Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
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There is "old" and outdated information still floating around about vacuum advance and recommending manifold vacuum for ALL applications.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!
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version