Edelbrock Slingshot air flow imbalance

[coxswain98]

Setup: .030 over, 77-B cam, dual points crab distributor, dual Holley 94s, 3.5 power valves, 51 jets in front carb 49 jets in the rear, both carbs rebuilt with tested floats and new needles. Currently no air cleaners to tune the engine.

I need some help!

I recently replaced to power valves since I was silly and had dual 7.5s causing bogging on throttle. With the 7.5s, I was able to sync the carbs just fine, both carbs pulling equal vacuum. It just died if I gave it gas, since the PVs were opening or were already open, flooding it.

Changed to 3.5 and while I was in there, checked my float settings and needles. Set the floats for a tilting engine. On startup, it idled like crap! But throttle response was amazing!

Cleaned the spark plugs of their blackened state and that helped with the idle. Idled even better at temp. When I grabbed the Unisync to start balancing, the rear carb is pulling a ton more air than the front. Nothing blocking it that I can see, and this wasn’t the case before the PV change.

Has anyone had this experience?

Additionally, I can see that the venturis are dripping fuel. My fuel pressure gauge says approx 2.75 psi and I’ve been tapping the needle housing to see if it is a stuck needle.

Any help would be appreciated!

[ford38v8]

There might well be other causes, but I'd guess that the 51 PV provides more fuel, thereby less vaccuum. Just a guess, I'm not a dual carb guy.

[Bored&Stroked]

Hopefully you purchased the correct power-valves that are designed for your carbs - from a place like Daytona. Modern Holley power valves will tend to leak around the gasket. You can modify some with a lathe to "fix" the issue - or just buy the right ones from Daytona or others that know what I'm talking about.

Now back to airflow . . .

You must have removed the carbs from the manifold to change the power valves, which means you also took the linkage off of the throttle shafts. So, when you put things back together the throttle plate alignment is not the same as before . . . which is normal.

Now you need to re-sync the carbs - via tweaks to the throttle linkage and/or linkage arms.

1) You should only have an idle stop screw in ONE carb - we'll call this the FIRST one. Back out the screw on the second one (you'll never use it).
2) Setup the linkage arms to be at about the 2 o-clock position on the throttle shafts. Make sure the arms are tight and do NOT slip on the shafts. Many of the repop arms are of cheap pot-metal and you cannot get them tight enough to hold their position. Sometimes I have to "pin them" with a small roll-pin.
3) Make sure the distance between the centers of the throttle shafts is as close to the same as possible to the distance between the centers of the throttle arms (the ends that attach to the linkage). In other words the lengths of the two sides of the linkage parallelogram need to be the same.
4) Start the car and adjust the idle stop screw and mixture screws on the first carb to get the engine to idle. You can adjust the idle mixture on the second carb if needed - or just turn the idle mixture screws 1 1/2 turns from seated for now.
5) Check the airflow with the Unisyn on the first carb. You need to adjust the air-control component to make sure the red indicator is not sitting on the bottom of the sight glass. I just make sure it is 1/2" or so up at this stage.
6) Adjust the linkage so the second carb pulls the same amount of vacuum. This is done with the linkage, not with the idle stop screw. The linkage relationship to the first carb will be responsible for controlling the air flow on the second carb.
7) Adjust the idle mixture screws on both carbs to get the best idle. Check the airflow and tweak the sync if needed.
8) Lock down the linkage adjustment shaft/nuts for now.
9) Temporarily turn in the idle stop screw in on the first carb to get the engine running at about 1500 - 2000 rpm.
10) Check airflow on both carbs and adjust the linkage so that the second carb pulls the same amount as the first. Lock the linkage down.
11) Back the idle stop screw out on the first carb until you are close to the idle RPM you want. Adjust the idle mixture screws on both carbs and the idle stop screw on the first to set your idle where you want it. If the airflow at idle is the same for both carbs, you should not see gas dripping from the main venturi boosters . . . there should be enough fuel coming from the idle circuits to run the engine.
12) Sometimes you might go through the above procedure a few times . . . especially if you are new to doing it.

You'll find that there is no exact/perfect flow match at all RPMs - so you have to pick a point and adjust them at a given RPM to be as close as possible. I usually do this at between 1500 and 2000 RPM and then recheck at other RPMs. Sometimes I need to schwag things a bit and look for the best match between a few different RPMs.

[drfromnc]

Not to hijack the thread but I’ll need to tune my dual 97s on my slingshot soon.

B&S, what uni-syn should I buy when I am ready to tune? The Edelbrock 4025?

https://www.speedwaymotors.com/Edelb...-Tool,267.html

[KiWinUS]

Originally Posted by drfromnc

Not to hijack the thread but I’ll need to tune my dual 97s on my slingshot soon.

B&S, what uni-syn should I buy when I am ready to tune? The Edelbrock 4025?

https://www.speedwaymotors.com/Edelb...-Tool,267.html

Doug a piece of garden hose and your ear will get you VERY close. I grew up using this method to tune twin carbs on Triumph motorcycles had never heard of a Unisync at that time. I may also have an extra pair of Unisync would have to check.
Also synced many sets of triple carbs on hot rod 6 cylinders years ago. I was the go to guy in my hometown.

[Bored&Stroked]

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Originally Posted by drfromnc

Not to hijack the thread but I’ll need to tune my dual 97s on my slingshot soon.

B&S, what uni-syn should I buy when I am ready to tune? The Edelbrock 4025?

https://www.speedwaymotors.com/Edelb...-Tool,267.html

Yes, that is the unit that I've had for about 50 years! LOL

[coxswain98]

B&S: Thanks for the details! I'll give that a try. I'm still not understanding what caused the change from before I changed the PVs. The airflows were very close to each other during that tune, and now one carb sounds like a jet and the other barely sucks my hand in when I cover it.

[Bored&Stroked]

Originally Posted by coxswain98

B&S: Thanks for the details! I'll give that a try. I'm still not understanding what caused the change from before I changed the PVs. The airflows were very close to each other during that tune, and now one carb sounds like a jet and the other barely sucks my hand in when I cover it.

Well, if you pulled the linkage off to work on the carbs . . . that is your answer.

[drfromnc]

Ok Tony you have to explain the garden hose method to me. You can text me direct if you want.

[Flathead Fever]

With the linkage off and a return spring on each carb adjust the air fuel mixture so that each carb's air fuel ratio is the same at idle and the idle speed of each carb is the same. You might have to go back and forth between the two carbs several times until they are both set equally the same. They both need to be adjusted identical. I use a carburetor Uni-Syn tool. If you cannot adjust the carbs evenly then something is wrong, like maybe a power valve is leaking on one carb. I first learned do this on my '66 Mini Cooper S with twin 1 1/2 SU carbs. I miss that car, lots of fun.

Once everything is okay with the carbs adjustment, then hook the linkage back up so you don't alter the carb's settings, adjust the linkage to fit the carbs, not the carbs to fit the linkage.

[Bored&Stroked]

I like Flathead Fever's methods as an initial starting point (having no linkage on the carbs and using the idle stop screws and mixture screws to adjust idle speed and initial air flow).

Once you've put the linkage back on, you need to back out the idle stop screw on ONE of the carbs and use the procedure I outlined to sync them.

The linkage must be responsible for the air-flow amount of the second carb (not the idle stop screw). The carbs need to be synched when the throttle plates are opened - off idle - and the linkage needs to be the one controlling this (which is why you adjust it).

One needs to consider that some linkages may have deflection or slop in the ends used to connect to the throttle arms (mini heim ends or whatever). With the linkage controlling the final air-flow through the second carb, you'll be creating a situation where the sync works the best it can at idle and off-idle.

Best of luck!
B&S

[coxswain98]

B&S: I have followed your instructions and I think I have it balanced nicely. Unisyn matches at increased rpms and idle, no dripping fuel at idle or off idle, small amount of starter fluid passed over the carbs does not bog or rev it. However, the mix is still rich. Spark plugs are black after cleaning and re-running the engine. Exhaust smells gassy.

I don’t think I’m understanding the mixture screws. I just put new power valves (3.5 Hg) and that solved my bogging issue on revving. I don’t think fuel is leaking past. The first carb mixture screws are about 1/2 turns out each, second carb (with opened throttle) are screwed all the way in.

I’ve tested to make sure all spark plugs are firing

Do you think it is the mixture screws making it rich?

Thanks for your help!

[Flathead Fever]

The mixture screws on both carbs should be turned out approximately the same amount if they are in balance. If you have a carb "at idle rpm" and you can turn the mixture screws in all the way which should completely shut off the fuel to that carb "at idle", it should run like crap with the screws shut off or not at all. If not, this means the carb is getting fuel from someplace else other than mixture screws at idle. The only fuel at idle should be going through the idle mixture screws. Something is wrong with that carburetor. A bad power valve, gasket, crack or something is leaking fuel into the engine at idle. Unless you have the throttle plate opened up too much on that one carb and it's pulling fuel from different circuit. The idle mixture screws are only for the idle adjustment. The throttle plates should be opened the same on both carbs, just barely opened. An un-syn tool is "only" used for the idle speed adjustment. It just verifies that both carbs throttle plates are opened exactly the same amount at "idle". You're just trying to make both carbs adjusted the same amount at "idle".

[petehoovie]

[Flathead Fever]

Here's a pre-war copy of ad from an SCTA program. I have most of the issues, some original and some copies, starting from the beginning up to around 1951.

[Bored&Stroked]

Normally one has to have the idle screws out on BOTH carbs to achieve a smooth idle. If they are both pulling the same air, then there is no reason you can't adjust the idle mixture screws to achieve a fuel balance between the two. You turn a mixture screw in until the engine starts to slow-down and lumber a bit - then turn it back out until the idle smooths back out. You should be able to do this on all four idle mixture screws.

Now, back to your statement that the "plugs are black". The fact that they are black when idling the car in the garage means nothing. Unless you have a O2 meter plugged into the exhaust system (at the right point), you can't really check for the fuel mixture being rich at idle or at any RPM point where the engine has been cruising around town. You have to do a load test on the road to determine if you have the right main jets and power valves installed.

Also, with today's fuels it is very difficult to read plugs like in the old days - they do not color the same. If you're not really versed in reading plugs (and cutting them apart), you're probably wasting your time.

If you want to attempt a plug read, then you have to take the car on the road, drive it for a few miles and then put it under hard acceleration and when you reach a higher RPM, then push the clutch in, kill the engine and coast to a stop. Then you can read the plugs. This might give you an idea as to the main jets - but only if you can decipher what you are seeing.

You cannot drive the car, come to a stop, drive home, idle around a bit, get into the garage and then check the plugs - this means zippo as far as main jets and power valve sizing

If you really want to know what is going on, then buy an O2 system, put the bungs in the right place and see what your AFRs are at different load conditions.

[coxswain98]

B&S: Thank you! That makes me feel better. I’ll work the mixture screws like you say.

I’m going to change the oil. Anything I should be on the lookout for?

[petehoovie]

Originally Posted by Flathead Fever

Here's a pre-war copy of ad from an SCTA program. I have most of the issues, some original and some copies, starting from the beginning up to around 1951.

[Flathead Fever]

The installation of an O2 sensor is a really good idea. You want the bung welded in as close to the engine as possible for the sensor. I have bungs welded in on my projects for 02 sensors, I just haven't finished any of them yet. I collected the cars and parts when I was working and then I had a stroke so I'm taking a break from them for a while. I had a smog license and dealt with carbs and O2 sensors for 30-years. But I've never had an O2 sensor on a hot rod yet. They are great for dialing the carbs in at idle, cruise and full throttle. Like Bored Stroked said, it's a much better way of tuning an engine than looking at the plugs because you can look at the O2 gauge while you are driving.

[Bored&Stroked]

Actually, you do not want the O2 bungs close to the block - ideally they should be about 3 feet back from the block and installed so the sensor will be at a 10:00 or 2:00 o'clock angle when installed.

But, since the flathead has internal exhaust passages (about 8" in length), then you can move the O2 bungs a bit closer (like 6" from the end of the tubular headers). What you need to make sure is that there are NO exhaust leaks in the system - as they can pull oxygen in and ruin the readings.

The other sensors that are installed right next to the block are EGT (exhaust gas temperature) sensors. I weld those little bungs (1/8" BPT) as close to the block as I can (the angle does not matter). I like to have one on the rear cylinder and one on the center (mixed) cylinder. These really only work with tubular headers - probably not really viable with stock manifolds.

When I setup a flathead exhaust system I put both EGT and O2 bungs in - so I have both to tune with. The O2 sensors are the most valuable, but it isn't a bad thing to see how the EGTs are reading - especially on a high-output blown engine.