Carb Jet Question

[Bulldog]

I have a GY6 150cc engine with a bigbore 62mm piston, port/polished large valve head, stroker crank and an A12 cam. I live in San Antonio and our elevation above sea level is between 700 - 800 feet.

The fuel is gravity fed from the fuel tank, via a 1/4" petcock, 1/4' Braided clear vinyl fuel line with a clear fuel filter secured inbetween the flow from the gas tank to the carb.

I installed a 30mm Koso flat-slide carb on a 30mm aluminum intake manifold and a 2.75" aluminum intake riser. I started out with a 145 main and 40 pilot jet.

Before starting the engine I turned the air/fuel mixture screw all the way in and backed it out 1 1/2 turns to start with.

The engine fired up right away. The idle is smooth but too high. Backing out the idle screw has no effect on reducing the idle. Backing out the air/fuel mixture screw has no effect on lowering the idle either. The only way I could get the idle to drop was to screw the air/fuel mixture screw all the way closed.

I am uncertain where to begin with regard to appropriately dialing in the carb. My first thought is to raise the C-clip on the slide needle one notch. My rational is that this would help reduce the idle by allowing less fuel into the carb at idle.

PGH or anyone have any guidance on where I might begin? Thank you.

[pghruby]

The slide carburetors are more finicky to set up, but the idle should be something simple to set. When looking at the throttle cable do you have any slack in it? I think it may be too tight resulting in the needle not fully closing. If the cable is slack and idle is still too high, you will need to pull the carburetor apart and look for obstructions that may be dragging the needle or or keeping it open. A 40 pilot is quite large for a 150cc based engine - You may need to drop down to a 37.5 or 35 (most of the sliders came with 32.5 or 35 pilots). If you still have a vacuum operated petcock, the vacuum line from the intake elbow will go directly to the petcock. If you have a manual valve, you need to cap the vacuum port on the elbow.

If you purchased the slider from a source I suggested, the quality will be good and support will be behind the sale. If it came from somewhere else, we will have assume nothing is OK and take one step at a time eliminating possibilities...

[Bulldog]

I purchased the carb from Mike at Buggypartsnw.com.

The fuel is gravity fed. I removed the vacuum barb from the intake manifold and replaced it with a bolt to plug the hole.

The cable has slack in it. Further, I removed the intake tube and confirmed that the slide was resting in a fully closed position. In otherwords, I could not push it down any further.

The carb is brand new, out-of-the-box and came with the 145 main and 40 pilot installed as a recommended starting point by Mike at Buggypartsnw.com with my current engine set up.

Is Buggypartsnw.com a reputable resource in your opinion?

Would a pilot jet, that is too large, cause the idle to run high?

Assuming the carb is clean and unobstructed on the inside, what do you recommend I do from here? Should I drop down to a 37.5 or go straight to a 35 pilot and see how this effects the idle? Additionally, am I correct that before I begin checking my plug color and tweaking the main jet my attention should initially be on getting the idle set correctly first?

[pghruby]

Mike will take care of you, but that carb sounds like it is jetted for a 250.

[Bulldog]

Yes, he is a good guy. What do you think about dropping the pilot jet down to 37.5 and the main to 140?

[pghruby]

Address one problem at a time - To get the idle down, try a 35 or 37.5 (38) pilot jet. Once idle is resolved, you will need to get some spark plug readings to see where the the main jet should be.

[Bulldog]

Okay. Good. I will do this. Thanks.

[Bulldog]

I pulled the bowl off the bottom of the carb this morning and installed a 140 main and a 38 pilot.

The idle screw responded to idle adjustment this time versus the 40 pilot that wouldn't. I was able to get the idle down to a smooth, steady and reasonable level. However, when I began to adjust the air/fuel mixture screw, by slowly turning it in from a starting point of 2 1/2 times out, it only increased the idle and would remain at a steady high idle until the screw was closed off and then it would drop. So I just left it at 2 1/2 times out and took it out for a spin. I stopped intermittently along the way to see if it would idle appropriately at rest. It wouldn't. It would always be too high. I would have to jab and release the gas pedal and sometimes it would drop down and idle lower and then stall. I could sit there for a few minutes adjusting the idle screw and jabbing the pedal and ultimately I could get it to idle low, smooth and steady and remain that way. But, once I rode it around a bit and stopped it would never drop down to the same low, smooth steady idle. It would otherwise be too high of an idle.

This is very frustrating. I read about a few others who have the same problem.

Do you think these are symptoms of having a pilot jet that is still a little too large? PGH, do you think I should drop down to a 35 pilot? Any other thoughts or recommendations?

Thanks.

[pghruby]

You're getting closer, so I would definitely try a 35 pilot. You're lucky you have a place to ride close to home. I have to tow my buggy somewhere to ride it and do the jetting in the field.

[Bulldog]

Well, I have been driving it through my neighborhood. So far nobody has complained. Most folks just look, smile and wave. I don't think they have ever seen a little buggy like it before. The kids really like too. I bought some ramps to drive it up into the rear of my Ford F250. I am looking forward to getting it out on some trails once I get it running right. I appreciate all your help. I am going to order and install a 35 pilot. Thank you!

[Bulldog]

I have not received the 35 pilot jet yet. But, I was wondering if you could use a floor jack to raise the rear end of the buggy off the ground and run it at WOT for a plug test if you didn't have a convenient straightaway do do it on the road otherwise?

[pghruby]

The chance of it slipping off the stands at WOT would cause major injury or damage. I cannot reccomend this as a safe method.

[Bulldog]

Boy, I never thought of that. You are right. That would be a very dangerous situation.

[pghruby]

You could remove the chain or CVT belt, but you are not testing the engine under load - You will be hitting red line before you get to WOT.

[Bulldog]

PGH,

I installed the 35 pilot and left the 140 main as is.

I started the engine with the air/fuel mixture screw 2 1/2 times backed out from the closed position. After letting the engine warm up for about 5 minutes I attempted to adjust the idle to an appropriate level. Unfortunately I cannot get the idle maintained at a reasonable low level. The sensitivity of the idle screw is such that either it idles too high and attempts to creep forward as the clutch is beginning to engage or I can back it off the slightest little bit and it stalls. There is no inbetween. Further, the air/fuel mixture screw does not not work at all. As I turn it inward towards a closed position I get no response. It isn't until it is fully closed before the engine reacts.

Any thoughts, recommendations or advice?