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Discussion Starter · #1 ·
My bike fell on it's right side a few days ago (while parked). It runs, but was backfiring real bad. And was running weird. Like if I held the throttle at 2k RPM, it would stay there and then all of a sudden it would run better and jump up to 3K rpm by itself.

So I figured some crud got in the rear carb.

So I replaced the carbs with some other carbs, the problem is still there. No matter what carb is on the rear cylinder, it will backfire in that carb.

I checked cable routing and it all looks good, according to the manual at least. Vacuum and fuel both.

Any ideas?
 

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Discussion Starter · #2 ·
Anyone?

I did some searching on the forum. It looks like most backfiring comes during decelleration and is because of vacuum leaks.

My backfiring happens all the time I think. I have the filter cover removed and I'm looking right into the carbs. The rear carb keeps having mini-explosions inside it. This happens with the original and the replacment carbs.
 

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Discussion Starter · #4 ·
Thanks for the response!
I haven't touched anything. Fuel mixture, carb synching, jetting, nothing has been touched. The bike purred nicely. Now it suddenly doesn't after the tipover. :(
 

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Did you ever figure your backfire out. I am having the same problem on a newly rebuilt engine. Rear cyl as well. Did syncing the carbs help? Mine runs great above 4k but spits and backfires at idle.
 

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Discussion Starter · #6 ·
No, I haven't figured it out.

Mine will backfire & spit & stumble at idle. I noticed that if I put my hand over the front carb neck, the bike will immediately die. But if I put my hand over the rear carb neck, then there is significant suction but the bike continues to run.
My hand does get pretty wet with gas when I do this.

Someone suggested the timing somehow got out of whack?

I don't have a carb sync tool to do the carbs. But I'm not sure that I suspect the carbs. How could two different sets of carbs both have the same problem?
 

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If you get fuel and air, next thing to check is spark...have you checked your rear plug to make sure it's firing? If it's firing have you checked your timing? Valve adjusment? it could be that it's jumped timing and the intake valves are'nt closing when they should be hence pushing fuel and air out of the carb instead of sucking it in...your carbs should never push gas away from the engine. Or it could be a burnt/bent/broken valve and it's just not holding back the pressure.

Hopefully it's a spark issue and just flooding the rear cylinder.
 

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These are all long shots but I draw a big blank on any reasonable explanation.

When the bike fell over, were you right there to pick it up, or did it lie on its side for a long time? If it was on its side for a long time, maybe oil got past the rings and fouled the rear plug only?

Maybe when the bike fell over, debris/moisture got stirred up in the tank and made its way to only the rear carb? After swapping carbs, perhaps some free debris again made it to the rear carb, and only the rear carb?

Maybe the tip over sensor is flaky, and falling on it's side was enough to make it really flake out and intermittently affect cut the fuel/ignition. How this would affect only the rear cylinder, I don't know.

Did absolutely anything else happen between working/not working besides the tip over? Are you 100% certain only the rear cylinder is affected?

When you figure this one out, please post the answer. Good luck!
 

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Discussion Starter · #9 ·
Thanks for the help, guys!



have you checked your rear plug to make sure it's firing?
I just checked and yes I'm getting spark on the rear spark plug wire. In fact, I noticed that with the rear spark plug cable removed, the bike runs smoother and doesn't backfire. It still won't hold an idle. But it didn't backfire at all and only "coughed" once.



If it's firing have you checked your timing? Valve adjustment? it could be that it's jumped timing and the intake valves aren't closing when they should be hence pushing fuel and air out of the carb instead of sucking it in...
I am not sure how to check the timing. I'm going to research that this afternoon.

I have not done the valve adjustment either. Bike has 49k miles (I have owned for last 1.5k miles) and it pulled 190'ish on both cyclinders in a dry compression test I did on it about 500 miles ago.

your carbs should never push gas away from the engine. Or it could be a burnt/bent/broken valve and it's just not holding back the pressure.
I'm not sure that it's pushing gas out? If I loosely hold my hand over the rear carb throat, it doesn't get wet. It only gets wet if I seal off the rear carb throught with my palm, which it then provides suction and gets my palm very wet.



When the bike fell over, were you right there to pick it up, or did it lie on its side for a long time? If it was on its side for a long time, maybe oil got past the rings and fouled the rear plug only?
It was on it's side for about 60-90 minutes? I was replacing my fork oil & seals. The front wheel was removed and the jackstand I was using to hold the bike up, had collapsed. Since I was alone, it took me awhile to get the bike back up again. I had to rig a hoist system from my ceiling.

Maybe when the bike fell over, debris/moisture got stirred up in the tank and made its way to only the rear carb? After swapping carbs, perhaps some free debris again made it to the rear carb, and only the rear carb?/
That would make sense, except that I've tried two different carbs with the same results. :)


Did absolutely anything else happen between working/not working besides the tip over? Are you 100% certain only the rear cylinder is affected?
All I did was replace the front fork oil & brake pads & put on stainles brake cables.


When you figure this one out, please post the answer. Good luck!
Yeah, I hate it when people ask for help and then never post back what fixed their issue. They don't realize or care that someone six months down the road is reading the thread, hoping it contains the fix to whatever problem they're currently having.
 

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I think you should be looking at the "|F" mark only. Can someone else verify this?

Front cylinder: Rotate the crank another 360 deg, and the proper cam marks should line up. Two crankshaft revolutions = one camshaft revolution, so I think you're OK, just 180deg off with the cam.

Rear cylinder: Set the crank to the "|F", not the the "|R" line, and try again.

Try this sanity check. Put your finger in the front spark plug hole and manually rotate the crankshaft. You should feel the compression stroke, followed by suction on the power stroke. Do the same for the rear cylinder. They should feel the same.

Was you bike making any funny noises before this happened, like a loose cam chain?

EDIT: Edited for "|F" mark.
 

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Discussion Starter · #12 · (Edited)
Hmm, when I put my thumb in the front spark plug hole, I feel the compression stroke, followed by suction on the intake stroke.
But when I put my thumb in the rear spark plug hole, I only get the blast of air (compression). I don't ever really get any serious suction?

This bike has plagued me with various mechanical & electrical problems ever since I got it. Just one problem after another. Included in that were some occasional problems where it would appear to run on just one cylinder, but I thought that was due to me sorting out some high performance carb linkage problems. And it would come and go.
The bike ran great the day before I tore it down for the fork oil swap, which is when it fell over and this problem occured.

Thanks for the tip on rotating the "|F" by 360. I did that and now this is what I get for "|F".

FRONT Cylinder:



REAR Cylinder:
 

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It's a little hard to tell with the cam chain guide in the way, but the front cams looks about right.

The rear cams look to be out 180 degrees. Rotate the crank 360 and show the rear again.

You said you did a compression test when you got the bike, is that right? You might try the test again, see what you get.

Where is Andy when you need him?
 

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The front looks correct. After checking the front the crank must be rotated one full turn and set back on the F again, to check the rear. Do that and take a pic of the rear.
 

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Discussion Starter · #16 · (Edited)
I just checked my valves and this is what I came up with.

Per them manual it should be within .1 to .2mm for intake and .2 to .3 for exhaust.

The printing on my journal covers say the intake is on the right and exhaust on the left.
So by that logic, I'm looking good except for the intake on my front cylinder, which is .001" out of spec.
 

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Odd bid of action, but engines make many noises we can't always hear while riding, perhaps prodding the motor in various places around the engine with an engine stethoscope while she's IDLING. I just don't think this issue is timing or valve actuation related.

Engines do talk.. even healthy ones have a lot to say.
 

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Thanks guys. I rotated the crank 360 degrees and set it to |F again. This picture shows what the rear cylinder looks like now -->

That looks about right, again, it's a little hard to tell exactly with the cam chain guard in the way. You need to see 16 chain pins (inclusive) from the '3' mark on the intake cam, and the '2' mark on the exhaust cam.

I think you might be barking up the wrong tree here though. On an earlier post, you said that the problem would "come and go", that defiantly isn't a cam timing problem.

Also I think now that the tip over probably has nothing to do with the backfire, but instead was coincidental.

Something easy to try is swapping the ignition coils front/back.
 

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I agree with TeeRiver, the cam timing looks a-ok. The two exhaust valves that measured .007" are a bit tight, should be between .008-.012". That's not causing the problem you're having. The problem is not under the cam covers, it's somewhere else.
 

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Discussion Starter · #20 ·
I'm a little worried about one thing. I just <finally!> found my Haynes service manual.

For checking the valves, it says to do the following.

1) Line up |F so that the front cylinder cam lobs are pointed away from each other (TDC).
2) Check valves.
3) Rotate engine 270* until |R is lined up and the cam lobes are pointed away from each other.

Problem is with step#3. At this point, my rear cylinder cam lobes are not pointed away from each other. They are instead, pointed upwards and slightly angled towards each other.

I have to then turn the engine 360*, line up |R again and now my cam lobs are angled slightly away from each other.
 
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