Holley Carb kicking my tailpipe!

[Jermz01]

A carburetor? I can barely spell the word - let alone admit to growing up in the generation that had to use them daily on their rides...

That being said - I've had a nice little crash course in "How to dial a carb in"...

Problem is... I'm at that last 10% and I just can't seem to get it...

The problem: off idle stumble (keep reading...)

I have a 4160 Holley 4-bbl (600cfm if I recall) with vacuum secondaries that came on my 1969 C10 (small block 350, mild to stock cam). Stock ignition system. When I bought the truck it had a nasty dead spot right off of idle... sitting at the stop light you'd stand on it - and - wait for it... NOTHING!

Swapped the accelerator pump (30cc)... had a nice little back fire when I was screwing around in the garage (stupid me)... since the carb is bit older - no "back fire protection"... so long power valve...

Pulled the primary and metering block... cleaned everything! New 7.5 power valve (the vacuum measured 15 inches of mercury). The valve I pulled out was a 6.5...

Cleaned everything up - put back together with the new 7.5 valve, another new 30cc accel pump, new gaskets... dialed the secondaries to just hang a hair off the slots... idle mixture screws are 1 and 1/2 turns out on both sides per Holley's recommendations...

Truck starts great... idle to WOT is great... idle to a very mild part load is terrible... the engine stumbles, stutters, shakes a little... at stop signs it's all or nothing with the pedal!!

Is my accel pump jacked (again)? This occurs in neutral, while in gear... even if you are cruising - back off the throttle and lightly tip in... I have the accelerator pump arm snug but not tight on the throttle linkage (sorry I don't recall all the correct names)... I pulled the accel pump diaphragm off and checked it (again)... no signs of wear, tear, etc. Spring looks and feels good.

I have a one inch phenolic spacer on the carb as well...
Older single plane Edelbrock (plan on changing it)...
350 turbo trans...
Vacuum attached to PCV off of valve cover, to the distributor and one to a vacuum can.

Open to ideas and suggestions???

[trusty72]

does it have the rich/lean dial on the vac secendaries? how are the floats set?

[silverstreak]

Here's my thoughts.

The mixture screw reccomendations (1 1/2 turns out) are basically guidelines to get the engine running, then you fine tune the idle mixture from there. Adjust them for the highest engine vacuum or use a wideband O2 sensor if you have access to one. An improperly adjusted idle mixture can have an effect as it transitions to part throttle.

Check the advance weights in your distributor to be sure they're free and not sticking or binding.

If you're running a single plane intake and a spacer with a mild / stock cam, that's not exactly the best combo. You should see an improvement by switching to a dual plane intake.

[Jermz01]

I'll give these a try...

If I recall - turning the mixture screws ccw = enriching... cw = leaning

[silverstreak]

Originally Posted by Jermz01

I'll give these a try...

If I recall - turning the mixture screws ccw = enriching... cw = leaning

Correct, clockwise (in) is lean
and unclockwise lol (out) is richer

just turn each one about an 1/8 of a turn at a time and try to sneak up on the best idle mix.

[Bad70sbchevy]

x2 on the idle mixture, also check your float levels, there real easy to check. Had those same symptoms on my 600 double pumper and found out the float levels were set way too high

[68C15]

single plane manifolds take quite a bit more accel. shot than dual planes. add in the factor of your 1" spacer and it gets worse.
you also need to realize that this manifold/spacer combo will not perform as well on the transition phase as a dual.
first I need to know what size shooters you have (there is a # stamped on it) and what type (does it have a small metal tube or not).
since this is a 4160 with a metering plate on the rear all the idle feed comes from the primaries, I would make sure the secondaries are closed at idle.
we need to address the transition circuit stumble in stages, ie:
timing of initial shot, velocity, location, duration, and volume.

if you want to discuss this in higher detail PM me for my #.

[68Stepbed]

Check float levels first. Check acc. pump linkage next. The linkage should be able to move approx. 1/32" before the pump shot starts. Also, check your acc. pump cam. Sometimes they can get worn or the screws come loose.

As 68C15 stated, also check your squirter. Find out what size yours is. It should be a
.31 Watch the squirter nozzle as you slowly work the throttle linkage all the way back to WOT with the enginne off. You should keep a steady stream until the linkage stops.

Let us know what these tips yield and we'll dig deeper if need be.

A dual plane intake will help alot too as mentioned already as well.

[Jermz01]

Checked float level... a small trickle from primary (matches Holley's recommendation)

Linkage has some play on accel. pump (a bit tight) - small adjustments being made to loosen it up

Pump shooter is a #25 on the primary (68step... why a 31?) [edit - there are no tubes coming out of the shooter]

Going to try the idle -> WOT (engine off) watching squirters... [edit - both small nozzles in squirter are spraying through linkage sweep]

As a side note - checked vacuum, no apparent leaks...

[Jermz01]

Interesting observation...
Tried the idle to part load shift while the engine was running... carefully watching the pump shooter. nothing, to a very small dribble, occurs when you start to tip in (engine stumbles - lean condition my guess since it's not fueling)... at a certain point the pump shooter beings to spray like expected...

accel pump is bad? I'd hate to think since I just replaced it with a brand new one!!!!

ideas? many thanks!

[prostreetC-10]

Originally Posted by Jermz01

Interesting observation...
Tried the idle to part load shift while the engine was running... carefully watching the pump shooter. nothing, to a very small dribble, occurs when you start to tip in (engine stumbles - lean condition my guess since it's not fueling)... at a certain point the pump shooter beings to spray like expected...

Not really sure what this info is really saying but this thread is getting all over the place. You said you have "stock ignition". Have you first checked your initial timing? Does this thing have points? HEI?

Quit changing your 30cc diaphragm! It's not the problem. If your accel circut it adjusted right you should be able to tap on the 3/8th nut on the bolt of your pump arm and get fuel to start to dribble out the shooter. Please first make sure that you can tap (or lightly push) on the pump arm and make gas come out the shooter.

Are we working on an 1850/80457? When you changed the power valve, does this carb have the trans tube between the metering block and the main body (this would be an older carb)? Is the correct metering block gasket being used? I've seen carbs come in where they picked the gasket for the trans tube type and put it on the newer block design. When you hit the gas, the pump shot washes into the power valve recessed area of the main body and not into the channel.

It seems that the things you are chasing are more for "fine" tuning and you have a bigger, basic problem going on. Where are you setting the primary shaft? It should lay about .020 on the trans slots which will make the slot look like a square. Will be as long as wide. Is your spacer a 4 hole or an open? Is the old intake a Torker? If the spacer is a 4 hole, it will actually help your situation. An open will add to the plenum problem which is already weakening the signal to the carb which is why they are 2,500+ rpm starting point intakes.

I really think you might want to take just a second to rest on the carb and really make sure everything else is up to par. Throw a light on this thing and make sure you are not at 4* 6* etc for initial timing. You should really be at 14*, 16* or higher.

Depending on how clean or dirty this carb is, just check your air bleeds to make sure they are not packed with dirt or a turd is stuck in the high speeds (the inside ones).

[68C15]

is the check valve missing from under the shooter? small cylinder shaped thing with a point on one end. point should face down away from shooter nozzle. are both gaskets in place? one under nozzle and another under screw head.

[Jermz01]

both gaskets are there... not sure about the cylinder shaped item. When I pulled the shooter off there was the shooter (#25) two small gaskets and the screw holding it in place... I didn't pay attention to what was under it... looks like I'll have to pull that back off and check!

Is there anywhere on the web that has good pictures? I've done some looking around and on the Holley site... and a lot of it is very good... but...

[GASoline71]

Try a different cam on the accelerator pump. Why do you keep replacing the pumps? 30cc is more than enough.

You just need a quicker shot of fuel off idle. Also, when you go up in squirter size... you need to go up at least 3 sizes. If you have a 25 in there... you need a 28.

Check out this link

http://www.holley.com/data/TechServi...ech%20Info.pdf

Gary

[Jermz01]

Hi Gas71... I've only replaced the accel pump diaphragm twice (the first time for an original off idle stumble). The second time was when the stumble popped it's nasty head up again... So - it's not being changed regularly.

We are rocking the "orange" cam on the carb... pretty "stock" I'd guess. I was just reading up on the shooter - and wonder if a #28 may be a solution to this problem.

68C15 - after a little investigation - I'm 99.9% sure the pump discharge check valve was in there (I didn't flip the carb over when I pulled the shooter out).

[Jermz01]

Man - ya change an accel pump once around here people go crazy ;-) haha

Actually - I think what I said does make sense:

Not really sure what this info is really saying but this thread is getting all over the place.

While the engine is running the pump shooter does not squirt fuel as you tip into the throttle (it does when the engine is off). AKA no fuel... so at idle the engine runs fine, under heavy acceleration it runs fine, at a very light load it stumbles... the shooter isn't squirting fuel... perhaps this is normal, but it didn't make sense to me. I haven't worked on carb's before.

The ignition is at 15° advance. It is not an HEI system but rather points.

The carb is an 1850-3 2537

To be honest when I changed the power valve I did not see a trans tube, but I claim ignorance for not really looking to be honest...

I have a 4 hole 1" spacer. It is an older Torker II intake.


When I changed the power valve I did clean the carb. I can not say 100% that something is not lodged somewhere....

[prostreetC-10]

I'll just throw out a few more things then I have to get on with my day.

First of all.....the part throttle and transition circut are the hardest to get right with a Holley. Your intake is not helping. You need a little Performer or Performer RPM for the street. Dual planes are very friendly down low. Also understand that diagnosis over a computer can be very hard where as leaning over the motor can get a quick solution to what is actually going on.

Do not worry about not seeing gas when you "ease" into the throttle. The shooter is fighting the velocity within the venturi. If the opening is slow enough, the booster and trans slot is usually good enough. Once again you have an intake that is not good down low. You list is supposed to have a 25 shooter but you can try a 28 or a 31. The 1850 did come with both a 25 and a 31 shooter in the later series carbs.

What are we running for jets in the front? Sometimes with a Holley you need to completely go through the carb and get it back to the stock settings in every way. Holley does a pretty good job of getting these pretty close out of the box. I have had 100's and 100's of Holleys run through my hands. I have seen everything. That carb should have 66's in the front. Sometimes that will be lowered based on someones elevation. My town is at 3,400 ft so things run a little fat and we have to step them down a little. Is this carb clean? Dirty?

Is your orange pump cam in the #1 or #2 position? It affects where at in the cam arc you are starting and ending your stroke. I actually like running the pink 3 hole cam in all my carbs and in the #1 position.

I realize this is hard to do. It's like asking someone on the internet "what's that noise?" lol

BTW....is this something that JUST started? Did you JUST get the truck? Have you had the bowls off? Did you do or consider a complete rebuild first?