Rough Idle Mystery

[KipDurran 18]

1969 F code convertible

 

 

I decided to rebuild the carb. I also got a timing light with tach so I could set everything up.

 

 

The carb is an Autolite 2100, original to the car.

 

 

The rebuild went well. Whoever rebuilt it before me set the float level all wrong-- It was too low.

 

 

One of the problems the car was having was stalling during turns, especially right hand turns. This is now fixed.

 

 

 

I reinstalled the carb and when started right up.

 

 

I got the timing set to 10 degrees initial and set the idle mix screws ( about 2.5 turns each). It was at the point that it was idling smoothly at the factory spec of 650 rpm. I didn't stall anymore. It had more power, etc. Good vacuum. Life was good.

 

 

 

This was all great for about 2 days and then it started running like crap. Rough idle, hesitation.

 

 

I took the carb apart thinking I messed something up but nothing doing. It all looked good and it is still doing it. When I look in the carb, there is fuel flowing through both barrels. The wet float level is good.

 

 

While it was idling I saw a little smoke (It is not a very clean engine atm, I thought maybe a bit of oil on the manifold and I had also cleaned up a bunch of dirt and grime back there) over by #8 so I took the plug wire off to see if it was loose. Funny thing is that it did not change the idle at all when I did that.

 

 

I had also recently replace the rotor, dist cap and plug wires. Trust me, I have check and double checked-- they are routed correctly. There is also spark to the plug.

 

 

So then I went to all of the other plugs and removed the wire one by one. Good spark on all.

 

 

Removing wires from 2,3,5,8 all have no effect on the idle. Removing 1,4,6,7 does affect the idle.

 

 

Also, moving the idle adjustment in and out on the driver's side has no effect whatsoever. The other one works as intended.

 

 

Coincidentally, I think the manifold routes the air to the cylinders in the above pattern. Do we have something going on here? Could it have developed a manifold vacuum leak on that side of the manifold?

 

 

 

 

What the heck?

 

 

Steve

[Stevewr54 12]

Fuel should not be dripping from the boosters at idle. All idle fuel should going in under

the plates from the holes behind the mixture screws. Pull the booster assembly out of

the carb. and clean the small brass idle fuel tubes. If you have a high float level running,

check fuel pressure. 5 lbs. max. is about all they will tolerate. Steve

[barnett468 418]

sounds to me like one side of your idle circuit is plugged or you have an air leak .m i would plug all vacuum ports on carb including pcv .. then spray flammable brake cleaner using long nozzle gently at carb base and intake at heads . . lower float level to 1/2 inch if it is higher than that . . shake float to see if it is getting gas inside . . remove the boost venturi block and place te cleaner nozzle against every port and clean the bejesus out of it . . spray into the tiny hokes in top of carb but not in the center hole . . plug vacuum advance and leave it plugged for now and check timing.d

[KipDurran 18]

Thanks for the replies.

 

I was quite down about it yesterday. Today I embarked upon a journey of self discovery and was victorious.

 

To be clear, there is not fuel dripping from the booster. I just meant to indicate that with throttle both squirted and there probably not plugged up (I changed my fuel sender and was concerned that possible sediment might have been stirred up. I replaced the fuel filter- again).

 

I any event, I took everything apart again today and I have it down to a science.

 

I spent about an hour and a half with all of the adjustments and I think I have it solved. These carbs are fiddley things.

 

Specs:

 

Warm idle: 670 RPM

 

Initial timing 12 degrees with the advance disconnected and plugged.

 

Vacuum : 21 inches and steady from the manifold port.

 

What I found is that the idle mix screws need to be out a bit further than the 2.5 turns. Turns out I was probably fuel starving the drivers side and that's why removing the plug wires had no effect. What I did was to keep turning them out with an eye on the vacuum gauge until I got a steady idle. The needles look to be in good shape and not damaged so I am not sure what is going on there. I didn't actually count how many turns out I ended up with because I did it by ear and by the vacuum gauge.

 

I have learned a lot today. Car is running great ATM. If there is any other advice please let me know.

[barnett468 418]

as i mentioned it sounded to me like an idle circuit was plugged . . having the screw in too far causes the same symptoms . .it makes no sense that it ran fine for 2 days but you had to change the screws after that to make it idle . . if you cleaned out the holes as i suggested then one might have been partially plugged as i suggested.

 

i'm telling you now that if your float level is higher than the 1/2" that i suggested, it may have problems under some conditions, especially when the weather gets hot.

 

also, if your jets are stock and the engine has been bored out and you are less than around 1000 feet below sea level they are likely too small.

 

from a stop, mash the pedal to the floor hard and fast . . if it bogs or hesitates you need tuning.

.

[KipDurran 18]

I have a brass bloat and it does not leak. I set the float dry to 3/8 inch and wet fuel level to 1/4 inch per factory specs. I also tried to make sure as best I could that all of the passages were cleaned out. I will double check them at another time.

 

No bogging with acceleration. In fact, I am having to get used to the better acceleration from stop than I have ever had.

 

I am sure that I will be tweaking things in the future but I am petty happy now.

 

 

So lets talk floats: What symptoms come from a float set too high or one set too low?

[barnett468 418]

of it runs fine don;t try to clean out anything.

 

the gas level can increase when the carb gets hot and cause fuel to go into the cylinders often while parked which can make it hard to start and do what some call vapor lock . . it can also cause stalling from flooding when stopping coming down a hill like a freeway off ramp . . it can also cause stalling on flat ground under hard braking . . this is a common prob on a holley if the floats are set to the factory level . . this is why i said to set them at 1/2" or even slightly lower but now that you have set it higher anyway, it doesn't really matter . . these are the kinds of questions that are best asked before dpimng something not after.

 

a slightly low fuel level will have absolutely no adverse affect.

[KipDurran 18]

I read the manual before doing this and figured that setting the float to factory specs would be ok. It is not a hard thing to change the float level if necessary.

 

Thanks for the help.

Edited January 5, 2015 by KipDurran

[1969_Mach1 333]

I'd leave the float at the stock setting and see if you have any issues. Lower it if you have any of the problems barnett468 mentions. If you have any issues, it probably won't be until the summer months. I've rebuilt probably 1000 carbs in my time and always set the floats at stock specs. To be fair to barnett468, most of those were GM carbs. So he probably knows more about the Autolite carbs.

 

Vapor lock is when the gas in the line between the fuel pump and carb starts to boil. As the fuel pump operated the gas vapors compress preventing fuel flow through the line.

[barnett468 418]

I read the manual before doing this and figured that setting the float to factory specs would be ok. It is not a hard thing to change the float level if necessary.

 

Thanks for the help.

 

ok well one shoukd always use the stock settings unless other info is avail but since i know that the stock level on the motorcraft and holley carbs and a bit on the autolites is deffinitely an issue i thought it would be helpful to mentiio it to save you from potential probs . .m. the float level is also more of an issue now with the corn gas they make us use than it is with real gas . . these days i prefer to use phenolicspacers and bockoff the exhaus cross overs but i live where t gets to 100 degrees f this is less of a concern at places like the north pole

[KipDurran 18]

I am in Seattle so the summers are a bit milder.

[barnett468 418]

I've rebuilt probably 1000 carbs in my time and always set the floats at stock specs. To be fair to barnett468, most of those were GM carbs. So he probably knows more about the Autolite carbs.

Whats a ...GM? :blink:

[1969_Mach1 333]

Whats a ...GM? :blink:

 

Ha Ha! Rochester not Motorcraft, stuff General Motors uses.

[barnett468 418]

Ha Ha! Rochester not Motorcraft, stuff General Motors uses.

 

lol, i know i was just "trying" to be being amusing . . i have probably done 30 of the edys, q jets a chesters and about 10 zillion combined of the other ones so pretty much the opposite of you it sounds like . . i think cliff ruggles has the q jet market cornered.

[1969_Mach1 333]

When I use to work as a GM tech and also at an independent shop, by the time carbureted cars and trucks came in for driveability issues, they were beyond simple tune ups so it was common to also have to rebuilt the carb.

 

I prefer Fords but I ended up the GM route because at the time GM had 2 year training programs at colleges and not trade schools so it seemed like a good path. You alternated between work at your sponsoring dealership and school. Even though I didn't stay in that trade it opened up doors.

Edited January 6, 2015 by 1969_Mach1

[KipDurran 18]

Well, I was persistent and I have figured out what was wrong. To those who suggested that there was a blockage somewhere-- ding... ding....ding... you were right.

 

However, it was not dirt or crud.

 

I took the thing apart again and soaked it overnight in parts cleaner and then blasted everything with carb cleaner. All of the passages were open and epeciall the idle circuit.

 

I then got to looking at the carb closely. There is a gasket that goes under the venturi cluster. As I am sure you all know, the idle circuit runs through here.

 

 

 

 

The two holes in the gasket were poorly cut and were partially blocking the flow through here, especially the drivers side circuit. I carefully cut larger holes here and test fit it to the venturi assembly to make sure that the idle circuit was not blocked.

 

The car idles smoothly now with just 2 turns out of the idle screws.

 

I appreciate all of the patience and the help.

 

 

[SparkyGT 26]

messing around with carbs is a science,  the good old skin eating carb cleaners used to be a lot better at cleaning out everything, now its a bit tougher on rebuild i have found, one of my buddies dad who worked at ford in the late 60's used to tune my big block with a holley all by ear, the thing ran awesome

[barnett468 418]

Well, I was persistent and I have figured out what was wrong. To those who suggested that there was a blockage somewhere-- ding... ding....ding... you were right.

 

However, it was not dirt or crud.

 

I took the thing apart again and soaked it overnight in parts cleaner and then blasted everything with carb cleaner. All of the passages were open and epeciall the idle circuit.

 

I then got to looking at the carb closely. There is a gasket that goes under the venturi cluster. As I am sure you all know, the idle circuit runs through here.

 

The two holes in the gasket were poorly cut and were partially blocking the flow through here, especially the drivers side circuit. I carefully cut larger holes here and test fit it to the venturi assembly to make sure that the idle circuit was not blocked.

 

The car idles smoothly now with just 2 turns out of the idle screws.

 

I appreciate all of the patience and the help.

 

lol, the old bad gasket trick . . good catch . . yeah i have had that idle symptom around a zillion times which is why i suggested a plugged idle . . glad that its better now.

 

 

[barnett468 418]

lol, the old bad gasket trick . . good catch . . yeah i have had that idle symptom around a zillion times which is why i suggested a plugged idle . . glad that its better now.

[KipDurran 18]

A bit of an update:

 

The car is idling like a sewing machine at 650 rpm in drive.

 

Initial timing at 12 degrees

 

21 inches of  vacuum at idle

 

I have also installed a Pertronix Ingnitor and Flamethower coil. This made a bit difference in improving the idle.

 

However, a new problem has shown up. It's always something.  With light acceleration the car will stumble. If I go very light, no problem. If I hit it hard, no problem. I thought that It might have been the mechanical advance so I took the dist apart and cleaned the mechanical advance really well. I then put the timing light on with the vacuum advance plugged and the mechanical advance seems to work fine.

 

I also replaced the vacuum advance module. When I put 10 inches of vacuum on it with a pump I get about 5 degrees of advance so this checks out ok.

 

New fuel filter. New plugs (gapped to 45 because of the Flamethrower) and new wires, cap and rotor. Just about everything that I can think of to replace.

 

I have a couple of ideas:

 

1) Carb jets. I do not know if they have ever been changed or if they are a stock size. I know that I have not changed them.

 

2) Accelerator pump-- I have confirmed that it works but there is an adjustment in the lever. I put it back to the stock locations but maybe an adjustment here?

 

3) Vacuum modulator on the transmission. I wonder if there is a vacuum leak here. I have not noticed any trans fluid in the line however.  It seems to shift ok. I am needing to take the car to the trans shop to get the seal around the shifter shaft replaced as this is leaking. I will have them replace the module regardless. C4 tranny.

 

4) I have not tried spraying WD40/Carb cleaner around to see if there is a vacuum leak. I will do this just to be complete. With 21 inches at idle I am not completely convinced that it is a vacuum leak but you never know.

 

Thoughts?

[KipDurran 18]

It just occurred to me that I ought to put the vacuum gauge on the vacuum modulator to try to diagnose a leak there. I will do that tonight.

[barnett468 418]

.

you can just suck on the hose it you do not have a vacuum pump . . a gauge aline will not tell you anything.

[KipDurran 18]

I have a pump.

[barnett468 418]

I have a pump.

 

thats better, lol . . oil can get on the hose at the modulator and deteriorate it or it can just get oul . . if you have a leak it might just be the hose on the valve.

[KipDurran 18]

So I put the vacuum gauge on the hose going to the modulator. It won't hold a vacuum at all. Also, after a few quick pumps I think I might have heard a bit of gurgling. I stongly suspect that it is bad and needs replacing.

 

Am I correct to assume that it should at least hold a vacuum?