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Brookville Motorsports
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BACKGROUND:

1988 GL1500 with 1200 original miles.Obviously in storage a lot. I have a miss and a backfire when letting off the throttle. the right exhaust pipe has turned blue where it comes out of the body.I have cleaned the carburators three times and installedcarb kits. Replaced vacuum lines, and the intake joints. Run seafoam through it. Synched the carbs,new spark plugs, new timing belts,new air and fuel filters. I sprayed either, and silicone spray, used propane and I can't find any vacuum leaks. Still no change. Spark seems good, plugs are brown in color EXCEPT for one which is WHITE- the right front plug. The compression is consistantat 138 lbs.

What is causing the miss and backfire-overrun? Why just one cylinder shows lean when the samecarb feeds all three cylinders?

BMS
 

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A white plug is lean mixture, regardless of how the other two plugs look. When you cleaned the carbs, did you set the mixture screw by the book, or did you leave that alone?
 

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if there is a white plug it is either not firing effectively washing the plug or it has a restriction in the intake not letting fuel in, think i would try the plug first, i have had plugs that would fire a nice blue spark outside the cylinder but not fire once installed



this could also produce the backfire when the unburned fuel gets in the hot exhaust
 

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Brookville Motorsports
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I set the mixture screw at 2 1/2 turns out.I will try a new plug.

Thank you,

BMS
 

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If carb., plug etc. seem ok then go to coolant supply or possible valve problem at the offending cyl.
Do a simple compression test . Your comp. test at consistant 138lbs. suggests your comp. test is faulty.
Start motor and achieve full op.temp.
Remove all plugs , disable ign. be sure to open and hold throttle fully open , no choke , test each cyl. and list each value . Let us know what you find.
 

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Brookville Motorsports
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Machinehead,

The compression showed a range of 136 lbs to 140 lbs. The cylinder with the white plug is at 138 lbs. The compression test was done with the same proceedure you have discribed. I'm also thinking that something in blocking or inhibiting fuel flow to that cylinder.

Thank you,

BMS
 

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Yours sounds like mine did about a week ago. I know that you have cleaned the carbs, but that means a lot of things to a lot of people.



Did you take out the low speed jets and clean them with something? I like to use a piece of copper wire just to loosen things up in there and it will not damage the jet like a torch tip cleaner. I know that I spent a good half hour on each of the low speed jets to get them looking acceptable. There was a lot of gunk in there and it did take a while to get them clean. I would do the same thing with the main jet as well.
 

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Brookville Motorsports
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sjy,

I have used torch tip cleaners on the jets and I'm aware that they can damage the jet so I was careful not to. I also cleaned the small passageway underneath the slow jet. I cleaned the carbs by completely disassembling them and then putting them in an ultrasonic cleaner which seems to do a good job.



NOTE:

Before I bought the bike the owner had taken the bike to his local honda shop and had the carbs cleaned. When he got it back the bike had this backfiring problem. they told him to run some carb cleaner through it and it would be fine.

When I have the carbs off again I will pay special attention to the slow jets as you have suggested.

I am grateful for all suggestions and when I get a list of them I'm going to tear it down and follow each one.

Thank you,

BMS
 

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BMS wrote:
I set the mixture screw at 2 1/2 turns out.I will try a new plug.

Thank you,

BMS
All else being equal, try setting the right front cylinder mix out another 1/2 turn. I had the opposite problem, one carb running too rich. Played with the mix a bit and ended up going in an extra 3/8 compared to the other three.

Just my $.02

J-
 

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BMS,



Could it be there is a valve problem? Your description sounded just what mine sounded like, but I also did not have a decent idle. Carb rebuild fixed all of my problems. Other than the backfire and blue pipe, how does it run? Any hesitation issues or lack of power? Rough idle?



Having just had mine apart last week, I know where you are at. The prospect of taking mine apart again would not excite me. Are the slides working correctly? Not being able to see it and hear it, it is kind of hard to diagnose.

I don't claim to be an expert by any means, but I have rebuilt many carbs and it is amazing what one small piece of **it can do.
 

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Brookville Motorsports
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SYJ207,

That is the conclusion I'm coming to. The valves. Would a valve not seating cause this? Would it effect the compression?

I appreciate your help.

BMS
 

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Brookville Motorsports
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SJY207-(I think I got it right this time)

When I rode the bike it ran fairly good but it has a miss in one cylinder. The miss is there at idle and revingit up. I don't notice the miss when riding. My son on the other hand says it cuts out while he rides it and looses power The backfire is always there. I now notice a little bachfire on acceleration as well as decceleratoin.

The slides seem to be working well.

There is no hesitation. Throttle is very responsive.

Thanks,

BMS
 

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I find that sometimes it is the easiest to go back to the basics and start there. Gas, compression, spark. Need to make sure you are getting all three and in the right quantity/quality. If you find no problems there, then start looking for different things. Bad intake gasktseal (can't remember what they seal with), vacuum leak, obstruction in the intake (seen lots of things get in there).

From your last description of the backfire being there all of the time, I would double check the compression in that cyclinder and then start back from there with the above.

Good luck.
 
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