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351w Idle problems

  #1  
Old 04-15-2018, 10:34 PM
Jghake
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351w Idle problems

Hey guys,

I have a 351W with Motocraft 2100/2150 carb, auto choke, and Duraspark. It ran like absolute crap when I got it...

I've got the auto choke adjusted and working properly, the truck will cold start and idle now, and finally drives around. Originally the mixture screws were something like 9 turns out, and the idle was so far out the adjustment screw wasn't even touching. Now the idle screws are at about 1.75 turns out, and it will idle at 800-1050 RPM.

It drives around but coming to a stop it will want to die. (replaced the power booster and master cylinder) It feels like if I slow down gradually, instead of abruptly that it behaves better. If you tap on the gas it will keep going. Previous owner mentioned that he might have put the smaller sender unit in when he replaced it so the pickup might be too short and I might not be getting fuel when the level was low. Dropped the tank today and replaced the sender/pickup but it looked exactly the same as the one I pulled out. Also have a new fuel pump, and the filter at the carb has been replaced, there is an inline clear filter in the engine bay and I can see fuel in it, but the level never gets above about 1/2 or so.

I also hear a chugging at idle and the idle bounces around like stated above.

Any ideas where I should look next?

 
  #2  
Old 04-16-2018, 06:58 AM
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Pull that vacuum line off the dist and put your thumb over the line so it will not suck air. Does it suck on your finger and does the idle drop? If it does, then the vacuum line is hooked to the wrong place, or the idle is still too high(the carb is open too far). The dist should not have vacuum on it at idle.

If the vacuum line is hooked to the correct spot, you should be able to turn the idle speed down till there is no vacuum on that line. If you can't make it run like that, you need to find out why you need the carb open so much to keep it running, Might want to check the timing with a light.

If you need it running for now, you can plug that line off with a screw and set the timing so it will run. You can then see if it still wants to stall. You can drive it like that with no harm done, just your fuel mileage will be down a little bit.
 
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Old 04-16-2018, 12:09 PM
Jghake
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So far this is what I've done, and in order.

1. Set timing to 8
2. Set mixture screws to 1.5 turns out (originally 8+)
3. Replaced Power Booster and Master Cylinder (possible vacuum leak)
4. Adjust auto-choke properly
5. Adjust idle screw (originally almost completely removed)

I'll pull the vacuum off like you suggested soon. One thing I did notice when I was setting the timing that the distributor was almost completely seized in the block. I was able to get the timing adjusted by spraying penetrating oil near the base of the distributor and cleaning up all the crap that collected on the top of the engine. Then I could use a closet rod dowel and a hammer on the distributor to get it to spin....

Is there anything I can do to make this easier to adjust?
 
  #4  
Old 04-16-2018, 06:48 PM
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Since the penetrating oil has been on there awhile, it might be better already. If not, keep spraying it. The engine gets a brown gummy residue inside it, and this freezes parts together, like the dist housing.

Check the vacuum line to the dist like I mentioned. If you have to turn the idle speed down to get rid of the vacuum on this line, report back how it runs. If you have to do this, then your idle mixture screws will start working again also. If the carb is too far open, then idle mixtures screws will not work right.
 
  #5  
Old 04-17-2018, 02:06 PM
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Originally Posted by Jghake
I've got the auto choke adjusted and working properly, the truck will cold start and idle now, and finally drives around. Originally the mixture screws were something like 9 turns out, and the idle was so far out the adjustment screw wasn't even touching. Now the idle screws are at about 1.75 turns out, and it will idle at 800-1050 RPM.

So far this is what I've done, and in order.

1. Set timing to 8
2. Set mixture screws to 1.5 turns out (originally 8+)

Your timing is too late, the idle speed is too high, and the idle mixture screws are too far out.


ENGINE OFF

1. - Adjust your idle mixture screws to 1-1/2 turns out on each side. (This is a bench setting to get the truck started, and is a bit rich.)

2. - Adjust the curb idle speed screw until it just touches the throttle lever and then turn it in one more complete turn.

3. - To prevent crossfire, route your spark plug wires like this:




ENGINE ON (Completely warmed up and choke completely off)

4. - Set your timing to 10 degrees BTDC.

5. - Adjust idle speed screw to 600 RPM in PARK.

6. - Turn the idle mixture screws in (one at a time) to lean the mixture. Your idle speed should increase when you do this. Adjust the idle speed back down to 600 RPM and repeat until no further change is observed.

7. - Adjust the idle speed screw to 750 RPM in PARK. This should get you back to 600 RPM when you put the truck in gear.


Capiche?
 
  #6  
Old 04-18-2018, 09:51 AM
matthewq4b
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Originally Posted by LARIAT 85 View Post

6. - Turn the idle mixture screws in (one at a time) to lean the mixture. Your idle speed should increase when you do this. Adjust the idle speed back down to 600 RPM and repeat until no further change is observed.

The Above is not correct When you turn in the idle air mix screws reduce the idle speed NOT increase it.
This step of the procedure is all wrong.

Set your base idle speed as mentioned. Idle air screws 1.5 turns out. Then back each screw individually out until max engine RPM is obtained. Then turn each in a 1/4 turn.

The technical proper way to do this is with a vacuum gauge and adjust the screws until maximum engine vacuum is obtained. The maximum engine RPM -1/4 turn will get you pretty near bang on to that.
 
  #7  
Old 04-18-2018, 10:18 AM
Jghake
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I guess I don't understand what "back each screw individually out until max engine RPM is obtained" means. Does that mean the RPM will be near redline while I'm adjusting the mixture screws?!
 
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Old 04-18-2018, 10:25 AM
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Originally Posted by Jghake View Post
I guess I don't understand what "back each screw individually out until max engine RPM is obtained" means. Does that mean the RPM will be near redline while I'm adjusting the mixture screws?!
LOL no. As you turn the idle air mixture screws out the engine idle will increase eventually it will stop increasing no matter how much more you back it out, it should only increase a few hundred- 500 RPM when you back the screws out. . So you back them out out till max engine is RPM achieved with the idle mixture screws then -1/4 turn / turn back in 1/4 turn.. Ideally these should be set with the Air cleaner in place but that is not always practical.
 
  #9  
Old 04-18-2018, 11:15 AM
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As mentioned before...set the mixture with a vacuum gauge - a most necessary tool - and you'll be ahead of the pack - along with a timing light and DVOM.
 
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Old 04-18-2018, 11:30 AM
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I have also heard of running #4 and 8 plug wire on the inside edge of the valve cover to the rear.
I would see that diagram as yesterday I just installed a set of make to length plug wires...
Dang.......
 
  #11  
Old 04-18-2018, 12:31 PM
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Originally Posted by matthewq4b View Post
The Above is not correct When you turn in the idle air mix screws reduce the idle speed NOT increase it.
This step of the procedure is all wrong.

Set your base idle speed as mentioned. Idle air screws 1.5 turns out. Then back each screw individually out until max engine RPM is obtained. Then turn each in a 1/4 turn.

The technical proper way to do this is with a vacuum gauge and adjust the screws until maximum engine vacuum is obtained. The maximum engine RPM -1/4 turn will get you pretty near bang on to that.
Wait...2 or 2-1/2 turns out might be the bench setting (Sorry - my mistake.) There is a bench setting to get the vehicle started and *should* be a little on the rich side, if you are using your stock carburetor. Turn the screws IN to lean the mixture. When that happens, the idle speed *should* increase.

Well, at least that is how it is on my Autolite 4100.

Yes, a vacuum gauge would be ideal and how I set mine. In fact, I also set my ignition timing with a vacuum gauge. Watch the gauge, and turn the mixture screws for maximum vacuum. When you accomplish this, turn the screw back out 1/4 turn. (I ended up with 21" of vacuum and the screws ended up being 1-1/4 turns out on one side, and 1 turn on the other.)
 
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Old 04-18-2018, 01:00 PM
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I griped about that the other day. Most people read the instructions on that initial setting on the mixture screws, and think they are supposed to leave them there. Like the other guys said, that is just to get you started, you adjust them afterward till the engine idles the best and the fastest with the screws. Keep going back and forth from one side to the other till you get it the best.

And as I mentioned before, if your carb is open too wide, the screws will have little to no affect when you turn them.
 
  #13  
Old 04-18-2018, 01:19 PM
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Originally Posted by LARIAT 85 View Post
Wait...2 or 2-1/2 turns out might be the bench setting (Sorry - my mistake.) There is a bench setting to get the vehicle started and *should* be a little on the rich side, if you are using your stock carburetor. Turn the screws IN to lean the mixture. When that happens, the idle speed *should* increase.

Well, at least that is how it is on my Autolite 4100.

Yes, a vacuum gauge would be ideal and how I set mine. In fact, I also set my ignition timing with a vacuum gauge. Watch the gauge, and turn the mixture screws for maximum vacuum. When you accomplish this, turn the screw back out 1/4 turn. (I ended up with 21" of vacuum and the screws ended up being 1-1/4 turns out on one side, and 1 turn on the other.)

You don't need the 1/4 turn if you set with a vacuum gage it is just max vacuum. Short of getting the gas analyzer out you are not do any better than that. The 1/4 turn in only applies to setting it with RPM as max vacuum is just hair leaner than max RPM.
 
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Old 04-18-2018, 03:17 PM
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Originally Posted by matthewq4b
The Above is not correct When you turn in the idle air mix screws reduce the idle speed NOT increase it.
This step of the procedure is all wrong.
I just re-checked my rebuild sheet for an Autolite 2100/Autolite 4100, and the initial bench setting for the idle mixture screws are, in fact, 1-1/2 turns out. That is, turn the idle mixture screws all the way in, and then turn them back out 1-1/2 turns. This should be slightly rich, but enough to get your truck started. Then, turn the screws in (1/4 turn increments) to lean the mixture. Being that the mixture was slightly rich when you started, the idle speed will INCREASE as you lean out the mixture screws. You will get to a point where the idle speed will stop increasing and start to decrease. When that happens, STOP and turn the mixture screw back to where it ran the best. Then, back out an additional 1/4 turn.

That is, in fact, the "right" procedure.

The idea here is to make the adjustment as easy as possible. You don't have to guess which way to turn the screws if you follow Ford's specifications and bench set the carburetor before making adjustments. When everything else is correct and you have the correct carburetor, the final settings of the mixture screws will not be too far from the initial bench setting.


Originally Posted by mathhewq4b
You don't need the 1/4 turn if you set with a vacuum gage it is just max vacuum. Short of getting the gas analyzer out you are not do any better than that. The 1/4 turn in only applies to setting it with RPM as max vacuum is just hair leaner than max RPM.
You just contradicted yourself. If the final 1/4 turn out only applies when tuning it by max RPM, and the max vacuum method is a "just a hair leaner" than max RPM, why would you think you don't need it?
 
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Old 04-18-2018, 06:11 PM
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Originally Posted by LARIAT 85 View Post
I just re-checked my rebuild sheet for an Autolite 2100/Autolite 4100, and the initial bench setting for the idle mixture screws are, in fact, 1-1/2 turns out. That is, turn the idle mixture screws all the way in, and then turn them back out 1-1/2 turns. This should be slightly rich, but enough to get your truck started. Then, turn the screws in (1/4 turn increments) to lean the mixture. Being that the mixture was slightly rich when you started, the idle speed will INCREASE as you lean out the mixture screws. You will get to a point where the idle speed will stop increasing and start to decrease. When that happens, STOP and turn the mixture screw back to where it ran the best. Then, back out an additional 1/4 turn.

That is, in fact, the "right" procedure.

The idea here is to make the adjustment as easy as possible. You don't have to guess which way to turn the screws if you follow Ford's specifications and bench set the carburetor before making adjustments. When everything else is correct and you have the correct carburetor, the final settings of the mixture screws will not be too far from the initial bench setting.




You just contradicted yourself. If the final 1/4 turn out only applies when tuning it by max RPM, and the max vacuum method is a "just a hair leaner" than max RPM, why would you think you don't need it?

The 1.5 out is bench mark to insure it will start and run reasonably smooth Some carbs you may have turn them in some may have to have them turned out 1.5 is generally a close to middle of the road.. I have seen them be set from any where from 1 turn out to 3 turns out. The 1.5 is just a rough setting. There are to many variables to say 1.5 out is going to be rich in all set ups. Air Cleaner type, altitude, If the float is not spot on , if the carb is not the original one ignition system condition (plugs) can all change these settings. That is why it is advisable to set your idle air yearly. This was part of the yearly tune up back in the day. With EFI people have forgotten these older vehicles should have at minimum a yearly tune up. When they were used as a primary it was not uncommon to have a summer and a winter tune up.

Setting by vacuum the best method to adjust them short of a gas analyzer. and even with a gas analyzer you will be with in an 1/8 of turn of max vacuum.

Max vacuum is NOT max engine RPM.

Turning them in leans the mix turning them out richens it.


To repeat

"You don't need the 1/4 turn if you set with a vacuum gage it is just max vacuum. Short of getting the gas analyzer out you are not do any better than that. The 1/4 turn IN only applies to setting it with RPM as max vacuum is just hair LEANER than max RPM."

Max vacuum is just a hair leaner than max RPM so that means you need to turn them IN not OUT when setting with max RPM.

Comprehension issues...........Re read the post
 

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