I have a 302 from a 74 in an older F-100, I put a rebuilt motorcraft 2bbl on and now its got some issues, I sure could use some advice.

When a accelerate gently, from a stop it stumbles, at times almost dies. If I put my foot into it a but more after the stumble it picks up and keeps going. Seems like the accelerator pump is not giving enough fuel? Can it be adjusted? What else might it be?

The Idle screws (one for each bbl?) All I know to do it start at 1 1/2, open each untill it idles smooth.. Whats the real thing to do?

The idle is also quite rough, I have been trying to track that one down for some time, it is the same with the old carb as it is with this one, so Im confused..

Any and all help very much appreciated!

 

Look to see if the Xcell pump circut is working by simply looking in the carb as you open the throttle by hand. If it gives a good solid squirt, then it's not the pump circut. If it just dribbles out, doesn't squirt, or only squirts for part of the cycle, then the carb needs rebuilt and cleaned. The pump diapham may be worn out?

Don't assume that it's the carb causing the problem. Other things such as the ignition system, or a worn out timing chain can cause this type of problem as well. If the problem presists with a different carb then most likely it's not the carb at all.

Most Motorcraft/Autolite carbs use a power valve, like a Holley, so the timing of the unrichment circut can become screwed up by a sticking power valve. This is usually a matter of the carb not being clean internally. If the PV doesn't open it will cause a lean stumble. The powervalve not closing, or blown can cause a rough idle and a bog.

I adjust the idle mixture screws by starting with them turned out a few turns, so it's plenty rich, and then I turn them in, until the idle starts getting too rough-then back out 1/4-1/2 turn. Then I fine tune the idle speed.

Some emissions type carbs are "reverse idle". This type is an adjustable air bleed rather than a fuel control. Out, makes the idle mixture leaner, rather than fatter.

 

It ran fine before I put this rebuilt carb on, I did this only to clear up a rough idle and now at a constant speed it rund bad/rough..So I am thinking its likely the carb. Is it not at all a min jet symptom?

 

So the Xcell pump system is working fine? If you suspect that the jetting is wrong, then pull the spark plugs and inspect them. This will tell you if it's too lean or too fat. To get a correct reading, first double check your intial ignition timing. On a stock 302, it should be 10*-12* advanced(with the vacum advance hose disconnected). If the timing isn't advanced enough, it will give a false rich reading, or if it's too advanced it will indicate too lean falsely. I like to install a fresh set of plugs to read, once everything is set. Drive it and then read the plugs. If they are white, then it's too lean and can cause a stumble. If they are black or dark brown, then it's too rich. Tan is what you want. You may need to get the jets from your other carb if you need other jets. You used to could get Motorcraft jets from the local Ford dealer in decades past, but they may not have any, or know what they are today. If you have the carb apart to change jets, then check the power valve at the same time.

 

I adjust idle screws by turning them out 3 or 4 turns. Turn them in until you see a drop in vacuum with a vacuum gauge. This is more accurate than trying to hear an idle change.

If you have put on a new carb. then you don't know if the main jets are the right size. Does the engine run better with the choke on? Will the engine continue to run with the idle screws turned all the way in?

You could have a vacuum leak. Vacuum gauge should read about 20 steady inches at idle.

 

Not sure if this applies, but have you cleaned the EGR valve? This can cause a rough idle if it's not sealing thus allowing exhaust gas to enter the system. Might be the source of the stumble as well........

As for the stumble....hum....power valve...or a vacuum leak......buy your self a vacuum gauge....doesn't need to be expensive, and adjust your idle for max vacuum. Should be at least 20+ on a stock engine......

 

Remember, this is a different carb, it ran fine before I changed it.

OK, so I have 22 steady inches at idle, needle moves a tiny tiny bit (normal??)

The plugs (clean set) had very little color at all, a slight white dusting on the electrode.

It starts ok, it DOES run better when choked a bit.

Going down the road it is rough, it acts like is is starving at a steady speed or accelleration, the acc pump squirts ok.

WHats next?

 

If it runs better with some choke then it isn't getting enough gas. Adjust the float level in the carb up a bit. That will give you more gas to the engine. If you need to adjust the float level too high to get enough gas to the engine then the main jet size needs to increase and then move the float level back down. You will know your float level is too high if you can't kill the engine by turning in your idle screws all the way. If you adjust the float level remember to measure the height of the float when you pull it apart. Then you have a reference to work from.

 

Ill try that! I do want to thank you all very much for the help, It is difficult to figure this stuff out without help, it is appreciated!

 

Im having a difficult time finding Motorcraft main jets, in the "old days" we used to drill out jets. I have one spare pair of jets, they are #54s just like what was in this unit (in a 302 with a rebuilt , new to me, carb) I drilled the #54s out with #52 bit, it seems to be somewhat better. First of all, can anyone tell me what # jets should be in a 302, 2bbl, ???I was told has mild cam (not sure if its true).

Second, anyone have a conversion chart, or just a good handle on what drill size = what jet size? if a #54 jet is the same as a #54 bit, (and it is) then when I drilled it with a #52 drill bit which is bigger, did I make it a #52? or a #56? ....is a #52 jet smaller than a #54? I am thinking about drilling one more size to a #50 bit, is this crazy too big? or is it reasonable that it may need this much fuel, it still seems starved, float level is good, vacuum is good, and drilling to a #52 did help a but... HELP!!

 

Don't know about jet sizes. Drill size would make since.

There are 2 main circuits in a carb. One is the idle and the other is the main circuit. The main draws gas thru the main jets and the float level and main jet size determine how much gas flows. The idle circuit can get its gas from a couple places but the air mixture screws and float level sets the amount of gas that mixes with air at idle. Unless you have an exhaust gas analyzer, about all you can do to set mixtures is to check the plugs for color or sniff the exhaust(yuck). The plugs will tell you what to do if you give them a chance. I sounds like you are on the right path. Make small changes and check it out for awhile.

 

When reading plugs the modern no-lead gas sometimes won't color the plugs. It depends on the formula mandated for your area, and the mix of additives used by the company. A few gallions of LL100 AV Gas or some 104 octane booster added to the tank will help color the plugs for reading, if that's the case. You may need to resort to drilling the jets. The power valve adds gas to the main circut too. So make sure that it's working right and will open when the vacum drops.

 

How does one check the power valve? I think this is an old one without a vac line going toit.

 

A crude but effective method is to just remove it and suck on it to make sure it's functional. If it's stuck you may be able to determine that by visual inspection. If the spring isn't functional or too weak, then vacum will close the valve, and probably it won't open creating a part throttle and wide open throttle too lean condition. It's been so long, that I don't recall if Motorcraft power valves are numbered to indicate at what vacum level they are supposed to open at. You may be able to get jets and power valves from a place that specialize in vintage Autolite type carbs, like Pony Carburetors. Try a web search.

I have some Motorcraft jets in plastic bag somewhere around here, but I havn't found them yet to see what size they are. They may not work for you, because most likely I got them to lean the carb on my old 400 powered F350 for high altitude, many years ago.

 

Thanks for that info, Ill have a look at it. I need the sizes larger than 54s, like the next couple of sizes, anyone have-em around and dont want em?