Carb help please
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Carb help please
Have an 86 f-150 with an 1981 f-150 4.9 / c4.. need to get carb adjusted to be able to get to my brother so he can fine tune it.. I finally got the lean/rich set but not sure of normal specs on low and high idle screws.. havent messed with them but truck cold seems to idle quite higher than needed and when warm a little low..
drivetrain only has 43,000 miles on it.. just need a baseline to go off of to be able to get it about an hour away..
drivetrain only has 43,000 miles on it.. just need a baseline to go off of to be able to get it about an hour away..
Tuned
Joined: Jun 2006
Posts: 363
Likes: 3
From: Graham WA
Basics
First let me say that I'm not and expert on carbs, but I have owned many cars with carbs and have toyed with carbs several times. Here is what I have learned:
There are 5 basic settings on a carb and you can only adjust 4 without carb disassembly.
1) Cold start and cold idle speed are adjusted by chnanges to the choke side settings. In your case when you step on the gas (once) to set the choke, the fast idle screw comes to rest on one of the higher steps of the fast idle cam. In your case it seems to be set to high. The adjustment screw (in most cases), rest on a stepped cam below the choke. While the engine is still cold, back that screw out until the idle is at a setting that seems reasonable for warming the engine(1200-2000 range). Once warm and the choke is fully open, this adjustment is no longer in play.
2) If the engine stalls during warm up, you may need to adjust the choke to come off sooner or stay on longer until the engine gets warm. This adjustment is made buy loosening screws that surround the choke housing (usually 3 on a holley). There may be a marker on the black plastic housing that is a place to start from. In any case, some trial and error, and playing with this setting is usually required for any given engine.
3) The warm idle speed adjustment screw is on the oposite side of the carb. One can adjust this setting after the engine is completely warm. Warm idle speeds are usually around 700 RPM. Check you specs. If you don't have access to a tach, you will have to guess here. If it is an automatic trans, the setting may have to be slightly higher in order to not stall coming off idle when you step on the go peddle.
4) The air/fuel adjustment screws should only be adjusted with the engine warm and the starting point is defined by your carb manufacturer. If you have no info, try 1 1/2 - 2 turns out from full closure (idle needle valves seated) and open more or less until the engine smooths out. Do not seat these screws hard! The idle seats are delicate and easily damaged This will be fine tuned with the help of a vacuum guage and/or engine analyser later. This adjustment has no affect on off idle performance, only the idle.
5) Performance off idle can be adjusted only by changing the internal jets, usually not necessary to get the engine to run. Fine tuned for economy or all out performance through experimentation if you desire. A professional that knows your engine and the mods it may have installed may be the best way to get the most out of your engine regarding jetting.
I hope this helps!
There are 5 basic settings on a carb and you can only adjust 4 without carb disassembly.
1) Cold start and cold idle speed are adjusted by chnanges to the choke side settings. In your case when you step on the gas (once) to set the choke, the fast idle screw comes to rest on one of the higher steps of the fast idle cam. In your case it seems to be set to high. The adjustment screw (in most cases), rest on a stepped cam below the choke. While the engine is still cold, back that screw out until the idle is at a setting that seems reasonable for warming the engine(1200-2000 range). Once warm and the choke is fully open, this adjustment is no longer in play.
2) If the engine stalls during warm up, you may need to adjust the choke to come off sooner or stay on longer until the engine gets warm. This adjustment is made buy loosening screws that surround the choke housing (usually 3 on a holley). There may be a marker on the black plastic housing that is a place to start from. In any case, some trial and error, and playing with this setting is usually required for any given engine.
3) The warm idle speed adjustment screw is on the oposite side of the carb. One can adjust this setting after the engine is completely warm. Warm idle speeds are usually around 700 RPM. Check you specs. If you don't have access to a tach, you will have to guess here. If it is an automatic trans, the setting may have to be slightly higher in order to not stall coming off idle when you step on the go peddle.
4) The air/fuel adjustment screws should only be adjusted with the engine warm and the starting point is defined by your carb manufacturer. If you have no info, try 1 1/2 - 2 turns out from full closure (idle needle valves seated) and open more or less until the engine smooths out. Do not seat these screws hard! The idle seats are delicate and easily damaged This will be fine tuned with the help of a vacuum guage and/or engine analyser later. This adjustment has no affect on off idle performance, only the idle.
5) Performance off idle can be adjusted only by changing the internal jets, usually not necessary to get the engine to run. Fine tuned for economy or all out performance through experimentation if you desire. A professional that knows your engine and the mods it may have installed may be the best way to get the most out of your engine regarding jetting.
I hope this helps!
Tuned
Joined: Jun 2006
Posts: 363
Likes: 3
From: Graham WA
Tuning Sequence
As I'm sure you know, but I should have mentioned anyway, that the carb is the last part of the tuning.
The first 2 steps are:
1) Verifying good mechanical condition including even compression on all cylinders and reasonable compression (like => 110psi) and no 2 cylinders separated by more than 15% (these are the minimums that most manuals speak of). Older engines will lower compression may run but never per original estimates of power for a fresh engine. If the cylinder balance is off, the engine will also run rough and may even backfire into the intake or fire into the exhaust ports because of leaky valves.
2) Ignition:
a) Strong (readily visable) spark at the plugs.
b) Be sure that your timing is set to TDC per common methods of doing that (if this engine is brand new), and once running adjusted to spec. If the engine was running before the carb went on then the timing is likely close already.
3) Fuel pressure exists and is adequate? For most carbs 4-6 psi. If you have no way of measuring pressure, at least be sure the carb is getting fuel. Open carb choke and work the throttle. Do you see and/or smell fuel spray in the throttle body. If not, you likely have non existent of inadequate fuel pressure.
The first 2 steps are:
1) Verifying good mechanical condition including even compression on all cylinders and reasonable compression (like => 110psi) and no 2 cylinders separated by more than 15% (these are the minimums that most manuals speak of). Older engines will lower compression may run but never per original estimates of power for a fresh engine. If the cylinder balance is off, the engine will also run rough and may even backfire into the intake or fire into the exhaust ports because of leaky valves.
2) Ignition:
a) Strong (readily visable) spark at the plugs.
b) Be sure that your timing is set to TDC per common methods of doing that (if this engine is brand new), and once running adjusted to spec. If the engine was running before the carb went on then the timing is likely close already.
3) Fuel pressure exists and is adequate? For most carbs 4-6 psi. If you have no way of measuring pressure, at least be sure the carb is getting fuel. Open carb choke and work the throttle. Do you see and/or smell fuel spray in the throttle body. If not, you likely have non existent of inadequate fuel pressure.
