Bud2

383 using a ProForm 750 vacuum secondaries carb. Below is a session which shows a very lean spike when floored. This session had a blue cam in the 2nd position which the Holley book says is a 20cc shot at 20 degrees of throttle opening.




So I slipped in a green cam in the 1st position which the book says is a 24cc shot at about 23 degrees throttle opening. Same road, same gear but without the RPM signal.




You can see that it indicates it is even leaner..... Why the hell is that?

I've also done this with the primaries and going to larger jets appears to make the LM1 thinks it is leaner. I thought I was doing something wrong. But now I just don't get it. It just seems to be opposite to what it should be reading. Keep in mind the A/F ratio is set at the factory and I can't change it in the software or hardware. And yes, it has been recently calibrated.

Any ideas?

Bud.

zwede

Too much fuel will cause a lean reading. You get so much fuel you have some cylinders misfire or have a bad burn. Incomplete burn leaves oxygen in the exhaust which is read as lean.

A lean spike on the gauge is normal when you floor it. Don't go by the gauge, tune the acc pump to where the transition is clean and you have good response. What the gauge shows in this instance is not relevant.

Bud2

Thanks for the reply zwede. I'm not sure that's what I wanted to hear but it gives me something to think about.

Bud.

MrForce

You can decide whether this applies to your situation or not. I had a customer bring me his self tuned 4 cyl turbo import. Laptop programmable EFI and a POS narrow band O2 w/ gauge, no datalogging. His problem? A bad stumble on acceleration with the gauge going lean at the stumble. His enrichment tables were jacked to the highest I'd ever seen them. The 800 lb gorilla he couldn't see? A bright yellow car with a back bumper and tail panel covered in fuel soot. I cut the enrichment tables by half and then by a third. The stumble went away and the amount of time the gauge spent at an indicated lean was greatly reduced.

Tune the car, not the gauge.

Rob

Crepitus

True^^miss fire will look lean, very easy to keep going fater and read leaner. Make sure the pump linkage is adjusted correctly. Do you still have the cam in position 1? Correct linkage adjustment, squirter size have more effect on tip in. Vac secondarys need a big squirt get your linkage set up set the original cam in pos 1 and try a couple bigger squiters. Look for a little puff of black smoke with the pump shot then back off a bit.

diehrd

I have to say that adjustable secondary system is about the coolest thing I have encountered on a carb . ( I use the quick fuel 750 vac )

If your lean , slowly turn the adjustment in until it seats counting how many turns it takes. It should be 1.5 turns . Now if your getting a bog the way it was , back it out 1/4 turn less then it was and run the car to see if the issue is helped. Do this a few times until your lean condition improves or goes away. If you get below 1 full turn you need a bigger pump shot.

The opposite applies to a rich condition . if you are to rich set adjustment past its base point 1/4 turn at a time and this will allow the back barrels to open sooner.

Belgian1979vette

I can tell you from experience a misfire can show up as too lean as well. Check out plug gaps first if necessary close them up. Read the plugs to know for sure what your engine is doing. Old fashion way is not always bad.

Bud2

Many thanks for the input. I can't feel a misfire. Nor does it lag at tip in. The plugs are too sooty to be lean. Plus even if the car is, in fact, running rich I've never seen a puff of black smoke come out of the back end. The engine feels strong but I know that there is more on offer if I can get things fine tunned. I'll start leaning the carb and see what that gives me.

To complicate things even further my MSD is suffering from electcrical interference and makes timing difficult to maintain.

I'm chasing my tail somewhat here.

Thanks again for the info.

Bud.

Belgian1979vette

Soot = too rich. Lean it out. IMO you should rev up your engine when stationary and see what your AFR's are doing. If too rich lean it out a little and don't watch the AFR's at idle. Should be around 13.5-14 something at around 2500-3000.

chevymans 77

Get your cruise AFR set and than work on WOT,

Keep the secondary’s disconnected and get the primaries working correct than move to the secondaries.

Get a cruise AFR at 2000 rpm and than at 3000 rpms.

Then we can work form there, at 2000 rpm you are still on the transition circuit and at 3000 rpm you are on the main jet.

Once you have this set move to the pump shot and WOT,

I’ve seen these lean spikes mainly on annular booster carbs, from what I’ve seen the main air bleed is enlarged to a point it causes a delay of the main circuit and a lean spike at full throttle (quick opening of the throttle). The PV circuit feeds through the main circuit so a delay of the main circuit will cause a lean spike when you slam the throttle open.

The old Hollies used main air bleeds in the area of .024”, in the hew carbs with annular boosters some are using .032”. The only way I could resolve my lean spike on an HP series Holley was to reduce the main air bleeds. This is only one cause of the lean spike as others have suggested pump cam and squirter size and phasing can also help.

Neal

diehrd

Get timing situated making sure it is solid and all in by 2800 rpm. And I just want to add , you may not bog but your going lean at wot then recovering , your changing pump cams BUT not nozzles .. Why ?

And why are you not using the vac pod adjustment on the secondaries to slow or speed up the back barrel deployment .

Bud2

Why? Well I guess I'm still learning how to deal with this more complex carb. The 4160 was much easier to work with. I'll get there - just not as fast as one who is experienced.

Bud.

ajrothm

I have chased lean spikes and lean stumbles for years on my car. Even though I may not "feel" anything, the log always shows a lean spike when wacking the throttle open from cruise.. No matter which cams, pumps, squirters, I have put in it... It always shows a spike..

I even have an actual stumble that shows up at the drag strip, going to wot from 1000 rpm idle, and I can't tune the stumble out of it...tried everything... (50cc pumps, hollow screws, pink cams, "50" squirters) still always does it.. Partially worse in cold air as expected.

Ironically, the car 60's better with the stumble, seems to hit the converter harder. I gave up on trying to tune it out..

Bud2

I don't feel so 'alone' now. Thanks.

Bud.

Crepitus

for me it helped a lot to make a huge rich change to a circuit to see where that particular circuit was kicking in. Be sure you dont have to much idle transfer slot exposed. Get the idle and transition squared away then go to the main, power valve and pump shot. I really dont know much about the vac secondaries.