Carbs and popping

[agrady1995]

Having the following issue on a 2000 Interstate:

Cleaned and synced carbs. Bike runs with all the power in the world, but the left bank pops (more on 2 and 4 than 6) when running, once or twice every 1-5 seconds intermittent.

Idle is nice and stable at 1000RPM once warmed up, however once warm, starts and idles at 1500-2krpm for about 10-15 seconds before going back to 1k. 

**This is the second bike with the exact symptoms, however I thought the popping on MY bike was a leak in the ex pipes as they are two brothers and very rusty looking.**

Bike had sat for a long time before I got to it, so I never got to hear it run before hand. First startup had the left side release a dark/blue puff of smoke that looked like oil, but I could be wrong and it was old gas.

I have checked spark, exhaust gaskets (replaced) and compression (nominal).

I have not done a leak down test to check for a valve problem but I suspect the carbs. I took them back apart  and made sure the parts are all clean. Not sure what I could be missing, but still does it. Air cut valves? all held vacuum but maybe not working like they should?

[Jims99]

Have you desmogged it? Sounds like a vacuum leak. I noticed when mine started to idle higher when warm, a vacuum cap was leaking. Looked good but had a crack in the back side. Second thought was exhaust which you checked already.

[agrady1995]

Yes desmog. I cleaned and put the kit in tight, so I don't think it's leaking but among (hopefully more than one reply) I will try this friday or saturday. Anyone else?

[Jersey]

Yes desmog. I cleaned and put the kit in tight, so I don't think it's leaking but among (hopefully more than one reply) I will try this friday or saturday. Anyone else?

You can chat with Dave Wilder as he's the Carb Guru. 
As others mentioned, popping is almost always a LEAN condition.  What's causing those carbs to be lean is the challenge.  Usually it's a very very small air leak in any number of places.  Check the whole air pathway, starting at the air cleaner.  Can use a propane bottle or other tricks to look for the leak (Turn on the bottle and run the tip around all of the connection points.  The leak will suck in the propane and increase the RPMs).  Could mean those carbs just need a mixture tweak? Could be an air leak on the exhaust heads?

Could also be related to the Air Cut Valve (16510-MZ2-650).  I have one go bad and cause similar popping and rise/drop of the RPMs when going to idle.

Jersey

[yrunvs]

I had to retighten my exhaust headers a Few times after installing new gaskets before they seated satifactory

[agrady1995]

Seeing as someone said the air cut valves can cause this rise/fall of rpm when going to idle, can anyone tell me if this is due to leaning on the side for so long? Maybe they gummed up.. I'll

[Mooskee]

Air cut valves? all held vacuum but maybe not working like they should?

They should hold vacuum, but you can test them to verify they open and close the valve when the diaphragm moves. I have found 2 of them that would hold vacuum, but did not operate the valve. If you have a Redeye rebuild kit, he has the tools and instructions for testing them with a mighty vac.

Checking the air cut valves at this point involves removing carbs and taking them back apart.

I would follow Jersey's advice and check for vacuum leaks. If there are no vacuum leaks, or exhaust gasket leaks, maybe try opening the pilot valves 1/4 turn on the cylinders that are popping. That will enrich the pilot circuit and may stop the popping. I am assuming that when you cleaned the carbs, you got the slow jets clean. If the slow jets are blocked or partially blocked it will cause a lean condition. Sitting on the side stand with fuel in the bowls for a long period could cause the jets to plug.

Good luck, and hang in there. You will figure it out.

[agrady1995]

10-4. Yes all air cut valves tested OK. I will try today (tonight) to do a propane leak test and then if that produces nothing I will check the jets again, I guess. Thanks again guys. Jets do seem likely as it was on its' side and popped.

I already (early on) tried enriching 1/4 turn and had no change.


Kilroy

[98valk]

10-4. Yes all air cut valves tested OK. I will try today (tonight) to do a propane leak test and then if that produces nothing I will check the jets again, I guess. Thanks again guys. Jets do seem likely as it was on its' side and popped.

I already (early on) tried enriching 1/4 turn and had no change.


Kilroy

how much popping?  IC engines do that when going off throttle to idle rpms.  colder temps and/or cold engine it will do it even more.  backfire loud?, then there is a problem.