Carburetor Fine Tuning, Trying to get my dual EMPI HPMX 44's as close to perfect as possible - Now running Spanish Weber 48 IDF's... Options

[sgetsiv]

I've been working on my carbs for the past few weeks to try and get them to run as close to perfect as possible. Here are the details at this point:

2315 Raby built engine

EMPI HPMX Dual 44's
52 idle jets
150 main jets
200 air correction jets
55 accelerator pump jets

New CB Performance Rotary Fuel Pump - 3.5 psi - stock location
No fuel pressure regulator - just a TEE fitting
Stack electrical fuel pressure gauge 0-15 psi - reads 3.1 psi when cold, 2.4 psi once everything warms up and is running
Wide band AFR gauge

Fuel lines flushed and fuel filters cleaned
Gas tank pulled and re-installed lines at base to be sure there are no kinks

I rebuilt the carbs with the EMPI rebuild kits which includes all new gaskets, accelerator pump diaphragm, new mini fuel filter, new float needle valve assembly, new washers for the accelerator pump jets, new o-rings for the idle jets and idle mixture screws. Floats are set to factory specifications as well.

I pulled the throttle plates and was extremely careful to be sure I put them back exactly the same way and that they do not hang up - their action is perfect and they cover the progression holes.

This project started a few weeks ago after running into lean issues under load - turned out to be the fuel pump relay was going bad. There was a significant voltage drop across the relay and it finally ended up failing completely during testing.

Once the engine is fully warmed up, my AFR is reading 12 to 13 at idle; adjusting the idle mixture screws can make a big difference in AFR readings without changing the idle significantly.

I started out with the accelerator pump jets set to factory setting of 7mm of thread exposed. During my test drives today, I turned them in (richer) to about 10mm. Response is better but still not perfect.

I'm getting AFR readings in the 12-14 range during "regular driving" on the highway. Going up hill under load at 60-70 mph in 4th gear (say around 3500-3800 rpm) and very slowly pushing on the throttle creates a lean condition. Once I step on it, the AFR drops down to 11-12 and then settles again in the 12-14 range.

I would really appreciate some input on adjusting the accelerator pump and any other suggestions on how to get the most of these carbs. Are the accelerator pump jets (55) big enough? They can certainly be reamed out. I did measure the output and got around 0.3 ml fairly consistently for each jet. All 4 jets spray straight right into the center of the carb in an even stream.

[McMark]

A little basic info, which I know Steve knows already. When you open the throttle a mechanical linkage presses on a diaphragm to pump fuel into the throttle body. Think of a turkey baster full of fuel - squeeze the bulb and it squirts. Lets go over the adjustments and jets. The pump system isn't as simple as most people think.

Adjustment Nut
The tiny adjustment nut on the linkage adjusts the 'at rest' position of the linkage. The linkage arm you're adjusting pivots in the center (green dot) so when you move the nut out, the arm goes in. The picture below isn't an HPMX/IDF/DRLA, but shows my point. The lower arrow is where the nut would be. Moving the nut in (tightening) makes the upper portion of the arm move out. When that arm moves out the purple diaphragm moves out allow move fuel (black) to sit in the pump body. This adjustment changes the overall quantity of fuel potentially being squirted. But you must pay attention to the nut/arm being too tight or too loose. If you tighten the nut all the way, trying to get more fuel, you can actually make the arm move away from the diaphragm and introduce a gap. The diaphragm has a limited motion and can stop even if the arm keeps going. If you end up with a gap between the arm and the diaphragm, there will be a 'pause' when you open the throttle because the arm is moving, but hasn't contacted the diaphragm yet.



Accelerator Pump Jet
This is the nozzle that squirts the fuel. It's the most obvious part of the system. A larger jet will allow a greater volume of fuel to spray out. This won't affect when the accel pump sprays, just how much. It's also worth noting that a small jet will squirt earlier and longer, whereas a HUGE jet will squirt later and shorter. Think of a hose that you turn on full blast for exactly 1 second then off again. A fully open hose will just burble out a little water. Now do that again with your finger on the end, and the water will actually squirt out across the yard, and it will squirt for a longer time. So a smaller jet can increase low throttle movement squirt characteristics.

Check Valve and Bleedoff Port
This piece is less widely known about. If you take the cover off your there is a small jet in the bottom of the float bowl. This is where the accelerator pump gets it's fuel supply. When you release the throttle, fuel is sucked through this port. There is a small ball inside that acts as a check valve (see pic). The purpose is so that when you open the throttle and pressurize the pump, it doesn't squirt all the fuel back into the bowl. This check ball valve must be operational. If you pull the piece out and shake it, you can hear the ball rattling inside a good one. If there is no rattle, it's not allowing fuel to pass and you need to get a replacement or clean it. If you flip the jet over there is a small plate (green in pic) that is pressed into place to retain the ball. On HPMX carbs this plate is too low. Because it takes fuel pressure in the pump to lift that ball and shut off flow back to the bowl, a ball that sits too low will take too long to close and will allow a lot of pump pressure to bleed off. On Steve's carbs (and any HPMX that comes through here) I pulled the jets out, tapped the retaining plate a bit deeper into the jet. This doesn't restrict fuel going into the pump, but makes the ball more responsive by reducing the amount of movement necessary to shut off the ball valve. Finally, there is a small bleed off port. You can get these check valves with a variety of sized bleed off ports. This port lets a small amount of pump pressure (fuel) to return to the bowl. This port mostly affects the amount of time the pump will squirt. If you reduce the size of the jet it will bleed off less fuel, therefore more fuel will come out the jet in the throat. The reverse as well, a larger jet will bleed off more pressure and reduce the amount that comes out at the throat.