Spitting flat spots and timing

[octanejunkie]

Ok.
The 383 in my car starts and runs perfectly at idle. Doesn't even seem to need the choke on a cold day.
Try to get the car moving though and its a different matter. At light throttle (maybe 1/3), and low revs, it bogs and spits back through the carb something terrible.
Once moving and up to speed, just when you think it should take off and really start pull, it just tails off with a giant flat spot.
These problems lessen slightly as the car warms up, but its still pretty bad.
Its running a Holley 750 with vacuum secondries, which is fairly new and in really good condition. That sits on a Weiand intake. Motor is a 68 engine out of a charger.
Heads have recently been removed and had all the valves ground, and re seated.
When i asked the previous owner if he set the timing he just said the distributor went back on the same as it came off....
Engine is super clean and quiet, and i don't have any worries with the internals.
This sound like a timing issue to me. How about you guys? Too retarded, or too advanced?
I'm going to get it timed and tuned up next weekend hopefully, but can anyone give me some tips on a rough tune just to get it driveable and through till next week?
I read about twisting the distributor incrementally and checking for pinking. Help?

P.S the accelerator pumps seem to firing in extra fuel ok

[morgan]

Symptoms match mine when running vac advance.... Spits and bangs dreadfully. Pop the vac advance pipe off (be sure to put a cap over the carb and dizzy end) - see what that does. Will take 2 minutes.

Do you have a timing gun ? Easy enough to check what you are set up at.

Good news is that if the engine was fine before it suggests the curve is good on the dizzy. Short of something split in the carb, not much left.

Thats what I would do first. Eliminate vacuum advance.

[Dave-R]

Yes timing.

Lots of posts on this forum on the subject. Use the search facility.

But in a nutshell the problem is this.

The original timing spec is no good for modern fuel.

You need a LOT of initial timing (something like 18-20 degrees) but no more that 34 degrees total at 3000 rpm.

Two ways to do this;

1) Modify the distributor and reduce the amount of advance timing it gives. This is more accurate.

2) Set the total timing without vacuum advance to 34 degrees and then connect the vacuum can to manifold vacuum instead of the original "timed" vacuum port. This will give you more advance at idle.

[Charger]

my 383 likes 20deg initial 34deg max all in at 2500rpm

Dave’s laws ...

ignition ...
http://challenger.mpoli.fi/board/viewto ... f=13&t=828

carb ...
http://challenger.mpoli.fi/board/viewto ... =11&t=5308

loads on here about timing ...

http://www.moparuk.com/forums/viewtopic.php?t=45873

http://www.moparuk.com/forums/viewtopic.php?t=35148

http://www.moparuk.com/forums/viewtopic.php?t=35679

my recent timing exploits ...

http://www.moparuk.com/forums/viewtopic.php?t=46913

and some more of the same ...

http://www.moparuk.com/forums/viewtopic.php?t=46806

[Steve]

Hi Jon....think we have all been there on this subject and think we have all found the timing is the issue. Really study those links Si posted and in particular, Daves very helpful threads. They really are good!

If you go down the limiter route, Jon ( Turnip on here) from FBO stocks limiter plates with new springs for about £25 that will help you to limit your total advance as you raise the initial well into the teens.

Happy reading matey, its all very interesting stuff!

cheers Steve

[octanejunkie]

Great replies guys.
I'm about to have a look through those posts.
Just a bit of an update.. I had a little fiddle with the distributor. Marked it for reference, then started advancing it just a little at a time. Car immediately started responding quicker and running better. I moved in total around 8-10mm and it stopped spitting back, almost, just an occasional pop.
With doing this though, another issue reared its head. The car became reluctant to idle with the vac advance connected. When i pulled it off and plugged it, it idled nicely. It seems the vacuum from the carb is quite strong at idle, and is just sucking the vac on full advance straight away. Many i've spoken to don't even bother with the vacuum advance, but i'd quite like to have it working, as its good for light throttle, and at present the car still feels very laboured and sluggish pulling away in first..

[Steve]

Just out of interest....what figures have you landed on for initial and total advance?? Dave said something really interesting a while back about timing.....we were discussing finding the ' sweet spot' by turning the dizzy and listening to the rpm increase. When the steep increase in revs starts to shallow out, thats there or thereabouts where the car wants to be. I wrongly thought it was when the rpm stopped increasing all together.

Wonder if its worth trying this, then limiting your total advance when youve got the initial sorted?? Just a thought....

[octanejunkie]

Well, i was only doing it by ear on the drive, and i never got to a point when i heard the revs noticeably increase.
Just doing it a bit at a time, then going round the block to see how it felt. I ran out of time, and burnt £20 worth of fuel, so stopped there.
Hopefully i'll get some figures later this week.

[Steve]

wonder if its too far advanced maybe?? youve marked it where you know it will start and sort of run so why dont you try setting it to 10 deg btc, try it, then just increase a deg or two and see what you find as the best setting? I did this with my Polara and worked a treat. Got rid of my little stumble off idle and pulls well now. Im assuming you are using a dial in type timing light and are happy that the TDC mark on the balancer is accurate with no 1 cylinder (always worth checking)....amazing what you can do with a 1/2" spanner and a timing light eh!!

keep us posted matey and good luck

[Dave-R]

You need to understand that there are two sources for vacuum advance that work differently. One above the throttle blades (called timed vacuum) and one below the throttle blades (manifold vacuum.

The factory advance port on the carb is above the throttle blades. There should be no vacuum at all on this port at idle. If there is it is because the throttle blades are open too far. Which means you are running on the main jets at "idle" instead of the idle circuits.

Increasing the initial timing will increase the idle speed and allow you to wind down the throttle back to 800rpm and get the throttle blades closed enough to get the idle circuits working.

So disconnect vacuum advance for now and get a vacuum gauge on the port. Get the throttle blades to close (no vacuum on the port and an idle speed of 800 rpm (in drive if it is an automatic - be careful!). Once the throttle blades are closed you can adjust the idle mixture screws to just rich enough to give you max rpm at idle. Re-adjust the speed again.

This will get you to a base line to work from.

[octanejunkie]

Its running on its idle circuit ok, as when you wind the screw in, it dies.
The vacuum is being taken off the base of the carb. there is another port further up the carb which has been blocked off, i'll try it from there.

The carb is a normal aftermarket Holley 750. Its running on a Weiand stealth crossplane manifold. Is 750cfm enough for this engine config?

[Dave-R]

Yeah 750 is spot on.

It is running with the timing on manifold vacuum then. In that case disconnect it, set the timing to 34 degrees at 3000rpm. Then re-connect the vacuum and set your idle speed and mixture. Should run OK like that.

[octanejunkie]

The problem is that manifold vacuum is so high it just sucks full advance straight away and then ticks over very lumpy when running from that carb base port.
Stick your thumb over it and it really tries to suck your skin off! Lol
With it plugged, i get a nice even 800 rpm idle.
I'll have another fiddle this afternoon.

[Dave-R]

The vacuum can on the dizzy may be the adjustable type if it is aftermarket. You put a hex key down the port on it to adjust the amount it can advance.