[BossHemi429]

Most of the Gas smells come from not having the carb tuned in as has been said by many. A simple rule of thumb that was told to me by Holley Tech Guys some 25 or 30 years ago still applies today. If you have any kind of performance engine then of course there is a performance cam installed. When you have a performance cam then the engine vacuum changes. Most Holley Carbs except the Performance Carbs like the HP and Ultra HP Series come with a number 65 power valve in the metering block. This is for pretty close to stock cam engines. Holley Tech Guys say you need about 12 to 13 inches of Vacuum for this size power valve to work properly. What it does is allow for more fuel to the engine for start ups and cold starts but a one point when the engine is warmed up the Vacuum Signal allows the power valve to close. When you have a bigger cam you may only have 10 inches of vacuum or maybe your engine only makes 5 inches of vacuum. If you don't have 12 to 13 inches of vacuum at least the number 65 Power Valve can't operate properly and that is when you have the extra Gas Smell. Your carb is in an over rich condition and your engine can't burn all the fuel it is getting. Changing the Jets don't help this much either because it is a different part of the carb that is causing the problem. The jets are close for the engine size. The power valve is the problem and is why you have to keep putting in new Spark Plugs too often. I ran a 460 in my 1973 Mach 1 for about 23 years on the street with two 660 Center Squirters and only changed the plugs one time. Once I tuned the carbs and got a good Crane Ignition System Installed it ran great and I could start it in NC winters after sitting for weeks at a time and get out of the car within a minute or less of warm up time and the 660s don't even have chokes.

The same problem is true for fuel injection too. If you have fuel injection you need about 17 to 18 inches of vacuum for the signals the whole system gets for the fuel injection to work well. Some cams may be designed to allow for more vacuum but lack of vacuum is the cause of more problems than guys realize.

Holley had a simple way to find out what size power valve that was needed for a particular engine. No matter what cam was installed. At least for Street Engines and Street Strip Engines.

Put a Vacuum Guage on the Full Vacuum Port on the Carb. Most Holley Carbs have a Full Port Nipple under the front of the carb. If you have a Port Nipple on the upper side of the Metering Block on the Passenger side this is Ported Vacuum for the Distributor. Anyway, with a Vacuum Guage connected to the Full Vacuum Port under the Carb or directly on the Intake Manifold your reading should be 12 to 13 inches if you are going to use the number 65 power valve that comes in the carb. If it is less than that you divide the number you get by half and install a Power Valve as close to that number as you can find. Example, if your vacuum reading is 10 inches of Vacuum then you install a number 5 power valve. If your reading is only 5 inches of Vacuum you install a number 2.5 Power Valve. Holley says even if there is no vacuum you should try to use at least the smallest Power Valve which is 2.5 because it is better to use a power Valve than not. They do offer Power Valve Block Off Plugs for Race Engines.

I like a carb myself and would only use a carb on a hot rod or performance engine. There are many carb companies today offering their version of the Holley carb with their own tricks. Holley now has an HP and an Ulta HP Carb and I can tell you first hand that the Ultra HP is an Awesome Carb. The Holley Instructions Tell You Right Off, Don't Adjust the Idle Screw. It is adjusted where they want it. Changes are made elsewhere. We had an Ultra HP on a 423 Stroker Small Block Chevy Eng a few weeks back on the Engine Dyno and it was such a great carb. It idled great, it responded great, it started the engine after sitting a day or two great. We didn't have to do anything out of the box to the carb and the throttle response was great. For a 500 to 600 HP 427 Cubic Inch Engine I would probably choose the Ultra HP 750. For anything less the Ultra HP 650 is good to 500 plus hp on smaller cubic inch engines. I called and spoke with the Holley Tech Guys a few weeks ago about two Ford 427 Stroker Engines I just built and one 331 Stroked 302. Both the 427 engines are built with AFD, (Air Flow Dynamics) Aluminum 351 Cleveland Style Heads I import from Australia. The 331 has World Products Windsor Sr Heads. All three engines have Crane Solid Roller Street Profile Cams and they all are Pump Gas Engines built to run but also to drive daily with good dependability. The other engine I am working on right now is a 350 Chevy Performance Engine for a 1955 Chevy Wagon. Holley recommended the Ultra HP 650 for this engine too.
I hope this info helps some of you who might have questions. I know there are some of you who know much more than I and hope you agree with what I have said.
Sorry if I got carried away and wrote too much. I am new to the site and sometimes I do get carried away with trying to put out too much info. Mike.