Technical Chasing A Light Stumble Holley

750 HP DP, light stumble 2500-3500 in first (4 speed). Above 3500 in first, no problem. 383, 4:11, 69 primaries.

I have gone to tube squirters, worked w/pump cams and helped a lot. Closed down LSAB 4 numbers, helped.

So, I’m thinking this ^^^^ richened up idle circuit a little and helped so changed primary jets 69-73. Seems to be the fix.

I have avoided richening jets due to plug condition. Did peppy cam cause carbon on plug and not necessarily a sign of rich jetting ?
Pic w/69 jets.

Have 160 t-stat and mostly 10 minute trips.

 

Probably not the same, but I had what seems like the same problem in a 4160 Holley in my Corvette a few years ago. I went through everything and could not find it. During one of my diagnostic attempts, I ran without the air cleaner. The stumble was gone! I bought a K&N filter, thinking it would flow more than the (new) Wix element I had been using, and it made the problem worse! After switching back and forth between the Wix, the K&N, and no filter several times, I came to the conclusion that the problem was airflow related.

I finally solved the problem by replacing the 3" high Wix element with a 4" high unit. Luckily, the 4" unit still fits under the hood, so I'm good. Since I'm not an anal-retentive "matching numbers" geek, I'm fine with it.

 

NOT a tune-up "pro" but......with a "peppy cam".....coupled with low speed/RPM stop & go driving plugs will get sooty. Keep in mind this "gas" we have to buy(blend/non-blend/winter blend/summer blend/eythanol/non-eythanol on-and-on) makes keeping these engines a real challange too. I've also found that ignition timing is the problem more so than a carb..........sometimes.
Keep in mine these "peppy-cams" WE love to run .....need A LOT.....of initial timing.
Give it PLENTY of hot spark as early as possible.........then fiddle with the "controlled-leak" we call a carburetor.
Is what I do.
6sally6

 

Yep, many carb problems are
best fixed by dialing in distributor!

Distributor is MSD Pro Billet, 25 initial, 35 all in at 1800. Manifold advance. Gas is 93 no corn. Idle speed is 950-1000.

 

What does your car idle at? If you have to have the idle up, 1000, 1200 rpm, back the idle down on the primaries and adjust the idle with the secondarys. If you are lucky and have a carb with a adjustment screw your lucky, if not I take the little adjustment screw, allen more than likely, out and put it in upside down where I can get to it easy. If to much transfer slot is showing, it'll foul plugs and have a rough idle along with a stumble. Hope this helps.

 

Idles smooth MOL @ 950-1000 where builder recommended, 4 Idle mixture screws are out 3/4 turn. I can kill engine by turning any idle screw in. Can also idle down w/idle screw until engine just dies out.

I put thumb screw w/spring on secondary idle. It’s adjusted closed then just a half turn or so to keep blades from sticking. My primary transfer slot is “less than square”.

I guess my ? is “Are there circumstances when plugs simply
won’t have that optimum tan
coating ?”

BYW, have never fouled a plug
or had a high speed miss, no black
exhaust smoke.

 

What's the ambient temperature where you live?
I would run a hotter thermostat myself.

 

50* + now to high 80’s in summer.
Didn’t notice light stumble in summer.

I have a hollowed out 2” aluminum HVH Super Sucker, air gap intake and 5/16 manifold gasket, trying to keep fuel cool as possible.

Primary jet change 69 to 73 took care of stumble plus stronger feeling. Surprised richer jets helped when plug
show some minor carbon (already a little rich ?)

 

Put a timing light on the motor and see if the distributor is advancing. I fought the same sort of problem, only to discover the vacuum diaphragm had failed, and was not advancing the distributor.

 

I have a dialback,
timing is all in
@ 1800, 35’.

 

Try changing the power valve to a lower one by 1 in .

 

I guess my ? is “Are there circumstances when plugs simply
won’t have that optimum tan
coating ?”

Absolutely

 

This would open PV a little later ?

 

1. What is the compression ratio of this engine?
2. Is that Autolite 3923 spark plug in your picture out of this engine?
3. Why is it the engine only sees 10 minutes of run time?

 

AHotRod,
Supposed to be 10-10,5
Yep, plug out of #1 cylinder
I’ll drive car to convenience store,
Grocery, etc in winter just for fun.
Summertime more like 45
minutes at a time.

 

The tan residue on the plug was mostly lead oxide back when I was a pup. Plugs just don't look the same with this modern stuff they call gas. Ten minutes run time isn't enough to burn the crap off the plug anyway. Takes a lot of heat to do that.

 

Explain "" light stumble 2500-3500 in first"". I see you have a spacer, may be good for top end power but drivability wise it will dampen carb "signals" some, and make owner/operator nuts. Try with out the spacer. Thats a ton of initial timing and your heads may not need 35° total because of chamber design. In by 1800 is a little early but with 4.11 gear not too bad. I would put initial at 20° and try [can always put more adv. in dist. if you still need more than 30° total.] first, second remove spacer and try. Adj idle mixture/speed on both trys. Be sure the spring you put on secondary idle speed screw is not letting the secondary's open some when primaries are off idle.

 

“Be sure the spring you put on secondary idle speed screw is not letting the secondary's open some when primaries are off idle.”

Forgot to mention,
secondaries open almost even w/primaries.

The spring is there to keep tension on thumbscrew so it doesn’t back out.

Thanks !

 

Why are you running a 4 corner idle holley on the street? If you have a stumble at 3500 rpm, your out of pump shot. Got 50 cc pumps? You are on the transition step. I think your out of pump by then. That nut is sure screwed down a long ways. What is the clearance between the pump and pump arm at closed throttle? If you have no stumble off idle and have it at 2500 to 3500, you are out of acc pump. Something else wrong. Lippy

 

And that plug looks like it's shivering. Cold. Especially for a ten mile run. Lippy JMO

 

Yes, you want it to try opening the power valve a little sooner, might get rich enough sooner to cover the stumble.

Do you know what number power valve is installed now, what is the engine vacuum is at idle.

 

How about a vacuum reading during your short cruse around the block?
I tend to read a spark plug on a full throttle run, not a drive around the block if the engine has a higher RPM operating range camshaft than a stock replacement.

 

You need larger squirters

 

Thanks guys,

A change from 69 to 73 primaries
(HP main body came w/73s) seems to have solved my minor stumble. Most folks w/750s favor 72-76 primaries.

Someone axe about 4 corner idle, aren’t all DPers 4 corner ?

I think I’m fixed !

 

Nope!... Not all... Maybe the HP versions do... My B/G "gold claw" has those....

 

Why wouldn’t I run carb w/4 corner idle on street ?

My carb is 750 Holley Ultra w/HP midbody upgrade. Has tubular squirters and Pro linkage.

Squirters are 31, 37 (from memory)
Green pump cams. 30cc pumps as suggested by Holley. Secondary squirter has hollow screw.

I had mistakenly installed 8.5 PV 4 door, corrected w/6.5, 4 door. Maybe 8.5 was blackening plugs ?

Jetted to 80’s primary (Holley furnishes 76’s w/upgrade). Way better performance !

Jetted secondaries 84 (Holley furnishes 82’s). I had been running this thing lean

Idles around 900-1000, idle mixture screws out 3/4 turn. If I turn one in 1/4, RPM drops. Removing full vacuum hose at idle causes RPM drop, as it should.

Bottom line much more performance, no stumble anywhere.

Recently, found out “no posts/comments/threads on anything built after 1965 allowed” As aluminum Ultra Holleys, air gap intakes, HVH SS, MSD Pro Billet distributors and 350 Chevrolet engines all fall outside of cutoff date, my apologies to those offended !

 

Can someone explain the pros and cons of 4 corner idle circuits on the street? My thoughts are the carb is idling on all 4 ?
With Google I got opinions when I searched (not real deep though)

 

4 Corner idle benefits
Cammed up motors.....


“I personally love 4 corner idle, but I'm a nitpicky tuner. It's really dependent on the manifod vacuum of the engine, basically how radical of a cam. With low manifold vacuum, its hard for the engine to pull enough fuel through only two ports. Most of the time, folks don't realize how good the engine can run with a little more controllable fuel. It's always worth it to me. Just my experiences. If you try it let us know how you like it!”

https://www.holley.com/blog/post/how_to_adjust_a_carburetor_4-corner_idle_system/

 

Update,

383/480, M 21, 4:11, 3500#’s,
750 DP Ultra, HP mid-body, Holley Pro
Linkage, 2” HVH Super Sucker, 26’ initial>36’, full manifold vacuum

Now,
72>74 P
82 S
35 to 31T to 28 P squirters
41T to 31 S
Pink in #1 P
Blue #2 in #2 S

By moving up 2 in P jets, I was able to greatly reduce squirter sizes, and less aggressive pump cams.

W/Holley Pro linkage, Holley suggests using blue and white pump cams on secondaries. They’re right !

Plugs look fine. I might try moving
82 S to 79 S. Engine was dynoed w/79’s but didn’t have Super Sucker installed and had a VS carb on it.

Car is less cold natured, less likely to flood when hot, better off idle, starts instantly like FI. Car will pull itself briefly on level ground w/o surging. Most importantly, is
quickest ever and borderline dangerous under hard acceleration.

Hope this helps someone w/similar set up.

 

I would not sweat it unless someone with clout (moderator) brings it up, besides this is the
"open discussion" side of the HAMB not the "traditional" hot rod side.
Not like you are posting photos of an EFI LS engine in a 74 Vega, if someone gets past the first photo of that intake spacer and are offended then they have already wasted too much of their precious time!