Help 70 Motorheads

[FlyingW]

Alright guys, I hit a wall with Tonka Trike and its really got me stumped. Why are the 70 motors so difficult to tune? First I rebuilt the carb by putting one of thise "Save a carb" kits from DRATV. It ran so so but I figured it was a worn out carb. I bought a brand new carb and installed it tonight. It started with a half pull and idled beautifully. 0-1/2 throttle was fine but when I gave it full gas the motor fell flat on its face and died. Also, when I give it the gas it's poping through the carb. The new carb is in its stock configuration in that it has a #58 main jet. I am running a K&N cleaner with an aftermarket pipe. I took the air cleaner off but it still ran the same. Any suggestions?

[225]

Flying the problem is your running a 70.:booty:
What you need is one of these.:D
http://i64.photobucket.com/albums/h196/edog_02/1502.jpg

Could be running rich.
Is the air screw adjusted properly?

[FlyingW]

Roger that 225. If I hadn't spent a ton of cash on Spontanious Combustion then I would have. The air screw is fine . It's the high speed circuit giving me problems 1/2-full throttle.

[vegasbret]

where can I buy a motor like that? how much$$ ?


Flying the problem is your running a 70.:booty:
What you need is one of these.:D
http://i64.photobucket.com/albums/h196/edog_02/1502.jpg

Could be running rich.
Is the air screw adjusted properly?

[Mr. Richard]

all the 70's i have needed a bigger main jet,, and i have retapered the needle in my cordless drill,,in an effort to get more fuel,, , they just run lean i guess,,,

[FlyingW]

I wonder if a #75 would be overkill?

[the.all.new.shaun]

i'm currently running a 78 main (the only jet i could find locally) with the c clip in the top, thats with a fast flow foam filter. its not brilliant on the idle when cold but still starts a runs very strong, it does spit back down the pipe when your easing off tho. other than that it does seem perfect, plus not showing signs of running to lean and it was a brand new plug when i changed the jet.

[scary132]

Mine is like that until it warms up. Starts right up and idles great, but if you try to ride it or give 1/2+ throttle it dies. Cold start it has to sit and idle at least 5 minutes or so till its hot, even after the choke is off. Then all of a sudden its fine the rest of the day.

[TravEX]

Yeah, you're running lean, with the K&N and aftermarket pipe. I run a clamp on filter on my daughters 70, and have a 64 main that is going in it tomorrow. It runs fine with her on it and the throttle screw adjusted down, but when I wanna rip on it, won't take WOT like you're saying. Kasey and others are right, but I dunno with jet you need, 64? They skip from 64 to 68, so I went with the 64 like others have used when they use the clamp on and stock exhaust.

[TravEX]

Also, if it won't take WOT with the choke 'off', but it will with the choke 'on', you're lean. Try that.

[FlyingW]

Yep TravEx, that's it. I've been thinking about it all day and considering all of your inputs one has to consider this. Each motor's ability to breath in and breath out is influenced by many factors. Since I'm baselining with a stock motor and stock carb then the true method is to start swapping main jets going up in numbers and use the plug chop method as the indicator of whether the mixture is right or not. The mitigating factor is the K&N air filter and the aftermerket pipe. These two components changed the motors ability to breath. Even if I took these two parts and installed them on another stock motor probably won't have the same effect on another motor. This is because the motors themselves have different breathing characteristics such as wear as a factor. With that said, what works for you doesn't necessarly mean it will work for me as far as what main jet to use. This is where the plug chop comes in. Other factors that effect what jet to use are factors such as carb wear, altitude, temp, timing and so on.

Also consider this, if I take an aftermarket pipe and put a couple of bends in it such as I have then I have just effected the motor's ability to breath. If I tuned the carb to the pipe before the bends then its possible I would have to rejet after bending the pipe because I've effected the motor's ability to breath. The Honda ATC70 motor intakes a small amount of air and fuel compared to say a 250 motor for example. Since the amount of air and fuel being injected in to the motor is so small then it doesn't take much to effect the motor's performance.

The carb condition as a whole also contributes to the motor's ability to breath. Gasoline is abrasive and carbs do wear out. Since I'm starting out with a brand new carb then tuning should be fairly straight forward. I know the #58 jet is too lean and I'm guessing that a #75 is going to be too rich. A #75 is what and 90cc motor would use as a stock main jet. Most guys running a K&N and aftermarket pipes are tuning somewhere between a #62 and a #70 main jet with varying needle positions. I wrote all this as a way of reasoning what I need to do mostly for myself but for the other guys who haven't had a lot of experience at carb tuning. Most guys would buy a bunch of jets and go up or down accordingly but since I have a few extra #58 main jets and an extra #75 main jet then I'm going to take a #58 and start step drilling it until I get a hole somewhere in between. If this fails then I'll buy a bunch of jets and do it that way but I have a pretty good set of micro drill bits and I think I will come out victoriously. I'll let you know how I make out.

[FlyingW]

I drilled the #58 with a .070 drill bit. i came to realize that the jet size stamped on the jet corrosponds to the hole size in thousants. The #58 has a .058 hole. The #75 jet has a .075 hole. Is this a right way of gaging the jet size.

Anyway, with a .070 hole it's running much better although a bit rich. i have a #65 standing by. Tomorrow I'll do the plug chop and see what the plug looks like.

[TravEX]

Sounds good. I'm about to head out and work on my 70's too. Gonna put the clamp on and 64 jet on mine and put my stock airbox on my daughter's with a 60 main in it. Maybe I can talk my wife into drag racing me in the yard to see which is stronger, hehehehehe.