Backfire through carb when warm

[joey4420]

I know it has been a while since I have been on here or posted for that matter.  

 

My 46 D24 original engine, had the carb rebuilt last year when the accelerator pump went bad.  Only driven it a few weekends since then maybe 200 miles tops.  

 

So I went for a drive today maybe 8 miles, drove to the gas station filled it up and drove home, was planning to go for a nice drive this afternoon if all went well, but on the way home it started backfiring through the carb in 3rd gear trying to accelerate up a hill.  It did this once in the spring as well when driving my youngest son and his date to the prom.  Once it got warmed up and trying to go at speed say 35-45 mph giving it gas it would backfire through the carb.

 

I am thinking maybe a fuel issue, pretty sure it is the original fuel pump as the car only has 51k miles on it.

 

Thoughts?

[Sharps40]

Not likely fuel unless its hog rich at speed and fuel is collecting in the intake.  The spark is igniting fuel/air in the intake manifold.  That is due to either or both of spark and valves doing what they do at the wrong time.

 

Check dizzy cap for cracks, check rotor, condenser and coil for function, check points, check engine ground, check dizy ground, check small flex wire inside dizy, check dizy plate for free movement by vacuum advance (if dizy is the kind with a vacuum advance), check plugs, check spark plug wires wires for leakage (at night, look for blue corona running along the wires) and cross firing, check static timing, and observe timing from idle up to total timing at 3200 rpm, check vacuum advance diaphragm for function, check vacuum hoses from carb/manifold to vacuum advance....if all that's good, then perhaps consider stuck valves, loose timing chain (unless motor internals are otherwise known good/new, etc.), and fuel system last.

Edited July 10, 2016 by Sharps40

[joey4420]

Wow, thanks Sharps.  Sounds like I need to just climb under the hood and start checking things.

[Sharps40]

Yep.  Get a good motors manual on ebay from the mid to late 50s for 20 bucks.  Good trouble shooting section.

 

Your rebuilt carb may still be a turd (or even a bad fuel pump) but backfiring through the exhaust is most often fuel/rich related.

 

Spitting/Coughing through the carb (as you describe) is most often ignition or valves.  It can be as simple as dizzy static or total timing too far advance....i.e. a sliped dizy due to a loose hold down bolt.  But, get it running and go through it one item at a time.  When an item passes muster, move to the next item.

Edited July 10, 2016 by Sharps40

[greg g]

From lack of use you may have a valve or two most likely intake sticking open allowing for compression to back into the intake then backfire when the spark plug lights off. I would start by pulling the plugs to see what they look like the doing a compression test, before doing any fiddling with carbs or distributors. The other thing is to check your ignition timing also.

[joey4420]

I will check compression and hope for not valve sticking, just went to check timing and it was off a bit.  Runs much smoother at idle now and when revving up at a stand still.  But timing seems perfect right at the point I can't move the distributor any farther.

 

I sadly am timing it by feel and sound at the moment but I am sure I can get it very close this way as I have before on many other vehicles.  I will take it for a drive and see what happens.

[Jeff Balazs]

You could try checking it with a vacuum gauge. This is a cheap and simple yet very effective diagnostic tool. It will generally tell you what you need to know to get through stuff like this.

 

Jeff

[_shel_ny]

  But timing seems perfect right at the point I can't move the distributor any farther.

 

Loosen hidden bolt on bottom of distributor for more movement.

[Sharps40]

Get a timing light. Even a 12v run on a separate battery. Check. By ear usually gets you too far advanced. Remember the inverse relationship between dwell and timing. Set the dwell (points gap) first as dwell changes alter timing but timing changes do NOT alter dwell.

[Fernando Mendes]

If backfiring(complaints,causes and remedies)

"This may be caused by an excessively rich mixture

b)Hard starting   

   (1)Hard starting when cold

       (a)Choke not closing properly.Repair or replace choke linkage and/or choke control cable.Centralize choke valve.

       (b)Bowl being drained.Loose plug in bottom of bowl.Tighten plug".

[joey4420]

Well I haven't attempted to adjust the timing past the point it stopped, as I haven't looked for the other nut that locks down the timing, but I can say it drove like an almost new car yesterday compared to what it has been driving like.  It made it to almost 60 MPH before it lost power and backfired through the carb.  Before I couldn't get past 30.  Not sure what changed, but what a difference.  Even took it for Ice Cream last night and nice short 30 mile round trip, it was wonderful.  So I will adjust a wee more tonight if I have time after work.

 

Now to just figure out why the clutch isn't releasing well going into first.  Ugh a never ending battle of fun in my garage.  I need to thin the herd is what I need to do, way too many projects.

[Sharps40]

Any update?

 

Sounds like you don't have a tune up manual.  Given that you twisted the dizzy and changed the timing it may be best to tune from scratch.  If you don't know the static timing perhaps proceed as follows (these cars will be timed 4 before to about 4 after TDC but TDC will get you running well with correct dwell....choose a points gap fo 16 to 18 thousands to get close to the right dwell.)

 

 

Bring the engine to TDC (motor pointer pointing at the zero mark on the hub)  on number one piston compression stroke and ensure dizzy rotor pointing at number one spark wire. 

 

Set point gap.  If you set the widest range, it'll provide dwell at the bottom of the range and allow the dwell to increase to its max setting as the rubbing block wears in.  If you don't know the points gap, 16 to 18 thousands will get you started and running.

 

Start engine, warm up.

 

Check dwell to ensure its in the right range, if you know what the dwell is supposed to be.  If its not in range, shut down and readjust points gap....wide gap = small dwell, narrow gap = large dwell.

 

Otherwise, with engine warm, remove vacuum advance tube from dizzy and plug the rubber hose leading back up to the carb.

 

Check timing with a light.  Set it somewhere between 4 Before TDC  to 4 After TDC by loosing the clamp on the base of the dizzy and slowly turning the dizzy till the timing pointer points at the timing you want to achieve.  0 (TDC) to about 4 before TDC should run best.

 

Tighten dizzy hold clamp at the base.

 

Remove timing light.

 

Reinstall vacuum advance hose to the advance pot on the dizzy.

 

Go driving.

 

Get a tune up manual and reset timing to the factory spec for the motor number and the dizzy number at a later date.  But, 0 to 4 before should run well if the motor and other systems (vacuum advance, carb, coil, plugs, wires, condenser, points, vacuum leaks, etc.) are not turds.

Edited July 22, 2016 by Sharps40