So Close! Carb problem????

Socopunk

XS650 Enthusiast
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Here's the LowDown:

'73 TX650-K&N airpods, stock headers, Shorty Mufflers. Main Jet 135, Pilots stock, Needle at 1 slot below middle. Mixture screw 1 turn out from seated. Everything else is stock.

Here's the Problem:

Bike starts on 1 or 2 kicks with choke on. Flip off the choke and it idles nicely at 1100rpm. If left alone will idle for a good 10mins and then dies. If after warm it is given throttle, from idle --> 1/2 throttle pulls great, at 1/2 --> 3/4 throttle backfires like a madman, loudly. Don't want to throw a piston so I don't try for 3/4 --> Full Throttle.

So whats the deal? I'm a newb here, but am slowly learning.

If anyone can help that would be Greatly Appreciated.

James
 
well the 1/2-3/4 throttle is controlled mainly by your needle and if it is backfiring it is too lean. Try raising your needles and see what happens. And I assume your using the stock 38s??
 
Motor dies after 10 minutes..............could be a plugged vent in the gas cap. When the motor quits, lift the gas cap just slightly and have your ear down next to the cap. If you hear a hissing as the air rushes into the tank, you need to clear the vent.

Ignition coils may work fine when cold, but as they heat up in 10 minutes the voltage could be dropping off. Old coils are bad news.
 
Check all the basics, valves, cam, plugs, points. Those tolerances change a bit when it warms up. Make sure you've eliminated everything else like non-fresh gas (over two weeks old it gets iffy), fuel strainer screens or filters, float levels, etc. Are the plug wires old? Old plug cables can leak and short to ground during throttle up.
 
I hope during that 10 minute warm up you have afan blowing on the engine if it is stationary and not moving. The diaphrams the diaphrams! Been there fixed it with replacing the diaphrams. Clogged vent worth looking into as well. Not for the backfiring, but the dieing after 10 minutes enogh time to consume the air in the tank and stop flow.
 
So, I pulled the carbs apart cleaned everything out, when both were apart I decided to throw a bigger main jet in (137.5). Also noticed that the pods I had on were blocking an airhole on the back of the carbs, so I tightened the pods on pulled out a little so the carbs can still breath. Not sure if it was the new Main or the airpod adjustment (or both) but the bike pulls through to full throttle beautifully now. No backfires or bogging at all. The idle seemed a little high (1300-1400) so I let out the screws a 1/4 turn and it helped but sounded like it was going to die any second. Sounds so much better though. Had the fan on the engine to keep it cool and like clockwork about 10mins in it dies. Didn't hear any hissing as I opened the gas cap. So. We're thinkin' coils right? Gonna check the coils tomorrow. If they're bad I'll replace. If not, what else could it be?
 
Soco, are you at 1 turn out on your mix screws now, or 1 1/4? I have a '73 with a similar setup and am still at 3/4 turn out.

As to your issue, it seems like if it can be clocked like that when it dies, might be electrical. Coils seem like a good possibility.
 
Startin' to think I need to change the thread name from Carb problem to Electrical problem. Bike is super hard to start now. 10 kicks and maybe. I'm at 1 1/4 turns on the mix screw. what are your jet settings TC? I'm thinkin' the coils are now shot. Gonna check in the morning.
 
Soco, I'm running stock jets for a '73... can't recall the numbers, though.

Yup, you may have an electrical problem. I spent weeks chasing an electrical problem through my carbs.
 
Can anyone lead me to some info on checking the coils. Blind idiot when it comes to electrical. What does the multimeter need to be set to. AC-ohm? RX1? pulled the coil wire to the spark plug and tested resistance between connection inside the coil and to the metal mounting plate and it is at 6ohms on both coils, cold. Most literature says it should be between .5 and 3. Are both coils bad?
 
Don't overlook your condenser. I used to own an old Honda Civic CVCC that developed this same problem. Coil was the first thing I thought of, but a new one didn't make any difference. Turns out the condenser was bad.
 
Socopunk;
If you have the stock coils then they should have a primary resistance around 4 to 4.5 ohms. Use the meter selected to Ohms and use the lowest scale, usuall 20 ohm scale. Disconnect the 2 small wires from the bullet connectors and measure the resistance of the coil. If you measure from the spark plug lead to one of the primary leads it should be around 8K ohms if I remember correctly.

Having said that, those original coils are old and they only had 10K to 13K volts when new. Its best to replace old weak coils with a new dual output coil.............much higher voltage to plugs. Points need a coil with greater tham 4 ohms primary.
 
So new problems here. Carb seems cursed. If it was just one problem to troubleshoot it might be easier, but it is switching up on me for no apparent reason. Bike starts easy now idles great when cold at 1000rpm. As the bike heats up the idle goes up (to about 1500). So I turned it back down with idle screws. Idle stayed good at 1000. When I pulled the throttle it took forever to come back down to 1000. The second time I pulled the throttle it never came down. Turned idle screws back 1/4 turn down to idle. At this point the idle was fluctuating from 1000 to 1500. Went back up to 1500 and then went down to 1000 and then bike died. WTF!?
 
A high hovering idle that won't come down:
-air leaks
-ATU sticking and weights not returning to closed position
-Mixture screws not adjusted correctly..............do the "dead cylinder method"
-carbs are not sychronized
- butterflys must be centred properly in the carb bores to allow them to fully close
 
Thanks for the info. Gentleman. Went down your list and found a vacuum leak on the right carb. pulled the carb and reset the carb. Also changed the spark plugs, thing runs great now. Now for all the little cosmetic things.
 
Runs great, but the plugs are really black. So it's rich now right? Will a small adjustment like the mix screws help lean it up? I don't want to go back to the popping.
 
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