atc70..carb terms explained please

[lilatv]

I have rebuilt the carb and all is well.

However, I have read a few articles and would like to put a name to the part. So bare with me.


On the 70 carb...what would be the carb slide?

There are two screws on the left side. What is the left one for and the right one for and what is their names? I simply kept track of the stock positions and set them to that point when I cleaned and reinstalled the parts.

thanks

[Dan Tenn]

Im no expert on 70 carbs, but I think I can give some help. The carb slide is the tube inside the carb that moves up and down, and is attached to the throttle cable. I slides up and down (carb slide, get it?). On most carbs they are round, but some have square corners and are called flat slide carbs. The two screws are for air/fuel mixture, and idle adjust. The idle adjust will generally extend out from the carb and be easy to adjust with fingers or screwdriver. It moves the slide up or down a little bit to control idle speed (in is faster, out is slower). The air/fuel mix screw changes the ratio of air to fuel by letting in more or less gas into the mix. Most of the time their flush with the carb, and only adjustable with a screw driver. I believe that in is less fuel (lean mix), and out is more fuel (rich, or fat mix). Hope this helps.

[lilatv]

Okay we are on the same page then...the only thing I didn't understand was the idle screw and how it moved the carb slide (kinda thought that was it, holds the needle).

So looking into the top of the carb, there is a small protrusion that the carb slides slit ..slides over. So the idle screw moves this up and down to adjust the idle speed, correct? Regardless I get that aspect.


What has me puzzled, everything is at stock settings, but appears the little three wheeler might be a little rich at idle , during cold starts not warm and the idle screw, the one with the pointed end, is bottomed out, lean.


Interesting, I am using a replacement autolite plug, maybe its the wrong heat range, I will use the recommeneded ngk and see if it improves.

Thanks for the reply

[TimSr]

Any machine runs richer while cold, so thats perfectly normal, and not necessarily something that needs "fixed".

I was just going to add that the air/fuel screw is called the "pilot air screw" or "idle air screw" and this screw only controls airflow through the pilot jet for teh idle circuit. It is to be adjusted for best throttle response off of idle, by blipping the throttle and turning it in 1/4 turn increments. It has no effect on your lean/rich carburetion once the throttle slide has begun to open. Probably the most common misadvice I have seen is directed towards when and how to use this adjustment. If you are "running rich" or fouling plugs while riding, this adjustment will not help you.

[lilatv]

Okay TimSr.


Lets make this a bit more clear...I may have a comprehension problem...so be patient.


Facing the carb...the screw on the right, sets idle by moving the carb slide/needle up or down.

The left screw adjust air flow through the idle jet and simply helps the transition from idle to part throttle. As a note, mine is bottomed out, if I turn it 1/4 turn out, the engine appears to wanna die or the rpms drop. I have been adjusting it by moving it a little here and there and listening to how the throttle sounds when blipped. Bottomed out seems best.

So a little smoke out the exhaust on cold start is okay, that goes away in a few minutes is okay? What if said the filter opening has some gas on it, is this normal? Could be collected vapors as it sits I suppose.

thanks

[TimSr]

I cant speak screw locations on this specific carb, but they are as Dan described them. As a general universal reference, you should start at about 1 1/2 turns out on the pilot air screw. If the RPMs drop below accepatable levels or it dies, then turn the idle speed screw in (to raise the slide) until it reaches and maintians an acceptable idle speed. Then blip the throttle, note the response let it level back out, and turn the pilot air screw 1/4 turn and blip it again, noting response. When it begins to decline, turn it back and go the other direction and test in 1/4 turn increments. Youll proably end up with a range - ex: if best throttle response is 1.25 turns and 1.75 turns, set it at 1.5 and youre done.

[lilatv]

1 1/2 turns out on the pilot air screw adjust idle screw until idle levels out or is normal, blip throttle and take note, turn pilot air screw 1/4 turn out again, blip throttle, take note..when declines put it about mid ways and be done with it. Gotchya...I will give it a shot later on and post the results...thanks

[TimSr]

Yes. You may make several 1/4 turns before it begins to decline. Then you go back to the 1.5 turn position, and do it again in the other direction.

[lilatv]

Well I was able to get to it sooner than later.

I turned the pilot screw out 1.5 turns and set the idle screw at a decent idle setting, one turn out. The throttle response wasn't so good even after I readjusted the idle setting. Turning the pilot screw out further caused rich smoke to come out the exhaust. I adjusted the pilot screw 1/4 turn increments in, blipped the throttle, let it idle a few seconds, turn 1/4 in blip the throttle etc....I ended up at 1/4 turn out with the idle setting still at one turn out and the throttle reponse was the best there or bottomed out..didn't make much difference...any more and it was sluggish if that makes sense. So I chased my tail on this one. The pilot screw is back to bottomed out and the idle screw is one turn out. That was the best smooth transition into the rpms with a steady introduction of throttle, without a noticable bog. Leave it alone or try another approach? Let me know. Thanks for the reply

[Dan Tenn]

Id say that when you rebuilt the card you didnt get the pilot jet cleaned good enough. The book is a guide, but not an absolute, but it shouldnt be all the way in. IMO, somethings wrong. On the other hand, if it runs good, and plug looks O.K. leave it alone.

[lilatv]

Well here is where I am at now.


First thing I did was take the brand new autolite plug back out, clean the original ngk and re install it.

Adjusted the carb using the pilot screw and found 1/4 turn out or bottomed out to have the best throttle response. The idle adjustment was untouched. At one point in the process the little atc was smoking badly out the exhaust, so it was definitely too rich. Reason I mention it was overly rich, is I only drove it maybe 100 ft after I set the pilot screw back to bottomed out and shut it off...checked the plug this morning and it was fine..no dry soot. The reason for my interest in the carb adjustment was the autolite plug had dry soot all over it, meaning rich condition or wrong heat range. So if putting the ngk plug in an overly rich condition,then setting it back and letting it run a 100 ft distance at a moderate speed is enough to read the plug, I would say the autolite was the wrong heat range. Do you feel it needs more run time or it was enough for a plug reading? I will check just looking for reassurance.

I am fairly certain that the pilot jet was clean, the carb, before I went through it, was very clean for something that sit for 15 years, no varnish, and no crap to speak of in the carb, just a trace of rust in the bottom of the bowl, IMO from sitting all this time dry. Luckily the choke was closed during all this time, so nothing was able to crawl in there and make a home...haha.

So what do you think? If you still suggest I pull the pilot jet and clean it again, I will do it, let me know.

[TimSr]

I agree with Dan Tenn. A carb looking clean means nothing, and the vast majority of clogs are NOT caused by varnish or buildup, and the vast majority of clogs will not be removed with carb cleaner. It takes one grain of sand to clog a pilot jet, and sand will not disolve with Gumout. For this reason, "cleaning" a carb should always include complete removal of all jets, and making sure every orifice will pass a shot of carb cleaner all the way through it.

Make sure its fully warmed up to normal operating temp before adjusting or reading anything. Ignore the exhaust when doing carb adjustments. Make sure you are starting with a correct plug. I clean and reuse plugs, but whatever method you use, the ceramic insulator around the center electrode must be completely clean, and bright white before you start. 100 ft is probably not nearly long enough for a plug reading. You need about 20-30 seconds. Speed is not relevant, throttle position is. You need to do readings at full throttle under load, and half throtle under load, but neither of these readings will have anything to do with the pilot air screw adjustment.

[lilatv]

Well to keep it simple. No matter what position I put the pilot screw in, other than 0 - 1/4 turn out, it revved poorly, there was a noticalbe hesitation or slight bog however a person wanted to look at it.

No sand around these parts of the woods. There was always a filter on it, probably never driven in mud or water.

When I had the carb apart I put fluid through every hole available and a nice clean steady stream came out the other end.

I would say given my speed it took every bit of 20-30 seconds to make the 100 ft trip last night.

I will check the pilot jet again, possibly later on, just so I can verify that its still clean. I can understand something could easily get caught in it, given the design of it.

I will let you know what I find out.... later


I have a side question since your the administrator, is it common to stop getting e-mail notificaton after the first post? Seems I received one and all the rest I find out by visiting the site.

[lilatv]

Well to build off my last post.

I went ahead and took the carb apart...gotta love the ease of the motorcycle design. I pulled the old pilot jet out, looked fine, blew air through it and it was fine, just so it wasn't a guess, I blew air through a brand new pilot jet, and noticed a very little improvement in the ability to put air through the new vs. old. The new jet is a screw in and the old a press in but they were both marked "38" and with minimal persuation the screw in went right into place. I double checked all holes/ports/etc. If there was a hole I put fluid through, it came out somewhere in a hurry, so its still clean. I started the atc let it get warm, then turned the screw out 1.5 turns, it wanted to bog about mid throttle, I was about to get it around 3/4 to 1 turn out with a good throttle response. If I took it past 1.5 turns this time it didn't smoke but you could smell gas through the exhaust, 1-1.5 turns got rid of that but 3/4-1 turn had the best response at idle. I will ride it or let someone ride it later and mess around with the pilot jet some to see if one setting appears better than another. I will also do a part throttle under load run, shut it off and take a look at the plug.


Appreciate all the replies

[OldSchoolin86]

If your bogging mid throttle then you have to do a needle adjustment. My guess would be that your mid is too lean. Get your scew set back to where it was good for idle to mid and then move the needle clip down one spot to raise the needle up. See if that helps. If it does but it's still not right move it again.

[lilatv]

Oldschoolin86


Funny you should mention that, as that is what I have done. Lemme add some details..and everyone can give me their thoughts.


Here is where I am at.

Carb clean, pilot jet is new.

First thing yesterday.

pilot turned 1.5 turns out, air/fuel one turn out....engine would run half the time....once stopped she died....high likely hood it wouldn't start until the pilot screw was turned all the way in. During this process the idle was not smooth very poor and in most cases it wouldn't run.

Another observation, the engine increased in rpms if I bottom the pilot jet out, telling me she is way too rich.

at this point I am thinking the pilot jet controls the idle, why would it be rich? Only other cause, the needle isn't seated in the main jet and leaking fuel at idle, probably another reason it would idle better with the pilot screw all the way in, so the needle was at two notches from the bottom, I put it at two notches from the top.

I immediatedly set the pilot screw at 1.5 turns, fired right up, even had to back the air/fuel screw out some, idle was smooth. Jump on it, take off in first get part throttle..man there is more power here...kick it into second go to full throttle....muffled pop and she dies. ??????....at this point I restart it and it wants to bogg down and die even in first...required a slow process of easing into the throttle letting off easing into the throttle letting off to get ti to run in first again, must have gotten way too much fuel or something...any thoughts? My guess is take the needle down one more notch from the top....that would put it one above the original setting. Sound about right? I became frustrated and put it up... so I could be clearer the next day. Seems like I made some progress though.


Any thoughts feel free to jump right in...sorry about the rambling posts.

I have noticed that this carb is very much simillar to the edelbrock/carter style carb on my truck, thats interesting.

thanks for the response

[lilatv]

Well I believe I found the culprit....I thought about it last night and came to the conclusion the needle must not be set right, then this morning I read Oldschoolin86's post and thought, what a coincidence, I will definitely persue that approach one more time and being I went from two notches from the bottom to two from the top, I went three from the top or three from the bottom (same thing) and it worked.

I adjusted the needle down one notch, third from top, set the pilot screw at 1.5 turns, fired the atc up, set the idle and she ran fine. I adjusted the pilot screw a little here and there, over 1.5 and the idle began to decline, no real power gain up or down, so I left it at 1.5 and called it good. Most noticable difference is the idle has smoothed out greatly. I was at the point were I was trying to perfect it from a performance aspect, then I had to stand back and say to myself...its only 70cc...I am sure everyone has been in a simillar situation...the gearhead disease...more power...haha.

More I think about this carb, I suppose the idle and main were drawing fuel primarily from the main jet/needle given the pilot jet could be turned all the way in. I guess these carbs are so simple that its more of a balancing act as to what provides how much fuel (pilot or main), than the carb being dependant upon one function over the other at idle. It rans simillar whether the pilot turned in or the pilot out and the needle readjusted, only improvement was the idle smoothed out when the pilot screw was used and the carb adjusted properly. Who knows?

Thanks for the replies

[Dan Tenn]

glad you got it fingered out. Seems like you learned a thing or two in the process. Good job.