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Index > Engine/Drivetrain > Thread: Carb (Hitachi) questions
Thread: Carb (Hitachi) questions [' This thread is 2 pages long: (1) 2 ']
Jeff20B


Moderator
Posts: 661
posted November 12, 2004 07:19 PM

Carb (Hitachi) questions

I've got what appears to be a '77 carb. Actually, I have two of them along with one '76 and one '74. Total of four carbs. The '74 is water damaged but may be rebuildable. Has anyone here ever messed with Hitachis?

The '77 Hitachi I'm working on at the moment came with 94 main fuel jets in the primaries and 145 jets in secondaries. Based on some advice I got from Nikki (12A) carb owners, I tried 160 in primary and 190 in secondary. Big mistake.

Should I try some slightly larger jets than stock? Say 100 primary and 150 or 155 secondary? Will the stock air bleeds/emulsion tubes etc be able to handle the increase in jet size? I've got to first solve a lean issue and then try to add more power like my other '77 Hitachi on my white REPU (it's even adequate on the streetported engine in it, which I find very cool).

Any advice would be welcome. Thank you.
____________
'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7

       
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Jeff20B


Moderator
Posts: 661
posted November 15, 2004 01:02 PM

I am in the process of rebuilding the slightly water damamged '74 Hitachi carb. It has 106 primaries and 140 secondary jets. I think I'll put the 106s in my '77 carb and try it again with 160s in the secondaries, and get a set of 110s for the primaries of the '74 carb and throw in a set of 155s.

I'm trying to work within my means and not spend too much money on this rebuild. It's sort of a side project while I work out the bugs in the '77 carb. If the '74 carb ends up working, I'll no longer be one carb short. :)

Ok, this is how the jet swapping will most likely proceed.
'77 goes from 94 to 106 primary and 145 to 160 secondary
'74 goes from 106 to 110 primary and 140 to 155 secondary

I'm trying to up the jet sizes by just a little to keep them on the slightly rich side since these carbs are going on street ported engines, and I don't have any air bleeds or emulsion tubes to play around with. The '77 was lean with 94s, so 100-105 is what I'd shoot for. Luckily the '74 carb had 106s in it.
____________
'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7

       
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klaus42


Rotorhead
Posts: 1877
posted November 15, 2004 04:45 PM
Edited By: klaus42 on 15 Nov 2004 16:52

Sorry it took me this long...

...to respond! I've been busy...
Yes, I've tinkered some with Hitachi's... (and, apparently, some more knowledgeable folks than I have done emulsion tube, etc. mods with some measure of success in the past)...
However, I would stick with Racing Beat's proven jetting recommendations --pulled from a now earlier version of their famous tech. manual/catalogue-- ...(page 16)...:

"All pollution-controlled Mazda rotaries through 1974 are jetted substantially rich by Mazda to provide excess fuel for burning in the thermal reactor. Leaning the main fuel jets to restore correct mixture can improve both performance and fuel mileage. The jet change has no effect on the amount of oil metered to the engine through the carburator. ...For some models, a change of all four jets is recommended, (two primaries and two secondaries), while for others, only the primaries need be changed. The following are our jet recommendations for the stock carburator on a stock engine. These changes generally result in a five percent increase in mileage, and a three to five percent increase in power. They ONLY apply to the early Mazda models, which were jetted significantly over-rich. If the car will operate above 3500 feet, use one step smaller primary jets. If the car will operate above 5000 feet, also use one step smaller secondary jets."

1974 rx-4 and REPU: (stock) 106M primary/140M secondary
(use): 105 / no change

Their timing recommendations (from P.33 of the same manual)
1974-1975 stock porting and intake 13B: 0 degree leading,
10 degrees ATC trailing.
1976-1978 stock porting and intake 13B: stock (both).

Note: Their recommendations for all street ported 13B's 1974-1978 running Holley, Dellorto, or Weber are identical to the stock porting '74-'75 recommendations above... while slightly different settings are recommended when running these non-stock carbs on stock-ported 13B's...

Also of note is that they never recommend running the stock carb. on any street-ported engines, as they require more flow... You might also want to ensure needed fuel delivery volume, as well as correct pressure, by replacing your fuel pump, or at least testing it, to ensure flow...
It also occurs to me that your #1 likely issue with the Hitachi's is the vacuum secondary diaphragm. These are seemingly unavailable... though at one time, i'd looked into the possibility of getting them remanufactured... (this is really only feasible if we put together enough of a group deal.)
You might end up needing to cut down the spring backing the diaphragm, and/or soldering shut the 'vacuum bleed off' jet that regulates vacuum to the diaphragm... both of these little 'tricks' help a bit to counteract the effects of 30-odd years of aging and inflexibility on behalf of the diaphragm...

Hope this helps! Gotta love Racing Beat... :) --K.



       
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Jeff20B


Moderator
Posts: 661
posted November 16, 2004 02:55 AM
Edited By: Jeff20B on 16 Nov 2004 02:58

Ah, finally some news I can use. The other forums I posted on weren't of as much help. Thank you.

I swapped some jets this evening before I read your post. I put the 106s and 155s in the '77 carb. How do you think it'll run? I can tell you that it has the same size primary air bleeds as the '74, so the RB rule of using 105s for better power ought to apply to the '77 carb. If the primary circuit of this carb finally has some get-up and go, I'll be happy as it;s always been a bit on the weak side; the only time it feels powerful is when the secondaries open. I'll probably go ahead and test the carb and then order a set of 105s for it and probably go back to the stock 145s in the secondaries, if the 155s aren't too good.

I'm a little confused by this part:
quote:
Note: Their recommendations for all street ported 13B's 1974-1978 running Holley, Dellorto, or Weber are identical to the stock porting '74-'75 recommendations above... while slightly different settings are recommended when running these non-stock carbs on stock-ported 13B's...
Are you saying that Racing Beat's defenition of a street ported 13B is one with the same size intake ports as a stock ported '74-'75 13B? If so, I ported mine a little larger. Should I use the 106 jets in this case? Or try to find a set 104s?
____________
'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7

       
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Jeff20B


Moderator
Posts: 661
posted November 16, 2004 12:41 PM

The 106s in the primaries of the '77 carb are very nice! This engine has never felt this powerful on the butt dyno before. It looks like I'll have to get a set of 105s soon.

As for the secondaries, this is where it gets kinda grey. I installed the 155s and I don't know if that's killing the power or improving it. Infact, as I was testdriving it, it's as if there were three positions in the gas pedal: normal, medium, and holy crap! I can only assume the secondaries are opening. The only thing I can compare it to is the REPU, and it's got a definite transition. In this car however, the farther I press the pedal down, the more power is produced without a normal sounding secondary transition. Maybe they are opening? It actually gets kinda scary because the brakes are bad. The car is also low to the ground so it feels faster than it actually is.

So what do you think I should do about the secondary jets? Go back to the stock 145s, stick with the 155s, or maybe even try the 160s? I can list the numbers of everything and let you Hitachi experts recommend stuff. :)

This is how my '76 Cosmo FSM lists Jets and Air bleeds
1. No. 2 secondary Air bleed
2. No. 1 secondary Air bleed
3. Secondary main air bleed
4. Primary main Air bleed
5. Accel. pump injection nozzle, weight and ball
6. Richer air bleed and jet (M/T only)
7. Vacuum jet

This is what the '77 carb has in it
1. 80
2. 100
3. 100
4. 80
5. (I haven't examined the AC pump squirter on this carb)
6. 150
7. 90

This is what the '74 carb has in it
1. no number
2. 150
3. 160
4. 80
5. (no weight inside AC pump squirter, and I don't know why)
6. no number
7. 120

As you can see, the primary main air bleeds of both carbs are the same size, so logically, they can both use the same size primary fuel jets. In other words, both carbs can take advantage of RB's suggested 105 jets. This explains why the test drive today was so successful.

As for the secondaries, I really don't know what I should do. I welcome any suggestions based on the numbers I posted above. Anyone ever experience anything similar with your Hitachis? Even educated guesses are welcome. :)
____________
'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7

       
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klaus42


Rotorhead
Posts: 1877
posted November 16, 2004 10:39 PM
Edited By: klaus42 on 16 Nov 2004 22:42

Without going outside to disassemble

...and refresh my memory of precisely what's in which of my little Hitachi collection...
(Which I really oughta do; this might actually get me motivated again!):

The part in my post you found confusing refers to RB's timing recommendations. They actually do list somewhat different timing for stock ported 13B's running Dellorto, Holley, or Weber:
'74-'75 13B 0 deg. leading; 8 deg. ATC trailing
'76-'78 13B 3 deg. ATC leading; 13 deg. ATC trailing

...(these were the potentially pertinent data omitted in my previous post.)

***I highly recommed getting their 'Tech. manual/Catalogue' in whatever edition available which is most applicable to the vehicles in your --or anyone's-- collection. They feature many 'tech tips' you'll likely find invaluable! If the good stuff isn't in their latest catalogues anymore, perhaps I can run off some copies or something...

As far as your secondaries: The 'holy crap!' experience during your butt-dyno session was indicative of what could, and certainly should be available to you at all times. My guess: your secondaries are not always popping open. (Typical old Hitachi problem!) Try all of the 'tricks' I listed in my previous post and see what that does. Have you inspected your vacuum diaphragm? If it's still flexible and 100% intact, you're fortunate!

If it's not that, then you've either got (possibly?) a tiny piece(s) of dirt, or something, periodically inhibiting performance... That, or (perhaps because of that?) fuel starvation under certain conditions?
Have you found and cleaned those two little cylindrical fuel screens at the float bowl inlets? They typically clog...

As far as secondary jetting goes, I dunno... all depends on your engine... whatever works. (Trust RB!)
Some folks just wire the linkage to make 'em mechanical...


       
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rotormunky


Redlining
Orlando, Florida
Posts: 424
posted November 16, 2004 11:04 PM

Anyone interested . . .

I have a completely rebuilt hitachi carb still in shrinkwrap sitting in my parts shed right now.

I snagged it off Ebay a couple years ago but have since decided that even if I stay with the 4-port I'll convert to the holley setup.

Just wondering if anyone would be interested in buying it, and what the approximate price would be?

If you like I can take pics, but it looks like a brand new carb. Spacer plates, etc.

Later guys.


____________
-Martin
Orlando, Florida

http://www.themonkeyhouse.org/REPU

'77 REPU (Some assembly required :)
'91 Cabrio (Battered and bruised, but she's still my baby.)

        Click here to visit rotormunky's homepage. 
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klaus42


Rotorhead
Posts: 1877
posted November 16, 2004 11:10 PM

Hot Damn!

I'll guess demand is gonna not be too extreme... but, I'd also wager you could get more for it than I'd be willing to give...

If Jeff doesn't need it way worse than I do, I'm certainly interested! How affordable could it be?

Anything you'd like or need more than mere money?

       
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rotormunky


Redlining
Orlando, Florida
Posts: 424
posted November 16, 2004 11:19 PM

quote:
I'll guess demand is gonna not be too extreme... but, I'd also wager you could get more for it than I'd be willing to give...

If Jeff doesn't need it way worse than I do, I'm certainly interested! How affordable could it be?

Anything you'd like or need more than mere money?


Nothing that can be easily shipped :)

Basically if I don't like the way it is in the truck, I'm ripping it out and building my own so . . .

I've no idea what the carb is worth, I'm not trying to be coy or anything. Its sat shrinkwrapped in my shed for two years now so it won't hurt it to sit there until someone offers something that seems reasonable. Technically I suppose I could throw in my used and dirty but completely functional hitachi off my truck since I won't be needing that either.

Frankly I'd rather it go to someone who is looking for original equipment than someone just buying it to put on a go-kart or something.

Its cozy where its at but if anyone is interested I can take pics.


____________
-Martin
Orlando, Florida

http://www.themonkeyhouse.org/REPU

'77 REPU (Some assembly required :)
'91 Cabrio (Battered and bruised, but she's still my baby.)

        Click here to visit rotormunky's homepage. 
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klaus42


Rotorhead
Posts: 1877
posted November 16, 2004 11:27 PM

I'd consider it a gift

at $50 plus shipping; for any more than $100 total, I'd have to think about my cash flow...

Sounds to me like Jeff might need it more; I'd hate to snag it if he really needs...

Then again, perhaps if you're parting with two, we could share...? (Question being, who'd get the nice one...?)

Like I said before-- no doubt it (they) are worth more... Don't mean to be insultingly low-ball.
I could afford more, non-cash...

Are you cerain I don't have something --whether for your truck, or perhaps something you could use as a gift this Christmas season--?

(A small glimpse of some of my glass can be viewed under Beautiful Visions at glassartists.org)

       
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Jeff20B


Moderator
Posts: 661
posted November 17, 2004 10:48 AM
Edited By: Jeff20B on 17 Nov 2004 11:25

I shouldn't need another carb for a while if I can get this '74-'75 to work. Thanks for your consideration.
____________
'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7

       
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klaus42


Rotorhead
Posts: 1877
posted November 18, 2004 12:14 PM

Perhaps...

...it would be more appropriate of me to offer you at least as much as it set you back when you bought it, Martin...(whatever that was)...?

My low-ball offer is probably more appropriate for your used/usable one.

That said, I don't 'NEED' another carb., either...
...it'd just be too nice/easy to be able to slap a fresh one on and run it!

If someone 'NEEDS' it worse than me, I understand... if not, I'm definitely interested in hearing how much you need for it/them.

Thanks!

       
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Jeff20B


Moderator
Posts: 661
posted November 18, 2004 01:04 PM

Two sets of 105 jets are on their way.

Hey, you know how RB seems to make a big deal out of switching from 106 to 105 jets in the primaries? It leaves me wondering about the secondaries, and if playing around with the jets is a wise thing to do without a dyno to test the changes. You said to trust RB's advice and leave them stock, but I just don't know.

I guess what I'm trying to say is that RB makes a big thing out of .01mm of a difference in jet size and here I am changing jet sizes in .10 or .15 increments. Am I way off base here?

I have four 4 port 13Bs which have four different sized ports. The engines range from stock '76 ports (small) to stock '74 ports, to slightly larger than '74 ports (standard streetport/mildport, I think) to even larger (an extend port, I think). The stock '76 engine has a stock '76 carb, so I'm not worried about it. The other three engines are what I'm working with right now. I've got two working '77 carbs and one untested '74 carb that I just completed my first rebuild on (well, first car carb rebuild anyway). The primary air bleeds of one of the '77 carbs and the '74 carb are the same, and testing of the '77 carb with the '74 carb's 106 jets in the primaries was succesful. This means they both should benefit from a set of 105 jets. I haven't seen inside the other '77 carb for a long time, and didn't write down any of the jet sizes the last time I was in it. If it's got the same size primary air bleeds as the other two carbs, I'm golden (it'll get 105s). :) If not, I'm back to square one.

As for the secondaries, these three carbs had different jet sizes. I have to assume the 150 jet I found in my friend's garage came from the '77 carb I got from him because the jet has an Hitachi symbol on it (kinda round looking). I have no idea what the primary jets are, but the carb always had a more powerful primary circuit than my other '77 carb (that is until the jet swap hehe, but I haven't done a side by side test on the same engine yet). Anyway, the previously weaker of the two '77 carbs had 145 secondaries and the '74 carb had 140 secondaries. Yep, that's 150, 145 and 140. Do you guys think I should install the carb that came stock with the largest secondary jets on my engine with the largest porting? If so, should I also put the 106 primary jets in it, you know, because the ports are so large? Or would it be a better idea to put the largest jets on the smallest ports? I'm rather confused about that. Or how about if I up the secondary jet sizes by 5 or 10 or possibly even 15? What could it hurt? I assume the farther I get from '74-'75 size ports, the less significant RB's recommended jetting becomes.

Should I trust RB's recommendation? Or simply up the jet sizes on the larger ported engines? Thanks for reading this longish post.
____________
'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7

       
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klaus42


Rotorhead
Posts: 1877
posted November 18, 2004 01:22 PM
Edited By: klaus42 on 18 Nov 2004 13:26

We're in the land of what-if's and opinions now.

My 2 cents: Main fuel jetting, in and of itself, is about relative mixture, rich-lean, at full throttle.

Yes, rotaries tend to 'like it' a bit rich, BUT...

It really all depends on various other fuel/ignition/engine/porting/driving style factors.

In and of itself, porting is about timing, and flow.

How these aspects interrelate, well... depends!

My guess: 'other factors' typical to different years/configurations/specific carb's and individual components/conditions might be at play.

Two good starting points:
1.) 100% clean, functional, utterly stock
2.) The above, modified/set to RB spec's

Your call from there on!

*Always good to consider the possible effects of age/compromised parts... like used jets that may have been reamed out at some point; 'less than ideal' emulsion tubes/castings; throttle plate bushing wear, etc., etc., etc. ...)

Best, K.

       
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Jeff20B


Moderator
Posts: 661
posted November 18, 2004 02:04 PM
Edited By: Jeff20B on 18 Nov 2004 14:06

Thanks for the reply. I guess the thing I'm most concerned with is which engine should get the 106 jets instead of the 105s. I don't think I can justify spending any money on a .01 smaller difference in jet size at the moment. The two sets of 105s on order will cover two carbs. I've gotta test them before getting any more.

Having stated that, and with a semi-educated guess, I think the engine with the biggest ports ought to get the carb with the biggest jets. The thing that throws me off though is the fact that an 'undersized' carb tends to run rich because the engine's vacuum is stronger than what the carb was calibrated for; it then sucks more fuel as it's got less resistance than the air coming is, due to restrictions etc. A bit like a choke I suppose.

I guess the same logic can apply to a carb that tends to lean out at high altitudes. ACVW Solex carbs tend to do that. There is mention in the RB catalogue of needing to jet the primaries slightly leaner if it will be operated above 3500', and then leaner secondaries if above 5000'. Hmm.

Maybe I'm too slow today to see the connection, but something keeps eating at me to get to the bottom of this jetting stuff that's confusing me so much. I'm near sea level, so standard jetting applies (scratch one variable off the list).

Anyway, I'm thinking that on streetported engines, they might want the leaner jetting because the carbs might tend to be too small at high RPM, and thus go rich. Perhaps this is why the two '77 carbs had larger secondary jets than the '74 carb? You know, because the porting was smaller from the factory beginning in '76. Does that mean that since these carbs are now on streetported engines, that I really should get smaller jets for them? I'm so confused.
____________
'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7

       
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klaus42


Rotorhead
Posts: 1877
posted November 18, 2004 02:58 PM

Anyone...?

Jeff, I'm sorry to say it's beyond me as well... I'm sure someone has all the answers, but I've offered all the best of my knowledge/opinions.

Any rotorcarb pro's out there care to enlighten us further?

       
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Jeff20B


Moderator
Posts: 661
posted November 18, 2004 04:57 PM

Thanks for your help. Hopefully we'll get this figured out.
____________
'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7

       
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klaus42


Rotorhead
Posts: 1877
posted November 19, 2004 01:51 PM

Some food for thought:

Just a few more ideas to add to the confusion; these coming from what I've learned about the flow of oxygen and propane to my surface-mix glass torch... perhaps just as applicable to the physics of automotive mixture intake, (let alone fuel flow from tank to carb)...:

Laminar flow --as opposed to turbulent flow-- allows us to achieve the cleanest, most efficient and complete combustion.

Laminar flow cannot be achieved if one is attempting to run too high a pressure (volume?) through either too narrow and long, or otherwise too restrictive a system...

Potential non-turbulent flow rates are determined/limited not only by the length and diameter of lines, tank to torch, (Fuel tank to combustion zone...), but, also, by the point(s) of greatest restriction.

Perhaps this explains why RB never recommends the stock carb for ported motors?

It's all about flow... and turbulence, (or the lack thereof), as regards efficiency. (I think!)

I'm sure you can google a physics formula for determining flow rates. If not, I have that around here 'somewhere'!

       
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Jeff20B


Moderator
Posts: 661
posted November 26, 2004 12:55 PM

I have two different types of propane torches. One has nice flow and the other seems turbulent. The turbulent one has a single jet that can be removed for a really large slow, yellow flame. :) Anyway, I think I see what you're talking about. The flame is very turbulent and doesn't heat stuff as efficiently as the smoother flowing one. Heh, I use MAPP gas in the smoother torch from time to time. The tip becomes red hot (it's stainless steel and seems to survive just fine).

Hey, I finished rebuilding the Hitachi and installed some 105s in the primaries and the stock 140s in the secondaries. I took it for test drive yesterday and the idle circuit, primary cicruit, and the choke/fast idle linkage was very cool! It fired right up within two seconds of cranking and idled at about 1500 right off the bat with the choke held open nby a penny (there is no electric choke holder in this car). It idles smooth with the occasional blip. Then when it had warmed up a bit, I release fthe choke and it almost idled right where it was supposed to be. I adjusted the fuel and air mixture screws and now it revs up nicely and returns to idle nicely with a smooth, strong idle.

The secondaries may not have opened, or if they did, felt weak due to the power of the primaries. :) Anyway, I replaced the spring with a shorter one and will take it for a test drive soon. If I still don't like when they open, I'll remove the spring as I've done on all my other Hitachis.
____________
'74 REPU
'76 Cosmo
'77 MG Midget 13B
'81 RX-7

       
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klaus42


Rotorhead
Posts: 1877
posted November 26, 2004 01:12 PM

Nice!

...typically, the secondaries won't budge until/unless you're under load at the 'proper' RPM's... On a good day, I can stick my foot in it at around three grand, and get 'em to pop in time for the 4K powerband...

       
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