TIME FOR A NEW CARBURETOR

QUESTION: I have been having a problem with engine hesitation while running especially when the boat hits turbulent water. Also, when I first start the engine up, it dies out four or five times before it will run at an idle speed of 900 to 1,000 rpm. This has been happening since I had the carburetor rebuilt two years ago. However, the boat runs well at 5,400 rpm.

I have a 750 Holley carburetor that is modified to accommodate a Weiand blower with a Crane cam and roller rocker arms. The carburetor is 12 years old and has been rebuilt twice during that period. The fuel pump, as well as all electrical systems, check out. I suspect the rebuild kit that was used may have been a generic and/or a counterfeit. Can the age of the carburetor be a factor, such as worn external linkage, etc?

ANSWER: If you are sure there is no problem with the ignition system or wire connections to the engine, the problem is most likely associated with the carburetor. The idling problem and the hesitation are probably two different problems. Before you attack the carburetor, make sure that all wire connections to the engine (including grounds) are good. Also, check to make sure the advance mechanism in the distributor is functioning properly. If the advance mechanism in the distributor is worn, or there is a problem with the springs, it is possible that the spark advance is fluctuating too much at idle speeds and that will deteriorate the engine’s idle quality.

I am assuming that your carburetor does not have a choke. If this is the case, the engine will be “cold blooded” for a couple of minutes when it is first started up. If the idle circuits are set too lean, it will have a tendency to die until the engine is warm. Most supercharged carbureted motors with the idle mixture screws set properly will idle with a slight cycling of rpm.

If you are noticing a hesitation, or miss when the throttle is advanced from the idle position, it is commonly caused by improper settings of the accelerator pumps. Supercharged engines need a little more fuel volume from the accelerator pumps as the throttle is advanced in order to prevent a momentary lean condition. It is best to set up the accelerator pump cams on the linkage so that the rate of the pump action is quick.

High-volume (50 cc) accelerator pumps work well on the primary side to provide adequate fuel during the transition off-idle. The hesitation caused by a lack of adequate fuel being provided by the accelerator pump is exaggerated when the engine is already lean at idle.

If your engine hesitates or loads up when the boat is hitting waves, it is likely that the fuel bowls are set too high and that results in fuel being spilled into the venturis every time the boat hits down. Sometimes this problem is worse when aftermarket (generic) needle and seat assemblies or bowl floats are used. If the water is rough, and you are only using fuel from the primary bowl, the secondary bowl starts to fill beyond the proper level each time the boat hits down and the float sinks momentarily.

If the boat is used often in rougher conditions where it is not likely that it will be run fast enough to use fuel from the secondary fuel bowl, it helps to set the fuel level in the secondary bowl a little lower.

As a carburetor gets older, other things can go wrong. It is possible that sealing surfaces are warped. It is common for the metering block gasket surfaces on the main housing to become warped to the point where only a marginal seal is achieved. It is very important that the correct gaskets are used to make sure the correct passages are lined up. For your carburetor, I would only use a Holley rebuild kit.

There are antisiphon tubes inside the metering blocks that can fall out internally and cause the engine to run weird. If you shake a metering block in your hand, you should not hear anything rattling around inside. If you do, the metering block is junk.

HOLLEY CARB KITS

QUESTION: I need help with getting the correct part number for rebuild kits for the carburetors that are on my Mercury Racing HP500 engines. A previous owner had the choke horns milled off the carburetors so the list numbers are missing. I e-mailed Holley but was told without the list number they could not help me. Can you help?

ANSWER: The Mercury Racing HP500 engine was originally equipped with an 800 CFM Holley dual-feed (4150 series) carburetor. This carburetor is similar to the list number 9022 carburetor. The Holley "Renew Kit" that will work for your carburetor, as well as most all of the 4150 series dual-feed, double-pumper carburetors is Holley's Part No. 37-485. Do not use rebuild kits offered by other companies. Many times, generic needle and seat assemblies supplied in off-brand kits are not capable of flowing enough fuel for high-performance engines.

Pay attention to the jet numbers in each position on your carburetor. If it has the original main jets installed by Mercury Racing, there are four different jet sizes and must be installed in the original location. The primary port jet is a number 81; the primary starboard jet is a number 75. On the secondary side, the port jet is a number 89 and the starboard jet is a number 93. The original carburetor used a 6.5 primary power valve. The secondary metering block has a power valve block-off plug.

The HP500 carburetor was equipped with the larger 50cc accelerator pumps on both the primary and secondary bowls. Pay attention to the position of the accelerator pump cams for proper lever rate. The primary accelerator pump cam should be light brown and be attached with the screw in the No. 2 hole in the lever and cam. The secondary accelerator cam should be brown and be attached with the screw in the No. 1 hole in the lever screwed into the top hole in the cam.

—Need some guidance from Bob Teague, Powerboat's lead test driver and owner of Teague Custom Marine? To send him an email click here.

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