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... |
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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... |
sgetsiv |
Jul 6 2014, 08:26 PM
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Former 914 Owner :( Group: Members Posts: 159 Joined: 20-June 08 From: Portland, OR Member No.: 9,192 Region Association: Pacific Northwest |
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 |
Jul 7 2014, 11:34 AM
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914 Freak! Group: Retired Admin Posts: 20,179 Joined: 13-March 03 From: Grand Rapids, MI Member No.: 419 Region Association: None |
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) |
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