Hey all, I was out performing a tune-up and carb synch today and found a source of constant frustration. I run initial timing at about 12 degrees and have a super-zap msd type ignition system. I get the car nice and hot, clean everything off with carb cleaner, pull the jets and clean those too, synch the carbs so they are all working together, and then comes the idle adjustment. I have always noticed my car has the occasional miss at idle. The engine will purr really well, and every once in a while I hear a tiny blip in the rhythm. Determined to get rid of this, I noodled around with the idle setting today for I don't know how long to try and get rid of it. The idle is really smooth, and throttle response is perfect, but the tiny and occasional miss at idle just won't go away. Is it safe to say this is just the nature of webers messy idle mixture and to just live with it? Or has anyone managed to 100% get rid of this nonsense.
I had the same problem at one time but tuned it out. As you knoe the most difficult part is finding what carb is causing the problem. I narrowed it down as soon as I saw a cut a way of an Ansa muffler. That told me that each bank was not mixed and because one side fed one exhaust tip I was able to determine what 2 carbs were the issues. After that it was a matter of playing with the air/fuel mixture screws. A quarter turn at a time listening for increased idle speed. I finally found the cylinder and by richening up the mixture it took the miss out. Just my experiance but it idles smooth at 1000rpm with equal sound out of each set of exhaust tips. By the way I have a 77 GTB and my timing is at 7* at idle. I have a pertronix set up in my front dizzy and am using Bosch Blue coils. Runs great all the way up to red line.
Many thanks for that. I also have an Ansa sport muffler. It's funny because one side will not miss for 4-5 seconds and then it will miss again. I changed the ignition advance to Euro settings so perhaps the amount of initial timing I am running is making things more difficult. Total timing was set up on a dyno so I know it is right, but being somewhat of a perfectionist with transition and idle I would like to have it perfect. I will mess around with it a bit more
I had a very similar issue just recently, and i traced it to a specific cylinder. After trying everything under the sun, it turns out the idle circuit was clogged. When i rebuild the carbs i left a little piece of lead in the circuit and it was clogging the flow of fuel. it ran great over 4k, but slight stumble on idle. doesnt sound like you have this issue since you havent rebuilt your carbs, but did you pull all the idle jets and blow them clean?
I see you have cleaned the idle jets. Have you isolated teh missing cylinder, or is it a random miss. you might try moving the idles jets around, a mechanic once told me if you overseat the idle jets they can crush down so they flow less...not sure if there is any merit to that or not.
Unless your trying to pass emissions, never adjust timing at idle speed except to get it running initially. You set final timing to 34 degrees at 5000 rpm, and "check" idle speed timing just to verify. If the idle timing is off to much, have the advance re-curved on a dizzy machine. IIRC, idle timing should be 6 degrees BTDC on a euro car. They seem to like to run rich at idle, I turned my idle mix screws out 5 turns and it stopped sputtering. Idled down to 700 and purred beautifully with crisp throttle response. You may want to check your brake booster hose and check valve. It lets in air and leans the mixture on number 4 when you use the brakes, but it there is a fault that cylinder will always be lean.
Thanks guys. Will look into this stuff a bit further. I did pull all the jets to clean them. I even pulled the floats from the bowls and cleaned everything in there really well too. Maybe I will just pull them all apart and scrub the passages with my little weber brushes. Euro cars ran 12-14 degrees initial timing. The advance weights in the distributor did not move quite as much as the other markets. They (S127G distributor) had 13 degrees distributor advance opposed to 16 for others.
Going through a similar problem at the moment with my car. A bit of carbie work and trying a few different ET's etc. I have just upped my idle jets from 55 to 60 as I to had to have them wound out about 4 turns. This works OK for idle but not for the run up to transition. All the advice still says that we should be aiming to have the idle mixture screws out around two turns or so. Anyway, with this misfiring etc I reckon it is more plug problems for me and maybe the odd hi tension cable. The trouble is when we start to tweak the carbs and adjusting idle etc the plugs tend to get fouled and then you have a few problems rather than just a carbie tweak problem. It has really been driving me crazy lately and running terribly rough. Last night swapped out the plugs and the problem seemd to disappear (nearly). Now I reckon I can get back to the carbie tweaking again and when it is all finished I will put in a set of new plugs. Upped my static advance today to 11 degrees. Keep in mind I have P6 cams. Steve
Get a set of hotter plugs to use when doing the carb tune. They do not foul as often and work good in setting the idle and max timing .
That's a good idea. I am running BP7ES right now which I can tell are not getting to the heat level required by just idling. Will throw in some 6s and see how she does.
John, are you sure you have a matched set of DCNFs? It's not uncommon for some prior goof ball 308 owner to have replaced a bad DCNF with a non-matching DCNF. If the DCNFs aren't matching then there will be different progression holes drilled in the throat near the butterfly...resulting in different mixture across the cylinders. Worth checking just to make sure.
I found that 5s and 6s actually cause light detonation at higher rpm. I tried backing off timing etc, but the issue was still there (dyno showed it). I yielded best bhp on the dyno with 7s by a factor of about 6bhp at the wheels. The only issue is the 7s have to be kept hot. The high energy ignition system helps but does not completely alleviate light fouling at idle. Once I have time, I will throw in 5s or 6s and just set the idle with those then make the swap. Once my high-perf engine build is done they should be hot all the time and I might even have to swap to 8s.
Ya know I poked around looking for this, and I am sure all the carbs are the same. I know the one and only mechanic who ever touched this car before me and the original owner very well. However, I did find what I believe to be a common Weber error. My F36 emulsion tubes are not all the same, but all are stamped F36. Two of the tubes look very much like F36s, but the holes are in different places all over.
