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Damn valve rattle wont leave my engine

bot0611

bot0611

1/2 ton status
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Oct 22, 2006
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Location
Chula Vista, Ca
9.5:1 compression 383
HEI dizzy with spark box
4160 750 holley

Ok so i have the vacuum advance to a full vacuum port and cruisin on the freeway it rattles at high speeds, today i decided to retard the timing a bit and at it felt better on the thotle at first then it felt weak and the rattle keeped on going, so i saw that the vacuum hose was flat i changed it advanced timing as it was and still the dam rattle

im running 87 octane fuel

so tomorrow il try ported vacuum

and would the vacuum advace on the dizzy be bad?????

what else can i check for?
 
The "rattle" is detonation and everytime you hear that noise it is melting a piston and or breaking piston rings. The vacuum advance should be hooked up to ported vacuum. You can check to see if the vacuum advance pod is working by sucking on the vacuum hose and watching the advance shaft for movement or with the engine running suck on the hose and hold the vacuum and the engine RPM should change.
 
well everyone has mixed opinions on the vacuum advance hose, i have it on full vacuum, i will get the timing right on it using a method i saw here earlyer using the vacuum gauge and change the hose to the ported vacuum and see

BTW i messed with the vacuum advance canister getting the allen key in but dont remember how much turns i gave it, can that affect??? if so how can i fix
 
Alrigh seems my vacuum canister was not working, sucked on it and the pin did not move so tried if it was screwed in and yes it was got it to half point, got the timing and the mixtures screws in right using the vacuum method and wow the thottle felt good, no test drive yet, now what efect will the vacuum canister have on the engine if the diagram is too retarded or advanced??
 
Why are you running 87 octane.......

With that ratio, (assuming it's correct or close enough) you should be running 93, especially with iron heads..
 
Compression ratio. you said it's 9.5.

What heads/size are you running?


I'm going to side with 4x4high on this, and say your having detonatin problems. But I feel it's too low of octane level.


I would ditch the vacuum canister, and just run a hose straight from the dist, to the carb.
 
Yeah thats around it 9.5-9.6 comp

wich a bigger octane fuel run better without the advance, or lets say the advace hooked on to the full vacuum?
 
Run the advance to the ported side of the carb, you don't want it advanced at idle, just when the vacuum drops as you accelerate.

Iron or aluminum heads?
 
The ported vacuum always made sense to me, dont know why everybody says to hook it up to a full vacuum.

Stock iron heads with mild port on them
 
You are fine with 87 octane fuel. You just need to "dial in" the tuning and you should be good to go. Make sure the vacuum is hooked to ported vacuum source though.
 
I don't get it folks. Cruising, manifold vacuum is the same as ported.

The only variable I can see here is how much timing the vac can is pulling based on the engine vacuum it is seeing. Is the vac can adjustable? Have you tuned the distributor curve? If anything, the vac can being bad would more than likely reduce cruise timing overall.

9.5:1 is certainly pushing it on 87, but on flat ground, light throttle, with good heads, you shouldn't be pinging.
 
Yes the vacuum canister is adjustable, no i have not move the curve, although it has stiff spings on the weights and the vacuum canister seemed to have the diafram all the way in i re set it half way like a new on i have here so i guess some tunning in there might be in order but how
 
With an adjustable vac advance on manifold vacuum, you SHOULD be able to crank up the base timing, while turning down the vacuum advance. If it starts easy, and you have a good idle, you should be set. Then find out if it still pings at cruise.

You could also go the other way, and reduce mechanical advance, while increasing vacuum advance. Should make starting easier while still making cruise better. Have to be careful with this though, as mechanical is all you have under heavy load (WOT, hills, towing) so if base timing is too low, you might lose power under those conditions.

To be honest, I haven't played with a carb for years (yay!) but I can tell you that GM pulls upwards of 20* of timing at IDLE with the stock EFI setups (because they can, and they idle far better at that setting) and also run upwards of 36* at cruise, sometimes into the 40's.
 
dyeager535 said:
I don't get it folks. Cruising, manifold vacuum is the same as ported.

The only variable I can see here is how much timing the vac can is pulling based on the engine vacuum it is seeing. Is the vac can adjustable? Have you tuned the distributor curve? If anything, the vac can being bad would more than likely reduce cruise timing overall.

9.5:1 is certainly pushing it on 87, but on flat ground, light throttle, with good heads, you shouldn't be pinging.
Click to expand...

Ported vacuum allows the vacuum advance to come on slowly throughout the RPM rather than FULL vacuum advance if it was hooked to manifold vacuum. While cruising it is the same but idle and light acceleration is where you want it on a ported source.
 
How does vacuum advance come on slowly? It can't change engine vacuum, which is it's source. The throttle plates below the slot mean no vacuum to the vac can, but once the plates are beyond the slot in the venturi wall, vacuum is the same in the intake or at the ported vac feed.

It's not like ported vacuum is a leak. The vac can is a dead end for vacuum, as soon as the ported vac slot is uncovered, the timing changes.
 
dyeager535 said:
How does vacuum advance come on slowly? It can't change engine vacuum, which is it's source. The throttle plates below the slot mean no vacuum to the vac can, but once the plates are beyond the slot in the venturi wall, vacuum is the same in the intake or at the ported vac feed.

It's not like ported vacuum is a leak. The vac can is a dead end for vacuum, as soon as the ported vac slot is uncovered, the timing changes.
Click to expand...

This is where you're wrong. Ported vacuum reacts to the postion of the throttle plates, the less they are open the less vacuum that is produced and the further the throttle is open the more vacuum that is produced (up to the max that can be made given cam selection which at some point will be the same as manifold vacuum). This happens because of the amount of air flow through the throttle bore. Velocity is controlled by the amount of the throttle opening.

To test what i'm saying hook a vacuum gauge to a ported vacuum and also another to manifold vacuum and you'll see that they don't match (except for a given steady RPM).
 
Ok why would you put the vacuum bell to a constant full vacuum?

The advance is suppose to get hooked up to a vacuum source that receives more vacuum as the throttle is increase, aka a port above the throttle plates on the carb.

Secondly your base timing should be in the range of 12-16*. The idea is that you want to tune your vacuum advance with an allen key in the bell until you bring the timing as far advance as you can get it without pinging.

Thirdly be careful. I would do the vacuum advance tuning on a nice hot day like the ones we're running out of because timing advance that doesnt ping in cold weather can possibly ping during a hot day. Thats why some guys on the octane border lines will run 89 or 91 in the summer and 87 in the winter.

And to be honest 89 octane wouldnt be a bad idea for an iron headed 9.5:1 motor. If your complaining about gas prices its a difference of about $3 a tank or you can kick yourself in the ass for building a motor you can afford to feed.
 
I got timiming and the idle mixture screws tunned and the truck seems to run good, i have heard no pinging, took it on a 25' road, hope i got it right, ill try and take to the highway and see how it performs.

Ported vacuum hook up now!

I might of have exagerated there at most $2 more for a better octane fuel:crazy:

Thanks for the help guys, anything else comes up ill post again.

Thanks again.:bow:
 
If you have an aftermarket camshaft, which is not degreed in, you should use a vacuum gauge connected to manifold vacuum to set initial timing, and not timing light.

With the vacuum gauge connected to a manifold source, advance the distributor (vacuum advanced disconnected) until it reaches maximum vacuum reading, and then back it off ½ an inch of vacuum. Then, lock the distributor down there, and connect the vacuum advance to a vacuum port located on the base plate of the carburetor (which is a manifold vacuum source).

I have always gone with manifold vacuum for ignition advance on my older trucks. Manifold vacuum is associated to throttle position, which is what dictates what the ignition advance should be for any given throttle position. Ported vacuum is mostly all on or all off.
 
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