Tip-In Stumble

[Joe C.]

This is the last "gremlin" I'm chasing prior to an impending engine tear-down. My purpose is to resolve/eliminate any possible fuel/carburation/ignition issues so that I can establish a valid baseline prior to a thorough blueprinting/upgrade. Car in question has 327/365, built "as stock". Maybe I'm asking too much of an engine with a big cam, but here goes!

This condition has been present from day one, with varying consistency, and to varying degrees. With car in motion, in any gear, at most RPM ranges, but especially when less than 2500, and ONLY when throttle is partially opened (light pressure on "go" pedal: i.e. not coasting, but gently cruising) Here's what happens:
1. When throttle is further depressed GRADUALLY AND SLOWLY, there is a stumble, and sometimes a backfire if revs are low enough.
2. When throttle is further depressed CRISPLY AND QUICKLY, throttle response is instantaneous, with no stumble.

Joe

[Wayne K.]

Re: Tip-In Stumble

Joe,

Have you ever tried making an adjustment to the accelerator pump ?

Wayne

[Duke W.]

Re: Tip-In Stumble

Accelerator pump adjustment, float level, and secondary opening spring are the things I would look at first. If they are not right on factory spec, set them as such for a baseline and go from there.

As overlap increases it's increasingly difficult to achieve good low rev behavior and response. I never had any issues with my 340 HP configuration. It would pull a 3.08 gear smoothly from a thousand revs in top gear, but the 30-30 cam has a lot more overlap than the Duntov.

...another reason to go with the LT-1 cam, which has about the same effective overlap as the Duntov, which is a lot less than the 30-30.

Duke

[Timothy B.]

Re: Tip-In Stumble

I second the accellerator pump adjustment but will say I once had a nova with a hesitation that turned out to be a vacuum leak with the hose at the vacuum advance because it did not fit tight.

[Joe C.]

Re: Tip-In Stumble

Joe,

Have you ever tried making an adjustment to the accelerator pump ?

Wayne

Pump has been set, reset, re-re set and re-re-reset to Holley recommended clearance of (IIRC) .020" fully depressed plunger.
Experimented with .028" and .030" nozzles, in place of the stock .025". Pump shot appears to be vigorous and forceful, very much like my own, if you know what I mean.
Pump cam is stock.

Joe

[Clem Z.]

Re: Tip-In Stumble

make sure the pump shot starts as soon as the throttle lever starts to move. i have seen where the arm has come loose on the shaft and when it was fixed the arm was not in the correct position to start the pump shot right away.

[Joe C.]

Re: Tip-In Stumble

Accelerator pump adjustment, float level, and secondary opening spring are the things I would look at first. If they are not right on factory spec, set them as such for a baseline and go from there.

As overlap increases it's increasingly difficult to achieve good low rev behavior and response. I never had any issues with my 340 HP configuration. It would pull a 3.08 gear smoothly from a thousand revs in top gear, but the 30-30 cam has a lot more overlap than the Duntov.

...another reason to go with the LT-1 cam, which has about the same effective overlap as the Duntov, which is a lot less than the 30-30.

Duke

Accelerator pump adjustment, float level, and secondary opening spring are the things I would look at first.

Accel. pump is AOK (see response to Wayne, above).

Float levels were probably somewhat low before, due to the bad fuel pump. Float levels have been verified and are NOW set to be 1/32" above spec. This change has made no difference.

Secondary opening rate has nothing to do with this condition. I'm talking about tip-in problems in the primary circuit. AYR, secondaries have been checked, and operate properly.

http://smg.photobucket.com/albums/v216/speedo1952/?action=view&current=10-13-07Alone1.flv

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...another reason to go with the LT-1 cam, which has about the same effective overlap as the Duntov, which is a lot less than the 30-30.

LT1 overlap is just about midway between Duntov and 30-30.
overlap @ .050" lobe lift:
Duntov - 8 degrees
LT1 - 16 degrees
30-30 - 26 degrees

Joe

[Duke W.]

Re: Tip-In Stumble

By effective overlap I am talking about the actual computed overlap in square-inch-degrees that is one of the outputs from EA, and this measurement also takes into account valve size and lash.

Comparing .050" lift overlap in degrees is meaningless due to the difference in clearance ramp designs. You cannot compare mechanical lifter cam .050" data to other mechanical lifter cam data unless the clearance ramp designs are identical, which is rarely the case, and for the same reason you cannot compare mechanical lifter cam .050" lift data to hydraulic lifter cams.

Duke

[Timothy B.]

Re: Tip-In Stumble

Joe,
Just a suggestion but you may want to open the secondary plates a bit more then reset idle and re-adjust the air/gas mixture screws, this should richen up idle mixture at the idle port and may be enough to help transistion to main circuit. You could also try pump cam on #2 hole and adjust the accellerator pump again but I don't know if this will give more shot.

[Joe C.]

Re: Tip-In Stumble

make sure the pump shot starts as soon as the throttle lever starts to move. i have seen where the arm has come loose on the shaft and when it was fixed the arm was not in the correct position to start the pump shot right away.

Clem,

There is no play in the arm, or the lever. The motion transmitted from the cam to the arm to the lever is immediate.
Pump shot begins as soon as throttle is moved.

[Clem Z.]

Re: Tip-In Stumble

do you have the correct power valve because too low a opening will cause this problem. if you have a 6.5 try a 7.5 or even 8.5. make sure the idle transfer slots are not open more than .020 at the idle setting or not completely closed

[Joe C.]

Re: Tip-In Stumble

Joe,
Just a suggestion but you may want to open the secondary plates a bit more then reset idle and re-adjust the air/gas mixture screws, this should richen up idle mixture at the idle port and may be enough to help transistion to main circuit. You could also try pump cam on #2 hole and adjust the accellerator pump again but I don't know if this will give more shot.

Timothy,

All good suggestions!
I have made certain that the idle transition slots are perfectly "square" as viewed from the underside of the primary throttle plates. This translates to a .020" length of exposed slot, which is exactly equal to its width.

The secondary throttle stop screw has been adjusted to balance the pri/sec throttle plate openings. This results in a curb idle of about 850-875 RPM.

Normally, when adjusted for highest vacuum and most stable idle, the idle mixture screws are both 1 3/8 turns off the stops. I tried to richen the idle mixture by backing the screws out in 1/4 turn increments, in four iterations, until I reached 2 3/8 turns, with a "nasty" rich idle. These adjustments made NO DIFFERENCE in the off-idle transition. I set them both back to 1 3/8 turns.

[Joe C.]

Re: Tip-In Stumble

do you have the correct power valve because too low a opening will cause this problem. if you have a 6.5 try a 7.5 or even 8.5. make sure the idle transfer slots are not open more than .020 at the idle setting or not completely closed

Clem,

Again, good suggestions!

The idle vac with my engine is about 9.5 in-hg @ about 850 RPM, using a compromise between Duke's dandy valve lash settings and GM recommended. Mine are set @ .025" (which I prefer to .030"). Holley calls for a 6.5 pv, and that's what I have in there. I had tried a 7.5 for kicks, but that had no effect on this condition. Holley likes the pv to be about 1/2 of idle vac, and when the 2818 with 6.5 pv is used with L79, that would be about right. When the 2818 is used with L76, the margin is less, and closer to opening off-idle (richer).

See reply to Timothy, above, for idle transfer slot explanation.

Before you say anything about the dandy lash settings, I'll tell you that the stumble existed in exactly the same way with the lash @ .030. That situation provided somewhat higher idle vac of, IIRC, about 10.5 in-hg @ 850.

[Timothy B.]

Re: Tip-In Stumble

Joe,

Sounds like the idle circuit is tuned good. At low speed cruise, mixture is available from idle port, transfer slots, and main circuit. Transfer slot is fixed and idle circuit sounds good. Do the plugs look like main jets could be increased two sizes?

I still think accellerator pump shot not timed properly. You need heavy shot with light movement of throttle. The volume is the same but timing needs to be increased by changing cam location or squirter nozzle size.

One other thing you probably have considered is timing. How much inital timing are you running because the centrifugal comes in quick with 365 HP engine. Engine is going lean and pop back through intake valve, I would think 12-14 is good.

[Bill M.]

Re: Tip-In Stumble

Could be the transition from idle circuit (vacuum) to main circuit (air flow) is the problem. I had this problem with a 327 carb. on a 283. The idle circuit responds to vacuum. The main circuit responds to air flow. The 327 main circuit was not flowing enough fuel (because the 283 air flow was less) off-idle as the vacuum signal for the idle circuit fell off.

I would try raising the float level to increase main circuit fuel flow to see if that helps. That would tell you it's probably a main circuit issue.