:: Yamaha Virago XV Forums That Rock!!! Tech Articles How To..... Tune Hitachi Carbs

#1:  How To..... Tune Hitachi Carbs Author: Mazzhole Post Posted: Mon Oct 22, 2007 10:50 pm

Ok here is my first attempt to write a how to on tuning Hitachi carbs!!!!! I am sure i will leave something out so please feel free to chime in gurus!!

Things you will need include
Allen to take off air filter and the bullet for the exhaust thingy,
12mm socket for bolt holding tank down
30mm to hold up tank
Snub nose screw driver
long flat head screw driver
carb adjustment tool makes things easy they are about 8$ at the auto parts store
pliers of some sort
and the sync tool now i know there is some controversy on this site about this but mine worked great

12 ft of 1/8" ID clear PVC tubing

1 yardstick

2 wire ties

2 plastic T's that fit vacuum lines

2 spare parts of vacuum lines

1 ink pen taken apart with the ink tube clean of ink.

The tubing was taped to the yardstick with transparent packing tape. The wire ties were put thru the hole in the yardstick to secure the tubing on each side at the top. If desired a machinists or carpenters square and a pencil can be used to extend the 1/8" markings to both sides of the stick or the level of the oil on one side can simply be noted before testing begins since it must return to this point to be in balance. Fill with oil to about the 26 or 27 inch mark (36 at the bottom). You may have to leave the unit sit for a day or so to get all the bubbles out. If you are careful not to introduce air when filling this is less of a problem. Now take the ink tube and cut it in half. At the top of the yard stick cut the PVC tubing and stick each end of the ink tube in the ends of the PVC tubing secure it with some RTV i used some high tack RTV. Let this try for 3 hours.

When in use with the engine running both lines must be connected, one to each carb. The oil will be sucked up and out of the manometer if only a single carb is connected and the other end is left open to the atmosphere.

Ok so now the actual procedure

Hook up the carb tool to the carb holders

Set you throttle stop screw on the front cylinder to about 1000 rpm

* Synching Carbs At Idle: You will first synch the carbs at idle. This is done by means of the two screw adjusters ("throttle stop screws") on the sides of you carbs. The left one is a set screw, and the right one is a thumb wheel screw. Loosen the retaining nut on the set screw so it can be turned. Working these two screws, set you carbs to where the oil is even in the towers with the idle at 1000 rpm on your tach. Leave the retaining nut loose for now.

* Adjusting Your Pilot Screws: These screws control the mixture in your pilot circuit, which controls off-throttle running (idle and deceleration) and running when the throttle is cracked open just a bit. The screws control fuel , not air! You turn right (in) for less fuel (leaner) and left (out) for more fuel (richer).

pics for you enjoyment

Front Pilot Screw

Rear Pilot Screw

WARNING! Never bottom these screws hard. You can injure the seats or the screws, making good adjustments impossible. If adjustments don't seem to get results, this may be your legacy from an earlier tinkerer. Very likely your pilot circuits are plugged.

These screws came set very lean from the factory (EPA stuff) and what we want to do is set them somewhat richer to help reduce warm-up time, improve low-end running, and inhibit a tendency to backfire.

With the engine idling, turn one screw in until the engine begins to slow due to fuel starvation in that cylinder. Then back out say 1 and 1/2 turns or until the engine runs its fastest and smoothest. Then do the other side. This should get you close. If you chose, you can try doing one cylinder at a time, that is, if the engine will run on the other cylinder. Pull one plug out and be sure to ground the plug wire you pull off. Ungrounded plug wires on running engines are hard on coils. You are searching for a smooth, strong idle with each cylinder firing nicely. Once this has been achieved, you'll then want to recheck your rpms at idle and your synch and readjust if necessary. When you have decided that your idle is solid, tighten the left side retaining nut on the set screw. If you experience backfiring, "spitting" through the carbs, or other signs of leaness, back the pilot screws out a tad more.

* Synching Carbs At 3-4000 rpm: Once you've got your idle squared away to your satisfaction, you now need to synch your carbs at higher rpms, where you will do your riding. This synch is accomplished by turning the synch screw located between the carbs in the back. You'll most likely need to remove the rear carb breather pipe to get to the screw, and move the tank up out of the way a bit using the 30mm socker works great if you have one. Once you get to it, it is usually quite simple to rev the bike to 3-4000rpm, hold it there, and check your synch. Then let go of the throttle, make a correction, and try again. A couple of adjustments should get it. Note that on some carbs the linkage between them has taken some serious wear and has slop in it which maybe difficult to take all the way out. Due to play in this linkage, the throttle valve of your right carb (where the throttle cable connects) will always open just a tad before the left carb throttle. This maybe noticeable at tiny throttle openings where your right cylinder will hit harder than the left, but if you are synched properly at idle (using the set screws) and at higher rpms using the synch screw, this effect will go away as the throttle is opened further, and is not usually bothersome.

This is the Screw you are looking for, it is at the end of the screw driver and difficult to adjust this is were the carb adjustment tool comes in well

I cannot take much of the credit for this i just added the following websites together and took some pics please let me know if i forgot something and i will add it as i see it!!!

#2:   Author: 84xv1000 Post Posted: Tue Oct 23, 2007 1:15 pm

You need to shrink the pics down some so that peeps can read this post without having to scroll side to side. It is definately not user friendly at this point.

#3:  Re: How To..... Tune Hitachi Carbs Author: Mazzhole Post Posted: Tue Oct 23, 2007 10:46 pm

will do guy thanx keep the suggestions coming, i want this to be as helpful as possible!!!!

i just tried to resize them and it did not work, i will try again tomorrow!!!!

#4:   Author: pachakutek Post Posted: Wed Oct 24, 2007 3:06 am

Nice... I was always wondering where to plug the vaccum meter thing.... great job! Thanks

#5:   Author: pachakutek Post Posted: Wed Oct 24, 2007 3:07 am

If possible make it an article too for the KB

#6:  Re: How To..... Tune Hitachi Carbs Author: grazingazer Post Posted: Wed Oct 24, 2007 9:03 am

terrific job mazzhole...keep up the good work

#7:   Author: mark75 Post Posted: Wed Oct 24, 2007 9:39 am

I've always been a "visual" learner. Good job.
The smaller pics are easier to use also. thumbs up

#8:   Author: yamaguzzi Post Posted: Wed Oct 24, 2007 11:34 am

mazz, ill bet a short youtube documentary would garner a lot of visits,,,thanks again for the work youve done yg

#9:  Re: How To..... Tune Hitachi Carbs Author: Mazzhole Post Posted: Fri Apr 25, 2008 3:49 pm

Bump for those who might need it

#10:  Re: How To..... Tune Hitachi Carbs Author: kjwhip73 Post Posted: Sun Apr 27, 2008 9:48 pm

Great job mazzhole. I didn't realize the pilot screws were phillips head. Just came from the garage to check them with a shorty flathead and couldn't seem to seat them. Couldn't find a short philups to try but will tomorrow.

#11:   Author: edviragoxv1000 Post Posted: Wed Aug 27, 2008 1:58 am

Bump , Just because this step by step is great. Thanks again Mazzhole

#12:  Re: How To..... Tune Hitachi Carbs Author: michel Post Posted: Mon Jan 12, 2009 8:19 am

Very helpful. Were your pilot screws covered with copper?


#13:   Author: funkamongus Post Posted: Sun Jan 18, 2009 10:40 am

yes very helpful, My pilot screws are covered with the little plug. You have to get a drill and just barely put a hole in it, being VERY careful not to chinger up the top of your idle screw itself, then pry it out with a "suitable drift" to expose the screw. I read its close if you start with a soft seat, (dont seat hard, will mess it up) then 2 and a half turns out. I wonder though if it matters what elevation your living in. I'll know soon, Im at 6500 ft.

#14:  Re: How To..... Tune Hitachi Carbs Author: michel Post Posted: Sun Jan 18, 2009 3:47 pm

Once I remove the caps I intend to use my Colortune plugs. the virago has the same size plugs as my beetle and I use the Colortune plugs to adjust the cars twin carbs.

Altitude does have an effect, you will be running richer as the air intake will be less dense and contains less oxygen. The Colortune plugs will take taht into consideration.


#15:   Author: grazingazer Post Posted: Sun Jan 18, 2009 4:02 pm

you should use a leaner adjustment at higher levels to compensate for the difference in air density funk but setting intially to 2 1/2 turns...turning in till the warm engine stumbles...then backing the screw out until the engine runs it's smoothest...adding 1/8 turn counterclockwise at that point...will give you what you need...if the engine stumbles when you crack the throttle lean it out...if it backfires richen it're looking for smooth throttle responce throughout the rpm range

#16:   Author: funkamongus Post Posted: Sun Jan 18, 2009 6:10 pm

thnx GG. Im on it. See my video yet? check out She runs at my you tube site,, I rode her today for the first time!!!!

#17:   Author: RusVirago Post Posted: Mon Jan 19, 2009 8:55 pm

funkamongus wrote (View Post):
thnx GG. Im on it. See my video yet? check out She runs at my you tube site,, I rode her today for the first time!!!!

Very nice! sounds awesome.....

#18:   Author: sirpug Post Posted: Wed Apr 01, 2009 11:39 am

Quick question; why did you use the ink pen tube at the top of the yard stick? Why not just leave the tube whole and plug it into the carbs? What am I missinig?

#19:   Author: funkamongus Post Posted: Wed Apr 01, 2009 11:49 am

its a restrictor.. I used a connector from an auto shop.. the restriction makes it easier to read, less bouncy. Also stops it from running so quickly into your engine if its way out!! make sure you have the vac connections on the same sized nips on the carb holders..

#20:   Author: funkamongus Post Posted: Wed Apr 01, 2009 11:51 am

Sirpug,, what mods did you do to the plug wires? and resistor? What did you get out of it???

#21:   Author: sirpug Post Posted: Wed Apr 01, 2009 12:51 pm

I built some new wires from stock you can buy in bulk (it's yellow) that is solid core. The only reason I removed the resistor from the boot (replaced it with a piece of copper rod the same length) was because I saw on here somewhere that the AP63s have the resistor built in and you'd be doubling the resistance, which will ive you a weaker spark, if you ran both. Search for plug mods and you'll find it.

#22:   Author: funkamongus Post Posted: Wed Apr 01, 2009 1:01 pm

gotcha.. may splain what Im seeing on the plug too. good one there pugsy!! thanx! (glad I noticed that your siggy and that you put it there for me to find!!) Im on it.

#23:   Author: funkamongus Post Posted: Wed Apr 01, 2009 1:02 pm

so ngks are better plug n play on the old wires until I upgrade? IOW (in other words) no resistor in that plug?

#24:   Author: sirpug Post Posted: Wed Apr 01, 2009 1:29 pm

Take a look at the tech article (1) in the red section of the nav links to the left (top). Matthew has a great article about this stuff in there about plug caps and coils. Check it out, should clear up some questions.

#25:   Author: sirpug Post Posted: Thu May 07, 2009 11:21 pm

So here we go. Need some serious help!

What is a good starting point if it's way out? I got my valves adjusted good and she fired right up, better then ever so I thought "HELL YA!!" but then I hooked up my tool (built here) and noticed that the rear cylinder was pulling a little more. The longer I watched the farther up the tube it went so I went and backed the set screw all the way out. Still pulled more than the front and I couldn't adjust my front down any because I lost my idle.

Now comes the dumb part. I read somewhere here that a good way to start is to back the middle linkage (high rpm adjustment) screw all the way off and and just focus on getting the idle set. So I did that. Now it won't even fire up. Screwed it all the way back in a it fired up but was way off. Where is a good starting point for this screw since it obviously affects the idle adjustment somewhat?

#26:  Re: How To..... Tune Hitachi Carbs Author: ridgerunner Post Posted: Fri May 08, 2009 12:17 am

I too have been subjected to the Hitachi carb syndrome. If they are working follow the grazingazer mode of not fixing them. Once they die all bets are off in my book.

Step 1
Remove hitachi carbs from your bike. These carburetors are obsolete and virtually no new parts are available. Mostly only other 25 year old diaphragms and jets. Discard. If you can sell them great.

Step 2
Purchase or otherwise obtain a set of new Mikuni carbs. or get a 1>2 manifold and use a single Mikuni. Pick the flavor that you want and buy the proper jets. The VM is super easy to adjust The TM is the newer carb that probably will work excellent. I think some riders are using a harley version of the TM.

The VM Mikuni has a large volume of data for single and dual carbs on various Triumph, BSA Norton and BMW twin cylinder motorcycles. The jetting is similar. A simple google search can get you in the ball park quickly. I have posted what works for my setup. No secrets. For the youngsters we could call it open architecture. I have posted a spread sheet that while cryptic is decipherable. I started with a new carb that was jetted for a 250 vintage MX bike.

Step 3
Go for a ride enjoy the excellent throttle response and performance.

If you need a different jet they are available for 3-12 dollars I have a pill bottle full of them from past racing 30 years ago.

Keep in mind that with a redline of 7000 rpm there is no need to supersize the carb throat. Keep it moderate. We don't have drag racers. Have fun.

#27:   Author: 1986 XV700 Post Posted: Fri May 08, 2009 12:24 am

I haven't really found that the position of the linkage screw was all that important, provided it was close before you started. If your not sure you might back it out a couple of turns. If the bike was running OK before you started and you back it out too much you'll need to slowly increase the throttle and adjust as needed when you start the high RPM adjustment. You will discover that it will quickly pull the oil to one carb when when you accelerate the engine it the high RPM adjustment is severely out.

Hopefull I haven't misunderstood your question but here's my two bits:

Especially at idle the sync tool is measuring the vacuum created between the engine and the butterfly in the carb. The idle adjustment screws on the sides of the carb rotate the butterflies. The more you loosen the idle adjustment screw the more the butterfly closes. The more the butterfly closes the greater the vacuum at the port where you connect the tubing for the sync tool. The more you loosen the idle adjustment screw the more oil gets pulled towards that carb. If one of the carbs is pulling more oil through the clear tube than the other, either tighten the screw on that carb or loosen the screw on the other carb.

If you are trying to adjust the idle more than about 100 RPMs, you will probably have to alternate adjusting the idle screws from one carb to the other several times until you get it where you want it just to keep the oil in the tube.

#28:   Author: shmalphy Post Posted: Tue Apr 27, 2010 10:39 am

I should be able to do this with the petcocks on prime, and just hook the tool to the carb boots, and plug the large nipple on the front carb boot right? seems cheaper/easier than getting the t's and extra vacuum lines, and should get the same result.

#29:   Author: funkamongus Post Posted: Tue Apr 27, 2010 11:27 am

Both Nipples or ports, the rear and the front one, should be the SAME size. One in the front is larger than the other,, the smaller one is the one you want. And Ridgerunner, Im a newbie, only been wrenching on a few bikes so far and I dont understand why you hate these carbs.. I have not had that experience. I followed the instructions, tuned it, and havent had a single problem since. Thats MY story.

#30:   Author: shmalphy Post Posted: Tue Apr 27, 2010 2:52 pm

I know I am hooking it up to the nipples that are the same size, but my point is that you do not need the t's and spare vacuum lines to also connect to the petcocks if you put them on prime, just plug the tool straight to the boots on each side, right?

I am going to build one of these today, and it seems unnecessary to connect it to the petcock as well as the boot.

#31:   Author: funkamongus Post Posted: Tue Apr 27, 2010 2:59 pm

correct, to the boot on both sides. Umm, you havent removed your MCV? I would.. especially if you have access to the mixture screws... Then, I would adjust the valves, THEN I would mix and sync...

#32:   Author: shmalphy Post Posted: Tue Apr 27, 2010 3:08 pm

I did remove the MCV, the large port is plugged, and i go boot to petcock on both sides.

I plan to adjust valves as well, I need to get a feeler gauge. I will do that before I go any further

#33:   Author: funkamongus Post Posted: Tue Apr 27, 2010 3:55 pm

Right on!!! Gonna be good! Youre gonna love her!! When you get em right, you can tell the difference... Tech Articles How To..... Tune Hitachi Carbs

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