Question: Ignition timing on a '75

[Thore826]

I've gotten lots of helpful advice here before, was hoping for some more.
My '75 L-48 (before I go on, I know its the lowest performance engine EVER, I know you can't get much more power out of it, but I can't afford a new engine right now, and don't want to messw with the internals on this one... "if it ain't broke, don't fix it")... I eliminated the Cats, replaced the stock exhaust manifolds with Heddman Headers, using Glasspacks for the mufflers (its loud, but legal). Was also toying with buying the Edelbrock Performer Intake Manifold, and rebuilding the QJet (hopefully) this weekend.
My problem... I'm getting alot of backfiring when coming out of high-rpm (highway to side-streets) and a bit at idle. I remember finding someone said that in '75, along with destroying everything else in the engine, they messed with the ignition timing curve, so I was thinking if I was able to do something with my stock-HEI or get an aftermarket one, would that help a bit with the backfiring?
Thanks

 

[Hib Halverson]

The word "backfire" is sometimes misused.

Do you mean backfiring out the carb air horn or do you mean "popping" from the exhaust?

When you say it backfires a bit at idle, do you mean you can start the engine and let it idle and while idling it backfires out the carb or pops out the exhaust?

 

[Crimson Thunder]

Your backfire sounds like the normal "popping" sounds glasspacks make on deceleration.

 

[Thore826]

ah, learn something new every day I guess, thanks! it is a "popping, it occurs usually when I'm getting on the off-ramp dropping from highway speeds back to around 30 or 40, and then the occasional "pops" when I'm idleing. I never even put a thought to the glasspacks, I figured it was from the changes to the flow. But now, as far as my fuel economy, do you think that I ruined it when I messed with the exhaust system? Would the lessened back pressure from headers and true dual help or hurt economy?
Thanks for the help so far, at least now I know it is a common occurence (but not bad...right?)

 

[Crimson Thunder]

The new exhaust probably threw off your A/F mixture a little bit. Your at the point now where it would be a good time for a full Chassis Dyno tune. Might be the best $150.00 you'll ever spend.

 

[Curtis]

Save your 150 Bucks.. Tune it yourself.. YOu can increase your timing advance a few degrees and improve your throttle response and fuel mileage..Advance it a little at a time until you get detonation at part throttle acceleration, then back it off a couple degrees. Use a vacuum guage and set your idle mixture screws for best lean idle. Strive to get the highest vacuum while working one mixture screw and then the other. AS you progress you may have to adjust your idle stop screw to slow the engine back down. Doing this will give you a smoother, cleaner idle and better throttle tip in. Its all easy to do and FREE !! You might be pleasantly surprised what just a small amount of tuning will do for an L-48.

 

[Rex S]

I've gotten lots of helpful advice here before, was hoping for some more.
My '75 L-48 (before I go on, I know its the lowest performance engine EVER, I know you can't get much more power out of it, but I can't afford a new engine right now, and don't want to messw with the internals on this one... "if it ain't broke, don't fix it")... I eliminated the Cats, replaced the stock exhaust manifolds with Heddman Headers, using Glasspacks for the mufflers (its loud, but legal). Was also toying with buying the Edelbrock Performer Intake Manifold, and rebuilding the QJet (hopefully) this weekend.
My problem... I'm getting alot of backfiring when coming out of high-rpm (highway to side-streets) and a bit at idle. I remember finding someone said that in '75, along with destroying everything else in the engine, they messed with the ignition timing curve, so I was thinking if I was able to do something with my stock-HEI or get an aftermarket one, would that help a bit with the backfiring?
Thanks

Qjet isn't the best carb to rebuild as a first timer. It isn't that you can't do it as everyone has to start on their first one but have someone who knows Qjets standing over your sholder. If there is any play in the front throttle shaft (part with the butterfly valves that opens when you push the throttle) don't even bother as the carb is worn out. All the air must go into the engine through the top of the carb or PCV connection. Any leaks at the shaft will make it impossible to set the idle mix.

Chassis dyno and stock L48 just don't go together. Use the vacuum gage, trace down all vacuum leaks (don't forget the brake booster and HVAC controlls) and live with it 'till you can afford a replacement engine.

You could build your first engine on a stand. That would allow you to drive the car while building as you can afford the parts and then drop in the fresh engine.

 

[Crimson Thunder]

How does a chassis dyno and a L-48 not go together?

By no means do i intend to disrepect the OP,but it seemed he was at the end of his tuning ablilities.

A chassis dyno is for more than measuring HP. A skilled operator can use it tune AF ratios and get the timing perfect.

 

[Rex S]

How does a chassis dyno and a L-48 not go together?

By no means do i intend to disrepect the OP,but it seemed he was at the end of his tuning ablilities.

A chassis dyno is for more than measuring HP. A skilled operator can use it tune AF ratios and get the timing perfect.

Niether did I try to disrepect him or his car. I just infered form his comments that money is a bit tight at this time. Dyno charges are at the whim of the operator. The money from a few dyno pulls adds up fast and could be better spent on parts.

 

[JohnZ]

it is a "popping, it occurs usually when I'm getting on the off-ramp dropping from highway speeds back to around 30 or 40, and then the occasional "pops" when I'm idleing.

Is your A.I.R. system still on the engine?



Edit: Oops, if you have headers, then the A.I.R. system has been removed.

 

[73shark]

Qjet isn't the best carb to rebuild as a first timer. It isn't that you can't do it as everyone has to start on their first one but have someone who knows Qjets standing over your sholder. If there is any play in the front throttle shaft (part with the butterfly valves that opens when you push the throttle) don't even bother as the carb is worn out. All the air must go into the engine through the top of the carb or PCV connection. Any leaks at the shaft will make it impossible to set the idle mix.

Q-jets aren't that hard to rebuild especially if you have a copy of Cliff Ruggles Rochester Q-jet book and/or HP Books Rochester Q-jet book. Both have excruciating detail.

As for the throttle shafts, there are reamer/bushing kits that facilitate fixing this problem.

I think JohnZ is on to something re: the A.I.R. system. Without it, you have a rich exhaust which is exacerbated on deceleration.

 

[Crimson Thunder]

Niether did I try to disrepect him or his car. I just infered form his comments that money is a bit tight at this time. Dyno charges are at the whim of the operator. The money from a few dyno pulls adds up fast and could be better spent on parts.

I agree on money being spent on parts,but why throw parts at a 75 with only 160 HP. Get it running right and save for a rebuild or a new motor.

 

[krscholz]

Get rid of the AIR. Pack it up and store it somewhere safe. You don't need it. Did you go with dual 2 1/4" exhaust system? Some thought perhaps towards a performance cam.

 

[Crimson Thunder]

A 75 L48 does not have enough compression for a perfomance cam,although a small duration (around 210) and a set of Vortec heads might make for a fun driver.

 

[tracksnblades1]

Are your hedman headers leaking at the heads? Recurving your distributor's mechanical advance (sooner) and limiting it to 8-9 distributor degrees may allow you to run more intial timing which tends to perk things up at lower rpms. If you're worried your mechnical advance timing (weaker springs) is overlapping excessively with your vacuuum advance, you might elect to change your vacuum advance. Chevrolet had many vacuum advances that ranged from as little as 12 degrees to as much as 24 degrees. Some z28 vacuum advances would drop advance at 9" and delay advance until 12". This was a useful tool back then when your performance cam was causing your timing to wonder at an idle. Your light springs can cause this too. Good luck tuning her in.