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Discussion Starter #1
I have a 74' 260z with 72' SU carbs. The car can be a bear to start when cold .....winter days especially. It actually helps to pump the gas while the choke is engaged in starting it and even then may take 3-5 attempts. Once started it runs great and I have no other drivabilty problems.The choke appears to engage/disengage normally.Any ideas? Thanks in advance!
 

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I don't believe SUs behave like other carbs with respect to adding fuel for cold starting by pumping the gas. Holding the throttle a bit open while cranking should help, but pumping won't do anything - there's no accelerator pump to squirt-squirt.

The choke on an SU simply lowers the main jet to give a rich mixture. You may want to check the spec on how much this travel should be and verify that this is working. Don't remember offhand the details for this setting. Maybe someone will help - try posting more specific question on this.

Check Ztherapy.com "Just SU" video for total carb tune-up instructions.

Later - Al
 

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Actually the choke mechanism does open the throttle just a bit to allow a faster idle with choke. The controlling mechanism is the 'dogleg' shaped link on the side of the carbs. This may be misadjusted in your case. With choke full on you should have a fast idle around 1500 to 2000 rpm. This may be why when you open the throttle by pumping you get to start easier. Also may take a while for pressure to build or gas to get to bowls. Are you running both elect and mechanical pumps? When I had both I started easier. After switching to electric, I now wait a bit for pressure to build after switching 'on'. Also might want to try a bit heavier oil (like stock 20wt) in the dampers for cold weather. ATF and Marvel may be ok for hot weather but may cause excessive leaness for winter. After playing with different oils for almost 30 years, I have finally gone back to straight 20 wt for all year long. Less lean pop during warm up and easier start/idle. I just couldn't justify the less than desireable warm up and driveability of the thinner oils for a little bit of performance enhancement when fully warm during warm weather. That's my story and I'm sticking to it!
 

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I have a 260z that gave me the same trouble and to put it simply my only trouble was that my choke was not closing enough to cause it to start. I found this out by disconnecting my choke cables an manually closing them one morning during a frustrating attempt to start it.

Hope this will work for you.

Rebel Z
 

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Discussion Starter #5
Thanks for all the advise! It does idle around 2k when choke is on and im only running the mechanical fuel pump.(I was told the electric one was redundent and unessesary). Ive had a 71 240z and 2 other 260zs in the past and never had so much touble starting them when cold. Again,once mine is started its fine..Im going to burn up my starter if this continues!
 

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Pumping Gas Does NOTHING!

Please read the post below on "how to use a choke" or "How do I use a choke".
Pumping the gas on an SU does ABSOUTELY NOTHING as there is NO accellerator pump.
The way you get started is that your "Redundant" electric pump starts pumping gas to your percolated-dry float bowls before you crank the engine. By using the electric pump, you prime the mechanical unit, and that's what it's there for.
By cranking and cranking, you are drawing fuel by dry-pumping the mechanical unit, eventually filling the float bowls and letting the car start.
Reconnect your electric pump, and don't listen to whomever told you to disconnect it in the first place. It's there for a reason. Otherwise they would've saved the $14 it cost to buy it and put it on in the first place!

Also, opening the throttle during starting further than the lever opens it, decreases intake manifold vacuum. Decreased vacuum decreases the ammount of fuel drawn up the main jet, causing it to run leaner, making it harder to start.

Common sense and a little research will bear these statements out, if you keep an open mind.
 

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Re: Pumping Gas Does NOTHING!

I had similar probs with my UK 260 which has 240Z carbs on it (and K&N's). Agree that pumping gas does nothing more than make your leg warm - no acclelerator pump y'see. I don't have an electric pump but will be fitting one as soon as I can - the benefit in pressurizing up to the mechanical pump has to be worth the small outlay!

I actually sorted my cold-start/choke problem by working the choke verrrrrrry slowwwwly and watching what the jet did. I found that at the limit of the choke (ie: fully on), the jet was as low as it would go and the butterflies cranked open a tad to raise the RPM. A bit of fiddling with the screw that operated the butterflies at this point worked wonders. It took a few attempts to get right, but it really improved the way the car started and ran when the engine was cold.

TL
 

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Discussion Starter #8
Re: Pumping Gas Does NOTHING!

Thank you!
It DOES make more sense to me that the electric fuel pump has a purpose.It also makes sense as to why it takes so long to start.
 
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