66 Buick Q-Jet

[Rhett]

I've got a 66 Q-jet, and  I am trying to dial out an off-idle stumble. I've got the the specs as such...when it was cooler it was almost gone, but now it seems to have gotten worse since the summer heat. It doesnt  load plugs or soot the pipes, but could I now be too rich at that throttle range?? The pump shot is good and I had to go big on the ICR to get it as good as it was in the spring. Any suggestions?

Thanks, Rhett

Fuel Inlet                                   0.145
Float Level                   0.250
Idle Tube                                   0.035
Idle Channel Restriction   0.060
Accel Pump Discharge   0.030
Main Air Bleed (upper)   0.070
Main Air Bleed (lower)   0.070
Idle Air Bleed (upper)   0.052
Idle Air Bleed (lower)   0.070
Mixture Hole                   0.094
Primary Jets                   0.074
Primary Rods                   0.044
Secondary Well restriction   0.032
Secondary POE Restriction   0.052
Secondary Tube Restriction   0.036
Secondary Rods              CK .0527 L
Secondary Valve Opening   1.130
Secondary Hanger                         M

[Cliff Ruggles]

I'd reduce the idle down channel restrictions, you have enough idle fuel for a "Pro Stock" engine!.....Cliff

[Rhett]

Agreed, although it doesnt load up at idle..so at least I've got that!

How can I increase the off idle fuel to get by the stumble? lengthen the slot? Open it up?

Opening the ICR worked to a point, and rods/jets messed with part throttle performance..

Is there anyting else that can be messed with?

[Cliff Ruggles]

Might want to stagger the jet/rod relationship a bit to bring in more fuel at cruise, which will offset how much fuel is needed from the idle system to get thru transition.  74 jets are plenty rich in that carb, might want to reduce the jet size a few numbers and go smaller on the metering rods by 31 or 32 numbers.....Cliff

[Rhett]

So something like .071 Jets and .039 rods?

[Cliff Ruggles]

71/39 might work.  If you don't want to make that much of a jump, try 73/42....Cliff

[Rhett]

Let me see if I've got this straight..

74 Jets and 44 Rods give me a total area of .00278031

The 73/42 Combo would give me .00279994 - Bigger for more cruise fuel...

Could the same thing be accomplished with leaving the 74 jets and going to a 43 rod for .0028464? More cruise fuel, but I did try that once and it didnt seem as crisp under part throttle.. is that too much cruise fuel for this car? (stock 401, auto, no cam, 2.5 exhaust)

By that yardstick, the 71/39 combo give me .0027646 - seems to be a step in the wrong direction......??

Thanks, Rhett

[Cliff Ruggles]

The mods I suggested were in combination with reducing the fuel being supplied by the idle system.  The idle system feeds the engine off idle and thru transition via the transfer slots and lower idle airbleeds......Cliff

[Rhett]

So to reduce the idle fuel, I'd have to drive in new ICR restrictions and resize them to .046 or so, correct?

If I'm hearing you right, you suspect the carb is too rich at off idle and cruise??

[Cliff Ruggles]

Correct.  To much idle fuel to the mixture screws and transfer slots can make the carb rich at part throttle....Cliff

[Rhett]

I get what you mean, the ICR is huge and it's jetted big..but it doesnt feel rich, it feels lean - in the sense it doesnt stumble or miss, there's just a slight "hole" in throttle range where nothing happens, and then it picks up when the cruise comes in. I know WOT and idle mixtures could cover any richness indicators, but no fouling, pipe soot or smoke throughout throttle range..

Could a temporary rich condition cause a "hole" like that?

[Cliff Ruggles]

Rich conditions usually don't give a lot of negative symptoms, just excessive fuel consumption.

The float level and fuel pressure play a big role here as well.  The early carburetors have the worst fuel inlet design and float hinge pin location. 

We avoid them here for any sort of performance use.  The fuel level in the bowl may be a tad low, and why you are having to put so much fuel to it to get good results.

You mentioned a .145 fuel inlet, most of the conversion kits are .110".  .250" is a "low" float setting for that carburetor, might want to move it up to 3/16" and see if it helps some?........Cliff

[Rhett]

Thanks for the feedback - I'll bump the float up and see if it helps

I got the .145 needle/seat from you to cover a fuel deliver issue I was having - would fall on it's face just after the shift. Maybe I didnt bring the float high enough.

I know these are the least desirable Q-Jets, but it's really a pretty good running carb, good throttle response and secondary transition, makes good power and idles well... just has that hole.

[Rhett]

Set the float at 1/8 - seems to have done the trick..


Thanks for your help.

Rhett

[Cliff Ruggles]

1/8 to 3/16" is usually what it takes to get those working correctly....good news!.....Cliff