help again

Ok everybody I changed my old points distributer to a HEI style, i got it changed and timed to TDC where it idles good but if you give it throttle she backfires. I am changing the spark plugs tomorrow am but what else can it be?

 

Backfires out the exhaust or intake??

 

shouldnt it be at least 5 deg. before top dead center. if its at tdc it sounds you should advance the timing a little

 

if its idling smooth the fireing order should be correct. iwas just saying that the timing should be bumped up a bit. you should have things set correctly before you try to diagnose a problem.

 

ok she backfires out the intake, she fire rigt up and idles good and smooth i tried to advance and retard but no change, she has th amc 304 and i put in a new 65k hei distributer wires and plugs

 

First, let me state "I don't know squat about HEI distributors". With that said, my first guess is the secondary spark is jumping to another cylinder. Since you changed both the wires and distributor, try and re-install the old wires (assuming they will fit) and see if the problem persists. If it does I would look inside the distributor for arc trails.

Good Luck.

 

ok now i regapped the plugs check all wires and retimed to 5 degrees advanced it is a little better but still backfires through the carb when acellerating, could my carb be bad or my vaccuum adv be off

thanks all

 

were do you have the vacuum advance hooked to? manifold or ported?

 

it comes off the carburator, i was also told the accelerator pump needs repacing so i may rebuild the whole carb and if that doesnt work possibly a new one?

 

ok now i rebuilt the carburator and it is better if i accelerate slowley it does not backfire, but if i hit it quick it does and now i see flames come out of the carb. it doesen't matter if i have it up in rpm's or not. i need help!!!

and thanks for it all
Richard

 

I wonder if your cam chain jumped a tooth. No tuning in the world can compensate for that.

Let's think this through. If you are getting fire out of the carb what would we conclude: advanced cam or retarded cam? If the cam is advanced so far that ignition would happen while the intake valve is still open you'd have a BIG backfire and no power at all. That would take a heck of a lot of advance, I think. If the cam is retarded the intake valve is opening while there is still some residual unburned exhaust gasses under cylinder pressure. Maybe at idle the fuel burns more completely because it has more time. At higher revs fuel still takes the same amount of time to burn but all of the valve events are happening faster so cylinder pressure exists for a wider angle of crank position, causing unburned residual gasses to flame up through the carb.

You can measure cam timing without pulling the cam cover but it requires some special tools such as a degree wheel for the crank and a mag base dial indicator. Not sure how to diagnose easily without the right tools but there is a lot I don't know. Getting to the cam chain is no small amount of work but if your engine has a lot of miles on it the cam is probably stretched even if it hasn't slipped. And if you change cam chains you can advance the cam by a few degrees and gain some performance. Use a true roller, double roller chain set to reduce friction and component wear. It'll hold timing longer than a regular chain.

 

if my timing chain jumped even just a bit would i still be able to get it timed, right noe i am 5 degrees advanced and it idles good and accelerates good as long as i do it slowley but as soon as i hit it it backfires

 

If you are truly timed at TDC, instead of 5 to 8 degrees BEFORE TDC (BTDC) you should have a lousy idle and you should have popping out the exhaust.

To properly time your engine,
You should first pull the vacuum line off the distributor, and plug it. (Golf 'T' works great for this).

You should time the engine at around 650 RPM to 750 RPM,
And you should have 5 to 8 Degrees of ADVANCE,
(Before TDC or BTDC).

Don't forget to hook the vacuum line back up when you are done!
---------------------

If you still get backfire or 'Popping' out the exhaust,
Then you have one (or more) of four problems,

1. Distributor not set in the correct position relative to the crank/cam timing.
This is pretty common to get the distributor one tooth off since most people don't take into account the rotation of the rotor during insertion.

The distributor gear is SPIRAL CUT, so the rotor will have to be 'Backed Up', turned counter clockwise about one spark plug terminal distance before you try to install the distributor.

As the distributor gear engages the camshaft gear, it will move towards the #1 plug terminal as it seats on the block and mates with the camshaft gear.

2. You have too much vacuum for the vacuum advance.
GM vehicles produce MUCH LESS Spark Ported Vacuum at idle and just off idle...
So when you use a GM Vacuum advance on a vehicle that has a Strong Spark Ported vacuum signal, you will get strange things happening when you attach a GM style vacuum advance to it.

3. Wrong Vacuum Source.
You should have the distributor connected to SPARK PORTED VACUUM... Not manifold vacuum.
Manifold vacuum signal works contrary to spark ported vacuum, and should never be used for distributor vacuum advance.

4. You may not have enough accelerator pump shot (fuel discharge) from the accelerator pump.
If the vehicle works fine if you accelerate very slowly,
But 'bogs' or 'Pops' and drops out, then comes back in when you start to accelerate,
The it could very well be you don't have enough accelerator pump shot.

 

For what it's worth, my 75 6cyl. is set at 7 degrees btdc, plugs gapped at .048,and 8mil wires. Vacuum line hooked to bottom of carb. Hope this may help.

 

i will try retarding my timing and adjust the carb tonight, how do i set my vaccuum advance and to what?

 

This may be a silly ?. Are you getting a full 12 volts at the coil ? Start and run.

 

That's actually a very good question, Rusty. Weak coils will not recover as quickly after each ignition event. It may recover fine at low revs but not so well at higher revs.

 

I don't know, I have assumed so but I better check that tonight also,