fuel/spark misfire problem

[winkie74]

I have a 78 atc 90, it has a 53mm big bore kit, dratv hot cam, ported and polished head and intake, 18" straight pipe exhaust, and a new 22mm carb courtesy of dratv. My old carb had a couple stripped screws and leaked everywhere so I ordered the 22mm carb with new fuel line, fuel shutoff, filter, 65 and 78 main jets and some other stuff unrelated to the engine. put the new carb on and advanced the timing a hair and it ran horrible. No lowend power whatsoever and it missed when I changed gear. So I thought since the stock jet is a 65 and the new carb has a 96 in it, it must be getting too much fuel, so I put the 78 main jet in and its much worse now, if I use very much throttle off the line it dies, and it stumbles, and backfires all over when I change gear at wot. It ran great, but a little lean, before the new carb and advanced timing but now it runs terrible. Don't know what to do now. Suggestions? when I turn the choke on it runs better and doesn't die but has no power across the entire rpm range.

[barnett468]

Hello

Until or unless someone comes along that runs that carb this may help you. The 22 mm carb is made in Taiwan so I don’t know how their jet sizes convert to Keihin jets irregardless it sounds like the new carb is simply too lean. I would do the following.

Start with a new or light colored plug, if it is black toss it.

1. QUICK TEST – Just put a 100-102 jet in it and see what happens.

2. GAS LEVEL TEST – If your gas level is low in the float bowl then your carb will be lean. Install clear plastic tube on bowl drain hold open end of tube level with top of carb and open bowl drain valve. The gas will flow into tube. It should be level to 3/16 below the bottom of the carb body where it meets the bowl. If it is incorrect then adjust float accordingly.

3. Reduce ign timing to previous setting, if it is worse than change it back.

4. Install an 85-87 main. It should make it better. If it does, take note of how it runs and install the 96. If it still runs better but not good enough go to next step.

5. Advance ign timing, if it is better leave it, if it is worse return it to previous setting.

6. Install the 96 main jet, if it is better but not perfect go to next step.

7. Install a 100 - 102 main, this should do it.

8. Take a plug color reading after 30 - 45 minutes.



DRILLING JETS – If you do not have the jets you need you can drill your existing jets up to two number drill sizes max. Drilling them more than that will reduce the chamfer angle surrounding the entry hole in the jet to a point where it might adversely affect the flow/performance of the jet. Number drills are NOT regular drills and can be purchased at any hardware store. Find a drill that fits the jet tightly then buy the next size number drill etc.

[barnett468]

Hello


If step 1 doesn't improve condition then do step 2, if it is still not better reinstall the 76 jet and go to step 3.

Sorry for the extra post I have no edit button.



Keep it clean and on topic!!
Howdy

[winkie74]

I just ordered a few jets ranging from 78-95 and a 102. Ill put the 102 in first and give her a good run and see how it goes. I backed the timing off back to stock and it was even worse yet so I put it back to the previous setting. This carb has a 36 idle jet in it now, think I should swap it for a 38 maybe?

[barnett468]

Hello winkie74



Ill put the 102 in first and give her a good run and see how it goes.

Yeah it’s quick and easy, lol.





I backed the timing off back to stock and it was even worse yet so I put it back to the previous setting.

That’s actually a little surprising to me. Are you saying it ran fine with this cam and the other carb? Are you positive the ignition is firing before the timing mark appears in the window? Since it runs better with it advanced you might advance it more than you had it and reinstall the 96 jet and see how it runs for now.

Are you using a timing lite for this? You can put white out on the marks to see them better if need be.





This carb has a 36 idle jet in it now, think I should swap it for a 38 maybe?

Again since that carb is Chinese I don’t know how their jet numbers convert but this is what I’d do, others would do it differently.

The way they are commonly set up is the following providing the fuel mix screw is set close to mfg spec.

If the air temp where the vehicle is is around 50 degrees ie outside, garage etc. it should require around 2/3- full choke and from 0 to 1/4 throttle to start quickly.

If it starts easily at that temp with NO choke and 1/4 throttle the pilot is too large.

If it needs FULL choke and several pulls etc to start the pilot is too small.

GAS LEVEL – Please do this test, it is a new carb so who knows what it is. If the level is off by much jetting may be difficult and will not cure all the carb problems. Just picture the gas level as another “jet”. If it’s not set right, it won’t run right.

[winkie74]

Got the 102 jet today so ill put it in tomorrow and see what happens. It always starts on the first or second pull with choke and some throttle. Ill be back on here if the 102 doesn't help the off idle throttle problem and try to track down a larger pilot jet.