BP7ES or BPR7ES?

Just did a plug check on my 82 XJ650 Maxim to see how she was burning since my last colortune, and I noticed it has BPR7ES plugs in it, yet the shop manual says it should have BP7ES.

I know the R stands for resistance, but how much? And is there a big difference? Can I just drop some BP7ES in there and go, or am I looking at another colortune after putting in the right plugs?

 

A lot of us run the BP7ES with 5k R caps.

 

My caps don't specify if they are 5k R or not. Even so, I'm not worried about radio interference, so does it really matter?

I read that the stray rfi signals will turn off your C. D. I. 8O

 

The resistors are there not just to help minimize RFI, but also to make the spark last longer, which should allow the mixture to be ignited more consistently... This also makes the spark slightly weaker.

You can unscrew your plug boots and measure them with a multimeter. The stock boots should be 5K. If your boots are 5K, and you're running R plugs, your overall resistance may be too high...

I can't imagine that running without resistors would hurt anything, but I can't help but wonder why Yamaha engineers decided to go with resistor boots to begin with.

 

Thats a question I've been meaning to ask the guru's about.
The stealership and other local wrenchers keep saying "you have to
run the resistor caps", but they can only give vauge answers like,
"thats how they designed it"....like that helps!
But to answer your question untill better minds chime in-go with what
the manual says-hard to get a better idea of where to start then that.

 

NGK resistor plugs are 5K ohms (when new).

XJ650 models are supposed to use non-resistor plugs and 5K plug caps.

You could use the BPR7ES (resistor) plugs and do away with the resistor plug caps (use zero-ohm caps, or just plug "boots"), and that way, every time you change plugs, you're "refreshing" the resistance value of the circuit, too.....

Increasing the resistance will lower the voltage at the plug, but in reality, the voltage at the plug is plenty high enough to fire an good engine in well-tuned condition, even if you up the secondary circuit resistance by 5K or so. The increased resistance causes a weaker spark but increases the "dwell time" of spark occurance....hence a weaker but "longer" spark.

For engines that are tuned a bit lean, especially "YICS" engines, the longer spark "time" may be of benefit for complete combustion, and that appears to be the route that Yamaha took on the later XJ700 water-cooled engines (up to 15K resistance, using 10K caps in combo with 5K resistor plugs). Those 5-valve engines can be thought of as "YICS" engines, except the 5-valve system gives the "YICS effect" over a much broader RPM range...

 

Chacal..I had the same ques as well..my X has DR8ESL at present. It ran good 2 days ago..but would not run ydy for some reason. I see spark on all plugs..but doubting may be its not strong enough. It does hav 5k ohms NGK caps. Could i go with D8EA plugs..if yes..will it make it easy to start !!
Thankxxxxxxxxxxxxxxxxxxx

 

http://www.ngksparkplugs.com/products/p ... covers.asp

 

Unfortunately, it's doubtful. If the bike will not start easily, then there is typically other issues (worn out engine, clogged starter fuel jets, choke system malfunctions, etc.).

 

Thnks chacl, I did take the carbs out for IInd time and cleaned the jets..n put it back on..but i did not know how to remove the choke plungers(4 of them) without a thin spanner as the shaft is on the way..i left it alone.
Upon unsucessful attemps..when i checked plugs..i see some water in the threads..dont know how it landed up there...the plugs were tight enough.
I do see fuel in the plug electrodes.....!!!!!!!!

 

Len, our bikes use low octane fuel, which when exposed to a spark goes BANG, unlike high octane which has a progressive burn, so how would a longer lasting spark help?
Also the E3 PLUG has a very short spark duration, but claims to burn the fuel more effectively.
Wiz.

 

Sorry to nit-pick but, high resistance = lowered amperage, not
voltage.
But thank you for explaining the reason yamaha has applied it
to our bikes.

 

Parts is Right

V=IR

This means that if you increase the resistance, for the same voltage you get a thinner spark. (The "fatness" of the spark is defined by the current, the distance the spark can travel is defined by the voltage.)

You only have a certain amount of electricity going into the plug in each pulse, so you can either have a fat spark and a small gap, or a thin spark and a wide gap.

The reason why the particular size spark plug is chosen by the manufacturers is that it's a pay off between the fatness of the spark and the size of the gap, they've basically looked at the amount of electricity you have, the amount of gas you have to ignite, and the amount of time you have to ignite it in and chosen the one that will give the best burn given this information over the lifetime of the plug (i.e. what they expect it to be like between service intervals. Of course, if you start fiddling with the amount of gas you have to ignite, but tweaking the carb jets, or are happy to change your plugs more often than standard, you may find that a different set up will yeild better results.

I'm not going to pretend to be a plug designer, most of the information you will get regarding plugs is from peoples own experience of trial and error, every bike is different, so there's no definitive answer, what I would suggest is that you change your plugs a little at a time. They're $2 each, $8 a set.. That's a small price to pay for better performnce, a big one for fuel economy!!