Hesitation And Stumbling Under Load and Starting Out

[Hubler13f]

So, I finally solved my problem after a week of researching and reading I must have read 100+ threads about Hesitation, Timing, Sputtering, Hard Starting, Plug Gap and so on and so forth, so I figured I would add my experience to maybe help someone out in the future. My issue began after I had blown a head gasket and replaced it. After I got it started it was running like crap, like you know when you spent all night out drinking keg beer and eating junk, Yeah that kind of CRAP! It was hard to start, and sounded like the timing was retarded and the starter could barely turn it over. When it would run, I could stand there and it would idle smooth I could blip the throttle and it would respond beautifully. But on the road it would run hot and would jump and buck and sputter starting out and going up hills or under any load at all. On the flats it felt like I was towing something with it. So at first I figured it had to be related to the head gasket problems, long story short it wasn't. I messed with the timing and it would get somewhat better when advanced but still wasn't good.

 After a week of trial and error I went through all of those threads and made a checklist and hit it hard. The following are things I read and did to try to solve the problem not necessarily in that order. Static timed, Timed with light, Timed with vacuum gauge, Road timed it, Cleaned the distributor, Replaced the tiny ground wire in the distributor, Checked the cap, Checked the rotor, Checked the wires, Re crimped the wire leads, Checked everywhere possible for vacuum leaks, Re torqued the plugs, Tightened every screw bolt and stud from the top of the carb to the block, Dumped Seafoam down the carb and let it sit over night, Cleaned the fuel filters, Unhooked my electric pump when the car was running allowing the mechanical to do all the work (my electric runs a 2.5-3 lbs. and seems to do pretty good at assisting the mechanical) Checked for black smoke, Smelled for gas in the oil, Watched the oil fill tube for steam or smoke, Broke the nose cone of the starter and replaced it with a spare,  

Pulled the plugs and they were all black and sooty, very sooty. So I opened up the carb and looked at the valve that is supposed to be pulled down by vacuum, I pushed very lightly and it was stuck in the up position so I pushed just a bit harder and heard it click as it dislodged and slid down, I exercised it a little, cleaned out the fuel bowl, just a little fine powdery dirt in the bottom, then made sure my float was adjusted right. Static timed it started it up and ran down the road and it ran like a dream. So all of that just to find out that little valve jet was stuck in the up position and dumping extra fuel into the motor. It was a lot of good reading and I appreciate all of you fellas for posting your previous problems. So here is my post I hope it helps someone some day.

 

Edit: I forgot to mention I also checked and tested the vacuum advance and it was good.

Edited June 27, 2017 by Hubler13f
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[medium_jon]

Glad you got it fixed. What a relief!

[greg g]

That little thing is in the power jet it functions to provide an enriched air fuel mix to sustain the needed jolt that the accelerator pump initially supplies when you open the throttle.  Under high manifold vacuum situations a vacuum signal from the manifold pulls the needle down against the spring holding the jet closed for idle and partial throttle cruise. When manifold vacuum drops, the spring opens the jet providing more gas.  I have seen a car go from poor running black smoker getting about 11mpg to to good runner getting 19 mpg, just by virtues of sorting this fuel circuit.  Glad you found it and fixed it. 

[Andydodge]

Now this is why I think extra carburettors are such a good thing..........3 or more preferably as you would then be able to play multiple choice as to what carby had the problem............lol...........life is good when you have a win..............andyd 

[DonaldSmith]

Step-up piston, between the venturi and the float:

The step-up piston parts:

There was a post some time ago about the aluminum body of the carburetor corroding and inhibing the free travel of this piston.  Check it from time to time. 

[DonaldSmith]

So, to check this step-up piston, take the top off the carburetor and push down on the middle part of that brass strap.  The piston and spring should move down freely.  The funky screw will stay in place. If the piston doesn't move down freely, or won't return up by spring power, the piston needs cleaning, or the little cylinder in the carb body needs to be refinished.  I remember using a pencil with fine sandpaper taped on, to clean the walls of the cylinder.  You might want the carb off the engine for this, to thoroughly clean out any sanding debris.     

[dpollo]

The trouble is always the last thing you check.    Many times , the power valve can be disabled by putting the wrong gasket under the carb.  This happens more often on Chevrolet sixes of the era because their carb gasket has only one slot and can easily be installed upside down or backward.  On Chrysler products there are 4 slots  BUT there are at least two different gaskets.      

[medium_jon]

2 hours ago, dpollo said:

 

The trouble is always the last thing you check

 

@dpollo: always is!

[Hubler13f]

All good info, and to Dpollo, your absolutely right. I'm not sure what made it stick, but it was running great before the head gasket blew it only sat for about a week until I got parts, maybe it backfired through the carb upon start up and lodged it up and open.  Who knows it's always something.

I have 3 other carbs tore down, 2 are soaking in carb cleaner buckets. So on a rainy day when I'm feeling patient I'll sit down and rebuild one, taking my time and honing and polishing everything. For now it's back to running good.

[48ply1stcar]

Where have you been all month.  I have blamed every possible component for my stumble at 50 MPH and at the end of 2nd gear.  Thanks for the information, hopefully this may solve my issues.

[Dodgeb4ya]

Don't forget there needs to be the proper sized O-Ring gasket down in the bore beneath the step up piston .

[DonaldSmith]

Update- I checked my step-up piston today.  It moves freely.  It moves a needle valve next to it.  The odd screw at the other end is a sort of guide.

Here's my thumb pushing the piston down. 

(In the background is the top of the carburetor, flipped back. I disconnected the rod from the Sisson choke but not the rod to the throttle linkage.)