very high revving at idle from cold start??

[escapedan]

Ok, I finally got the beast started...definately a home run for the night, but now I have a different situation.

When I start the car, the engine revs very high and stays that way. I have twin SU's on a 1973. This is the first time starting since removing all the smog equipment. I have never worked on an SU carb before but I can tell you that the piston inside the carb is all the way "down". If I manually move the piston up, the engine revs slow considerably and then the engine dies. What am I missing??

I have filled the carbs with SAE20 wt oil and inserted the dampner rods as described in the haynes (useless) book. When the dampner rods are screwed in, it is pretty hard to move the piston up.

As always, thanks in advance. The advice is always sincerely appreciated.

Dan

[Zedrally]

Have you checked your chokes linkages?

[Chino 240Z]

What he said.

Chokes may be pulled on alittle, choke linkage to the nozzles under the carb my need some tweaking, they may be binding and causing the nozzles to be in a choke position.

With the oil dash pot rods tight (or what ever we call em) and about 2 to 3cc's of oil, the pistons are suppose to feel sluggish to push up and will drop slowly. This is the oil working in the cylinders squeezing by the brass bulb in the cylinder giving the piston a soften or dampened rise and fall with more or less throttle. Sounds like they are working right.

Do you know which screw is your idle screw? tight close to the carb, one on each carb. They will be the ones that will open the butterfly in each carb. The car should idle with the butterflies open just a crack, to much will allow to much air flow and will give high idle. These screws only work when it is idling. When you push the throttle the linkage begins to open the butterflies the rest of the way. A uni-syn flow meter is used to measure the air flow of the 2 carbs to help match one with the other. You can survive without one, but for 25 to 30 dollars they are great! You will try to get the opening of both butterflies about the same for a smooth idle, but that doesn't mean they will both be set "exactly" the same amount for best results, one may be a tad more than the other.

Once the idle is okay, you can begin with adjusting the mixture nut under the carbs. Start 2.5 to 3 turns open, adjust up or down small, equal increments at a time until you get the quickest idle before it starts to rough up the idle. Too high will make a lean condition and too low will give a rich condition, you need to play with it for the best result, and again may need to close or open the butterflies a tad with the idle screws you were adjusting earlier, kinda back and forth till you feel it is at its best. After a couple iterations back and forth it should be at a smooth and low idle. Final idle is about 800 to 1000 RPM's.

Here is a easy mixture test: On the underside of each carb you will see a small lifting pin with a spring on it. You will notice that if you push it up and down with your finger the piston will rise up as you push and drop down as you release your finger.

With car warmed and running, carbs adjusted the best to your ability, push the lifting pin up about 1/16" or more:

*PRM's rise and stay up, that carb is rich.

*RPM's rise briefly, then drop, mix is about right.

*RPM's fall, engine get rougher, mix is lean.

It's gona take several readings, some looking at the carbs to find what moves what, and several attempts to get it, until you get the hang of it.

Now Go For IT!

[escapedan]

WOW!!! THAT IS QUITE POSSIBLY THE BEST RESPONSE I COULD EVER HAVE ASKED FOR!!! I'm printing out your advice as we speak and I'm going to follow it to the letter as soon as I get off work tomorrow.

I should have mentioned in my orig post, but the choke cables were removed from the car. I can see where the linkage hooked up, but the cables are not installed....or even there at all. I dont think this will hurt anything as I wont be driving the car in cold weather....furthermore, I live in California and out here 60 degress is considered cold! =) Do I need to secure the choke linkage somehow, or can it be left alone?

I recall seeing the idle screws, but i will make sure I'm looking in the right place. Thats great news about the pistons sounding like they are working correctly, I was really thinking that they were the problem. So the idle screws should only be adjusted when the car is running right?

Also, my mixture screw is currently only 1/2 turn open from the fully closed position, I will open this up as you suggest and start from there.

Thanks again for the awesome reply, I'm determined to learn enough about these cars to be able to reply on this board instead of always posting questions to it!!!! =)

Dan

[Zedrally]

From your reply it's obvious that the choke is your problem.

Whilst you can still drive without it being attached, I strongly advise you to either re-attach them or replace them if they are missing.

This will also increase your fuel economy!

[Chino 240Z]

Hi Dan, Chokes in California may help on cold mornings to get the idle up a tad to help warm up the engine, after which you would push in the cable or turn off the choke. Now I don't get the chance the run my car often only when getting ready to go to the track and it starts, bang everytime, but we'll see this winter? No choke cables here either and the linkage on the carbs are left alone intack.

I have to stop and think, but the cables are attached to the fuel nozzles under each carb. In cold areas a fuel rich car will start better with alittle extra fuel and warm up with a higher idle quicker. When you pull the cable it pulls down the nozzles a tiny bit,this allows a little more fuel to be drawn up from the nozzle in the bottom of your carb. Kinda like temporarily raises the fuel level and riching up the fuel for cold starts. When you push the cable in the spring on the nozzle returns the nozzle back to the normal position. Just like when you turn the mixture nuts on the nozzles, turn up leans, turn down richens, choke on pulls nozzle down to richin. I think if you leave the choke turn on you would be running rich and using more fuel, probably not running good either. But not having the choke cables won't hurt things.

It is nice to keep parts on the car if you want to keep it's value and at stock condition. I like that idea of keeping anything you take off or don't want to run with, incase you sell the car. But for racing or just goofing around with, if it don't need it? I take it off.