Günther-C3

Hi all!

I have 68 with 427ci BB and a 770cfm Holley 4150 carb on the top. The gas milage is verry poor, so I thought maybe the jets of the primary are too big.
So I bought a a/f gauge to see, what´s going on and hooked it up. Originally the primary jets were 72. Before I hooked up the a/f gauge I was at a shop and they told me, the engine runs too rich. I should put in smaller jets.

Today I put out the 72 jets and put in the 62 jets. Since I have changed the jets to the smaller 62, I have e few of problems:

• the engine boggs at cruising
• the engine is pinging

As I know bogging is a sign that the engine gets no or too less fuel. That´s curious, because the a/f gauge shows 13,5:1 at idle, 12:1 at cruising and aprox. 16:1 at WOT.

So the gauge says that the engine runs too rich at cruise, although it´s bogging and it is too lean at wot.

Please help guys, wtf is going on here and what should be my next step?

MelWff

Das isnt nicht gut! The jet size change was way too large. 62 ist sere klein. It would help to know the carburetor list number to see what the original jets were. A better change would have been going from 72 to 70. What are the sizes of the power valve or valves?

Timsride

Too big a jump in one shot. Also was the power valve checked? I would also start learning to read plugs they don't lie. That's why the big boys still do it. I don't know how to access happy faces from my phone so Cheers!

boltnut

I've heard many say that it's hard to read plugs with new hei systems and ethanol fuel. Not sure if you got the ethanol crap where you are though. You should try no more than 3 steps at a time with jet changes. What makes you think you have a rich condition other than fuel milage ?

chevymans 77

What is your cruise RPM?

You are more than likely not on the main jet while cruising. The transition slot and idle circuit is in control of the AFR when below around 2500 to 2700 rpm's, its different for each combo.

What's the list # of the carb so we can look it up and see how its set up from Holley.

More than likely its rich at cruise due to still being on the idle and transition circuit. Its lean at WOT due to taking out so much main jet on the primary side.

I would put the primary jet back at 72 and see what the WOT AFR is then if you are comfortable with operating on the carb we can lean the cruise to 13.5 to 14.

To lean the transition circuit you will need to take it apart and operate on it. More than likely adding some wire in the Idle Feed Restrictions (IFR).

I would normally start with disconnecting the secondaries and tune just the primaries first. Get the fuel curve as flat as possible on the primaries and than use the secondaries to tune WOT.

Not sure how fine you want to tune your carb so let us know.

Do you have an idea of your timing curve? It needs to be close before tuning the carb.

Neal

birdsmith

Obviously you're way lean at WOT; before you make any more of those runs you should correct that issue before you burn a piston...hopefully you haven't done any serious damage already. From looking at your carb's listing it apparently has secondary jets instead of the metering plate that some 4150's come with; you should step those up about 4 sizes. As stated by others, 10 sizes is a huge jump down on the primaries; to stay safe you should bump those back up to, say, 68 and then work your way down a size at a time from there. Before you take it back apart though you need to take a vacuum reading at idle to make sure you have the correct power valve. From whatever vacuum reading you get the power valve should be 1 1/2 inches below that number, so if you're reading 9 inches of vacuum at idle you should install a 7.5 power valve.
The tuning sequence should be as follows:
(1) Begin with the carb initially jetted on the rich side to be safe, then
(2) Adjust the accelerator pump arm so it's just barely touching the pump lever but not opening it at idle.
(3) Set the idle mixture screws 2 1/2 turns out from being screwed all the way in
(4) Start the car and set the float levels so that the fuel is just level with the sight plug on each bowl.
(5) Get it hot and set the idle mixture screws 1/2 turn out from where you get the best idle, then adjust the idle speed with the idle speed screw, then finally
(6) You can finally begin tuning the running mixture via power valve and main jet adjustments. It's really not possible to properly tune a Holley until you've gone through steps 1-5 first; that is what sets the foundation for everything else.
Perhaps others will chime in with some detailed info about playing with squirters, secondary springs, and pump cams, but if you follow the steps above you should be able to get your Holley tuned pretty close to where you need it as long as you're not dealing with any other serious issues such as big (or small) vacuum leaks, etc.
I have a suspicion that right now you have a power valve issue, maybe a torn diaphragm (have you ever had it backfire?) or mislocated/missing/torn power valve gasket which is steadily leaking fuel, perhaps a very small amount since you're not complainig about idle issues but enough to make it run rich on the primaries...just a hunch. If all goes well you shouldn't have to tear into it more than 3-4 times to get it where it needs to be. The great thing about Holleys, though, is that you can do these things without always having to remove the carburetor from the car and you can disassemble the carb a section at a time. Enjoy!

Günther-C3

Thanks guys for your help!

My car is a Holley 80770. It is originally jetted with 72 for the primaries and 74 for the secondaries.
@birdsmith: the steps 1-5 are done and I put in the 68 jets. The power valve is a 6,5 and it is new. I also changed it since the carb was apart, as well as the fuel bowl gasket.

If the weather is fine I gonna make a testdrive tomorrow and let you know.

Thanks again, Günther

MelWff

The Holley website shows the original secondary jets should be 75.

Günther-C3

Today I could make a testdrive with the 68 jets in it. The engine runs great, no bogging at all
But what I really don´t understand is, that the a/f gauge still shows 12:1 at cruise. Why didn´t the afr change since I changed the jets???

Günther

chevymans 77

You are more than likely not on the main jet while cruising. The transition slot and idle circuit is in control of the AFR when below around 2500 to 2700 rpm's, its different for each combo.

Neal

mr.beachcomber

Diagnosing a carb problem long distance is rather chancy; but here are two suggestions. Use a vacuum gauge to measure your manifold vacuum (below the throttle plate) at cruise to see if you're at or below 6.5" Hg (same as your power valve). If so, you'll need to change out the power valve to a lower activation vacuum than what you see at cruise, say 4.5" Hg.

Another possibility is a high float level allowing fuel to be pulled out the booster Venturi at cruising speeds. Your Holley is rated for 5-7 lbs. of fuel pressure. If you have more pressure at cruising RPMs, your float won't be able to cut off the fuel flow through the needle and seat allowing fuel "creep" where the fuel level slowly rises above the stock level. When it's really bad globules of raw fuel drip out the booster Venturi causing the type of bogging you originally experienced before changing the jets. (You can check for this problem by sdjusting the idle speed up to or near cruising speed with the air cleaner off. If you see raw fuel spilling out of the booster Venturi, you fuel level is too high!)

Hope this helps!

Hope this helps.

Günther-C3

Can you give me an internet-link where to buy such a vacuum-gauge?

Thanks, Günther

mr.beachcomber

This link will give you an idea of what's available in the market place. I prefer a unit that I can plug into the distributor's vacuum advance port using a tee fitting to monitor manifold vacuum. Then the unit can be either taped/positioned near the
windshield for monitoring or you can use additional hose to bring the unit into the passenger compartment.

Günther-C3

Thank you for the link, I gonna buy one

@MelWff: you are right, the secondary´s should be and are 75 jets.

Thanks, Günther

Günther-C3

When I get the vacuum gauge, where do I have to hook it up? On the port on the carb for the distributor-vacuum?

scottyp99

You can hook it up anywhere that it will see full manifold vacuum. Here are a couple links that will help you out.

http://en.wikipedia.org/wiki/Manifold_vacuum

http://www.imperialclub.com/Repair/F...auge/index.htm

http://www.linnbenton.edu/auto/perform/vacuum.html


Scott

mr.beachcomber

Here's a link to Holley's Installation, Tuning, and Adjustment Manual for your carburetor. All the information you need as to carburetor specifications and tuning is contained therein.

Two items of note within the manual that might pertain to your situation. The first is Holley's admonition to accurately adjust the rear float level to preclude fuel pull over in the rear barrels. The second is Holley's statement that the stock jetting is baselined for sea level and that jetting should be reduced one jet size for every 2,000 ft. (Salzburg is listed as 424 m (1391 ft.) above sea level so you're on the cusp of reducing your jetting one number as a matter of course.)

If you do find that your power valve is being activated during cruise conditions and change to a lower value power valve, you'll need to watch your A/F ratio at cruise to ensure that your primary jetting isn't too lean without the additional fuel flow from the old power valve.

Take Care!

69 Chevy

Another Holley tuning feature is the accelerator pump cam. There are different profiles available and at least my #4776 has 2 different mounting holes. A typical bog problem is usually associated with the accelerator pump's profile.

Günther-C3

Thanks for your help guys
So what I gonna is to set the idle mixture with the vacuum gauge.
About the power-valve: you said, the power valve number should be 1,5 lower than the vacuum I read off of the gauge. Is this right?

On youtube there is a video and they say, that I have the measured vacuum/2, and that is the number of the power valve I need.

And what else I don´t know is, which rpm´s should the engine rev when I read off of the vacuum gauge?

Sorry for my bad english

Günther

mr.beachcomber

That was just an example based upon my own experience with big block Chevys. You should use a power valve that doesn't start opening (adding extra fuel) while cruising. Typically, a Holley 6.5 power valve fulfills this requirement for a street-driven Corvette. Therefore, before you buy another power valve, remove the old one and inspect it. If the power valve is damaged and/or doesn't seal the metering block from the fuel bowl, that may be the source of the problem.

That is a good rule of thumb; however, consider the type of driving your Corvette sees. For example, if your driving includes hilly terrain, you may want to replace the ideal power valve with one that activates slightly earlier (closer to cruise vacuum) to provide extra fuel as the engine starts working to climb the hill.

Typically, I would use the lowest measured manifold vacuum I saw while cruising at highway speeds in high gear.

Your English is fine. On the other hand, my German is atrocious!