Virago XV1000 Carburetor Tuning.

Paul Sutton

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The stock exhaust system has rusted out on the inside so I have bought a Black Widow replacement. All is well except the rear cylinder is on the lean side, not excessively, but the insulator on the BP7ES plug remains perfectly white with no sign of blistering. Although I was able to dial in the front cylinder using the pilot screw this approach is insufficient for the rear cylinder.

My Virago (1988 SE model) has two Hitachi HSC40 carburetors and they have been thoroughly cleaned in the ultra-sound and all jets blown clean. The bike is currently working and pulls well without any popping on deceleration.

The Hitachi HSC40s I have were their latest models prior to the change over to Mukini. Due to EPA requirements these carburetors have limitations as to which jets can be replaced/fiddled with. The idle circuit has an air jet which is pressed in and sealed away plus a fixed 0.41mm fuel jet. The 0.41mm fuel jet is pressed in and not easily removed. There is also a main air jet pressed in and not easily accessed and the main fuel jet which is easily accessed like on the BS34s. The needle can also be shimmed if needed.

Regarding the 0.41mm idle circuit fuel jet, I have found that Joe Minton had recommended drilling this out to 0.5mm. But on the Virago forums it is not recommended to do this because it makes the mixture too rich and this cannot be corrected with the pilot screw.

What might be a simple solution?
  1. I could just change my rear plug to a BP8ES or BP9ES to avoid over heating the electrode.
  2. Increase the size of the main jet.
  3. Shim the needle a little.
  4. Do a Minton mod and only drill out the 0.41mm jet to 0.45mm
What are your views on this?

Thank you Guys.

Note: All my riding is down at small throttle openings so increasing the main jet or shimming the needle may not have much effect??
 

grizld1

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Yes indeedy; if the Hitachis behave anything like Mikunis of the same generation, lifting the needle will have a strong effect on PJ tuning. But I'd also want to do the WOT roll on to ensure that mains aren't too lean. Go up a step if the motor misfires, then repeat the exercise.
 

Paul Sutton

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Thank you for the thoughts.

Grizld1, at 3000 rpm in 5th the bike pulls away strongly if the throttle is suddenly opened up fully.

Bosco659, the rear exhaust plug looks brand new and there is no darkening at the base of the insulator. Will take a photo later today.
 

Paul Sutton

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A photo of the rear spark plug (BP7ES):
plug.JPG
Not a lot visible. The insulator is white all the way down inside just like a new plug i.e. no 1/10" darkened ring at the bottom. The insulator is also matt white in appearance like a new plug. If that is the heat line on the ground strap then that seems not too lean. Perhaps I should stick it in the lathe and cut the insulator free???

Edit: See Entry #11 below where I back track a bit, a lot...
 
Last edited:

5twins

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As mentioned, I would experiment with shimming the needle. The needle setting has a big effect on plug color because it controls midrange and that's where you do most of your riding. I thought many of these bikes came with the rear carb jetted slightly richer than the front one? I think it was to aid in cooling the rear cylinder and usually consisted of a one size larger main. Along those lines, you might try shimming just the needle in the rear carb.
 

Paul Sutton

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Thank you 5twins on the shin comment.

Dump ass or what?? I conclude the ground strap heat line is OK so decided to recheck my premise and chop the plug in the lathe:
Plug 1.JPG

So what has happened to my white "new appearance" insulator? I guess this is why they chop plugs.
What do we conclude now?
 

Paul Sutton

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Is it too lean? If I shim the needle, what would be a good value to try, 0.5mm or higher?

Thank you.
 

Paul Sutton

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As mentioned, I would experiment with shimming the needle. The needle setting has a big effect on plug color because it controls midrange and that's where you do most of your riding. I thought many of these bikes came with the rear carb jetted slightly richer than the front one? I think it was to aid in cooling the rear cylinder and usually consisted of a one size larger main. Along those lines, you might try shimming just the needle in the rear carb.

Thank you 5twins. To help the rear cylinder run cooler it has a 124 main compared to a 132 in the front cylinder. This seems a bit counter intuitive so I must check further in case my manual is wrong. It is easy to change a main jet on the Hitachi, but would increasing the rear main jet a bit benefit me or is shimming the better option? Shimming means carburetor removal which costs about an extra 1.5 hours work. But its relaxing work and I will do what's necessary if the plug analysis indicates the rear cylinder to be too lean.

Thanks.
 

grizld1

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I'm a firm believer in tuning by the behavior of the motor rather than the appearance of the plugs. Leaner MJ in the hotter cylinder is indeed counterintuitive, and that big a jump in stagger jetting has me scratching my head too. Are those the MJs that you actually found in place?
 

Paul Sutton

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Grizld1, the main jets the carbs came with were 132 (Front) and 124 (Rear). This matches with the Haynes Manual.for XV1000SE Shaft Drive. I just went through my manual and find the variation in jet sze, and as to which is bigger, front or rear, varies with the Virago engine size. With the 1100 running Mikuni carbs the difference is 1 size and the rear is the smaller. But on a XV920 the smaller is on the front. I will double check a manual from a Virago forum.

Thanks.
 

5twins

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I happen to have a PDF copy of the Haynes Virago manual and sure enough, they list the larger main in front for all models but the XV920 and 1000 chain drive models. That does seem odd but I always thought that was an odd bike, lol. My buddy had an early model 750 ('81 I think) and I had to clean his carbs several times. I hated working on those Hitachi carbs, lol, and getting them in/out was quite the puzzle.
 

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Thank you 5twins on the shin comment.
Dump ass or what?? I conclude the ground strap heat line is OK so decided to recheck my premise and chop the plug in the lathe:
So what has happened to my white "new appearance" insulator? I guess this is why they chop plugs.
What do we conclude now?

Looks ok to me. Thing is, you've opened a can of worms and doubt has crept in. Doesn't matter what is done.

Xs850s have Hitachi carbs. They have the same problem the jets are not removable. I haven't done it, and spent a bit of time yesterday looking for the information on a removal process and changing the jet size, but couldn't find where it is.
 

Paul Sutton

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650Skull, thank you and I get what you mean about doubt.The bike runs well without any hessiitation, cold start is excellent, the earth strap heat mark is good at approx. 50%, the insulator and electrode is not blistering or eroding and the insulator is not perfectly white as I originally thought - So it looks like a duck and quacks like a duck.

There is a jet removal video on YouTube where they use a small bolt extractor to remove the jets. But this would be a last resort for me. I will just run a BP8ES on that cylinder as some do and monitor the situation. But I think all will be fine for now.

Thank you everyone for your thoughts and I will try shimming the needle at some point just to see what happens out of interest..
 

5twins

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Yes, for shims I use M3 washers. They're about a half MM thick. On an adjustable needle, the clip slots are usually 1mm apart so adding the washer is like moving the clip a half step.

iEVv8Mf.jpg
 
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