Carb help please

[Guest]

I bought a 78 gs750 and I've done the carb rebuild on it, cleaned everything with an ultrasonic cleaner and carb cleaner, new ignition coils, new plugs, new manifold gaskets, running pods (Stock filter and box where toast),upgraded my main jets from 102.5 to 115, and raised the needle one notch. When I got the bike it was difficult to start but now it starts reasonably well and idles well too. It doesnt need choke to run, when I throttle the bike it jumps to 2500rpm well and then totally dies. I've torn the carbs out at least 1 dozen times and checked everything even set the floats to factory height. Any suggestions on what I can do next would be greatly appreciated.

BTW- I love the restoration process thus far and the resources on this site.

Thanks,
Justin

[tom203]

Have you changed the o-rings at cylinder head/carb bootinterface? part #17 in this diagram?

https://www.partsoutlaw.com/oemparts/a/suz/50d3fec4f8700230d8b4dca7/air-cleaner

[sam000lee]

Have you adjusted the valves?

When you cleaned the carbs, did you replace/check O-rings?

If you haven't seen this yet, cylcleorings.com is the go to place for this.

[BurtyMcSquirt]

Yes I've replaced all rings and gaskets from the engine back. I haven't done anything with the valves yet though. Would that cause the bike to fall flat like that?

[hannibal]

The valve clearances get tighter over time. Eventually they can get so tight, they won't fully close amymore. Not sure how this could cause your issues, but it's a must do item.

I'd also suggest a proper vacuum sync of the carbs.

[BurtyMcSquirt]

You're right, eventually the valves will have to be addressed and I'm currently making my own version of a carb balancer so I'll see what that can tell me. Have you ever use a colortuner by chance?

[Nessism]

You have bumped up the main jets 5 sizes. What led you to that decision? Seems kind of drastic.

Where are the pilot fuel and pilot air screws set? The pilot fuel should be about 1 turn open and the pilot air roughly double that.

Did you get high quality pods like K&N's or are you using the cheap generic pods like Emgo's?

[BurtyMcSquirt]

I bumped it up based off of previously viewed posts (too much?). before the new jets it wouldn't start easily and would only run on full choke. I do not have K&N pods nor emgos but I'd guess they're not much different then the emgos as per the recommendation of a friend thats done several cafe racer builds. My pilot fuels are at 1.5 turns and pilot airs at 1. Thanks again for the assistance.

[Steve]

With the fuel screws at 1.5 and the air screws at 1, the pilot fuel mixture is WAY too rich. If your valves are tight (the usual situation), the intake valve closes while the piston is coming up, which lets some air out. The only place for it to go is back through the carb. It picks up a bit more fuel, because the carbs don't really care which way the air flows through, they are going to add fuel. On that cylinder's next intake cycle, the valve opens, the piston goes down and, you guessed it, you get a THIRD dose of fuel in the mixture.

Switch your numbers around for a starting point, set the FUEL screws to 1 turn, the AIR screws to 1.5, then get the valves adjusted.

K&N pods are generally accepted as the best, but they ARE expensive. APE pods are just about the same, they just don't have the K&N logo, but are about half the cost. EMGO pods are cheap "throw-away" pods. They are hard to tune for because their airflow characteristics keep changing. Not sure what you have, but it might be worth looking into at least the APE pods.

.

[BurtyMcSquirt]

Great info thanks. Should I reinstall the factory main jets as well and drop my needle back down a notch? I do get a lot of backfires through carbs and exhaust so multiple doses of fuel would explain that.

I'll need new shims for the valve adjustment correct?

[Nessism]

Stock jets won't work, but 5 steps may be too much. It's hard to say right now. I would have suggested 3 steps on the mains and then try. You can always tape up the outside of the pods to further richen the mixture so you can figure out which way to go. With the mains at 5 steps your next step to try, after doing the valves like Steve says, is to remove the pods completely if the bike doesn't run better.

[BurtyMcSquirt]

Ok. So I'll get 110 mains and work on the valves in the mean time until they come. What would you suggest for the throttle needle?

[Nessism]

I'd leave it where you are. You are going to need a little extra richness just off idle since the pilot jets are still stock.

BTW, getting some of those APE pods was a solid suggestion too...

[BurtyMcSquirt]

Thank you. I'll let you know what happens.

[allojohn]

There are a few here with '78 750's with K&N pods. I run stock pilot 15 and main jet at 117.5, e-clip at position 4.
Fuel screw 1 full turn out, air at 1.5 ish, with highest idle. Syncing the carbs will make a huge difference so after you've adjusted the valves borrow a Morgan Carbtune and git er done!