Help with air screw setting on RB-carb

[joey11]

I recently got my bike back together after and had my carb done by RB-designs. It started right up, but has a bog when you wick the thottle and it seems like it does not want to clean out. It also has a bit of oil coming out of the pv drain tube.
I was wondering if anyone could help me out and let me know what the "ballpark" setting would be for the air screw. I played with it and had no luck getting rid of the bog.
I'm at about 600ft above sea level, running 50:1 with 94 octane.

[Treesmacker]

I bet you have a lean condition because of the cooler temps.
Try raising the needle one clip position (clip position #2 to #3).

[joey11]

Yeah it is cold here. Temps were in the upper thirties yesterday, but i had 175 main in it before the mod and I believe he put in a 178.
I'll give that a try though.

[rpduc]

I agree about the lean condition due to cold. The needle move fixed that same exact behavior in mine, but I had already rejetted according to the chart and correction factor in the manual.

32deg. calls for a 1.04 correction (sea level) so if you started with a 175 you should have a 182 in there. The needle shift will fix the midrange, but you might still be lean on top with a 178.

I run a 178 midsummer in my MC250.

[webmaster]

Where is the bog? Is it stumbling or burbling right off the bottom? Or is it the transition from bottom to mid?

With the RB-Designs carb -> I know the sensitivity of the air screw is much greater. Keep this in mind if tweaking and taking advice on air screw settings from others, you could get off in the weeds pretty fast.

I would note the baseline air screw setting that Ron set up for you - and then go in an 1/8 to 1/4 turn to see if it cleans it up.

I rode Magellan's bike with the RB-Designs Carb - I was very, very impressed. Very Clean and Very Fast. This was without the head mod too.

Oh and sounds like you need to clean out your pv cover and check to make sure the pv linkage is working right.

jeff

[rpduc]

I don't want to hijack joey's thread, but Jeff you mention stumbling and burbling right off the bottom. Mine did that Sat. I didn't want to deal with it out on the ice, so just lived with it, but what is the cure for that?

I assumed joey was talking more like a bog than a stumble. The bog I was talking about was definitely a lean situation fixed by dropping the clip on the needle. Conversely this stumbling seems like a rich condition, but maybe I'm wrong...

[webmaster]

Jetting - kinda like talking politics in a bar... Not sure why I am responding... ;-)

In general the air screw should get rid of a burble right off the bottom. But there are usually other things going on as well. The needle position, idle setting, and air screw setting need to all work together in order to have good running throughout the rpm range AND adjustability that "works right".

That being said, Ron's fixes to the carb means that you should just be able to do a slight tweak to fix a burble off the bottom. He has a three pronged approach - blades to improve air velocity at lower rpm and mid transition, internally the carb circuits are modifed and the needle, main and pilot jets are set to match the other mods.

On stock carbs with a variety of needles, etc. The circuits aren't as "well defined" - e.g. the pilot is leaned out to 38-ish and idle screw position is dialed inward to compensate so that they start to dominate the fuel mixture into lower throttle settings and the air screw adjustment has less effect.

Bottom line - on rpduc's carb - with it's needle, pilot and idle setting, pulling the needle clip up will work for him on a given day. For joey, his RB-Designs mod'ed carb will give him greater control via the air screw to just tweak it and go. But if conditions vary greatly from the baseline, he might have to adjust the needle position as well.

This is based on what I understand - Ron has a much high level of understanding than I do. So I should stop before I get in over my head and into trouble.

Since it sounds like I am pumping up Ron, I should say that I don't have any association with Ron at RB-Designs other than he does live near me and I stopped by his shop to chat with him one day. Meticulous old school guy who has done work for factory race teams, etc. etc. He does sponsor the site here and I view him as a great asset to the gasgas community. He is on the very short list (2 people) that I would let work on my bike.

jeff
webmaster
gasgas riders club

p.s. If your bike has some time on it you should send it to him to replace the nozzle (where the needle fits into in carb) - it wears and makes the bike hard to jet over time - you will just start 'chasing your tail'. If your budget allows, have his other carb mods done and the head mod too. Best bang for the buck.

[rpduc]

Jetting - kinda like talking politics in a bar... Not sure why I am responding... ;-)

Thanks for the info. Somewhere between ice and dirt (March?) I'll have to get my carb to Ron. I've seen nothing but good reports.