Bolting on a new carburetor, especially a Holley, is a great feeling. The motor immediately sounds healther, throttle response becomes crisper, and of course power and fuel economy improve. However just like other mechanical parts, a carb needs periodic rebuilding to keep it metering efficiently. Holley carbs, if properly tuned and maintained, will virtually last forever. Our 650cfm double pumper had served us well for over 4 years, and had put up with hundreds of jet changes, before it started running a little rough.We had noticed some part throttle popping, rough idle, and even inconsistant ET's at the track, so we decided it was time for an overhaul. In this article we'll show you how easy it is to rebuild your Holley, and we'll also give you some tips on tuning it for maximum performance.

Holley "Trick Kits" and "Renew" kits are the easiest way to revive your carb. They come with everything you'll need, including gaskets, power valves, and other hard and soft parts. We picked up the 37-485 kit for rebuiding most Holley 4150 and 4160 carburetors. Be sure to get the specific kit for your carb. All Holley carbs come with the part number stamped on the choke tower, so use this to order the correct kit.

When it comes to Holleys,
4150 and 4160 refer to the two most popular models of Holley carbs. A 4150 has both primary and secondary metering blocks, allowing you to change jetting for both the primary and secondary circuits. The 4160 models utilize a metering plate in the secondary , which has fixed, non-adjustable, metering orifices. All 4160 models have vacuum actuated secondary throttle plates. The 4150 models come in either dual mechanical throttle plates or with vacuum secondaries. All 4150's have dual accelerator pumps, and dual fuel inlets hence the term "double pumper." The difference between vacuum and mechanical secondaries is simply in how the carb actuates the secondary metering system. In vacuum actuated carbs, the secondaries open up only when the vacuum across the primary venturis reaches a certain level. A diaphragm housing on the side of the carb contains a changeable spring which controls precisely when the secondaries open. On a mechanical secondary carb the secondaries open when the throttle lever reaches a certain position. The simplest way to tell which type you have is to look down the throat of the carb, with the engine off, and pull back on the throttle lever. If the secondaries open, its a mechanical secondary. )

The beauty of Holley's is their simple and universal design.Not only are they a snap to tune, but as we learned, they are just as easy to rebuild.
Follow along as we rebuild and tune our double pumper.
Click for larger image!1. Before you start, be certain to have containers to hold and organize all the small parts which will result from the disassembly. Spray can caps work great for this purpose. A tuning tip: Notice how we've spliced in a small section of rubber hose into our cheapo chrome fuel line. This makes jet changes very simple; instead of having to disconnect the fuel line at the bowl fittings, the rubber hose will flex enough to get the bowl off and allow you to access the metering block to change the jets. Click for Larger Image!14. Start by placing the base gasket on the throttle base. Do not use any sealer or adhesive on any of the gaskets. Be sure all the passage holes line up.
Click for Larger Image!2.Start by draining all the fuel out of the carb. Remove just one of the lower bowl screws and let the fuel drain out. Again, a plastic spray can top works great to catch the fuel. We always carry one with us to the track to avoid spilling fuel all over the intake. Click for Larger Image!15. Screw the base to the carb body. Use a dab of red locktite on the eight screws to keep them from vibrating loose and falling into the motor! Do not overtighten the screws as the carb body will strip easily.
Click for Larger Image!3.Remove the fuel bowls and take out the floats, and needle/seat assembly. Remove the metering blocks, and unscrewthe power valve in the primary metering block. Some Dominator style Holley's will have a powervalve in the secondary metering block as well. Click for Larger Image!16. Replace the fuel bowl umbrella check valves with the new ones provided. Be carefull not to tear them apart.

Click for Larger Image!4.Take note of the powervalve that came out. The powervalve will be stamped with the vacuum rating, though its difficult to tell in this photo, the numbers on the right are a 6 on top, and a 5 below, identifying it as opening at 6.5 inches-Hg. Click for Larger Image! 17. The kit comes with new accelerator pump diaphragms and springs, so swap those out too. If you have a vacuum secondary carb, the kit comes with a new secondary diaphragm, seal, and spring.

Click for Larger Image!5.Unscrew the accelerator pumps, and carefully turn the carb over; two tiny needles will slide out of the accelerator pump bores. Don't loose these!

Click for Larger Image! 18. Swap out the old needles and seats for the new ones. The new ones comes with Viton tips which seal better. Be certain to replace the small gasket under the adjuster screw or you'll get fuel squirting all over the engine.
Click for Larger Image!6. All that remains is the carb body and throttle base plate. Eight screws in the base plate will separate it from the carb body.

Click for Larger Image!19. With the needles and seats in, re-attach the floats. Setting Holley float levels is somewhat tricky, especially with the engine running. We recommend setting it now. Adjust the screw so that the top of the float is level with the bottom of the sight hole. Note that we have replaced the stock hollow brass floats with Nitrophyl floats. This is not neccesary, but chances are if your carb is as old as ours, the brass has corroded and may have pin hole leaks, or the brazing has separated. Upgrading now is cheap insurance against a hard to find fuel level problem.
Click for Larger Image!7. Tuning tip: As you may have noticed, our carb does not have a choke. Milling or cutting the choke horn off is a popular and worthwhile modification which will increase and smooth out airflow into the carb. This is the perfect time to do it, since you'll be cleaning the carb thoroughly afterwards. The metal is very soft, and a die grinder, dremel tool, or even a small hacksaw can get most of the choke horn off. We have not had any problems, even on cold mornings, running without a choke. Click for Larger Image!20. Replace the powervalve with the new one which comes in the kit. Note that the kit only comes with the stock 6.5" power valve. For most engines this works well. Tuning tip: The power valve functions to provided added fuel during temporary low vacuum conditions. This helps eliminate flat spots during part throttle, low vacuum conditions. The easiest way to determine which power valve you need is to take your engine vacuum reading at idle. Generally a power valve half this reading will work well.
Click for Larger Image!8. Another tuning tip: Most of the thottle bores have casting ridges that disrupt airflow. Using a light sanding roll will smooth these out.

Click for Larger Image!21. Idle adjustment screws are one of the first things to wear on a carb because they are so frequently adjusted. When replacing these be sure to fish out the small cork gasket, it serves to seal the screw and keep it from loosening and changing the adjustment.
Click for Larger Image!9. Use a light file across the carb body to clean up any burrs across the metering block surface.

Click for Larger Image! 22. Eventhough the blue non-stick gaskets are good, we recommend spraying them with Pam or similar non-stick spray. Our last set lasted several years, and never stuck or tore on us!
Click for Larger Image!10. Most carbs will not need any further disassembly of the base plate. Inspect the base plate for cracks, and use a straight edge to check squareness of the mounting surface. If it is anything but dead flat, you should replace it. Check the linkage for smooth operation. If there is any binding, or play in the throttle shafts, or they leaked fuel, you should either replace them with a Holley throttle shaft service kit. Click for Larger Image!23. Replace the fuel bowl vent whistles. They are held in place with tiny self tapping screws, so hopefully you didn't lose them.
Click for Larger Image!11. With the carb completely disassembled, and all grinding completed, start the cleaning. This is the most important part of the rebuild because it is the varnish and grime in the passages that causes a poor running carb. We found that the carb body was too big to fit into the carb dip, so we used spray solvent to clean it. We let the metering blocks and bowls soak in the dip over night. A word of caution: Most carb dips and solvents will corrode aluminum, so use only spray cleaner to clean the base plate. Click for Larger Image!24. With the metering block gasket oriented correctly, place it on the carb body, and then attach the fuel bowl.
Click for Larger Image! 12. Use an air compressor to blow out all the passages. If you dont have a compressor at home, you can use compressed air in a can, available at most computer and office supply stores. We found the straw tip works great to get in to all the metering passages and blow out any dirt or gunk. Click for Larger Image!25. We like using these Moroso nylon bowl screw washers instead of the paper gaskets. They dont leak, or break apart, and again are great if you make alot of jet changes.
Click for Larger Image! 13. With the carb thoroughly cleaned, we can start reassembly. The Trick Kit comes with a variety of gaskets for different carbs. So be sure to carefully compare your old gaskets to the new ones. The nice thing about the Holley kit is that you get the blue non-stick gaskets. If you make frequent jet changes, these gaskets hold up great, and don't flake apart clogging up the passages.
Click for Larger Image! 26. Last but not least screw down the shooters.Be sure to drop in the discharge needles, point facing down. Note that their are two gaskets, one below the shooter, and inside (under the head of the screw.) Also shown here are the two types of shooters, one with nozzle extenstions, the other without. They both perform the same function, but the ones with the nozzles are said to be more accurate...will you gain a tenth? Probably not.
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