Timing on a 400 Pontiac

455gto · Poncho Master!

Timing on a 400 Pontiac

apairofponchos · Veteran Member

Is it stock? What year?

From my 68 Manual...

Centrifugal

Start: 0-2 degrees 500 RPM

Intermediate: 5-7 degrees 1000 RPM

Max: 15-17 degrees 2500

Vaccum

10-12 in Hg to start advance.

17 in Hg for full advance (which is 10 degrees)

There were numbers for different flavors of Pontiac ignitions, but

they should all be within a degree or two of timing and an inch or two

of vaccum. You may end up with 36 to 40 degrees total timing before you have timing where the engine likes it best.

455gto · Poncho Master!

It's a 1970 GTO.When I got the car it has an excel coil,headers,a very lumpy cam and an Edelbrock intake.I put on a Edelbrock 600 performer carb on.
The inline fuel filter is losing pressure,so I'm going to steel lines and the Edelbrock inline filter.
The vucuum adance is toast and I'm ordering a new one.
The car was at 18 degrees and I had trouble starting when hot.Now it's at 8 degrees and no power at 3000rpm.If the vacuum advance was working....might solve that problem.

macsleuth · Guru

The correct amount of timing is what the engine likes.
Try a few different settings and see what works the best for you.
The factory settings are a compromise between performance and fuel economy.

apairofponchos · Veteran Member

It is very possible that the difficulty starting when hot is the starter (solenoid) being overheated by close proximity to exhaust pipe and the fact that these engines do tend to run hot. Common problem with (US) Pontiac V8 engines. The advice I received when I had this problem was to either a) go with an external solenoid; b) replace the existing starter with an aftermarket "mini" starter which will allow for more clearance; or c) install a heat shield between the starter and exhaust. This may or may not be what is causing your problem, but it's worth considering.

455gto · Poncho Master!

I also thought it was a hot start problem,but after I get the car started when hot. If I shut it off it starts right up each time.
So I'm thinking timing?Maybe play with that like macsleuth said.
Or it's fuel.In the carb,line or fuel pump?
Take one thing at a time and eliminate it as the problem.Timing is the cheapest to fix biggrin

Ghost Post · Poncho Master!

So the car ran fine at higher revs when the timing was bumped ahead?.
And now doesn't w/ it knocked back and vacuum advance isn't working for sure?.

Check your centrifugal advance isn't stuck in or out, change the vacuum advance and set your distrib. just back from where you get the pinging of too much advance..

Doubtful but best confirm there's no exhaust obstructions also that your timing chain hasn't jumped and/or has been installed off a tooth..

Also look for the possibility of holes in the fuel line between the tank and pump and check to confirm your cam lobe is Ok..
Sometimes it's good to use a temporary electric pump to isolate fuel pump problems.. If no elec pump available, create a gravity feed to the carb from a container suspended higher than the carb..
Make sure there's not an intake leak too...mist some methal hydrate out of an old Windex bottle around suspect areas at idle and listen for or have a helper watch a tach for any increase in revs..

Be sure you keep a fire extingusher at hand w/ the above fuel test methods...or anytime!.

Good luck!.

-- Edited by Ghost Post at 16:47, 2008-08-13

455gto · Poncho Master!

Ghost Post wrote:

So the car ran fine at higher revs when the timing was bumped ahead?.
And now doesn't w/ it knocked back and vacuum advance isn't working for sure?.

The car ran good at 18 degrees,but was popping at higher rpm.Like it wasn't getting enough fuel.Low rpm to 3500 was fine.Also hard starting when hot.

Now with it knocked back.It starts fine when hot.Seems to have alittle more horses at bottom end,but now starts craping at 2900 rpm.

I got a guy to check my vacuum with a gauge.I soon as he would pump it up.It would drop just as fast.

BILTIT · Veteran Member

Sounds like you have more than one problem.

I assume you are talking about initial timing (8&18 degrees)? Initial should be fine anywhere between 6-12. Total should be around 32-34.

Whats the compression? heads? cam? intake?

Pontiac motors do not like the edelbrock carbs, the secondary flap does not open at the proper rate and are non adjustable in the Non-AVS carbs.

As mentioned check all timing components for proper working order. Check fuel lines, pump, and filter.

Make sure your timing works first then move on to the carb.

Vacuum advance has nothing to do with power, its only for economy at cruise and can help at idle. I ran my 455 without vacuum advance for many years, i finally hooked it up to help with the idle due to a lumpier cam.

-- Edited by BILTIT at 14:16, 2008-08-14

455gto · Poncho Master!

BILTIT wrote:

Sounds like you have more than one problem.

sounds like it.

I assume you are talking about initial timing (8&18 degrees)? Initial should be fine anywhere between 6-12. Total should be around 32-34.

Yes on the intial timing from 18 to 8 degrees

Whats the compression? heads? cam? intake?

The compression in 160 across the board.The heads are #15.The cam is unknown,but sounds lumpy.The intake is also Edelbrock.

Pontiac motors do not like the edelbrock carbs, the secondary flap does not open at the proper rate and are non adjustable in the Non-AVS carbs.

As mentioned check all timing components for proper working order. Check fuel lines, pump, and filter.

Make sure your timing works first then move on to the carb.

Vacuum advance has nothing to do with power, its only for economy at cruise and can help at idle. I ran my 455 without vacuum advance for many years, i finally hooked it up to help with the idle due to a lumpier cam.

I thought the vacuum adavance works at the higher RPMs ?

-- Edited by BILTIT at 14:16, 2008-08-14

I'm changing the fuel lines from the pump to carb.As they are rubber lines...I,m changing to steel.Also the plastic filter to steel as well.A new vacuum advance as well tommorow.Take it for a run and then play with the timing.Hopefully this will work.If not I'll be going over the advice and start checking it off the list

455gto · Poncho Master!

I think I found out the problem.My cam is worn flat on the exhaust valve on #5 cylinder. cry

BILTIT you were right.A new vacuum advance did nothing.