Carb Tuning, surging and hanging idle

[ottoguy]

Changed the plugs a little bit ago, just cleaned and put in new gaskets/orings in the carbs. The bike has some brand of jet kit put in by the previous owner. The pilot screw is set to 2.75 on all carbs. Also put in a new air filter. I did a benchtop sync before I put the carbs back on.

Issues: Hanging RPM's when shifting, can't get the idle up to 1,500 rpm's without it spiking up to 3-4k. If I adjust the idle screw up it jumps from like 1.2k-1.4k to 3-4k. Also have some surging at like 3-5krpms. I started trying to sync the carbs, but I can't get the idle up to the 1750 it recommends without going up to 3-4k? The sync when it's running at like 1.2k is pretty close.

Any suggestions? Bike is a 1999 Katana 750.

[steves]

Well, cleaning the carbs is usually the first think I would suggest.

Second would be to look for a vac leak.

[Kreylyn]

Changed the plugs a little bit ago, just cleaned and put in new gaskets/orings in the carbs. The bike has some brand of jet kit put in by the previous owner. The pilot screw is set to 2.75 on all carbs. Also put in a new air filter. I did a benchtop sync before I put the carbs back on.

Issues: Hanging RPM's when shifting, can't get the idle up to 1,500 rpm's without it spiking up to 3-4k. If I adjust the idle screw up it jumps from like 1.2k-1.4k to 3-4k. Also have some surging at like 3-5krpms. I started trying to sync the carbs, but I can't get the idle up to the 1750 it recommends without going up to 3-4k? The sync when it's running at like 1.2k is pretty close.

Any suggestions? Bike is a 1999 Katana 750.

With the sync tool connected, are all other sync ports blocked off (vacuum hose for the petcock if it's still connected, etc..)? Airbox and filter on and not leaking? No other leaks?

If you can't get it to hold at 2k, then try 3k or as low as it will go but over 1800 rpms. As you sync it, adjust again and try to get lower to the 1800 rpms range. Do both banks of carbs, then sync the pair of banks. Adjust again, and try for the 1800 rpms. If you can get the 1800 then... check all of them again at 1800 rpms and fine tune.

If at this point you can't get the bike to idle properly... you have other issues that need to be fixed. Probably a need to further clean the carbs, or find a vacuum/air leak issue.

Krey

[ottoguy]

Thanks for the quick replies. Unfortunately I have to sit all day at work before I can go home and try any of these options.

The filter and box are both on, and all other vacuum ports are closed off. I will have to try some carb cleaner, or starter fluid to look for leaks. I will try syncing them with it idling high. I didn't want to do too much this morning at 7am, as I didn't want to wake the neighbors.

[Kreylyn]

Don't forget to make sure the engine is warm before trying to sync it.

Krey

[ottoguy]

Carb sync really helped but I still have a little hanging of the idle, and some back firing or after burn on deceleration.

It idles around 1k, if I turn it up the idle screw the hanging issue becomes more pronounced. It's pretty close at this point, but any suggestions are welcome.

I sprayed carb cleaner all around the carbs and boots while it was running and was unable to find a leak.

[steves]

Popping, or literally a backfire?

That's usually a lean condition; however some popping on decel, especially after heavy application of the throttle is normal as unburnt fumes detonate.

Don't mess with the idle screw too much.

It's a fueling issue.

[ottoguy]

I turned out the A/F screws to 3.25. It ran like crap one night, then I put in some Lucas Fuel treatment in the tank and road it on the highway to work. It seems to idle much better now. Still a little surging and idle hang but not too bad.

I also just read the DynoJet install instructions and it says the A/F screw should be out 3.5 turns, so I will have to give that a try. I never checked the DJ instructions because I thought 2-3 was standard. I may replace the DJ kit sometime, but it's what was on the bike so for now it's working.

Thanks for everyone's help, just wanted to put out my results.