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.

[Mikey914]

Looks like classic thread to me

[jmill]

Looks like classic thread to me

(IMG:style_emoticons/default/agree.gif) Best write-up I've seen explaining the accelerator pump circuit.

Most don't have the patience or knowledge to delve into tuning the accelerator pump circuit. Marks info goes a long way in providing it.

I know it's the lazy way out but have you tried a larger main and AC jet (like a 155 main and 210 or 220 AC)?

[sgetsiv]

I haven't tried a larger main jet yet but I have run both 140 and 145 mains so I can certainly testify that these smaller mains do make a significant difference in AFR readings between 2500 and 3500 rpm's. Much leaner.

Note that the AC jets actually make the car run leaner with an increased size - since I am running 200's now, I could drop to a 190 or 180 to richen the mixture above 3500-4000 rpms.

It's also true that there is overlap between jets all the way across the rpm spectrum - idle jets affect rpm ranges up to 3000 rpm, mains from 2000 to 4000 rpm, and air correction jets from approx. 3500 on up. These rpm numbers are just estimates based on what I have read. Somebody with more experience may want to describe how the transitions between idle, main and AC actually work.

[jmill]

Note that the AC jets actually make the car run leaner with an increased size - since I am running 200's now, I could drop to a 190 or 180 to richen the mixture above 3500-4000 rpms.

True. You didn't mention you had a lean condition at WOT. That lead me to believe your AFR at high velocities was in line. Increasing the main jet size would enrichen the mixture at medium velocity/flow and the larger AC jet would lean it back out at high velocity/flow. The exact point at which each jet is most affected has more to do with velocity/flow (read cylinder volume, venturi size, and RPM plus butterfly position for progression ports). The quoted RPMs are an average and vary in a case by case basis.

[ChrisFoley]

Increasing the size of air correction jets will lower the rpm where the mains come in, reducing the midrange lean transition.
Bigger AC jets means bigger mains will be needed to maintain good wot mixture.

[McMark]

Idle mixture screw on the side adjusts fuel flow when the throttle plate is closed and the progression holes are blocked. Once the throttle opens (or if the idle speed screw is open too far, or if the throttle plates are mismatched and don't close evenly) the idle mixture screw stops functioning. Not a hard and fast drop off, but it drops off in relation to throttle opening.

Idle jet meters fuel flowing through the progression holes. Once the throttle plate passes the last progression hole they pretty much drop out and the idle jet does nothing. This is more of a throttle opening condition than an air flow condition.

Main jets come in when the vacuum created by the main venturi is great enough to pull fuel up and into the throat. This is a matter of engine displacement, cam timing, venturi size and float level.

Air correction jets come in when the main jet starts maxing out its flow. But air correction jets play a part whenever the main jet is in action. As mentioned the air correction jet adds air to the fuel flow. This air displaces fuel, therefore leaning the mixture.

The emulsion tube has holes along its shaft. These holes are where the main jet fuel and air correction jet air mix. The location of the holes affects the characteristics of how and when the air and fuel mix. Fuel flows along the outside of the emulsion tube, and air flows along the inside of the emulsion tube (until it bubbles out the holes). Typically, higher holes will come in sooner, and lower holes delay air corrector influence.

As mentioned, the main venturi affects the amount of vacuum created. On the same size engine, a smaller venturi will make vacuum at a lower rpm and therefore pull the main jet into action at a lower RPM. If it's too low it will overlap the idle jet function too much. A larger venturi will raise the rpm where the main jet comes in. Too large and you have a 'flat spot' where the idle jets have cut out, but the main jets haven't come in. Similarly, a larger engine (with the same size carbs and venturi) will suck more air per cycle and will therefore increase vacuum in the same carb setup. A smaller engine will suck less. This is why a 2270 with 40mm carbs will take a larger venturi than a 2056 or 1911 would.

Also as mentioned, the float level affects when the main jet comes in. If the float level is too high, then it doesn't take much vacuum to cause the fuel to be pulled in. Too low and it takes more vacuum to pull the fuel. This would be related the the venturi function. Theoretically, you could run 'too small' venturi with a low fuel level and be okay. The small venturi increase vacuum, but the low fuel level increases the fuels resistance to flow.

Here's a picture I stole off the internet that is pertinent, although only shows one aspect of this complex function. This picture is not a complete reference.
(IMG:http://www.914world.com/bbs2/uploads_offsite/farm9.staticflickr.com-419-1404754461.1.jpg)

[rhodyguy]

i spoke with steve this am. asked all the trivial ?s. got to air bypass screw setup. one on each carb set at 1ish? and the other 2 at 6 (IMG:style_emoticons/default/dry.gif) turns out. i believe this is contributing to his lean condition and transition stumble. steve plans to go the the pre run setup as per tomlinson's directions.

[McMark]

The HPMXs can have misaligned throttle shafts/butterflies.

[matthepcat]

Great thread....bookmarked.

In all honesty the proposition of running aftermarket fuel injection is not any more complicated than a guru level of carburetor tuning.

[rhodyguy]

mike blizzard learned all too well about the HPMX 'webers'.

[sgetsiv]

Just wait. I'll be posting some additional information about transition points between idle jets, main jets and AC jets once I have a little more data.

As Kevin indicated, the air bypass screws will be reset per the Tomlinson book - 1/2 a turn out as a baseline point. I'll be testing for other potential air leaks before making any changes to the air bypass screws.

The next step will be moving up or down from the 57 idle jets to see how those affect AF ratios at transition to mains - both slow transition and fast, pedal to the metal transition. My gut says that I will need to increase the idle jet size to get a smoother transition to the mains because the accelerator pump is not able to quite keep up.

Stay tuned...

[jmill]

You could also try the F7 ET. The F11 provides a leaner transition whereas the F7 provides a richer one. They aren't cheap (about $85 for the set). It's hard to justify the cost until you're sure that's your problem.

[ChrisFoley]

The lean transition can be cured with larger AC jets.
Fuel in the well will emulsify sooner.
Emulsified fuel moves more readily than liquid, thereby transitioning to the mains sooner.

Yes, you need larger Mains to go with the larger ACs.

Larger idle jets will make it too rich in part throttle running.

[sgetsiv]

Just a quick update...

Idle jets are now 57's. Running mid 12's at idle on the AFR.
I have the air bypass screws all set at exactly 1/2 turn out.
All 4 barrels are pulling the same at idle and at 1400 rpm.
The manifold bolts needed a bit more tightening as well.
I have moved the accelerator pump screws in to about 10mm on both sides - just slightly richer than it was before.

Response is nice and smooth.
Flat highway cruising runs in the mid to high 12's on the AFR.
Uphill highway cruising goes leaner but never gets above 15. Same RPM, just more load and more throttle.
WOT at 2500-3000 rpm goes slightly lean - maybe 16-17 but just for 1/2 a second or so before dropping to 11ish once the accelerator pump jets kick in. It quickly settles in around 12.5 to 13.
If I remember correctly, as I accelerate with WOT above 3000 rpm, the AFR readings are between 12 and 14.
I can drive continuously at 4000-5000 rpm and am getting consistent 12-14 readings as well.

I still think the transition from idle to mains is a bit heavy - I can probably move the accelerator pump screws in to 11 or 12mm to completely avoid the lean situation with sudden WOT. I was wondering if larger mains and larger AC's would allow the mains to come in sooner and help to eliminate that lean / hesitation condition? Or would that just make the mid range richer and the top end leaner?

[sgetsiv]

Installed 160 main jets and 220 air correction jets yesterday. Still running the 57 idle jets. No changes made to accelerator pump.

First impressions are that the car runs leaner above 3000 rpm, even just cruising. 13 to 13.5 typically. A little throttle at 2500-3500 rpm drops the AFR slightly as the accelerator pump kicks in. WOT is above 12.5 once the accelerator pump circuit has finished its work. Runs pretty nice in the 4000-5000 rpm range with AFR from 12.5-13.5.

CHT's were below 210ish on the front two cylindrs and 240ish on the back two. It's probably still slightly rich and that's helping to keep the cylinders cool.

I would like the mains to kick in sooner so I'm going to try 165 to 170 mains next. Will likely need to increase the AC's as well. I understand from the Tomlinson book that the ratio is 3:1 - so if I increase the mains by 5 then I should increase the AC's by 15.

My spark plugs were all a bit loose so I torqued them to 15 ft-lbs (with my new handy-dandy torque wrench). I finally broke down and bought one after all these years. Thanks HF.

I'm driving Scarlett every day this week since my sister is in town and borrowed our other daily driver...

[wndsrfr]

Great thread....bookmarked.

In all honesty the proposition of running aftermarket fuel injection is not any more complicated than a guru level of carburetor tuning.

This thread might be a candidate for the Classics.....

[jmill]

I would like the mains to kick in sooner so I'm going to try 165 to 170 mains next. Will likely need to increase the AC's as well. I understand from the Tomlinson book that the ratio is 3:1 - so if I increase the mains by 5 then I should increase the AC's by 15.

I've always selected smaller vents or went with a richer ET to eliminate lean transition. I've never tried the larger AC jet and I'm very curious to find out if that solves it.

If you increase your mains by 5 and your AC jet by 15 you should have a similar AFR to what you have now. What Tomlinson's statement also means is that increasing the mains by 5 is similar to reducing the AC jet by 15 or increasing the AC jet by 15 is similar to decreasing the main by 5. I've found it's not exactly the same but close.

[sgetsiv]

I've been driving Scarlett everyday for the last 3 weeks. I read in the Tomlinson book about the best way to clean out the idle circuit - you remove both the idle jets and the idle mixture screws and then blow compressed air through. I know I had some junk in there and once it was all cleaned out, everything was much smoother.

I'm now down to 52 idle jets, 145 mains and 220 AC jets. Running really nice with cruising and WOT in the mid 13's. It does go a bit rich between 1500 and 2500 but that's mostly due to the accelerator pump doing its thing. I could back out the pump screws a few more turns - now running 8mm of thread.

It's just amazing to me how important it is to have the carbs synched in order to avoid popping and backfire on decel.

I do have some jet doctors on order so I don't plan to mess with the carbs anymore until I have them installed.

Oh, and the car does seem to have more power throughout it's range running in the 13-14 AFR range.