ATC70 valve lash question

[thestud25]

Quick question and I am pretty sure I know the answer. If my 70 is back firing through the carb and has an erratic idle, it means the intake valve lash clearance is too tight, correct? I set it at a tight .003 clearance.

[briano]

It could also be the points. But yes .003 or .004 is where I set mine

[fastatc70]

I set the valves to the Honda xr50 setting that is .005 and .007 on the exhaust.

Also check for a air leak at the intake. The other problem it could be in running to lean.

[thestud25]

I have the carb dialed in. I will adjust the lash on the intake valve again and see what happens.

[Daddio]

Did you just rebuild the engine? Just a silly question.

[Daddio]

Quick question and I am pretty sure I know the answer. If my 70 is back firing through the carb and has an erratic idle, it means the intake valve lash clearance is too tight, correct? I set it at a tight .003 clearance.

Actually the Honda repair manual says .002 inches or .05 mm.
I personally set mine to .004 inches. I give mine a little more because when I have it at the track running it wide open for five plus minutes at a time they tend to get very hot.
Sorry I had too many zeros. I fixed it.

[thestud25]

Did you just rebuild the engine? Just a silly question.

No, didn't rebuild it. I bought it not running, adjusted the last and got it running, but still back firing through carb and erratic idle.

[Daddio]

If you would have had it apart, it would be a little easier to diagnose. I'm not too sure what to say. I guess eliminate all of the easiest things first then start looking inside for stuff like carbon build up, bent valve and so on.

[thestud25]

If you would have had it apart, it would be a little easier to diagnose. I'm not too sure what to say. I guess eliminate all of the easiest things first then start looking inside for stuff like carbon build up, bent valve and so on.

Will do, thanks man!

[jb2wheels]

How are the points and condensor? 70s are very sensitive since the point gap also sets the timing. Even if they're right, worn points will give you issues.

[thestud25]

It has been over 15 years since I have dealt with points. It has monster spark though. What clearance should I gap the points at?

[Daddio]

I don't have my manual here at work and I can't remember right off the top of my head. There might be a free downloadable manual here on this site though.

[Daddio]

Here is the link. Just about everything you can think.

http://www.oscarmayer.net/atc/manuals/

[tysonhorn]

I am currently dealing with a similar issue. One of my 70s used to run great... then it sat for a year and I can hardly give it any throttle without it bogging down and dying. I have tried different jetting, different carbs, replaced a couple gaskets. Finally I checked the valves. Intake was at .004 so I left it. Exhaust was at .007 so I brought it down to .004 and it made a huge difference. Now it only dies on me every once and a while if I get on it too hard. I replaced gas and checked points as well. Im wondering if adjusting exhaust valve to .002 would do better or worse.

[shortline10]

The only time the valves would be causing your issue is if it was adjusted to tight or possibly leaking , to loose would just make them a little noisy . A simple compression test will tell you the health of the motor .
I would bet it was more of a points or condenser issue .