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Old 03-15-2012, 04:06 PM   #1
alexk243 OP
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1983 Honda Shadow running on one cylinder

I have been trying to troubleshoot this for a while now and just cant figure it out.

My buddy has a 1983 Honda Shadow VT750 (I know, not my adventure bike). It is a V-twin, dual carb, dual pipe, harley replica. Last year it started bogging and appearing to be running on one cylinder once it hit 4000 RPM.

I took apart the carbs, cleaned really well (did not rebuild with new parts however) and put it all back together. Running it in the garage recently without the gas tank or air filter I noticed that it clearly was not running on both cylinders. The rear cylinder seemed to not be firing, or it was just lagging. This bike has 2 plugs per cylinder and all of them were sparking.

After the bike warmed up (running on one cylinder) the second cylinder began to chime in, but when the gas was applied fire would come out of the exhaust and the carbs of the issue cylinder.

I am lost, dont know where to look. Spark seems to be ok and carbs seem to be ok.... What should I check next? or is there something specific in the carbs I should look at?

Oh and this bike ran great for 2-3 years prior, ran really well, but he stored it outside... in the winter... in Chicago... so not surprised something went bad, but this went bad during the riding season last year.
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Old 03-15-2012, 05:05 PM   #2
VTSteve
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Are you sure the plugs are firing all the time? It sounds to me like you might have intermittent or weak spark in the bad cylinder, as if it's loading up on gas and then finally firing after it gets hot enough. You can try swapping the coil and or CDI boxes between the 2 cylinders and see if the problem follows those components. This is assuming the plugs are good.
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Old 03-15-2012, 05:09 PM   #3
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I forgot to mention, if it's been sitting outside for a while, it's more likely corrosion on a connector instead of a bad component. Disconnect the plugs on the CDI units and clean them up first if you haven't already.
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Old 03-16-2012, 08:38 AM   #4
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Quote:
Originally Posted by VTSteve View Post
I forgot to mention, if it's been sitting outside for a while, it's more likely corrosion on a connector instead of a bad component. Disconnect the plugs on the CDI units and clean them up first if you haven't already.
I swapped the CDIs with no change. Pulled one of the plugs and tested it while the bike was running and spark never had an issue. Toward the end of our testing last night we got it to run fine and cycle through all the gears on a stand, but I know from experience last year that if we test ride it the bike will loose power and go to one cylinder under load.

And even when we had it running, full power was intermittent.

What else is there I can check?
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Old 03-16-2012, 09:10 AM   #5
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Is there an inline fuel filter? It could be plugged up and restricting fuel flow when you really get on the throttle. I'm guessing being an 83 it probably doesn't have a vaccum petcock, though there's probably a small screen filter in it as well. Disconnect the fuel line feeding the carbs and make sure you have good fuel flow.
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Old 03-16-2012, 11:40 AM   #6
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Get out your meter and start checking the coils, wires, and the ignition triggers.
If you don't know how, get on YouTube and do a search for motorcycle coil check and watch one of several vids on there.

Stop running the bike with a forked up ignition before you break something expensive. And stop replacing parts instead of diagnosing.

Sorry, but you're gonna have to engage your critical thinking skills to fix this or pay someone who can.
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Old 03-18-2012, 12:46 PM   #7
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Quote:
Originally Posted by VTSteve View Post
Is there an inline fuel filter? It could be plugged up and restricting fuel flow when you really get on the throttle. I'm guessing being an 83 it probably doesn't have a vaccum petcock, though there's probably a small screen filter in it as well. Disconnect the fuel line feeding the carbs and make sure you have good fuel flow.
It has a low sitting reserve tank that feeds into a electric fuel pump (weird for 83 I know) which is totally functional. You can unplug the gas lines at the carb and turn on the key and gas will shoot 1-2 feet out of the line... so its good.
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Old 03-18-2012, 12:48 PM   #8
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Quote:
Originally Posted by kraven View Post
Get out your meter and start checking the coils, wires, and the ignition triggers.
If you don't know how, get on YouTube and do a search for motorcycle coil check and watch one of several vids on there.

Stop running the bike with a forked up ignition before you break something expensive. And stop replacing parts instead of diagnosing.

Sorry, but you're gonna have to engage your critical thinking skills to fix this or pay someone who can.
Yea ill pull out the volt-meter and take a look at the coils, but as it appears to be getting sparks and seems to run I kinda rulled out an ignition issue and had my focus on gas delivery. But Ill retrace my steps and look back at ignition.
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Old 03-18-2012, 01:12 PM   #9
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Had a friend with a similar issue with a 96' model except it was the front cylinder. I know I pulled the carbs 3 times. Had fire from all 4 plugs. Had fuel from the tank and fuel pump. Played with it for a few days only to find out that there was no fuel getting to the fuel bowl on the front carb. Seemed fuel line going to the front cylinder carb was old and restricting fuel flow to the front carb. New fuel lines and problem solved.

Don't know if this could be an issue with yours or not.

Ray
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Old 05-01-2012, 09:11 PM   #10
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Replaced all the fuel line. With that T connector it does tend to kink, but we have kept and eye on it and it does not seem to be an issue. Been messing around with the bike a bit recently. Rebuilt the carbs with new float valves... no difference. Time to retrace my steps with the ignition.
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Old 05-01-2012, 10:38 PM   #11
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Coils are at 2.2 ohms each.... is that an issue?
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Old 05-01-2012, 10:54 PM   #12
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Have you checked the timing?
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Old 05-01-2012, 11:25 PM   #13
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Have you checked the timing?
No... could that go bad from sitting?
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Old 07-21-2014, 02:33 PM   #14
toofast4uu
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Hey there, I know its an old thread..but im having a similar challenge...what was the final verdict? Thanks
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