Just another annoying carb!

[Alpha Reign]

So today I finally found time to work on the carb. I raised to needle jet from the third position to the fifth. It worked well until I got to full throttle. So then I switch from a 105 to 110 main jet. I then started the bike again and . It happens when I go from idle to half throttle fairly quick.

I then decided to drop the needle jet down to the fourth position. It stopped smoking but this . This happens when I go from idle and apply throttle fairly fast. If I do it slowly then there is little problems. Also this could be due to the bike not being fully warmed up, but I ran out of time to test.

Any ideas on where to go from here?

[Megadan]

First off, let me state that you should only be judging carb adjustments based off of an engine that is fully up to temp. Trying to judge the state of tune before that point will only lead to false diagnostics. The carb is tuned for when the engine is at operating temps. Anything below that is why carbs have chokes or enrichment circuits.

Second, be aware that this type of carb does not lend itself well to snap throttle changes. Add in an engine that is not up to temp and it can lean out so much that it will cut out.

Also, keep in mind that each jet and setting affects a specific throttle position.

Now, assuming the valve lash is set properly my recommendation to you would be to get the engine nice and hot before making any changes. Once it is fully up to temp (say a 10-15 minute ride at moderate road speeds) start by judging if the idle mixture is set appropriately. A nice rock steady idle at 1500rpm, which to my ears in the video your idle sounds a little low. If the idle is too low it can also contribute to cutting out when you snap the throttle open.

The needle is only going to mainly affect 1/4 to 3/4 throttle position, and the main from 3/4 to wide open. It is hard to judge which jet you are trying to sort out with rapid throttle changes.

[Alpha Reign]

Hmm, guess I was wrong. I was thinking you were suppose to adjust the carb when it was cold. I understand the different positions and what jets affect them. I will try tomorrow and take it for a short ride.

[Megadan]

Originally Posted by Alpha Reign

Hmm, guess I was wrong. I was thinking you were suppose to adjust the carb when it was cold. I understand the different positions and what jets affect them. I will try tomorrow and take it for a short ride.

Yeah, tuning a carb on a cold engine will only lead to it being too rich once up to temp. All carb tuning is done for an engine at operating temp for this reason, with the choke/enrichment being in place to add additional fueling or restricting the airflow to help it run better when it is cold until it can reach operating temps.

Just another question. You have a Mikuni clone, but I see no mention of what pilot jet size you are using?

[Alpha Reign]

Hey Dan, do you know why it was smoking bad? I thought it might be way to rich. I'm using the stock jet the mikuni comes with. I haven't found a place where I can get decent shipping on a pack of them. That is what I want to try next but waiting a month and a half sucks.

[Megadan]

Originally Posted by Alpha Reign

Hey Dan, do you know why it was smoking bad? I thought it might be way to rich.

Generally a backfire (an ignition event back through the carb) like that is due to a too lean mixture and/or poorly sealed valves. Very common to see that on a cold engine when the choke or enrichment is not engaged. If the valve lash is not set properly it can also contribute to this condition, as well as exhaust after firing.

[Megadan]

Just my 2 cents on the carb setup with the stock exhaust and a pod filter given the air temps right now, I would be looking at a 25 pilot and a 110 main jet with the needle in the 4th notch. Then it is just a matter of setting the idle mixture and idle speed, preferably with the idle drop method if you can access the mixture screw while it is running at up to temp.

[Sochin]

1st throw away that airfilter, they are garbage. I ordered what looked like the same one, and the narrow part where the filter body goes on is smaller than the carb intake. Put on the stock hose and filter rather than that one. Try run just the carb and see if it does the same? I bet it will run fine.

[Bruces]

You can buy single jets locally from any power sport dealer ,they are not expensive .

[Cravin01]

Idle is low, you'll get a better throttle response bringing it up a little also. you can use a piece of cardboard around the muffler to remove the carb while it's hot to play with the jetting at temp. don't be afraid to take big steps in jet size. It will still run and you'll dial it in faster.

[Douglass]

A few questions.

What size pilot jet are you using, and how many turns open on the low speed air/fuel screw?

What is you elevation?

In the video the engine sounds like the idle is a little low.

For reference, I am at sea level, I have a Mikuni Carb, 27.5 pilot 2.5 turns out, needle clip in the middle, and a 120 main jet.

If you plan on running that air filter off road, you probably want to look into getting a foam pre-filter. When I ran a similar style air filter on my bike it didn't do much to keeping the dirt out of the carb.

[roundhouse]

Mine does that too and it is fairly well dialed in. Once warm all my probs go away.

[Alpha Reign]

Originally Posted by Douglass

A few questions.

What size pilot jet are you using, and how many turns open on the low speed air/fuel screw?

What is you elevation?

In the video the engine sounds like the idle is a little low.

For reference, I am at sea level, I have a Mikuni Carb, 27.5 pilot 2.5 turns out, needle clip in the middle, and a 120 main jet.

If you plan on running that air filter off road, you probably want to look into getting a foam pre-filter. When I ran a similar style air filter on my bike it didn't do much to keeping the dirt out of the carb.

I'm using the stock pilot jet the mikuni clone came with. I have not been able to get my hands on any. The screw is 2 turns out if I remember right. I do agree the idle sounds low. My elevation is 750. Also, I am going to put the stock air filter back on because I use it for off road.

Thanks for the info!

[DualSport]

I did 1.5 turns out on mix screw, changed jets to 110 and 20. I ride 600-800 feet elevation and it's been 23F-70F lately. I usually start full or half choke whichever works at the time and after a few minutes open choke fully and ride all day.