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RSRBOB, Technician
Category: Motorcycle
Satisfied Customers: 1078
Experience:  Former Factory Service Rep, Dlr Line Tech, Service Manager, General Manager, Store Owner
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1998 Yamaha Virago Freshly rebuilt and synchronized carbs.

Customer Question

Customer: 1998 Yamaha Virago Freshly rebuilt and synchronized carbs. Fresh gas and plugs. Bike stumbles after 25 minutes of riding and drops the front cylinder. Condition is worse when going up or down hills. Fat blue spark on both plugs. Rev the engine in neutral and it is ok again for 5 minutes.
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Customer: will gladly pay $18 for a true solution. Have 40 hours into the bike now.
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Submitted: 1 year ago.
Category: Motorcycle
Customer: replied 1 year ago.
Now what?
Expert:  RSRBOB replied 1 year ago.

Hi and thank you for your question.

This sounds like the front cylinder is running rich. Anytime a bike starts running worse as it warms up, the first thought to come to mind is it is rich. It is a fact that as an engine warms up, it runs richer.

I would suggest confirming this by pulling the front spark plug out and looking at it after you have experienced the symptoms again. Sadly, that could be the easy part of this equation. The difficult part could be determining what has failed and is causing the symptoms.

My question to you is, who did the rebuild? Also, to eliminate any errors in interpretation, would you be very specific about what was rebuilt, especially saying what parts were replaced with new parts.

It would be a good idea to do a compression test on the engine just to make sure both cylinders have within 10% compression of each other.

Some of the potential problems that could cause this symptom is an improperly set float level, worn jets, float needle and seat wear issues, things such as that. If you intend to dig into the carbs, we are going to just have to go into them and inspect and scrutinize each individual component in the carb and take it from there.

Let me know the answers to those questions and then decide what to do next.



PS, if you have not taken carbs off a Virago before, it is not the easiest job at all. It is difficult and tedious. However, if you are up to the task, I will gladly walk you through it.

Customer: replied 1 year ago.
I removed disassembled and serviced the carbs. New pilot jets were installed as the existing ones were 1(wrong size) and 2 (broken jet head) mains were correct size and were soaked in carb cleaner. Diffusion tubes, jet needles float needle and floats were all in good shape. Gaskets on jet block were replaced as originals were bad. All passageways were blown out with carb cleaner and compressed air. Diaphragms were good. Compression is 156 and 158 psi with carbs on and butterfly's open no air filter. I vacuum synchronized the carbs at idle and while revving. Pilot air screws are 2.5 turns out. Float levels set to factory spec via clear tube method. The bike starts and runs flawless for the first 25 minutes. Then bogs and drops the front cylinder. I can put it in neutral rev it to WO throttle and get 5 more minutes of riding time until it boggs again. Front plug is fouled. I clean the plug and it does the exact same thing the next ride. If I ride it very hard it will flood out quicker but it will not run longer than 25 minutes without flooding out.
Expert:  RSRBOB replied 1 year ago.

Thank you for the additional information.

Without questioning what you have done so far, I suggest taking a magnified look at your jet needle and needle jet for signs of wear. What you are looking for on the needle is signs of wear in the center, where the needle would be at mid range, such as looking smaller in the middle than the top with an unnatural arc to it.

In the needle jet, look at the top orifice for signs of wearing in an oval pattern, specifically worn in an oval towards the intake valve on that carb.

Exactly what did you set the fuel height to in mm?

Also, there is no such thing as cleaning a fouled plug. Once a plug has fouled, it is going to be very susceptible to a repeat fouling, usually in a shorter time frame than the original fouling.

Also, what spark plugs are you running in the bike?



Customer: replied 1 year ago.
Float height set to 13.5 mm. Plugs are NGK BP7ES I am getting a set of Autolite AP63's to see if that helps. The bike only has 1200 miles on it and the needles look like new with no wear on them or the opening to the tubes they sit in. No ovaling. Just bizarre how well it runs for the first few minutes... Could it be electrical?
Expert:  RSRBOB replied 1 year ago.

Thank you for the additional information.

You mentioned previously you set the fuel level by using the clear tube on the outside of the float bowl. Where does the 13.5mm measurement come from?

It could be electrical, but unlikely. That is easy to check. Get an inductive timing light, run the bike to failure and put the timing light on the front cylinder. If it has lost spark or become intermittent, the timing light will show that.

This still seems fuel related to me. I am going to think on it awhile. I am also curious how you came up with a Virago with 1200 original miles. That can create its own set of problems too.


Customer: replied 1 year ago.
See attached for float height. Friend had the bike in his garage for years. Trying to get it running for him. Just trying to do him a favor. I now have 40+ hours into it and am thoroughly perplexed. When cold it runs like a champ. As soon as it is warm it boggs badly.
Expert:  RSRBOB replied 1 year ago.

Thank you.

Please understand, when I ask a question I am not asking what the service manual says. I am well familiar with how Yamaha wants it done. What I need to find out is where is the difference is between either what it used to be or what you are doing different than what the service manual says.

Currently, there are 2 problems.

The first is your spec. It does not agree with a fuel level spec. That is more like a float height setting. Fuel level is a result of float height but they are not interchangeable, nor are the specs.

The next problem is you sent me a drawing of V Star carbs which are not Virago carbs. That creates another problem. Are you working on a Virago or a V Star. Yamaha made both models in 1998. If the carbs you are working on look exactly like the drawing you sent me, it is a V Star. If the float bowls are on the bottoms of the carbs, it is a Virago. Obviously my next question is which bike are you actually working on? Both are air cooled V Twins and a V Star is a like a new and improved version of the Virago, there are key differences.

Lets start over armed with this info and get on with getting your buddy back in the wind for more than 25 minutes at a time.

The last question would be, is this an 1100cc?



Customer: replied 1 year ago.
It is a Virago 535. I just wanted to show you the clear tube method I was using for float height. I raised the tang height approximately 1mm ( lowering the fuel level in the bowls) and brought it out for a ride. It was much worse. Ran bad immediately. New plugs came out white after 7 miles so I assume I am way lean at this point. Question : how important is diaphragm slide orientation? I noticed that the diaphragms rotate on the slides. I read some ware that the hole in the bottom of the slide must align with the discharge side of the carbs. I aligned them that way on assembly. I am worried that the slides may be rotating during operation. I marked them with a sharpie so I can see if they are moving when I pull the carbs out again. ( which I hope I can avoid). Right now I lowered the float tangs .5mm and will ride it again this morning. Only issue is 32 degree temps. Brrr.Thoughts
Expert:  RSRBOB replied 1 year ago.

Ohhhhh, a 535. OK. That explains the carb setup.

If you are adjusting the float height, you should do it by measuring the float height and not by tab movement. A .5 mm movement at the tab could be 5mm at the float. You could go from dead lean to dead rich.

The other point that came to mind was did you compare the fuel level of the front carb to the rear carb? If it is the same, the problem is NOT float height or fuel level. If it were, both cylinders would exhibit the same problem. The other point here too is don't deviate from the factory service specs. Fuel level 13.5~14.5 mm below the cast line on the float bowl. If the bike will not run correctly at that setting, then something else is wrong. Any attempt to solve a problem outside those specs is merely putting a band aid on the real problem.

You may have hit on something with the slide twisting in the diaphragm. Again, your best bet is to compare the frt cylinder to the rear. The service manual does not specify the location of the lift hole but if memory serves me, it is towards the intake in line with the needle. You can attempt a spot of super glue to hold it in place, but there is no guarantee. The only sure solution is to replace it.

Check those things out, the slides, fuel level frt to rear, and dynamic spark while in the fail mode, and let me know where we are at.



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