Holley 650 vs stock rebuilt carb
#16
11-10-2011, 05:55 PM
"The vehicle would still run because we are only talking about the primary jets (which predominently is for idle)"
The jets are for the cruise and power circuit and have nothing to do with the idle circuit. The idle system is completely separate. The engine will even idle without jets at all. (Discussed on the Speed Talk Forum some time ago) If the idle AFR cannot be adjusted with the idle mix screws the problem may be with the float level or there is something else going on. At any rate, it's not the jets.
The jets are for the cruise and power circuit and have nothing to do with the idle circuit. The idle system is completely separate. The engine will even idle without jets at all. (Discussed on the Speed Talk Forum some time ago) If the idle AFR cannot be adjusted with the idle mix screws the problem may be with the float level or there is something else going on. At any rate, it's not the jets.
#17
11-11-2011, 04:35 PM
I'm not sure where you're getting your information from.
I have a 600cfm carb on my 300 right now. Stock jetting, as said, was 66 size jets.
I can easily get it to idle low enough to lug at around 400 RPMs.
Size 66 jets was a little rich for my altitude. A wideband O2 sensor in the exhaust came out at around 13.5 - 14.0 on the interstate.
Size 65 went to 14.5 - 15.0
Size 64 went to around 15.5 - 16.0 Plenty lean for an interstate speed cruise.
For the heck of it, while testing some other fuel circuits, I put size 59 jets in it (only 7 sizes below the stock setting) and the only time it would be able to do ANYTHING was when it was functioning on the idle circuit only. VERY VERY gentle acceleration, and my max speed was around 35 - 40 because after that it used the main jets for cruising and the vehicle surged and jerked so bad from being lean that it would barely drive, and I could smell it in the cab.
There is absolutely no way that dropping the main jets 11 sizes would set a 600cfm carb correctly for a 300. I also don't have extras like a performance cam/valves, etc.
The primary jets are also not just for idle. Sure, fuel goes through them for the idle circuit, but once the RPMs start to increase, almost all the fuel is metered through the primary jets for acceleration and cruise.
You set up the primary jets based on the amount of air that can move through the carb. A bigger carb can have more air going through it, which is why it needs bigger jets. The size of the engine doesn't factor into that at all.
A larger engine simply draws on the carb more, but the air to fuel ratio needs to stay the same regardless of engine size.
I have a 600cfm carb on my 300 right now. Stock jetting, as said, was 66 size jets.
I can easily get it to idle low enough to lug at around 400 RPMs.
Size 66 jets was a little rich for my altitude. A wideband O2 sensor in the exhaust came out at around 13.5 - 14.0 on the interstate.
Size 65 went to 14.5 - 15.0
Size 64 went to around 15.5 - 16.0 Plenty lean for an interstate speed cruise.
For the heck of it, while testing some other fuel circuits, I put size 59 jets in it (only 7 sizes below the stock setting) and the only time it would be able to do ANYTHING was when it was functioning on the idle circuit only. VERY VERY gentle acceleration, and my max speed was around 35 - 40 because after that it used the main jets for cruising and the vehicle surged and jerked so bad from being lean that it would barely drive, and I could smell it in the cab.
There is absolutely no way that dropping the main jets 11 sizes would set a 600cfm carb correctly for a 300. I also don't have extras like a performance cam/valves, etc.
The primary jets are also not just for idle. Sure, fuel goes through them for the idle circuit, but once the RPMs start to increase, almost all the fuel is metered through the primary jets for acceleration and cruise.
You set up the primary jets based on the amount of air that can move through the carb. A bigger carb can have more air going through it, which is why it needs bigger jets. The size of the engine doesn't factor into that at all.
A larger engine simply draws on the carb more, but the air to fuel ratio needs to stay the same regardless of engine size.
#18
11-11-2011, 04:36 PM
"The vehicle would still run because we are only talking about the primary jets (which predominently is for idle)"
The jets are for the cruise and power circuit and have nothing to do with the idle circuit. The idle system is completely separate. The engine will even idle without jets at all. (Discussed on the Speed Talk Forum some time ago) If the idle AFR cannot be adjusted with the idle mix screws the problem may be with the float level or there is something else going on. At any rate, it's not the jets.
The jets are for the cruise and power circuit and have nothing to do with the idle circuit. The idle system is completely separate. The engine will even idle without jets at all. (Discussed on the Speed Talk Forum some time ago) If the idle AFR cannot be adjusted with the idle mix screws the problem may be with the float level or there is something else going on. At any rate, it's not the jets.
#19
11-11-2011, 08:08 PM
Postmaster
You couldn't drive a Holley equipped vehicle without main jets but it would idle just fine. The idle circuit is completely separate from the main jets.
It sounds counter-intuitive, but if you did put a 1000cfm mechanical secondary carb on a 300 you'd probably have to jet it richer than stock to get it to run at cruise. There's not air flow to pull enough fuel through the boosters at low speed. But then when you open it up and turn revs it's stupid rich. Yet another reason to avoid a "too big" carburetor.
Then again, I wouldn't recommend a mechanical secondary carb of any size in a truck. At least not if you want any kind of economy. A patient man with a wideband O2 sensor could squeeze a few more ponies out on the top end but it wouldn't be nearly as thrifty around town as an Edelbrock or 4160.
It sounds counter-intuitive, but if you did put a 1000cfm mechanical secondary carb on a 300 you'd probably have to jet it richer than stock to get it to run at cruise. There's not air flow to pull enough fuel through the boosters at low speed. But then when you open it up and turn revs it's stupid rich. Yet another reason to avoid a "too big" carburetor.
Then again, I wouldn't recommend a mechanical secondary carb of any size in a truck. At least not if you want any kind of economy. A patient man with a wideband O2 sensor could squeeze a few more ponies out on the top end but it wouldn't be nearly as thrifty around town as an Edelbrock or 4160.
#20
11-11-2011, 09:29 PM
#21
11-13-2011, 12:06 PM
Post Fiend
However, when dealing with race engines that have long duration-high lift cams that simply cannot hold an idle as low as a street engine, the main jets *can* come into play at "idle", but that's usually over 1,000 RPM.
Its the reverse of the old wives tail about screwing the idle mix screws into will lean out the engine. It will only lean out the idle, and leave everything above idle alone.
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