Odd backfire

[squid]

Ah.. I forgot to add this info.  The car has had the vacuum advance capped off since I bought it last summer.

I really haven't messed with it since I have never had a problem with the car up to this point.  There always has been a barely perceptable ping when I really get on it, running premium pump gas, though.

-squid

[71383bee]

IMO theat carb is too small for your needs.  I think you would have been better off with rebuilding the 750 DP. Vacum secondaries on a 4 speed don't mix too well.  That's probably causing your hesitation at shift points.

Without timing and vacum numbers it will really be hard to dial in your combo.  Set the timing first and then move to the carb.  If it's pinging under load it may need to be retarded a bit, but it's tough to tell without any #'s.

[Grancoupe]

I agree with 71383bee about the carb being too small. The pinging could be lean burn as well as timing. Running a little fat could have helped push the spark curve up a bit with todays wonderful gas.

[squid]

I still haven't had a chance to check the timing, but I thought I would add an update from today's symptoms.

I have noticed that the backfire occurs mostly under modest acceleration.

Also, the "flat spot" only occurs under load.  If you open the throttle in neutral, there is no flat spot.

I probably won't get to the timing until this weekend.

I probably should have consulted you all before listening to the Holley guy.  Oh well... live and learn

The upside is I no longer have black soot patches behind the car when idling cold (at least my wife likes that!)

-squid

[squid]

Ok.  I replaced the balance resistor and the coil.  The backfire is still there, but the mild "missing" or hesitation I was feeling at speed seems gone.

I have checked the timing, meaning without the vacuum advance.  Which, if you've read the thread, you know has been capped off since I bought the car.  The timing measures about 12 degrees.

So, I decided to hook up the vacuum advance to see what I get.  Again, with the advance connected, I'm still getting 12 degrees.

I should say I believe this is 12 degrees.  It shows between the 2nd and 3rd "hash marks" from zero.  Someone wrote +10 next to the second hash with a marker.  So I guess the question is, does this seem right?

The upside is, that aside from the backfire, the car is running the best it ever has.

-squid

[keithcuda]

It happens when you are near home? Like when your nearly there??
Damn, your car knows, it obviously doesn't want to go home. Take a different route maybe one you havn't taken for a while!! LOL

Sorry, , i think timing is the possible problem.

[squid]

Well near home was how it seemed to be happening.  As I have been driving it more, it appears not to be related to the length of time I drive.

Basically, it is like it cuts out for a fraction of a second.  I said 2 or 3 seconds previously, but I have learned it is much shorter because it is too fast for me too look at the tach.

When it happens, if i can get off the throttle and get the clutch in, it won't backfire.  If I'm not fast enough, it will.

-squid

[71383bee]

Do your self a favor and get a dial advance timing light and/or a timing tape to apply to the balancer.  They are worth it.

The million dollar question is where the Total timing is at.  You can't measure this without either of the items mentioned above.  Most mopars like to have around 34-38 BTDC at around 2200-2800 RPM.  To check this you need to have someone rev the motor until the distributor stops advancing.  At this RPM the distributor has reached its total advance...now check the timing.  If it's within the range specified then you are in good shape.  Initial is just as you checked...initial advance at idle. 

The difference between initial and total advance is known as the mechanical advance  and it is controlled by a series of slots and weights within the distributor.  Different engines have different timing requirements.  Some cars like a ton of initial and only about 16 degrees of mechanical advance.  Others like less initial and 20 degrees of advance.  Adjusting these points is known as curving the distributor.  The only way to do this is to open up the distributor and adjust the springs (controls the rate of advance) and/or the advance slots (controls the amount of mechanical advance).  There are some shops tha will do this for you as well.  I had Don at FBO do mine on my car. 

By the way always leave your vacum advance disconected during timing adjustment.  It will add additional advance to the motor and throw off your marks.  You can reconect it later, but watch out because the vacum advance will add additiona advance during part throttle which adds additional advance to you engine which may cause detonation. 

[rallyechall]

You may want to check for arching in your electrical syatem and plug wires. It is free, requires you to run your car at night with the hood open. Make sure there are no overhead lights. It has to be dark. I had new wires cause problems for me, and this is how I found the problem.

[squid]

I took a look at the wires in the dark.  No arcing...

I will get the timing tape tomorrow, and post what I find.

-squid

[squid]

Okay.  I searched the parts stores here in Memphis, and no one has any timing tape.  It looks like I will have to go online to find some.

However,  I have had some success!

I took the advice, about the carb.  First, I went back and took a look at my old 750 DP.  And for the first time (keep in mind I haven't had this car too long) I noticed that it had vacuum secondaries!  I guess Holley made an earlier 750 DP with vacuum secondaries because I confirmed the stamped part number is for a 750 DP.

Anyway, the 750 DP with mechanical secondaries cleared things up immensely.  The car is burning clean, and the performance between gears has improved by leaps and bounds.  I don't quite understand why, but this has solved by backfiring problem too!   If the backfiring was from running too rich, then the 670 should have solved it..  Hmmm...

Thanks to all for the help!  I will let you know what I find with the timing (if I can ever find some tape).

-squid

[MEK-Dangerfield]

Yeee-Haaa!!   

[Chryco Psycho]

put a martk on the damper 2 3/8" clockwise from TDC , that is 36 * advanced