How much timing help

[jrr]

The builder of my new carb wants me to dial in more initial timing, going from 12-13 to 18-20 degrees. He feels that with my cam, I am "firing and dragging in about 10% exhaust gas". I am getting a below 3200 rmp lack of sharp response and gas mileage is around 6 mpg from normal 12. Plugs are black on edges and a little damp.

The set up is a little 351/385hp FMS crate. GT 40X 305s, Vic jr, Z303 cam 228/228. Vacuum advance set up with total at 3K of 34 deg. Basically street use at this time.

What do you folks think about the increase in initial suggestion???

Thanks

[JCoop]

Try setting it with the vacuum DISCONNECTED to 34 max at 3500RPM. With the vac connected, it may yield up to 45 degrees at 3500 but that's without load. Also, the vac take-off for the dizzy should be hooked up to the carb base, or the proper port, depending on the carb, not the direct manifold port. You could also just run it without vacuum advance if you're emission exempt and don't care too much about a little loss in mpg. I had problems with a Weber setup and it boiled down to ignition timing to get a proper baseline. Very critical on small blocks, especially when you start upgrading the induction, I think. Your new carb may be setup too rich as well. What carb is it?

The other way is the old school way I was taught: keep dialing in advance until it pings, then back off a bit. Good luck and let us know how it goes.

[Mike Simard]

Initial timing at 18 or so is a good thing on motors like these. Your motor will seem crisper off idle and will burn cleaner. You may need to limit the centrifucal advance travel in order to keep it between 18-34. I use to use an MSD distributor and had an adjustable vacuum advance modified so that it only provided 8 degrees travel, that also improved driveability.

[jrr]

Ray:
Timing was set with vac line plugged to 34. The side elec read shows more than that since it is clean further 1/2 way down the base of the side elec. Everything is hooked up properly for vac, valve cover to rear vac line and front vac line to dizzy. We are jetting down since carb came with 76/86 and 35 idle feeds. The new carb is a 4150 dp, flowed to 780cfm true since I will be changing the heads soon. Ray in all seriousness, I couldn't hear pinging in this thing with good ears. Plugs don't show any signs of detonation though.
By way let me thank you again for the great mirrors.

Mike
FMS says not to go over 16 initial but the more I search I think they are wrong. Biggest issue is I have a duralight set up with FMS/MSD ignition box. Only way I can dial more initial into it is to find someone with the vanishing distributor machine. I must be old since I remember when all the shops had them. I am looking at the Crane set up since it will also handle vac advance initial/total adj with simple screw adjustments. Knowing what I know now I would go straight mechanical but carb builder says I can't retro the carb. Must be more than just plugging the vacuum inlet/outlets.

Thanks
Jack

[Jac Mac]

Jack,

Ray has a supply of upmarket vacuum line blockoff apparatus that he could sell you if you need one!! I dont want to stand on your carb man's toes, but I cant see a problem with plugging the vac until you sort out the mech advance curve. If your duraspark is the one Im familiar with shortening up the mech adv amount is not a major and can be done on the car with the aid of a timing light and helper to call out the rpm for you, if your familiar with what needs to be done.

When did you get 12mpg, with the old carb? or this new one?

Jac Mac

[jrr]

Jac Mac
Thanks for the reply. 12 was with the old carb, 6 with the new one. Have jetted down to 74/84s and plugged metering bowl leaks finally. Car runs appreciably better. Like Ray said, timing is set with Vac plugged at 12-13 initial, 34 total. Signs that with Vac attached I am getting a deal more. Simple fix is I am going to a new Crane set up and mechanical advance.
Jack

[Jerry Clayton]

If you were only getting 12 mpg before you had a problem--if only 6 now you have a bigger problem--Is your carb a Holley or Demon?

If you want to put a smile on your face, put a holley 3310 on it but reduce the primarys to #70s.

You are jetted way too rich for a 780 carb---
What dist do you have?

Jerry Clayton

[jrr]

Jerry
Have a Duralite with FMS/MSD ignition box. Actually plugs look pretty good after several shots at reading with the new jets. The old carb was a 750 4150 with 71/81s. Keep in mind that my engine set up along with gearing makes cruising a little difficult. Only 5 speed used like a toploader.
Car is going to LaRocca dyno as soon as we can fit it in. I will live with rich rather than alternative until we can get some controlled tuning done.

[Jac Mac]

Jack,

We have Westcott Cobra about 15 threads down with a tail of woe about a crane dizzy, hope it was a one of. Looks like you need to get your carb dialed in properly before attacking the ignition, for my money & NZ 2cents worth stick with the current ignition. You sound as though your in my age bracket and may have access to some of the old Ford Muscle Parts books [circa 1970] etc. The info in these should still apply in regard to altering the mech adv curve in your distributor.

Regards
Jac Mac

[rob frink]

I think 18-20 initial is a good move. Get rid of the vacuum advance and use just mechanical adv. If you start with 20 initial, then you only need about 14 mechanical to peak at 34 total. Your mech adv mechanism will need to be adjusted or modified to limit the range of advance to 14 degrees. The MSD billet distributors use bushings, the duraspark style require that you bend the little "ears" that stop the advance mechanism from turning.

20 initial provides more time for the fuel to burn at low engine speeds. You will notice that the rpms increase by simply advancing the idle timing. This allows you to close the carb butterflies to lower your idle speed. By closing the butterflies more, you increase idle vacuum and low-speed velocity through the carb. This allows for better mixing, better low speed combustion and better throttle response. This is how most modern carbs are designed to work. You cannot dial-in your carb until you address the initial timing issue. 20 intial helps the engine run at idle..otherwise, you play games with the carb's idle circuits to try to help the engine run at idle....since the carb's idle circuits have a big impact on the mid and high-speed circuits, the engine will run poorly throughout it's range if you don't get the idle squared away first. Lots of initial timing is used to simply help the motor run at low speeds with agressive cams by providing more "burn time".

[Cobra 29]

Jack, I have a 385HP FMS crate engine. It had the Duraspark distributor and the FMS/MSD ignition box. The distributor was not worth the effort to recalibrate and I wanted an adjustable curve. I installed a small dia MSD distributor and 6AL box.

Wanting as much initial timing as possible and limiting to 34 degrees, I tried to keep the vaccuum advance module for more MPG. The vac line was connected the ventra port on my 4150 DP 650 carb. I could not find a way to limit the travel of the vaccuum module (degrees) so I removed it.

I now have about 15 degrees of initial timing and installed the 18 degree bushing in the MSD distributor. It works great.

Mike, I would love to know where to get a vaccuum advance module for an MSD distributor where you can adjust the travel so limit the number of additional degrees. Most on the market allow you to change the rate of travel (degrees) by changing the spring resisitance against the vaccuum diafram.

[vettestr]

Jrr.
I am just a sideline observer and I know nothing about all details but GO with your carb Pro. If he is a GOD or a tweaker is not in question as you already picked him. Too be fair to the tunner he did not select components and can only tune what is there but unless he told you the combination was not tunable before starting it is on him to make right. If you just brought him carb in a box to rebuild then his job is very narrow.
I am not trying to come off as a smart guy or say anything has been done wrong by anybody, maybe I should have simply asked you to give him free hand to make these almost free changes to finish his contracted project. If it is not right it is on him and if you do not let him finish it is on you.

I edited to say BTW> I believe most engines could improve performance with more timing unless already tweaked to find the max for that engine. Cobra29 = You do not need to worry about exceeding 34 degrees from vac advance under high manifold vac condition. 50 degrees total under max vac conditions would be OK as it goes away the moment a load or loss of manifold vac is sensed. The point it drops out is adjustable with aftermarket advance units but most stock vac advance units will work fine if checked.

[jrr]

Rob
I am tending to agree with you on more initial. It definitely is stubbling below 3000 rpm. Much more sensitive to the cam/intake setup which make power at 3200 and up. Even putting in smaller jets, two numbers or 9% reduction, didn't help, in fact made the low speed bog worse.
Cobra 29
Actually the new Crane unit allows for optically changing the initial on vac advance. Guess I will just bite the bullet. You are right about the FMS setup, simple but it does present some challenges.
Jeff
You are right about following the carb builder, I started this thread to get some educated opinions. The changes are on me since he built the carb not the combo. I do have faith in him, especially when I get the cam on at 3000 the thing screams. Local police just seem to dislike it when I shake the neighbor up.

I do appreciate all of the opinions/suggestions.

Jack