308 carb problem, idle and deceleration

Is there a deceleration valve? This may just be a fuel injection part only, though. Carb linkage operating smoothly & returning to its stops?

 

It sounds like your linkage is binding and the butterflies are not completely closing. I think it would be impossible to run 3000 rpm with closed butterflies, so using a mirror and someone else on the throttle pedal, check that all of the throttle butterflies are equally opening and return to completely closed on their own.

The possibility that it is your ignition advance not returning to the original setting is also possible, but less likely - ensure that the flyweights and springs are all working freely and lubed up well.

Good luck and let us know.

 

Could it be carpet or mat stuck under the pedal, my mat used to always get hung up, keep it back now with mat clips.

I always check the stupid stuff first..........

 

With the linkage disconnected and the car warmed up and idleing, hand operate each carb separately to accelerate the motor. Release the throttle lever quickly and the one that doesn't decelerate is the one that may be the culprit.

 

Looking at the '78 Owner's Manual, pages 34 & 35, Starting The Engine. First, is the choke released (blue spot)? Second, it states that the carbs are connected to a fast idle device that is controlled by engine water temperature, so is that system operating correctly? Maybe your water temp. sensor or wiring is bad.

 

If this were the situation, wouldn't the engine idle kind of lumpy, shake a bit, maybe? Since one cylinder is getting more fuel than the others.

 

Check the WUR screw adjustment between the carbs. I had messed with mine and it was actually sliding past the base it was supposed to hit, then catching on the screw threads on the way back, holding the throttle open.

Also, check that the cable moves freely.

 

Toby
That's a good question and my first reaction is that mine didn't have the "lumpy" idle you suggested. One of my carbs was hanging up and affected the return to 1000rpm's. When I reread this thread I see that Bernie said his idle was 3000 rpm's. Over the past couple of years I've read the posts on idle problems and usually the fast idle problem was more like 1500 to 2000 rpm's. A hanging throttle plate may not cause an idle of 3000, but it wouldn't hurt to check. When a plate is open too far (past the idle circuit) it starts incorporating fuel from the progressive circuit and can cause a higher idle speeds. I adjusted my carbs by the Weber book which inpart says to use the idle stop screws (AKA throttle stop screw) to balance the carbs. If your engine is in top shape this is fine but if there is a noticeable imbalance in the carbs requiring adjustment of the idle stops to compensate... this can open one carb closer to the progressive circuit. Now the vacuum leaks should be corrected but if they are between say the intake and block or if there are compression differences in cylinders then unless your going to rebuild the engine (or carbs) you may have to cope with the cards your dealt. If you use the standard approach to balance the carbs you might for ex. get one carb with 0 turns in of the idle stop, the next could be 1/2 turn in, the next 3/4 turn, and the forth 1 turn so now there is a built in discrepancy between all carb plates. Now if you hookup the linkage properly the differences in the plates are locked into your setup. This is what I did for months and I would get a good idle but when I would blip the throttle the idle would hangup to 2000 rpm's. I finally called Mike at Pierce Manifolds (who balances carbs two days a week) and he said to close all throttle plates all the way. Adjust the throttle stop screws to first contact then in 1/2 turn on all carbs. This is to hold the plates off the throat walls and he does not move them from that point. All plates are now equal during function. Balance among carbs is achieved with the air bypass screws and air/fuel screws. Now this approach may not set well with some members but I got my car balanced and the problem with the 2000 rpm idle went away. I now can enjoy my 308. To be truthful one or two throttle stop screws may have to be tweaked after this approach has been attempted if absolutely necessary but not more than 1/4 turn otherwise the problem has to be found. Just some thoughts for you and Bernie to ponder. Bernie hope you get your problem straightened out regardless of how you tune your carbs.

 

Correction on the above post # 12. Where I said," This is to hold the plates off the throat walls and he does not move them from that point". I should have said...once all the throttle plates are in the same position then that relationship to one another should not change. However to correct the idle speed these screws can be changed as long as they are changed the same amount. Ex. to decrease the rpm's turn all the idle screws out 1/8 turn thus maintaining the same relationship. Sorry.

 

Bernie,
I have that problem when the outside temp is cold, basically a sticking throttle cable issue.
Assuming the car was running ok before this work, it has to be related to the work that was done. Need to go back one step at a time to unravel the mystery.
I would also wonder why your mixture screws are out 5.5 turns, they should be 2 to 2.5 turns if all else is correct.
I would start with ignition timing, check to see what the advance is and that it is advancing/retarding smoothly at time of 'failure'. Once ignition is correct (all plugs firing well and cleanly) then move to the carbs. I'm sure your mechaninc knows this already but might be worth a reminder.

Gerrit
http://dino308gt4.com

 

That sounds way rich to me. I grew up with carbs, but over the last 15 to 20 years with all the fuelie cars I forgot a great deal. This 308 I have driving now gave me a few fits and teething troubles until I opened up books and came here and read, listened, and re-learned.

Initially I had my mix screws turned out two turns. It idled like crap, hunting idle speed, and would hang up around 1500 or so and burble and pop before dropping back down. Plugs were black, exhaust was black and nasty, all indications it was running rich.

But the books and the guys here all said to begin with 3 turns out. Now dangit, its already way rich, how is this going to help? In frustration I tried it. I took all the linkage off so the car just ran on its individual carb idle stop screws. I then started turning the mix screws out in 1/4 turn increments, and as I did I noticed the idle speed climbing. A lot. I didnt keep track, but by the time I got the mix screws out to 3 turns I had probably dropped the idle speed screws a couple turns or so.

Look at a cross section diagram of a carb. At the point where the throttle plate closes, the mix screw has a hole opened into the bore. You will also notice other small holes of varying size above that point. These are idle progression holes. They allow the mixture to stay near optimum as the plates are opened. When the mix screw is lean, your forced to open the throttle into this area to regain idle speed, and its impossible to get a clean mixture. When the throttle drops from high speed, the high vacuum pulls lots of extra fuel in and it takes a while for idle speed to recover.

If your running way rich, you probably have the idle screws farther the other way, so your throttle plates are closed even further. So now the majority of air the engine needs to run is now pulling through the mix screw. The slightest piece of grit somewhere that could hold one carb even slightly more open, and with all that extra fuel that can pour through your way to far open mix screws, the speed comes up. And think about it, 3000 rpm will create much higher vacuum at all the carbs, and will effect them all undesirably.

The point is, there is a direct mechanical and dynamic relationship between the throttle plate position, the progression holes, and the amount the mix screw is open. Set the mix screws at 3 turns and adjust everything else to compensate. Make absolutely sure, before doing anything on the carbs, that the ignition is operating correctly, and your advance is not hanging. A timing light can easily confirm that. The rest of the syncronising work fills pages, and no need to go into it here, just DO NOT turn the mix screws away from three turns until the motor is idling properly through all other settings and adjustments. And once you do, dont touch anything else again. Mix screws are for "FINAL" mixture corrections, not to alter idle speed or syncronization. Always return the mix screws to 3 turns out before syncronization and idle speed adjustment, just like the books all say to do.

 

Just a note: there are two types of fuel mixture screws used on Weber carbs. (maybe more) 1. the short stubby tapered screws which usually take 2 1/2 to 3 turns out and 2. the longer tapered screws which are out 4 1/2 to 5 turns. The longer type require more turns for a response and will give a finer adjustment wheres the shorter tapered ones respond faster to rotation and do not allow as precise adjustments. I was told that the long taper type were Euro ones however my US car was them.

 

On mine, when that happened (2 mode idling), the problem was the float valves were old on a carb causing one of the valves to never close all the way, and excess fuel dumping down the carb to run at 2500 RPM instead of 1000 rpm.

Changing out the needle valve solved that problem. It may or may not be your case.

 

Egads!!! Please keep us posted.

 

What I can tell you about what I did with my 308 carb... is that when I got it:

- idle at 1,300 rpm with A/C comp. on and idle at 1,500 rpm w/o A/C comp. anything lower, the engine just die.
- no acceleration, deceleration very slow, sometime need to pull gas pedel back to normal position.

Now:....- idle at 950 rpm sharp with A/C on, w/o A/C 1,000 rpm sharp.
- very good acceleration, deceleration fast as soon as I let go pedel, decelerate all the way.

What I did was... rebuild carbs, new coils, new cap, new rotor, new ignition cable, Distributer impulse unit, new spark plug, new accelerater cable... and my is as good as new.

Basicaly, the electrical system was old and rotten.

 

Bernie
I mentioned in an earlier post that there were two types of fuel mixture screws, the short stubby ones which work best around with 2 1/2 - 3 turns out and the longer thin tapper which requires 4 1/2 - 5 turns out so it sounds like your in the ballpark (with long tapper). I would think that if your car idles at 800 rpm's, purrs, and leaps and growls then I'd be happy and leave will enough alone. BTW I said that mine had the long taper and that it was a US car, well I forgot that it is in fact a Canadian car. Also someone said the Euro cars had the long taper so maybe the Canadian cars do also and yours has the long tapper screws. If so 5 turns is OK. Does you car have a speedometer in km/h or miles/h? Mine is in km's.

 

What happened to the cylinder with zero compression?

 

Just another thing to check when tracking down a fast idle. Air leaks from under the carbs can raise the idle speed. There are some threads about testing for this.

When I was 'poking around' looking for vacuum leads, I just happend to put a wrench on on of the carb hold down nuts and it was loose. I checked a few more and they were not a tight as I had originally had them when I rebuilt the carbs. Re-torque of all the nuts solved my fast idle problem.