81GS650L Misfire

[DaveP]

Received my new NGK spark plug boots and tachometer cable so I could continue trying to get the bike running again after a few years of sitting. It is running now, but has a misfire at idle, not the higher rpm's.

Here is a link to a 20 second video of the timing of the pops: http://www.abcdpinc.com/81gs650l/20seconds.wmv

In this video the pop occurs at 9 and 13 seconds. The original video was 1 minute and the pop occurred again at 50 seconds and then again shortly after I stopped videoing.

The backfire/misfire is on cylinders 2 and 4 which I could tell because I had my carburetor vacuum gauge hooked up and it was pushing the hoses off of those ports when I returned the rpm to idle range.

What do yo think?

Thanks,
Dave

 

[tom203]

My iPad mini choked on the wmv file!
But if it doesn't do this (misfire) at 2000 rpm and above, I'd suspect the idle circuits on carbs 2 and 4 ain't right and mixture is too lean- how much out on your idle mixture screws?

 

[DaveP]

I have been turning the screws in which seemed to improve the situation.

When I rebuilt the carbs, I started at 2.5 turns out. I went to 3 turns out when trying to get it started earlier. Now I am at about 1.5 turns out.

If I am understanding this chart correctly, http://members.dslextreme.com/users/bikecliff/images/carbspec_float_height.html, the GS650 pilot screw is originally set at 1.5 turns out.

I don't think the wmv format will play on apple devices. Here it is again converted to avi: http://www.abcdpinc.com/81gs650l/20seconds.avi

I believe the carbs are clean. Everything was poked and carb cleaner sprayed through all of the passages before rebuilding.

Dave
http://members.dslextreme.com/users/bikecliff/images/carbspec_float_height.html

 

[tom203]

From what I read on here, carbs probably left japan with screws at 1.5 turns out to keep EPA happy,so very lean. After rebuild/cleaning, the advice is to start with them at 2.5 to 3 turns out and then adjust down.
Since you said, the misfire just seems there at idle on 2 and 4, I'm still thinking clogged in idle circuits.this might clean out with a little running when engine gets hot. I'd probably try a dose of Seafoam or equivalent and go riding IF you can ride safely somewhere. When riding,everytime you close throttle , high vacuum is pulling on the idle circuits- they just might clean up,but no guarantees!

 

[DaveP]

Couple of new points.

Carbtune: Hooked up the Carbtune to check the sync. When I first hooked it up it wasn't registering any vacuum with 8 cm Hg being as low as the gauge goes. I had to turn it upside down so that 18 becomes zero. The gauge was only moving to about 21 with the engine idling at about 2,000 rpm. Not much vacuum.

Colortune: picked up one of those clear spark plugs so that I could try and see what was happening in each cylinder. 1 and 3 have a nice blue burn, but the pilot screw can't make it turn yellow/orange. 2 and 4 really aren't running very much except for the occasional firing. You can see the spark occurring consistently, just nothing burning, even at higher RPM.

Spark plugs in 2 and 4 appear wet, could it be a timing issue on just those two cylinders?

 

[tom203]

"2 and 4 really aren't running very much except for the occasional firing. You can see the spark occurring consistently, just nothing burning, even at higher RPM."

Sure sounds like carbs to me.But you said previously that misfiring on 2 and 4 seems only at idle- and now the color tune says otherwise. Revving it up in neutral still feeds off idle circuit.