Engine knocking after carbs cleaned

[bjrehill]

Hey all. This may be carb related. I'm not too sure. The engine is knocking at idle. It sounde like 1 of the cylinders is our of whack. The engine also runs rough, but idles nicely at 1100 rpm

I'll give the background. First 1980 suzuki GS850GL stock air box, foam filter clean with oil, the carbs had pods on them at one time, but I never had the bike running that way.

I just got done rebuilding the cam chain tightener, so while I had the carbs off I thought I'd give them a cleaning. I had some rough running leading up to this project. It sounded like 1 or more cylinders were misfiring. I found that the boot going from the air box to the carb on Cylinder 1 (left side facing the front) was cracked big time. The crack was about 1/2 inch wide and 1 inch long. It was on the bottom inside corner so was not visible without a mirror. I also replaced the boot O rings on the carb boots leading to the engine. they had been sealed with gasket sealer before. I sprayed wd 40 to see if there was any leak after this. All seems fine

I also put in a fuel filter, thinking that junk may be getting in the tank.

After putting things together I started the bike and began adjusting the air screws (Stock carbs) to get highest RPM's. The only cylinder I can affect the RPM's is 1. Air screws are between 1 and 2.5 turns out. The plugs look rich. Plug 3 looks almost black. Plug 1 and 4 are dark brown. Before all this started plug 1 was white all others dark brown.

So the questions I have are where to look? What may cause the knocking? All cylinders are firing, just not well. All pipes get equally hot. How do I adjust the carbs to fix it? Is it the carbs?

By the way last year the bike was running fine but began surging and slowing down at 4000 RPM's in 5th gear. This year was about the same, till it began back firing while excellerating.

Thanks for any and all help.

[Katamaniac]

It sounds like your carbs are jetted too rich and out of synch.

Did you rejet back to original specs when you removed the pods and put the air box back on?

[Anonymous]

The engine also runs rough, but idles nicely at 1100 rpm

Did you synchronize the carbs after your clean up? Your problem sounds to me like the synch is out of whack. When the carbs are not synch'd it idles rough (especially when cold) because the cylinders are fighting each other. Synching them will smooth that out and get everything pulling equally.

By the way last year the bike was running fine but began surging and slowing down at 4000 RPM's in 5th gear. This year was about the same, till it began back firing while excellerating.

That sounds like your leaking intake boot causing an air leak and lean condition. It should be corrected now that you replaced the boot and O-rings.


Mark

[bjrehill]

I did not rejet back to original. Actually I don't know what size jets are in the carbs. I have 2 sets of carbs. One that was on the bike that I have not torn apart, but the bike would not run with and the other from a parts bike that had pods on it. These are the ones that I cleaned. I did not do a complete dip. They were in good shape when I opened them. No buildup in the bowls, gaskets in good shape, all jets were clear.

I am pretty sure that since I replaced the leaking boot and O rings and also tightened up the air box (put weather strip where seal was missing) that there is less air getting into the carbs. Last year the air screws were in tight. This year they are further out.

I did not resynch. I am trying to get the mixture set first if possible.

[Anonymous]

I did not rejet back to original. Actually I don't know what size jets are in the carbs.

To get the pilot and needle set you don't need to worry about your main jets at all. It sounds like you are still rich from the pod settings. Do you have CV carbs or the VM carbs? If they are CV's, I would set the mixture screws to 2 turns out and test ride it to see what that gives you for a plug reading. The CV's are not as sensitive on the pilot circuit for using the highest rpm method. I didn't bother with that on my 1100. I test rode it and went by how it ran (abrupt/soft throttle resonse, starting and transition onto needles) and what the plugs showed.

If you have VM carbs, I am not much help with them. Keith Krause would be the man to answer all of those questions. And comment on my info on the CV's, too, for that matter.

I did not resynch. I am trying to get the mixture set first if possible.

I understand. I did all my jetting for pods without synching the carbs and now that I am happy with it, I will be synching them when I get a chance in the next few days. A good bench synch should get them close enough to run OK and get your jetting sorted. My 1100 is certainly not running badly, despite not being perfectly synched. I expect the idle to smooth out and some vibration to go away when I get it done, but it will not be a huge difference in how it runs overall, just a bit smoother.


Mark

[bjrehill]

I have CV carbs. I am just trying to get that knocking to stop. It does stop at higher RPM's It does seem to be coming from either cylinders 1 or 4 or both. These are the 2 easiest to adjust anyway. I removed the lead for plug 2 and then 3 to see if they were the ones knocking but the sound continued.

Thanks fro all the suggestions.