ATC70 Dies At Wot (headgasket?)

[JohnR.]

My daughter's bone stock 70 will just fall flat on its face if I crack the throttle unless the choke is on. If I ease into it I can get it to tach out but it doesn't pull like it should. The problem started last year and I pretty much parked it after replacing the points and rebuilding the carb didn't help at all. I had the day off today and was determined to get it sorted out. Here's a run down of what I've done so far:

1. new OEM points and condenser from Dr. ATV and installed as per service manual (tried gap from .012"-.016" and bigger gap seemed to make it run better).

2. new spark plug and boot

3. rebuilt the carb with a kit from Dr. ATV

4. adjusted valve clearance to .002" as per service manual

5. replaced gaskets on head side and carb side of intake manifold and replaced plastic spacer.

It got noticeably better when I opened up the gap on the points some but its still wasn't right. Since its acting like its lean I got out a can of ether with a "straw" on the end to see if I could find an intake tract leak. Here's where things get a little weird. I sprayed where the carb meets the intake mani and no change in idle speed. I sprayed some on where the mani meets the head and again, no change in idle speed. Then, as I did that last test one more time, some of the ether ran down where the head and cylinder mate (i.e. headgasket) and the idle jumped up momentarily. I sprayed some directly on that spot and the idle jumped up again. I thought maybe it was just getting sucked into the intake port area so I wrapped a rag around the headgasket so no ether could run down there and the idle didn't come up. I pulled the rag out and gave the spot on the headgasket just behind the intake port a small shot and once again the idle came up.

So, my question is has anyone had a bad headgasket act like an intake leak? I've got 12 years experience as a mechanic but this is a new one to me. I'd really like to get this little 70 straightened out. I've honestly spent more time chasing this stupid problem than my daughter has spent riding the damn thing in the 3 years we've owned it.

Thanks.

John

[Derek]

I would convert it to cdi, you can use a pit bike stator off of eBay then you also get a lighting coil too. You can get a wire harness and a performance cdi box. You can get all you need for under $50.

[JohnR.]

The problem with that is that the ignition pickup and flywheel from a pitbike won't fit under the side cover so I'd have to replace that as well. I'm pretty convinced its some type of intake leak.

John

[CRAZY70MAN]

atorand flywheel good, I use light steel wool, make sure no rust or oil from chain or leaking seal. How did you time? Use testlight. 70's are very sensitive to timing. Hook test light piont to black power wire out of case and pos. from charger to clip or ground on light. Rotate and when light dims, points are JUST beginning to open, align marks on flywheel and case cover. Also, just because carb was rebuilt does'nt mean there is some mor junk in jets or carb? Did you set float height? Where is needle clip? Put it in the middle to start. Those 70's are real sensitive to adjustment, take your time and it will work out for ya' I hope. Also use your manual for carb adjustments....... hang in there, hope this helps.... keep us updated. :beer :beer

[JohnR.]

I set the points to .012" first and then tried going up to .016" since I heard that adjusting the gap can help with problems like I'm having. I've read a big article over on GlamisDunes.com about using the test light method but I figured doing it by the factory manual should be OK. When I rebuilt the carb it was clean to begin with but I went through all the ports with carb cleaner and compressed air. I'm pretty sure that the rebuild kit came with new jets (it was a while ago so I forget). I'm pretty sure the needle is in the middle but I'll check it again when I get back to it (hopefully tomorrow night). I cleaned the inside of the flywheel and the stator plates with steel wool and cleaned them off with electric contact cleaner when I replaced the points. No leaks in there either.

Thanks for the input so far guys. Keep the ideas coming. I would be out there right now fooling with it some more but I'm working in my shed and the wind is brutal out there. Its blowing right through the building. I want to get this done so I can get back to wrapping up my 70 that I've been slowly putting back together.

John

[riverrat]

Head gasket should show up with a compression test. What's the compression?

Double check the carb rebuild, since adding choke changes the problem. That's a clue. My 250 street bike needed 3 rebuilds before I got one passage clean.

Yes, the wind is brutal, I would be out there also, lol.

[JohnR.]

The choke is helping because the bike is lean. When I spray ether near the HG area the idle comes up so there has to be a leak somewhere down there. I'm going to look at it some more tomorrow. I have a head and base gasket out of a gasket set that I bought for my other 70. I might just tear the head and cylinder off tomorrow and replace the gaskets just to eliminate them as potential problems.

John

[riverrat]

I had a 1/8" tear in my intake boot on the 250, sprayed either up there, and there was no change. I have had little luck with spraying either.

I understand the bike is lean. Adding choke helps. So that's a bike clue, and a starting point to find out what's going on.

I doubt a head gasket could cause a lean condition, but anything is possible. I would still double check the carb. Make sure the float is still floating.

Maybe you shouldn't even listen to me, I tried to tune my 250R with the choke on, lol. Anyway, that's how I solve problems. My own problems are always the hardest to solve, when it's someone else, I usually get it right away. This is from a guy who trouble shoots for a living.

Fuel filter is clean?

[riverrat]

Sometimes just talking it out helps you figure it out on your own anyway, even if the advice is wrong.

[riverrat]

You know how to do a leak down test? I don't but I think that will show a head gasket leak.

Maybe fill the cylinder with kerosene near TDC on the intake stroke, and slowly turn it over, and see if kerosene runs out the head gasket?

[oneroom42]

timing timing timing!! i had the same problem with mine. it ran better with the choke on so what does the timing have to do with the choke right? thats what i thought. another guy on here told me to adjust my timing. i did have it set but i messed with it again. i adjusted the points so that when the F on the fly wheel reaches the notch up top the points begin to open. i didnt use anything but a screw driver, pen light, and my eyeballs. i know its not by the book but it works. what a difference the timing makes. id be willing to bet your timing is off. be sure to let us know how you make out.

[riverrat]

timing timing timing!! i had the same problem with mine. it ran better with the choke on so what does the timing have to do with the choke right? thats what i thought. another guy on here told me to adjust my timing. i did have it set but i messed with it again. i adjusted the points so that when the F on the fly wheel reaches the notch up top the points begin to open. i didnt use anything but a screw driver, pen light, and my eyeballs. i know its not by the book but it works. what a difference the timing makes. id be willing to bet your timing is off. be sure to let us know how you make out.

I was thinking that last night also. Not only timing, but check the cam chain timing.

[JohnR.]

I'll check the timing again as soon as I get a chance (hopefully tomorrow) but if it is the timing how do you explain the idle coming up when I give the headgasket right behind the intake port a little shot of ether? I'll check the timing one more time but if it doesn't change it I'm going to pull the head off and see what it looks like. I've got an extra gasket here so I can just slam it back together.

John