1985 Honda ATC 70 - Jeting Help

[stewmills]

Hi. I am new to this forum and just got a new (to me...used of course) 1985 Honda ATC 70. Needed work when I got it and I got it running, but it was missing the stock air box so I replaced it with one of the little cone air filters off fleabay. I understand the rationale for rejetting up since there is more air, but can't seem to dial it in. I have played around with the needle, main, and pilot a couple of sizes up but it still has this loud crackling backfire at the carb and it seems very weak on the pull. I didn't replace the points and have never replaced points before (but am relatively handy).

Could it be that the points need replacing or am I just not hitting the right jet settings and need to keep my focus there? I don't want to spend $100 in jets to try a gazillion options but I really want to get it running smooth.

For comparison, if I leave the stock jets in along with the cone air filter, it seems to run near perfect on FULL choke, but I know that's not good for it.

Thanks in advance for any help you can offer!

[webbch]

Not familiar specifically with the ATC 70 - I have a 110, 185 and a couple 200's, suspect the 70 is comparable...

I work on jetting LAST after verifying everything else...valve adustment, timing chain adjustment, timing, check for air leaks (in both intake and exhaust - my experience with backfiring has generally been due to exhaust leaks), new spark plug, disassemble and clean the carb.

Once all those items are in order, I start with the stock jetting and fuel mixture screw (pilot screw) in the stock setting. I generally increase the idle speed as high as is needed (within reason) to get the engine to idle without me having to keep my thumb on the throttle. If I need the choke on to run, so be it - it's a starting point. Work on slow/idle jet first. Rule of thumb is if you have turn the pilot screw more than 1 full turn off the stock setting, it's time to go up/down on the pilot jet, depending on whether it's too lean or too rich. When you're in the ballpark, you should be able to make the engine run poorly at either extreme end of the pilot screw range. If your jetting is way off, you won't be able to do that starting out. Assuming you've checked out the items above before starting to jet the carb, sounds like you may need to go up at least 2, maybe 3 sizes in the slow jet as a starting point in order to be able to run without full choke.

Another item worth mentioning is that whether rich or lean, even with relatively poor jetting, the engine should run fairly consistently. Surging or failure to return from a rev fairly quickly has been indicative of air leaks or fuel leaks past the float valve in my experience.

FYI - I installed my needle seat backwards last night on my 200x - was idling pretty decent, just couldn't throttle it up at all (plus the carb slide had trouble seating because the needle was binding on the seat in the lower position)...threw off the required pilot screw setting by about a turn as well :) Flipped it back around correctly and ran like a charm.

Last little item - when I'm tuning the carb, I use my timing light...it has an RPM readout on it, which I use to set the RPM to spec values. While making pilot screw adjustments, everyone always says to find the midpoint setting between where the engine runs poorly on the lean/rich ends. When I've done it, that point always seems to correspond to the place where the engine runs at peak RPM for a given throttle position, so now I just watch the for the highest RPM as I turn the pilot screw.