slight hesitation off idle

[Marx3]

Hi, recently build a 17085227 for a buddy

tubes: 038
DCR: 052
Low. Idle: 076
Upp. Idle: 069
dont remember by-pass, sized according to the other idle-sizes.

Single main airbleed: 085
Jets: 77
Rods: 50M

Dont remember acc.pump discharge.

sec. rods: CE
Hanger : R


pull-off about 2 seconds.

after messing alot with the APT ( it is currently at 3.5 turns form bottom ) and changing the hanger from J to R. the engine started to pull stronger all over the rpm range, but it still has a little problem off idle, or when flooring it from rolling at a slow speed. like it sputters or hesitates a few times, not so much that you can feel it in the car, but enough to be annoying...

engine is a Buick '71 Buick 455 with a dual plane rpm intake, milled heads ( cr should be about 9:1 )
cam is the 68002 Lunati

# Advertised Duration (Int/Exh): 268/276
# Duration @ .050 (Int/Exh): 227/233
# Gross Valve Lift (Int/Exh): .505/.521
# LSA/ICL: 110/106

rear-end is stock, propably about 2.73 and converter is stock also...

I am not sure if the problem is to blame on the converter ( it is beeing changes shortly to a 2200 stall )

or if the jet/rod combo is slightly lean... I mean, the combined fuel-flow area equals the smaller jet/rod combo for recipe 2... Am I wrong on the fuel supply, or should I just wait for the new converter and try again?

[Marx3]

btw, car is a '71 riviera...

[Marx3]

oh yeah, and the idle seems to have begun to fall off slowly, when holding still i D. It did not do this a couple of weeks ago, when the carb was fist installed... Again, I am suspecting the jets/rods?

[Cliff Ruggles]

Hesitation at light throttle off idle and when flooring it?

Going to the floor involves the secondaries, so go after the choke pull-off release time, spring tension on the secondary airflaps, secondary plastic cam (common problem worn or loose on the shaft), accl pump shot, hanger height.

Light throttle or off idle hesitation look at the APT height.  A 50M metering rod may not be quite large enough on the upper section for the .077" main jets.  It could be just a tad rich at part thottle.....Cliff

[Marx3]

It seems the problem occurs when ever the engine is suddenly asked to work, from at standing still, or cruising at 40 with a trailer, the engine seems to just make noise with out pulling. And yes, it is when flooring it.

The pull-off is about 2 sec. and the spring is 3/4. The carb is build with your kit, so cam etc is new :-)

I will see if I can find a pair of 52 rods, or jets a couple of thousands smaller to try with!

moving the acc pump arm from the inner to the outer hole and back had no effect.

Lowering the apt to 1.5 turns seemed to make it worse though..?
I wil try a lower hanger as soon as we can find the time to do some testing again.

[Cliff Ruggles]

When the engine is suddenly asked to work, vacuum falls off sharply.  This causes the power piston to raise up as spring tension overcomes the vacuum signal to it.  APT adjustment would have no effect here.

I would make sure that the metering rods are correctly located in the jets, and well up on the small tips of the rods at full lift.

The height of the metering rods (secondary hanger) and which metering rods are being used will have a big impact on fuel delivery when the secondaries are brought on quickly.

The accellator pump plays a role here, but it appears from your description that it is working correctly.

Seconary POE is another player in that system as well.......Cliff

[Marx3]

I will check up on the rod-location!

regarding acc-pump.. it seems to be working exactly like it should, I dont remember the size of the shot-holes, but it was built as close to recipe 2 as possible. And thing is... if you step on the throttle two times when flooring it, the hesitation disappears, and the engine just pulls freely.

POE restr. are .058

should I go up or down on hanger height and sec. rod size? and slower or faster on the pull-off? Just to have guidelines you know...

[Marx3]

Hmm, putting in a 2200 stall converter seems to have practically eliminated the problem...
new problem: it runs out of gas at some point in 3 gear :-) time to do something about the stock fuelpump :-)

We are getting the engine dyno'd next week, and I just can't wait to see the results!!! Especially in the fuelratio department! I will post the results!

[Marx3]

power as measured at the wheels: 257 hp / 390 ft.lbs. AF-ratio was 13.6 at 60 mph, and 12.7 at wot.
tried installing 52 rods, but that bumped AF-ratio to 14.9/15 at cruise, so we stayed with the 50's.
So all in all, great succes!