Hitachi help

bowjangles · 10-26-2013, 04:32 AM

Just rebuilt a 74-76 Hitachi 4bbl carb. The directions that came with the kit
for adjusting the carb are terrible. Can someone explain how to adjust the 2 screws on the side of the carbo- the top one is big and the lower one is smaller. What does each screw do exactly-What are the initial settings ?

djmtsu · 10-26-2013, 10:30 AM

I'll let you know when I get around to getting mine running

Unless you get to it first!

Rotary Evolution · 10-27-2013, 11:24 AM

the lower screw is for idle mixture adjustments, the larger top screw is for idle speed adjustments.

adjust the top one to get near desired idle speed then for the bottom one: warm up the engine to operating temp then begin turning it clockwise until the engine idle speed raises to peak and then begins to stumble then turn it back out 1/8th of a turn. once the bottom screw is adjusted then you can set the idle speed with the upper screw.

make mixture adjustments rather slowly, if you try to go too fast you may overshoot where it should be.

if the idle is still poor then there are other issues to resolve. cleaning the metering jets in the hitachi and nikki carbs is not easy to do and sometimes plugged up jets are overlooked. vacuum leaks will also cause you to beat your head against the wall in trying to fine tune the carb.

bowjangles · 10-28-2013, 05:14 AM

Thanks Rotary Evolution u r the best!

bowjangles · 10-29-2013, 12:43 AM

Wow, engine runs better now . Idle mixture is set about 5 turns out. Idle speed screw doesn't make any difference where it is set -doesn't change idle speed at all. Something wrong there ?

Rotary Evolution · 10-29-2013, 11:25 AM

yes, normally the idle mixture screw is set at about 2.5 turns out on a stock carb, it's possible someone mixed up nikki main jets for the primaries or used the carb on a 12a and rejetted it. at this point in time it's difficult to know what has happened to the carb over the years.

i forget offhand the stock main jet sizes but i want to say 60 primaries and 90 secondaries but that could be nikki jets. if i get a chance i will see if i have the numbers written down on my carb sheets.

the idle screw isn't critical, you can also use the butterfly cam stop to set the idle as well if it is giving you difficulty. the cam stop is located under the throttle cable on the carb cast iron base.

bowjangles · 11-05-2013, 06:21 AM

Will try-Thanx

10-26-2013, 04:32 AM	#1
bowjangles Rotary Fan in Training Join Date: Sep 2013 iTrader: (0) Posts: 62 Rep Power: 12	Hitachi help Just rebuilt a 74-76 Hitachi 4bbl carb. The directions that came with the kit for adjusting the carb are terrible. Can someone explain how to adjust the 2 screws on the side of the carbo- the top one is big and the lower one is smaller. What does each screw do exactly-What are the initial settings ?

10-26-2013, 10:30 AM	#2
djmtsu My minds tellin' me no... Join Date: Feb 2008 Location: Murfreesboro, TN iTrader: (10) Posts: 4,043 Rep Power: 21	I'll let you know when I get around to getting mine running Unless you get to it first! __________________ 1976 Mazda Cosmo RX-5 1976 Mazda Cosmo RX-5 2003 Toyota Tundra TRD 2015 Toyota 4Runner SR5

10-27-2013, 11:24 AM	#3
Rotary Evolution rotaryevolution.net Join Date: Feb 2011 Location: Las Vegas, NV iTrader: (0) Posts: 248 Rep Power: 14	the lower screw is for idle mixture adjustments, the larger top screw is for idle speed adjustments. adjust the top one to get near desired idle speed then for the bottom one: warm up the engine to operating temp then begin turning it clockwise until the engine idle speed raises to peak and then begins to stumble then turn it back out 1/8th of a turn. once the bottom screw is adjusted then you can set the idle speed with the upper screw. make mixture adjustments rather slowly, if you try to go too fast you may overshoot where it should be. if the idle is still poor then there are other issues to resolve. cleaning the metering jets in the hitachi and nikki carbs is not easy to do and sometimes plugged up jets are overlooked. vacuum leaks will also cause you to beat your head against the wall in trying to fine tune the carb. __________________ http://rotaryevolution.net Last edited by Rotary Evolution; 10-27-2013 at 11:30 AM..

10-28-2013, 05:14 AM	#4
bowjangles Rotary Fan in Training Join Date: Sep 2013 iTrader: (0) Posts: 62 Rep Power: 12	Hitachi help Thanks Rotary Evolution u r the best!

10-29-2013, 12:43 AM	#5
bowjangles Rotary Fan in Training Join Date: Sep 2013 iTrader: (0) Posts: 62 Rep Power: 12	Hitachi help Wow, engine runs better now . Idle mixture is set about 5 turns out. Idle speed screw doesn't make any difference where it is set -doesn't change idle speed at all. Something wrong there ?

10-29-2013, 11:25 AM	#6
Rotary Evolution rotaryevolution.net Join Date: Feb 2011 Location: Las Vegas, NV iTrader: (0) Posts: 248 Rep Power: 14	yes, normally the idle mixture screw is set at about 2.5 turns out on a stock carb, it's possible someone mixed up nikki main jets for the primaries or used the carb on a 12a and rejetted it. at this point in time it's difficult to know what has happened to the carb over the years. i forget offhand the stock main jet sizes but i want to say 60 primaries and 90 secondaries but that could be nikki jets. if i get a chance i will see if i have the numbers written down on my carb sheets. the idle screw isn't critical, you can also use the butterfly cam stop to set the idle as well if it is giving you difficulty. the cam stop is located under the throttle cable on the carb cast iron base. __________________ http://rotaryevolution.net Last edited by Rotary Evolution; 10-29-2013 at 11:28 AM..

11-05-2013, 06:21 AM	#7
bowjangles Rotary Fan in Training Join Date: Sep 2013 iTrader: (0) Posts: 62 Rep Power: 12	Will try-Thanx