Holley carb and performance, what do you advise me to do ?

Hello guys, I just mounted the 4V Holley 1850-3 carb in my 72 Mustang Grande.

The car has a 351C engine and there was a 2V Motorcraft carb in it.

So I bought a Weiand 4V intake and that used Holley 600 CFM carb. Now everything is set OK and the car runs great, a bit less HP at lower engine RPM, but when I go over 3000 RPM it opens nicely and the engine delivers a lot more power.

I'd like to try something (in the future) that can deliver more power at lower RPM, what do you advise me to do ? New carb, more CFM ?

The engine in the car isn't the original one, it's a 1971 351C 2V (now turned into a 4V).

You don't want to go bigger cfm to get better low end power.

Check the power valve to see what vacuum it is set at. (You will have to take off the front bowl to check). A good rule of thumb is 1/2 of whatever your idle/cruise manifold vacuum is. It's possible it isn't opening soon enough (or too soon). Do you see any black smoke from the exaust at the lower RPM's?

You can also play with the spring settings on the vacuum secondaries. Believe it or not, they could be opening 'too fast'. Often people will say they can 'feel the secondaries' opening but what they really feel is the engine catching up to the added available air flow. There is a spring kit you can buy with several different springs. Do not try to modify the spring, do not remove the check ball.

You might look at the timing and how fast you are getting to about 34 degrees advance. Clevelands like a lot of initial timing and they like to get it in before 3000 rpm (general rule). Do you have a vacuum advanced distributor? Is it hooked up to manifold or ported vacuum?

Only change one thing at a time.

Thanks, that's what I thought, so maybe it's that spring thing that has to be adjusted. Which spring kits do you advise me to get ?

You don't want to go bigger cfm to get better low end power.

Check the power valve to see what vacuum it is set at. (You will have to take off the front bowl to check). A good rule of thumb is 1/2 of whatever your idle/cruise manifold vacuum is. It's possible it isn't opening soon enough (or too soon). Do you see any black smoke from the exaust at the lower RPM's?

You can also play with the spring settings on the vacuum secondaries. Believe it or not, they could be opening 'too fast'. Often people will say they can 'feel the secondaries' opening but what they really feel is the engine catching up to the added available air flow. There is a spring kit you can buy with several different springs. Do not try to modify the spring, do not remove the check ball.

You might look at the timing and how fast you are getting to about 34 degrees advance. Clevelands like a lot of initial timing and they like to get it in before 3000 rpm (general rule). Do you have a vacuum advanced distributor? Is it hooked up to manifold or ported vacuum?

Only change one thing at a time.

Click to expand...

http://www.holley.com/20-13.asp

They are available from a lot of on line places. Here in the states they are at some local auto parts stores as well.

Did you adjust the timing at all when you put the new carb on?

Is this the intake? http://www.summitracing.com/parts/WND-7516/?rtype=10

If it is, the torque loss at lower RPMs is due to the intake manifold being a single plane design. A larger carb would just make the low RPM torque worse.

Chuck

Yes timing is adjusted and yes this is the IM I have, anyway I don't really understand how a dual plane IM would do better ?

Thanks.

Yes timing is adjusted and yes this is the IM I have, anyway I don't really understand how a dual plane IM would do better ?

Thanks.

Click to expand...

This is not the most technical explanation but, I think it will explain why the dual plane would work better for your application. Chuck http://www.popularhotrodding.com/enginemasters/articles/hardcore/0601em_intake/viewall.html

Fantastic article!!!

Thanks ! That's great input !

That is a really good article. I loved the last line:

"While logic dictates that a single four can meet any street engine's requirements, emotions ensure that multi-carbs are here to stay."

Guilty as charged...

The car runs a lot better right now ! Still sometimes have the backfire in the exhaust but it's a lot more powerful after a fine tuning !

humm, do you have a dual Vacuum advance hooked up?

backfire still means a bit retarded on the timing.

don't go by the book on engines you have to feel them out so they run right.

a dual advance will retard timing depending on engine load. not sure if you have one but it could be causing problems on deceleration.

baring that, are you running any Vacuum advance on the motor or just mechanical?

check total timing you want it 32-36 degs total.

were the springs for the mechanical advance changed to lighter ones or are they stock?

are you using the stock distributor?

when initial timing was set did you lower idle Rpms in addition to blocking off the vaccum advance. one mistake is sometimes people set the initial timing at normal idle rpms lets say 675 rpms. at that rpms the mechanical may open slightly. what can happen is when you slow down the timing may drop below your initial setting as the mechanical advance comes completely off so it can cause a backfire. you want the engine timing set with the vaccum advance off and plugged, and the idle rpms so low the engine is barely running smoothly. use the timing light and lock it down at that point, then raise rpms up and have somebody sit in the car, put the car in gear "D" and then set initial Rpms under load, then set the idle air bleed mixture screws for max vacuum, you will have to go back and forth keeping the idle steady as you lean or richen the bleed screws.

once you find max vacuum at the correct idle rpms richen the screws just slightly 1/16 of a turn.

that will insure deceleration is set well, if you still find some backfire, a lot of articles talk about a a 1-2 degree timing advance when changing your intake or installing headers.

if you can increase timing i would try it.

how is the engine when its cold, is it really 'bitchy' some hesitation and a slight stumble you can feel in your pants? i was locked into 10 degrees initial for a few years, i tried everything to ditch this stumble in between the the choke plate opening all the way and hitting correct engine temperature i finally gave up and just bumped up 2 degrees and it helped a lot, then a read an article stating there was a mistake with my setup and the recommended initial 10 was incorrect and the new standard became 12 initial, go figure.