Pesky soft spot

[eventhorizon66]

Okay so I've had the new 350 (stock with headers, edelbrock intake/carb) in for a while and it runs pretty good overall.  But I have a "soft spot" from about 1000 to 1500 rpm while gently accelerating in top gear.  No matter how lightly you roll on the throttle there is some noticeable hesitation.  It almost feels like my old 305 used to feel when it was under heavy load/light throttle and just pinging away, but in this case I can't hear any pinging.  Hit the throttle hard enough to drop it down a gear and it runs fine.  1500 seems to be the magic threshold.  Also, I've never been 100% staisfied with the power.  That may be my unrealistic expectations for a stock 350 (but I can barely chirp my 235/75-15 all seasons from a dig, even with 3.73 gears and a 3.06 first).

I've checked very thoroughly for vacuum leaks.  My vacuum gauge indicates about 19" at 850 idle in park, but the needle slowly fluctuates between about 18.5" and 20".  My timing is about 14* initial with about 35* total (in by 3000), my vacuum advance adds an additional 15* and its hooked up to ported vacuum (I tried full vacuum a while back and it didn't seem to make any difference, also I must pass the sniffer test every year).  When I was running stock .075" x .047" rods in .098" jets, there was a tiny lean surge while cruising at highway speeds, big hesitation off the line, and occasional popping under heavy throttle.  I switched to .073" x .042" rods and it ran much better.  I adjusted the floats (as per Edelbrock specs) some time after the rod change and a very small amount of the off-the-line hesitation came back, but the soft spot didn't change in severity.  I've tried all the way to a .070" x .037" rod; the off-the-line hesitation disappears, but the soft spot is still there and the gas mileage drops noticeably.

To make things even more interesting I pulled all my plugs today and they all indictate a lean mixture, except my #8 which is almost perfect.  Here's a link for a pic of my #5 and #8 plugs for comparison (all the others look similar to #5).  The resistance in the plug wires checks out OK (#5 = 676 ohms, #8 = 644 ohms).  Any thoughts would be appreciated.

[VileZambonie]

I would run manifold vacuum to the vacuum advance and also try giving it a few more degrees.

Did you unplug the torque converter clutch to see if it makes a difference? Perhaps that's all you are feeling.

[eventhorizon66]

Thanks for the response Vile.  I'll give full vacuum another try and play with timing a bit more.  But any ideas why one plug could be different from the rest?  Also I am running R45TS plugs (as per the installation guide that came with my engine), but I see you recommend an R43TS quite a bit.  Could I benefit from a slightly colder plug?

[VileZambonie]

Your plugs look ok. If it's not pinging with the plugs you're running don't worry about it.

[eventhorizon66]

If it turns out to be the TCC that is causing the prob, what can be done to correct this?  Is a switch to manually unlock the converter the only answer?  The TCC vacuum switch is new and hooked to ported vacuum.  The stall rating of the converter won't affect this, will it?

[Blue 82]

connect your tcc to manifold vacuum along with your vac advance
you can tap the brake pedal to unlock the converter
try it upon accellleration and see if the conv unlocks/locks
you may also need a better accelerator pump shot to get rid of that soft spot

[77 step]

It seems to me I had to increase the stroke on the accelerator pump on mine.  Here is a quote from the manual.

"CALIBRATING THE PUMP
If you encounter any hesitations or stumbles that do not seem to be related to the basic metering or have not responded to changes in the basic metering, move the
pump drive link to one of the holes closer to the carburetor body. This will increase the stroke length of the plunger and result in more pump delivery."

Here is a link to the manualhttp://www.edelbrock.com/automotive_new/mc/carbs_acc/pdf/carb_owners_manual.pdf

Good luck!

[eventhorizon66]

Just thought I'd give you guys an update.  The soft spot was the torque converter locking up at too low a speed (37 mph).  I guess the engine doesn't like to pull my truck at nearly 40 mph from 1100 rpm.  I put a toggle switch in so that when I'm toodling around town it is always unlocked.  When I hit the open highway I just flip a switch and the rpms drop another 250 rpm or so.

Also I switched back to manifold vacuum for the advance canister.  What a difference in throttle response!  I guess the first time I tried it, the jetting was too lean for me to feel a difference.  Now my idle is a rock steady 20.5" of vacuum.  Also I played with the accelerator pump and discovered that it got less responsive when I moved it to the hole closest to the carb and reached max responsiveness  in the hole furthest away from the carb.  Now it really does "hit like fuel injection."  On top of that, after an 80% highway (~65 mph), 20% city trip the other day, I calculated about 17.5 mpg.  Yee-Haw!

Thanks for all the help and suggestions.

[Blue 82]

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