Same Big Block Question, Same Big Block Problem!

[savatreatabvr]

I've asked questions about why my mid 70's C20 454 has no power many times on this forum and I've gotten great answers and advise but nothing really seems to help much. If you click on the attached video you'll see my truck running with a vacuum gauge hooked to it, as far as I can tell the vacuum gauge is showing my truck is running pretty good! Ok I can live with that but why is it still lacking power? Shouldn't the gauge be reading low vacuum or be unstable or something if there's no power?



Sorry about the video quality, it was taken with my cell phone.

[Old Truck Man]

Check your timing chain. A loose timing chain will allow the cam to be retarded and it wont make much power. Some of the cheep roller chains stretch very quickly. A good tune up and compression test , check out the vacuum advance. check for clogged tail pipes. Just hard to diagnose something without being where its at.

[Jake Wade]

Start with ignition timing. What is the initial timing at idle with the vacuum advance disconnected? Is the vacuum advance working? How much advance does it give? What is your total timing with vacuum advance disconnected? and at what RPM do you reach peak advance?

IDK the details of your 454 but, a start would be 10º-12º initial timing at idle with a mild camshaft with vacuum advance disconnected and 34º-36º total all in at 2800-3200 rpm.

Your vacuum advance should supply an additional 10º-15º

[ChevDog1]

Does your engine have the original camshaft? Many 454's from this era suffered from bad cams that went flat. I have replaced a couple from 76-79. Even had a bad one in my 1979 Z28. Might want to pull the valve covers and check lift?

[savatreatabvr]

Originally Posted by ChevDog1

Does your engine have the original camshaft? Many 454's from this era suffered from bad cams that went flat. I have replaced a couple from 76-79. Even had a bad one in my 1979 Z28. Might want to pull the valve covers and check lift?

I've adjusted the valves a few times, (running adjustment) and they seemed fine. All the rocker arms seemed to have the same lift. I've heard the same thing about the cams going flat, not sure how many miles because the 454 is not the original engine so I'll post a video with the valve covers off and running, I could be missing something.

[mud.man.rj]

With the vacumn advance unhooked, run the engine and advance the timing until it goes to it's highest vacumn reading that is stable then back off the timing (retard) 2.5" on the vacumn gauge and should be spot on. Then check your timing marks to see how close you are. The balancers on any old style Gm have a rubber seal between inner and outer halves and can slip over time so the timing marks will not be accurate. Give it a shot and see how it goes. Also make sure your valves are not set a bit to tight, will be a bit of a dog because the valves won't totally close all the way and seal properly so will have a vacumn leak, not usually a problem but just trying all bases.

[savatreatabvr]

Originally Posted by mud.man.rj

With the vacumn advance unhooked, run the engine and advance the timing until it goes to it's highest vacumn reading that is stable then back off the timing (retard) 2.5" on the vacumn gauge and should be spot on. Then check your timing marks to see how close you are. The balancers on any old style Gm have a rubber seal between inner and outer halves and can slip over time so the timing marks will not be accurate. Give it a shot and see how it goes. Also make sure your valves are not set a bit to tight, will be a bit of a dog because the valves won't totally close all the way and seal properly so will have a vacumn leak, not usually a problem but just trying all bases.

The harmonic balancer is probably the original one so I'm sure it needs to be replaced. The timing chain has a chrome cover on it so I'm assuming/hoping it was replaced but I will check it just in case. Don't get me wrong, the truck runs good enough to be a daily driver, it accelerates through the gears fine and cruises fine but it still feels like I'm dragging a boat anchor so I'm just trying to find cheap ways to put some life back in the old work horse.

[savatreatabvr]

Originally Posted by Jake Wade

Start with ignition timing. What is the initial timing at idle with the vacuum advance disconnected? Is the vacuum advance working? How much advance does it give? What is your total timing with vacuum advance disconnected? and at what RPM do you reach peak advance?

IDK the details of your 454 but, a start would be 10º-12º initial timing at idle with a mild camshaft with vacuum advance disconnected and 34º-36º total all in at 2800-3200 rpm.

Your vacuum advance should supply an additional 10º-15º

The only aftermarket parts on the 454 are a Edelbrock 1407, a 1" spreadbore to squarebore adapter, a chrome timing chain cover, a chrome oil pan and chrome MOROSO valve covers. Would the throttle shafts on the 1407 leaking hurt performance drastically or would I just have a rough idle? When I pull the vacuum advance hose the idle slows and then raises once put back on so I'm assuming the vacuum advance is working properly but I will take your advice and check it thoroughly and post the results.

[boggerboy]

sounds like your vacuum advance is working. and it seems like your cam is fine to if your driving it around daily with out any real issues like un stable idle or severe bog. and it doesn't sound like you have noises coming from the top end as if something was excessively worn out or not properly adjusted. how about try pulling your distributor out and inspecting the gear on the end for wear. and then shining your light down the hole and inspect the cams teeth for wear. very common problem and its often over looked.

also have you ever pulled your spark plugs out to see what the cylinders are doing after running for awhile. maybe a compression test on all the cylinders as well just so you know first hand what's going on. a wide band o2 sensor will tell you a lot of facts about what your engine is doing as well. cheers

[mud.man.rj]

So first off if the engine is at a low idle around 650/700rpm the vacumn advance isn't supposed to work, it actually operates off of vacumn once the throttle blades start to open so make sure you don't have full vacumn to it at all times, supposed to be ported vacumn, which does nothing at idle until the blades start to open then the vacumn canister operates and gives the engine some advance, as you see with full vacumn it would throw your base timing off because it would already be advancing the timing and give you a higher idle. So check that you have it on the right port. How about exh, what is your system like, fairly stock? If it is size it up or at the very least install a freeer flowing muffler, take the cork out. One more thing using an adapter plate to a stock intake is not a great idea because if you look down into the intake and at your throttle blades it is a complete mismatch, so the fuel actually puddles in the cross webbing in the top of the intake causeing poor performance, well work what an alum intake is worth to change it up to a more modern intake to match carb, say a performer or performer RPM and that will help quite a bit.

[mud.man.rj]

One more thing, not sure in the video if you have a old style points distributor or HEI, if old points get rid of it ans swap in an HEI and open up the plug gaps to .045 from the .028, get some spark to that baby.

[savatreatabvr]

Originally Posted by Jake Wade

Start with ignition timing. What is the initial timing at idle with the vacuum advance disconnected? Is the vacuum advance working? How much advance does it give? What is your total timing with vacuum advance disconnected? and at what RPM do you reach peak advance?

IDK the details of your 454 but, a start would be 10º-12º initial timing at idle with a mild camshaft with vacuum advance disconnected and 34º-36º total all in at 2800-3200 rpm.

Your vacuum advance should supply an additional 10º-15º

I checked the timing this morning, (cell phone dead so I couldn't make a video) with the vacuum advance off and full vacuum port plugged I've got 14º timing. With the vacuum advance plugged in the mark on the harmonic balancer is about an inch above the 16º timing mark (top mark on the tab) so I'm assuming I've got about 28º - 30º total timing? I neglected to raise the RPMs to 2800-3200 for that reading but how do I accurately read the timing if the harmonic balancer mark is way off the tab? It seems to run best at 14º but I believe that's to high so I retarded the timing to 12º to see how it drives. I'll post the results.

[Corts60]

You need a timing light that has advance timing on it to check total timing.

There is nothing wrong with running vacuum advance off of manifold vacuum. Many engines are happy that way. I would set your initial timing to 10-12 degrees as mentioned above (without advance hooked up) and then try running with the vacuum advance on ported vacuum. See how it responds, and then try running again on manifold vacuum and compare the performance between the two. With a factory or mild cam, you should still be pulling around 18-20" of vacuum at idle. Adjust timing, then hook a vacuum gauge up and adjust your idle-mixture screws one at a time until you achieve your highest vacuum reading.

[Old Truck Man]

Reach down and grab the front crankshaft pulley with your hands. turn it back and forth if the timing chain has slack you will be able to tell. Cheep import timing chains are sometimes loose when brand new. and they really get slack in a short time.

[mrolds88]

+1 on the timing chain. Start there. Ported vacuum is the way to go. At idle it doesn't advance the timing, on the intake it does. When you reach a certain point of throttle, it opens the port and adds vacuum to the advance, on the intake it removes vacuum from the advance.

[savatreatabvr]

Originally Posted by Old Truck Man

Reach down and grab the front crankshaft pulley with your hands. turn it back and forth if the timing chain has slack you will be able to tell. Cheep import timing chains are sometimes loose when brand new. and they really get slack in a short time.

I took your advice and put a socket on the harmonic balancer bolt with the distributor cap off and I've got some serious chain slack! The harmonic balancer moved a good 1/4" back and forth before the rotor would move. I guess I'm in the market for a chain set, lol!

[savatreatabvr]

Originally Posted by mrolds88

+1 on the timing chain. Start there. Ported vacuum is the way to go. At idle it doesn't advance the timing, on the intake it does. When you reach a certain point of throttle, it opens the port and adds vacuum to the advance, on the intake it removes vacuum from the advance.

I thought having full vacuum at idle would give me more response but after reading your post it makes since to use ported vacuum. I'll try it out and post the results!

[BigBlocksRule]

While you're at it, pull that Edeljunk carb and add a 650 Holley vac sec.

And without a cam, headers and a tune, that engine is never going to feel like a big powerhouse. Compression is somewhere under 8:1, heads are peanut port.

[imjeff]

Those mid 70s big blocks put out less than 250 hp. Has it ever felt more powerful? I doubt a new chain is going to get you much as it sounds like it's running well.

[savatreatabvr]

Originally Posted by imjeff

Those mid 70s big blocks put out less than 250 hp. Has it ever felt more powerful? I doubt a new chain is going to get you much as it sounds like it's running well.

It's a whole new ball game now I'd replace the chain set as preventive maintenance rather than better performance, I'd hate for the chain to break when it's 118" here in Phoenix in a few months. Next question, how to install the oil pan to timing chain cover seal, not the crank seal the timing chain cover to oil pan seal? I've done one before but it leaked even using a gasket sealer, any tricks on sealing it properly?

[nekkidhillbilly]

Originally Posted by imjeff

Those mid 70s big blocks put out less than 250 hp. Has it ever felt more powerful? I doubt a new chain is going to get you much as it sounds like it's running well.

my thoughts were around 230 hp stock.

[savatreatabvr]

Originally Posted by BigBlocksRule

While you're at it, pull that Edeljunk carb and add a 650 Holley vac sec.

And without a cam, headers and a tune, that engine is never going to feel like a big powerhouse. Compression is somewhere under 8:1, heads are peanut port.

Me and Holley's don't get along, lol, I've never had good luck with them. I used to be a Quadrajet man until I replaced a 800 cfm Quadrajet with the Edelbrock 1407, the only issue so far is I think the throttle shafts may be leaking and the Edelbrock doesn't use throttle shaft bushing like Quads do so not sure what to do about that?

[imjeff]

Originally Posted by savatreatabvr

Next question, how to install the oil pan to timing chain cover seal, not the crank seal the timing chain cover to oil pan seal? I've done one before but it leaked even using a gasket sealer, any tricks on sealing it properly?

Pull the pan, get everything clean, buy a good gasket set, and don't over torque it. If you try to replace only the front of the pan gasket it will leak.

Jeff

[ChevDog1]

Originally Posted by nekkidhillbilly

my thoughts were around 230 hp stock.

They may have only made 230-250hp, but I bet the torque was 400+. My very first truck was a 1978 GMC 1/2 ton SWB with the factory 454 (this was back in 1985, so the truck was still pretty new then). I ran low 14's in the 1/4 with this truck - completely stock! With a posi rear end and fat 60 series rubber, it would smoke the tires through all three gears.

It's a crappy cell pic of an actual photo - but this is
the truck.

[nekkidhillbilly]

Originally Posted by ChevDog1

They may have only made 230-250hp, but I bet the torque was 400+. My very first truck was a 1978 GMC 1/2 ton SWB with the factory 454 (this was back in 1985, so the truck was still pretty new then). I ran low 14's in the 1/4 with this truck - completely stock! With a posi rear end and fat 60 series rubber, it would smoke the tires through all three gears.

It's a crappy cell pic of an actual photo - but this is
the truck.

not stock they werent the 99 vortec ones only made around 400 ft lbs. most of the ones in the smog years made about 360 ft lbs.

1974

LS4 454 235hp 360flbt

thats the chevelle truck im sure was less. my gray 01 4wd with a 4.8 would run a 15 flat all day with 285 tires. it made about 295 ft lbs. cousin had a 5.3 03 that would run low 14s all day was bone stock. made 320 ft lbs