1970 CUTLASS, BACKFIRING 350 ENGINE

[shaks 442 clone]

just 3 nights ago out of the blue my car started backfiring im baffled

i did a vacuum test off of the carb it was normal not flickering beyond normal

the car has an msd 6a box, upgraded ignition to h.e.i, stock intake, edelbrock carb 600cfm, it has a mechanical fuel pump and a piggyback electric one by the tank (it came like that when i bought it)

any help with things i should check to help get down to the root of the problem would be really appreciated


also one last thing what should the timing of a 1970 350 olds engine be set at?

thanks
~shak

[$tryker]

Timing should be 10 or 14 degrees, depending on the horse power you have. Although you could squeeze a couple more degrees with the HEI.

You didn't state the location of the backfire. Was it a pop through the carb, or in the exhaust. Check the timing first. An exhaust backfire could even be air entering the exhaust system. Let us know how things are going.

[shaks 442 clone]

Originally Posted by $tryker

Timing should be 10 or 14 degrees, depending on the horse power you have. Although you could squeeze a couple more degrees with the HEI.

You didn't state the location of the backfire. Was it a pop through the carb, or in the exhaust. Check the timing first. An exhaust backfire could even be air entering the exhaust system. Let us know how things are going.

well it pops in both the carb and the exit of the exhaust

and also to add not sure if its important but the filler neck on my gas tank is damaged when i step on it gas pours out no matter how tight the cap is

so i purchased a chevelle gas tank today i decided to put the chevelle gas tanks cap on my cutlass and the backfiring seem to have gotten worse

could the filler neck be my problem

the only thing i dont understand is the filler neck has been like that for a year now and this just started like 4 days ago

hope someone can lead me in the direction of the anwser

[Jokers69]

does it run well enough to drive it?
does it cough through the carb once or twice when throttling up then smooth out?
if your cruising and whack the throttle like your passing what happens?
under what conditions does it backfire through the exaust?

[shaks 442 clone]

yes it runs well enough to drive i drive the car everyday even thto it is backfiring, also is that bad for the car ?

it does bacfire through the carb when i really wanna get on it, so i have to slowly move up in speed instead of just gunning it and gaining speed the fast way

if i gun it like i said the backfiring will get worse it will backfire out of the carb, i will have the loss of power feeling like the car dies out for a second then comes back and then dies out again

when i am coasting on the freeway at like 60ish thats when i was having the most exhaust backfires

[Jokers69]

try disconnecting the vacuum advance where it goes into the carb AND PLUG it off AT THE CARB. Drive it and see if that makes any changes. If not hook it back up.

This might sound like a silly question, but did you put gas in it around the time this started happening? If so, it may have picked up some water, pour in a bottle or two of dry gas.

[Oldsguy]

Backfire is caused by incorrect timing. Could be that it just needs a tune-up, new rotor and distributor cap, could be the distributor hold-down clamp is loose, could be the timing belt slipped a link, or vacuum advance unit is bad. First thing to do is get a good tune up and verify timing. Also, make sure you have no vacuum leaks that would not allow full advance upon hard acceleration.

[88 coupe]

Originally Posted by Oldsguy

........ or vacuum advance unit is bad ........

Nothing to do with the problem.

It's primary purpose is to help increase fuel economy and it's secondary is to enhance "off idle" throttle response. At anything above idle, it only works under high vacuum (cruise) conditions.

A faulty diaphragm, could be the source of a vacuum leak, but it could not cause his symptoms.

........ make sure you have no vacuum leaks that would not allow full advance upon hard acceleration ........

When the engine is under load, there is not enough vacuum to move the diaphragm.

Other than that, you are dead on.

Norm

[Redog]

Timing, or 2 wires are mixed up

[millwrightrice]

If its a old engine you could have a flat lobe on the cam.GM is famous for this.

[Oldsguy]

Originally Posted by 88 coupe

Nothing to do with the problem.

It's primary purpose is to help increase fuel economy and it's secondary is to enhance "off idle" throttle response. At anything above idle, it only works under high vacuum (cruise) conditions.

A faulty diaphragm, could be the source of a vacuum leak, but it could not cause his symptoms.


When the engine is under load, there is not enough vacuum to move the diaphragm.

Other than that, you are dead on.

Norm

Yeah, of course. The centrifugal advance would be kicking in at higher rpm where the engine would be under a load like that. I didn't think that one out first. Thanks for the correction.

[88 coupe]

Originally Posted by Redog

........ 2 wires are mixed up

Not likely.

Originally Posted by millwrightrice

........ could have a flat lobe on the cam ........

Also, not likely.

Norm

[Jokers69]

Originally Posted by 88 coupe

Not likely.


Also, not likely.

Norm

so, instead of telling us whats not likely, how about what is likely

[shaks 442 clone]

well i opened up the cap and found that the carbon button had melted

so im going to replace that either tonight or tomorrow and see what it does

also if you are not familiar with the name carbon button it is the brass piece that sits under the tiny spring of a gm hei cap

[88 coupe]

Originally Posted by Jokers69

so, instead of telling us whats not likely ........

Troubleshooting of this kind, is a process of elimination. Need I say more?

Originally Posted by Jokers69

........ how about what is likely

When I have something better than I posted previously.

Originally Posted by 88 coupe

........ Other than that, you are dead on.

When/if I think of something else that actually fits his description, I will post it.

I am finished with your hijack.

Norm

[Jokers69]

Originally Posted by shaks 442 clone

well i opened up the cap and found that the carbon button had melted

so im going to replace that either tonight or tomorrow and see what it does

also if you are not familiar with the name carbon button it is the brass piece that sits under the tiny spring of a gm hei cap

cool. let us know how you make out..

[Jokers69]

Originally Posted by 88 coupe

I am finished with your hijack.

Norm

It's not a hijack, oh great one. It's a commentary on a very arrogant posting style. Where is the line to kiss your ring, I'll make sure I don't confuse that one with the beer line.

[Oldsguy]

Good to hear it was the rotor. Always check the easiest and simplest first in these cases. A good tune-up goes a long way. Be sure to change the cap too, I bet the eight tower conductors are corroded and worn too.

[shaks 442 clone]

after replacing the cap and rotor (both msd parts) it is still backfiring

there has to be an underlying problem else where

i am going to try replacing bought collector and exhasut manifold gaskets next

by the way does anyone know a good way to get proper timing when you are running msd?

[Oldsguy]

I think it would be the same way as with a stock distributor. disconnect vacuum advance and plug the source so there is no leak. Run car at idle and use a timing light to set the timing, adjust it by loosening the distributor clamp enough to rotate the distributor to get the desired setting. Are you using an MSD control box also? If so, I am not sure if there is anything to be done with it as I have no experience with that. I do know that the control box is supposed to change the advance curve for certain conditions. I also know there are different models, the MSD6 and the MSD7 but that is about as far as my knowledge of them goes. If you are using one it might be helpful to figure that out as well.

[88 coupe]

Oldsguy: He has a basic 6A box.

Originally Posted by shaks 442 clone

after replacing the cap and rotor ........

While you had the cap off, did you check the automatic advance for free movement?

........ (both msd parts) ........

All Delco ignition parts are up to the job. Any difference, in cost, was a waste of money.

........ going to try replacing bought collector and exhaust manifold gaskets next ........

If it leaks, it needs to be fixed.

Finish the tuneup (including a compression test) first.

........ good way to get proper timing when you are running msd?

MSD box is not a factor. Set it in the normal manner. 20° should work with your HEI, until your problem is solved.

The (unnecessary) extra impulses from the MSD, can cause erratic readings from some lights. If so, you can try a different light, or you can disconnect/remove the box.

Norm

[shaks 442 clone]

Originally Posted by 88 coupe

Oldsguy: He has a basic 6A box.


While you had the cap off, did you check the automatic advance for free movement?


All Delco ignition parts are up to the job. Any difference, in cost, was a waste of money.


If it leaks, it needs to be fixed.

Finish the tuneup (including a compression test) first.


MSD box is not a factor. Set it in the normal manner. 20° should work with your HEI, until your problem is solved.

The (unnecessary) extra impulses from the MSD, can cause erratic readings from some lights. If so, you can try a different light, or you can disconnect/remove the box.

Norm

what is the automatic advance? is it the thing that the cap screws down too?

[shaks 442 clone]

also all i have is a msd 6a box nothing fancy just the basic one with no rev limiter

[88 coupe]

Originally Posted by shaks 442 clone

........ is it the thing that the cap screws down too? ........

The spring/weight mechanism that is located under the rotor. It's also called mechanical or centrifugal, depending which manual you are reading.

Norm