How to Tune your Hitachi or Nikki Carb....






Part 1- Adjusting the Float Level inside the Carburettor

Part 2- Adjusting the Idle Mixture



First of all lets do some checks and find out if in fact we need to play with the Carby at all...


After running the vehicle, stop the Engine and check the float level in the glass windows on the fron and rear of the Carburettor. They may require cleaning. In the Centre of the glass you should notice a white dot. The fuel level should line up with this dot. On my Rx5 the fuel level was about 2mm below this dot. The level of fuel inside the fuel bowl is like a rich / lean adjustment for the entire carby. A high float level will make the car run richer thoughout the rev range... a low one.. like mine.. means the car is a real bitch when cold.... without the choke on.. forget about running..... plus a lean mixture is not good for power or heat in the engine...

If the level is slightly over the dot by say a millimeter or 2, then this is ok also.. as long as it is the same front to back... as each Rotor basically has its own 2bbl carby. Both haves have there own fuel bowl.. everything is duplicated twice.

If the level of fuel in both front and rear is within the range of the sight glass, but not lined up with the mark then proceed to Part 1

If the level in one sight glass is ok, but the other is off the scale, then it is recommended to follow the proceedure in Part1, plus also Replace both needle and seat assembly's in the top of the Carby, unless you find some other reason for the level being so high.. such as a cracked or leaking float.


Common Problems:

If you cant see the fuel level in either glass... then either:

A: You have no fuel in the carburettor (Car will not start). Check the fuel filter for blockage. Check to see if you have any fuel left in the tank.... its been done before :) When you turn the ignition on can you hear the fuel pump running. Factory fuel pumps have been changed on most old Rx's.. if yours still has the original then it might be time for a new one.


B:You have too much fuel in the carburettor. Can also be intermittant (like on my 12A Rx2)... Happened maybe three times a year. Fuel bowls would fill up and fuel would flow over the sides pouring down the throat of the Carby... Even after changing the Needle and Seat. Cause: aftermarket fuel pump provinding either too much pressure, or pulses in the fuel pressure. Even though my 12A Rx2 and my 13B Rx2 had identical fuel pumps... the 12A had new needle and seat... and did the overflow trick three times a year.... 13B never had a problem.... It normally happened within 10 minutes of starting the car.... the car would still run.... but when idling at the lights... it would run like a bridgeport at 2000 rpm!! cool. You had to drive fast until it cleared.... otherwise it would try to die... and if it stalled... forget it.... 1/2 hour of fiddling to get it going.... pull the plugs and cleaning them etc.... The fuel pumps I am talking about are the Solenoid, Facet style fuel pumps... they look like a transformer. If this happens to you, or you are buying one... then try a different... non solenoid type. Buy one that goes Wizzzzzzzz... not tick tick tick tick :)




Part 1

Fuel Level Adjustment.

So you need to adjust the fuel level... well lets do it...


both float levels off the scale....


Firstly remove the Air Filter from the Carby.

Undo the four clips on the lid, and remove the wingnut from the centre of the Carby. Remove any hoses from the Air Filter Body, and the blue Securing bracket at the Rear (if its still there).


No 1 rule... dont drop anything metal into the carby otherwise you will destroy your engine! next time it starts... put a clean rag over the centre of the carb. If you do drop something... remove the 4 12mm nuts and take the carby off.... do not open the throttle body!


Now the Carby should be visible like this....


1. Remove the Stud from the Centre of the carby

2. Undo the screw holing the Accelerator pump linkage, and remove the linkage from the top cover. Just lie it down on the engine.

Nikki 12A carbs are even easier.. you dont have to take off the accelerator pump.. so skip this step.

3. Remove the Accelerator cable (2 *12mm nuts), and also the Throttle return spring.

4. If still fitted.... remove one end of the choke linkage by removing the tiny Split Pin and Spring.

5. If fitted... remove the Choke cable.

6. Ok now its just the 9 screws holding the top cover of the Carby on... Remove these and the cover should lift off.

On 13B hitachi models.. be careful of the accelerator pump... push it through the rubber sheath so it stays in the carby body.

Flip the top cover straight back, as shown in the pics.... not to the front or back of the car.. as there is nothing holding in the pins which secure the floats.

The fuel lines can stay connected.


Ok it should look like the picture shown below now:

Get a ruler and measure the distance between the (upsidedown) float and the aluminium of the carby top. Mine was 7mm. Then you can bent the tabs with long nose pliers to adjust the gap. I bent mine so the Gap was reduced to 5mm... to hopfully bring up the fuel level a couple of mm. ensure the gap is even on both floats. The Rx4 factory manual states this gap sould be 11mm, but this was not the case for mine.

Once adjusted, put the top back on the carby, and install a couple of bolts to hold the top down. Now turn the ignition on to run the fuel pump for 5 seconds or so. Now you can re-check the fuel level in the sight glass. If you like, install the throttle return spring... and maybe the accelerator cable and start the car to get a more accurate idea of the fuel level with the car running. You may also have to block off any pipes that go to the air filter... and the car may be harder to start as the air filter is not present. Also if you want to do this you must warm up the engine before you start stripping the carby.

This picture shows the white dot on the sight glass, where the fuel level should be steady at.

Here is the Carby body with the cover off.

If you Major Carby troubles... then you will want to remove the entrire carby from the manifold. From the underneath just tighten the 6 retaining bolts holding the butterflys to the carby body. Then from the view in this picture, simply remove the Brass jets one by one... and blow compressed air through each one to ensure they are not blocked. If you dont have compressed air then your mouth works good.... just tastes bad :) and then hold them up to the light. The jets in the top whick can be seen in this picture... You will only need to check if..... the car has been run with No air filter.... (or the filter is that rooted that it may as well have none).. or the carby is off a jap import motor... sometimes these jets get blocked up.

**The Most Important thing to look for is the main jets in the Fuel Bowl. These cause many problems.... intermittant problems... problems where the car runs fine on secondaries but runs like a bridgeport on primaries..... blocked main jet... extremely common. There are 4 plugs on the fuel bowl you need to remove... and yes the fuel will drain out. Then you can use a big flat blade screwdriver to remove these jets. This happens when fuel filters are not changed... or you get a dodgy batch of fuel.

If you do get rubbish somehow in the fuel system... a common senario is that overnight it may settle to the bottom of the fuel bowl... then first time you start the car the next day..... bam sucked straight into the primary main fuel jet.... then :( car runs very poorly.... if at all


Ready for test run.. without air filter, accelerator pump, or choke.

Back together now. This Pic. also shows location of Idle Mixture Screw & Throttle Stop screw which adjusts Idle speed.

Air filter on... When buying a filter... buy a Ryco A31 gas filter as they are half the price of the A267x or the listed one for this car. The only difference is the centre diameter is slightly larger... so just centralise the filter in the housing before you put the lid on.

All finished.


Part 2:

Idle Mixture Screw Adjustment.

This is very easy to adjust and only affects the mixture at idle and a tiny amount at small throttle openings. There are two steps, just go back and forth between the two. Do not look to this adjustment as the cause of any major problems with engine idling.

To make the car idle leaner... adjust the idle screw in say 1/2 to 1 turn.... then adjust the idle speed back to specs with the throttle stop screw. the car is now running leaner at idle. You should adjust the idle screw in until you notice the car running rough, and the back the screw out (richen up the mixture) 1/2 to 1 turn. Re-check the idle speed (approx 800 - 900 rpm for a stock engine)... and thats it.. done!


On the Rx4's they have a large Air bleed screw directly above the mixture screw. On this model you use this to adjust the Idle speed, rather than the throttle stop screw.

If your idle speed is like 1500rpm + and your car is running like shit... then the mixture screw probably won't do squat. You need to fix some other things and come back to the mixture screw later.





Any Questions? Click here...


Take me Back to the Home Page