Need help getting my car to Idle properly

Check for any vaccum leak as this will cause to idle high.

 

Although your DCNF carbs may be syncronized (i.e., all barrels providing equal mass airflow at idle), have you confirmed that the air bypass screws are opened as little as possible to equalize the two barrels on each carb (per the usual instructions, either both should be fully closed on each carb, or one closed and one partially open to match the closed side)?

As a side note, 4 deg BTDC is not a lot of advance at idle for a "R1 only" set-up -- you could/should easily run OK at the stock 7 deg BTDC (although this will raise the idle speed even more).

PS Are these the replacement 40DCNF12 carb (the amount of idle screw opening would indicate that these are not the stock F units)?

 

Check your timming Make sure it is right on the $$$.

Then!!!

Your idle mixture screw should be set with an o2 sensor, as 2.5 turns is a starting point.

Your throttle stops ????

These three will affect idle too.

Good luck!!

 

Craig -- Comments understood. Are you confirming that both banks are at 4 deg BTDC at idle (i.e., the physical relationship between the two Pertronix units is OK)?

 

You need to go back over your carbs again and make absolutely sure they are throttling down. Tripple check your throttle cable and linkage rods, make absolutely sure they are not holding the carbs open, or in some way hanging up somewhere. Unless the carbs are "all" leaking internally, if you close all four carbs down with the speed screws, regardless of ignition or anything else, it wont even run.

 

Craig I have Don's set up on my 77. I have no problem bring it down to 700RPM at idel if I want to but I have it adjusted to 1100RPM which gives me 1000rpm with the AC on. Also I am at 7*BTDC as told by Don. I check both dizzys for timing just to make sure. Also I assume Don did the Pertronix's set up in 1 dizzy so the 2nd dizzy needs to be set also per Don's recommendations. So at 1500rpm if you are reading 4* BTDC then at a 1000rpm you would be at maybe 0*. Also check the advance as you rev the motor and see if is working correctly. Next I would disconnect all of the carb linkage. From accelerator cable to linkage from each carb. Also disengage the fast idle cam so it does not work and back that screw all the way out. Disconnect the choke cables and make sure that the chokes are closed. Then start it up and see how it runs. Also back out the idle stop screws and give them a 1/2 turn in. Start it up and see what you get. If you are still at 1500 then I would take some Carb cleaner spray and go around each carb and see if the idle increases looking for a vacuum leak. Buy the way disconect and plug the brake vacuum and carb vacuum hoses to the smog stuff (mine had been removed) . See what you have and go from there. Buy the way my air mixture screws are somewhere between 6 and 7 turns out and not the 2 1/2 that the weber books talk about. My car runs rich at idle but runes like a raped ape up to redline. I run NGK 5 ES which for my driving runs great in city and highway speeds. Kep at it and you'll get it. Buy the way are you sure that the accelerator peddle cable is loose at idle. You should be able to move the cable with your finger without moving the throttle linkage. Good luck and let us know how you make out.

 

I had a problem with getting my idle down to 1000 RPM and found that I had a leak in the manifold. The previous owner was using coolant that had some phosphates in it. The result was corrosion between the manifold and the head. I ended up putting material on the maniflods and then milling it back to spec.

Later I had a problem with getting the idle down and it turned out to be the carbs were running too rich. I screwed the mixtures in a turn and now the idle is down around 6 to 700 RPM's. I also put in a set of colder plugs and regapped them at .020. They were gapped at .030 before.

My car is a '77 GTB. This is probably not your problem, but it's worth checking out. Good luck with it.

Bob Wassam

 

Nautique
If your throttle stop screws are all the way out and not touching anything put your finger into the carb throat and push the throttle plate towards the closed position. Watch to see if there is any movement of the plates or linkage levers. If so the plates are not closing completely and may be hanging up on the plates, plate shaft, or linkage levers. This interference should be eliminated. It would be best if the linkage was disconnected. If you are going to try the carb cleaner to check for vacuum leaks be sure to use the thin tube that usually comes with the can. Also shield the intakes of the carbs. It is too easy to get stray cleaner fumes into the throats of carbs which would give a false reading. Best to allow the air to clear after each test spray as the cleaner is volatile and lingering spray will go into the venturi. Also as someone mentioned above you may be a little rich on the fuel/air mixture screws. You might try turning all of them in an 1/8 turn to see if the rpm's drop. Then maybe another 1/8th. If all are adjusted equally then you can always turn them back to the original position if necessary. Watch you don't get too lean. Good luck.

 

Some out may call me crazy, it won't be the first time but here goes. The way I pinpoint vacuum leaks is to take a propane torch, remove the nozzel and attach a two foot piece of hose that will slip over the tip. Crack open the valve on the propane bottle and point the end or the hose at suspected leakage areas. It works better that carb cleaner at locating leaks because as soon as you remove the hose from the area the leak comes back. This must be done in a well ventilated area, but I have done this for many years and find it to be very effective at locating vacuum leaks.

From experience, another area to check is the bushings where the throttle shafts go through the carb housings.

Also, have you verified your RPM readings with another tach. My 79 GTB reads about 1350 on the factory tach when the engine is at 1000.

 

Craig
You said you have had your 308 for 8 yrs so I assume it idled correctly at sometime. The list of items you noted in your first post... were they done before or after the idle changed? You also said you repacked the throttle bearings and replaced the manifolds to carb gaskets. Did you rebuilt the carbs? Sometimes the small lead plugs in the body of the carb can become dislogged and needed restaked. If they are out or loose this causes a vacuum leak. Did the car just start having the idle problem all of a sudden? What might have been done to the car that may have caused it to act up? All the suggestions the guys have given to date are good ones and should have help to detect the problem. Did you check the fast idle cam between the carbs as Steve suggested? If it idled OK before then it will idle correctly again. Think positive.

 

Craig,

i have had a few carb 308s. i am running dual Crane XR3000's in one dizzy. Crane reccommends at least .50 or.60 plug gap. i opened my gaps up and the car pulls better and idles smoother. i have found BP5s to be bit hot. i use bp6's in my 77 gtb. with all that you have said, i would be sure to disconnect the linkages and be sure there is no binding in the joints. i have had to replace several of the the ball ends on my carb linkage on all 4 308s. it seems they wear out the ball and socket joints and the carbs will bind slightly and hang. driver side carb linkage seems to always be worse for wear. i am not sure why. to see this wear in the linkage you must undo the safety wire or cotter pins and look closely at the joints. it took some fine tuning , but i finally have my black GTB running near perfect for a multi carb car.

i wish you the best and please keep us updated, i would love to be of more help.

 

If you use carb cleaner to locate a vacuum leak, be sure to have a compressed air nozzle or garden hose handy to put out the possible fire. Happened to me with a Jag v-12 when the cleaner wet a leaking plug wire. Big ball of fire. A quick water spray put it out with no damage, but if I didn't have the hose close by, ?. Took a while to dry the engine. Propane is better. Good luck.
Mike

 

OK , progress. So is this a problem with 1 carb or 2 or 3 or all of them. Since you have to remove the bad carb anyway it is fairly easy to check for flatness . Get a good straight edge like maybe a 6" steel ruler and lay it edge wise on the carb base and then hold it up to a light and see if it is flat. Also check the manifold while you are at it. If it is warped then you have a few options. You can have it machined flat or you could make a thin gasket that you could put in between the carb and the thick base gasket. So check the flattness 1st and go from there.

 

I would highly recommend pulling your intake manifolds too while you are in there, they most likely look like this underneath. You'll sleep better at night.
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Most likely it was caused by antifreeze with phosphates in it. Bob Wassm

 

For your information.

I've been talking to people in the auto technician business and have found that most people use brake fluid to find vacumm leaks. I have been burned by gasoline so the idea of spraying something on a running engine that could flash scares the heck out of me. One person I know told me that he uses a water spray bottle to find leaks. He says that water plugs the leaks and when that happens the idle gets stronger and then you know where the leak is at. Another person told me that he uses a stethoscope to find the leaks. He mentioned that it's really easy to find it that way. Still another person told me that if he believes that the leak is around the manifold he'll spray a little oil around the bases and he can see the oil being sucked into the leak. Oil doesn't flash at engine surface temps so it's pretty safe.

Bob Wassam