383 Vacuum Setting/Check Issue?

[ashley_j]

Hey Guys
I need some advice on my 383 and its weak vacuum issue. I have a 383:
30 over
MP Cam 284/484  241 @ 50
906 Heads
Stock Manifold (I like the way it looks on the engine plus I already had it!! )
10:1 Compression
Eddy 800 cfm
FBO Dist/ components
(Timing per FBO Init-16 Mech-18 Vac-14 at 14"? I am not sure if this number)
I know some of these components may not be the best choice but I never intend to race or get good ET's. This is just for the street and some playing  It runs fairly well but I am not sure if it would be better if the vac issue was solved. I only have 90 miles on the rebuild. I have checked with vac gauage and timing with a light.
I have not been able to get any more than 9 Hg at idle. Am I looking for to much or am I over looking an adjustment/check that I missed? Also I am not sure on the Vac timing check/set. Any Help is Appreciated!!!!

 

[spitnl]

Hi,

Have you checked the compression, because it's unlikely to be 10:1 with the 906 heads, more likely to be somewhere around 8.5:1, especially if the pistons are a lot below deck.
If you calculate the compression don't trust the advertised cc's, the open chamber 906 are over 90cc's uncut.

With just 8.5:1 compression the MP 284/484 is a lot of cam.

You could run the dizzy on manifold vacuum if you haven't done so already.
This will give more advance at idle helping the engine making more vacuum.

You could also advance the cam a couple of degrees.
This is more work however, cause you need to take the timing cover off.
Was the camshaft degreed in when you installed it, or did you just line up the dots?

Or install a "smaller" camshaft with less duration and earlier intake valve closing.

Cheers,

Frederick

[ashley_j]

Yes the cam was installed with the degree wheel by the builder. We doubled checked it to make sure of the readings. The 10:1 is an est. but it is no lower than 9.75 to 10. We used KB 400 Solid Dome pistons. The calculations with the cc test put the compression up around 10. The last compression test was fine according to the builder but I do not have the numbers handy. I also want to make sure I understand the vacumm recommendation made. I should bypass the carb vac port and run the dist off the manifold port. Bear with me as I learn with this new engine. I have never gone this far over stock and all the tuning required to get it right. So if the questions sound dumb  just bear with me.       
Thanks

[spitnl]

Ok, looking at the front of the carb you should see 2 vacuum take-off points.
The left one, which sits slightly higher, is for ported vacuum
The right one, which sits slightly lower, is for manifold vacuum.

You currently have your vacuum line on one port, the other is blocked off with a plastic cap.

Put the vacuum line on the take-off for manifold vacuum, if it isn't so already, and put the cap on the other.

Frederick

[ashley_j]

That is the port I am using for the advance. But still only about 9Hg at idle. I am going to do some more testing in different combinations to see what the range is at this point. I hope I do not have to take the timing cover off  .

Thanks for the advice!! 

 

[spitnl]

What is you initial ignition timing?

[ashley_j]

I included the numbers that were recommended to by Don at FBO for the distributor he curved for the engine. I used a dial timing light to set the numbers.
Timing per FBO Init-16 Mech-18 Vac-14 at 14"? I am not sure if this number-How to measure. 

Jeff

[spitnl]

That would be 16 degrees initial timing, vacuum disconnected.
18 degrees centrifugal advance giving a total of 32degrees.
And 14degrees of advance at 14" of vacuum.
At least thats what I make of it, anybody want to chime in?

Use the timing light to check the advance at idle, with the vacuum port disconnecter and blocked.
Should be 16degrees.

Frederick

[Chryco Psycho]

16* at idle is correct the rest is tuned into the dist

[ashley_j]

Thanks Guys!!!!!!!!!!!       On to adjusting the carb....yeah?