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Hammmerhead

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Discussion starter · #1 ·
1983 ATC 70 . New Gas tank. New Piston, new jug, bike now has 115 lbs of compression. Valves are good and set at .003. New stator, new points, new condenser. I have two carbs. The original Keihin (sp) and an aftermarket copy. (Bike runs rough with both) Both carbs have a 58 main and I think a 35 or 38 idle jet. Also running an open UNI filter. Anyway the bike spits and pops out the carb and bogs and dies when you hammer the gas and hold it down, let off and it idles fine. It runs excellent with choke on or a least much better than when it is off. The engine runs the best with the idle screw 1/4 turn out from seated. At first I was thinking a lean condition with an open filter and the spark arrestor removed from pipe, but the plug is wet with fuel. My points seem to be set right as they just start open at the F mark and should be close enough to run better than this Anyone want to take a shot at this?

I failed to mention the after market carb is new and there is no trash in tank or fuel line
 
#4 ·
Rotate the flywheel until you can see that the points are open their farthest and set them so that they're .012"-.016" at that point.Setting the gap at "F" will result in the points opening too soon, too far and the timing being too advanced.
tmas
 
Discussion starter · #5 ·
tmas said:
Rotate the flywheel until you can see that the points are open their farthest and set them so that they're .012"-.016" at that point.Setting the gap at "F" will result in the points opening too soon, too far and the timing being too advanced.
tmas
Click to expand...
I just tried them at .012 and .016 at TDC. It seemed like it actually ran worse. I have tried two different coils because I was worried one might be a 12V coil but don't know if that would even make a difference. No change in the swap though. Im thinking I may need to pull the flywheel and see if I have a wire shorting or something. After messing with the points again it seems to be even worse. Its missing much more on the idle to 1/4 throttle. It will idle fine when leaned down the 1/4 turn out, but give it a little gas and it spits and pops out the carb and exhaust, but more out the carb. When I back the air screw out further it just gets worse and I have to screw the idle screw in so far to keep it running it just starts loading up and dies.
 
Discussion starter · #6 ·
Well I checked the timing again with test light and a battery and it was right. For the heck of it I backed the air/fuel screw out three turns and it would run strong in neutral. At three turns it didn't idle the smoothest or the highest RPM, but it didn't pop and die when you hammered it. When I tried to ride it, it was a little boggy uphill, but didn't die. I could turn the choke on it and it wouldn't idle, but if you kept it running it ran a little stronger with the choke on. Since I have an open air filter and no exhaust baffle, Im thinking I need to go with bigger jets. Any thoughts on how bigger I need to go on the jets. Since the 70 trail bikes have such bigger jets, did Honda intentionally restrict the power on the ATC 70 with that horrible stock air box?
 
Discussion starter · #7 ·
I just tried various Jets, but haven't test ridden with them, just bench ran it. With the Uni filter and baffle pulled out of the exhaust, it acted much better with a 45 idle and a 65 main. Im thinking I will be ale to drop to a 42idle and possibly a 62 main. I have talked to an old timer that said the 70 will never run right without a little intake restriction an/or exhaust restriction. Anyone have thought on this?
 
#8 ·
The stock airbox is configured into the jetting...so you removing it and keeping everything else stock will make it run pretty poorly. You lean it out too much. Remove the baffle...it leans out further. You can get them to run just fine without the airbox with jetting...they don't need it, but they are much torquey if you leave it all intact.
 
Discussion starter · #9 ·
fatcaaat said:
The stock airbox is configured into the jetting...so you removing it and keeping everything else stock will make it run pretty poorly. You lean it out too much. Remove the baffle...it leans out further. You can get them to run just fine without the airbox with jetting...they don't need it, but they are much torquey if you leave it all intact.
Click to expand...
Thanks Fatcaat. I have a baffle on the way for my muffler, so I hope this will help bring things closer. With all the reading I have done on the net with open filters and the ATC 70, I am wondering if it is actually possible to get it to run as strong and correct with an open air filter and pipe. I would love to talk to someone with an open air and exhaust that has actually got the thing running right. I have tried a 40/42 and 45 on the idle and a 60/62 and 65 on the main and the dang thing still runs better with the choke on. It wont idle with the choke, but still will pull strong without the popping of the carb. It also runs better with the air/fuel screw turned out 2+ turns or so even though this isn't the highest idle position of the screw setting. It idles higher turned in further but pops in transition when you hammer it.
 
Discussion starter · #11 ·
Cam timing is spot on and just put a new cylinder and piston in. There are no grounds on a ATC 70 at least that Im aware of, other than the ground from the kill switch next to the coil. I know theoretically jetting can cure this, but I have yet to hear of anyone that steps up and says I have a 70 with this issue and this what I did to fix it. Everywhere I look on the net, problems with open air filters on the ATC 70.
 
#12 ·
Well, I have two TRX70's with open air filters. One does run much better than the other, and it's the one that the baffle is in the exhaust. Without the airbox, the pilot at 38 over the stock 35 corrects the idle and one size over stock on the main is all that's needed. I'd surmise if you are not getting good results with those changes, that perhaps, you need to look at your electrical system, timing, points, plug, etc.
 
Discussion starter · #13 ·
fatcaaat said:
Well, I have two TRX70's with open air filters. One does run much better than the other, and it's the one that the baffle is in the exhaust. Without the airbox, the pilot at 38 over the stock 35 corrects the idle and one size over stock on the main is all that's needed. I'd surmise if you are not getting good results with those changes, that perhaps, you need to look at your electrical system, timing, points, plug, etc.
Click to expand...
thanks for the info fatcaat!
For the heck of it, I just stuck my old air box on which has all the filter frame guts in it, but not the foam element. Immediate improvement even with a 45 idle and a 65 main. No pop when it is gunned, no black smoke which I thought I would get choking it down with the big jets and it revs quickly. When my baffle gets here next week I really hope it helps things enough so I can run an open air filter. Even if it doesn't, I will be glad the thing is quiet and I will go back to the stock air box. These little things are loud without the baffle! I know the neighbors will be happy and my son and I can ease around with it and not draw attention.

BTW, where is your airscrew at fatcaat? Thx!
 
Discussion starter · #14 ·
Finally got it right after $50 in jets. What I thought would just take a few bumps in jets sizes for a uni filter on the 70, ended up being much more. Final jets were a 42 idle and a 75 main as compared to the stock 38 and 58. There is a noticeable bump in low end and top end as well. No more coughing and backfiring, just a strong, fast rev when the throttle is gunned. Air screw seems to be the best at 1 to 1 1/8 turns from seated. Thanks for everyones comments as persistence paid off.
 
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Hammmerhead
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