synchronizing hitachi 1st generation [1981-1983] virago hsc40 carburetors 

Description article submitted by sirpug
Author grazingazer Date Tue Jun 02, 2009 7:48 am Type Tech Article
Category Tech Tips
Comments [14 - Post your comments]
Views 878  [Rate Article]

This how-to is a conglomeration of a bunch of different posts from various threads that I found while I was doing frustration based research here. This was after I tried Mazz’s how-to tune Hitachi carbs but, because my bike was so far off, I couldn’t get it to work right. All the oil got sucked into the rear cylinder as soon as I started it and I ended up getting everything way off trying to sync the carbs at idle to stop that from happening.

This procedure was all based off of posts I’ve seen by Grazingazer, Club_C and Eaglebeak.

Disclaimer: This was done on my ’82 750 with Hitachi carbs. Other bike/carbs may be different.

Tools :
Flat head screw driver
Phillips screw driver
A small screw driver to adjust your mixture screws (mine a flat head, yours could be different)
Two vacuum gauges (I bought two vacuum test kits from Harbor Freight, came with everything, for $12.99 ea)
Tools as required to loosen your left hand idle set screw (if you have it, mine’s a flat screw driver and an 8mm wrench)
Zip ties
Also, I have read that an adjustable aquarium air valve will reduce the fluctuations but I didn’t worry about it. We don’t sync at idle and at 3000 rpms they were both pretty smooth and easy to read.

Pre adjustment steps:
It would be a good idea to set your valve clearances before you perform this procedure. I recommend doing this since a valve being out of adjustment could cause issues. You can find a good procedure by Matthew here:

If you’re able to run your bike, warm it up to operating temps. The choke needs to be full off BEFORE you can do this procedure.

Adjust your idle mixture screws to 2.5 turns out from SOFT SEAT and set your idle speed i to roughly 1000 rpms.

Run at least one fan before starting this procedure to combat over heating. The more the better.

OK, here we go…..

1 – Back the idle set screw (not on all bikes; ignore this step if you don’t have it) on the left hand carb ALL the way out so that the throttle arm is not touching. At this point your idle speed will be set only with the thumb wheel on the right side. Left carb will mirror through the linkage.

2 – Remove the right carb air intake boot (or otherwise gain excess to the linkage adjustment screw). If you remove the boot make sure to keep it handy. It will need to be reinstalled when you’re running to keep the conditions the same between both cylinders (mine wouldn’t run with it off anyway).

3 – Back the linkage adjustment screw all the way out (well at least until it’s loose; please don’t remove it all the way). Slowly tighten the screw until it JUST makes contact with the linkage going to the left carb. I then slowly rotated the screw in to remove any slop between the individual carb throttle linkages (when I started you could move the RH throttle about an 1/8” before the LH throttle started to move; now they move at the exact same time). Re-install intake boot if removed.

4 – Hook up your vacuum gauges and secure the hoses with zip ties to ensure no leaks. I have manual petcocks so I just installed them to the old petcock vacuum points on my carb mounts. How they will hook up will depend on you personal circumstances. Just make sure that they are both hooked to the same size ports on the carb mounts.

5 – Start your engine. Mine took a little more petting then usual to get started with the gauges hooked up even though she was warm. I think this is because of the larger vacuum area in the lines and the gauges. Give everything a second to stabilize after start.

6 – Proceed to 3000 rpms. Let stabilize for a few seconds and then record your vacuum readings. From this point I just played with the linkage screw until I was dialed in. It’s kind of monotonous at this point but well worth the effort. Take a reading, shut down, remove intake boot, adjust screw, re-install intake boot, start, take reading, wash, rinse, repeat; always repeat. I was able to tell if I went the wrong way or too far etc.

7 – After you get the sync dialed in allow her to idle for a second and set the idle speed to 1000 rpms.

8 – Adjust your idle mixture screws by turning them in until the cylinder starts to sputter and then back them out until it runs smoothly. This was hard for me to hear and I’m not sure I have mine set perfectly so I ordered a Colortune kit.

9 – Double check your idle speed is still at 1000 rpms and set the set screw to the point that there is about a hair of daylight between the throttle lever on the left carb and the top of the screw and locked it down.

10 – Reinstall the intake boot and remove the vacuum gauges.

11 – MOST IMPORTANT!!!! Go on a test ride. Make sure you hit the highway for a good ride. Monitor the throttle response for proper idle mixture settings and the feel of the bike.

After I was done with everything I noticed that the fluctuations on the vacuum gauges at idle were about the same. When I started, it was noticeable that one carb was pulling more vacuum at idle then the other one since the ranges were so different.


User comments 
grazingazer Posted: Mon Jun 01, 2009 7:20 am    Post subject

sirpug is the one to be congratulated...he composed the work...i was only the editor of the piece
2Hoops Posted: Mon Jun 01, 2009 7:28 am    Post subject Re: [ KB ] synchronizing hitachi 1st generation ['81-'

Just a little thing, It referes to a article for adjusting valves by matthew, was there supposed to be a Hyperlink on the word "here" ?

Thanks for all the effort Sirpug. I'll be using this for sure.
grazingazer Posted: Mon Jun 01, 2009 7:42 am    Post subject [ KB ]

[ Article updated...grazingazer ]

Category: Tech Tips
Type: Tech Article

Article Name: synchronizing hitachi 1st generation [\'81-\'83] virago hsc40 carburetors
Author: grazingazer
Description: article submitted by sirpug

>>Read Full Article
grazingazer Posted: Mon Jun 01, 2009 7:43 am    Post subject

Quote: › just a little thing, It refers to an article for adjusting valves by matthew, was there supposed to be a Hyperlink on the word "here" ?

the url has been inserted...thanx for the info
scootermcq Posted: Mon Jun 01, 2009 11:11 am    Post subject

grazingazer wrote (View Post): ›
sirpug is the one to be congratulated...he composed the work...i was only the editor of the piece

Nice Job Sirpug.... sorry I didn't give credit where credit was due.
sirpug Posted: Mon Jun 01, 2009 11:48 am    Post subject

Thanks, just thought it might help.
rhysvando Posted: Wed Dec 30, 2009 6:04 pm    Post subject

is there any particular vacuum pressure you should get on the guage? or just set them up so they are equal?
funkamongus Posted: Wed Dec 30, 2009 6:17 pm    Post subject

equal to each other. on idle and off idle. at idle its the Idle screws on either carb, off idle is the center screw between the carbs. (the one thats real hard to get to)
anaman51 Posted: Wed Dec 30, 2009 6:44 pm    Post subject

This does a far better job of explanation than the Clymer manual, and the pix are fantastic. Other processes I've read make this far harder than it has to be. Good job, Sirpug!
rhysvando Posted: Wed Dec 30, 2009 8:45 pm    Post subject

thanks mate
rhysvando Posted: Wed Jan 06, 2010 5:14 am    Post subject Re: [ KB ] synchronizing hitachi 1st generation ['81-'

hey guys
im running standard setup (with vacuum to petcock to carby manifold) do i need to "t" piec e this to remain as per usual? or just bypass the vacuum hose to petcock and just fit guage straight upto manifold?
grazingazer Posted: Wed Jan 06, 2010 9:08 am    Post subject Re: [ KB ] synchronizing hitachi 1st generation ['81-'

run on prime and hook to manifold for best results [don't leave it on prime when the engine's not running]
packman Posted: Wed Jan 06, 2010 2:49 pm    Post subject

Good one Graz. My wife turned the knob to prime thinking it must stand for "PARKED" or something like that. Glad I was in the garage later when gas started dripping out to stop it.
rhysvando Posted: Wed Jan 06, 2010 6:24 pm    Post subject

yer learnt from that after rebuild my brother left it on prime while he was fiddling and filled the case with petrol! thanks for the reply!

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