Thread: Won't start - Will backfire :-(

I just purchased a 1974 CT70 (K3). When I kick it a few times, it mostly spins a few revolutions but sometimes it backfires (from the tailpipe) about once out of 15 kicks.

Here is what I’ve done so far;
Replaced the carb with a new aftermarket, jetted for 70CC.
Replaced the battery, charged it and installed it.
Checked the points & cap.(look pretty new), measured points gap at .015”
New spark plug, gapped at .025”
Drained all the gas out of the tank & lines and replaced with new
Kicked it with the spark plug out leaning against motor fins, spark looks OK
Removed spark plug after about 10 kicks, smells like gas on it.
Horn, turn signals, brake light all seem fine
Remove, disassembled, cleaned, re-soldered RUN/STOP handle switch.
Removed the new carb and made sure the bowl had gas in it
Checked parts of the K3 wiring diagram against the bike, looks OK
Double checked neutral switch and neutral light is on.
Checked regulator diode
Wiring under the seat looks OK. Wiring in headlight looks OK

It is possible there is a problem with the timing? How could the cylinder fire with the exhaust piston open?

I’ve been working on it for about 5 evenings now. Any suggestions (besides throwing it into the Puget Sound)?

Thanks,
Scott
Seattle, WA

Reset the points gap to .013 and check your intake and exhaust valve gaps. Set them at .002 in

Looks like you have done quite a bit to it the bike already. After checking the valve gaps, instead of kick starting it try to push start it. Do this on a slight decline with the bike in second gear, make sure you hold the shift lever all the way down when coasting as this will make the transmission be in a "neutral". After getting some speed up just let up on the shift lever. This will make sure your motor is getting a good spin for a start up.
I too had to use this technique when I fired my K3 up last year after sitting for over 5 years! Mine started after about 5 feet of engaging. After the initial start I had NO trouble starting it again with the kick start. Good Luck and let us all know how it works out for ya.

OK, 2 good suggestions. Thanks bjatwood and Hondaman!

I will check closer the points gap and adjust if not at .013.

I never thought about the push start method. I'll give that a try too.

Do either one of you know how to set the timing? I read through the manual and quite a few posts and came up as useless as the Mariners today (an 11-1 loss to TB) as I took a break from fixing up the CT70.

I'll give these 2 a shot first and report back.

Scott

Hook up a timing light to a seperate 12V. battery source, hook the plug wire up to spark plug wire, take off the timing cover and see if it shows fire (light) on the F mark when you kick it. Also check that the cam timing is right. I had a ct70 once that had a bent exaust valve. (it stuck open after sitting a long time) and was doing what you discribe. Do you have good comp? if so its not a valve. good luck Grant. Also to change timing on a 70 you change the point gap a little. closer advances it, wider retards it.

bjatwood, I tried your starting method on a 30' of downhill blacktop. It ended up sounding like one long kick with about 3x backfires in the 15' of turning the motor over down hill with the key on.

Hondaman, I checked the gap closer and I think it was close to .019", I reset to .012". That measurement was with the "T" on the flywheel at the notch on the motor case. I could just barely get at the points screw through the side opening on the flywheel, the maximum points lift was right at the end of the slot. I did not attempt to check/set the valve gaps yet, mostly because I've not got it straight in my mind yet. I will do so and check.

Grant's trail, I have been working with the cover off this whole time, but I have not had the flywheel off yet. So far, I've been working with the "T" mark and not the "F" mark. I'm not sure the differrence between the two. I'm not sure if the valve is bent or not and I have not checked the compression, but it is much harder to kick start the minibike with the spark plug in as compared to it out. I will see if I can perform your timing suggestion.

Thanks for the input,
Scott
Maybe it's time to find a first aid station for my K3 in Seattle.

The T mark is top dead center of the piston in the cylinder bore, The F mark is the timing mark you should see light up with a timing light when its running. I had one do the same thing kick it a bunch of times then a spudder out the tail pipe push it and a couple of back fires, I had a bent exaust valve, but a exaust valve set to tight would do it too. good luck, Grant, PS yyou don't have to take off the flywheel to do this.

When the F mark is aligned with the notch on the casing your points should just begin to open. Align the F mark and look at your points through the opening in the flywheel. You should see the points just beginning to open. Rotate the flywheel back and forth and look closely. You also better check those valve clearances. That is one of the main reasons a bike will not start along with bad, dirty, or misgapped points.

How about a couple more simple things that can be easy to overlook:
- Check the air filter. From what you've said it sounds like you're getting gas, but you've got to have air, too.
- You could try loosening the exhaust at the head. If it starts it will make a lot of racket, but that would indicate that the exhaust is clogged.

Good Luck

-kevin

Originally Posted by FatAirKiter

New spark plug, gapped at .025”

Have you checked spark?

I would change plug, try again. It's cheap and at times very effective.