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Tuning Hitachis on a 1981 XV750 Brat

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Tuning Hitachis on a 1981 XV750 Brat

PostAuthor: rcolmer » Thu May 18, 2017 10:04 pm

I want to start this post by saying a huge THANK YOU to all the posters and contributors on Viragotechforum, you have all taught me so many things whilst building my bike; saving me countless hours and probably thousands in mechanic bills.

Here it goes, because I've (almost) exhausted all my options and DEFINITELY most of my patience. I've read enough posts here and phoned enough mechanics to know the hitachis are a nightmare to tune with a modded engine but this is what I have and these are the issues I'm facing.

It started as a stock 1981 XV750 with 39k on it. Among numerous mods not to do with this question (tank, front end swap etc) I've added one pod filter which replaces the stock filter and airbox, and fabricated short straight pipes. It ran, understandably, very lean. No Low, mid or top end power whatsoever and you could hear the engine breath and choke as it tried to rev in neutral. So this is what I've done to remedy the problem thus far, please stay with me:

-Drilled the 41 pilot with a #76

^This actually did fix the idle to 1/4 throttle. It ran perfectly in the aforementioned throttle position, but still no mid or top end power (as I didn't touch anything else).

-Changed main jets to 128
-Used two nylon washers from home depot to shim the non adjustable needles up a few millimeters (they fit surprisingly perfectly, still allowing the needle its small amount of spring travel)

^This really worked. 1/4 - wide open was PERFECT. Very smooth and fast acceleration, no sputters, no pops, no bluish rich smoke or smell. HOWEVER, with this set up the idle - 1/4 throttle is now TOO RICH, and sputters through its position. AND fuel screw does nothing, even all the way turned in bike idles happily (until you apply a little throttle). But as soon as the throttle is cracked past 1/4, BOOM, opens right up no problem.

SO I removed one shim from each needle, keeping everything else the same. Hopeful that this might lean out the available gas when bike is idling or throttle is hardly cracked. Didn't work, Low end still sucked, and now mid range suffered from being too lean as well.

AGAIN opened it up, added a half shim, changed pilot back to undrilled original #41, kept main at 128... and then snapped a bolt head off the flange bolts. So I don't know if this set up worked or not.

What possible combinations still exist? If this set up doesnt work after I extract the bolt and fire it up, what are the next things you'd try?

I am told there is a chance it is simply impossible to tune the Hitachi well, and that Mikunis would just be the wiser less frustrating option. I agree, but I would like to exhaust every option at my disposal with these Hitachis because I have the tools and don't want to give up until I'm positive it wont work.

Additional info:
-I have main jets 122, 124, 126, 128 & 130, and two sets of pilot jets (original, and drilled with a #76)
-Carburetor is spotless clean
-brand new plugs came off BLACK after first mentioned set up (great mid-top end, terrible bottom end). This indicates rich, but we know idle is rich and it idled much much longer than the 3 minute ride I took it on to discover the great top and mid end power.
-I tried to restrict the pod filter but even a small restriction of the airflow seems to have a negative affect on whatever performance I'm getting so I've simply left it free of obstruction
-No leaks in the system, I check with quickstart everytime I reinstall the carburetor and again before removing it after any test ride.
-Plugs are brand new
- and YES I know dynojet makes a stage 1 kit with adjustable needles included, but this kit is $120 CAD and I only need the adjustable needles out of it. Plus, the caps on the needles are made of a near identical nylon, and IT WORKS.

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Re: Tuning Hitachis on a 1981 XV750 Brat

PostAuthor: DGA » Thu May 18, 2017 11:59 pm

The air filter makes a huge difference, but the stock one does not restrict the flow at idle and very low speeds, so that stands to reason that the bottom end jetting does not need to be changed much, certainly don't need to drill the pilots out. But do open the idle mixture screws, they were to lean from the get go, go to 3 or 3.5 turns open from a soft seat. Shimming the needles up is good, as you found, I went as high as .125" but it would be blubbery off throttle, good under power, so I went down to .080" of shim. Also, get the smaller needle and make both carbs the same. Main Jets you can go as high as 140. Try 135 maybe. Very important to have the carbs synched, adjust both idle stops down until they have gaps then raise them both to make it idle. The synch screw between is for open throttle settings, also is useful to adjust for linkage wear. Adjust the screw until it JUST lifts the LH idle arm, then back it off slightly.
'81 XV750, clip ons, 18" rear wheel, lowered 920 forks with twin discs and 1.5" spring spacers, 1100 brake master cylinder with CNC billet cap, -3 braided brake lines, lowered instruments and headlight, CNC billet mirror mounts, small billet turn signals with running lights, Koso tach/ speedo, stainless 2 into 1 stepped headers with reverse cone muffler, starter heat shield, 4 guage starter cables, rear exhaust heat shield, 4" round open air filter with billet CNC mount, TM38 flat slide Mikuni carbs, XV700 ported heads, BT-C05 cams, colder BP8ES NGK plugs, stiffened swingarm, polished engine side covers, polished rear peg mounts with carbon cover panels, single horn relocated, Sparto tail light with 3D printed lens, BSA tank kneepads, one off carbon fiber cafe seat with brown cover, fork gaiters, KDX200 rear shock, single tach/speedo with carbon fiber dash, red LED under tank and seat lights, black pearl paint with silver accents.
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Re: Tuning Hitachis on a 1981 XV750 Brat

PostAuthor: chrismalm » Fri May 19, 2017 12:54 pm


If you're going to drill the pilot jets, the recommended size is #50, which I believe is .50mm . I think #76 would be too rich. With .50mm pilot jets, the idle mixture screw range is only 0-1 turn before it's too rich. #76 probably puts the idle-mixture screw out of adjustment range.

I had a 750 air filter on a 920 and it was too restrictive at WOT causing bogging. I removed the snorkel, problem solved. Try a test without the pod to confirm.
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