OK I have a question to ask here and I am looking to see where this topic leads. I have done some conversions which include 25-30 Homelite's, 31 Ryobi's, 33-38-45 Homelite's, 46 Poulan's, and my latest one was a 55 Craftsman Chainsaw. I did some reading here when I first started doing these engines and it seems to come to the conclusion that the spark plug of choice is the Champion RCJ7Y or a NGK, Bosch, Denso equivalent. I use mostly the Champion plugs myself because for me they are easy to get and affordable. I have had no problems with these plugs and all my engines run just fine with the RCJ7Y, or the RDJ7Y which is the tapered seat version. Here is my question, has anyone tried going up or down one heat range with their choice of plug, Champion, NGK, etc. and what kind of results have they had? Not worth the effort? I understand there will be some smoke from two stroke fuel, thats a given, but will a change in heat range take away some of that with out costing horsepower? Will going up or down in the heat range create to much heat or chance fouling? For those of you wondering, Yes I do run all my engines on a test stand and have used a tach on all of them and adjusted the carbs to get the most from them. I am just wondering the results with out buying a bunch of different plugs and going through the trial and error part myself. I'm sure with all the people here doing conversions, both as a hobby and a possible side business, this has been tested by some. Just curious where every stands on this subject. Thanks to all that reply, everyone of you has helped me learn allot about the conversion process.

Just a quick note... Stihl uses only 1 spark plug in all of their chainsaws.

Has any one tried this plug in their conversion?

Choice of plug has more to do with engine operating temp. than anything else. for instance, a weedwacker that only runs at half throttle with little load on it would soon foul because the plug doesn't run hot enough. Other end of the spectrum, chainsaw working under full load with fins half full of sawdust would need a cooler plug to keep from detonating. Or raising the comp ratio for more power would cause the engine to run hotter, requiring a colder plug. These are just a few reasons. But if the average temp is around 300 degrees F. RCJ7Y is fine.

hi! i just like to ask if NGK BMR6A would be ok for a homelite 25cc?
thanks!

Ernest:
Can't find my old NGK book that has all the equivalent plug numbers in it. Although it sounds the same as the RCJ7Y but not sure whether it has the extended nose.

it is used in a kawasaki 43cc brush cutter. i have seen it in a fax flyer by a hardware store here. its so hard to find a resistor plug here, most of the store only got dj6y and other non resistor plug. i am almost tempted to buy non resistor, but i dont want to compromise safety.

www.sparkplugs.com

i think i wont order online because shipping is too much. a single sparkplug costing $2-4 needs $20-30 for shipping. wow.
RC1 thanks for that link. NGK BMR6A in cross ref is Champion RCJ8. will this work on homelite 25?

what does the Y in RCJ8Y in champion plug means? and what does the P in NKG BPMR7A means?

don't know about the Y, but the "P" is projected tip...A BMR plug is shorter than a BPMR...Only RC engine I know of that uses a BMR plug is a G23..A BPMR in a G23 will result in the piston hitting the side electrode and mysteriously causing the engine to stop when the gap closes..

alw phantom, just wondered how the 55cc craftsman came out?? eazy to convert? what was the performance like? Im thinking about doing one of these soon. got any pics?( sorry to get off the subject ) thanks!

Bill I sent you a PM, I would like to not turn this into an engine discussion. I would really like to get more opinions on plugs, choices, and why.

You really won't find any gain in changing around spark plugs and/or their heat ranges. If the heat range of the plug is correct and the plug is burning clean, that's as good as it gets. As mentioned, a modified engine might like a plug one heat range colder, and a motor that is lightly used might like one one range hotter. What matters is that the electrodes are not eroding, and that the plug is not fouling. This of course assumes that the mixture is correct to begin with. A good looking plug will have a light brown center insulator, and no evidence of high heat such as blueing and/or electrode erosion.

You can not change the burn characteristics of an engine, nor it's power capability by changing spark plugs given that is has the correct one to begin with. I've had great luck with Champion plugs through many years of hot rodding various engines, though often in two strokes the NGK plugs work well also. Just my experience, as I think really most any name brand is fine.

Good luck,
AV8TOR

The way the heat range is mechanically determined is the distance the heat has to travel from the end of the centre electrode to the head. If you take a cold plug and a hot plug side by side you'll see a very short ceramic on a cold plug and vice versa on a hot. A loose plug can lead to engine damage, as it loses it's heat transfer to the head and acts like a very hot plug which leads to detonation. Hope this helps.

I have been involved with engines pretty much for the last 20 years so I know the basics of plugs. What I was looking for here is if someone took a specific engine and a specific plug, then went up of down one heat range and noticed any improvment or not. I got a few different plugs yesterday and I guess I will check into it myself and see what I come up with. Thanks for all the replys to everyone.

Optimum plug is the hottest one you can run without detonating. Let us know what you find.

Guys I was under the impression that any ol' spark at the right time was good enough. Is it the heat of the spark that changes from plug to plug? How does it effect engine performance?

It's the ability of the plug to handle / dissipate the heat.

As said above, to hot a plug can detonate. When close to the edge, as in slow traffic on a hot day and shut the motor off you can get run on where a piece of carbon can glow and cause the engine to run on acting like a glow plug. I remember when I was an apprentice airplane mechanic in the 50s, A private plane pulled up to the pump for gas, plane wes the predecessor to the harvard, mostly plywood with an inverted 6 cyl Ranger inline aircooled engine, some of the old farts can jog my memory as to what it was. Anyway, when they shut the mags off the engine slowed down and almost quit, then reved up like you turned the mag back on, but not so. When the back cyls got hot as it slowed down, theplugs would start glowing and rev the engine back up. When it got a blast of cold air when the engine reved up the back cyls would cool off and shut off again. This would go on as long as it had fuel. the only way to shut it off was to shut the fuel off.

Another point. If your flying along with a big grin on your face happier than a pig in sh## and all of a sudden the engine shuts off you may be running to cold of a plug. What happens is the plug doesn't run hot enough to burn the carbon away. Builds up inside the plug, a piece breaks off and shorts across the electrodes shutting the motor off like a switch. Sometime you can take the plug out, remove the carbon bridge put the plug back in and carry on. You have to look close as sometimes it's about the size of a hair. Hope you don't mind my rambling.

Ramble away ,,,,, it's good stuff !

Thanks that answers my question[8D]

Edit: but feel free to ramble on

Everyone is pretty much on track here, but just to clarify:

The "heat range" of a plug has nothing to do whatsoever with it's spark. As mentioned, it is the ability of the plug to transfer heat away from the tip and into the surrounding head surface in order to cool itself. This is governed by the design of the plug, and usually the length of the center electrode/porcelin. The ideal situation is when the plug is running just warm enough to consistently burn off any combustion deposits and keep itself clean and unfouled.

With the proper air/fuel mixture, and a quality oil at the proper mixture being a given, then the plug heat range is correct when the plug neither fouls, nor runs hot as indicated by plug appearance. Running too hot is dangerous, also as mentioned, as it can cause preignition and engine damage. I've seen holes burned right through pistons caused by this. Note that over advanced ignition timing can sometimes make a plug appear as if it is too hot also.

Once again, there is no power nor other benefits to be gained by changing heat ranges above or below that which works properly. You can not reduce the amount of oil smoke by jacking plugs around, but if you run a plug hot enough you will make a different kind of smoke!

Take care,
AV8TOR

Guys,
I'd be very grateful if anybody could recommend the correct plug for my recently acquired Ryobi 31cc short-shaft engine? It came with some weird numbered thing that doesn't have the 'R' for resistor.

Thanks!
Blue