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Discussion Starter #1
I have a 351W with a rebuilt Holley 650 DP, that I'm trying to jet. I just want to get in the ballpark with the jetting so I figured I ask what others have found to be a good jetting combination, and general carb set up such as power valve sizing also. I know what the ball park jetting on a 750 DP would be, but I inherited the 650 and I'm gonna leave it on the car since I drive this car A LOT.
Thanks in advance for the replys.
 

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Because your going to get a huge vacume signal on the small carb your going to run smaller jets than you would on a 750. Its tough to "jet" over the internet because your eleveation will play a roll in how big or small your going to need.

I would start with a 72/78 spread, and figure your heading down from there. If your sea level you will probably end up about 68/74.
 

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Yes, Im sure your going to end up with some real small jets compared to "650" people. On a 408 I wouldn't be at all surprised if you dont end up with something like a 58 primary and a 64 secondary.

Plug reading is the only way your going to know. Lots of the NASCAR 390's are drilled to provide an even higher fuel signal and flow at even lower vacume levels. With the BIG cams the vacume signal below 5000 RPM would/could lead to lean out otherwise. Metering blocks as well as booster passages are drilled to ensure high volumes of fuel are available once its pulled through the jet.
 

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Discussion Starter #6
Kim what my car is doing is it has a lean stutter if I jet the carb back to stock jetting. It goes away when I put 71's in the front. I can pull a plug with either the 67's or the 71's and the plugs are carbon black. The carb has a 6.5 power valve and the engine has a Edelbrock Performer RPM flat tappet camshaft (224int.-232exh. @.050 duration and I think .496-.520 lift. I don't have a vacuum gauge, so I haven't checked engine vacuum yet, but I thought that with the 650 on the 351 it would be fine with the 6.5, though now I'm thinking I might try a 4.5 just to see what it does.
 

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Idle mixture adjustment... blown power valve.

On a 351 you should be able to run an 8.5 or higher, espeacially with that small cam. In about an hour here, Ill be home and post for the Umpth time on tunning a carb in Paul's post. I typed it all up last night, but between the Corral, and something in my computer I was locked out at 11:30pm, and 5:00 am comes awful early. Need to start with a vacume gauge, a tach, and get the idle tune right to go any further.

Going to low on the vacume rateing on the power valve you might lose out on fuel flow at near WOT, without running BIG jets. The power valve lets you pull down about 6 jet sizes from optimum jetting for WOT. It gives you better fuel economy.

The dual power valve combinations are real nice for part throttle conservation, and WOT performance.
 

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650 on a 351 with 73's square is too much jet.

Secondary idle screw??? You mean the throttle stop screw accessed from the bottom of the carb?

First and foremost, you should really get a book if you dot have someone there that knows about carbs.

But, from the beginning, the thing should start with a set of 72/76 jets (just to be safe, but your probably going to end up at about 68/74 give or take two jet sizes)

Second the throttle stop for the secondaries should set the secondaries one throttle blade thickness below the transition slot. That will allow you to get an idle without pushing the primaries well up into the transition slot. Ideally you should run drilled primary blades if the idle speed setting required has you tipping the primaries into the transition slot. The transition slot is non-adjustable fuel flow...

Then once you have things set so the throttle blades are positioned half way decent, you can adjust your idle mixture. Use a vacuum gauge, and tweak the idle mixture screws and the idle speed screw to set the idle at the speed you want, and the highest vacuum reading you can get. Should be about 5/8 turn out on both idle mixture screws. (When tweaking on them adjust by about a 1/8 turn at a time.)

Once you get an idle that won’t blind you. Time for a couple boxes of plugs and a day at the drag strip with a jet kit.

Get a plug reading with a WOT pass, and NO idle time. Read the ground strap for proper jetting.... The ground strap should be discolored from the tip towards the bend about twice the thickness of a center electrode. Check the porcelain (use a 10x magnifying glass and a light to see) for detonation (specs of aluminum) and read the plug threads for proper heat range. Should see about two threads with turned color.

Your bog and miss sounds like the big jets, and the power valve dropping open. Its fuel fowling a plug or two.
 

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Discussion Starter #9
I have a Holley manual, and that is where I have been getting some of my information. I haven't used a vacuum gauge to set the idle mixture screws, but the book says that you can use the tach. I turned the screws in until the engine choked down at idle and then started screwing them out a 1/8 of a turn each at a time until I got the highest rpm reading at idle. The Holley manual for my carb says that the front jets should be 67's and the rear jets should be 73's, and the power valve is a 6.5. I have taken it to the track, and I get the best plug reading with 69's in the front and 74's in the rear. Even with me doing that, at WOT when I pull the plugs they look good, but say I drive 18 miles to my job,under normal driving conditions and I pull a plug, it will be carbon black. The car idles like a champ, I have no problem with that, but when I use a set of jets less than #71 in the front primaries I have a lean stutter between the rpms of 1200-1800, especially when I have the engine under a load. The smaller the size jets, the worse the condition is. I have replaced several power valves in it thinking that they were the culprit...only using 6.5's and it doesn't change anything. I have messed with several carburetors over the years on various cars of mine and haven't seen one run like this. I at first thought it could possibly be a vacuum leak, but I would think that the plugs would show a lean plug, not one that is carbon black. I understand what you are saying about the transfer slots, but I have always associated that with a carb that has bad idling characteristics that you cannot resolve with the idling mixture screws. I am by no means a know it all, or "the man", but I usually have no problems tuning a carburetor.
 

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Check your float setting. I wager your dumping raw fuel from the vent tube. Are the venturies of primary or secondary side dark with soot or carbon?

Use the vacume gauge to set idle mixture, save a few pennies in the gas purchase arena, your rich.

The lean pop/bog is an accelerator pump shot issue. Dont run the thing fat to tune around what should be a more agressive pump shot.

Reading plugs... DONT read the porcaline. I think I described plug reading in that diatribe ment for another post. The ground strap is how you tell if your rich or lean at WOT.

A new ground strap should discolor about the length of two center electrodes to indicate proper combustion processes.

Move the pump shot cam to the #2 hole, on both the arm and the cam. Then look at moveing up in squirter size of the bog isnt cured. Your leaning out when the throttle blades are moving, not at steady cruise.
 

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Good info Kim, as usual.

I have a carb book, but I guess I haven't had it out in the shop and followed it step by step when I was actually doing the adjustments.

Yes I was talking about the secondary throttle stop screw under the side of the carb. The reason we opened them up more was to get the primaries back down to a more reasonable level.

I thought that you were supposed to have a power valve with half the rating of your idle vacuum? What should I change mine to, like a 7.5 or 8.5? Idle vacuum (right now) is about 10"

So on the idle I want to set the speed I want, then adjust the mixture screws until I get the highest idle speed, then lower the speed and re-adjust again until I get to my desired idle? Or adjust it to the highest vacuum and then lower the speed?

I'll try putting some different jets in it and see how it reacts. I think I may have the power valve opening, I only ran the engine for maybe 30min and I sucked down my fuel can about 8-10"
 

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Manual Trans car 2" below Idle. Auto car, 2" belwo standard cruise is recomended. Unless its got a MONSTER cam, usually something between 6.5 (stock) and 8.5 will suffice.


Idle speed/mixture setting, adjust mixture, reset idle speed. Adjust mixture, reset idle speed etc... till ya get highest vacume.

Ideally when you pull your gauge, and initate a vacume leak, the idle will pick up about 200 RPM. Proper mixture is just an eyelash RICH.
 

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Sounds like I should just wait til I road-test the car to mess with the power valve, since its an auto and I don't really know what the cruise vacuum is going to be.

When adjusting the idle mixture in each step, what are you shooing for before you re-adjust the idle speed? I guess thats what I'm not getting yet. You want the highest vacuum at the end, so do you adjust the mixture at each step for the highest vacuum at that speed as well?
 

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Set your tolerable/desired idle speed. Then play with the mixture screws to achieve max vacume. Odds are idle speed will change, readjust the idle speed screw, then readjust the mixture screws. Usually get it right in two cycles.
 

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Discussion Starter #15
Kim, the front float was what I thought was the problem originally, so I had double checked the float height, and went as far as to buy and replace the front float. The primaries and secondaries look very clean with no signs of soot, or carbon. When I had the carb jetted for best performance(69's front-74's rear) at the track, the porcelain was Pecan tan and the electrode had discoloration on the strap. The guy that I bought the car from had size 31 squirters in the primary side, but I changed them back to 28's when I first started messing with the carb because I thought it would be easier to put everything back to Holley spec and start tuning from there. Tomorrow morning I'm going to hook a vacuum gauge up to the engine and set the idle mixture screws, and check vacuum. I am also going to change out the squirter on the primary and see if that helps.

Thanks Kim
 

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Discussion Starter #16
I hooked the vacuum gauge up this morning and got the highest reading (11.5)with the screws turned out 1 1/2 turns. At first with the car idling in the driveway the rpm didn't change, but as soon as I pulled out and came to a stop sign the idle moved up almost 100 rpms. The car seems to be driving fine, but I have yet to pull a plug to see what they look like. If they are still carbon black under normal driving, should I just accept it or should I continue to tune? The reason I am trying to get the best tune for daily driving is that this is my primary vehicle and is on the road a lot since I have to drive 36 miles to work, then when I get off work I have to drive 32 miles to pick my son up from day care. I have a race car that I race, so the ultimate 1/4 mile tune is not ideal for my daily driver (even though it does see the track from time to time).
Thanks for you input Kim, you have been patient and very helpful.
 

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1 1/2 turns out?? Wholly christ what cam is in that thing? You have a vacume leak!!!!!!

I have never seen a car take more than 3/4 turn out on a gasoline engine.

On alcohol I think Ive got all 4 corners (times 2 carbs) at 1 1/2 turns out. (Air bleeds removed from the idle passage)


You got something else going on and are masking it with copious amounts of fuel.
 

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I think mine were over 1 turn out also, and the only vacuum lines hooked up are 1 to the distributor (the port out the side of the primary block) and one to a PCV valve off the rear port.
 

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Discussion Starter #19
I thought that I might have a vacuum leak, but I have already checked and didn't find one....I even tried the old trick of putting my hands over the primary bowl to see if the engine speed picks up, and it choked the motor like it's supposed to, so I don't think that I have a vacuum leak.
 

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That test would only work if the carb hadn't been adjusted around the vacume leak. Ie... the carb and car were running fine and then got the vacume leak.

Best way, pull all the vacume conections from the manifold/carb. THen tune the thing. Use carb cleaner to check that base and or EGR plate, or carb spacer etc.... is sealed. A propane torch (unlit) works real well if your careful and doesnt make a mess.

Once you have it tuned with all vacume accessories removed, start adding things one at a time until you find the AIR source. Odds are its a brake booster, or PCV valve.
 
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