carb jetting (4160)
Ok so this is my first time messing with the internals of my carb. I have a Holley 4160 750cfm carb. I think I have been needing to change the jetting because it has been running awful rich. Today i pulled the bowls off, I see two jets on the primary side bowl and none in the secondary (just a plate thing). This is a newb question, but are both jets in the primary bowl for primary or is one secondary? How does that work?
Also, the two jets were both 72's... what number jets should I switch to? It ran ok, but stumbled a bit when I floored it at low rpm in higher gears, also lined the combustion chambers of the heads with black sut as well as header ports.
To add to the equation, while the engine is apart im upgrading from the regular edelbrock performer to the performer RPM manifold as well as gasket matching and home-porting the heads. The heads were from a 305 for higher compression and didn't notice the intake runners didn't match the intake manifold's runners until now. I am considering upgrading to 1.6 ratio rockers as well to get a little more cam lift. I don't know if this will be enough of a difference so that the engine will actually be able to use the #72 jets or if I should still use smaller??
Here's my engine's specs:
Holley 750cfm 4160 (vac sec)
Edelbrock performer rpm intake
Comp XE274 cam, 230/236, .490/.490 lift
#450 heads, 64cc chambers, 2.02/1.60 valves, home porting
Comp ratio 9:1
Manual 5sp tranny (NV3500)
4160 series Holley means only 1 metering plate with changeable jets. The rear is a plate ( which can be changed.......complete plate for plate )
Holley sells a kit to convert your 4160 to a 4150. The kit has another metering plate for the secondary (rear) side with changeable jets. I did this to mine.
I would just convert the 4160 to a 4150, drop 2 jet sizes and try it....
Holley also sells a jet assortment box...for about $40...with 30 sets or so
Lot less expensive than buying them at $5 a set
I agree. Just drop two jet sizes and see how it reacts. Then you can go down one or two more if it needs it. 68 is not an uncommon primary jet to run.
I would also offer a word of caution on the head porting. Dont do anything extravegant. Search for "pocket porting" or "bowl porting" on this site and you will find lots of information. That and the gasket matching are all you want to do without a flow bench. Just hogging out the ports could end you up with a real dog or useless heads.
Thanks guys. I'm not going to have the money to convert to a 4150 right now because of the rest of my engine I had to throw money into rebuilding but I will check everything out in there.
People keep telling me that 72's arent that big and pretty standard, so I dunno but from the get-go I always figured the 750 was a little big for my combo anyways so maybe thats why its running rich. I'm hoping my little mods here and there will allow me to pick up some extra power and put that 750 to use.
As far as the porting goes, ive done quite a bit of reading about it. I don't plan on hogging out the runners at all, a light polish on the intake side and a smoother polish on the exaust. My idea is cleaning out the port a little, grind enough to match the intake runners between intake manif. and heads (top of head runners come in and are actually smaller), and deshroud the valves as well as polish up the bowl. The heads were from a 305 so when the 2.02/1.6 valves were installed they have a good bit of shoruding
RE: Carb Jetting
I know you were asking about carb jetting but after looking at your engine specs I think you should go in another direction. If the # of your head is 367450, my books show that is off a 75-76 262 ci. It is for a very small smog engine, the port volumes are so small they will never move enough air to take advantage of the cam you picked. That cam runs from 1600-6000 rpm with a healthy duration, cyl heads and cam is where you want to spend the money for performance. Those heads were designed for a very small bore, I assume you are building a 350, so those chambers will over hang in a 4.00 or 4.030 bore. Also since you have a manual trans, forget the 4160--get a 650 or 750 DP which will make more power, but I would replace those heads if it was possible.
Those heads will work fine if you open the runners up, raise the roof of the port as much as possible for the entire length of the runner, narrow the port divider to 1/8" and work in from there being careful not to cut into the headbolt hole, leave the port floor alone except for blending to short turn into the bowl, match your intake to the new roof height and divider width. Unshroud those valves out to the bore line not gasket line. Blend your lower cut below the seat into the bowl. They'll work fine and probably end up at a 170ish intake port volume. The set I did the chambers CC'd at exactly 64 after the deshrouding and a .030 mill. I did spend a ridiculous 30+ hours on the ports though :D . If you do the port work make some go/no go guages out of heavy wire so you can check dimensions as you cut- this way the ports will be more equal when done.
The 750 VS carbs do seem to have a really rich idle circuit and with your cam the overlap isn't helping much. I bet if you open up the secondary throttle blades just a tad with the setscrew adjustment in the baseplate and readjust your air/fuel mixture screws the richness will drop quite a bit. If this problem is related to rpms below 2500rpms I'd say it's more than likely an idle circuit problem. Make sure your idle mixture is on before making jet changes. JMO's Bob
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