Need help with Holley carburetor tune.

[My427stang]

That carb seems to be the craziest conglomeration of changes.  To be honest, if I couldn't go back to pure stock on the LIST number because of booster changes or something like that, I'd run the 850.  It's a decent match anyway

As far as the exhaust, the core change should help, but it'll be loud!  In the end, Cobra headers usually don't work well anyway, but it doesn't make them flatten out badly.

Have you verified that you are getting full ignition advance, full throttle and are you confident in your technique in degreeing of the cams, both in process and desired centerline?  Any of those would flatten it out above 5000

[Barry_R]

Plugging the secondary power valve AND pulling four jets out is a really radical lean move.  Dont know what the secondary PVCR is, but that restriction in the exhaust might be the only thing keeping the pistons inside of the engine.

The "dig" on the old 4781 850s is/was a soft mid range.  The reality is that they are low on signal due to the booster/venturi relationship.   This makes them nonresponsive to the normal "jet change" mentality.   You might not even feel a two jet change because the draw is softer.

[Drew Pojedinec]

That is why we all love the 4780 so much

[338Raptor]

That’s what it is. 
Idle feed is 27 on the stock 850DP. I bought it new 25 years ago and didn’t modify it until now.
Idle feed was 33 on the custom 850HP, it’s back at the builder now.

I didn’t degree the cams (original builder did that) so it’s possible that could be an issue.
I’m getting full advance and full throttle.  AFR at WOT is 12.7 to 13.0. I only have one wide band so I’m only reading 1 side of the engine. I’m assuming the other side is doing the same thing. 

As a reminder I’m Burning non-ethanol.

[My427stang]

That’s what it is. 
Idle feed is 27 on the stock 850DP. I bought it new 25 years ago and didn’t modify it until now.
Idle feed was 33 on the custom 850HP, it’s back at the builder now.

I didn’t degree the cams (original builder did that) so it’s possible that could be an issue.
I’m getting full advance and full throttle.  AFR at WOT is 12.7 to 13.0. I only have one wide band so I’m only reading 1 side of the engine. I’m assuming the other side is doing the same thing. 

As a reminder I’m Burning non-ethanol.

I thought the problem carb was a 780?  Did I mix things up?  If so, in general (lots of things play into it) an 850 would be about 80/88 jetting without a rear PV.

IMO I think you are putting more stock into the fuel blend than really matters.  Even ethanol blends vary widely, and don't cause the craziness you had been seeing, and although there can be some jetting required when alcohol is involved LOL  I have never heard anyone complain that new versus old non-ethanol caused issues. 

With that being said, I realize I am a comment or two from you telling me to butt out, but as a long time carb guy who dove pretty deep into the HP series, I feel your pain.  They can be funky, but when it comes back, remember that fixing them with pump shot is not where you want to be.   I also think you have to remember that your "old shelf carb" which was designed for old fuel, did pretty good as-is, that's a good indicator that it is a carb problem, not a fuel problem.

As far as the cams, Jay can talk MUCH smarter on this than me, but due to the design of the SOHC valve train, cams are much milder than the numbers seem in a SOHC.  If your cams are milder and advanced, that could be killing off the top end charge (assuming you don't have some odd air filter choking it or the lid too close to the carb)

[338Raptor]

When I started the thread I thought the custom carb was a 780 because that’s what the original owner told me.  After a little research I learned it was a custom 850-HP.  After a dozen unsuccessful adjustments I pulled that carb and installed a 25 year old 850 double pumper.  Both carbs started on the engine is stock form. 

The 850-HP I started with (that I originally thought was a 780) had the builders original settings in it and it would run 20 to 1 lean at mid throttle then 10 to 1 rich at WOT. I pulled this carb and sent it back to the builder for rebuild/recalibration. 

Next I installed my rebuilt/cleaned out totally stock 850 double pumper that I originally purchased 25 years ago. This carb ran a little lean at part throttle and less rich at WOT. After pulling the secondary PV and changing some jets it ran MUCH better than the first carb ever did.  After the changes indicated a few comments ago it is now running 12.7 to 13.0 at WOT, idling at 13.5, cruising at 13 to 1 (this is why I’m interested in dropping the idle jet 1 number), and mid throttle (just before PV opens) at 14.5 to 1.  Then 12.2 to 1 when PV opens.

As you can see none of these numbers are lean despite all the small jet sizes. 

However, after all this, the motor pulls strong up to 5000 RPM then feels held back. RPM climb slows down and the sensation of acceleration seems to slow down.
 I’m thinking the 2” restriction in my side pipes may fully explain this.  The cam timing could be a huge part but I’ll know more after I replace the side pipes.  New pipes are ordered and will be on the car soon. I also anticipate the new exhaust may let me put the carburetor tune back to normal.  I hope.
Basically I’m going to stop chasing my tail until the new less restrictive exhaust is installed.

I just mentioned I’m running non-ethanol fuel because I thought some where thinking the variations in ethanol content was complicating my tune.

[Cyclone03]

As I think I said above,before doing “jet” changes to correct low speed low load cruising I lean the mixture screws 1/2 turn then test. The mixture screws do have some effect on low throttle angle Cruise and early transition circuit operation. With fairy large engines with strong pull on the mains you could be past the transition and be in the main, conversely you may not start pulling on the main till higher than expected throttle angle and RPM. This light load tuning area is where a vacuum gauge really helps.

[338Raptor]

I didn’t mention it but I did what you suggested. Turned the idle screws in 1/2 turn and no difference in how it behaved and nothing noticeable with AFR while driving. The only change was the Poor idle

I ran with my vacuum gauge attached and got a pretty good idea what readings where at different events. When the engine is running super lean at mid throttle it had approx 10” or more vacuum. Idle vacuum = 11. Cruise vacuum is 11-20 depending on load. And I could see the PV open at exactly 6.5.

I checked my timing, it was 10 at 850rpm, 12 at 1000, and 32 at 3000 rpm.

And I checked my valve lash. It was .018 and .022 cold. None needed readjusted.

[Barry_R]

That seems like really low timing numbers, especially at 3000...

[1968galaxie]

How high does one normally run the timing in a SOHC?

[Barry_R]

Not enough data to give any sort of hard recommendation, but the 32 number feels about 6 or more degrees low.  The 32 is what I might expect for a modern, efficient chamber - the SOHC ain't that...

[ToddK]

29 degrees total is all my iron headed, 10.3 to 1 SOHC engine will tolerate on our premium pump fuel, which is equivalent to 93 octane in the US.

[My427stang]

Where do they usually make max power on the dyno?

Could be too little but too much initial and comes in too quickly if pinging on 29.  Seems to me that it's a big open chamber with little obstruction, so shouldn't need a lot of timing, but never worked with the hemi style chambers