Fleetwood

Can't seem to find whats wrong with my car.I have adjusted the timing at 6,8,12degs.BTDC,the carbs been overhauled & setup,I have checked for vacuum leaks.If I accelerate quickly flames come out of the carb,If I accelerate slowly,it seems to rev up OK.This is with the car stationary,& in park.Could the catalytic convertor be blocked?
Thanks Neil.

any4xx

Has it run SINCE the rebuild, or is this "first fire?" If it's a fresh rebuild, I'd suspect that the cam wasn't installed properly. Either that or seriously bad timing. Have you checked to see that TDC on the balancer is correct?

stingray4me

I agree to, its the timming or camshaft off. One time I singed my eyebrows when it shot some flames up the carb. :iagree:

Fleetwood

The engine itself has not been overhauled just the carburettor,& yes there is a definate timing groove on the balancer.It does seem to point to timing,but would it read 8deg.BTDC with the timing light,if the timing was so far off? :confused:

any4xx

Hmmm...

The timing mark is sometimes off as compared to actual TDC, but if you've only had the carb. off, I'd say that your problem is obviously there, assuming all was well before the carburetor rebuild. Is it possible that you have a stuck float or leaky power valve that is allowing the manifold to fill up with gas? Anything other than that & I'm as stumped as you are.

Tom

Tom454

Assuming you didn't mess with the valve adjustment, and you didn't mess with the spark plug wires, and you didn't mess with the timing chain & gears, and assuming you don't have a Fluidamper, then maybe your harmonic balancer outer ring has slipped since the last time it was properly timed.

Fleetwood

Tom,thanks for your imput,I may have to take the top off the carb to check the float level,or I may just take it into a mechanic.

Tom454,I have not messed with this engine,the harmonic balancer is fine.I may be wrong but if the valve timing were out,surely you would not be able to set the timing at 8degs BTDC with a timing light.

Thanks Neil.

Light84vette

I had the same problem when I installed the 406. Problem came out to be that the cat convertor was plugged up from the oil the old 350 was throwing out. After I had dual 3inch exhaust done it has never done that again.

jyounane

Do you have the vacuum advance disconnected when checking timing? Some set-ups need this done to get mechanical advance at idle set correctly.

Cheers,
Joe

Fleetwood

LIGHT84VETTE,I have heard that blocked cats can cause this problem,thats why I first asked the question.Looking at mine it seems to be an original one,I may just make a test pipe to see if it makes any difference.Thanks for your imput.

Joe,Yes I had the advance disconnected & plugged when I checked the timing,mechanical advance seems to be working when I accelerate as well.

Jughead

When I was having the flame thru carb syndrome, It was recommended to me to richen up the fuel mixture. They told me I was running too lean. Didn't make any sense to me but I did it anyway. I turned the mixture screws a bit and it's been fine ever since. Worth a try. :thumbs:

Tom454

"Tom454,I have not messed with this engine,the harmonic balancer is fine.I may be wrong but if the valve timing were out,surely you would not be able to set the timing at 8degs BTDC with a timing light."

Neil... when the outer ring of the balancer separates from the inner hub, the timing mark moves to a new position. Depending on how far it moves, you can still "set" the timing to what you THINK is the correct number. But... it will be wrong. So, a separated damper can cause backfiring by virtue of the fact that the timing is set incorrectly, even though it reads correctly with a timing light.

It is physically impossible for exploding fuel to go past a closed/seated intake valve up through the carburetor.

Backfiring through the carb can only happen if the fuel in the cylinder gets ignited (at the wrong time) when an intake valve is open. This means that the two events are out of sequence.

Either you have an ignition event that is being triggered at the wrong tme, or you have an intake valve that is being opened at the wrong time. Since you have not messed with the mechanicals, I would focus on the timing & plug wires etc.

"Backfiring" through the exhaust is similarly linked to an exhaust valve event.
Unspent fuel is ignited when an exhaust valve is open, and the resulting sound is heard at the tailpipe.

Exploding unspent fuel ignited by high temps in the exhaust system, when everything else is correct, is not usually referred to as "backfiring" by a mechanic.

Tommy Lee


PS- I don't think you mentioned how many miles are on the engine... if it has high miles or rough service, check the timing chain & gears for excessive play. Remove the distributor cap so you can see the rotor. With the car in neutral (and preferably with the plugs removed) put a socket on the crank bolt. Rotate the engine in one direction until you see the rotor just begin to move. Now rotate the engine in the other direction.... if the rotor takes a "long" time to respond in the reverse direction, the chain & gears are worn too much to time the valve events properly & need to be replaced.





[Modified by Tom454, 9:24 AM 5/2/2004]

Fleetwood

Tom,I did go out to the car & look at the balancer after reading your post,& cannot see any seperation of the rubber between the centre & outer ring.I will bring the motor up to 8degs BTDC again with #1 at the top tomorrow & check it again.If when #1 piston is at top dead centre,& the rotor points to #1 lead on the cap,& the balancer is in line with 8degs BTDC,you would assume the balancer was OK, right?
This is not the original motor,but what I am led to believe is a crate motor,so there is no way of me telling the actual mileage.But I will check the chain as you mentioned,thanks.

any4xx

Let me chime in here again;

"Testing" the correlation between the balancer mark and the rotor as you have described does nothing more than what you have already done with a timing light. What you NEED to do is to take out the #1 spark plug and rotate the engine with some sort of a probe in the spark plug hole. A piece of coat hanger will get you close, but if you want to really get accurate, you'll need some sort of a shallow 'stop' to keep the piston from going all the way to the top of the cylinder. Rotate the engine one way, slowly and carefully, until the piston hits the stop. Mark the balancer where it lines up with the TDC mark on your timing pointer. Rotate the engine the other way until it stops again. Mark the balancer again. Your actual TDC mark on your balancer should be exactly half way between your two marks that you made. If it's not, there's your problem.

Hope this helps,
(other) Tom.

Fleetwood

Thanks Tom,I have been doing that with a thin screwdriver in the #1 plug hole,making sure I am on the compression stroke.But I will try doing it again,just to make sure.

Neil.

any4xx

When checking for TDC, it doesn't matter if you're on the compression or exhaust stroke.

DO keep us posted on what you end up finding out here. I HATE problems like this and am always happy to see the solution.

LemansBlue68

Are you getting a decent pump shot from your accelerator pump when you flip the throttle open? You should see a healthy shot of fuel squirting into the primaries when you open the throttle. The accelerator pump prevents the momentary lean condition that occurs when the throttle is opened.

Reasons that can cause backfiring through the carb are: too retarded spark, spark plug wires out of sequence, improper cam timing (could be you've lost a tooth on a timing gear or the timing chain is extremely slack), too lean A/F mixture.

The reason the lean mixture can cause a back fire through the carb is that lean mixtures burn slowly. The burn is lasting past the exhaust stroke and into the next intake stroke, igniting the charge in the intake manifold.

Vesa

Guys you all got it wrong! He just has a mean maschine a real beast :smash:

Fleetwood

I have just been out to the car again,pulled out #1 plug,& by kicking the engine over,felt the compression stroke coming up,with my finger over the plug hole.I have then put a thin screwdriver in the plug hole & turned the engine over by hand until the piston was at top dead centre.I then lined up the timing mark on the balancer with the 8degs.BTDC mark,pulled the cap off the distributor & noted that the rotor was pointing at the #1 spark plug lead.
This would rule out timing.It's also idling very rough.
Would it also rule out valve timing,or is that something else?How do you check that.
I will make up a test pipe at work to rule out the catalytic convertor,& maybe pull the top off the carb & check the accelerator pump.

Thanks Neil.

any4xx

A far as valve timing, you'll need a dial indicator. You "CAN" run a rough check visually by simply pulling off a valve cover and watching your valvetrain work one cylinder while turning the engine, but all you'll get there is a rough idea if things are working in their intended order, and if the engine will run at all, they obviously are. At this point, I'd suggest a compression check. If your cam timing is off, it should show up a low readings throughout.

But back to the track here... This whole thing started with a carb. rebuild, right? And it ran fine before that, right? Go back to the carb!


[Modified by tpiini, 8:06 AM 5/3/2004]