Tunning SU's

[ZD44TT]

Howdy all,

 

can anyone tell me if you can tune the air/fuel mixture for round top SU's?

 

Drove to work today, about 38km in peak hr traffic and it ate up 1/4tank.......AHHAHhahah WTF........is that normal?

 

 

if so where abouts is it on this pic?

 

I need to get it on a dyno and do it I guess.

[gav240z]

The fuel mixture nut is at the bottom of the carb. If you look underneath you will see a spring loaded nut that can turn up and down. I think from memory down is richer up is leaner.

 

To tell if you are running rich. Pull the spark plugs if they are black in colour it's running too rich. If it's a really light brown it's too lean. Just right is a nice chocolate brown colour.

 

If the throttle shaft on the SU's is stuffed and needs rebushing you'll never get them (the SU's) to sync up correctly. I learnt the hard way.

 

There are several books out there on tuning SU's look on Amazon.com if you are interested.

 

The SU's you have on your Z appear to be of the British Type. Early 240z Hitachi carbs look slightly different and have an Hitachi symbol on the top of the fuel bowl FWIW.

 

You can also generally tell if your too rich when you jump on the gas and you get a large cloud of continuous black smoke. If your are too lean you will notice it go very flat in the top end.

 

If your car is out of tune it's best to go easy on her until you get it in tune.

 

Your electrical timing could also cause poor fuel consumption. My 240z with an L28 lightened flywheel, mild cam, electronic dizzy & 240z carbs is really quite good on fuel.

 

When I drove up to the 2005 Nationals in Goulburn from Victoria, I drove with Sulio in his then new Ford Falcon Wagon (straight 6 4ltr) he was towing also, but our fuel consumption was fairly close. Albeit the Z did drink a bit more. However not bad for an old Z.

[ZD44TT]

hmmm might wack on a cam so it can be tuned asap.

 

 

how much power are you making on your set up?

[nat0_240_chevZ]

Some questions.....

1-which 1/4 of your tank did you notice it use?? up high or low?

 

2-does the car idle slightly higher when you put your fingers (spread to allow air through but not heaps) infront of the intake rams?

 

3-do you have much play in the throttle linkage where it enters the carby?

 

4-generally how well does the car run?

 

5- how fast is it idling??

 

 

and also of course it chews fuel when stuck in traffic, stop start stop start, big sqirt everytime the pedal is pumped.

 

answer a few of those q's and we'll go from there i reckon, also do you have a timing light?? And a dyno session isnt going to net you many extra HPs unless they have/are a specialised carby tuner along with vacuum and mechanical advance experience, most places only plug into your ecu and edit a few parameters re VE curves etc.

 

nato

[ZD44TT]

Some questions.....

1-which 1/4 of your tank did you notice it use?? up high or low?

 

2-does the car idle slightly higher when you put your fingers (spread to allow air through but not heaps) infront of the intake rams?

 

3-do you have much play in the throttle linkage where it enters the carby?

 

4-generally how well does the car run?

 

5- how fast is it idling??

 

 

and also of course it chews fuel when stuck in traffic' date=' stop start stop start, big sqirt everytime the pedal is pumped.

 

answer a few of those q's and we'll go from there i reckon, also do you have a timing light?? And a dyno session isnt going to net you many extra HPs unless they have/are a specialised carby tuner along with vacuum and mechanical advance experience, most places only plug into your ecu and edit a few parameters re VE curves etc.

 

nato[/quote']

 

 

1. one notch below half way.....i travel around 80km and it's down to one notch on top of empty.......consumption is more like 1/2tank for a 80km trip.

 

2. haven't tried this but will do.

 

3. yes abit of play in linkages.

 

4. Car runs very well..........revs nice.......off throttle abit of back fire popping all the time........engine never skips a beat........sits in traffic pefectly.......on acceleration it's smooth.........

 

5. Car idles around 950 - 1050 rpm. idles smooth.......doesnt go up or down.........normally it sits at 1000rpm.

 

I would love to sit it on a dyno and have to lightly tuned.

[ZD44TT]

does anyone know what the Air/Fuel ratio maps roughly look/read like for a stock SU'd LS motor?

 

honda's are tuned to 12.8a/f.....

 

Wonder if it's worth getting a light tune on it as is before the cams get backed in a couple of months.

 

Thanks.

[zedevan]

you'll find the fuel guage isn't very linear, so atleast use a full and see how far you get before worrying about getting it tuned, you should get a bit over 400km depending on how ur driving

 

i can't remember where the part of faster travel on my guage is to be ablt to help much more, but i'm pretty sure its once it gets to half way it drops faster, or mayb thats the other car i've been driving... :roll:

[ZD44TT]

you'll find the fuel guage isn't very linear' date=' so atleast use a full and see how far you get before worrying about getting it tuned, you should get a bit over 400km depending on how ur driving

 

i can't remember where the part of faster travel on my guage is to be ablt to help much more, but i'm pretty sure its once it gets to half way it drops faster, or mayb thats the other car i've been driving... :roll:[/quote']

 

 

I'm confident in saying I'm getting close to 170km/tank of 98 octane : (.

[nat0_240_chevZ]

check for leaks, when you fill it right up.

leaks in fuel lines.

u running an electric fuel pump? with no return line?

take off aircleaner and see if you can see fuel spewing out the main jet (with a mierror of course) where the needles goes to.

 

that sort of fuel economy would challenge the bugatti at full noise!!

 

nato

[Rod F]

My L28 with twin round top Hitachi's gets about 260km's to a tank with city driving on regular rubbish fuel. I'm driving to Ballarat tomorrow so I'll see what highway driving returns.

[ZD44TT]

check for leaks' date=' when you fill it right up.

leaks in fuel lines.

u running an electric fuel pump? with no return line?

take off aircleaner and see if you can see fuel spewing out the main jet (with a mierror of course) where the needles goes to.

 

that sort of fuel economy would challenge the bugatti at full noise!!

 

nato[/quote']

 

no leaks anywhere in the enginebay or tank in the rear.

 

after i've leaned out the carbs the car seems to be pulling nicely all the way to redline now........strange..........makes pretty dam good power now...........after a good hour road tune/testing?..........hehhehe

 

but economy is still pretty bad i think.........halfway gave me about 140km..........we'll see what this tank does.........good mix of hwy and traffic.

[dazzed]

stick em on the railway line

[Zedman240®]

I think the most important thing to check again, I think it was mentioned by Gavin before, is the plug colour! Don't run the engine to lean or meltdown! If they are a coffee colour you are doing well. Its the most accurate way of seeing how well setup the fuel system is. Unless you have a Motec sniffer sensor in the exhaust! But do it after a high speed run not after crawling in traffic tho...Proper way is to go for a few high speed runs and before you let the engine idle, shut it off and then check. Do this in a quiet industrial area etc on a weekend Last thing Melbourne needs is another train disaster.....

[nat0_240_chevZ]

first half of the tank gauge reading is only about 30% of the tank volume, hence the 1st half always goes quick!!!

[chris240]

just on another note, can you modify SU's to apply mechanical advance ?

In the old days I had a 13b- RX4 (with standard downdraught carb) that I had the secondaries modified to mechanical advance instead of vacuum, it was a 1/2 hour job and made a great difference.

Ive got series 6 Su's (less than 6 months old) and they have run faultlessly apart for some minor timing issues.

[Zedman240®]

SU's don't have a secondary throat. They are a variable venturi carby. You can always try a thinner oil in the top reservoir. As long as they are balanced and in good order, it should be fine.