atc70 pilot screw questions

[Tomcall]

My 73 project 70 has the full big bore kit with the 20mm carb. I also added the cdi set up. When trying to tune the carb this is what I run into. My understanding was the general setting for the pilot screw was about 1 and 1/2 turns out after bottoming out. Then play with it to find the highest rpm spot and leave it. Then adjust the idle screw. What I am finding is that the highest rpm spot is with the pilot screwed all the way in.:wondering I always read that the engine should quit at some point when turning it in to the right as well as when turning it out to the left "too far"
By the way it will run thru the gears nicely with the pilot turning in all the way. I am stumped.
can anyone explain what may be going on or do I misunderstand how this is done??
Thanks for any help.

[fallguy666]

I believe the pilot jet may be to big.

[Tomcall]

Ok, I will need to open up the carb to see what size it is.

[barnett468]

Hello Tomcall;


How does it start and run?

What color is the plug, lite tan, dark tan or black?


If you have the orig style carb there will be a gas valve on the carb. If this is the case and your fuel mix screw is near the front of the carb just above the gas valve then your fuel mix screw is probably damaged and needs replacing because the engine should die with it in all the way but if the screw looks like it has a sharp point, then you can try a 1 size smaller pilot jet. .

If the screw is towards the rear of the carb, then your pilot is too small by 1 size and needs a bigger one .

This is providing you have no air leaks.


I would also check the gas level by doing the following:

Get a 10” long piece of clear plastic.

Connect it to the fitting on the bottom of the carb.

Hold it close to the carb with the open end even with the top of carb.

Open the gas valve and float bowl drain valve.

The gas level should be from even with the bottom of the carb where it meets the float bowl to around 3/16” below that. Set float level to correct it if it is out of spec.

[DohcBikes]

If you have the proper mix screw in, it should die when its buried.

If the idle is at its highest when the screw is buried, then what rpm are you able to set the idle at before it dies out? Anything above 1500 rpm and you are likely skipping the idle circuit to get it to run.

If it does idle at a reasonable rpm when its buried, then my guess is that you are missing the o ring for the mix screw, the o ring is damaged, or possibly the tip is bent or broken and missing.

The first thing I would do personally is pull the carb, clean it well. Check the components, parts and circuits in the carb for proper operation and set the float to spec per the manual.

Take the mix screw all the way out and inspect it. Check the carb body for the o ring if it is not on the screw.

EDIT: and I just realized that Barnett said most of this already. That's OK I owed him one LOL.....

[barnett468]

If you have the proper mix screw in, it should die when its buried.Hey R O O K I E, what is the difference between a PROPER mix screw and an IMPROPER one? :p

If you were smart like eye iz, would have looked it up in the manual and noticed that PRIOR to around 1978, the idle mix fuel thingamajiggy screw is on the cylinder side of the slide, which in this particular case, would make it a fuel regulating screw so to speak. Around 1978, they moved the location of the thingamajiggy screw, to the rearward side of the slide, which in this case, makes it an air regulating screw so to speak, even though the more air you let in the leaner the mixture becomes which means it is also a fuel mix screw so to speak sort of. Now, if was one of those lovely PJ carbs…

Now, had you not only read my post but UNDERSTOOD what it meant two [2], I wouldn’t be making you look like you did NOT stay at a Holiday Inn Express last night right now!




EDIT: and I just realized that Barnett said most of this already. That's OK I owed him one LOL.....How ya likin that EDIT button? I see ya usin it often enough. :lol:


:beer

[Tomcall]

This is a brand new carb that came with the full big bore kit from hondaatc70.com. I had checked the float level already using the method barnett 468 mentioned. I did have to adjust it. I have feeling that the o ring may be the issue. Just seems most likely. I'll let you all know.
Thanks for the good feedback.

[barnett468]

Hello Tomcal;


Jest keep in mind the location of the fuel mix screw thingamajiggy as to whether it is on the cylinder or air cleaner side of the slide, because aside from the screw setting, the location of the screw will tell you if you need a bigger or smaller jet.

[fastatc70]

Is this a case of maybe he has an air leak, the idle screw is in so far that the air screw is no longer a major part of the carb circuit. Does the motor run erratic? Idle up and down? Surging ? Like Barrett said what does the spark plug look like?
Possibly a bent or broken air screw.
If the air screw is all the way in the motor should die. That's why I was mentioning the air leak.

[Tomcall]

Here is a pic. of the pilot screw. Is anything missing on it?

[barnett468]

Well it's hard to tell in the photo.

Is it buggered up. it does not look like it is extremely bad or has been modified etc.

EXACTLY where is it located on the carb?

[barnett468]

Here is a pic. of the pilot screw. Is anything missing on it?Nothing is missing but I might put a little grease or oil on the o ring.

[Tomcall]

It looking buggered up is probably the pic being sharpened in my ps program. I thought they normally have a small washer and o ring above the spring?

[barnett468]

hello;

i asked if it was buggered up.

your carb is not orig style therefore the screw is simply a different style. the o ring position is likely correct for what you have.

you did not tell us where the screw is located.

[Tomcall]

It is located on the cylinder side. I am not sure what you mean about how the location makes a difference?
Thanks

[barnett468]

please re-read post 4 carefully, it's a bit complicated. a forward mounted screw typically works just the opposite of a rearward mounted one. both affect the idle.

your screw does not appear to be shutting off the gas at idle like it seems it should so this is bizarre. this is why it seemed like your screw might be damaged or leaking air. if it is not, then it is possible the screw is designed to work like a rearward mounted one. it is possible to design it this way but it would be extremely unusual in my experience.

at this point i would simply try one size smaller pilot. if you then need to turn the screw OUT a little to achieve the highest idle, the screw functions like a typical forward mounted screw does. if you still need the screw all the way in then try a pilot that is one size larger and see what happens.

what does the carb say on it?

do you know if it is chinese?

[Tomcall]

This is a photo of the carb. I am dealing with. I need to note that I have a 74 atc70 and I installed the exact same big bore kit that includes this carb. That carb acts normal. I should pull the pilot screw out of that one and compare, duh! should have thought of that sooner.

Question: when you say a larger or smaller pilot do you mean the screw it self? Do they come with different size ends on them?
Thanks

[Tomcall]

The pilot screw on the right is from the 73 having the problem. The screw on the left is from the 74 which works fine. Note the o-ring on the right is a little damaged. Is that enough to cause a problem?

[barnett468]

the o ring may or may not be leaking. i can not tell by looking at it.

since both screws are the same why don't you simply put the good one in the bad bike and see what happens?


don't forget to oil or grease it or it might bet damaged.

you can get o rings at any motorcucle dealer or most hardware stores.

[oscarmayer]

chances of the o-ring causing a problem is very unlikely. you have a Chinese carb. they are very inconsistent with their quality. i would say that since the one on the right has a different shaped tip and that could me the problem. it is slightly bigger than the one on the left.so it may not be even entering the metering hole where it seats properly. This would cause an issue as they are mating surfaces to seal and control flow.

[Tomcall]

I see what you mean. I'm not sure they are different sizes. The photo may be a distortion. I will look at it closer tonight. I will try exchanging the screws and see what happens.