Has anyone else had this happen? Over the past few days, after the car gets (very) warm, when I slow down to idle, the engine just dies, unless I keep my foot in the accelerator. Normally, the idle is fine, right at 1000rpm. The carbs checked sync'ed. And everything runs fine until I get it really warm and it is in the HEAT of the day. Then it gets tricky. When it gets to this point, even when it's not at idle, if I get on the accelerator, It feels like a few cylinders are cutting out; almost like it's flooding. Then I get the occasional backfire. It never backfires. If I take it easy on the accelerator, it pulls fine. Shut it off for awhile and let it cool, everything is fine. I've never had this happen. Has anyone else? What was the fix?
have you done anything to the car recently that resulted in this behavior or did it just start happening?
What do you mean by hot. I see you have a 79 so asking the standard questions like , still have points , still have cats , how old are the spark plug wires , what plug heat range are you running , when were the carbs sync'ed last and what has been done to the car lately.
I see you're from the Houston area, I'm in Dallas. IMO, occasional vapor lock seems to be a hazard of driving with a carbureted engine during the very hot months here in TX. Over several incidents in my 330 2+2, I observed that it always happened when I restarted the engine after it was fully warmed up and then drove off quickly. The problem disappeared after I began to let the engine run a couple of minutes before driving after a shutdown and restart.
Recently: Re-installed AC. Have been fine tuning it (in the drive-way at idle - 3000 rpm for long periods while adjusting freon levels in relation to head pressure for the optimum amount of coolant (pressure v. maximum Temp change). Also new fuel/vapor lines. New fuel filter. Good pressure from Fuel Pump. Checked the sync after I finished the AC. Checked to make sure all cylinders firing--they are. Idles at 1000. No points. Less than 500 miles on wires. BP6ES (from memory, I'd have to go check, but there is about 1000 miles between me and my car right now). It has cats, no air pump system/manifolds. Running 135 mains, F36, 200 ac and 55 idle jets. Temp outside has been running mid 90s. When it starts running rough (after running about 30-45 minutes) at idle, it tends to backfire. Again this hasn't happened since I first got the car and had my first lesson in dual distributor systems (timed the rear cam bank to the PM5-8 marks, doh!). And I havn't messed with timing at all lately, other than checking to make sure it is still on. Is it possible the fuel return line from the last bank of carbs is running too close to the AC compressor to condenser line, therefore causing vapor lock? I would think that any additional heat would have to closer to the fuel feed rather than fuel return to cause this. Thoughts?
90% of "carb problems" are ignition!! It sounds like you have a flaky plug wire and you are losing a cylinder intermittently, which would create the backfires and the engine slow down. Was the idle adjusted with the car GOOD and warm? They always idle slower when cold and when really hot than they do in the "sweet spot" of engine temperature (195--the middle of your gauge.) I just put new wires on my car because I had a misfire. The ohm meter said the offending wire was fine, but it was cross-talking to another wire due to breakdown of the insulation on the wire. They are only 3 year old 8mm Taylor wires, and I had a hard time accepting a bad wire theory until I swapped wires between cylinders and the misfire moved along with the wire to the other cylinder. High voltage is weird. It doesn't always act the way you think it will based on the resistance of the wire at low voltage. DO NOT assume it's not your wires just because they are new. Pull the airbox cover off, and try to figure out which cylinder is popping through the carb when this happens. That will tell you which wire is suspect. You can hold your hand right over the carbs and you can feel the popping back through the throat of the carb. Birdman
Vapor lock can ONLY happen in a diaphragm type fuel pump (which yours is not), not in the carbs, not in the fuel line to the carbs, possibly in the line in the fuel line to the fuel pump IF the fuel inlet at the tank has a restriction, but that is really rare. Doug
Idle was not changed. It is backfiring through the exhaust, not popping through the carbs. I will check the wires again, but it is definitely firing. (I can hear the popping when I pull a HT lead off and it is still close enough to jump the gap. Using gloves and standing on rubber mat, of course.) sounds like I have some more troubleshooting to do when I get home...
I want to resurrect this old thread. Was the problem ever found? I have an old Porsche 911T that is doing the exact same thing.
Actually the car has new plug wires, newly cleaned up carbs (Zenith), well balanced, new point, condenser, and dizzy cap, tuned with Colortune to be blue (instead of yellow), no popping when running well. Just that when it is uber hot, it stalls when coming to a stop.
I know this is getting off topic a little, but I just spent a bunch of time fixing "weeping" problems on my 356's Zeniths. I won't got through it all, but turns out the float level was too high (even thought it always measured right on spec) ... Because of the pressure drop in the venturi, a carb runs colder than ambient, but when the flow slows or stops, they heat up quickly. Gas boils at like 95deg so if the float level is too high, it will overflow into the throats resulting in a very rich condition. Try an additional washer under the needle and seat valve in the top piece to lower the level and see if that helps. Is your carb wet at the base 45min or so after turning it off? I had the same problem with a hot idle - especially if I shut the car off for 15 min or so. A warm start was difficult and took a bit to "clean" it out. It was rich. If I sat at a light idling for a bit the idle would slow and slow enough that I had to pull the idle stop out a bit. Now it idles on the tick regardless of temp (after warmed up of course). To the OP - As mentioned above, you can only have vapor lock if you are pulling fuel, not pushing it, so that would be the short hose from the tank to the pump. After that, it is all under pressure greater than atmosphere. Pops out of the carbs say lean, out of the muffler say rich. The easiest way to check for bad plug wires is to drive to somewhere where it is pitch black and open the deck lid (with your lights off - be careful of traffic!). Any cross talk you will see as blue flashes. I found one on the 356 coil wire on 3 year-old wires. You'll also hear the dull "puhh" miss. Yelcab - feel free to PM me if you like. This is still pretty fresh for me. Franny
"Just that when it is uber hot, it stalls when coming to a stop." I know this is not directly applicable but US cars traditionally had a dashpot on the carb for exactly that reason. Stalling when coming to a stop was typically a bad dashpot. I know Ferrari's didn't use one...maybe they should have!
It prevents the throttle from closing instantly when you release the accelerator pedal. It has a diaphragm and bleed hole so that when you close the throttle, the carb butterfly closes to basically a high idle position, then, as the air bleeds out of the dashpot, the throttle closes the rest of the way to the normal idle position. They were common in carb cars. Here's a pic of one: http://www.ratwell.com/technical/DualCarbs/15-AdjDashpot.jpg
Mike, I think you're on to the solution here. I experience the same problem but only when the temperature outside is above 86 degrees and the car gets really hot in traffic type situations.
Actually, 1978-79 US 308 do have a dashpot on the throttle linkage mechanism -- item 53 here (which gets acted on by adjustment screw 45 in the bottom of arm 23: Image Unavailable, Please Login If the OP's '79 is a US version it's something to check for proper operation/adjustment (or presence )...
Just to add to the database: Definitely doesn't work like it should. It was great for several months then one day it turns on me. Haven't driven the car because it want's to stall. Been checking lines, replaced lines, etc. No change. Simple example: Start the car and idle speed by tach is a little over 1000rpm. Rev engine to 3500 and lift off throttle. Engine then drops below 500rpm and recovers but just barely. On the road it will stall under braking. Others have had similar issues and suspect vacuum leak. I haven't found one yet.
Have you checked your compression on either bank? could be one bank (yes all all four cylinders) is low
A couple of points when synchronising the carburettors. When you set up the carburettor idle air flow ( with the air bypass screws fully in ) there must be no tension on the carburettor linkages from front to back. The carburettor throttle blades should always be just cracked open on the stop screws not just held open by the linkages when at idle. Good luck Rob
If the two banks are off significantly, Your timing belt is off by a tooth. This can happen as there may be some Slack It's a thought if you have tried everything
