Hi.
I've just about tried every tune-up on my bug. A recent matter of concern, whenever I accelerate heavy, I get backfires (and seems it comes from the carb...though not sure).
what could be the reason to it?
Backfire on heavy acceleration
Backfire on heavy acceleration
Been reading through reasons/causes of backfires on engines.
I have done significant adjustments on my distributor (really advancing it) and carb (tightening on the air intake screw to the point the engine hunts slightly)
Basically, what I'm trying to achieve is the point where the engine runs EFFICIENTLY. seems like the backfire is just telling me that---(My results on the matter).
Kindly school me on this one. I dont mind undoing everything to get it RIGHT!!!
Thanks.
I have done significant adjustments on my distributor (really advancing it) and carb (tightening on the air intake screw to the point the engine hunts slightly)
Basically, what I'm trying to achieve is the point where the engine runs EFFICIENTLY. seems like the backfire is just telling me that---(My results on the matter).
Kindly school me on this one. I dont mind undoing everything to get it RIGHT!!!
Thanks.
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Blue Baron
- Senior Member
- Posts: 879
- Joined: Sun Jan 18, 2009 8:34 pm
- Location: Tampa, Florida
The engine should never backfire under acceleration, only deceleration. Sounds like the timing is off.
Start by static timing the engine to 7.5 degrees before-top-dead-center. If you have two marks on the pulley, it's the mark to the left. If you have three marks on the pulley, it's the one in the middle. (And if only one mark, that's the one to use.)
You will need a small 12-volt test lamp.
Remove the distributor cap and turn the engine by hand until the rotor is pointing toward No. 1 spark plug wire and the 7.5 BTDC mark on the pulley is at 12 o'clock (even with the engine case seam.) Attach the test light to the side of the coil that the distributor wire attaches to and ground the other end.
Loosen the distributor clamp and turn on the ignition switch. Rotate the distributor clockwise until the light goes out, and then counterclockwise until the light first goes on. Tighten the clamp. You have now set your timing. (Remember to put your distributor cap back on.)
If there are two marks, the second mark (the one on the right) is 10 degrees before-top-dead-center. If the engine runs sluggish, you may want to time it slightly closer to the 10 degree mark to add a little more initial advance. Experiment to see what works best. My dead stock engines like 7.5 degrees; my engines with an 009 distributor seem to prefer around 9 degrees intitial advance.
If you have a degree pulley and a timing light, you should set the timing to 28 degrees total advance (that's revving the engine to full distributor advance.)
Start by static timing the engine to 7.5 degrees before-top-dead-center. If you have two marks on the pulley, it's the mark to the left. If you have three marks on the pulley, it's the one in the middle. (And if only one mark, that's the one to use.)
You will need a small 12-volt test lamp.
Remove the distributor cap and turn the engine by hand until the rotor is pointing toward No. 1 spark plug wire and the 7.5 BTDC mark on the pulley is at 12 o'clock (even with the engine case seam.) Attach the test light to the side of the coil that the distributor wire attaches to and ground the other end.
Loosen the distributor clamp and turn on the ignition switch. Rotate the distributor clockwise until the light goes out, and then counterclockwise until the light first goes on. Tighten the clamp. You have now set your timing. (Remember to put your distributor cap back on.)
If there are two marks, the second mark (the one on the right) is 10 degrees before-top-dead-center. If the engine runs sluggish, you may want to time it slightly closer to the 10 degree mark to add a little more initial advance. Experiment to see what works best. My dead stock engines like 7.5 degrees; my engines with an 009 distributor seem to prefer around 9 degrees intitial advance.
If you have a degree pulley and a timing light, you should set the timing to 28 degrees total advance (that's revving the engine to full distributor advance.)
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Blue Baron
- Senior Member
- Posts: 879
- Joined: Sun Jan 18, 2009 8:34 pm
- Location: Tampa, Florida
As to adjusting the carb idle and mixture, that's easy.
With the engine running, open up the small screw on the left side, and then open the big screw above it to set idle where you want it. Once the idle is set, tighten the small screw until the idle starts to slow. Open the small screw one full turn and you're golden.
Edit: I see from your other post that the carb is a 28 Pict 2. My instructions are for a 30 pict. The adjustment is basically the same, but instead of a small recessed mixture screw, there's an external mixture screw on the side, and instead of the large idle screw (air bypass) you should set the idle at the butterfly.
With the engine running, open up the small screw on the left side, and then open the big screw above it to set idle where you want it. Once the idle is set, tighten the small screw until the idle starts to slow. Open the small screw one full turn and you're golden.
Edit: I see from your other post that the carb is a 28 Pict 2. My instructions are for a 30 pict. The adjustment is basically the same, but instead of a small recessed mixture screw, there's an external mixture screw on the side, and instead of the large idle screw (air bypass) you should set the idle at the butterfly.
Last edited by Blue Baron on Wed Jul 17, 2013 4:09 pm, edited 2 times in total.
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Blue Baron
- Senior Member
- Posts: 879
- Joined: Sun Jan 18, 2009 8:34 pm
- Location: Tampa, Florida
Backfire on heavy acceleration
Hey Blue Baron
I must say..... Thank you for the elaborate instructions on tuning the engine...in particular, the timing (7.5/10 deg btdc). I am more in the know.
I was fortunate to get to the root of the problem....which is related to your last point on compression issues.
My manifold is leaking at either one or both heads. I must have had it come out off position...this once being a problem I had fitting it back after working on my heat risers a few months ago.
Was fortunate to have thought to test as I was returning my carb after the service when I thought to blow through the manifold. I was surprised the engine even started at such a leak as what I had.
Please check my new post for advice on matters I'm considering. Thanks.
I must say..... Thank you for the elaborate instructions on tuning the engine...in particular, the timing (7.5/10 deg btdc). I am more in the know.
I was fortunate to get to the root of the problem....which is related to your last point on compression issues.
My manifold is leaking at either one or both heads. I must have had it come out off position...this once being a problem I had fitting it back after working on my heat risers a few months ago.
Was fortunate to have thought to test as I was returning my carb after the service when I thought to blow through the manifold. I was surprised the engine even started at such a leak as what I had.
Please check my new post for advice on matters I'm considering. Thanks.
Backfire on heavy acceleration
Hey,
No manifold leak.... I was about to drop the engine to refit the manifold. On re-checking, the manifold was ok. (I hadn't known when you blow through the manifold, depending on where the intake/exhaust valves are, you could either blow through or get a closed system thus blowing through the manifold will get no air passing. Isn't there much to learn)
I suspect that the problem was as a result of insufficient air through the carb (with the choke valve being disfunctional and was almost shut.) This is only my suspicion. Good to say, on returning all, no more backfire.
As for the distributor, I had to advance to achieve smooth sail.
Thanks Blue Baron.
No manifold leak.... I was about to drop the engine to refit the manifold. On re-checking, the manifold was ok. (I hadn't known when you blow through the manifold, depending on where the intake/exhaust valves are, you could either blow through or get a closed system thus blowing through the manifold will get no air passing. Isn't there much to learn)
I suspect that the problem was as a result of insufficient air through the carb (with the choke valve being disfunctional and was almost shut.) This is only my suspicion. Good to say, on returning all, no more backfire.
As for the distributor, I had to advance to achieve smooth sail.
Thanks Blue Baron.
-
Blue Baron
- Senior Member
- Posts: 879
- Joined: Sun Jan 18, 2009 8:34 pm
- Location: Tampa, Florida
Easy way to test for manifold leaks: Buy a can of engine starting fluid, and with the engine running, spray a light amount around where the manifold attaches to the head. If the idle increases, you have a manifold leak.
Note: Engine starting fluid is flammable, so have a fire extinguisher at the ready!
Note: Engine starting fluid is flammable, so have a fire extinguisher at the ready!