Hard Starting When Cold...

[240Z240Z240Z]

My car tends to buck and pop when I try and start it.

In the morning I pull the choke on full, turn the engine over and when it finally fires it sound like it's only running on one carb, then the other one kicks in and the RPMs jump so I bring it down with the choke and the thing flutters and dies. This happens a few times until its able to idle with the choke on. Generally, at start up it's either over rev'ing, or stumbling until it gets going well enough to keep it running high RPMs with lots of choke.

Everything is in good operating condition, I just suspect something is out of adjustment, or needs upgrading. I just installed a Pertronix unit and that doesn't seem to have any affect on helping it start any easier.

I have to wonder if the fuel in my float bowls is too low, or if my mechanical fuel pump is not delivering enough fuel. Whatever the case, something seems out of whack.

Any ideas ?

[xray]

I think it's worth checking the float bowel fuel level, make sure the choke works properly without binding, but also check your plugs. I experienced some similar symptoms awhile back, and after cleaning and regapping the plugs the cold starting/bucking/popping was resolved.

Did it have the same problems prior to your Pertronix install?

[240Z240Z240Z]

Did it have the same problems prior to your Pertronix install?[/QUOTE]

Yes, it did. I should check the float level again, but it's almost like the mechanical fuel pump doesn't put out enough at starting RPM's to ensure the bowls stay full, and as a result the thing dies. The pump isn't that old and I would gladly convert it to electronic it that would solve the problem. Is there any conversions kits that blank off the old pump mount adn provide a place to mount a Facet electronic pump ?

[d240zx2]

35 years ago, Doug, I found the mechanical pump to be limited in output. I removed it, built a blanking plate out of 1/4" mild steel plate (a bit of overkill, I know) and installed a Stewart-Warner thumper pump at the tank. It's still "thumping" and delivering fuel to triple Mikunis at about 5 psi under any throttle condition.

If you decide to do something similar, you'll need to run a hot wire to the pump and insure that the pump mounting hardware is grounded well. I also blocked off the fuel return line to insure adequate pressure, but I'm not sure if that was really required. Again, a bit of overkill, but good fuel pressure and never a problem with vapor lock.

FWIW.

Frank

[beandip]

Your Z is prewired at the factory. Just look up the harness for the sending unit at the tank. You will find two wires taped with blue tape ,green is hot black is ground. Next to the console where it meets the dash on the passenger side , you will find a in line fuse . Green wire is the hot.
Now I doubt the mechanical pump is your problem. When fuel pumps fail it is usually first noticed when the engine is at higher RPM or when pulling a grade. Have you balanced your carbs and adjusted the mixture. Sounds to me your engine is running lean and the carbs are not balanced. I have been running a electric RX-7 fuel pump for years . They put out 5 psi and enough to supply a set of triples on a stroker. $15.00 at the local u-pull-it. Silent also, just make sure it is a pump for a carbed engine, not injected. You can run both pumps if you wish. The electric is a pusher pump. It must be mounted back by the gas tank. Gary

[240Z240Z240Z]

I'm going to try adjusting the floats and some other bits and see if I still have the problem. If so, I'll temporarily hook up an electronic pump and see if that fixes the problem. Thanks for the ideas, I'll report back when I know something new!

[JonnyRock]

Sounds to me like it's running too lean and you need to enrich the mixture. Does your Z have the round top SU's?

[240Z240Z240Z]

Sounds to me like it's running too lean and you need to enrich the mixture. Does your Z have the round top SU's?

I suspect the same, its too lean.

I have the 4 screw dome top SU's.

[JonnyRock]

All you need to do to enrich the mixture is turn the nickel-sized knob on the bottom clockwise. Do it while the car is running. There's a little nub on each one so you know how far you've turned it. Start with a half a turn on each, and proceed until it's running well. Then dial them back until the mixture is just barely too lean. The engine will bog and hesitate when you try the accelerator. Then simply enrich them one quarter turn and off you go!

[240Z240Z240Z]

All you need to do to enrich the mixture is turn the nickel-sized knob on the bottom clockwise. Do it while the car is running. There's a little nub on each one so you know how far you've turned it. Start with a half a turn on each, and proceed until it's running well. Then dial them back until the mixture is just barely too lean. The engine will bog and hesitate when you try the accelerator. Then simply enrich them one quarter turn and off you go!

I'm good at tuning them and have a Unisyn, 3 Colortunes, vacuum gauges and everything. I just need to get the floats set right first, or I'll never get it to start and accelerate well. I have a discussion going in the Carb part of this forum currently....Thanks.

[240Z240Z240Z]

Follow up,

I discovered several problems and I've got it pruning pretty good now. I found two main problems, a vacuum leak through the brake booster pipe, and the carbon electrode in the center of the ignition cap had a corroded spring. It was rusted and it just fell apart when I pulled on it. I'm still trying to get the floats set correctly, then its just a matter of getting the carbs dialed in and the ignition timing set to the sweet spot. I think I'll have to buy some new wires and plugs, any suggestions ?

[beandip]

One thing that will cause a stumble when coming off idle is if you are using ATF or similar thin oil in the carbs . This causes a lean mixture just coming off idle. Switch to 20wt and this will eliminate this . You may have other things going on but this will help a lot. ATF is 5wt too thin. The light oil causes the vacuum piston of the dampener to rise quickly. This is the opposite of what the dampener is designed to do. Gary

[240Z240Z240Z]

One thing that will cause a stumble when coming off idle is if you are using ATF or similar thin oil in the carbs . This causes a lean mixture just coming off idle. Switch to 20wt and this will eliminate this . You may have other things going on but this will help a lot. ATF is 5wt too thin. The light oil causes the vacuum piston of the dampener to rise quickly. This is the opposite of what the dampener is designed to do. Gary

Agreed. I only use 20wt.