Confucius Say....Carbs--WTF???

gregthall

gregthall

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Backstory: '71 2800 CS Stock Zenith Carbs, Stock Distributor, NO Pertronix.

So, I have finally got my coupe purring like a kitten. When cold, there is no hesitation present, I can give it gas and she immediately responds....but when warm (operating temp) from idle there is a slight gulp/suck when I hit the gas. It will not stall, but sucks a little and then responds. This is a bit disconcerting when you want the uninterrupted power in first to accelerate from a stop. It is NOT noticeable in 2nd through 4th. Like I said, before the thermostat opens it is fine, but once it is at operating temp it stumbles. I have resorted to gassing it a little before I start. Is this the "water" induced choke?
 
How about accelerator pumps? When cold the engine is rich due to chokes. When hot and chokes off it needs the squirt. Look down each carb's primary barrel to see if you are getting a good spray from both when throttles are opened off idle.
 
I will check in the morning.... but interestingly enough-- the car that would start with barely the depress of an accelerator, has been very sleepy at the spark...literally since I have noticed this glitch. It is a FULL depress, half accelerator until idle-- I was just attributing that to the change in temp and humidity-- much like the piano I need to have tuned.
 
Carbs

Also, when checking the spray from the accelerator pump nozzle, make sure that the nozzle is pointing in the right direction and position. The tip of the nozzle should be 10 to 11 mm from the outside edge of the venturi, 1.5 mm fromthe top edge of the venturi. The spray pattern should be a nice conical spray ( not a dribble or straight spray, this is for good atomization ) this spray should hit just below the outside edge of the venturi just below the tightest point of the venturi. There is a measurement of how much it should be but as long as you can see a decent spray, you will be OK. To check all this look down the primary barrel and push the accelerator all the way down.

If you are not getting a good spray, it may be your accelerator pump is faulty or the nozzle is plugged. Watch that the nozzle is not loose, this happens some time

This would bethe first thing that I would check

Staring first thing inthe AM should be full throttle and then let off, fast idle speed cold should be around 1800-2100 rpm. When hot , the car requires some throttle, each car is a little different as to how much

Thanks, Rick
 
and this is why I would never convert to carb, just too complex for me. i would totally be in the hands of mechanics

the sound of carbs and power when tunes perfectly is great but....
 
gregthall said:
Backstory: '71 2800 CS Stock Zenith Carbs, Stock Distributor, NO Pertronix.

So, I have finally got my coupe purring like a kitten. When cold, there is no hesitation present, I can give it gas and she immediately responds....but when warm (operating temp) from idle there is a slight gulp/suck when I hit the gas. It will not stall, but sucks a little and then responds. This is a bit disconcerting when you want the uninterrupted power in first to accelerate from a stop. It is NOT noticeable in 2nd through 4th. Like I said, before the thermostat opens it is fine, but once it is at operating temp it stumbles. I have resorted to gassing it a little before I start. Is this the "water" induced choke?
Click to expand...

confucius is dead, i am sorry to be the one to tell you,...;-)

zenith carbs are the most beautiful form of pouring gas into an M30 engine, but carbs are not alone in this battle,...oh, and we are here to help you in the right religion, god provides,...

please define hessitation and relate it to rpms not to gears, i found an issue in my transition, and it was totally related to ignition, so, good distributor, good spark, good points set to 0.35 to 0.4mm, flywheel ball aligned at 1700rpms without advance, have you checked all that ?

equal air flow in both carbs at iddle, so good synchro both at 900 and at 1700 rpms

test to remove sequentially the spark plugs to check variations in engine iddle to detect a faulty spark

the message is, the carbs can be faulty, yes, but they can be good too

IMHO

hope it helps
 
I will try to post a video of the situation cold and at operating temp. to visually and aurally tell the story.

Redid all the plugs (gapped), wires, points (gapped), timed the ball at 1700RPM, Carbmated the carbs, but I did not unlink them, which is why I initially thought this may be a linkage problem-- but it doesn't do it cold, only when warm.

The issue is only really a problem in first gear when RPMs are below a thousand, it resolves itself when they are raised above say 1100 - 1200, which is why it is not an issue at speed thru the other gears.

Rick- I will look at the spray again, I recall that it was more like a dribble a while back- not an atomized spray. I'll be honest, I have never messed with the jets... Benny or on the West Side. Is it a risky operation?

Greg

deQuincey said:
confucius is dead, i am sorry to be the one to tell you,...;-)

zenith carbs are the most beautiful form of pouring gas into an M30 engine, but carbs are not alone in this battle,...oh, and we are here to help you in the right religion, god provides,...

please define hessitation and relate it to rpms not to gears, i found an issue in my transition, and it was totally related to ignition, so, good distributor, good spark, good points set to 0.35 to 0.4mm, flywheel ball aligned at 1700rpms without advance, have you checked all that ?

equal air flow in both carbs at iddle, so good synchro both at 900 and at 1700 rpms

test to remove sequentially the spark plugs to check variations in engine iddle to detect a faulty spark

the message is, the carbs can be faulty, yes, but they can be good too

IMHO

hope it helps
Click to expand...
 
should not be a dribble - should be a stream that hits the wall of the venturi. When the engine is running the intake air will turn it into more of a spray. The accel pumps can be found, they are a mercedes part also and I saw a pair on ebay not long ago. The linkage can also be adjusted to make them come on sooner by bending it.
 
gregthall said:
I will try to post a video of the situation cold and at operating temp. to visually and aurally tell the story.

Redid all the plugs (gapped), wires, points (gapped), timed the ball at 1700RPM, Carbmated the carbs, but I did not unlink them, which is why I initially thought this may be a linkage problem-- but it doesn't do it cold, only when warm.

The issue is only really a problem in first gear when RPMs are below a thousand, it resolves itself when they are raised above say 1100 - 1200, which is why it is not an issue at speed thru the other gears.

Rick- I will look at the spray again, I recall that it was more like a dribble a while back- not an atomized spray. I'll be honest, I have never messed with the jets... Benny or on the West Side. Is it a risky operation?

Greg
Click to expand...

pardon me, below 1000 ? where is you iddle normally ?
transition is higher in revs between 1700 to 1800, so i bet it is not a transition issue
waiting for your videos
 
The hesitation only seems to happen when at idle...sub 1000 rpm. Cruising and pinning the pedal is no issue.

deQuincey said:
yes, that should be the ideal iddle
you said this issue is not happening when other gears are engaged, as we are focusing on the kickdown pumps, may i ask what happens when driving in, say, third gear, you suddenly kick the accel pedal to overtake ?
Click to expand...
 
Experts abound on this site so I will gladly bow my head if I am wrong but as stated below, it seems that the issue is coming off idle and a common culprit may be the accelerator pump. This is very easy to diagnose and fix if that is the problem; with the engine idling and the air cleaner off look down the throats of the carbs and push down on the linkage, checking for uniformity of the "squirt" in each carb. I suspect that the problem may still be there at cruising speed though much less pronounced with the speed of the engine and associated vacuum drawing fuel steadily. If you find that the squirt of gas from the accelerator pump is not uniform and/or not spraying correctly you can replace the accelerator pumps in a few minutes without removing the carbs and while you're there use some cleaner and/or compressed air to clean out the pump tube that carries the fuel to the carb throat (or venturi). Again, just a thought and was kind of covered below.

Good luck!
 
Your off choke idle

may be lean by what you're describing.

Several suggestions:

1. Determine which carb or both. On top of the first cover near the primary throat is the idle air opeing to the idle air jet. Closing that hole will enrich the mixture. In doing so, the engine will speed up indicating a lean condition or stumble (slower and rougher) indicating a rich condition.

2. Lean- first reduce slightly the butterfly opening causing less air and therefore richen the idle mixture. ( Think of degrees of the turning to do so. Not mutch.) There are very small holes in the side wall of the throat where there is a relationship of the butterfly plate edge can not be in it's optimum position closing off the idle mixture going down the carb to the engine. It does exactly what you are describing.

3. Idle mixture screw O ring is leaking. A stethesope or peice of tube to your ear will tell you if you need to replace it. Hardware store is where I bought the last ones.

4. The one that's harder and often misdiagnosed- secondary butterflys are almost closed meaning there should be little to no air over that throat. Simply place your hand very briefly over that throat at idle. If your hand gets sucked down then there's an issue with that setting.

5. Turn out the idle screw or both for more fuel and richen up the mixture. Again, small increments.

850 rpm +- is your target rpm. 900 and up and you're not much in the initial idle circuit.

Then the squirters work to get you past that circuit to the main idle circuit. Squirt not dribble.
 
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