I have several gas engines, small to large. They all have 3/4" socket head plugs. The plugs have gotten switched around over the years and some engines do not seem to perform as well as they did. I know all about carbs, electronic ignition timing, fuel, ect... The compression and bearings are good on the engines in question. Is there a rule of thumb on what spark plugs should be used on certain size or brand of engines? They all have CH or Rcexel electronic ignitions on good 4.8 v batteries. Seems to loose about 800 rpms after running full throttle for a minute and I know I'm not running them lean. Richening it up a bit on top end makes it less peak rpm. When making a diving run it doesn't seem to unload like it used to. The fuel used is good and doesn't affect other engines. I'm having this problem on a BME 44 and on a 4.2 Sachs engine. All carbs kits and filters are new and clean. They run smoothly at all RPMs so there is no ignition or carb setting problem. Would the wrong plug cause this because I'm stumped at this point. Any input helps. thanks, Paul

Hmmm! That would have me stumped too. Just speculating, maybe gap? How old are those two engines?
Edwin

There are two spark plugs that fit these engines. The plugs are interchangeable in the engines but not in the shielded caps. The treads and reach are the same on both spark plugs.

The smaller exterior spark plug has a tapered seat. NGK BPMR7F

The larger exterior size spark plug has a gasketed seat. NGK BPMR7A

The NGK BPMR7A spark plug should be in stock at most any auto parts store that carries NGK. The BPMR7F is usually not stocked locally but they can get it for you - often by the next day

All my engines have Champion, Bosch, even some with no name but just numbers. Some have 8's or 7's on the number. Are the plugs not just the size, but different heat ranges to them? Will the NGK BPMR7A work on any engine as long as it fits the hole and the cap? I have US 41, Quadra, Poulan, Zenoah, BME, Husqvarna... They mostly have different plug numbers but they are different brands too, so that's why I'm confused.

All the ones that used the old Champion RCJ6Y, RCJ7Y, Bosch WSR6, etc

Keep in mind that heat ranges between NGK and Champion are backwards from each other

The "R" is for resistor type plug.

Most gasoline engines that we use on our planes use a 5-7 heat range plug. These are medium/cool heat plugs. Watch out running the 2's and the 4's because they are a hotter plug and I have heard that you can burn a hole in the top of the piston after extended use. As far as "unloading the prop" goes going downline or in a dive and having poor performance issues I would take another look at how I re-assembled the carb...sounds like fuel to me. Exploded views are available for our Walbro carbs at www.ereplacements.com
Good luck, I am just trying to help you.

I had a problem with a stock carb years back that turned out to be the fuel pressure regulator adjustment was wrong. It was starving my engine. Took 5 times of trial and error adjusting the fork on the float type valve but it was finally running better than it ever had out of the box. Its rare for that, but not impossible. That was a last resort test, after I had already changed everything ignition, changed timing, checked for air leaks all to no avail. Ordered a new carb but fixed it before it arrived.
Edwin

Just my 2 cents on this, use the plug designed for the application. If it was designed to run on a Champion XYZ, use the Champion XYZ, and don't deviate. Small engine ignition systems aren't exactly picky in the same way automotive ignition systems have become but its still one less thing to worry about. Since you've interchanged plugs I wouldn't be worried so much about the brands as the differences in things like heat ranges and electrode depth etc.. having been installed in engines designed for something else.