Author Topic: A Little rich at WOT  (Read 2463 times)

Offline 77l48

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A Little rich at WOT
« on: March 28, 2014, 09:51:37 AM »
I have  Chevy 350 that I warmed up last year with a 219@.050 on a 108 LSA cam 9.9:1 CR , Dart SHP heads ported, and the performer intake (hood clearance restriction). I'm also running long tube 7/8" primaries into side pipes with STS baffles. So a low restriction exhaust.

I live at a 4000 ft altitude so jetting is a bit different perhaps due to higher altitude.

I used Cliff's book to rebuild and modify initially on the stock motor and it ran great.
Now it is a bit rich at WOT. I used Cliff's second recipe to accommodate the lower vacuum cam that I'm using. About 13" @ 600 and 15" at 900 rpm.
I suspected that it was rich last season but didn't have enough time to really nail it down before winter hit.

After the rebuild I started with a .073 main as recommended by the book. That was way too rich. blubbering at WOT and not right at any portion of the rpm really. So I knocked it down to a .071 main jets. This ran much better. But like I said  still seemed too rich.
At WOT I have a flat spot between 2500 to 3000 or so. This also happens to be where the torque converter stall is at.
Played with the float level a bit but still rich.
Also played with the secondary flap spring. It wanted to be wound really tight to not blubber at WOT, which seemed to indicate that it was rich on the secondaries. However given the RPM range that is was a bit flat I suspect it's the primaries that are still rich.

So today it was nice enough to change the jets and go for a test drive. On this day DA was 5200 ft.
I dropped to .070 mains.
This ran much better and acts like I could go leaner. ???

Should I now lean the secondary circuit a bit by bending the hanger down a little or go leaner still on the primary side?

The Primary metering rods are .044" rods. Given this I should be running a .073 or .074 main.
This is an APT carb and the recommendation is for .029" larger main than the metering rod as I understand it.

On the secondary side I have the flap 3/4 wound. and the secondary hanger is hanging .08" lower than a G hanger. I tried higher initially and that made things worse, as was predictable.
I have adjusted them some in the process of chasing the rich WOT.
I also have Cliffs .044 tipped secondary metering rods. These when compared to the stock rods have a skinnier profile, this would make them run richer.

So why is this combo wanting to run so lean? Is it due to low DCR at high altitude or something along those lines? Or have I got my carb mods wrong.
Once it hits 3000 to 3500 it doesn't care and accelerates nicely but would like to tune out the flat spot.
Am I going in the right direction with leaning the primary side or should I be focusing on the secondary side at 2500 to 3000 rpm?

Here are all my carb specs.

Carb # 17057204

Idle down channel .052
Lower idle bleed .070
upper idle bleed .070
accel pump discharge holes .028 and accel pump arm set to outer hole. This seemed to run better than the inner hole at initial full throttle application.
Main Air Bleed main body .070
Main Air Bleed air horn .070
Main jet (now) .070
Secondary POE well restriction .040
Secondary POE restriction .056
secondary tube restriction .036
Air flap opening distance 1.30"
Choke pull off 1.7 seconds
Choke deleted from carb.

Total timing 34* @ 2600 rpm. 19* initial, 51* cruise.

I did set the rpm to 2200 with the idle screw and unplugged a vacuum line and the RPM increased. This indicates primaries still rich ???


Offline 77l48

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Re: A Little rich at WOT
« Reply #1 on: March 29, 2014, 05:40:48 PM »
It was a nice day to day so got some more time to tune the carb a bit.

I forgot that this winter I had changed the secondary POE restriction  and opened it up to .056. Previously it was still stock dimension.
Remembering that, I decided that the secondaries were perhaps a bigger player in this problem than I had thought.

First I tuned in the primary side dead on for the conditions today. 72* F at 4000 ft.
I used the tip in method to tune the APT and at 2120 RPM it dropped to 2070 RPM using this method. So that seems right on.
Next I tuned the idle jets until I got the highest RPM possible from both sides. Ended up being 2 1/4 turns out on both sides for an 850 RPM idle. At least they are balanced side to side it that's worth anything. 2 1/4 turns out seems about right to me.
Set the RPM back up to 2120 rpm and opened a vacuum line. RPM went up to 2220 RPM. This is telling me still too rich on the primary?
I'm happy with the way it is running now on the .070 primary jets. The exhaust is much cleaner now and the idle responds to idle screw settings as well as ATP changes. However if it wants to be leaner then......?

Next I took the hanger and bent them to a "J" hanger dimension with Cliff .044 rods installed. I initially found them bent down to a "V" dimension. So I guess I had already tried leaning up the secondaries last season.
Took it out for a test drive and at WOT from a dead stop it blubbered between 2500 and about 3200 RPM or so.
Still too rich. Tried changing flap spring tension and weaker seemed better.
So weaker spring tension is leaner?
Took it back home and decided I was going to re install the original CH secondary needles. They are fatter and have a bigger tip so should be lots leaner.
Made that change only and took it back out.
From a stop no blubbering, just slight hesitation now.  8)
So I'm going in the right direction. Leaner is better.
Should I try .069 mains? or should I get fatter secondary needles? or could I just bend the hanger for a leaner pull?
Could be a week before I get another chance to make any changes so gives me time to order needles If I need to.

Offline Cliff Ruggles

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Re: A Little rich at WOT
« Reply #2 on: April 16, 2014, 04:51:38 AM »
WOT A/F is not tuned with primary jets.

Tune the primary side for best heavy throttle and light part throttle WITHOUT the secondaries.  When you have determined the best jet/rod combo, done.

Then go after the secondaries with different metering rods to find the best full throttle A/F.

Did you install a new secondary cam/spring in the carb?  Might want to try different pull-off rates, too quick will almost always result in a stumble/bog no matter how tight you wind the spring.

Hanger height plays a role here as well. 

Secondaries are easy to tune, as all the adjustments are external. 

On really fast accelerating vehicles it may be necessary to vent the POE wells internally, and fill the vent/slot at the back of the choke housing, as some fuel can spew over into the secondaries on a hard launch, simulating an out of tune condition, when what's really going on is lack of fuel control.....Cliff

Offline 77l48

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Re: A Little rich at WOT
« Reply #3 on: April 18, 2014, 04:12:37 PM »
Thanks for the info Cliff! Heard you were out for a bit . I hope you're on the mend.
I basically leaned the primaries with the thought that most of the fuel at that rpm was coming from the primaries and would have the greatest effect until the secondaries were farther open. This is only a mildly warmed up 350. I'm guessing how far open the secondaries are at 3500 rpm.
I have since installed .069 primaries and tuned them for good running.
Idle mixture screws out 2 1/2 turns and APT out 4 1/4 turns. Rpms still rise unplugging a vac line at 2000 rpm but less now, around 80 or so. Also re installed the .044 tipped secondary rods and bent the hanger  a little leaner than with the CH rods to partially compensate for the richer .044 rods.
It runs well with what I have now, seems slightly weak in the area of 3000 to 3500 though. I'm not using an AFR gauge, just all seat of the pants and listening, smelling and reading the plugs.

Does the engine care at WOT where the fuel is coming from or just the total fuel delivered? It' a dual plane intake. Are the front cylinders going to be a little leaner than the rears due to lean primary jetting?  Sounds like a silly question since each cylinder pulls individually, but the distance from each cylinder to the secondaries varies.
It seems to me it could be slightly leaner in the 3000 to 3500 range with the same or more fuel at higher rpms.

This is not a fast accelerating car, relatively speaking, it a 70's corvette with a th- 350 and a 3.08 rear on it.


Offline Cliff Ruggles

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Re: A Little rich at WOT
« Reply #4 on: April 25, 2014, 04:20:02 AM »
Cars that accelerate slow can be more difficult to tune than those with better power to weight ratios as the transition is much longer.

As mentioned above, spend some time tuning the primary side to get it perfect, then go after the secondaries for all full throttle tuning.

I'm curious why the choke would be deleted, an electric choke is easy on that carburetor, and a very good feature for anything that is street driven.....Cliff

Offline 77l48

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Re: A Little rich at WOT
« Reply #5 on: May 08, 2014, 04:55:08 PM »
I deleated the choke for simplicity. It had the hot air choke system. I retained the idle kick  up (terminology?) portion of the choke system, but removed the choke itself. It is not needed as near as I can figure.
 This is a summer only car and even at 40*, 20 seconds of me holding the throttle is enough for it to idle. At any summer temp it's not needed at all and will idle fine with the idle kick up function until it warms up.
 I also considered it an obstruction to flow on the primary side even after it was open. Is that incorrect?

Offline Cliff Ruggles

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Re: A Little rich at WOT
« Reply #6 on: May 16, 2014, 03:37:43 AM »
There are no negatives from having the choke and all the hardware in place for tuning or performance.

I like a working choke, especially if the engine is ever started in really cold weather. 

Those carbs are great candidates for an electric choke.  I ran my 1977 carb for years without one, but the older I get the less I could tolerate pumping the accl on cold days and "feathering" the throttle till it warmed up.  I re-installed the E-choke a few years ago, instant cold starts and fast idle so I don't have to sit there and keep it running.

Makes moving the car in the Winter time a LOT easier as well.....Cliff
« Last Edit: June 02, 2014, 04:08:50 AM by Cliff Ruggles »

Offline 77l48

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Re: A Little rich at WOT
« Reply #7 on: May 18, 2014, 07:04:33 PM »
Thanks Cliff, maybe I'll get a choke in the future if I drive this car more often.
Got a good chance to try out the primaries on a steep 15% grade while staying on the primaries only. It seemed slightly lean with a slight hint of hitching on the pull.
Opened up the APT another 1/2 turn to make slightly richer and it seemed to like that better.
I am wondering what is the usefulness range of operation on the APT with your .044 primary rods.
Right now I'm 5 full turns out and must be approaching the point at which I've pulled the tapered section out of the jet orfice. At some point more turns out will not enriched the mixture, right?

Offline Cliff Ruggles

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Re: A Little rich at WOT
« Reply #8 on: May 28, 2014, 02:39:00 AM »
Nothing at all wrong with being up 5 or more turns and getting on the faster tapered portion of the rods.  This also sets the PP for much less movement to get up on the smaller .026" tips, so faster throttle response, etc. 

That's why we use the full tapered rods in lieu of "stepped" rods like the "K" and "P" series.....Cliff

Offline 77l48

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Re: A Little rich at WOT
« Reply #9 on: May 31, 2014, 07:52:47 AM »
Thanks cliff! That gives me some room to go richer when the temps cool or I drive to a lower alt without having to change the jets then.
I have it tuned now so that it runs strong through the entire RPM range even the 2500 to 3500 range. It just wanted less fuel. It's always a little unnerving when approaching leaner and leaner conditions.
I have side pipes and the noise from them I afraid could be covering up pinging that may occur if I go too lean. So that leaves me checking the plugs fairly frequently during jetting changes.
Thank you very much for your advice!
Also really enjoyed your book. Very well written with excellent photos and step by step procedures as well as the recipes in the back. Awesome read!
It's the only reason my Q-jet runs as good as it does now. :) :) 8)

Offline Cliff Ruggles

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Re: A Little rich at WOT
« Reply #10 on: June 02, 2014, 04:11:56 AM »
Thanks.  Tuning can be very rewarding, as much as it can be frustrating.

I really push folks to buy our rebuild kits, and tuning parts.  Sure we need to make money to keep the doors open, but a good end result starts with good parts.  We also have vast experience with tuning, so can cut quickly to the chase with tuning components based on carburetor number, and the engine/drivetrain/vehicle specs.......Cliff