Fluid drive stumble in "fourth" gear

[ChrisP]

So all of a sudden my 48 Desoto has developed a "stumble" that only seems to manifest itself when im driving in "high high" range....or 'fourth" on the fluid drive. That is what im observing: no prob in either low range gear, and "third" seems to provide good smooth acceleration...but in fourth, big time rough spot

 

Car stumbles, hesitates, even wants to backfire.

 

I know there is a second accelerator pump in the carb for the fluid drive...anyone able to shed some light on what this symptom might be? Vacume advance?

 

Maybe im imagining it and need to drive it out to further observe but it seems like it's only demonstrating the problem in fourth

 

Any ideas from the brain trust greatly appreciated

 

CP

 

[greg g]

Check the wiring from the trans to the coil.  There is a circuit that grounds the coil to allow the drive train to have some slack during the shift process.  If this wire has an inadvertent ground from worn insulation, it might be causing your problem.  You may also check the wire inside the distributor that connects the points to the coil lead.  These flex a lot and can become compromised within the insulation or can rub off insulation and ground to the body of the dist.  Also there is an insulator block where the coil lead attaches to the dist.  Check this to make sure it is intact and not allowing a ground to occur there.  If all those are good, then I would start looking at the fuel supply to assure you have enough flow to support cruising speeds.I had a car that would run fine in first and second, but a partially blocked fuel filter would not support speeds over 40 in high.

[De Soto Frank]

I would check the transmission wiring harness for chafed insulation and intermittent shorts to ground, as Greg suggested.

 

If it is popping-back through the carb, that is an indication of fuel starvation.

 

Does this happen as you step on the accelerator after the upshift to high-speed,  when already in high, and needing addition throttle to pass or pull a hill, or what ?

 

 

That "second accelerator pump", whiile similar in construction, is actually a dash-pot, to cause the throttle to close gradually, to avoid stalling of the engine.  It has nothing to do with running lean or a flat-spot.

 

Also check your fuel filter / lines and make sure they are not restricted.

 

And try running with the gas-cap removed, to see if that makes a difference: sometimes the vent int he cas cap gets plugged / restricted, causing fuel starvation under high demand.

 

 

De Soto Frank

[greg g]

Also your rubber line between the inlet of the fuel pump and the frame cross member may be collapsing under extended flow.  They will look fine outside but under sustained suction they will partially collapse restricting proper flow volume.  I swapped mine out with a short length of steel fuel line cut in half with a 360 degree loop of flexible modern fuel line spliced in the middle (to accommodate engine movement/vibration).  do the standard flow test to check if the pump is working at proper flow.

[ChrisP]

thanks for all the good suggestions guys....could be a simpler explanation: car was sitting around for 2 or 3 weeks with low fuel (dumb)....temp changes here in New England may have caused condensation in tank, because i filled up today and problem seems to have disappeared...may have been water in fuel?

 

i'll keep an eye on it, many thanks again i learned some things...

[Tom Skinner]

Chris,

I believe if its stumbling right after the upshift - say at 14-20 mph it could very well be the vacume advance on its way out.

If on the other hand it is stumbling at higher speeds say 20-35 then fuel or a bare wire may be to blame.

The Vacumm Advance does its work until enough speed is reached that the weights in the distributor kick in.

A Vacumm Advance that has quit working dogs everything down from the git go say 3-15 mph. One that is on its way out

could very well cause your symptom. Try checking your vacumn at Idle with a Vacumm Gauge. Is it a steady hand at say 19-20"?

When you rev the engine does the hand drop then resume its reading? I think your on to the right part check.

Remove the Vacumm Advance and test it or replace it. Also how is your Timing? Set it before you check your vacumm.

Let us know what you came up with.

Tom

[De Soto Frank]

If the problem seems to have "gone away", then it may well have been soem condensation in the fuel... are you subject to gas-a-hol up there in Mass?

 

Another possibility, that would go along with colder weather, would be if the exhaust heat-riser damper is stuck or the thermostatic spring is missing / broken / unhooked:  if the damper is not closed during warm-up, this can cause a stumble until the engine gets good and warm...

 

 

De Soto Frank