Edelbrock 850 performer RPM Q-jet

[Cliff Ruggles]

There is "old" and outdated information still floating around about vacuum advance and recommending manifold vacuum for ALL applications.

There are just some set-ups that will not want, like, need or respond well to a butt-ton of timing at idle speed.  Most of these engines will have higher compression and very well chosen cams in them making decent vacuum at idle speed with only about 8-12 degrees (some even less) timing in them. 

In a lot of cases where folks find themselves adding a lot of timing at idle to make the engine happy you simply don't have enough idle fuel to the mixture screws.  I recommend getting the carburetors idle system up to par for the application BEFORE running a bunch of timing at idle speed.

I'll also add here that efficient engines, no matter what make, size, or parts used will want LESS timing and fuel everyplace to be happy.  If you end up with an engine build that needs a lot of timing at idle, like 25 degrees or more just to salvage decent idle quality from it you simply missed the mark someplace in the engine build.  Typically this means that the compression ratio is too low, cam too big, LSA too tight, or a little of both. 

Cam companies do NOT help you here and the trend in recent years is to tighten up the LSA so the engine has some "attitude" in the exhaust note.  Overlap is not your friend with these things and a lot of it makes for poor idle quality and stinky exhaust.

Just some "basics" on this topic, but in any and all cases give the engine the timing and fuel at idle that it needs to be happy.  IF you plan on using manifold vacuum to the advance you may have to get an adjustable unit to make sure it stays "all-in" at idle speed so you don't get a BIG drop in engine RPM's when a slight load is placed on it like putting the transmission in gear (auto).

I'd also add here that these POS spring/weight kits they sell aren't worth two squirts of duck poop and can/will cause all sorts of idle tuning with these engines.  ALWAYS make sure that no timing is in at idle speed and the advance curve isn't starting till about 900-1000rpm's and smooth and steady till full advance. 

IF your engine likes, wants and needs ALL the timing in by 1500rpms or so once again you just didn't make good choices for it when it comes to compression and cam selection.....FWIW.....Cliff

[bry593]

For the original rich problem, which probably is still there, I'd recommend setting your float to 3/8".  Edelbrock says the 1910 is .300", but I found this is too high if used with typical mechanical fuel pumps.  I measured fuel pressure on an AC Delco ($50) pump to be 0-9psi.  I then installed a DelPhi pump ($20) and the pressure was 5-9psi.  The point being, Quadrajets are supposed to run at a maximum of 7 psi.  That extra 2 psi amounts to 22% more force trying to lift the needle off the seat.  So, you need more force pressing down on the needle.  I found a 3/8" float level shuts off 9 psi and fills the 1910 to about 3/8" from the gasket.

By the way, my rich problem did not reveal itself when first starting the engine and idling.  But once you had driven and came to a stop light, the engine would load up and try to die.  It would also be hard to hot start when shut off for just a few minutes. The other dead give away was the airhorn gasket soaked with fuel after a drive.

As for lean, your carb set for awhile.  Maybe some crud got into the idle tubes (like idle jets in a motorcycle).  I'd recommend pulling the idle mixture screws and blowing compressed air back through the idle circuit.  This should free up any loose crud.  If it runs better, you know you probably need to pull the tubes and clean the idle circuit.

[Cliff Ruggles]

The 1910's came with a .149" fuel inlet seat.  They can be problematic with some fuel delivery systems especially a mechanical pump putting out higher pressure w/o a return to the tank.

Very few engines will need a .149" fuel inlet seat to keep the carb full on hard runs and I seldom use those here unless it's a pretty high HP application installed in a car that leaves really hard.

For most set-ups it is better to go to a smaller fuel inlet seat for improved fuel control.  I run a later model carburetor (17057274) on my engine making over 550hp/600tq and use a .145" seat with 7.5psi fuel pressure.  It has been flawless for over 20 years, but I'm using an electric fuel pump behind the tank with 8AN lines/fittings everyplace with the regulator pretty close to the carburetor.  I also have a return back to the tank from the regulator to take some load off the pump.  I'd add here that I tested smaller N/S assemblies and varying fuel pressures and didn't run in to any issues until I went smaller than .135" and less than 6psi.  When I moved up to the .145" seat it ran the same as the .135/.140" seats but gave no troubles so I stopped there and have been running it that was over 20 years now.

Electric pumps are more forgiving for using higher fuel pressure as they are smooth flow vs the stop/start flow from a mechanical pump.  I've seen folks run into issues with some of these high output mechanical pumps especially kicking the pressure up with them and not using a return.  U suspect the stop/start action and no return may be "pounding" on the fuel inlet seat/needle some and pushing some fuel past it when it shouldn't.

In any case we often see conflicting results so there is no exact science with this sort of thing.  Some folks get away with less and some can barely get it done with more, but bottom line here is that you MUST have adequate fuel delivery for the power level of the engine and performance potential of the vehicle.  Also keep in mind there that when you find good traction and leave really hard ALL the fuel is trying to run back to the tank, so the game changes some and what you might get away with on the street spinning tires will NOT do well at the track with good traction.

For decades folks have regurgitated inaccurate information on the Quadrajet that the fuel bowl is too small for high performance work.  That is utterly absurd if you think about it for a second.  It wouldn't matter if it held on cup or a quart you absolutely MUST keep it full on a hard run or metering will not be consistent. 

The biggest problem is actually only having one N/S assembly so ALL the fuel has to come past one small hole to feed the engine.  The Q-jet actually has some nice advantages compared to other designs which include a center located fuel bowl with the jets on the bottom of the bowl (harder to uncover them on a hard launch).  Venting is excellent for the most part and easy to make it better if your car leaves hard enough to give you a nose-bleed.

......continued

[Cliff Ruggles]

Q-jets are such a good design that off-roaders and hill-climbers often prefer them to other designs and I've set up quite a few for folks who use them in pulling trucks as they fair very well in that environment as well.  Most recently they are becoming increasingly popular in some circle track racing classes since they lightened up the rules some and no longer mandate 2bbl intake manifolds in some of the "stock" classes.

Anyhow, I sell fuel inlet seats in .135", .140", .145" and .149".  In the early carburetors (short hinge pin/large float) I don't recommend the larger seats and they do NOT like a lot of fuel pressure either.  That design gave the entire line of Q-jets an undeserved bad reputation.  The later model carburetors are fine with larger fuel inlet seats and higher fuel pressure.  I've done a LOT of testing in that area and don't mind setting folks up with what they need and recommending a fuel delivery system and float level if you call the shop......Cliff

[bry593]

Yes, my discussion about 3/8 fuel level for a 1910 assumes a .135 seat.  A larger seat would just magnify the problem since the force pushing on the needle is proportional to the area of the seat hole.  A .149 has 31% more force trying to push the needle than a .135.  Crazy, eh?

F=P*A
A=(pi*d^2)/4
Canceling out the constants of pressure, pi and 4; the ratio and increase in force is:
.149^2/.135^2=1.31 => 31% more force!

[Cliff Ruggles]

You can NOT use the original posted dated for the 1910 for float level recommendations unless the original float is being used.  They used a larger float in the 1910's with more leverage than the replacement variety.

I've set up several hundred Edelbrock 1910's at this point and have only ran into issues a couple of times.  Coincidentally it was from customers using high output mechanical fuel pumps to them without any type of return.  One in particular was pushing past the N/S assembly no matter what he tried for float level (or size of the float) or fuel inlet seat diameter.  He was also using a "pusher" pump to his high output mechanical pump.  He refused to install a return (been doing things that way since Moby Dick was a minnow), until I showed him that the carb was fine with my fuel system past 10psi.

For most "high performance" applications I've used the smaller .135" seat at 9/32" and recommend 6-8 psi fuel pressure.  That will usually get it done with an good fuel delivery system to at least 500hp.

For really high HP applications, like one of customer and good friend uses with their 735hp engine pushing his car to 9.80's over 136mph we use a larger .145" seat, same 9/32" float level, fuel cell, electric pump fed by 10AN, then 8AN to the nose of the carb with a return style regulator in between........Cliff

[bry593]

Cliff,

Are you saying they had a special float, just for Edelbrock?  Now I know there are two off-the-shelf nitro-floats that can be used in the 170 series.  The first is slightly larger, but has the heel closer to the fulcrum (what my 1903 had stock), the second is slightly smaller, but has the heel farther away from the fulcrum (what I'm running now).  I tested both in my open bowl at 9psi and was shocked that they shut off at the same time when set to the same level.  I really thought the bigger float would shut off sooner, but it did not.

[Cliff Ruggles]

No, they weren't special just for Edelbrock, but they used the larger variety in their later style APT units.  The larger floats showed up randomly in post 1974 later style q-jets, but I never kept track of exactly which carb numbers I found them in.

The hinge pin location is excellent on the later carbs and they are fine with the smaller float. 

I've been running a later 1977 carb for quite a few years, small float, .145" seat, 9/32" float height and 7.5 psi fuel pressure.  It will NOT flood clear up to the 14psi rated pump pressure and has been absolutely FLAWLESS for over 20 years with the current fuel delivery system.

The ONLY time I had issues with the carb was way back around 1988-1990 when I installed a 428 into the Ventura and a Carter 120gph mechanical pump at the same time, dead-head.  It was OK most of the time but on occasion, especially hitting the brakes really hard it would flood momentarily.  I had a .149" seat in the carb at that time. 

I also noticed the gasket was wet all the time and I'd see fuel laying at the top of the accl pump hole at times.   So I decided to check the fuel pressure at one point and it was fluctuating between 6 and 10 psi with an occasion "spike" even higher.  I was NOT using a return and to this day suspect the HP Carter pump just didn't like to be dead-head even though Carter says they are OK in that configuration.

Jump ahead to 1999 when I installed a larger 455 making 1hp/CID.  I ended up sumping the factory tank, 140gph electric pump behind the tank, and going 8AN lines/fittings everyplace with a return to the tank from the regulator to cure fuel delivery issues. 

Since then I've moved on to two more powerful 455's and have never once had a fuel delivery or any other problem, and no flooding, wet top gasket or fuel laying around the accl pump hole.

I see some folks get away with less, but that's what it takes to support the power level of my engine and performance of the vehicle at the track with good traction.  In full street trim the car runs low 7's at 95-97mph in the 1/8th mile.  60' times are in the 1.59-1.64 range for most runs.

I don't run 1/4 mile because it's well below the legal roll bar rule of 11.49.  I have made a few 1/4 mile blasts before being asked to leave and the car has went really low 11's over 120mph and that was in pretty hot humid weather.

I did not dyno the current engine but did dyno one very close to it, just lower compression at 10.2 to 1.  It was also a 455 with the same heads and cam, and made 552hp/604tq, so mine will make at least that and probably a bit more since the compression ratio is 11.3 to 1.......Cliff

[bry593]

Nice.  Sounds like a pretty awesome Poncho.

[Cliff Ruggles]

Thanks. 

The current engine was about 30 years in the making.  I started out with a low compression 400, TH400, stock converter and 2.73 gears.  It ran 14.0's around 99-100 mph in that configuration  with a best ever 13.87 @ 101mph.

Two 428's and two 455's later, 10" custom converter and 3.42 gears it's running quite a bit quicker, but remarkably just as mild and easy to drive as the "little" 400 was way back in the mid-1980's........

[1965C-10]

I looked with a mirror and see a puddle of fuel, also after shutting off engine fog comes out.

Took video of it running with the fuel and a couple of pics of fog coming out of primary side, it says my files are too large, maybe I'm uploading the wrong way by clicking attachments and other option?

I'll see if I can upload video to youtube, but any suggestions on pictures?

[1965C-10]

https://youtube.com/shorts/PQ848GOarbc