2 bbl. Motorcraft & a dwell cam

Dave Noll · 11-12-2013, 04:16 PM

On my 351C 240hp combo, the cam that the grinder sent me dwell's @ max lift for 60 degrees. The lowest the idle will go down to is about 1600 which isn't the issue. To get that idle with that cam I have the idle plate screw down about as far as it will go. which has the transition slots completely uncovered & I have a 1/4" hole drilled in each throttle plate. With so much air going thru there @ idle it's pulling fuel out of the boosters. I closed the needle valves, lowered the float level, & drilled out the fuel inlet (seat) to .125. The idle is cleaner now but the plugs are still dark.

Do I just live with that & jet for down track or am I just completely out in left field ?

Ed Wright · 11-12-2013, 05:13 PM

Don, you may also have to plug the power valve and use larger jets. You may be able to use a lower number (maybe a 35?) Holley power valve. If you plugged it right now you may be able to close the throttle blades enough to cover the transition slots. You likely don't have enough manifold vacuum to hold the power valve shut.
They look about the same, but as many years, and as many Holley & Ford carbs I have rebuilt I never tried a Holley valve in a Motorcraft 2bl. I would verify the Holley valve will work. I would probably just plug it myself. May have to tweak it some to clean it up on the return road.

Good luck.

Dave Noll · 11-12-2013, 05:29 PM

Forgot to mention that, It's already blocked (with a plug from a holley) & I have .066" jets in it.

Billy Nees · 11-12-2013, 06:52 PM

What is your duration @ .050? Lobe sep? Cam grinders tend to want you to put WAY too big a cam in 2V motors. No sense in putting a 7500 RPM cam in a 6000 RPM motor.

Dave Noll · 11-12-2013, 07:22 PM

Billy,
250 @ .050", 299@ .006", 105 lobe separation installed @ 103. Lift right now is .375" with the stock rockers but should be .407". They told me the cam wouldn't be the problem, the carb would. I protested a little cause it sounded big to me but they assured me this was a good one. (it's a Cleveland, run it like one).

G Anderson · 11-12-2013, 07:44 PM

Rochester 2G totally different but I would expose those trans slots play with the float level-need full timing at idle(On mine 39*) my cam is 260@.050 on the intake 107 c/l (installed to 106ic)runs good to about 71-7200 rpm ??? oh and would close these holes to about 5/32"

Ed Wright · 11-12-2013, 08:38 PM

If your spark plugs are dark, I'm guessing it idles best with the idle mixture screws all the way in?

Billy Nees · 11-12-2013, 09:59 PM

First rule of any engine combo, find the restriction, build to the restriction. You've got way too much cylinder head and valve area, lots of valve lift and as I recall way too little compression so you can't be giving up cylinder pressure with a big cam. You've got a 350 CFM carb that's not going to make power at any kind of RPM so why spin it up there? It sounds to me like that big cam isn't making any vacuum.

jmarkaudio · 11-12-2013, 11:19 PM

You need to figure out how to generate more idle vacuum so you can close the throttle blades up. The t-slots need to be covered or you will have a tough time getting it to idle without closing the mixture screws all the way, and even then it won't be enough. More initial timing might help, if you run a distributor with a vacuum advance using the vacuum to add timing under no load can help. If you find that you have all the timing it will stand, I would look at advancing the cam.

As a last resort, make the idle feed restriction smaller, drop it to the point where it's tolerable at idle and doesn't cause a stumble coming up on the throttle. Being a race car you should be able to find the happy medium. You can try to solder the restriction after measuring it, it's the small tube in the middle of the larger one in the booster cluster. You might want to get a spare to play with to do this. The other place looking at what I can find is there may be an idle down channel restriction, making this smaller
will help as well. Again these are last resorts, making more idle vacuum to allow the blades to close is your best bet. Here is a picture to give you an idea on what is going on.

B/S 428 · 11-13-2013, 12:08 AM

i see alot of good input on your thread

11-12-2013, 04:16 PM	#1
Dave Noll Senior Member Join Date: Nov 2007 Location: Div. 6, Eastern, WA Posts: 709 Likes: 2,639 Liked 238 Times in 111 Posts	2 bbl. Motorcraft & a dwell cam On my 351C 240hp combo, the cam that the grinder sent me dwell's @ max lift for 60 degrees. The lowest the idle will go down to is about 1600 which isn't the issue. To get that idle with that cam I have the idle plate screw down about as far as it will go. which has the transition slots completely uncovered & I have a 1/4" hole drilled in each throttle plate. With so much air going thru there @ idle it's pulling fuel out of the boosters. I closed the needle valves, lowered the float level, & drilled out the fuel inlet (seat) to .125. The idle is cleaner now but the plugs are still dark. Do I just live with that & jet for down track or am I just completely out in left field ? __________________ Dave Noll, EF/S ,?/SA 6526

11-12-2013, 05:13 PM	#2
Ed Wright Veteran Member Join Date: Mar 2005 Location: Sand Springs, OK Posts: 8,132 Likes: 896 Liked 390 Times in 170 Posts	Re: 2 bbl. Motorcraft & a dwell cam Don, you may also have to plug the power valve and use larger jets. You may be able to use a lower number (maybe a 35?) Holley power valve. If you plugged it right now you may be able to close the throttle blades enough to cover the transition slots. You likely don't have enough manifold vacuum to hold the power valve shut. They look about the same, but as many years, and as many Holley & Ford carbs I have rebuilt I never tried a Holley valve in a Motorcraft 2bl. I would verify the Holley valve will work. I would probably just plug it myself. May have to tweak it some to clean it up on the return road. Good luck. __________________ Ed Wright 4156 SS/JA

11-12-2013, 05:29 PM	#3
Dave Noll Senior Member Join Date: Nov 2007 Location: Div. 6, Eastern, WA Posts: 709 Likes: 2,639 Liked 238 Times in 111 Posts	Re: 2 bbl. Motorcraft & a dwell cam Forgot to mention that, It's already blocked (with a plug from a holley) & I have .066" jets in it. __________________ Dave Noll, EF/S ,?/SA 6526

11-12-2013, 06:52 PM	#4
Billy Nees VIP Member Join Date: Oct 2002 Location: On a hilltop in Pa. Posts: 4,481 Likes: 3,579 Liked 7,650 Times in 1,726 Posts	Re: 2 bbl. Motorcraft & a dwell cam What is your duration @ .050? Lobe sep? Cam grinders tend to want you to put WAY too big a cam in 2V motors. No sense in putting a 7500 RPM cam in a 6000 RPM motor. __________________ Billy Nees 1188 STK, SS I'm not spending 100K to win 2K

11-12-2013, 07:22 PM	#5
Dave Noll Senior Member Join Date: Nov 2007 Location: Div. 6, Eastern, WA Posts: 709 Likes: 2,639 Liked 238 Times in 111 Posts	Re: 2 bbl. Motorcraft & a dwell cam Billy, 250 @ .050", 299@ .006", 105 lobe separation installed @ 103. Lift right now is .375" with the stock rockers but should be .407". They told me the cam wouldn't be the problem, the carb would. I protested a little cause it sounded big to me but they assured me this was a good one. (it's a Cleveland, run it like one). __________________ Dave Noll, EF/S ,?/SA 6526

11-12-2013, 07:44 PM	#6
G Anderson Member Join Date: Dec 2009 Location: Iowa Posts: 105 Likes: 13 Liked 4 Times in 3 Posts	Re: 2 bbl. Motorcraft & a dwell cam Rochester 2G totally different but I would expose those trans slots play with the float level-need full timing at idle(On mine 39) my cam is 260@.050 on the intake 107 c/l (installed to 106ic)runs good to about 71-7200 rpm ??? oh and would close these holes to about 5/32" Last edited by G Anderson; 11-12-2013 at 07:46 PM.*

11-12-2013, 08:38 PM	#7
Ed Wright Veteran Member Join Date: Mar 2005 Location: Sand Springs, OK Posts: 8,132 Likes: 896 Liked 390 Times in 170 Posts	Re: 2 bbl. Motorcraft & a dwell cam If your spark plugs are dark, I'm guessing it idles best with the idle mixture screws all the way in? __________________ Ed Wright 4156 SS/JA

11-12-2013, 09:59 PM	#8
Billy Nees VIP Member Join Date: Oct 2002 Location: On a hilltop in Pa. Posts: 4,481 Likes: 3,579 Liked 7,650 Times in 1,726 Posts	Re: 2 bbl. Motorcraft & a dwell cam First rule of any engine combo, find the restriction, build to the restriction. You've got way too much cylinder head and valve area, lots of valve lift and as I recall way too little compression so you can't be giving up cylinder pressure with a big cam. You've got a 350 CFM carb that's not going to make power at any kind of RPM so why spin it up there? It sounds to me like that big cam isn't making any vacuum. __________________ Billy Nees 1188 STK, SS I'm not spending 100K to win 2K Last edited by Billy Nees; 11-12-2013 at 10:02 PM.

11-12-2013, 11:19 PM	#9
jmarkaudio Junior Member Join Date: Apr 2008 Posts: 83 Likes: 0 Liked 0 Times in 0 Posts	Re: 2 bbl. Motorcraft & a dwell cam You need to figure out how to generate more idle vacuum so you can close the throttle blades up. The t-slots need to be covered or you will have a tough time getting it to idle without closing the mixture screws all the way, and even then it won't be enough. More initial timing might help, if you run a distributor with a vacuum advance using the vacuum to add timing under no load can help. If you find that you have all the timing it will stand, I would look at advancing the cam. As a last resort, make the idle feed restriction smaller, drop it to the point where it's tolerable at idle and doesn't cause a stumble coming up on the throttle. Being a race car you should be able to find the happy medium. You can try to solder the restriction after measuring it, it's the small tube in the middle of the larger one in the booster cluster. You might want to get a spare to play with to do this. The other place looking at what I can find is there may be an idle down channel restriction, making this smaller will help as well. Again these are last resorts, making more idle vacuum to allow the blades to close is your best bet. Here is a picture to give you an idea on what is going on. Attached Thumbnails

11-13-2013, 12:08 AM	#10
B/S 428 Junior Member Join Date: Jan 2011 Posts: 87 Likes: 18 Liked 154 Times in 34 Posts	Re: 2 bbl. Motorcraft & a dwell cam i see alot of good input on your thread Last edited by B/S 428; 11-13-2013 at 02:50 AM.