modified engine Rich mix, average peformance

[Joshua]

Hi all, I recently got my first Z (1975 260z) and learning how the more vintage style system works.

It is running rich at the moment (Very 20L per 100km) and Im wondering if I cant get a proper tune if;

1. SU carburetors are running stock piston spring (Reconditioned)

2.Engine(L26) is running a lumpy cam (Not sure on spec) & High compression (10.5:1)

I am tuning using the uni-syn tool trying to lean as much as i can but idle becomes weak below 900-1000 rpm and at a higher 5000 rpm+ and more heavy foot throttle position, operation becomes average and struggles to accelerate.

I have read on different spring types like red and green but what difference will this make to the operation?

Many thanks.

 

 

Edited January 17, 2019 by Joshua

[gav240z]

Do you mean the springs in the piston domes? For the piston / dampener? This acts as your accelerator pump in an SU carbs, by creating a slight delay at small throttle inputs the idea is that the additional vacuum will suck more fuel into the motor creating a slightly rich mixture.

However at constant throttle or full throttle or idle it doesn't do anything that would impact fuel consumption.

Leaning out the mixture to the point of poor upper end performance or weak idle is not good either.

You say the SUs are rebuilt? By whom? What was done to them? What needle spec are you running?

Have you looked at your spark plug colour to determine mixture setting?

How about ignition? Are you running stock points based ignition? Still running vacuum advance?

What fuel pump? Mechanical? Electric?

What is your driving style?

[Joshua]

It was purchased just under a week ago and everything was done before i got it. so unsure when and who did it but asked the question in case it could delay or speed up enriching the fuel mix by the damper spring rate, I was hoping that if this was the case i could have the leaner mix for the idle to keep it smooth then rich it up later on for a smooth rpm build towards redline .

It has a mechanical fuel pump, and not sure on the advance.

I normally drive casually and once in a while give it the beans

When I've been playing around the spark plugs were black but replaced them few days ago, hasn't affected how the engine runs.

I don't like running it rich or lean cause I know it damages the engine so when I attempted to lean it out I readjusted it back to about the original setting so the idle was smooth at 800-900 rpm, but the top end is still affected. So would the needle be worn or something? allowing excess fuel to slip past?

Also when cruising around once in a while the engine stutters almost like its starving (even with the rich mix) but the tacho and rev gauge needles goes haywire when it stutters but only lasts for a couple seconds.

Would that with the excessive rich air fuel mix would prevent me from reaching the top end smoothly? and have a good power band with half decent economy?

Thanks.

[1600dave]

What carbs are you running (earlier Datsun Hitachi SU's / flat top 260Z carbs / genuine British SU's) ? Is the needle  appropriate spec ?

To adjust mixture at only one point (eg at idle) you need to fit differently tapered needles. Winding the jet up or down will richen or lean it across the whole rev range.

Rough idle at low revs could be as simple as the cam - what is "lumpy" ? You may no get a perfect idle if cam is too big.

Is the float level correct ?

Tacho going haywire at same time as engine stuttering sounds electrical to me.