Carb Technical ? Jet Sizes
 

I am ready to make a switch from a 650 Holley to a 650 Speed Demon. I have a 302 Crate Engine. I have visited the Demon site and did some research. My question is this. Why would a Ford 302 need larger jetting than a Chevy 350-ZZ4. The other settings in both carbs are close to being the same.
I am using 64 & 67 in my Holley now. They could easily go up a few sizes without fear of fouling the plugs.

MODEL # 650 Vortec 650 ZZ4 650 Ford 320
PART # 1282818C330 1282010CZZ4 1282010F320
VENTURI SIZE: 1.280 1.280 1.280
IDLE AIR BLEEDS PRI: .059 .059 .059
IDLE AIR BLEEDS SEC: .063 .063 .063
H.S. AIR BLEEDS: .041 .041 .041
BOOSTER TYPE: DOWN LEG DOWNLEG DOWNLEG
BOOSTER LEG: .140 1.40 1.40
SQUIRTER SIZE: 31 STD. 31 STD. 31 STD.
BUTTERFLY SIZE: 1-11/16 1-11/16 1-11/16
AIR HOLE SIZE: NONE NONE NONE
PUMP CAM # # 330 # 330 # 330
PUMP CAM COLOR: PINK PINK PINK
VACUUM SEC. SPRING: N/A N/A N/A
METERING BLOCK # P. A-12400 A-12400 A-12400
METERING BLOCK # S. A-12401 A-12401 A-12401
SIPHON BREAK: .028 .028 .028
E-HOLE POS. #1 .031 .031 .031
E-HOLE POS. #2 N/A N/A N/A
E-HOLE POS. #3 .031 .031 .031
E-HOLE POS. #4 N/A N/A N/A
E-HOLE POS. #5 .031 .031 .033
P.V.C.R. SIZE: .059 .059 .059
TRANSFER SLOT RES: OPEN OPEN .OPEN
IDLE FEED RESTRICTOR .033 .033 .035
MAIN JETS PRI: # 70 # 76 # 85
MAIN JETS SEC: # 78 # 83 # 93
POWER VALVE PRI: 6.5 STD. 6.5 STD. 6.5 STD.
POWER VALVE SEC: N/A N/A N/A
LINKAGE TYPE: PROG. PROG. PROG.
N&S SIZE/TYPE: .110-VITON .110-VITON .110-VITON
ACC. PUMP DIA. SIZE 30CC 30CC 30CC

Bud

Nothing to add! The post was close to falling off the page with no answers.

Most carbs come rich out of the box. My 650 speed demon came with the 70/78 jets. I have a 351W and have jetted down to 66/74.

Roscoe

Hi Roscoe,

I agree with what you are saying 100%. That is why I am puzzled why BG would send out a carb that is supposed to be bench flowed to match the needs of s 302 crate and come with 85 & 93.

I'll agree that most out of the box run rich, but those sizes seem extreme to me.

As you can tell from the chart, 70 & 78 are recomended for 350 SBC.

Bud

That soundz like alot. My 460 w/a bg 850 has 85/93 and I might go doen a size or two....

I ran the thread about "hope they fix it but if not" concerning carb problems with my engine.. I'm not running a 302 but rather a roush 351W however, did have at first a 650 speed demon on it. As a matter of fact we put a couple of them on it trying to get it right. ended up blowing up the motor..well sort of. it was running so rich because of a few other factors that caused the power valve to be open 99.9 percent of the time the engine was running that it ultimately turned the oil into water, well it might as well have been for all the lubrication it was providing. Anyway I never changed the jets from stock on mine but the builders advice once he put it back together was to run primary and secondary as close to "square" as possible meaning jet sizes the same on both sides. I did however make another change that you probably won't have to since you engine is "crate" I went to a 750 mighty demon.. basically a holly 4150HP with mechanical secondaries vice the speed demon with vaccuum secondaries.. The cam profile I am using produces next to zero vaccuum from the moment you touch the accellerator so the vaccuum pot was useless but it also produced the problem that led to destruction.. causing the power valve to stay open so I've also plugged the primary power valve. I am also running 81 in the primary and 82 in the secondary. I don't know for sure but going all the way to 60's on yours might be a bit on the lean side, just my opinion. If it were me I think I'd stay low 70's and try to set them as I said "square". Just my point of view. take care and good luck.

With a Demon carb, I always thought the term "as close to square as possible" refered to the transfer slot opening rather than the size of the jets. If you take the carb off and turn it upside down, there's a transfer slot running vertically down the venturi. The butterflies are supposed to be adjusted so the visible part of that transfer slot is square rather than rectangular. If you have low vacuum, the power valve will stay open at idle causing a very rich condition. The natural tendency of someone incorrectly tuning the carb is to turn the idle butterfly screws in causing the butterflies to open. This will help the engine idle but it causes the transfer slots to be very rectangular rather than square and the engine will continue to run very rich. Instead of opening the butterfly idle screws, a correctly sized power valve should be installed (1-2 inches lower than idle vacuum). The transfer slots should then be squared up and the idle mixture screws should be adjusted to get the idle as close to perfect (fast) as possible. Only then should the butterfly idle screws be adjusted a very small amount to fine tune the idle.

Bud - Holley or BG, those jets sound way too rich. It's possible the carb you received got in the wrong box at shipping.

If you turn the carb up side down and look into the venturi, the small round holes, near the base of the carb, are the idle circuit vacuum feeds. With the butterflies closed at idle, engine vacuum draws air/fuel mixture from the idle circuit. Higher in the venturi you will see the slots. They are the vacuum draw for the transfer circuit. This circuit provides additional air/fuel as you make the transfer from idle circuit to main jet circuit.

Stock engines have loads of vacuum at idle. Butterflies can be almost entirely shut and the engine will idle, and idle richness adjustments can be made properly with the needle screws. High performance engines with overlap cams have very low vacuum at idle, simply because the intake and exhaust valves are open at the same time. Often, to make the car run at all, you have to open the primary butterflies so much that the butterfly exposes too much of the transfer slot and causes problems. One, since you are now running off of the idle and transfer circuits, idle adjustments you make at the needle screws have little or no effect on idle mixture, therefore you cannot get the correct idle mixture. To rich is the problem. Two, since you are using the transfer and idle circuits for idle, movement into the main jet circuit occurs too quick and all kinds of stumbling and bogging things begin to happen. Again, usually too rich. Engine stumbles, fumes burn your eyes, and the overload of fuel dumping into the engine washes oil from your cylinder walls, and further pollutes and breaks down your oil.

One cure for the idle problem is to open the secondary butterflies to allow more air into the engine so it will idle. To make this adjustment you have to have the carb off of the manifold. Turn the carb up side down and you can adjust the small screw in the base of the carb to open your secondary a small amount. This is where the square issue comes in. Adjust both primary and secondary butterflies so the amount of the transfer slot you can see from the bottom of the carb, appears to be square, not rectangular. This adjustment will give you more air at idle, without the need to open the primary so far you expose too much of the transfer slot. Another cure, is to drill small holes in the butterflies to allow more air to be drawn into the engine for idle purposes. Usually, you can get enough air with the secondary butterfly adjustment to cure the problem. Drilling butterflies should be done by someone with carb knowledge.

Most high performance engines do not have enough vacuum below 1,000 rpm for your carb to work correctly, or for you to be able to make correct adjustments. My 460 needs to idle at 1,050 rpm to make 8.5 inches of vacuum. Below 900 rpm the engine will hardly run at all, i.e., no vacuum.

Holley, and/or BG will tell you to run a power valve in the primary. If you do not, the lean condition that occurs between idle and main jet circuit can/will damage your engine, not to mention performance loss. Example, if you have 9 inches vacuum at idle, you need a PV several numbers lower. I run a 5.5 in my 460. As you accelerate, vacuum drops into the opening range of the PV, giving you additional mixture to prevent a lean condition until the main jet circuit takes over. If Holley or BG recommended blocking the primary PV, they would ship them this way.

Out of the box carbs can have a PV rated too high. 6.5 is usually what they come with. If your idle vacuum is too near the same number, the PV can open or fluctuate and flood your engine. If the PV is blown, fuel will constantly be supplied. This may have contributed to Gerry's problems.

Have said too much. Hope I have given you some food for thought. If you are not comfortable making adjustments, you will be way ahead to go to someone who knows exactly what to do. If your carb is off, performance is nil, and all the stumbling and bogging makes the car no fun to drive.

Lynn

Well the up date is that I got to talk to Don Gould at FBO Systems today on the phone. Good news to catch him in the office and not out at a race track on his cell phone.

I did ask about my questions concerning the jets BG plublished for the 302 Crate. Typo error

The bad news is that my new 650 Demon I received today has both the vacuum connections broken off during shipping. No problem with the claim, just an unfortunite delay. I was hoping that if I needed more tech help, I would take Don up on his offer to meet him with my the car at Maple Grove Dragway in a couple of weeks.

During our conversation a few interesting things did surface. He told me that it is not unusual to have as many as four different jets sizes in a carb. I know I have always changed them in pairs.

Also raising the float level on the primeries may help some stumble conditions. He recommended 2/3 to 3/4 level from the start. Some things I never knew.

Lynn,
Just about condition you mentioned I have with a few exceptions. My intake vacuum is 14-15" and idle screws did allow adjusting the idle mixture. The stumble is unbelievable. Indle fumes burn your eyes out of their sockets, etc. I have done everything anyone has suggested to try and retify the problem, just short of drilling the rear throttle plates. Very fusrtating to say the least. I finally just decided to swith to the Demon in hopes of retifing at least some of the problems. Tuning is easier and most can be done with the carb on the intake. We'll see?

CJ428CJ,
I have adjusted the rear idle screw on the holley more times than I really want to admit. I was at the point of going broke buying carb gaskets. Demon sure is nice with the rear idle screw.

I do regret that I did not do a little more research on Ford Crate Engines before making my decision. The B303 cam appears to be a poor choice for a carburated engine, due to it's large overlap.

Thanks to all and I will post the results when we get the Demon installed. Lets hope it's positive.

George,
The float levels on my Speed Demon are just over half on the primary and just under half on the secondary. The primary float level is very sensitive and if a bit high or low will effect performance.
Roscoe

George - If it is that bad at idle, you very well could have a blown primary power valve. Did you check the PV to see if it was blown? A simple way to do it is to remove it from the carb and place it up to your mouth on the flat side and suck on it. If you can draw air through it continuously, it is blown. If you can draw air through it just enough to make the valve open, and not more, it is okay.

I had this condition once, and could not crank the car in the garage, the fumes were so bad.

Lynn

Lynn,

I did it all! Power valve up (9.5), down (4.5),& Standard (6.5).

Rear throttle plates closed, cracked (squared) &open.

Paper clips in fuel metering circuits (different sizes).

Timing lower (10), higher (16), & Normal?(12).

idles find at 850, vacuum 14-15. Eyes burn though(rich?). Lean idle and the stumble is horriable.

The carb just has trouble going from idle circuit to main circuit as you stated eariler.

I have a Speed Demon on my Vette and it is a marriage made in heaven. Idle set with a vacuum gauge was at 13.97, only 3 hundreths off when we checked it with the o2 senser will doing a Chassis Dyno Run. Throttle response is smooth as silk. One jet change up on seconderies and tuning was done( street car tune).
Hopefully I can get the same results in the Cobra.
Bud

Lynn,

I did it all! Power valve up (9.5), down (4.5),& Standard (6.5).

Rear throttle plates closed, cracked (squared) &open.

Paper clips in fuel metering circuits (different sizes).

Timing lower (10), higher (16), & Normal?(12).

idles find at 850, vacuum 14-15. Eyes burn though(rich?). Lean idle and the stumble is horrible.

The carb just has trouble going from idle circuit to main circuit as you stated eariler.

I have a Speed Demon on my Vette and it is a marriage made in heaven. Idle set with a vacuum gauge was at 13.97, only 3 hundreths off when we checked it with the o2 senser will doing a Chassis Dyno Run. Throttle response is smooth as silk. One jet change up on seconderies and tuning was done( street car tune).
Hopefully I can get the same results in the Cobra.
Bud

You probably have done this, but just in case, check the play you have on primary pump arm. Move the throttle with your hand and notice whether or not the pump arm activates the pump instantly, delivering a squirt of gas into the venturi. If there is any play in this area, a stumble or bog will definitely occur. Same applies to secondary pump, but it is not as noticeable as what occurs on the primary side when you are just starting to move.

Lynn

George;

Just as a side not; I'm running a stard Holley 600 dp on a 350hp 351-W,been "tuning" my carb lately,got it nailed down yesterday,everything is "stock" except that I milled off the choke horn.... Factory jets are 66 front and 76 rear,after three jet changes I found that 64 front and 74 rear are just perfect,plugs are nice and brown throughout and no stumble or hesitation anytime....

A 650 dp should not be too different and on a 302 I would guess jets around these numbers should be a good stating point.... I have about 16 to 17 inches of vacum at idle and I'm using a 6.5 power vavle....idles at about 800 to 900 rpms.

David

Lynn,
Yes.
The adjustment for the accelerator pumps are snug against the pump levers. The diaphrams are all OK. Good thought and thanks!

David,
Your jets sound correct. I am using 64 & 67 in the Holley now. It appears that I am running a little on the lean side when inspected the plugs. I am not too concerned with it as I am also running 89 octane gas. It seemed like a good idea for a little insurance until I was able to get the correct jetting and timimg set, especially the total timig. I need to avoid any detenation problems.

I can drop back to 87 when this is finalized, 9.0 CR. I do realize that the 87 will have a small effect idle caused by the fuel burning increased amount.

To be perfectly honest, I also simply decided to stop any more investment in jets for a carb I anticipated replacing. The Holley is eight years old and old technology.

Demon came with 70 & 78 standard. I am thinking it a waste of time to start with jets that high. I am considering say, 66 front & 70s or 72s rear. Timing at 16 initial, total 36-37 in at 2,600 -2,800.
I just need Air Borne to get the claim settled so I can get the new base plate shipped in from BG and get started.
Bud