1978 GS750 Backfire Out Carbs

[ZakDanger]

1. Has K&N pods, standard exhaust

2. Carbs have been jetted 2 sizes up, needle clip moved one notch down

3. Carbs have been dipped, blown, sprayed, all o-rings replaced. (intake gaskets replaced as well)

4. Carbs have been vacuum synced.

5. Valves are within clearance

Hard as hell to cold start, most times have to push start it. Some backfiring when it idles, nothing major. Get her up to 40-50, starts backfiring out the pods like crazy. Almost no power in 5th gear. She can take the load in 1st, 2nd, and 3rd, but once she gets to 4th she starts backfiring, and then in 5th its almost all backfire and no power.

I have some ideas, and I'm tuning her pretty much every time I take her out to find that sweet spot. Pretty sure she just needs to be tuned in via my adjustable screws, but I'm open to suggestions. So basically, am I missing something here?

[Guest]

A lean condition will have them spitting out of the carbs. It looks like you have covered all the bases. Something isn't adding up the way it is running. Might want to go back and retrace your steps again.

[BassCliff]

Hi,

Just a wild guess, but I think you'll have to go up 4-6 sizes on the main jets to compensate for the pods. Plug chops will tell you.


Thank you for your indulgence,

BassCliff

[MisterCinders]

For pods and a 4-1, I went from 100 stock mains to 122.5. With pods and stock exhaust, you might not need to jump that far up the range. When you are at 40-50, what is your throttle position? Throttle determines mixture, not MPH or RPMs.

If you are struggling at mid-throttle, do a plug chop and adjust the needle setting. When you moved the needle "one notch down" where did you end up? I think the stock needle setting was slot 2 or 3. You should try the needle in slot 4. If it needs more adjustment, you can shim it higher there, or drop back to 3 and shim from there.

[ZakDanger]

Bass, by plug chops do you mean pulling the plugs and checking the color?

Also, a buddy of mine says retarding the timing just a hair would help out with the cold starting. What do the experts say?

[DannyMotor]

Dont play with the timing. It may only mask the symptoms of a larger carb issue.

[MisterCinders]

Dont play with the timing. It may only mask the symptoms of a larger carb issue.

I would check that your timing is adjusted properly, though. Timing that is out of whack will give lean and/or rich symptoms that will send you in circles on the carb side. If your timing is set correctly, however, changing it to "fix" a mixture problem seems unwise.

[BassCliff]

Hi,

Plug Chops-Highest RPM Methods
(by Mr. tkent02 and Mr. psyguy)


Thank you for your indulgence,

BassCliff

[MisterCinders]

Bass, by plug chops do you mean pulling the plugs and checking the color?

Also, a buddy of mine says retarding the timing just a hair would help out with the cold starting. What do the experts say?

Yes, plug chops involve runnning the engine under load at the throttle position you want to check, then killing it with kill switch and coasting in neutral (i.e., clutch in) to a stop. Then you can pull the plug to see what that throttle setting is doing for mixture.

You have to do the kill/neutral bit, or the engine braking and idling will throw off the results.

[gearheadE30]

It also might be a good idea to check to make sure that you got the choke fingers put back on the right way. They aren't symmetrical, and I did mine wrong, causing hard starting, backfiring in the carbs, and some odd running issues.

[ZakDanger]

Good stuff. Ordered two more sizes up on each my pilot and my main. I'll check the choke fingers now, good call. I raised my needles as much as I could, and I definitely got more performance when I'm at 3/4 throttle, so I'm leaning (heh) toward needing bigger jets. Anybody wanna buy some vm26 jets? On the cheap? I got 100 to 105 and 15 to 20.

Also, if you might have the following parts and be willing to part with them...

1. Tach Gauge
2. Fuel cap
3. Seat lock

Electric question too:

Had some aftermarket blinkers that I threw on just the tail, kept the front stock, now no longer blinks. Lights up, stays on. Not really surprised by this, two different bulbs, different need for wattage, weak circut, not enough to flip the relay... right? I plan on switching the blinks in front to the same aftermarket blinkers, just want to know why, cuz i'm curious, like a cat, thats why my friends call me whiskers.

[ZakDanger]

Also, did a plug chop. Lean.