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#1:  Back Fire on deceleration and down-shifting (Solved) Author: uthick Post Posted: Tue Jun 08, 2010 9:42 am

On my 82 920 i have been trying to solve the back firing issue, this winter i took the carbs and gave them a good cleaning installed a dyno jet kit because i have after market pipes on it and i don't know what they are, replaced the butterfly shaft seals and as far as i can tell i don't have any vacuum leaks and it still back fires on deceleration and down shifting, I notice that the valve adjustment cover on the rear jug is leaking a little oil didn't know with that seal being bad is causing my problem and also noticed that there is no exhaust gasket between the mufflers and the head pipes don't really know if there should be one or not. Any ideas or suggestions would be greatly appreciated.



Last edited by uthick on Mon Jun 21, 2010 6:52 am; edited 1 time in total

#2:  Re: Back Fire on deceleration and down-shifting Author: eddiethetech Post Posted: Tue Jun 08, 2010 9:59 am

How did you verify no vacuum leaks? A good test is to spray carb cleaner around all potential leaks and listen for the revs to pick up. The mounting boots get stretched, dry and cracked. The carb should fit in snug with the inside ridge seating firmly into the groove on the carb horn. If the MCV is still connected it can cause your problem as well. Run the petcock vacuum line straight from the intake boot to the petcock, then plug the large port with a scrap piece of plugged vacuum line. Take it for a quick spin and see if it still backfires. If not then make a more permanent plug for that port. Use a zip tie or clamp to hold it on the nipple so she doesn't blow off at speed. If your vacuum hoses are at all dry and stiff they should be replaced. Just a couple thoughts to get you started.

#3:   Author: slowpoke Post Posted: Tue Jun 08, 2010 10:35 am

I would say it is either a vacuum leak or you need to play around with the idle air screw (for lack of a better term) Use some wd-40 and spray the manifold flanges where they meet the heads. Listen for any differences in the rpms. If there is then the o-rings are shot, had it happen too many times with mine so I made my own gaskets using gasket material from the auto store. Also remove the AIS from the front cylinder (the two hoses from the manifold adapter) and cap the vacuum ports. I found that the caps you get from the parts store last about one ride so I rtv'd the large port closed and made my own cap for the small one with a piece of vacuum line and rtv'd the inside of one end.

Have you adjusted your idle circuit? (the smll screws on the cylinder side/ bottom of the carb). if not set properly could also cause a lean condition when at non-throttle. But I'd be willing to bet it a vacuum leak.

#4:   Author: bstig60 Post Posted: Tue Jun 08, 2010 7:30 pm

An exhaust leak can make it pop as well. Is it just a popping or a real BACKFIRE from the exhaust? Take a look at your plugs and see what color they are. That will tell you if you are running rich or lean and give an idea if you are dealing with only one cylinder or both. Either condition can cause a backfire. What criteria did you use to determine what jets to use when you installed the carb kit?
Post a pic of your pipes and maybe we can figure out what they are. I would hazard a guess that they are either Jardine's or Mac's, but we will know more if we can see a good pic or two of them.

#5:   Author: funkamongus Post Posted: Tue Jun 08, 2010 10:33 pm

Yes, your rear may be leaking. I have jardines. The rear has been giving me problems and I havent fixed it yet. I can adjust the muffler higher onto the header, but when tightened and driven, it vibrates back down. I can see the leak, too, as a darkness around the bottom of the header pipe. Im thinking a dog food can, cut, wrapped, and with 2 clamps

#6:  Re: Back Fire on deceleration and down-shifting Author: uthick Post Posted: Wed Jun 09, 2010 1:25 pm

I am in the process of replacing both exhaust gaskets, removed the mcv and caped them at manifold, and replacing the line that goes to the petcock and as far as i can tell that should be the only vacuum line on the bike now?? tired the carb cleaner around the carb boots and it did not rev up, I'm just wondering since i almost have the bike tore apart again taking the carb boots off and putting some hi-temp gasket sealer on them to make sure there not leaking, as for the jets in the dyno- jet kit there were two sets of jets I used the ones they recommend for after market exhaust, I will try and post some pics in the next couple days of the pipes, Thanks for all the suggestions and ill let everyone now what happens after i make the repairs


#7:   Author: funkamongus Post Posted: Wed Jun 09, 2010 1:30 pm

Yes, the only vacuum line. Good luck!!

#8:   Author: slowpoke Post Posted: Fri Jun 11, 2010 7:43 am

You don't want to use a RTV type sealer at the manifolds. Get some gasket material from the parts store and make your own gaskets. RTV will smear when sliding the manifold in place.

#9:   Author: drkstr47 Post Posted: Fri Jun 11, 2010 7:46 am

I kinda think of the popping as a heads up for all the cages around me that "HI, I'M HERE!!" hot

#10:   Author: uthick Post Posted: Fri Jun 11, 2010 12:09 pm

I got the bike back together and it still pops, didn't mess with the carb boots, took carb clean and sprayed all around and it didn't rev up, I did notice that around 2,000 rpm the motor kind of stumbles and then clears up with more rpm's and i did notice on down shifting its around 2,000 rpm's it pops, I seen a post on here about there should be spacers on the jet needle, is that true?? there was none on mine. One other thing i noticed i had there rear plug wire off just sitting on the jug ( was trying to adjust the the pilot screw on the front carb) and it was arcing out on the jug half way or more on the boot looked for cracks and could see any. Thanks again for the help!

#11:   Author: anaman51 Post Posted: Fri Jun 11, 2010 2:48 pm

Sounds like you're doing all the right things. Those pops are sometimes hard to get rid of. I know---I'm still chasing the source of mine. I think I've got the same problem Funky has, and I had come up with the same solution---old tin can and a couple of good clamps on the back pipe connection; just haven't done it yet. Each of us has to hunt down this demon separately, as it's likely the cause is slightly different for each of us. All we can do is keep sealing things off and tightening things up until it quits. Keep an eye on the condition of your plugs as you do this, and don't let the mixture lean out.


#12:   Author: uthick Post Posted: Fri Jun 11, 2010 10:10 pm

Tonight I took the mixture screws out to 3 turns out from 2 1/2 and put some high-temp rtv on the rear pipe connection ( I think this will probably melt off over time but maybe not) and took it for a spin and no pops, got back and noticed the bike was only idling at 800 rpm so I adjusted the idle up to 1,000 rpm and the stumbling came back at the 2,000 rpm backed the idle down to 900 rpm and it went away and then it started to pour down rain so I didn’t take it for another ride yet, was wondering if this is normal or if I got something wrong with the carbs or something else?


#13:   Author: funkamongus Post Posted: Fri Jun 11, 2010 10:18 pm

Hmmm. maybe a vac leak at the valve adjustment cover? Get yourself a new O ring, I just did all mine, I got them from ... Did you check your rear header going in to your muffler like we've been saying? I think it may be a typical problem with the Jardines not sealing to the rear header. RTV will NOT seal it. Not even temporarily. It will blow right off after it melts.

#14:   Author: slowpoke Post Posted: Fri Jun 11, 2010 11:29 pm

I'm wondering if the butterfly seal on the carbs are shot. were they ever dipped? If they were then the solvent ate the seals. You also have to remember you have a 28 year old bike that sat for most of its life. Sitting is the worst for machines. Seals and gaskets dry out from lack of oil to keep them "hydrated"

Good luck I hope you find the gremlin

#15:   Author: uthick Post Posted: Mon Jun 21, 2010 6:55 am

Hi-Temp RTV'd the rear head pipe muffler connection and no poping rtv seems to be holding up, Thanks for the help!

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