Holley carb cam
 

I just got my engine going and I am playing with the carb. Its a 750 double pumper. I added a 50cc accelerator pump and 0.037 nozzle as a starting point.
My question is what color cam and what position?

The engine is a 427 stroked to 482. I am not looking for the end answer, just a baseline and as others have this engine maybe I can get close and then tune from there.

Big engine, lightweight car... What is your most stable idle RPM?

If the idle is in the 900-1100RPM range, then with 037 nozzles I'm going to say Orange cam, position 2 to start...? (just guessin')

I've got a Quickfuel 780 on mine so can't help you but I would start in the middle maybe green or orange. I think you want a lighter, quicker shot on a light car with big engine.

Adding a 50cc pump has no benefit using 30cc cams.

I would run a runs a 30cc pump, start at orange pos 2.

The nozzle size determines duration.

Run the smallest nozzle and cam profile that gives no stumble at WOT from a standing start.

Thanks guys. The 50cc kit comes with brown and yellow. So I started with Yellow in the #1 spot and got rid of the hesitation. I am still doing some tuning and If my idle must stay 8-900 then I am moving it to the #2. I have a surge/buck at a steady throttle to sort out. Going over all basic adjustments again and then taking lots of vacuum readings.

The engine was dyno'd and I was told to go to a 50cc, but now it has my air cleaner and exhaust.

Who knows, maybe I can get good MPG ....... yeah right.....NOT!

Can we get some more info on your engine combo, and what Holley you have, list number etc?

I have run a 4779 750 dp on many engines smaller than yours, with 30cc pumps on both sides.

If you need to run 50cc pumps, then I would say there are other calibration issues that you are trying to cover up with excessive pump shot.

Troy Patterson from TMP carbs is one of many tuners who aims to get the other circuits to interact better and rely less on accelerator pump volume.

I prefer to run annular booster carbs on the street.

Gary

I've got a 408, running at 13.5 at idle, that stumbled off idle until I went up to the blue cam (started at rd). I guess when I hit the throttle, it would take a bit to get the air moving enough to draw a vacuum on the venturis. A big motor took a bit to get going and needs a big early pump shot until the carb could catch up. Still a 30cc shot.

[quote=Gaz64;1437161]Can we get some more info on your engine combo, and what Holley you have, list number etc?

Ok, her goes. it is a 482 (stroked 427) Has the Performer RPM intake and a 1/2 spacer. K&N air cleaner. The engine was dynoed and main jets were changed at that time.

The carb is a a 750 double pumper PN#4779C – Model 4150. For the changes done to it, the list number shouldn't make any difference. Except age and its a 2005.
Primary jet 78, secondary 80 (done by the engine builder)
Front Accelerator pump 50cc with .37 nozzle (done at the engine builder recommendation)
Rear Accelerator pump 30cc with .37 nozzle
it has 11 inches of vacuum at idle. There are two schools of thought on power valve. manifold vacuum divided by 2 or manifold vacuum minus 2. I am installing a 6.5 power valve this weekend. I may also be trying a 5.5.
It has a high volume (250 GPH) fuel pump regulated to 6psi with a return to the tank. All fuel line including the log was increased to 3/8. My builder recommends that the return line be reduced, so return pressure is greater than the 6 psi yet still keeps the fuel circulating.

I am playing with an intermittent surge and having fun dialing it in. If any throttle is applied, it just goes. It did hesitate prior to the 50cc. Surprisingly it has averaged 12 MPG in the first 500 miles... look out Miata.

2bars - Do you have vacuum advance? Just curious how you are getting 12 mpg...I get 8 mpg on my 482 with QFT 780.

No I do not have a vacuum advance. I have had a lot of highway driving averaging 70 mph. 3:73 gears, 0.64 5th gear, and 28 inch tall tires.

Thanks. I have 3:54 gears, 0.64 5th, and 26 inch talk tires. My mileage doesn't seem to vary city versus highway. Always 8...

I am sure once the engine is broken in, my mileage will have a significant drop along with increased tire wear.

Mine gets 10 mpg. 482, 3.54 with 0.82 5th. Quickfuel 780.

I guess I need to nickname mine Thirsty.

8, 10, 12 - either way we don't pass many gas stations!

Jegs has a cam kit (about $15) which comes with a chart showing the amount of cam lift with each color of cam and some tuning tips. I have tried up and down the scale and landed on orange as the best for my application. Mostly its trial and error. Its so easy that experimenting and changing out takes only 5-10 minutes so enjoy the process and the car.

That's the right way to do it... get every circuit at steady speed cooperating... then see how it accelerates and THAT is when you work on refining the accelerator pump.

I prefer annular boosters for street applications too.

Nothing against Holley personally, but any early model like a 4779 is old school.
Does not have the adjustability of other carbs now, and has porous castings versus machined billet metering blocks etc.

Even a modified 4010 of the 90s, can produce a more even fuel curve with better atomization, earlier main system start up at lower airflows, requiring smaller accelerator pump nozzles and cams.

In a Holley, my favourite is a HP or XP in 750 to 950 cfm.

I like the early BG stuff, just for the BG bowls and metering blocks.

My next carb will be a BG early bowls, billet blocks, RS main body with billet annular boosters, and idle-eze throttle body.

Gary

Well the whole FE thing is old school. I do appreciate the input.
My 428 had this on it with no issues so I think it is just a matter of tune

Follow up on the end result.
The cam wound up being brown in the #2 position with the 50cc pump and 0.40 spray nozzle.
I did turn the secondary idle set screw 1/2 turn and that allowed the primary idle screw to be backed off enough so the idle mixture screws would adjust while at 1000 rpm.

Hesitation...gone