Carb talk

[xrocket21]

Bike starts kinda hard, and idles low when its cold.

Warm, it starts beautifully.

Hot it seems to idle a bit high.

The carb is a Del Orto? Or something like that.

Theres an idle adjustment, and an idle air adjustment. The idle air/fuel screw is set stock right now.

I thionk I can adjust the needle too by moving a c-clip.

Im thinking too rich on the low end?

[Kurlon]

Throttle closed you tune using the idle mix screw and pilot jet. From your description it sounds a bit lean on the pilot circuit.

[xrocket21]

Throttle closed you tune using the idle mix screw and pilot jet. From your description it sounds a bit lean on the pilot circuit.

gah you lost me already.

I have an idle adjustment screw, and an idle air/fuel screw. Then the main needle itself can be adjusted for what I assume is the mid to top end air/fuel.

Youre saying richen up my idle air/fuel screw?

what increments should I go in? 1/2 turn? 1/4?

fucking dinosaur technology

[Kurlon]

Yup, sounds like you want to be playing with that idle air/fuel screw.

http://www.thumpertalk.com/forum/sho...d.php?t=327405 has a basic idea of what you're going for. Take your time, sometimes even just an 1/8th of a turn makes a difference.

What's the bike BTW?

[xrocket21]

Yup, sounds like you want to be playing with that idle air/fuel screw.

http://www.thumpertalk.com/forum/sho...d.php?t=327405 has a basic idea of what you're going for. Take your time, sometimes even just an 1/8th of a turn makes a difference.

What's the bike BTW?

Okay, thanks for the help. I used to tune carbs on my RC cars with nitro engines, but that was easy, you could really hear the difference. WIth this big thumper, I cant tell shit. Same with my chainsaw, it seems a lot more obvious if its rich or lean.

Bike is a 1997 ATK 605DS

[breakdirt916]

I don't know anything about carbs, but I wouldn't change the needle position

IIRC, the needle position only changes the 1/4-3/4 throttle position response



As Mr. Kurlon had described, you will be playing with the air/fuel screw (which makes slight alterations to how much fuel is made into a vapor at closed-1/8 throttle) and the idle speed adjustment screw

is everything else on the bike tops?

[xrocket21]

This has been a wealth of information, thanks so much.

[Kurlon]

Biiiiig thumpy!

Make sure the bike is warm before mucking with the carb, and sometimes raising the idle speed a touch with the other screw will help you hear the difference as you play with the mixture screw. The first time you muck with it, turn the screw in till you feel resistance and count the turns. Never TIGHTEN the screw, when it bottoms out STOP. Once you know how many turns it's currently at go wild as you know you can always return to where you are now. Try running it with the screw all the way in, 3 turns out, etc. That should produce dramatic changes in the idle to give you an idea of what the extremes sound like.

[xrocket21]

I have the manual for it, and it stated I think 2 or 2.5 turns out to start with, and thats where it is now. I tweaked it a half turn here and there but heard no difference so put it back.

[stoinkythepig]

I think the fuel enrichener (choke) is not working correctly and someone compensated by richening up the idle and increasing the idle speed. Get idle mixture and speed set correctly hot and then figure out why it won't start cold.

[xrocket21]

The choke sees to be working okay, i can see the cable move on both ends, and it definitely makes a difference whether I choke it or not

[breakdirt916]

what size pilot jet is in there? did ya ever pull carb and clean it?

how are the valves on the engine?

[xrocket21]

I pulled the carb and cleaned it when I got it, it looked pretty good though. No clue on the valves, bike only have 2k miles on it.

[Expat]

Try going up a size on the pilot, thay are cheap and it may do the trick, also, when you think it's all clean, clean it again! Get a magnifying glass and check all the jets for build up, use carb cleaner and a strand off a wire brush to really get the job done!

[xrocket21]

seems like the most common jetting issue that comes up are pilot circuit related.the following is a sure method to choose the needed changes.

with the bike warm and idleing turn the fuel screw in till the idle drops/misses.then go back out till the idle peaks/smooths.

this should happen between 1 and 2.5 turns on a fcr carb and 1 and 3 turns on a cv.

if you end up at less than 1 turn you need a smaller pilot jet.more than 2.5 (or 3 turns on a cv) you need a bigger pilot jet.

choose the appropriate size and retest.

I went a half turn, looks like I need a smaller jet. fucking hate carbs

[Paul_E_D]

Wirelessly posted (Mozilla/5.0 (Linux; U; Android 2.3.4; en-us; Nexus S 4G Build/GRJ22) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1)

I would clean the pilot again using compressed air to blow it out after cleaning, then see where you're at. Doubt it needs anything.

[xrocket21]

Wirelessly posted (Mozilla/5.0 (Linux; U; Android 2.3.4; en-us; Nexus S 4G Build/GRJ22) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1)

I would clean the pilot again using compressed air to blow it out after cleaning, then see where you're at. Doubt it needs anything.

The post I read indicated I need a smaller jet, not bigger. If I needed a bigger jet, I could see that it could be clogged and just needed to be cleaned.

I agree that I dont thibnk it needs anything, since its a stock carb on a stock bike.

[SSearchVT]

Wirelessly posted (Mozilla/5.0 (Linux; U; Android 2.3.4; en-us; Nexus S 4G Build/GRJ22) AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1)

I would clean the pilot again using compressed air to blow it out after cleaning, then see where you're at. Doubt it needs anything.

+1... In addition to the pilot and main jets that screw into the carb there are usually a few small holes drilled through the body of the carb itself. These are almost always bound to be filled with gunk. If you bought the bike used - make sure you've got the right jets in it in the first place.

Check the condition of the boot between the carb and the motor. If you've got a leak through a small crack or pinhole there it'll make you think carb...

[Paul_E_D]

rich and lean can have the same symptoms, so I would always suspect the most common issue, which is plugged jets. Carbs always "look" clean. It's the tiniest amount of varnish in the tiniest openings that causes the issues you're having.

[xrocket21]

+1... In addition to the pilot and main jets that screw into the carb there are usually a few small holes drilled through the body of the carb itself. These are almost always bound to be filled with gunk. If you bought the bike used - make sure you've got the right jets in it in the first place.

Check the condition of the boot between the carb and the motor. If you've got a leak through a small crack or pinhole there it'll make you think carb...

I put a brand new carb boot and air filter on

[Kurlon]

I went a half turn, looks like I need a smaller jet. fucking hate carbs

Your Amal may have an air bleed instead of a fuel bleed, in which case the mix screw works reverse of what was posted. I'm not familiar enough with those carbs to say for certain which way it works.

[xrocket21]

Your Amal may have an air bleed instead of a fuel bleed, in which case the mix screw works reverse of what was posted. I'm not familiar enough with those carbs to say for certain which way it works.

the plot thickens!

[Kurlon]

http://www.thumpertalk.com/forum/sho...d.php?t=170435

So, you likely have a Dellorto in there. Mix screw should be a fuel bleed.

Check the valves, check the slide, then muck with the mixture.

[xrocket21]

holy shit, I definitely have a dellorto, it says it right on it, but this guy is describing EXACTLY what my bike is doing

Thanks for the reply. I also have the stock jetting. Where are the typical air leaks coming from? I've had everything tight, because that was one of the suggestions for a solution. My problem is VERY bad running when cold. Specifically, the idle could be held up a bit, and it will kind of slow itself down, then choke. It is also extremely inconsistent when held at a higher RPM during warmup and while warm. When warm, the throttle will act like it is stuck open when I pull into the pits or stop at a light. If I hold the brake and let out the clutch to load the engine, it will return to a normal RPM and stay there when I pull the clutch again. If I even touch the throttle to raise the R's so I can get it out of gear, it will shoot up again and stay there. Then I either shut it off or load the engine again.

Someone mentioned to me that these carbs wear out... not just to the point of rebuilding, but to the point of replacing. This is a 1997, and I think has been raced it's whole life. The only parts that could wear and aren't replaceable are the plunger and the main body that it rides in. The plunger has quite the ding in it where it has been hitting the threads of the idle needle. This is simply adjusting the point where it starts, so has no affect on anything. Any experience with those symptoms?

I will try what you suggested for settings.

Thanks again!

[Paul_E_D]

Well, that's a bit more drastic than I originally thought. It does imply an air leak somewhere. Start looking for a mikuni flatslide on ebay?