backfiring through exhaust


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mgneeiey
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backfiring through exhaust

Postby mgneeiey » Wed Jan 29, 2014 4:48 pm



86 gl1200 and backfires through exhaust when giving it throttle. could this be a timi9ng problem? all vaccuum lines has been checked for leaks and none. could this possibly be the afterburner underneath the carbs? should you be able to suck air through the afterburner hoses? 2 hoses you can and two you cant. stumped in az. want to get this running since this is perfect riding weather. thx.



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virgilmobile
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Re: backfiring through exhaust

Postby virgilmobile » Thu Jan 30, 2014 2:14 pm

Backfiring in the exhaust indicates more fuel is passing through the system than the cylinders can burn and is igniting in the muffler.
Timing should not be a issue,however there are ways to test both the mechanical and vacuum advance parts...they do make a difference in the power curve.

I would suggest first to remove the spark plugs one at a time and inspect...Search "how to read your spark plugs"
See if one is sooty/wet...or more than one...this tells the overall condition..eg..one cylinder not firing well or a pair ....
With this info,you can determine closer to the cause...
Example..if it's just one cylinder,look at the spark itself,compression,and possibly work on that one carb(stuck slide or fuel level too high)
If it's a pair(fronts or rears) look at the spark..compare it to a good cylinder...(a bad coil,weak spark).
Cold start the bike and keep it under 1200 rpm for 8 seconds...lightly tap each header...they should all be equal...If you have a cold cylinder(s),sync the carbs first before repairing them...

As far as the "valve" you refer to...it's vacuum activated during deceleration only and is used to provide a bit more air flow over the main jet enriching the fuel.

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Re: backfiring through exhaust

Postby WingAdmin » Thu Jan 30, 2014 4:32 pm

What Virgil said. What's happening is when you are accelerating, the spark isn't igniting the fuel/air mixture enough, or at all. It then gets dumped, unburned, into the exhaust, where it ignites and makes a big bang. That could be caused by a fuel/air mixture that is the wrong ratio (carb problem) or not enough spark (electrical or plug problem). First thing to do is to figure out which cylinder is the culprit.

mgneeiey
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Re: backfiring through exhaust

Postby mgneeiey » Thu Jan 30, 2014 8:31 pm

ok thx guys, appreciate the point in the right direction. i will get back to you and let you know what i found. thx again.

mgneeiey
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Re: backfiring through exhaust

Postby mgneeiey » Fri Jan 31, 2014 7:55 pm

well i pulled the carbs again and sure it was getting too much fuel, the guy i had do the work on them didnt tighten one jet up and it was inside the bowl. ^$#@$%#@^ workmanship! so i checked the rest of the carbs and they were ok. put back in and now im getting backfire on decelaration. still seems like its getting too much fuel. rev up to about 5000 and it starts to bog down, then have to wait 10-20 sec. before i can get the rpm up that high again. any thoughts other then beating the dude with the carbs?

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virgilmobile
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Re: backfiring through exhaust

Postby virgilmobile » Fri Jan 31, 2014 10:27 pm

Just use boxing gloves If you go that route. :lol:
Your probably not gonna like what I suggest....
If you want the best running bike,you have 2 choices..... take the whole bike to someone who has done this before or do it yourself.

The carbs are a bit different than a single Carb from a car.The float levels must be right on the money.Even a few MM off and the mixture runs rich or lean.The slides must be smooth and the diaphragms clean with no pinholes..Bench testing and pre-sync is a must.
With the now decell backfire,a complete test of the hoses and valve is in order too.

mgneeiey
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Re: backfiring through exhaust

Postby mgneeiey » Fri Jan 31, 2014 11:02 pm

would you be talking the reed valve body on the underside of the carburator? i did notice that i could suck air through two of the hoses that connect that afterburner or reed valve. two hoses i couldnt suck no air through. could this be a problem being able to suck air through the hoses connected to the reed valve?

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virgilmobile
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Re: backfiring through exhaust

Postby virgilmobile » Sat Feb 01, 2014 7:54 pm

Not that cluster...there's another valve under there.Usually does Not fail tho,the vacuum lines to it dry out or get hooked up wrong.
I don't have access to my mainframe computer for a few days,but I and others have posted pictures of the correct plumbing of the hoses.
The valve is under the Plenum box,bolted to it.There's a hearty spring attached to a rubber diaphragm. It must move smooth and no tears in the rubber

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86pearlblue
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Re: backfiring through exhaust

Postby 86pearlblue » Sat Feb 01, 2014 8:20 pm

I bike mechanic I trust pointed me in the right direction with a similar problem... That's exhaust leaks. Check where the manifold pipes go into the crossover pipes. Ensure that the "donuts" are still there. Any leaks that close to the heads means it also sucks raw air into the exhaust, creating an oxygen rich environment where there should be none.

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virgilmobile
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Re: backfiring through exhaust

Postby virgilmobile » Mon Feb 03, 2014 4:41 pm

This post shows the vacuum lines..viewtopic.php?t=10576&p=51142

mgneeiey
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Re: backfiring through exhaust

Postby mgneeiey » Tue Feb 11, 2014 3:16 pm

ok guys i went ahead and did a compression test and cyl. 2 and 4 were 120psi, 1 and 3 were 65psi. now this has to be a big problem right? im about ready to throw in the towel. :evil:

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Re: backfiring through exhaust

Postby virgilmobile » Tue Feb 11, 2014 4:27 pm

Before you chunk it..at least identify why the compression is so much lower than the other side.

Without knowing the history,there's a few things you can check....
Assuming the compression test was done "Wide open throttle" for both sides,there are a limited number of things that will cause this...
Timing belt a couple of teeth off(maybe)....Not hard to verify...
Valve lash too tight.Not hard to check either.
Blown head gasket.Usually causes overheating too.
Rusted cylinders...Pull the plugs and eyeball inside.
Stuck rings...A overnight dose of 50/50 mix of acetone and ATF in the jug (bike tilted on it's side) can loosen these up.Just a shot glass in each one.
A very restricted exhaust...It can be unbolted at the head and test compression again.

It's odd that one side differs so much..
This leads me to problems other than just a worn out engine.
How did the original plugs look on that side....any rust???
Valve lash usually isn't messed with but who knows about the previous owner....
Timing belt is a good starting point..too far off and it can bend a valve just a little....

Generally,if there's no rust in the cylinders,the block/rings are OK.The low compression is related to the heads/valves....If a valve contacted the piston,it usually just "eyebrows"it.
If it's really bad it pokes a hole in it and you loose all compression....

mgneeiey
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Re: backfiring through exhaust

Postby mgneeiey » Thu Feb 20, 2014 5:08 pm

my bad cylinders 3 an 4 are only testing at 65 psi an 1 an 2 are reading 120 psi, i pulled the valve covers an everything seems to be ok from what i see valves look as though they functioning properly but i cant see if they are closeing completely should i pull the heads or is there something else i can do, the top of the pistons seem fine also

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86pearlblue
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Re: backfiring through exhaust

Postby 86pearlblue » Thu Feb 20, 2014 9:28 pm

I would. There could be carbon build up on the valves. The heads are fairly easy to take off. Just pay attention to the bolts and where they came from. They are not interchangeable. Get new gaskets.

mgneeiey
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Re: backfiring through exhaust

Postby mgneeiey » Fri Mar 21, 2014 4:54 pm

ok guys took both heads off and took to a machine shop, they told me that face on the heads were wore out and the valve guides had alot of slop in them. so i went down and picked up two more heads and had the machine shop put the valves in the new used heads. picked them up tuesday and fired her up today, running better but its not right. i can pull the number 2 plug wire off and no change while its running. i can also pull number 3 plug wire while its running with no change. now if i pull 1 or 4 the bike will die immediately. so i thought maybe weak spark and i took the coils off and had them tested. they were fine. came back and did a compression check and number 3 only hit 60 psi. all the rest were 120 psi. so iam taking the head back off now so i can take it back to the machine shop. any clue why there was no change in idle when i pulled number 2 wire off? i checked timing and it is dead nuts. i always check by hand cranking motor at least 2 revolutions and make sure nothing hit just in case. maybe start looking at ignition control mod. i am not sure.

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Re: backfiring through exhaust

Postby 86pearlblue » Fri Mar 21, 2014 6:04 pm

After you get the compression back up, I would get the carbs synced up. It sounds like only 1 and 4 are pulling enough fuel to keep the bike running at idle. 120 psi sounds good on the other three. Let's hope you don't have to replace the rings! You can also try adding Lucas heavy duty stabilizer to your oil. It may help get the compression up.

mgneeiey
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Re: backfiring through exhaust

Postby mgneeiey » Wed Mar 26, 2014 2:54 pm

ok guys got compression back. runs great at idle and going down the road till i hit about 3 to 3.5x grand and it starts wanting to die. but if im sitting still i could run it up to 7 or 8 grand. its when its going down the road when it happens. just need a little more help and i will be riding again. thx again guys for all your help in advance. :D

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Re: backfiring through exhaust

Postby virgilmobile » Wed Mar 26, 2014 3:20 pm

There's a huge demand for fuel while running down the road that is not there when just twisting the throttle.I would suggest a fuel delivery test first.
A plugged fuel filter,weak fuel pump,plugged fuel(tank) cap(leave it loose and run it again)
Collapsed exhaust manifold or restriction in the exhaust.

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virgilmobile
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Re: backfiring through exhaust

Postby virgilmobile » Wed Mar 26, 2014 3:26 pm

You can test the low fuel delivery possibility by this test.
Run the bike till it starts to dog off.Back off the throttle,keeping the rpm's up,reduced load.
Let it run several seconds there and then open the throttle back up.
This lag would allow the carbs to fill back up.
If it pulls up OK,the bike simply doesn't have enough fuel for the demand.

mgneeiey
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Re: backfiring through exhaust

Postby mgneeiey » Sat Mar 29, 2014 2:32 am

well took it out for a couple mile ride and still started dogging at about 3 and a half to 4 grand. so when i got back home, i was sitting idling and tried to get it above 4 grand but no luck until ......... :lol: lol i shook the steering wheel back and forth and all of a sudden it shot up to 6 grand then came back down till i shook the bike again. im thinking a readjustment of the floats are in order. what do you guys think?

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86pearlblue
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Re: backfiring through exhaust

Postby 86pearlblue » Sat Mar 29, 2014 7:23 am

It could be sediment in the bowls or in tank. You could also try some Seafoam in with the gas to clean the carb jets. But you're making progress! Once you get this ironed out, here's to many happy, trouble free miles!

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virgilmobile
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Re: backfiring through exhaust

Postby virgilmobile » Sat Mar 29, 2014 12:18 pm

Is it actually running on all 4 ? Sounds like a twin.a failed spark on one pair sounds like your problem.
Before you do the carbs again,fire it up from cold.Bring the rpm up till it starts to quit,Pull a plug wire,one at a time.are both sparking on one side?
The bike actually will run fair on 2 cylinders.

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virgilmobile
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Re: backfiring through exhaust

Postby virgilmobile » Sat Mar 29, 2014 1:42 pm

Another easy test.Unplug the 2 front spark plugs.Will it start?
Plug them back in and unplug the rears.Will it start.?

mgneeiey
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Re: backfiring through exhaust

Postby mgneeiey » Sat Mar 29, 2014 2:26 pm

ok pulled the front two plugs and it started, also puklled the rears and it started. then had it start dogging down and pulled each wire seperately and could tell the difference on each plug wire pulled. also last night i cleaned the fuel tank to make sure there was nothing to restrict gas flow. got alot of fine rust in there. changed the fuel filter and still the same, dogging down at about 4 grand.

mgneeiey
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Re: backfiring through exhaust

Postby mgneeiey » Sat Mar 29, 2014 2:28 pm

also changed the fuel pump recently and aqlso checked the gpm to make sure it was pumping enough and tested ok.




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