Pro sytems carb fine tune?

Help Support 7173Mustangs.com:

Omie01

Well-known member
Joined
Oct 4, 2014
Messages
907
Reaction score
27
Location
Minnesota, USA
My Car
1972 H code fastback Boss 351 clone
I took my baby out for some cruising time last night, and found that I have a slight hesitation when I first push the pedal, a small bit of pedal feathering gets past it and then she launches. But if I just push the gas pedal it will die. Any ideas? This is the Pro Systems carb I bought. The idle mixture screws are out about 1 full turn, float level is just a smidge above the bottom lip of the sight window and idle when warm is between 900-1000 rpm. Keep in mind I am just starting in the final tune stuff as I feel the rings should just be seated, and everything else should be mostly "Broke-In". My initial timing is 8 degree BTDC. Feel free to ask any questions if I did not give enough info. PS, I do have the 2 1/2" exhaust now so nothing should be changing after this point.

 

TommyK

VIP Members
Joined
Sep 19, 2010
Messages
1,664
Reaction score
46
Location
CT
My Car
73 Mustang Convertible
Before doing anything to the carb I would try adding more initial timing and see if it helps.

 

Qcode351mach

VIP Members
Joined
Dec 24, 2010
Messages
2,888
Reaction score
43
Location
U.S.A.
My Car
1972 Q code Mach 4spd
2004 350z
2005 Corvette
1971 Camaro SS/RS
Before doing anything to the carb I would try adding more initial timing and see if it helps.
Agreed you need more than 8

 

c9zx

VIP Members
Joined
Sep 30, 2010
Messages
5,345
Reaction score
273
Location
Mustang, OK.
My Car
1972 Mach 1 Q code
2007 GT
1969 Cougar Eliminator B302
CSX 7000 Shelby Cobra FIA
2020 Edge ST
2002 F250 V10
Assuming your cam is at least as aggressive as the CJ cam, I'd try 16 degrees initial with distributor vacuum lines disconnected and plugged, idle speed low enough to not have any mechanical advance engaged. You will need to check total initial+mechanical advance and limit it to 36 degrees. If you have vacuum advance, lit it to about 6-8 degrees. If the problem still exist, check the adjustment of the accelerator pump and discharge nozzle size.

Chuck

 

barnett468

Well-known member
Joined
Nov 3, 2014
Messages
1,097
Reaction score
10
Location
us
My Car
i have a collection of mustangs
Remove both front spark plugs and post a close up photo of the end.

Post the number of the plug.

Post the intake type.

Post the cam number or all the specs.

Below is one of the things I would do.

SETTING TIMING CURVE

Before you start driving it normally, I would set the timing curve so it is optimum for your particular setup . Below is just one way to do that . In general, the goal is to run as much timing as possible without it pinging under heavy load.

The more timing you have, the more vacuum it will have until the timing reaches its optimum level.

The more vacuum it has, the smother it will idle and the more you can close the butterflies.

If the butterflies are too far open, you will be into the transition circuit of the carb which can cause idle problems . One of the indicators that the butterflies are too far open at idle is when the carb has vacuum at the ported vacuum port.

1. Disconnect and plug the vacuum hoses to the dist if you have any and leave them plugged permanently or until further notice.

2. Reduce idle as much as possible then start your timing at 8 degrees BTDC.

3. With the engine warm and idling, advance the timing 4 degrees . Listen for a slight but noticeable increase in rpm and irregular/rough running.

4. If the rpm increases and it still runs smoothly, reset the idle speed then increase the timing 2 more degrees and check for the same things.

5. Retard timing back to 8 degrees btdc.

6. Reset the idle speed.

7. Increase the rpm to around 2000 then advance the timing 4 degrees . Listen for an increase in rpm and irregular/rough running.

8. If the rpm increases and it still runs smoothly, reset the engine speed to 2000 rpm then increase the timing 2 more degrees and check for the same things.

POST RESULTS

TEST DRIVING

After setting the timing curve you can do the following test to see if you have too much advance.

Get the engine up to operating temp.

Drive at around 20 mph in second gear for a few seconds then floor the gas pedal as fast as you can until you reach around 30 mph and listen for even the faintest pinging sound coming from the engine . If it pings, you have too much timing for the octane gas you are using . You can either reduce the timing some or use a higher octane . The highest timing level you can run without it pinging and/or running erratic will SAFELY provide the most power.

It may ping in hot weather even if it does not in cold weather . If you find this to be the case, the easiest thing to do is reduce the timing until it stops or try higher octane gas . If it still pings with the highest octane gas, you can reduce the timing then.

 
Last edited by a moderator:

Omie01

Well-known member
Joined
Oct 4, 2014
Messages
907
Reaction score
27
Location
Minnesota, USA
My Car
1972 H code fastback Boss 351 clone
I re-checked the plugs last night and they have actually started to get to the golden brown point. I did advance timing a little, reset float level, and reset idle, pretty much got rid of hesitation but now when its at operating temp and I'm sitting at a stop sign the engine starts surging a little bit, it feels like if I sit there long enough the engine would eventually die, up and to this point its running pretty smooth, and is rolling down the street good.

 

piper62j

Well-known member
Joined
Jul 6, 2014
Messages
333
Reaction score
7
Location
34613
My Car
1973 Q-Code Mach 1. Full restoration in progress.
Readjust the idle mixture.. One turn out on the screws doesn't seem to be enough fuel at idle. Usual starting point for idle mixture screw setup is to turn the screws out 1 1/2 turns from seated.. Do not tighten the screws down or you will damage the seat.. Just gently turn them in until they stop then back them out. Engine not running of course..

Start the engine and let it get up to temperature.. Adjust each mixture screw in or out to obtain the best idle.. There are two ways to do this. I use a vacuum gauge to indicate the highest vacuum while adjusting.. The other way is to obtain the highest rpm. As the rpm increases in both cases, adjust the throttle idle screw to keep the rpm within specs.. If you are running a performance cam, keep the idle with the manufacturers specs while making your adjustments..

 

TommyK

VIP Members
Joined
Sep 19, 2010
Messages
1,664
Reaction score
46
Location
CT
My Car
73 Mustang Convertible
Can you post the build sheet for the carb?

I assume it is a 4 corner idle carb?

 

barnett468

Well-known member
Joined
Nov 3, 2014
Messages
1,097
Reaction score
10
Location
us
My Car
i have a collection of mustangs
I did advance timing a little, reset float level, and reset idle, pretty much got rid of hesitation but now when its at operating temp and I'm sitting at a stop sign the engine starts surging a little bit, it feels like if I sit there long enough the engine would eventually die, up and to this point its running pretty smooth, and is rolling down the street good.
Exactly what did you reset the fuel level to?

Advancing the timing will not properly set the timing curve or distributor vacuum can.

In addition to incorrectly set idle mix screws, erratic idle can be caused by high fuel level or too much ignition timing or air leaks etc.

 
Top