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.
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.
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?
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.
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.
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.
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.
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
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.
Hey there, I know its an old thread..but im having a similar challenge...what was the final verdict? Thanks
I'm also having very similar issues. I'm working on an 88 Shadow 600 and I'm getting frustrated. Both cylinders are getting nice blue spark and compression is 150-160 psi in both. It wasn't running when I got it. The intake was removed and th PO claims he had the carbs off a few times and gave up. I thoroughly cleaned the carbs and tank. I found a cracked diaphragm so I replaced it, the other one looked good so I saved the money there (maybe I shouldve replaced both). It barely fires up and it sounds like its idling on one cylinder and wont rev. When I force gas down the line with a syringe, I believe the other cylinder gets fuel and itll rev up and sounds good. As soon as this fuel gets consumed, it goes back to the single cylinder low idle with an occasional backfire. The PO eliminated the fuel pump and claims it ran without it but I had my doubts so I picked up a low pressure Mr Gasket 42S pump from Autozone. It supposedly pumps out 2-3.5 psi. I didnt wan to overpower the float needles with too much pressure and Ive seen others use this pump successfully. I also threw on a new fuel filter. It seems to be getting gas to both carbs because it pours out of both float bowls when I open the drain screws. After priming the system, I had the bike running and idling perfectly. I thought I was done and turned it off. Last night I tried starting it up again and it was back to the same old low single cylinder idle. I pulled the plugs after a couple minutes of this. The plug from the front cylinder was wet with little carbon and the plug from the rear cylinder was dry with some carbon buildup. This must be a fuel delivery issue. I think my first step is to to get rid of any air in the system. Next, I will recheck the fuel lines to both carbs and possibly pull off the carbs again to check the floats. I would really appreciate some help with this. Im getting aggravated. I've been searching through many forums for answers. Since I sold my Tengai I haven't been on here much but I'm hoping the ADV community can help me out!
E-mailed my buddy and this was his reponse: "Carb Jet size and air pressure in air box. That's what the mechanic said" I forgot that we actually ended taking it into a mechanic... He did what we called a "$300 magic trick" I think he pointed out that gas has changed since it was made so it needed different jets, also it had some issue with the airbox... dont really remember what caused it.
Cool..thanks...I found out that my problem was a faulty ground...I had relocated the ground wire under the seat and it wasnt a solid connection. .. My bike ia using the same mr gasket fule pump...no issues...good luck
Listen I'm having the same issue and its weird. When I got the bike some idiots owned it. I got the same thing and one of the prongs on the plugs was mashed down. I changed all the plugs just because. That's what I do. I cleaned it all up. It ran like shot bc it had been sitting lord knows how long. I ran it and ran it. Shot all came out. Ran great till last week when it started doing some weird things. Like I noticed that one of the pipes was discovering at the head. So I enlightened it a bit. Then a couple days later it started not firing on one cylinder. So I ripped the carbs out. People were saying oh that it's the obvious coil problem. Absolutely not. I'm a mechanic on cars. But a gas engine is all the same. I said no. a fuel problem some where. Could are good. I ripped the carbs apart down to everything. Cleaned out every crevice even though they were so damn clean. Put them all back together. It runs amazing. But, now the rear cylinder isn't "firing" no the plugs are soaked the cylinder isn't firing bc it can't. What u are hearing is it eventually lighting off that gas that its soaked with. I'm gonna go with the diaphragm. Maybe there is something that we can't see. But, I talked to my uncle and he says that he's seen that and its normal. My one cylinder is 120 and the other is 94. So I'm not concerned about compression at all. I'm not gonna do rings when they are good. Bc I can tell u now. It has power. No lag. Could it have more power of it was firing right yes. Timing is not the issue here if u have compression and the plugs are firing. Fuel filter there's no way if one is getting too much a and the other getting the right amount. Fuel pump maybe. Could it honestly have enough pressure to open the valve? Who knows. But I'm thinking that bc I mixed up the carb parts which I'm sure I did bc now its on a different cylinder.I'm going with diaphram. There's a diaphragm on the side of each carb that has something to do with it. So that's what I would go for. Have I fixed it absolutely not. Its shouting bc u can feel it. U know that its not running right. My buddy says to lean it out a bit. But, u can even tell bc by the time u can mess with the carbs they are so soaked. People are buying new carbs. Stop. I've even went to cheap plugs. I always use ngk. But I went to cheap autolites I think there is a difference there. But that might just be my imagination. Its obviously not enough to hydro lock. Nothing could possibly go wrong by driving it. It smells. But I'm not worried about it. Looks like a diesel but hey. When winter comes soon. Ill tear it apart and do all gaskets and seals ill mess with it then when I have nothing to do for winter