Problems on Freeway in cold weather (SU Motor)

[Hugh 240z]

First off, a disclaimer: I am an EFI guy, so these carbs blow my mind.

I've got a 240z with an L28 and the SU carbs. This motor is on its last legs... but still runs pretty strong. It burns a lot of oil, however. I am going hybrid with this car eventually, but I need it to get around until that time.

It seriously runs nice and strong everyday, but I've noticed a problem late at night when its really cold out (southern california, not snow mind you)

I'll be cruising in high gear on the freeway and it just cuts out. It does it mostly when I apply the throttle under heavy load, like high gear uphill, or when passing cars. If I try to throw a lot of throttle at it quickly, its even worse. Sometimes when just going uphill without trying to accelerate, it cuts out then, too.

When it cuts, it pops and I can hear like a backfire out the carbs or something. It just sounds like the pressure is blowing out the intake.

This only happens when its really cold... and I can see my water temp is low (like less than 150 degrees) while cruising on the freeway between 50-75mph.

At high RPM, low gears, low load, etc... its running great, regardless of the temp outside.

[ChrisA]

Dont know about the weather differences for you, but the L28 will require more fuel under heavy load than the L24. I had to increase the fuel delivery on my setup (same as yours L28 w/SU's). The float bowls on my car were running dry and the car would buck and lurch and have no power. Once I tweaked the floats and the mixture and made sure I had good pressures, then I had no more problems.

FWIW,

Chris

[Hugh 240z]

Well, cold air is more dense, making more power... requiring more fuel... but I don't know anything about carburetors, so I don't know if they compensate for temperature changes like EFI or not?

[ChrisA]

Well, cold air is more dense, making more power... requiring more fuel... but I don't know anything about carburetors, so I don't know if they compensate for temperature changes like EFI or not?

Give www.ztherapy.com a look see. They can help you. The carbs wont compensate for anything except your right foot!! I still think its a lack of fuel issue but you never know.

Chris

[Hugh 240z]

Thanks a lot, I'm reading there now.

[Daniel]

Welcome Hugh, Saw your post # was low. The cross your fingers easy fix would be the heat to the carbs. Are your running a factory air cleaner housing? If so you'll see the location that the foil type duct attaches. If not then you need to try to get some exhaust heat to the carbs. The too easy is to change your fuel filter. I've had it work. The next question would be is the fan clutch locked on and over cooling the car ? huh?

[Hugh 240z]

Thanks for the welcome. I've been surfing around trying to find some people who understand the SU setup for help.:stupid:

I don't think the clutch is locked, because I don't hear the fan all the time like as if it were locked up.

The fuel filter could seriously need changing... I will give that a shot.

The air filter housing is some sort of aftermarket piece. It doesn't have the orange duct thing on it... its just an oval filter with a top cover that holds it on with a wing nut hanging on the side.

It also has an aftermarket header on it, so I don't think there's anything that routes exhaust heat back to the intake.

My problem is I don't want to invest any money into repairing this engine, since it will be coming out in the next few months. So the cheapo fix sounds good to me... I'll see what I can come up with.

[e_racer1999]

what part of so cal are you in?

[beandip]

If the air temp is cold and the engine temp is lower than 150 try adding some choke. It is most likely starving from a lean mixture when cold . Keep the choke on untill the engine is warmed up . The backfire could be a ''lean pop'', common if the mixture is lean and you open the throttle wide open. Another thing use some 20w in the carb dampeners ATF is 5w and it also causes a lean mix. 20w is what the factory recommended when new, it slows the rise of the piston in the carb slightly and this results in a richer mixture. The SU carb has no accelerater pump to shoot extra fuel into the airstream when the throttle is opened wide. It relies on retarding the rise of the piston to richen the mixture to accomplish the same thing. This is only for a 1/2 secound but it is sufficent. Try the 20w. My 2c Gary

[e_racer1999]

^^ what gary said is absolutely true. ATF is what a lot of POs put in the carbs because they heard somewhere it is good.... i switched over to 10W30 w/ some 30W tossed in there (just go for 20W) and it works MUCH better.....

[Hugh 240z]

Oh, I'm in Oceanside, let me go fix my location now...

[e_racer1999]

oh... if you were closer to SD i would try and help you out.....