93 virago same problem for 3 weeks

[93virago]

bought this bike used in great condition.  Brought it home rode it for 20-30 miles running great, then backfire and loss of power, chugging, at low RPMS starts and gets worse by the mile.  Ran fine on highway or above 3000 RPMs.  I was told bad plugs, changed them.  No help.  Was told that carbs had gummed up, ran fuel injector cleaner through half a tank.  No help.  

The guy I bought it from had the carbs completely rebuilt last fall, but then didn't ride it much after that. His mechanic comfirms that he had just rebuilt the carbs last fall.  So off to my local mechanic.  They say the carbs need rebuilt (without taking the gas tank off) and the tank needs to  be cleaned out due to rust build up.  I agree to this because the seller will pay for it. (local mechanic knows they were just rebuilt last fall)  I pick the bike up yesterday and take it on a ride.  Ran great for about 10 miles in town, then the exact same thing happens again.  Below 3000 RPM chugs and backfires on any idle or de-accelleration.  Runs great on the highway or accelleration above 2500.  I trust the local mechanic did what he said he did as I am friends with him.  

I took it back and dropped it off this morning.  He seems to not be able to figure out the issue.  Says compression is 110 and 120.  One plug (both are brand new) is burning clean and the other looks as if it is burning lean.   Another post on this site seemed to have the same issue resolved by an electrical problem.  Any help or advice would be appreciated, it is a very nice looking bike in great shape and runs fantastic for about 15 miles, then tanks.  

thanks

[bstig60]

Since it is happening below 2500 to 3000 RPM, I suspect you have a pilot jet passage plugged on the carb that is running lean. It may be something air won't get out. I have used a fine, flexible wire such as a small guitar string to clean these out in the past. Obviously you will need to take the carbs off and take them apart to check this. I would suggest cleaning both of them and bench testing the float levels before you put them back on. It is not uncommon for this to happen if you have any kind of debris in the fuel tank. My suggestion is to put an additional inline fuel filter between the fuel pump and the carb fuel lines to filter out any further debris.
Let us know what happens.

[93virago]

thanks, i will pass this along.  

[93virago]

bistig, they checked the pilot valve and said it was ok.  We also checked the float levels and they were about as close as they could be. One cylinder is not sparking/running the same as the other.  We tested the plug connector itself and it was fine.  He cut off the end of the plug wire and put it back into the connector to make sure that wasn't the problem.  3 different mechanics drove the bike, they said that if they had not been looking for exactly what I had described, that they wouldn't have notice a thing wrong.  That is crap.  I had my neighbor (40 year yamaha rider himself) and he noticed it right away.  He seems to think it is an electrical problem, but that is what he thought 2 weeks ago too.  

The mechanics left me with trying to replace the exhaust, as the mufflers apparently have been completely gutted/baffled out.  That maybe the lack of back pressure is the problem.  They also thought that maybe it was a plugged air intake for that cylinder.  They were supposed to check it, but I get the feeling they didn't.  

Brought the bike home and put in some seafoam.  After my neighbor took it out for a ride I took it out for a 15 mile ride on the highway.  Man it runs great 55-80, release the gas not a pop, sounds great.  Back in town, under 3000 rpms not running smooth but the loud backfire is completely gone.  

On the highway running it 4000 rpms, there is no change in the engine at all, very smooth.  Still under 3000 rpms in town, it will surge a bit and back off while holding the throttle steady.  Almost as if it is missing just a bit in the one cylinder.  

Any suggestions on the air intake for that cylinder?  They showed me where it is and how to check it.  What are your thoughts on the exhaust?  I can't imagine that would cause the problem under 3000 rpms and not over.  Lastly, could my neighbor be correct.  Might it be an electrical issue?  Again, I would think that would show up at any rpm.  

thanks for your time and information, the seafoam seems to be helping as it is better than before, but just not right.  

[bstig60]

Anything is possible! At lower RPM there is less backpressure on a straight pipe than there would be at higher RPM. Unless the carbs have been rejetted for the straight pipes, the bike will not run as well as it was meant to. I have a stock set of mufflers and pipes which came off a 700. I believe they are the same part numbers as the 1100. If you are interested in changing them out, we can talk about it.
What kind of plugs are you running in the bike? If the plug was black and sooty, it is either getting too much fuel or the plug is missfiring. Have you checked your coil resistance? If you are running NGK plugs, get rid of them and put in "Autolite AP63 Platinum's". Gap them at .028 thousands and then see how it runs.

[93virago]

I am using NGK since I was told they were better than champion, etc.  My bike is a 750, but I think the pipes are the same for 700-1100 from like 93 to 2002.  (according to an ebay dealer) I will swap out the plugs tomorrow and keep running the seafoam through it and get back to you on the pipes.  Mine are very nice, but if that is in fact the problem then they have to be changed.  Plugs are much cheaper that pipes.  

thanks again for your input.  I am in a bit over my head, I know cars and their engines and such, but bikes are different.  I appreciate your patience.  

[93virago]

more information for you to think about.  Late yesterday I took it out on the highway for about 15 mile, 60-70 mph.  Got back in town and ran perfect.  Rode it again a bit later last night, same thing, running perfect.  

Took it out this morning about 15 mile ride in town and highway, running perfect.  Stopped to pick up the plugs you suggested, back to running poorly in town.  So I took it out on the highway for 5 miles (always runs ok on the highway) back in town running poorly still.  Put in the autolite platinums.  Back cylinder plug was very black, front cylinder plug clean as a whistle.  

Took it out for a 10 mile ride.  Fine in town, great on the highway.  Back in town it was running just horrible.  I nailed the throttle 2-3 times to 4000 rpms, then it ran perfectly.  Checked the plugs and same thing, front plug clean, back plug black with carbonlike build up.  

It would seem to me that if the exhaust is the problem then it would never run perfect, then bad, then perfect.  The mechanics mentioned that perhaps there is a nest of some sort inside the air intake near the frame for that cylinder.  They showed me were it was at and how to get at it.  It sat in a barn for 5 years.  What do you think?  

thanks again.

[bstig60]

It sounds like the rear cylinder is loading up with fuel at lower rpm's. It's possible there could be an obstruction there and it wouldn't hurt to take a look. However, at higher RPM's more air is required, so it would seem more likely to flood out at higher rev's than at low rev's. Since the plugs didn't help, I would start looking for an ignition problem assuming the carbs have been illiminated. I don't recall if the later models have the plug wires molded into the coils or not. One way to find out if it is coil related is to switch the coils and see if the problem follows the coil. Since it has set for several years, I would suggest pulling off all the connectors on the coils and TCI and cleaning them. Also check to make sure there isn't any corrosion on your battery ground cable where it connects to the frame and make sure all connections are tight.
If you don't have a service manual, you need to get a Clymer's or Hayne's Manual. Do some searching on the internet and you may be able to download one for free.

[bstig60]

Here's a link for a Clymer's manual.   http://www.filestube.com/a3060dfd4f2fd79803e9/go.html 

[93virago]

thanks.  The only other piece of info that I can give you is that the backfire has completely gone away since they rebuilt the carbs and I started adding Seafoam. Thought that might give you another hint.  I will check the air intake and switch the coils and get back to you.  It did run good this evening after I cleaned that rear plug.  

[bstig60]

Dump in about 1/2 can of Seafoam and run that though it. The fact that it has improved since you added it before tells me there is still some passages in the carbs that weren't totally cleaned out. SeaFoam can work wonders. Also, the fact that it ran OK after cleaning the fouled plug says it probably isn't ignition, but it is good to check the things I mentioned to be sure.

[pofarm]

Here is a download for the manual.
http://www.carlsalter.com/aaman/Yamaha_XV_V-Twins_81-94.pdf

[93virago]

the plug wires are molded into the coils.  On my older bike they were not, 1979.  Since the plug wires are not long enough to switch the entire assemblies I put it back together.  I can, however, switch the 2 electrical wires coming into the coils.  Should that be something I consider switching?  The leads on both coils looked clean and I checked the batter connections and they looked good too.  

Did you mean half a can of seafoam in 1 tank full of gas?  I just filled it up today.  The bike does continue to run better more than not.  The plug was black again today so I cleaned it again.  Afterwards, took it for a ride and the exhaust had a much more throaty sound to it, although it didn't run perfectly.  

thanks again for taking the time to help me out.  I am learning quite a bit

[bstig60]

Yes, there are two wires coming into the coils, one is the power RW and the other is a gray on one cylinder and orange on the other. You can switch these connectors and it should accomplish the same thing as swapping the plug wires.
Seafoam: Yes, half a can in a full tank.

[93virago]

ok great i will run out and switch those wires and dump HALF can of seafoam in.

If the coil is bad and they are still running to the same cylinder/plug how can that tell me which one is bad? All I am doing is changing the power source coming into them aren't I?  Not switching which cylinder the coils are feeding.  I think that makes sense. if i can't switch the cylinders that the coils are feeding how can I tell if one of the coils is bad?  If the power souce going into the coils is a problem, that would show up, but if both power sources/leads going into the coils are good then the same cylinder has the same problem, correct?

[bstig60]

Sorry about that, You are correct. You would need to switch the plug wires too. Try this....... get a piece of solid copper plug wire at your auto supply store and build an extension to put on the rear plug. Then switch the power and trigger wires. That should tell you if you have a bad coil. Another thing that you can do is measure the resistance on the primary and secondary windings using a good volt-ohmeter; each coil  should read close to the same on each winding. If they do not, then there is a problem with one of the coil windings.

[93virago]

it wouldnt start when i switched the wires feeding the coils.  

[93virago]

on another note.  I am currently downloading the manual. As i stated in one of my first posts the compressions were 120 and 110 for the 2 cylinders.  That seems very low according to the manual.  The mechanics said it was fine.  What do you think?

[bstig60]

Sorry! Had a Brain Fart! Of course it wouldn't start, because that would have thrown off the timing and it would have been trying to fire on the exhaust stroke on each cylinder.
Compression: According to the manual the compression should be up around 150, so it is a little low, but I suspect it is probably OK as long as it runs well. If it was too low you wouldn't be able to get it to start and if you could, it would run at higher R's and not want to idle at all. The compression reading you get depends on how you do the test. The engine needs to be warm and the throttle wide open when you run the test. If one cylinder reads low, squirt some 30W oil into the cylinder and run it again and note the difference. The cylinder compression should be close to each other.