Hesitation on exceleration

[Drakeule]

My '60 Invicta does fine,except when I really step on the gas; feels like a flat spot .Rebuilt Rocester 4GC less than a year ago. 

 

Am I back to looking hard at the pump in the carb again?

 

[Ben Bruce aka First Born]

33 minutes ago, Drakeule said:

My '60 Invicta does fine,except when I really step on the gas; feels like a flat spot .Rebuilt Rocester 4GC less than a year ago. 

 

Am I back to looking hard at the pump in the carb again?

 

 

  Is the carburetor heat blocked off on the intake manifold? 

 

  Ben

[carbking]

Does it hesitate at ALL RPM's, or just from idle?

 

Jon

[Drakeule]

Sometimes from idle; but this has (mostly) resolved. Flat spot on serious acceleration; especially uphill.

 

P.S. "exceleration".....spelling was never my strong suite!!

[Wayne R]

Go check all your electrics first ,distributed, points, cap, condenser, wiring from all of these , plus another item to check is spark plugs the problem that you have can be caused by spark plugs with not the corrrect heat range. — to cold .

[60s GM Fan]

Do you happen to use any Octane boosters? 

[Ben Bruce aka First Born]

Bottom line, I think, is too much air for fuel available.   OR not enough fuel for the air.  Yeah, I know.  Same thing.   But are they?

 

  Try easing in and see what happens.

 

  One reason I went EFI years ago.

 

  Ben

[NTX5467]

If the hesitation happens only on heavier throttle movements, it can be more related to the accel pump not having enough shot.

 

As to the ign, set the dwell first, then base timing . . . in that order.  Is hot base idle speed to spec? 

 

What happens when you "leave fast" from a stop sign?  It thinks about it for a second, then goes?  Coughs? Dies?  THAT could be accel pump issues.  Like on a 1/2 throttle punch.  Slamming the throttle from idle to WOT, after a stop, normally does not work well, no matter what.

 

If it falls flat on accel up a hill, that could be the power valve not kicking in on cue.

 

Please advise,

NTX5467

Edited September 9 by NTX5467 (see edit history)

[Wayne R]

1 hour ago, 60s GM Fan said:

Do you happen to use any Octane boosters? 

Never used it —no need if your cars tuned correct.

[Wayne R]

Forgot to mention on Buicks when ever changing  or fitting new spark plugs the 2 plugs at front passenger are sometimes very hard to get to, and what can happen is it’s easy to crack the white porcelain on the spark  plug—- this can cause a miss under load.

[carbking]

First check to see if someone downgraded the ignition to electronic, forget everything that follows and reinstall points and condenser.

 

If the ignition is electronic, and you are reluctant to upgrade to points and condenser, do you have a generator or alternator? If generator, and you are in love with the electronics, upgrade the generator to an alternator.

 

Second thing I would check would be the distributor advance mechanism. If the distributor does not properly advance, then the engine will definitely have a flat spot.

 

Check the rest of the ignition as posted by several above.

 

If the ignition is given a clean bill-of-health, and you are relatively sure the carburetor is the culprit:

 

First, test the accelerator pump.

 

(1) start engine

(2) stop engine

(3) remove air cleaner

(4) hold choke butterfly open

(5) locate pump discharge nozzle in primary area of carburetor

(6) work throttle linkage to wide open quickly, while observing the pump discharge nozzle for 2 squirts of gasoline

(7) repeat (6) and compare volume of squirts (assuming you got squirts) of the first and second tests

 

If no squirts, carb needs to come apart, and work on the pump circuit. If this is necessary, a leather pump is superior to any of the modern wonder materials.

 

If the second squirt is much greater than the first, the pump may not have the internal check valve. Not all Rochesters do (you didn't mention tag number), and this can cause a flat spot. Do you remember if the pump has the internal check valve?

 

Does this Rochester have the auxiliary air valve? If so, did you replace the air valve spring, and adjust it when rebuilding the carb? THIS SHOULD ALWAYS BE DONE DURING A REBUILD!

 

Someone mentioned the power valve; is it the correct valve? Rochester used about 20 different valves in these carbs? Different size orifices, and different length activation plungers. Did you test the function of the power valve actuating valve with a vacuum source? Was the actuating valve removed, and the passage checked for "bumps"?

 

Hesitation from idle is often idle mixture screw adjustment. The idle mixture screws should be adjusted between 1 turn and 1 and 1/4 turns unless the engine burns as much oil as it does gas; in which case the idle mixture screws may be adjusted to a maximum of 1 and 3/4 turns.

 

EDIT: I have not seen an answer to Ben's question about the heat crossover.

 

Jon

Edited September 10 by carbking (see edit history)

[56 Buick]

Don't want to highjack the post, but what impact does an open heat port in the intake have. I ask as the port in my car is not blocked off and I am not sure if that is a good thing or bad thing?

 

Thanks

 

Drew

[Ben Bruce aka First Born]

Not hijacking, I think.    We all know creating a vacuum in a space [ intake manifold, in this case ] chills the space.  AT IDLE,  the vacuum is at it's highest [lowest absolute pressure].  The low pressure, low temperature CAN cause gas to condense into liquid and just lie there.  I'll bet if one could feel the bottom of the intake manifold one would notice the cool surface.   An inline engine manifold is easily seen as frost CAN , depending on conditions, be seen.   When the throttle is opened the intake manifold pressure  is increased [ vacuum DECRERASES ]  , air flow increases and the accumulated gas is sucked into the cylinders causing a momentary rich, flooding condition.

  

 Anyway, those were my thoughts when I asked the question.

 

  Ben

[56 Buick]

Thanks Ben.

[carbking]

Carburetion 101:

 

There are three basic considerations for fuel atomization efficiency:

 

(1) Velocity

(2) Heat

(3) Fuel

 

Reducing the heat means a corresponding increase in at least one of the other two (think of the choke increasing both velocity and fuel on a cold engine).

Increasing the velocity can mean a decrease in the necessary amount of heat required (a race engine at WOT does not need heat).

 

It really is not rocket science if one thinks about what is happening.

 

Blocking the heat crossover has become increasingly popular in the last 30 years for a number of reasons:

 

(1) blocking the crossover will not burn the paint from the intake manifold crossover area, thus keeping the show engine prettier longer.

(2) blocking the crossover MAY reduce issues when driving in city traffic with high ambient temperatures

(3) the racers do it (tests apparently have shown blocking the crossover is good for an average of 1.5 % power increase at WOT, so a 400 HP engine would increase 6 HP)

(4) the growth of the internet with some folks promoting any/all of the above

 

But one should also consider the "Law of Unintended Consequenses", thus:

 

(5) blocking the crossover can cause hesitation / stalling on a cold engine in city traffic for a time, sometimes severe

(6) blocking the crossover can cause much poorer atomization and burning of fuel, allowing fuel to bypass the piston rings and contaminate the oil

(7) blocking the crossover can reduce power, and will certainly increase fuel usage for a time

 

Personal experience:

 

I have been working on/with carburetors since 1959. In that time, have made lots of mistakes around carburetors, but the three worst, in order:

 

(1) installing an electric choke on the wife's daily driver with automatic transmission because I didn't have time (I thought) to fix the hot air choke.

(2) thinking I was sufficiently competant to make a Pontiac 301 in a TransAm (son's car) run as well as a basically stock 1969 Toyota 4 cylinder

(3) listening to the internet, and blocking the crossover on my Pontiac 350 because I needed (ha, ha) that extra 1.5 percent power

 

The solutions:

 

(1) fix the hot air choke (I really did have the time  )

(2) buy a different car (a factory turbocharger and reworked carburetor helped, but not enough!)

(3) (mandatory for the first 30 minutes of city driving) install a carburetor with a manual choke (what I did) or take the engine apart and unblock the crossover. With the manual choke, I could activate the choke, increasing both the amount of fuel and fuel velocity, during the first 30 minutes of city driving, and the car ceased to STALL at every stop sign/light.

 

Jon

[old-tank]

2 hours ago, carbking said:

6) blocking the crossover can cause much poorer atomization and burning of fuel, allowing fuel to bypass the piston rings and contaminate the oil

I am not seeing that with today's highly volatile fuel.

[60FlatTop]

2 hours ago, carbking said:

In that time, have made lots of mistakes around carburetors,

I actually live 25 minutes from the Rochester carburetor plant. In 1974 I thought the QuadraJet was too complicated so I bought the newly released Holley Spreadbore 650 double pumper for my 430 Buick Riviera.

 

I am pretty good with Q-jets today. Hesitation? First check the vacuum advance then deeper into the ignition.

 

When you consider how much activity there is in the ignition and the carburetor is just a pot of gasoline sitting on top of the engine with a few bleed ports, I m heading for the ignition first.

[Drakeule]

Boy. a lot to chew on here; like mentioned by some. I'll check out those electricals before proceeding.

Thanks all!