I have a 72 scamp 225 with a blow through turbo set up and I am experiencing a little sputter in low gear until RPM’s and boost comes up. It appears to be rich when this takes place. Once the turbo spools up and it shifts to second it pulls hard. The carb is an off the shelf Holley 600 with vacuum secondary from Advance Auto. The floats were changed out to prevent crushing problems, power valve port worked, boost reference fuel pressure regulator, and the choke was removed. Any ideals what could cause the rich sputter at lower rpm and only first gear??? The build is an offy intake, custom header 3into1/3into1 and the ones to turbo, waste gates both sides, turbo T03/T04 T04E, HTS blow off,ebay intercooler, Lunati cam, 904 trans shift kit, 2500 stall,and head has mild port and polish. Tore it down this weekend to fix an exhaust leak every thing looks good in the head. Found a few other odds and ends to fix hope I find the problem. The sputter was caused by a vacuum cap with a tear in it. Runs great now !!!!!

I'm thinking the sputter is a lean condition unless you've installed larger than original jets and/or a larger accelerator pump squirter. A 600 Holley is a little bit too large for good low speed response on a low compression, moderately cammed 225 especially with a stock torque converter. Most folks use a 390 CFM Holley or a 500 Edelbrock.

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Joshua

I agree with Josh, lean condition. Never heard of anyone using a vacuum sec carb in a blow thru. I would assume you would need to boost ref the secondary vacuum pot. Also tag the front PV with boost referance. Otherwise get a looser convertor, a wideband and a holley jet kit. We need more info on the build also. Later
Ryan

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Yes, i would be interested in finding out how his blow through works with a vacuum operated secondary too. Although he may never get into the secondaries as it is. ..here goes, you disconnect the secondary linkage that holds the secondaries closed when the primaries are closed,its just a little drag link on the side. You take the vacuum pot apart and put the spring in the other side of the diaphragm, so now the spring is pushing the...well, Ill let this guy tell the theory:
Diaphram-operated carburetors usually have an interlock to help pull the secondary throttles closed as the primaries close. The is type of actuation is used on the Holley 4150/60-series carburetors and is easily modified for turbo use.
The interlock between the primaries and secondaries is removed completely. The diaphram assembly is taken apart and the spring moved to the opposite side of the diaphram. the linkage is reworked so pressure on the diaphram opens the secondary throttle and the spring closes it.
With this modificaiton, the secondaries open only when the engine is supercharged. Consequently, it is possible to select main jets and power valves for the primary to give the ideal fuel/air ratio of naturally aspirated conditions. Secondary system jets can be sized for a rick condition more compatible to the high intake-manifold pressures, or boost.
When a carburetor is modified this way, there is no mechanical linkage between the primary and the secondary butterflies. The engine operates using only the primaries until the intake-manifold pressure overcomes teh spring in the diaphram. this opens the secondary butterflies.
If no other modifications were made, the secondaries would not close when the driver's foot was removed from the accelerator pedal; the engine would continue to run supercharged on the secondaries alone. To prevent this, a small three-way Mead Valve is placed in the sensing line and mounted on the carburetor.
The Mead Valve is actuated only when the primaries are fully opened. As soon as the primaries close, the valve vents the secondary diaphram to the atmosphere; the secondaries close immediately. This Mead valve is the same size and shape as a standard microswitch and is actuated in the same manner.
This setup gives a different feeling to the car than the standard setup. the secondaries will not open until manifold pressure reached about 1 psig. when this happens, there is not only a sudden jump in manifold pressure but a definite surge in power.

A vacuum operated secondary carb should not be used. It may be working now but how do they close reliably under boost?

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Whatever the guy says in the post, it seems that a light lift on the throttle aint gonna close those secondaries. only a full lift off and then only the diaphragm spring is going to close them per the mead valve. Not my first choice in carbs for sure. Looks like a 1/4 mile setup to me.

It works much like what is described by Turbo66Valiant stated in his article. With every set up there are adjustments that need to be made. I have been able to dail it in and tested it at several different boost settings and it responds well. Hope to get some photos posted soon.

Welcome, Mr. Scamp! Nice to see another boosted Slant out there. Maybe we can entice you to a Slant 6 drag race this year? Also, some of us are hoping to meet at Maxton Mile on June 25.

Best,

Lou (formerly of Chapel Hill, NC)

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