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1970 Chevelle Malibu "400" Clone

Fighting a low speed buck again. I added a 37 squirter, from the 31 it had and moved the pink cam to hole 2 (which is wrong according to the chart). I will likely drill a 25 to .035 after work and go to the blue cam. It helped the acceleration buck above 2000 but the deceleration is BAD.

Current timing:
22 deg initial at 950 rpm
38 total (initial + mech @ 3200)
9 deg of vac advance (limited the total opening but it is fully open at the 11in/hg it has at idle)
so it has 31 deg at idle (initial + vac)
I am sure it is timing related but not sure how to attack it right now.

38 is probably a bit high with the chambers it has and I likely should go back to 36 total which will make the initial 20 but I can add more vac advance.

The 9 extra deg of vac does make the idle better. I would ASS-U-ME it would idle awesome locked out at 36 but I am afraid it will ping with the stick. Plus the starter isn't awesome so I need to replace that too.

I may will pull the vac advance for the trip home to see what that changes.
 
In the video it looks and sounds good and you weren't 20 ft away.... :waytogo:

80' 427 said:
Fighting a low speed buck again. I added a 37 squirter, from the 31 it had and moved the pink cam to hole 2 (which is wrong according to the chart). I will likely drill a 25 to .035 after work and go to the blue cam. It helped the acceleration buck above 2000 but the deceleration is BAD.

Current timing:
22 deg initial at 950 rpm
38 total (initial + mech @ 3200)
9 deg of vac advance (limited the total opening but it is fully open at the 11in/hg it has at idle)
so it has 31 deg at idle (initial + vac)
I am sure it is timing related but not sure how to attack it right now.

38 is probably a bit high with the chambers it has and I likely should go back to 36 total which will make the initial 20 but I can add more vac advance.

The 9 extra deg of vac does make the idle better. I would ASS-U-ME it would idle awesome locked out at 36 but I am afraid it will ping with the stick. Plus the starter isn't awesome so I need to replace that too.

I may will pull the vac advance for the trip home to see what that changes.
Click to expand...

Your thinking is exactly on par from my perspective.

Are you sure that the vacuum is all in at idle? Because from my experience those hex looking adjustable cans are not all in with that vacuum. I've bought different cans at Napa and then tack weld and filed back on the flat bar to limit the max advance so its all in by 8" mercury but still not too much advance(some give you over 20). I worry it's not all in and that it's fluctuating on you with your cam lope. I use one of those hand vacuum pumps with the gauge so I can test the vacuum advance with the engine running in the driveway.

But I would likely try more vacuum advance to smooth it out, up to around 50-52 degrees total above 3200 RPM but with the vacuum all in at idle too. So if you are at 36 total w/out add 14 - 16 vacuum but have it all in at idle. Obviously 2 degrees less at 38.

This will give you the more advance it likely wants at low load but not too much under load.

Also, I agree, some of those charts I have found are way off on the pump cams too.
 
folkenheath said:
In the video it looks and sounds good and you weren't 20 ft away.... :waytogo:



Your thinking is exactly on par from my perspective.

Are you sure that the vacuum is all in at idle? Because from my experience those hex looking adjustable cans are not all in with that vacuum. I've bought different cans at Napa and then tack weld and filed back on the flat bar to limit the max advance so its all in by 8" mercury but still not too much advance(some give you over 20). I worry it's not all in and that it's fluctuating on you with your cam lope. I use one of those hand vacuum pumps with the gauge so I can test the vacuum advance with the engine running in the driveway.

But I would likely try more vacuum advance to smooth it out, up to around 50-52 degrees total above 3200 RPM but with the vacuum all in at idle too. So if you are at 36 total w/out add 14 - 16 vacuum but have it all in at idle. Obviously 2 degrees less at 38.

This will give you the more advance it likely wants at low load but not too much under load.

Also, I agree, some of those charts I have found are way off on the pump cams too.
Click to expand...
I pulled the can off and used my HF mity vac copy to see when it was pulled in at. I loosened the hex till it was fully pulled in at 10in/hg then added a crane limit cam but drilled a hole in the arm so it works opposite of how it is designed. I attached a picture similar to what I did off the interwebs. I can let it have more timing by moving it to the next notch. I agree I need to get close to 50 total. I am working slowly to get there.

I retarded the timing to 34 yesterday. I was getting just a slight rattle at about 3000. It still has 6 month old gas and it was 105 yesterday so that doesn't help.

I need to put in my wide band so I know I am not fighting a lean surge or something else.

I changed the accelerator cam to a blue cam and noticed the secondary arm was loose. I had to bend the arm straight to get it to touch the cam and it was at the end of the screw. I ended up installing a black cam in hole 1 which is a little taller.

Have you ever moved the Idle fuel restrictor to the low position from the high? I have never had one in the high position before.

crane limit cam.jpg
 
Never mind, the VC1862 looks pretty good. 16 deg of crank by 6-8 in/hg will get me to 36 at idle and 50 cruising, if I don't limit the advance
 
80' 427 said:
I pulled the can off and used my HF mity vac copy to see when it was pulled in at. I loosened the hex till it was fully pulled in at 10in/hg then added a crane limit cam but drilled a hole in the arm so it works opposite of how it is designed. I attached a picture similar to what I did off the interwebs. I can let it have more timing by moving it to the next notch. I agree I need to get close to 50 total. I am working slowly to get there.

I retarded the timing to 34 yesterday. I was getting just a slight rattle at about 3000. It still has 6 month old gas and it was 105 yesterday so that doesn't help.

I need to put in my wide band so I know I am not fighting a lean surge or something else.

I changed the accelerator cam to a blue cam and noticed the secondary arm was loose. I had to bend the arm straight to get it to touch the cam and it was at the end of the screw. I ended up installing a black cam in hole 1 which is a little taller.

Have you ever moved the Idle fuel restrictor to the low position from the high? I have never had one in the high position before.
Click to expand...
Nice job on the vacuum can. Do you think you are limiting yourself to how much advance can be all in at 10" with that can? Because if you tune more in its not all in? And I agree, I had some can # from napa that was all in by 8 inHg, but I don't remember which one. I'll see if I can find it. These days I just use the computer to control timing after I sync the ECU with the timing light, and I haven't touched the vacuum can in the truck since I limited the napa can 13 years ago.

I have not moved the idle restrictor location. I would just tune it to max vacuum with the idle screws, while not allowing the rpm to increase with the throttle blade screw, because when you tune the idle ratio as you get closer it likes it and the RPM increases, but you don't want to tune the idle fuel at the wrong RPM. I did convert some to 4 corner idle that weren't 4 corner idle.

I have never had an O2 hooked up while running a carb, would of been interesting, I would just tune by how it felt, ran, smelled, how the plugs looked, and 1/4 mile passes if it fit the application, etc.
 
I was going to sneak up on the timing at idle added by the vac. I will mess around with this can a bit more and probably buy the can I listed above if I fail.
My car has a vac gauge and I have a o2 port so I can watch the carb work. Understand how rich the idle feed restrictor is, watch when the carb moves to the main circuit. See when the power valve opens, power valve restrictor size being correct, etc. On double pumpers the ifr is usually so fat it kills mpg and soots up plugs, and the pvrc is so small that you end up with the main circuit over jetted to get the proper WOT A/F. There is a reason I have LOTS of brass set screws and tiny drill bits.
 
That's been my experience too with the tradeoff between max power or MPG and street driving. The power valves and staggered jets and restrictors helps, for 2 decades I played with Holley's and have built them from just pieces. Now I EFI and big block everything so I can tune it for both a good idle and MPG at low load and max power at the same time all without dumping the fuel bowls 20 times and making a mess.

Then when you add a power adder like nitrous or a supercharger the transition between vacuum and boost is uneventful and with dry nitrous I can vary the fuel curve with the injectors to better match what the engine wants instead of being a fixed fuel jet.

I give you props for sticking with the carbs though, Holley carbs are intricate pieces that deserve respect. Too many people blow them off because they don't understand them, it's the only carb I would use. Though I admit I was taught with Holley's and have zero experience with Quadrajets or Carters.
 
Qjets have lots of cool tuning stuff but they have their limits. I am not scared of FI, I just don't think the juice is worth the squeeze at the point I am at. Even my "Fast" car just has a dominator. My buddy that builds and dynos crazy stuff has been pushing FI but until I add a turbo or whatever it was just too expensive. That said, I have never had aftermarket fuel injection so I can't really claim I know what it would be like. That also being said I am less than impressed with every TBI style FI system.
 
TBI style has its limits, most of them self learning only and many limited in fuel flow. Especially the early units.

Holley now has 8 injector TBI units capable of 1200HP of fuel with full tuning capability.

Plus without the venturi restriction a 4150 can flow over 1000 cfm, some over 1200. And a dominator is 2200 cfm or more.

But like you said, with NA you won't really get more power, but you can have full power and still run clean down low pretty easy, but it's expensive, especially if you already have a carb.

I prefer sequential MPFI, I'll be converting my 489 to sequential with DIS using some D585 truck coils when I do this blazer project. I'm adding dry nitrous and I like the freedom of timing without the limits of rotor phasing.

I agree, a dominator carb is nothing but power potential, dominators rule powerful carbs.

Sorry about the sidetrack. I didn't mean to get sidetracked with EFI when we starting talking about tuning carbs. I have just used carbs for so long I much prefer the modern EFI at this point, the best of both worlds except cost.

Back to Chevelle awesomeness. I agree, I would totally recommend to lock your distributor out at 36 if it wasn't for the stick shift, because of that I would try to make the vacuum advance work better. It will get you the advance it wants at low load without too much advance at high load, like it was meant to do when you hook it up to full manifold vacuum. Too many people hook it to ported vacuum like they did in the emissions days of vacuum advance.
 
SO, I hooked up my wide band. Figured out the idle screws were set too tight. Backed it out to 1 full turn and it is happier (12.7-13.1 a/f). Some of the decel buck (which I would assume was idle surge) is gone (was going into the 18-19s on decel). The idle circuit is super fat (11.0-10.5) so I need to look at changing the ifr or the idle airbleed.

Ordered the vac advance can from Napa ($35!). I will try backing the VA stop off a couple of notches in the mean time.
 
Martin told me he wished you would of painted the engine a different color, installed a TH350, and put some 24" rims on there.


















On a more serious note, I am surprised if you can can get decent idle and low RPM readings with an O2 sensor and your cam. I would think it would mix up too much exhaust gas with the overlap at idle and tell you its lean when it's not. I would certainly trust a rich reading, but a lean reading I would trust your gut more than the O2 at idle. Of course if it likes it, it likes it, that's what matters.
 
I don't "trust" it but I use it to help clue me in. A lean reading with a buck that clears up with adding fuel indicates it might be partially accurate. The idle mixture should never set with a O2 (I set it to be happy with vac), but I backed out the screws and it made difference and it helped the buck and idles better so I suspect it was right. I am guessing I messed with timing and I should have reset the air screws.
The IFR is easy to see at 1500-2000 rpm. I hate to just change the air bleed because it can change the point of transition of the idle into the main circuit, which can lead to a rich overlap. I really need to dyno it so I can hold it at steady load and sort out all the circuits then get to WOT.

O2s are funny. A missing cylinder reads lean because it will have excessive unconsumed air (is that right?) Overlap will mess with it. It is an instrument and all good petro/chem guys know you can't trust 2 things instruments and women. You need to look at what the car is telling you

Funny story. I had that 74 Malibu and be just a year out of college I was pretty broke. I bought the old Doug Roe Rochester book that has lots of good tips for the 2gc. I was trying things that indicated good on the O2 but the car was definitely unhappy. It finally occurred to me that I had left the air pump functional in the exhaust. After I removed the pump inlet, it showed just how rich I was. I leaned it out to crusing at 15s and WOT was 13.0s. It gained a bunch of drivability over what I had tuned it too and what it was stock. It gained like 2-3 mpg and went from a doggy flat footing of the throttle from a stop to about a 1/2 a turn of the tire and a hard pull (for a 2bbl 350 and 2.73 gears).

I guess that lesson told me to not just look at one thing and the best tool of all is your ass and ears. If something is telling you one thing but the car is telling you another, listen to the car.
 
You're right on, too many people trust the computer too much, just like at the parts store. If the computer doesn't have it, they are clueless unless you get a guy with real world experience. Same with EFI tuning, if they can't read plugs or tell what the engine wants without the 02 readings, they shouldn't be tuning for other people.

I've tuned EFI with a procharged big block before with no O2, we just went by what the engine wanted, and the exhaust, and the plugs. Ran very well!

You are doing a fantastic job, in case I am not coming off that way. Most of your tuning and thinking is pretty much exactly like my train of thought.
 
I need to pull the carb tonight, pin all the orifices and make sure the throttle blades are square. I will likely drill a smaller ifr but I will see what it has.
 
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