

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Advise on carb setup. Running rich I think

Post  Nm767 on Sun Feb 02, 2014 2:53 am

So I finally got the starting down but the bike appears to be running rich. Due to just purchasing this bike my father and I checked the valves, plug, and jets. The plus was black and sooty and wet.

The valves were in spec except the right exhaust valve was at about a .011. We tightened it up to spec.

Here's the jets in my carb. Using a FMF power up lot with FMF slipon, uncorked.
170 main jet
68s pilot jet
FMF needle on 3rd clip.

According to jet guides that's all correct. I live at 1200 ft. Most riding will be at 1000-3000 ft but all has been at 1200 or below so far.

Bike is backfiring (well after firing). While idling, slightly while on the throttle , and a lot while decelerating. Also while has the throttle a a slightly faster idle I occasionally hear a noise like a poof of air coming out the pipe. Completely different the the exhaust noise.

Any suggestions on what might be causing this? I believe this is running to rich but not sure why.
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Re: Advise on carb setup. Running rich I think

Post  Mauser on Sun Feb 02, 2014 9:14 am

Have you set the mixture screw as per book ?.....

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Re: Advise on carb setup. Running rich I think

Post  Mad Frosty on Sun Feb 02, 2014 11:59 am

The jetting guide its not the law written in stone , its just a guide to get you somewhere close ,  every bike is slightly different you might need  go up or down one size , buy  some spare jets and try one change at a time . Going by your description it sounds that you are running a little rich , i had the same problem with the stock carburettor , it was slightly rich with the recomended jetting ,  also since i have changed the exhaust system and sealed it well , the backfireing/popping on deceleration dissapeared completely , shame really because i liked the " racing " sound of the stock exhaust system as i believe its part of the pigs character .
 BRP smile


Last edited by Mad Frosty on Mon Feb 03, 2014 7:42 pm; edited 1 time in total
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Re: Advise on carb setup. Running rich I think

Post  AURORA on Mon Feb 03, 2014 6:36 am

Check your exhaust maniflold gaskets for any leaks.
You might be running lean.


Pilot jet/mixture screw:

If you can start it cold with no choke, the pilot circuit is too rich.
If, when warm, the idle seems to hang when blipping the throttle, the pilot circuit is too lean.

The best way to set a pilot circuit is with an RPM guage. Warm the bike up and turn the mixture screw to where you get the highest RPM. If it's below 3/4 turns, or above 2.5 turns, change the pilot jet and try again.

On a 2-stroke - You should be able to ride in 3rd gear, throtlle BARELY cracked open, and it should cruise smoothly. If it sputters and crackles, the pilot is too rich. If it bogs, the pilot is too lean.

Ona 4-stroke - If it stalls in corners, flames out, and is really darn tough to start, the pilot is too lean. If it feels a little dead until you wind it out, the pilot is too rich. Another test it to rev it out a little in 2nd and then let the throttle snap shut. As it's decelerating, there should be very little backfire or popping - if it pops the whole way down, the pilot circuit is lean.

Needle Clip:

On a 2-stroke - Riding in 3rd gear, with a warm engine and the throttle BARELY cracked open, roll the throttle to 1/2. If the bike sputters and crackles, and you feel like you have to keep rolling on the throttle to smooth it out, the needle is too rich. If, on the otherhand, you get the dreaded 'buhhhhhhwaaaaa', the needle is too lean.

On a 4-stroke - Riding in 3rd with the throttle barely cracked open and roll the throttle open to 1/2. The engine should pull smoothly... if it hesitates and threatens to stall, then the needle is too lean. If it feels 'dead' and won't pick up RPM quickly, then the needle is too rich. An overheating thumper that doesn't have a radiator problem typically is an indicator of a lean needle.

Main jet:

On a 2-stroke - Riding in 3rd, with the throttle BARELY cracked open and cruising along, whack the throttle wide open. If you end up with a set of handlebars impacting your nose, or you loop out, the main is perfect! If it crackles, smokes, and won't get 'on the pipe' quickly, then the main is too rich. If it gives a 'buuuuhhhhwwaaa' sound and feels like it's sucking for air, then the main is too lean.

On a 4-stroke. If the engine feels like it's run into a wall and won't pull full throttle - the engine just sounds dead - then the main is too rich. If, on the other hand, it surges, the main is lean. A lean condition will also give you some 'pinging' and a pure white plug.

Advanced Topics:

I will continue to add to this FAQ as time allows. I'll start with the needle because that's the circuit that is 'in play' the most.

The needle regulates the mixture from around 1/4 - 3/4 throttle. Most people are familiar with the clip position, as it's the most common adjustment, but there's much more to the needle. The jet needle is a long rod that fits into the needle jet. On most carbs, both are replaceable with different sized components. As the throttle is opened, the jet needle is retracted from the needle jet and this creates space between the two for gas to flow through. The more you open the throttle, the more the jet needle is pulled out of the needle jet, and consequently the more gas can pass through the increasing space between them. Below I'll outline the various parts of the jet needle.

Length - The relative length of the needle is adjustable by raising or lowering the clip. If you lower the needle (by raising the clip), then the needle sits deeper in the needle jet. This leans out the mixture across the range of the needle. Conversly, if you raise the needle (by lowering the clip), then the needle is further retracted from the needle jet, and this richens the mixture across the needle's range. Needles are offered in various lenghths. If you have a needle which is still too rich, even though it's in clip position 1, then you need to order a longer needle. For example, needle 'A' in clip position 1 is the exact same relative length as needle 'B' in clip position 3. If you had needle A in your bike, and it was still rich - even though you had the clip in position 1, then you could change to needle 'B' and lean things out by going to clip position 2.

Root Diameter - Needles are offered in several different root diameters. The jet needle sits in a hole in the needle jet, as mentioned. The clip position determines how deep it sits in the hole. The root diameter, on the otherhand, is the diameter of the needle at it's pointy end. The wider the root diameter, the smaller the space between the needle and the hole in the needle jet. Therefore, I needle with a larger root diameter will be leaner than a needle with a smaller root diameter. The root diameter overlaps with the slide cutaway, which is to say that it affects primarily 1/8th to 1/4 throttle mixture. Typically you would swap for a needle with a larger root diameter to compensate for high altitude (or extreme heat).

Needle taper - Needles taper from top to bottom. As with all principles regarding the needle, the taper is relative to the diameter of the hole in the needle jet. Tapers are rarely changed, but here's a condition which warrants a taper change. Let's say the jetting is perfect at 1/4 throttle, but becomes increasingly leaner as you approach 3/4 throttle. In that case, you would want a needle with a shallower taper. Conversly, if the mixture is great at 1/4 throttle, but getting richer and richer as you approach 3/4 throttle, then the needle taper needs to be steeper. In my experience, needle taper only needs to be changed when the factory mis-spec'd it to begin with. Under very rare circumstances, big modifications to the motor - such as an overbore kit - will require a change in needle taper.


Remember that jetting needs to be adjusted for every 2000' elevation change and every 15 degree temperature change. If it was jetted right this summer, it's sure to be too lean during the winter. If you rejet it now, when it's cold out, make sure to lean it out a bit in the spring.

Good luck!
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Re: Advise on carb setup. Running rich I think

Post  Nm767 on Wed Feb 12, 2014 6:53 am

Here's the plug I pulled. And the picture makes it look lighter and dryer. It was dark and wet to touch.

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Re: Advise on carb setup. Running rich I think

Post  Mauser on Wed Feb 12, 2014 8:10 am

Would indicate she is running rich, is the airfilter all good and choke operating as it should, did you check mixture screw ?

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jetting

Post  temporary kiwi on Wed Feb 12, 2014 9:08 pm

does look rich, the way to adjust the jets is to take note of how the mtr responds at different throttle openings, i put marks on the twist grip so i know how far open to have the throttle, i do this to check each range of jet [ 0-1/8th = idle circuit 0-1/4 pilot, 1/4-3/4 needle, 3/4- full = main jet], in combo with cleaning the plug and then riding the bike as much as possible in that throttle range, then a plug chop in that range, adjust only 1 jet at a time, also need to make sure that the float is set a little low to prevent it affecting the readings, the idle quality is important for riding as it stops the mtr stalling, i usually start here and work my way up, each jet range overlaps to some degree, i usually set main jet 1-2 sizes smaller than what gives best top speed, be patient and you will get good results, p.s. take notes of each change, helps analysis

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