SU's got me stumped

[steve91tt]

I have a pair of Z Therapy carbs on my 240Z. These carbs worked perfectly until I took them off to rebuild my engine (2.4l to 3.0l, same head). Since I reinstalled them on the new engine the float on the front carb seems to be sticking.

I followed the cleaning and alignment procedure in the Z Therapy DVD 3 times over and still the same result. The carbs are clean inside and out and seem to align fine in the garage. The slides move up and down and return to the base of the carbs with a nice mechanical clank. Both slides fall at the same rate and the carbs are synced at both idle and 3000 RPM.

The car runs great 90% of the time. Strong idle, excellent throttle response, good power etc. However, once in a while the idle goes lumpy or sometimes the car dies. When this happens I can take off the air filter move the front slide up and down a couple of times and all is back to normal.

Any ideas?

[olzed]

Did you fit different needles to accomodate the greater requirment for fuel of the 3 litre over the 2.4? You may be using the same head but the larger engine needs more fuel/air. I would doubt from what you describe that there is anything wrong with the carbs.

[steve91tt]

Did you fit different needles to accomodate the greater requirment for fuel of the 3 litre over the 2.4? You may be using the same head but the larger engine needs more fuel/air. I would doubt from what you describe that there is anything wrong with the carbs.

Same needles. The car runs very strong 90% of the time and very badly 10% of the time. When it is running badly the issue is at idle or light throttle. I would have thought that if the needles were the issue then the problem would occur consistently and at higher throttle. No?

[olzed]

Does it run better with the chokes on a little.

If so it's running lean and you need to think about different needles.

3 litre needs more fuel and gas than a 2.4. What colour are your plugs?

Hopefully Bruce from Z Therapy will chime in here.

[madkaw]

Well it's not the floats if you can remedy the issue by moving the PISTON. Sounds like the jet is not centered and the PISTON needle is getting hung-up. You need to call Bruce and see about getting the jig they use to center the jet in the carb------IMHO:)

[steve91tt]

Does it run better with the chokes on a little.

I use the choke to keep it running when it's acting up but I don't think it's a mixture thing as much as a way to keep the RPM up. The plugs are new and a nice grey brown across all 6 cylinders.

I think it may be temperature related. It started to get cold here in Texas about the same time that the trouble started. We had frost this morning and the car ran really badly all the way to work. It stalled every time I took my foot off of the gas (without choke). However, as soon as I pulled into the parking lot it started running normally. I experience the similar when I work on it in the garage. It usually runs great with the hood up and I'm working on it. When it fails it usually does so during a test drive.

[Gary in NJ]

Well it's not the floats if you can remedy the issue by moving the PISTON.

That's exactly what I was thinking. A cleaning of the nozzles and perhaps new needles are in order.

[steve91tt]

That's exactly what I was thinking. A cleaning of the nozzles and perhaps new needles are in order.

I spoke to Z Therapy and Rebbelo about the needles and they both agree that the ones I have (SM?) should be fine for this engine.

I cleaned the needles, slides, domes and carb bodies but dirty nozzles is not something that I have considered. Good suggestion.

[Bruce Palmer]

Steve,

What's the humidity like right now.

Sounds to me based on what you just said that the carbs are icing up. Things get cold and humid at the same time and the air rushing thru the mouth of the carbs will ice up reducing the size of the venturi and your mixture goes fat and the car runs like hell. You pull off the freeway to take a look, and turn the motor off and before you can get the hood up to see what's what the ice has melted and the motor sits there running fine laughing at you. Ask me how I know this.

I'd say take the air cleaner off and drive around for a bit with the hood popped. Stop and quickly lift the hood and look for any evidence of ice in the throat of the carbs.

This is the only thing that I can point to that would be not constantly going on.

[Gary in NJ]

Bruce, doesn't a '73 have the winter/summer flap on the airbox. Assuming that it's connected and properly functioning, that reduce the possibility of carb ice. Steve, have you tried the flap?

I know from flying that carb ice can occur in temperatures as high as 80 degrees if there is high humidity. As a SOP, we add carb heat every time we are at 1/4 throttle or less.

[steve91tt]

Steve,

What's the humidity like right now.

Sounds to me based on what you just said that the carbs are icing up. Things get cold and humid at the same time and the air rushing thru the mouth of the carbs will ice up reducing the size of the venturi and your mixture goes fat and the car runs like hell.

Very interesting. I never considered the possibility of ice up. The humidity is currently 74% and the temperature is 48°F.

The summer/winter flap thingy is not currently hooked up to the manifold. I never really thought that it would qualify as winter weather here on the gulf coast.

[FastWoman]

The humidity is currently 74% and the temperature is 48°F.

That would make it a pretty cold, dry day in Houston!