Idles Great, Won't Take Any Throttle

[Craftman]

I have a 1979 ATC 90 which I have been working on to use as a pitbike at motorcross races. I just rebuilt the motor with: new piston ect, new valves, guides and springs, new stator, points, condensor, plug wire and cap. It also uses a BBR knock off pipe, 110 intake and carb, and K&N.

It starts and Idles great (sounds like a 300ex) but will not take any throttle. Using the 90 intake and carb does not change anything. The plug is black and sooty. I have almost a year in a frame up overhaul and now very near to completion I have hit this stopper. ANY suggestions?... coil? , internal oil leak?

[sandpuppi101]

Sound's too me like a needle seat,when you give it gas it just fall on it's face I bet.Get a new carb kit for her and you should be good.You mite also try getting a 1or 2 sized bigger main also,with that set-up your running.If it does'nt pop on Decelleration,I would'nt assume it would be point's or eletrical.My 2 cent's..

[Craftman]

Yea, when you give it any gas at all it falls on its face and will backfire if you persist. Which does seem like a carb issue.

Do you mean the float needle? Are you suggesting an overflowing carb?

The 110 carb has only been cleaned. The 90 carb has been completely rebuilt with a rebuild kit. The reason I was thinking electrical is because the problem is the same with either carb.

Oh, by the way: adjusting the timing statically while running only makes it idle differently or sometimes run away (during which it will respond to throttle). I havn't set the timing dynamically because I dont have a timing light.

[Gag_Halfront]

Is the advance working? IIRC those have a centrifugal advance. Setting the points a hair advanced shouldn't hurt.

What do you mean "run away"? Does it just idle really high? Could be you need to fix the timing then dial the idle down and you'll be set. I wouldn't be terribly surprised to hear it. My 90 had to be timed a bit outside the marks to run right. Alternatively, did you double check the point and plug gaps?

Usually when something "bogs" or "falls on its face" it is _way_ too rich. The black sooty plug also says "way too rich". When it "breaks up" or "stutters" it's usually lean. If it's idling fine and the very very beginning of the throttle feels ok (before the taper of the needle really starts to leave the jet) then maybe you have the jetting on the main jet a bit big. (Sorry to disagree, sandpuppi).

Does it seem like you have to have your idle screw cranked way in to get it to idle? How far in is it? How high is it holding the slide from the bottom of the carb? When you look down the throat of the carb and back the idle all the way out, how far does the slide drop? Another possibility is that you have the idle way to lean and the carb is way out of tune trying to compensate. Somethimes that happens depending on who's messed with the carb.

Without more info, it's kind of hard to tell, but at least you should have some ideas to get started.

Good luck!

[beets442]

Make sure coil wire isn't grounding out some wheres.or defective coil? That causes backfire too. Beets

[Craftman]

Well I went and fiddled with it some more. It works a lot better with the timing advanced moderately. The timing marks were pointed directlly line to line. When the timing plate was rotated clockwise (more retarded, right?) the idle would slow and it would die. I moved it counterclockwise so that the right side of the timingplate mark was just past the left side of the housing mark. Also honda says points should be 12-14 thousandths but mine seems to work better at about 16 . Strangely enough it seems a little lean now but I haven't ridden it yet. Stock 110 carb jetting seems like it sould be within the ballpark of a piped 90.

Do you 90 guys have similar timing and points settings? Also, how many turns out do you run the air screw? It is 1.75 turns out right now.
The point arc sometimes too, is this normal?

[elcamino12sec]

Same thing happened to me, thought it was a carb problem but the clip came off the needle inside of the slide so it was not getting any fuel. Inspect that first before you go throwing any more money at it.

[dansvan]

Also check the petcock to make sure it's not plugged up

[Gag_Halfront]

My experience was that those motors liked a bit of advance. I always had my 90 timed outside the marks like you describe. Those marks are a guideline. They're a "best guess". If you've got to be outside the marks, be outside the marks. I'm also thinking that if you've added some gap you've also added some dwell and probably, again, gotten a bit more advance out of it. I bet if you closed the gap back down and advanced them a skosh more, you'd get about the same effect.

If you're getting arcing at the points, there's two things I'd suspect. First would be dirty points. Use a matchbook cover to clean them. Never clean your points with anything that can scratch them like an emery board or anything like that. It will leave microscopic debris on the points that will burn over time and pit the surfaces which will cause more burning, etc... The second thing that could cause the arcing would be a bad condenser, but you said you just replaced it. That's a bit of a head scratcher. Check the connections, maybe.

Speaking of matchbooks, Not only do I consider a matchbook an essential tool for cleaning the points, I also used the matchbook as a gap tool on my 90's points. :-)