Back in June of 2011 I bought this 1984 Yamaha Virago XV 700 with a 1983 XV 750 motor in it. It took me a couple of days to get it running well, but I did.
I discovered pretty quickly that I hated the stock handlebars so I ordered drag bars and switched them over. I got some saddle bags from a couple I know and put them on. These little cosmetic things don't mean much but they add to my comfort on the bike.
I noticed after a couple of weeks and two cans of Sea Foam that the gas milage was pretty horrible. It was running about 34 mile per gallon. Thinking that was not good I looked up the specs for the engine. Optimal it should get 45 so if it had been a little closer to 40 I would not have bothered with it. It was running good but it was burning gas pretty quickly and the exhaust smelled strongly of unburnt gas. I resigned to pull the carbs and clean them thoroughly. They were not as bad as I had expected, but I had already run two cans of seafoam through them. What I discovered was that the carbs had the jets for the XV 700 in them. The 700 had two different jets, a 128 and a 132. The 750 calls for a 122 in each carb. I went ahead and ordered a pair of jets for it from JetsRUs.com who actually had Hitachi style aftermarket jets.
Before my jets could get here, I started having a new problem. If I twist the throttle too fast at take off, I lose a cylinder until I either back off the throttle or the RPM's get up to around 2500. If I ease the throttle up, no problems. I get smooth acceleration all throught the powerband. Not knowing what could be the cause of such a sudden problem, I back tracked what I had done to the bike. I had cleaned the carbs, replaced the sparkplugs with Autolite AC 63's (that's another irritating point), and changed the oil. I decided that it must have been something I did when I cleaned the carbs, so I pulled them back out and checked everything. The slides were a bit sticky because I had cleaned the lubricant off of them. Knowing how important it is for slides to move quickly, I figured that was the problem. I got some good engine parts lube and doused them with it. After I put the carbs back together and put everything back on the bike, I figured it would run like a top again... No such luck.
The jets came in and I pulled the carbs AGAIN. (I hate pulling those things. I would dearly love to go to a single carb setup.) I installed the new jets and put everything back together. It fired right up. I took it out of the driveway and... lost a cylinder. I have now gone through and recleaned the carbs, reset the pilot screws, replaced the plugs and the main jets and cleaned the carbs again. I still have not solved this problem, though I am getting much better gas mileage.
About the spark plug story. When I went to check the plugs the first time to see what kind of shape they were in, I started with the front plug (#2 cylinder) and it looked fine. I went to pull the rear plug (#1 cylinder) to find that it had been seriously cross threaded. This was before I started having any trouble with the bike loosing a cylinder. Even though both plugs looked okay, I went ahead and got new Autolite AC 63's because I couldn't find the NGK BP7ES plugs that were called for. Now I have the question of resistor/non-resistor plugs and if the plug wire/coil units have resistors built in to them. I can't get the wire boots off without destroying the wires because they are fused on, so I can't check for resistors behind the connector. The rear plug is well seated even thought those threads are so messed up. There is no compression loss around the plug. Oh well. Back to the current problem.
So let me give a detailed description of the problem here. If the bike is sitting in neutral and I crank the throttle, there is a slight miss at around 1200 rpm. That's it. No other problems anywhere in the whole range of the throttle. If I pull one plug and idle it on the other (done with each plug in turn, same results) I get the same thing... Right at 1200 rpm there is a slight miss if you crank the throttle but if you smoothly ease it up there is no miss. Under power at 1200 rpm it feels like I lose a cylinder. I'm not 100% sure that's what is happening, but that's what it feels and sounds like. If I keep the throttle steady , I get lots of backfiring throught the air cleaner. If I back the throttle off to less than 1000rpm, it smooths back out and I can try again. If I plough through the missing part, I get the cylinder back at around 2500 rpm after a few backfires through the aircleaner. Again, if I ease the throttle up slow and steady, no misses of any kind occur. I'm starting to think electrical, but am still confused about its sudden occurence. I'll keep you updated as I work on the problem.
Bikes Blues and BBQ is here and I have done all I can with this engine. I went and got higher end E3.46 sparkplugs and put them in, then did a full valve adjustment. The missing problem seemed to go away for a few hours, then it came back, just not as bad. I tried to re-balance the carbs and that reduced it a little more. Now I only have the miss from around 1200rpm to around 1800 rpm and little to no backfiring through the air cleaner. I have decided to try the original 700 motor after BBBBQ is over and see if the problem carries over (then it would be carb or electrical) or completely goes away.
So, I feel foolish. I think I found the problem. On a whim I turned the idle mix screws two full turns further out. I had been thinking it was running too rich, but the leaner I turned the screws, the worse things got. It's like the old carpenter joke, I kept cutting a little more off the board each time but it was still too short. So the front one is 3 3/4 turns out and the rear one is 4 1/2 turns out. The backfiring through the air filter completely stopped and so did the missing at 1200 rpm's. Ahem. Well, dumb-ass me. Now I just have to fine tune the mix and the bike will be okay again... I hope.
Bikes Blues and BBQ is over. On October first, the last day of the bike rally, I wanted to kick my Virago over into a ditch. The missing on a cylinder and backfiring through the air cleaner came back with a vengance. On top of that, it didn't want to start once it would cool down. What do I try next? I guess I'll try tracing the coil wires on my next days off. Maybe one of them is worn or melted. I have determined that the backfiring is coming from the rear cylinder.
Things done so far to the Virago
Changed handlebars from stock to drag bars
Cleaned carbs with SeaFoam twice
Changed Spark Plugs from Autolite AC 63 to Autolite AC 63 gapped to .028
LED lights and switch
Tightened friction clips in starter system (needs to be done again)
Cleaned carbs thoroughly with SeaFoam and carb cleaner spray for tight passages
Fixed some wiring issues with turn signals
Removed strange vacuum cannister so my fuel valve would work again (had been running it on PRIME)
New main jets from 132/128 to 122/122
New Air Filter
Checked thoroughly for vacuum leaks
Ordered new rear tire - Chen Shin C-907 130/90-15
Checked plugs and regapped them at .028 ( they weren't out of gap and looked pretty clean with a golden brown color)
Complete valve adjustment according to Clymer specs.
New spark plugs again, this time I got some E3.46 plugs with a triangular firing ring.
Replaced vacuum cannister to see if that would help anything... it didn't.
Rebalanced carbs with home made manometer.
Richened the mix at the idle mix screws... duh.
Added Razorback stickers (necesary for smooth power at high rpm's)
Cussed at it alot.
An update on my Virago. I ordered a single carburetor manifold from a man named Robert Kayga. He makes and sells these manifolds specifically for Viragos. You can find them on eBay for $165.00 plus $15.00 shipping. If you contact him directly at firstname.lastname@example.org he will ship it for free.
I got the left side manifold, meaning that the carb sits to the left of the engine and angles slightly foreward. The quality of the manifold is VERY good. It is aluminum and he cuts and welds the pieces himself. The welds are not the prettiest, but they are strong and mine had no leaks. It comes with gaskets, all you supply is the sealer and the carburetor and boot. It took me less than fifteen minutes to get my old stuff off and get the new manifold gasketed and installed with carb boot, carb and cables all installed.I had previously bought a Keihin PD 36mm carb off of ebay. It is not a CV, it is a slide carb. I am currently running a 60 pilot and a 140 main jet.I am still working on the fine tune, but it looks like this setup will work very well. I still have to open it back up and raise the needle a bit. For any of you that have trouble with your dual carb setups, this is the way to go. The single carb manifold lets you use whatever carb you want. Eventually I would like to find a good Harley Keihin CV carb. For all of you Harley enthusiast out there, just to let you know, Keihin is made by Honda. Mikuni is also Japanese and you will find many Harleys with these two Carburetors on them. S & S is american made and Edelbrock of course is. Ideally I would like to have an Edelbrock Qwik Silver II with the accel pump and the idle jet. those are hard to find and far out of my very low budget though. I have it on good authority that if you have one and want to put it on a 750 Virago, it needs a 13-0 Needle.