1st Post so be gentle...

[SteveMelchiorre]

I've been doing a lot of reading lately both on this forum and in the Falcon handbook but I've hit a bit of a wall so here goes...

I have a 69' Mustang w/ a 250 / Carter RBS & stock dizzy.
The head reads as D7BE 6090 AB
The block reads as D8BE 6015 JA
I have no idea why (and neither does the PO) why a newer I6 was dropped into this car. Marti report says it came w/ a 250.

I'm having trouble getting rid of a slight surge during idle in park and when I hit the gas from a standstill the car will often stumble at low speeds and occasionally stall.

-Vacuum gauge reads just under 20 but will surge up/down 1 or 2
-Plugs new but quickly become black and sooty with a wee bit of oil on the threads of #2,3,& 4
-Plugs are gapped at .040
-Timing advance is at 16 - and I can see the surge as the timing mark moves back and forth a bit.
-Air mixture screw is out 2 and 1/2 turns
-I recently rebuilt the Carter RBS carb thinking it was the accelerator pump but no appreciable difference was noticed.
-I've checked for vacuum leaks with a hose to my ear. (I'll spray some soapy water around the carb tomorrow and see what I find) but cannot find any.
-Idle is set at about 800RPM while in drive (w/ EBrake on)

Overall the car is driveable but I know it can be much better -- and I'm only getting approx 13mpg.
I do plan on picking up a DUI soon to eliminate any ignition problems.

Any other ideas / suggestions ?

Steve....

 

[fordconvert]

Currently running a points dizzy? If so is the dwell set with a dwell meter? Is the reading steady through the RPM range? It should be. If it wanders around thats a sign that the dizzy shaft is worn so the shaft is floating which causes the dwell to change which does not let the coil do its thing properly. 40 is wider gap than I usually see for points systems. I assume this is not a Load O? Meaning it has weights and springs and vacuum advance? The load o's have just the vacuum but still have springs so it can be confusing. With the vacuum line not hooked up do you still get advance with rpm? If so then its not a load O. If no it could be a load O or the weights are stuck. Assuming no load o where is the vacuum hooked? 16 seems on the higher side.

I dont know that carb very well, can you easily disable the accelerator pump? If its easy to do may be worth a try just in case too much fuel is the issue.

 

[rbohm]

8) for a points controlled ignition, .040 gap is too big, cut that back to .035. the soot indicates that the cylinders are running rich. and the variance in timing indicates spark scatter, usually caused by as indicated worn bushings in the distributor. the vacuum variance of 1-2" is nothing to worry about as i have yet to see any engine run rock steady vacuum readings. if you had 5-6" variance, then i would start looking for a reason why.

 

[SteveMelchiorre]

Here's some more info...

I re-gapped the plugs to .035 and upon initial start up runs smoother and idles nicely.
I checked for vacuum leaks around the carb w/ soapy water. None found.

Took her out for a test drive. Starts out fine but after a few minutes the car will start to hesitate and bog down unless I give it some gas. If left alone at idle it will eventually stutter and stall.
I pulled back in the garage and quickly hopped out, pulled off the air cleaner looked in the carb and the throttle plate is filling with gas. If I rev it, the gas clears, might sputter, and then proceeds to fill again until it eventually dies.
So....I adjusted the accelerator pump linkage until it "almost" stopped doing this. I say almost because I did not want to go too far.... So what I'm looking for is someone that has experience w/ a Carter RBS carb and can tell me how much travel the accelerator pump should have. I started this w/ just over 3/8th's of an inch of travel and stopped adjusting when I hit 1/4 inch of travel. The rebuild kit that I purchased for this carb did not specify how much travel it should have just how to measure it.
Or am I missing something else?
Steve...

 

[ludwig]

Or am I missing something else?

Your needle valve may not be seating all the way. The fuel spilling into the carb throat means that it is not being cut off. There could be dirt or contamination in there OR the float needs to be adjusted. If you have a brass float, it may be filling or partially filling with gas through a corrosion pinhole. OTOH, if it is a rubber or foam float, if you are running E85 gas, it could be melting or saturating the foam pores because of the alcohol (AKA: ethanol or solvent).

 

[JT81stang]

You say the air mixture screw is out 2 1/2 turns, it should only be out 1 1/2 turns. Put a vacumn gage on it & re-do the idle mixture setting untill you get the highest vacumn reading.

 

[ludwig]

You say the air mixture screw is out 2 1/2 turns, it should only be out 1 1/2 turns. Put a vacumn gage on it & re-do the idle mixture setting untill you get the highest vacumn reading.

Using a vacuum gauge connected to the manifold vacuum, turn the idle mix screw IN until the engine just starts to stumble. Then back the screw out slowly until you achieve highest vacuum. Should be 18* or better. You can do this a couple times over and under to get the correct point. When you have max vacuum, then back the screw off a quarter more. Bet it won't be 2 1/2 turns by that time.

 

[rbohm]

8) i would also check to make the the float level is set right. often times when you get flooding from a rebuilt carb the float level is too high. i also agree with resetting the idle mixture screws, 2 1/2 turns out is too much.

 

[mugsy]

Sounds like the fuel level in the bowl is too high. That causes fuel to spill over and go places it shouldn't and also can block critical air passages. Its either junk in the needle/seat not allowing it to close off correctly or the float level was set wrong at the rebuild (join the club!! )

After you verify/fix the flood condition then reset the idle needle. Personally I use the highest vacuum method. The FSM for my '65 states to use the highest idle speed but I still go with the highest vacuum reading.

 

[ludwig]

The FSM for my '65 states to use the highest idle speed but I still go with the highest vacuum reading.

Good vacuum is essential. It gets you a strong idle and then you don't have to bump the idle speed. Higher idle speed gives a hard clunk when you drop it in gear. Plus, if you get a good vacuum reading, the rest is cake. You can get a more accurate base timing when you do it with a tach and light. You can see if the timing mark has slid on the balancer.

 

[jamyers]

hair-splitting, it's what we do best. :mrgreen: 99.9% of the time, the highest vacuum will yield the highest idle speed. And if the idle speed is high enough to bang the tranny, you just back the idle speed screw out to lower it down to a workable level.

 

[ludwig]

I'm getting it now:
1) Highest idle speed = tuning by ear ($ 0).

2) Vacuum = general instrument tuning, then idle tuning by ear ($ 20 to $25 for vac gauge).

3) Timing with light and dwell tach = most accurate tuning (if the other stuff in on the numbers) ($35 to $150 depending....).

4) If not, fiddle with something and then start at the top again.

5) Ultimately, the gauge and light tuning will yield the most accurate timing (what I found).

 

[mugsy]

Jamyers,

I found that the highest vacuum and the highest idle (by ear) are not the same point on the mixture screw. On my engine, it hit the highest vac point first, then as the idle screw came out more it hit the highest idle (by ear). I was surprised and figured that they should be the same point on the mixture screw...

A vac gauge is a very helpful tool. Go buy one and you'll never regret it.

Before you dork around with the carb make sure the distributor is set correctly. Make sure the dwell and initial timing are correct or where you want them. If either is out their symptoms are the same as an out of adjustment carb. Don't ask why I know!! :roll: Also make sure the advance works and there isn't any vacuum leaks.

 

[ludwig]

I found that the highest vacuum and the highest idle (by ear) are not the same point on the mixture screw. On my engine, it hit the highest vac point first, then as the idle screw came out more it hit the highest idle (by ear).

Makes sense though since the max vacuum is probably optimum efficiency at that point. Then, as the mixture gets richer, the efficiency falls off by degrees.

I have found that whatever I did to get the engine in better tune, I ended up backing the idle off some. You want a strong idle but not too fast so that it doesn't clunk when you drop into gear and it doesn't fall off and stall when you stop at a light.

 

[The Plankster Prankster]

I checked for vacuum leaks around the carb w/ soapy water. None found.

do you realize that the soapy water test in only effective on systems under positive pressure, not vacuum pressure? a high pressure system pushes air bubbles into the soap and appears, but a vacuum leak would suck in the soapy water and not have a visible effect.
a better test is to use a propane torch (not lit), and flow gas from the torch around the areas you want to check for leaks. any noticeable difference in engine speed indicates a leak, especially when you can do it repeatedly in the same place

 

[SteveMelchiorre]

Thanks for all the info.
It's been kinda hard to get the air/fuel screw right when she keeps stumbling on me but it was sitting around 20. I'll start by going over the carb one more time and cleaning her out.
The advance is working (I believe) since if I disconnect the line from the carb my RPMs go up.
...and I will check the dwell before I remove the carb.

I'll report back with what I find over the weekend once I'm not a work.

Steve...

 

[fordconvert]

Normally the RPM would drop when you remove the vacuum line to the dizzy. With the line unhooked when you rev it do you still see it advance with the timing light? Im wondering if you have a load o mattic and maybe the wrong carb?

 

[mugsy]

If the rmps increase when the hose is taken off then the engine likes the additional air. I bet the carb is running rich at idle and the "air leak" is leaning out the mixture closer to stoch.

 

[ludwig]

I bet the carb is running rich at idle

He said the F/A mix screw was out 2 1/2 turns. That's rich right there.

and the "air leak" is leaning out the mixture closer to stoch.

and the choke may be either fixed half open or not coming off all the way until the high volume air flow pulls it off.
That or there is a problem in the idle/ run transition position.

 

[mugsy]

All of the carb adjustments should be done with the engine warmed up to operating temperature and the choke off.