timing

[jseremba]

My 1984 4 plus 3 is bogging above 3000 rpm. So I checked the fuel pressure. It is above 15 psi. So decide to check the timing. I have some small misses at idle. But the car idles at 700 rpm when it is warmed up and frankly runs good. So I put a timing light and cannot find the mark. I shut the car remark the mark and restart the car. Still no mark and I did disconnect the EST wire.
So bring the engine up to TDC, pull the cap and the rotor points to number one. So I screwed with the timing and now have a rough idle and more misses and bogging at lower RPM.
I am thinking of putting the light on number six. And also lifting the car putting the light on number one and getting under the car to find the mark. The engine has 74 K on it. I set the timing. USING the mark about a month ago. Car ran fine except for some bogging at high revs.

Thank you for the help

Joe

 

[vetteoz]

I put a timing light and cannot find the mark. remark the mark and restart the car. Still no mark

Maybe the outer ring with the timing mark has moved from the correct position; not uncommon for the rubber to break down with age allowing
the outer ring to move.
Only way to verify that TDC indicated on the balancer is actually true TDC is with a piston stop in the spark plug hole

 

[jseremba]

Maybe the outer ring with the timing mark has moved from the correct position; not uncommon for the rubber to break down with age allowing
the outer ring to move.
Only way to verify that TDC indicated on the balancer is actually true TDC is with a piston stop in the spark plug hole

So this is how stupid I am. The dial on the back of my timing light was not on zero. this is now the second time I forgot to check that. The other time I remembered before having a panic attack.
But I am still breaking up over 3000 rpm. I am thinking that I need new injectors. I am thinking of going to truck TBI's. Any suggestion as to which ones I should use.

Thank you

 

[vetteoz]

But I am still breaking up over 3000 rpm.

Breaking up under load is more usually a spark /Ign problem
How good are your plug wires?

 

[jseremba]

Breaking up under load is more usually a spark /Ign problem
How good are your plug wires?

Plug wires are good. What is the timing on the 84 Corvette at 3000 rpm. I am thinking that because my timing light has the advance knob I can set it to that number and bring the car to 3000 rpm. This way I can check the advance at high rpm, correct?

 

[geekinavette]

Been a while but hopefully my memory isn't totally gone, and this is all assuming a bone stock engine...

Set your timing at 6 deg BTDC with the EST disconnected and forget about it. (but don't forget to plug the EST back in) Beyond that all you can really check is that the mark goes advanced when you plug the EST back in...and chances are you will need to tweak the advance knob to even see the mark. As long as that's happening then the chances that the computer is screwing up the timing are pretty slim (unless it's not a stock PROM?)

You mentioned checking fuel pressure...under what conditions? The 84 pump is notorious for "appearing" to work just fine at low loads...but open her up and it falls on its face. Upgrading to a later TPI pump can only do good. Also assuming that you've checked/replaced the fuel filter with a GOOD one? (hey it's bit me in the ass more than once!)

DO NOT replace the injectors with anything other than the stock injectors! The flow rate of the injectors is a critical factor in the fuel mixture delivered to the engine. Changing the injector flow rate requires re-tuning of the fuel tables in the computer PROM. If do you at some point want to go down this road...get it running right in stock form FIRST, else you'll chase your tail forever trying to get it to run right.

The most common issues that will cause the symptoms you describe are imbalance between the throttle bodies, and/or a vacuum leak somewhere (intake lid, throttle shaft holes, EGR valve are all common culprits) causing erratic idle issues. The puking at higher RPM could easily be caused by a pump giving up.

You also mentioned the wires were "ok". Determined how? Not to make assumptions here...but often a DC ohm-meter is used to do this check...and this alone is not sufficient to determine if a plug wire is good or bad.

Definitely verify that the timing mark is at TDC where it should be as vetteoz mentioned. This is VERY common.

 

[jseremba]

Been a while but hopefully my memory isn't totally gone, and this is all assuming a bone stock engine...

Set your timing at 6 deg BTDC with the EST disconnected and forget about it. (but don't forget to plug the EST back in) Beyond that all you can really check is that the mark goes advanced when you plug the EST back in...and chances are you will need to tweak the advance knob to even see the mark. As long as that's happening then the chances that the computer is screwing up the timing are pretty slim (unless it's not a stock PROM?)

You mentioned checking fuel pressure...under what conditions? The 84 pump is notorious for "appearing" to work just fine at low loads...but open her up and it falls on its face. Upgrading to a later TPI pump can only do good. Also assuming that you've checked/replaced the fuel filter with a GOOD one? (hey it's bit me in the ass more than once!)

DO NOT replace the injectors with anything other than the stock injectors! The flow rate of the injectors is a critical factor in the fuel mixture delivered to the engine. Changing the injector flow rate requires re-tuning of the fuel tables in the computer PROM. If do you at some point want to go down this road...get it running right in stock form FIRST, else you'll chase your tail forever trying to get it to run right.

The most common issues that will cause the symptoms you describe are imbalance between the throttle bodies, and/or a vacuum leak somewhere (intake lid, throttle shaft holes, EGR valve are all common culprits) causing erratic idle issues. The puking at higher RPM could easily be caused by a pump giving up.

You also mentioned the wires were "ok". Determined how? Not to make assumptions here...but often a DC ohm-meter is used to do this check...and this alone is not sufficient to determine if a plug wire is good or bad.

Definitely verify that the timing mark is at TDC where it should be as vetteoz mentioned. This is VERY common.

First of all thank you for the help.
I tried to balance the TBIs but used the wrong vacuum port and kept drinking up all my fluid, doh!
ass far as the ignition system. The cap an rotor have about 500 miles on them and the wires have about 1500 miles on them. and the mark is in the correct place. Brought the engine up to TDC and pulled the cap. Which is why i was so terrified. I brought the mark up and then lost it when I ran the car. Forgot all about the advance dial on the timing light.
Setting the timing. How far should the timing advance when you connect the EST. Right now it is 8 before TDC wit the EST removed.
As far as replacing the fuel pump. The TPI pump produces twice the pressure of the CFI pump. So if I switch do I have to use an inline regulator?
and thanks again

 

[geekinavette]

There are a lot of variables that determine total timing...who knows where it will end up for a given set of conditions.

Fuel pump does not control pressure, it provides volume. There is a variable restriction inside one of the TB units (drivers side I think) that controls pressure. Changing to a pump with better volume capability has no impact on pressure. This has been done by many 82 and 84 owners, including myself when I had one.

 

[jseremba]

There are a lot of variables that determine total timing...who knows where it will end up for a given set of conditions.

Fuel pump does not control pressure, it provides volume. There is a variable restriction inside one of the TB units (drivers side I think) that controls pressure. Changing to a pump with better volume capability has no impact on pressure. This has been done by many 82 and 84 owners, including myself when I had one.

The reason I asked about the timing is because I think I recall, I think, that when I reconnected the EST the timing at idle did not move. I will recheck, that. And thank you for the info with respect to the fuel pump. I will have to change it. I really do think it is fuel supply.
thank you

 

[geekinavette]

Well it HAS been a while...but as I recall you should see some additional advance even at idle, but under certain conditions maybe not I just don't remember that well. However if you give it even a little RPM increase you should see a very noticeable increase in advance. If that is not happening then yes you definitely have a problem. Do this with the engine at full operating temperature!

Once you get everything working properly you'll be amazed at the difference a higher volume pump will make. The stock pumps were just junk...barely able to keep up.

 

[jseremba]

Well it HAS been a while...but as I recall you should see some additional advance even at idle, but under certain conditions maybe not I just don't remember that well. However if you give it even a little RPM increase you should see a very noticeable increase in advance. If that is not happening then yes you definitely have a problem. Do this with the engine at full operating temperature!

Once you get everything working properly you'll be amazed at the difference a higher volume pump will make. The stock pumps were just junk...barely able to keep up.

I will once the temperature gets above holy#$%^ cold go out and check that the timing advances correctly. I believe you are correct about the fuel volume. So I wil be picking up a pump soon. And I willl definetly give balancing them another shot. I am pretty sure I know which vacuum port to use.

 

[geekinavette]

I believe it's the center port on the rear of the TB that has the restriction. It should be directly in-line with a visible very small hole just under the throttle plate in the bore of the TB.

If your TBs have been overbored at all then this restriction is gone and you will not be able to measure a single TB...but you can always do it differentially by simply connecting each end of the manometer to a TB vacuum port and adjusting until the water is centered. (if its too far off one way or the other you'll still suck the water into the engine...but at least you'll know which one is pulling too much!).

Honestly I would pursue the fuel pump first...as an imbalance in the TBs is not going to do much at higher RPM where you are experiencing most of your trouble. This will only manifest itself taking off from a stop.

Tackle one problem at a time.

 

[jseremba]

I believe it's the center port on the rear of the TB that has the restriction. It should be directly in-line with a visible very small hole just under the throttle plate in the bore of the TB.

If your TBs have been overbored at all then this restriction is gone and you will not be able to measure a single TB...but you can always do it differentially by simply connecting each end of the manometer to a TB vacuum port and adjusting until the water is centered. (if its too far off one way or the other you'll still suck the water into the engine...but at least you'll know which one is pulling too much!).

Honestly I would pursue the fuel pump first...as an imbalance in the TBs is not going to do much at higher RPM where you are experiencing most of your trouble. This will only manifest itself taking off from a stop.

Tackle one problem at a time.

You are correct. doing the fuel pump is probably the correct thing to do first. The TBI's are original so the balance should work. Thanks again.