Virago XV1000 Carburetor Tuning.

I Had a totally clogged tube in one of the BS 34 -s Nothing coming through.

The tube going into the red circle on # 56
Ultrasonic cleaner ( Long times --but i have only one Cheap small one ) and the Carb cleaner fluid opened it.
Using the straw and can pressure.
Warm water and washing up liquid in the water.

I let the cleaner fluid stay in there plugging the exit with a matchstick . Doing it many times

I May have to buy a compressor .Just dont know how large one needs.
 
I got brave and went for it. Made a simple collet but it kept slipping. I then took a scrap of brass and drilled a hole through at 4.5mm, cleaned the outside of the jet, applied flux and soldered the brass to it.

Carb2.JPG

I gripped the brass with vice grips but it would not budge. I then applied heat to the carb body at the base of the jet, gave it a gentle twist with the vice grips and out popped the jet - marvellous stuff that heat!

Carb3.JPG

I then heated the jet + brass and slid the jet out. A wipe with a cloth removed the excess solder and left the jet sparkling. Now to proceed with Jan_P's suggestion above but I'm out off carb cleaner. Perhaps paint stripper??
 
Paint stripper tends to be caustic not great with aluminium. My go to for cleaning carbs is lemon juice it works really well. it softens the dried fuel varnish really well. It also helps that my lemon tree is loaded with fruit most of the time.

Lemon
High content of citric acid
 
After many hours I have made progress. Both carburetors have the 0.5mm washers removed now. Although placing a 0.5mm washer under the needles improved performance and got me out of the lean zone it appears they were just bypassing the real issue of partially blocked Pilot Jets.

Today the temperature jumped to 12 Celsius so I decide to check the carbon monoxide levels using a Gunson 4125 analyser. I successfully set both carbs to approximately 4% CO. The final carb adjustments will be made based on plug readings.


Just a few comments on the Gunson testing gear:

Gunson Colortune Plugs: These are very easy to use and allow you to quickly set the carbs to a ball park area where the mixture is not excessively rich. Rich is very obvious as it shows as an orange flame whereas lean and "just right" is not so obvious with both being blue in color. Certainly Colortune will get your engine running soot free then followed up with final adjustments based on plug color.

Gunson G4125 Carbon Monoxide Analyzer: This device uses the exhaust pressure wave to drive a diaphragm pump which feeds the exhaust gas to the analyzer. Some exhaust pipes may not provide a sufficiently strong exhaust pulse. With my bike each silencer has a large (about 1" diameter) central exhaust outlet surrounded by four smaller (approx 8mm) outlets. Using the large outlet gave very inconsistent results - all over the place. The analyzer would only work well when plugged into one of the small outlets. Also, the PVC tubing on the analyzer is too stiff so I replaced it with silicone tubing which gives a much better fit and seal. Once I had sorted out these issues the analyser is simple to use and requires no removal of spark plugs.

My results on Hitachi HSC40 Carburetors:

Colortune: Pilot screw 1 1/2 turns open and greater gave an orange flame.
Colortune: Pilot screw 1 1/4 turns open gave a blue flame.

G4125 Analyzer: 1 1/4 turns open gave 10 - 11% Carbon Monoxide.
G4125 Analyser: 1/2 turn open gave 4% Carbon Monoxide.

My conclusion: The G4125 analyzer gave me more confidence compared to the Colortune. I say this because a pilot screw setting of 1/2 turn seems too low at first glance compared with my experience of BS34s.
 
Proof of the pudding is in the Eating: I have rechecked the Carbon Monoxide levels and both carbs are running at 2.8 - 3.2%. After running up the motorway then killing the ignition and rolling to a stop both sparkplugs looked good, not to lean and not to rich. After 71 miles today of mixed riding the plugs look just the same as when killing the ignition on the motorway:

Plugs.JPG


Peering down the porcelain both have a dark ring on the last 1/4 - 1/3 near the base. I am happy with the results and glad I took my time over Jan - March to find what the real issue was. Total cost to get here was £29 for a new second hand Hitachi carburetor body and £125 for the Gunson G4125 carbon monoxide detector which will be useful for the annual carb checkups. The rest was a lot of time and patience.

Thank you to all who have compiled their experience on this forum which has helped me greatly.

Edit on 23 Feb 2023: 300 miles later and the plugs look the same. The carbon monoxide levels remain stable at

Front Cylinder =3.2% (Pilot Screw 1/2 turn)
Rear Cylinder = 3.0% (Pilot Screw 3/8 turn)

Since addressing my carburetors I have 55 - 57 mpg, prior to this work it was 50 - 52 mpg. If I were to switch to 0% ethanol fuel then I should see an increase to about 60 mpg. Every 5% Ethanol decreases your efficiency by approximately 3 mpg.


Edit on 18 June 2023: After running a few months at 3% without issue and good plug colour I decided to try 4% Carbon Monoxide just out of curiosity. Adjusted the Pilot Screws using Gunson G4125:

Front Cylinder = 4% (Pilot Screw 9/16 turn)
Rear Cylinder = 4% (Pilot Screw 7/16 turn)

The bike seemed fine to ride but examination of the plugs after a 65 mile outing showed dark grey fouling on both cylinders. I also noticed a rich smell from the exhaust. So back we go to 3.0% Carbon Monoxide just like Gunson recommend...:hump:

Video regarding function of the air induction and reasoning behind the differences in main jet size:
 
Last edited:
Just thinking out loud here with virtually no knowledge on carburetors.
Any risk for rear cylinder running hot and lean on warm summer days ?
If I get it right a richer mixture helps with the cooling
 
Jan-P,
The Virago deals with this by having different size jets on the two carbs. The rear cylinder has a 124 main compared to a 132 in the front cylinder. The needles are also different sizes but the idle circuits are the same.

Interesting how the mains would make it seem leaner. I guess the reasoning is the rear gets less fuel so ultimately there is less energy released at the rear under heavy loads.
 
Last edited:
Back
Top