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Discussion Starter #1
I'm getting backfiring, but only in the horizontal cylinder. Bike is a '94 CR with JE Hi Comp pistons and FCR's.

Yesterday I ran the bike about 60 miles with no issues, this morning I topped off my tank with about 4 gallons of gas (I keep 15 gallons of clear gas in the garage for the various bikes, this batch is about 2-3 weeks old) and headed out for a ride. After warming up and getting about 1/2 mile, the bike stalled while idling at a traffic light and was quite difficult to restart. The amount of time it ran seems to coincide with about how long it would take for the new gas to cycle down to the carb. I was able to push it home and eventually coaxed it back into idling, but just barely.

I adjusted the Air and Fuel screws from a 1/4 turn leaner than stock to about 1/2 turn richer. The richer adjustment seemed to help, but only a little bit.

I decided to take a video to capture better what's happening. It appears to be a true backfire with a visible flame in the carb throat. You can also hear a little stumbling after-fire as well, I assume on the same cylinder.

https://www.youtube.com/watch?v=qVbn3moaU74

I tried running some Seafoam through it, but now I can't even get it to start at all. Is there anything I should be looking at before I give up and tear the carbs apart?

I may need to break down and rebuild the entire fueling system but I'd rather not right in the middle of the riding season. Visual inspection of the tank itself seems clean and corrosion free.
 

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Are you sure your valve clearances are ok ? Do you have a healthy spark, good plugs, good wires and caps ?
 

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I had a similar problem some time back - stopped for gas, rode just far enough for the new gas to get to the carbs - and it cut out on one cylinder. Managed to limp it back home, then spent some time pulling carbs and jaets apart - with no luck. Then I did a compression test - and it was way down on one cylinder (vert). Pulled the cam belt covers off to check things - and one belt had snapped. Yours probably hasn't snapped - but it may have stretched and jumped a tooth... Worth checking!

BTW - my failure was due to one of the belt tensioner roller bearings failing.
 

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Discussion Starter #5
Are you sure your valve clearances are ok ? Do you have a healthy spark, good plugs, good wires and caps ?
Thanks for the suggestions guys! Already checked spark and it's good. Plugs, wires, coils and igniters were all replaced about 1500 miles ago, along with battery/ground cables, alternator, rectifier and belts.

I'll be checking belts and valve timing shortly. I'll also spray some starter fluid around potential air leaks to check for issues. I did recently tighten up the belts, maybe I didn't get a good torque on the tensioner/adjuster. Since it's localized to the H cylinder and that's the easier one to check, I'll try going through valve clearances too. If they're way off I suppose I'll do all eight.
 

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Discussion Starter #6
Thanks for the suggestions guys! Already checked spark and it's good. Plugs, wires, coils and igniters were all replaced about 1500 miles ago, along with battery/ground cables, alternator, rectifier and belts.

I'll be checking belts and valve timing shortly. I'll also spray some starter fluid around potential air leaks to check for issues. I did recently tighten up the belts, maybe I didn't get a good torque on the tensioner/adjuster. Since it's localized to the H cylinder and that's the easier one to check, I'll try going through valve clearances too. If they're way off I suppose I'll do all eight.
OK - The belts look good, are correctly tensioned (using the 5mm and 6mm gap method) and the timing marks all line up correctly. Checked valve clearances only on the horizontal cylinder and used the "loaded gap" method to determine opener gap:

Intake
Opener - .004"
Closer - .003" (measured at .007")

Exhaust
Opener - .003"
Closer - .003" (measured at .006")

So the closers look loose. This is the first time I've done this though. Would this be the cause of my issue, or would it still run this way?
 

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Discussion Starter #7
So I went back and re-measured. Apparently I need practice using feeler gauges, I got some fairly different but consistent measurements. I took three measurements and between each measurement I rotated the engine 360 degrees.

Intake
Closer .0045/.0035/.0035
Opener .0020/.0010/.0010

Exhaust
Closer .0030/.0035/.0035
Opener .0005/.0000/.0000

Going by this second round of measurements things look much closer to spec. Maybe the intake opener is a little on the loose side, but should be serviceable.

I didn't get a chance to do any leak testing, I probably won't get to that until Sunday/Monday.
 

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Uhhhhhhh........... not sure how you are doing your measuring but I would like to see ....
intake openers @ .004"
Exhaust openers @ .005"
closers @ <.0005" (zero is preferred but you can have no drag)

Any chance you mixed up your opener and closer numbers?

You should be able to push the feeler gauge through, without forcing it. Some people feel if they can jam it in there it is the size, this is not so. If you force it in you might be measuring things like flex, or closer when you want opener.
if you need practice to get the feel set a micrometer at the feeler gauge thickness and test the feel.
 

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Discussion Starter #9
Any chance you mixed up your opener and closer numbers?
Oh, damn. My typing brain got ahead of my logic brain as I was reviewing my notes. You are correct, I transposed the Opener/Closer numbers.My feel was based on needing more effort than 0 effort to get the feeler through, but without actually feeling specific resistance. One reference I saw was "what it would feel like if you slipped the feeler into a phone book," and I tried to stick to that.
 

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sounds lean on one do you have fuel getting to the carbs?
pinched fuel line?
air leak?
I think this is in the ballpark, i.e. that its a fueling problem. When I had the problem it was not resolved by my home cleaning solutions. Pulling the carbs and having them cleaned by a dealer with an ultrasonic cleaner did the job. Usually a fuel additive will solve the problem, but occasionally it takes more agressive action.

Bob
 

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i reread the original post, mechanical clearances very slowly go from adjusted to out of tolerance. It rarely causes a running-not running situation that changes abruptly. Unlikely valve train as it ran fine - then did not.

It's possibly electrical, but with the SS, the vast proponderence of issues like this are fueling. The fact that it ran properly, you added gas and then it acted up, to me points to bad fuel as top probability. Could be water, could be particulates. If you have a proper fuel filter then particulates are not likely the issue at the carb. I suppose contamination could have been really bad and blocked your filter. Possibles are stuck float, fuel pump, fuel line issues. These carbs have a rep for sticking floats, flooding bottom end.

Could you have gotten water/contamination in the fuel somewhere?

Bob
 

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Discussion Starter #12
i reread the original post, mechanical clearances very slowly go from adjusted to out of tolerance. It rarely causes a running-not running situation that changes abruptly. Unlikely valve train as it ran fine - then did not.

It's possibly electrical, but with the SS, the vast proponderence of issues like this are fueling. The fact that it ran properly, you added gas and then it acted up, to me points to bad fuel as top probability. Could be water, could be particulates. If you have a proper fuel filter then particulates are not likely the issue at the carb. I suppose contamination could have been really bad and blocked your filter. Possibles are stuck float, fuel pump, fuel line issues. These carbs have a rep for sticking floats, flooding bottom end.

Could you have gotten water/contamination in the fuel somewhere?

Bob
Thanks Bob, I'm coming around to probably a fouled carb as well. I'm running the bike (and shot that video) with the tank up, so it's not a pinched line. I keep 10-15 gallons of non-Ethanol gasoline in the garage for running the bikes, and this fill up was from that and other bikes have been run from it without issue. The tank interior *looks* clean, dull grey steel in all of the visible areas, only one spot of chalking on one of the screws, but no rust or other evidence of corrosion - but inspected without removing the filler cap.

I had another issue that smacked of fuel starvation not long ago that I had originally chalked up to a sticky breather valve, but I'm guessing the issues run deeper, as they seem to do with everything else on this bike. I'm going to try getting some Seafoam run through and keep my fingers crossed. If not I'll move on to cleaning/rebuilding the carbs - and failing that it will be refreshing/rebuilding the 22 year old fuel system with new lines, new pump, converting to an in-line filter and in-line petcock.

I really didn't want to do this during peak riding here, the season is spectacular but short in the Pacific NW. This is why I also own a '15 Thruxton. Funny that I would be turning to a British bike to be the one that starts every day. Sometimes it's more fun to ride a slow bike fast than it is to ride a fast one faster.
 
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Oh, damn. My typing brain got ahead of my logic brain as I was reviewing my notes. You are correct, I transposed the Opener/Closer numbers.My feel was based on needing more effort than 0 effort to get the feeler through, but without actually feeling specific resistance. One reference I saw was "what it would feel like if you slipped the feeler into a phone book," and I tried to stick to that.
What I do when there's a size range (e.g. .002 - .005), is have it so the smallest size feeler gauge slips through, but the larger one sticks or just doesn't fit.
 
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Remember when you clean the carb, every passage goes somewhere, and just spraying with cleaner won't always clear it out. I use welding tip cleaners and an assortment of tiny drill bits , sewing needles etc . To make sure every passage sprays forcefully out of the other end, Especially the low speed passage from the adjuster screw to the center of the Venturi. I have a tiny bent needle to clean the Venturi hole . Also the formed gasket under the fuel seat and under the jet holder can leak. If that happens you can pull unmetered fuel into the float bowl or engine.
 

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Discussion Starter #17
More clues today. Drained the float bowls and the fuel was clear and clean. Double check the plugs as it's been getting harder to start and everything looked good. Reset the fuel and air screws to slightly rich (I think both are at 1 turn right now).

1. The bike wouldn't start unless I had the idle adjustment screwed all the way in. This is a clue

Once warmed up this would have pegged the engine at 3000 RPM or more. Ran the bike for 5 minutes to get it warm, then ran it up and down the tach for another five minutes to get it hot. Had fun watching the fuel plume up in the carb throat with the air filter off :) but the idle never rose past 1100 RPM except:

2. Had a few instances of the idle hanging at ~2200 RPM. Now that I think about it, this had been happening more frequently on the street as well. This is also a clue.

The clues tell me that fuel delivery and quality isn't an issue. They also tell me that I need to clean these carbs. Have a friend who works on vintage Volvo's who doesn't have an ultrasonic, but did offer to let me at his barrel of Berryman's Carb Dip, with the caveat that it will dissolve everything that isn't metal. Do I take him up on it, or try to find an ultrasonic?
 

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I've never used the carb dip solution, I've gone to the ultrasonic if seafoam doesn't clear it up(which usually seems to work for me). I run a few oz in every other tank whether it needs it or not. This cleared up issues for me long term.

These are brand K carbs right? Don't they have an accelerator pump and no choke?

They are richened by twisting the throttle To squirt extra fuel for starting. I had brand M on my SS which had a choke. Someone here should direct you on set up. I know they shouldn't want to idle without help of the right wrist until you get a little heat in the motor. Are they set up properly, In addition to carb issues, I mean?

Bob
 

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OP said FCRs, ornery to start, lean drop after warm - a rather sudden change to this behavior after a short storage time. Sounds familiar.

Plugs the right color, check - but probably not enough miles since the problem started to show up there. So I am not thinking electrical.

Lets get back to the lean idle drop, follow the testing/cure for that and see what those procedures brings to light.

Any reason to think the exhaust could be plugged? A big assed tarantula crawled into my buddies Honda 350 bike exhaust pipe one time and died in there stuffing it all up with her and her nest. Enough starting fluid one time cranking splattered her all over the far garage wall. Ran great after that. His wife never went in the garage again,...

BTW, Ultrasonic is awesome, effective, and doesn't screw anything else up. My 2cents,...
 

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Discussion Starter #20
OP said FCRs, ornery to start, lean drop after warm - a rather sudden change to this behavior after a short storage time. Sounds familiar.

Plugs the right color, check - but probably not enough miles since the problem started to show up there. So I am not thinking electrical.

Lets get back to the lean idle drop, follow the testing/cure for that and see what those procedures brings to light.

Any reason to think the exhaust could be plugged? A big assed tarantula crawled into my buddies Honda 350 bike exhaust pipe one time and died in there stuffing it all up with her and her nest. Enough starting fluid one time cranking splattered her all over the far garage wall. Ran great after that. His wife never went in the garage again,...

BTW, Ultrasonic is awesome, effective, and doesn't screw anything else up. My 2cents,...
I'm still new to the fold, I thank you for your patience. Why does this sound familiar?

All I've seen about idle droop/hang issues were caused by lean mixtures and to enrichen the idle and 1/8th throttle mixture, which had worked wonderfully for the last 1400 miles. Had the air screw at 3/4 turn and the fuel screw at 1-1/4. One pump on the throttle, hit the starter and it would catch on the third rotation. What other T/S procedures are there, other than re-jetting?

Where do I look for air leaks on these? Spraying some starter fluid around the runner/adapter/connection didn't have any change, but I tightened the screws down a half turn or so just to be sure. The nuts on the intake runners to the head are tight. Is there a vacuum port somewhere that may be leaking by?

Haven't specifically checked, but running it up to 8K repeatedly this afternoon should have cleared anything out of the exhaust. Don't have any little ones around to muck things up or drop tennis balls where they don't belong.
 
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