73 F Engine Aisan Carb Help Needed

[Supra88]

My new-to-me '73 with F engine Aisan carb (Pertronix Ignition) is not running like it should. I have a delay in acceleration when I step on the accelerator pedal, it especially worse from a cold start up. In fact it wants to stall when I go to take off. Under WOT I get serious sputtering or what sounds like spark knock with very little power.

I've searched and read many posts on here to help steer me down the right path. Here's what I've done so far:

1. I pulled the air cleaner assembly off and placed cardboard over the inlet while running. It died immediately, so I'm assuming I don't have any vacuum leaks. Is this a safe assumption?

2. I checked idle vacuum and found it to be around 15.5 in Hg.

3. The fuel bowl is at the set point, although I haven't looked at it immediately after the sputtering/flat spot.

Overall it seems to idle fine, audibly it seems rather low in fact. I need to get a tach to determine where I'm at. The problem kicks in when I go from no throttle to throttle and under load I get the sputtering/spark knock with very little power.

I assume that I'm running lean, but being a newb to carburetors I'm a bit lost. In fact, I thought I was adjusting the idle speed and it turns out I was adjusting the throttle adjusting screw? I know, I know. I'm reading the FSM and I still cannot figure out where the idle speed screw is?

Please help!

Here are some pics -

I was adjuting the lower left screw - oops (I'm guessing this is choke/high idle?). Is the idle speed screw the lower screw to the right in this picture?

 

[Splangy]

The idle speed screw is the one furthest left in the first picture

 

[LAMBCRUSHER]

closest to carb body at rear side of carb is Idle speed(throttle plate position) closest to fire wall at rear of carb is high idle(choked speed) around the corner closest to the valve cover is the idle mixture screw. Idle mix only affects at idle.
I would check timing and then look for a vacuum/intake leak. Could be PCV valve stuck.....Is this set up for smog, or has it been hacked up? is there a vacuum line to the dizzy from the carb? you should have 16.5" at idle. When was the last valve adjustment done?

 

[Pighead]

1) No
2) That's a little low.

I cannot take a pic of my idle speed adjusting screw, I can barely see it. It's hidden way down at the base of the carb ('74 carb). I just know where it is and how to get a screwdriver on it. Search for the lowest screw on the carb. If you can't see it, that's probably it.

What's your timing set at?

 

[Supra88]

Thanks for the replies. Timing is/was around 10 degrees. If I run it more advanced the spark knock gets worse - retarded and I get no knock, but also no power. My vacuum is sub 13 now. I'm going to spray some water to see if I can find a leak.

Dwell is at 30, Rpm an unsteady 650, and coil volts measured 2.

What next?

 

[Pighead]

Set the point gap per factory spec and forget the dwell.
Is your dizzy advance working?

 

[Pin_Head]

It sounds like you have two problems: bogging down on transition from idle to accelerate and running out of fuel under high engine load.

For the first problem, make sure that you are not pulling fuel out of the primary nozzle at idle. Look inside with a flashlight at idle and check for fuel dripping out. If it is pulling fuel out of the main nozzle at idle, then chances are you have a vacuum lead. Check for fuel leaks and fix. The other possibility is that you are not getting any idle fuel. Check the solenoid valve for operation and see if turning the idle fuel mixture (brass screw on valve cover side) can make is stall or lower the idle speed. If no effect of mixture screw, maybe no idle fuel.

For the bucking under load, make sure that your fuel tank is not full of rust or dirt and make sure that the fuel pump can pump a quarter cup of fuel in 15 seconds of cranking.

 

[Supra88]

Ok - I'm not seeing any fuel in the carb at idle. When I turn the key I'm not hearing anything from the idle solenoid. How can I confirm this is toast?

Also, what's the best way to run fuel manually out of the fuel pump to verify flow?

Many thanks for the help.

 

[Supra88]

Set the point gap per factory spec and forget the dwell.
Is your dizzy advance working?

It's running a Pertronix system so no points, right? My meter pulled the dwell number so I thought I'd throw it as a point of data in case in meant something.

 

[LAMBCRUSHER]

Ok - I'm not seeing any fuel in the carb at idle. When I turn the key I'm not hearing anything from the idle solenoid. How can I confirm this is toast?

Also, what's the best way to run fuel manually our of the fuel pump?

Many thanks for the help.

no click when the key is turned to ON means open circuit to the idle solenoid. there is a wire at top back valve cover side of carb, no? should have 12VDC at this wire to ground when key is on. this opens the solenoid and allows idle fuel to flow. no fuel in carb at idle where? At sightglass?>means clogged feed line, incorrect port(feed or return)or stuck float, or bad pump. Take hose from carb side of pump loose from the metal line and put it into a container, crank motor (w/o starting it-disconnect coil primary so you don't get sparks!!!)for 15 seconds and measure fuel volume in container. your carb should have fuel level in bowl half way up or down the sight glass. if there is no level line visible, you are either all the way full(float stuck down) or all the way empty(float stuck up or other lack of fuel into carb>needle valve filter screen filter in line pump hoses or just plain out of gas)You need to get the bowl level correct/fix fuel delivery. you need to have 12VDC at the idle solenoid and a good working solenoid. If you mean no fuel at idle when looking down the barrel, that is OK as the idle fuel comes thru ports that draw from below the plate, if there is visible fuel flowing down the barrel at idle with the plates closed, it will run like poo. check the idle solenoid wire(disconnected from harness)to ground with an ohm meter. there should be some resistance. more than 0 and less than?10k. if it has resistance(a circuit) and it can get power, then it should pull open the idle port stop valve. you could have a mechanically bad solenoid valve and that will require replacing it.HTH

 

[Supra88]

no click when the key is turned to ON means open circuit to the idle solenoid. there is a wire at top back valve cover side of carb, no? should have 12VDC at this wire to ground when key is on. this opens the solenoid and allows idle fuel to flow. no fuel in carb at idle where? At sightglass?>means clogged feed line, incorrect port(feed or return)or stuck float, or bad pump. Take hose from carb side of pump loose from the metal line and put it into a container, crank motor (w/o starting it-disconnect coil primary so you don't get sparks!!!)for 15 seconds and measure fuel volume in container. your carb should have fuel level in bowl half way up or down the sight glass. if there is no level line visible, you are either all the way full(float stuck down) or all the way empty(float stuck up or other lack of fuel into carb>needle valve filter screen filter in line pump hoses or just plain out of gas)You need to get the bowl level correct/fix fuel delivery. you need to have 12VDC at the idle solenoid and a good working solenoid. If you mean no fuel at idle when looking down the barrel, that is OK as the idle fuel comes thru ports that draw from below the plate, if there is visible fuel flowing down the barrel at idle with the plates closed, it will run like poo. check the idle solenoid wire(disconnected from harness)to ground with an ohm meter. there should be some resistance. more than 0 and less than?10k. if it has resistance(a circuit) and it can get power, then it should pull open the idle port stop valve. you could have a mechanically bad solenoid valve and that will require replacing it.HTH

I'm having a difficult time getting a meter on the solenoid. I know the truck idles and doesn't shut off, albeit the vacuum and rpm jump around a bit. Would the truck idle if the solenoid wasn't working correctly?

When I test to see if I can hear the solenoid opening, it appears the bowl fills up beyond the halfway point. Does this mean the solenoid is opening and filling the bowl with "idle" fuel?

I'm going to do the fuel flow test tomorrow and see what I find. Thanks for the instructions on how to do this.

I have not been able to find a vacuum leak anywhere. I've checked the manifold and all of the vacuum hoses (not that many), what am I missing?

Last but not least, should my rpms be dancing all over the place? I'm at 650 +/- 75 rpms. Is this normal? If not, does this provide any clues?

I'm frustrated, but It helps getting everyone's input, thank you very much.

 

[Pighead]

The truck would idle without a working solenoid if the choke was pulled all the way closed. Also, one way to defeat a bad solenoid is to just cut the plunger off...
If the bowl fills beyond the halfway point of the porthole, it's the float adjustment or a bad needle valve..
Idle RPM should not jump around, but it seems to do that at low RPMs. Maybe speed it up to 750 RPM and see if it will hold steady. Otherwise another vote for vacuum leak.
Have you re-torqued your manifold bolts/studs?

 

[Pin_Head]

When I test to see if I can hear the solenoid opening, it appears the bowl fills up beyond the halfway point. Does this mean the solenoid is opening and filling the bowl with "idle" fuel?

No. It just means that when the engine isn't running that the bowl can fill up to normal levels. Do you have an electric fuel pump?

I have not been able to find a vacuum leak anywhere. I've checked the manifold and all of the vacuum hoses (not that many), what am I missing?

It could be either a vacuum leak or lack of idle fuel. In either of these situations, people often compensate by adjusting the idle speed screw so the truck will idle off of the main jet and main nozzle, which is why I asked you if you can see fuel dripping out of the nozzle (which some people mistakenly call venturi).

Last but not least, should my rpms be dancing all over the place? I'm at 650 +/- 75 rpms. Is this normal? If not, does this provide any clues?

No, it is not normal but it is consistent with an idle fuel or vacuum leak problem along with it bogging down at transition. Lack of idle fuel or vaccum leak is also consistent with your low and unstable manifold vacuum.

PS If I had to guess, I would guess that your two problems are related by dirt in your fuel tank that causes insufficient fuel flow and has made its way into your idle circuit plugging it and preventing idle fuel delivery.

 

[Supra88]

It's a new OEM mechanical fuel pump. Just to double check - when 12v goes to the idle solenoid, should you hear it actuate? I hear nothing, neither does my wife and she hears everything.

I see no drips of fuel of the nozzle when idling, in fact I see nothing.

Another point of data - I smell fuel vapors coming out of the carb with the air cleaner off and the truck off, is this normal?

I'll double check the manifold bolts tomorrow.

How can I clean out the idle circuit?

 

[Pin_Head]

Make sure the solenoid is working properly first. It is normal to smell gasoline with the air cleaner off.

 

[Pighead]

Dwell is at 30,

It's running a Pertronix system so no points, right?

So, no dwell either.

 

[Supra88]

Good morning and happy father's day. I pulled the idle solenoid and tested it. It strokes about a 1/4" or so, so I take it that it's working. Should I proceed with "nuking" the idle solenoid port and idle screw port with carb cleaner and compressed air?

Edit: tightened down all the manifold bolts (anyone know the torque spec, couldn't find in FSM?)

 

[LAMBCRUSHER]

Check the FAQs for the torque values. Many of them have been compiled by different sources, so stick with knoiwn goodies like FSM or Max Ellery. Haynes has a decent torque value table, but has also got other inconsistencies. Even Max ellery has been caught out to lunch, so don't just look at one source, go thru them all to find consensus about the values. Okay, the idle solenoid has stroke toit....but does it work with 12 volts? Did you check the solenoid by hooking +12VDC to the wire and - 12VDC to the body of the solenoid? If you fed it power, and it opened, then next is to make certain that the harness is delivering +12VDC to the solenoid when you turn the key to on. If the harness is not delivering 12 VDC, then even a working solenoid wont do it job, cause it's never asked to.(make sense?) sounds to me like you can get idle solenoid working, now make sure you have delivery to the carb. get truck running and using carb cleaner, spray at the interfaces of vacuum hose to tube connection points, at the brake booster valve, base of carb, intake to head interface- anywhere you think air could enter the vacuum stream. It takes a couple of seconds from the time of the fluid burst till the engine will react, so spray wait listen, spray wait listen. You are checking to see if at any of these points you spray flammable fluid, the idle quality smooths out as a result of combustible material being drawn in thru a LEAK POINT. Once you find your leak, repair and then retest. It could be time for a carb rebuild....

 

[Supra88]

I checked the solenoid hooked up to the vehicle harness and it worked once I turn on the key and made a ground connection.

I screwed the idle mix screw in before pulling it to baseline where I was at. It was as 1.5 turns out! Seems low to me? Part of the problem?

I'm going to hook up a vac gauge later today and run through the tuning again after I check for leaks again.

You're right, it might be time for a rebuild.

 

[LAMBCRUSHER]

yeah I tried to start at 1.5 turns out and really wanted to keep that opening as small as possible while retaining decent idle, but I think I ended up about 3.5 turns out when I was done adjusting. You might try taking the mix screw out a bit more. also use an ohm meter to check for continuity to ground on the carb body(where the solenoid gets its ground from) Have you confirmed fuel delivery to carb via pump and lines? and the float bowl level activity also seems a bit hinkey. try to figure out if it is or is not maintaining its proper level.....\