Of spark plugs and power valves.

[Vic]

I recently posted the following in the oriental bikes forum. I got one partial answer on the pv question and quite a bit of discussion, but nothing definitive on the spark plug question. I know it's taboo to post the same thing in different forums, but I'm willing to face public humiliation and disgrace in my pursuit of enlightenment.
I know there's someone out there who knows the answers.

Questions for RM experts.
I picked up my ' 02 RM 125 yesterday and I'm really liking it, but I have a couple of questions.

Spark plug- The stock plug is an NGK R6918B-8 . Since I refuse to pay thirty bucks for a spark plug that won't clean, gap or change itself, I'm wondering if a BR8ES is an acceptable alternative. Particularly, is it the same heat range?

Power valve spring preload adjuster- The only thing I can find about it in the manual is that it's supposed to be turned in one turn. How far in is it acceptable to turn it? I assume that increasing the preload on the spring will either delay the opening of the power valve or limit how far the valve will open. Which assumption is correct? I want to experiment with it, but I don't want to damage anything by tightening it too much. I already tried turning it in one half turn, which really smoothed out the power. I like the reduced hit for woods riding. Am I limiting the top end by doing this? Can I safely turn it in even further?

Thanks in advance for any help. :cool:

 

[Old CR goat]

Vic, I'm certainly no expert, but I did stay at a Holiday Inn Express once!:)
I had a 95 RM 250, that I ran a BR8EGV and BR8ES Had no problems.
That bike also called for a special construction plug. I used the others at the advice of a local Suziki dealer that I trusted.
I checked the NGK site and best I can tell thay are the same heat range.


(maybe that was a BP express when I slept in the car):confused:

 

[RM_guy]

Vic,
This is a note pertaining to the sparkplug issue that I sent to BigLou last spring:

<snip>
I have always wondered about the reason for using the expensive plug. Luckily I never owned a RM that called for it. So I called Eric Gorr for the straight scoop. The expensive plug uses a straight over ground strap as opposed to the bent ground strap on an ES plug. I guess Suzuki stresses the plug quite a bit and the bent strap doesn't hold up as well and breaks off over time. Eric said the 125 had the problem and he didn't think the 250 did. They probably spec the same plug to be on the safe side. Eric did say that ND makes a comparable straight over ground strap plug that is cheaper. The crossover is probably available at their web site.
If he does decide to run the ES plug anyway, I'd replace it frequently.
<snip>
I never found the ND website but if someone does, please post it here.

On the powervalve question. You won’t break anything by over per-loading the powervalve spring. The stock pre-load will let the PV open at a set RPM. The spring pre-load keeps the valve closed until the RPM have built up enough to overcome the spring force. As you add pre-load (turn the adjuster clockwise) the PV will open later. Reducing the pre-load will open the PV sooner. How much pre-load you need is based on how you want the motor to act. Through trial and error you can optimize the timing of the valve opening base on your riding conditions to have a smooth transition (typically lower pre-load) or a “hit” as the top end power comes on.

There is a point where you can have too much or too little pre-load. Too much pre-load and there will be a flat spot in the power that acts like a lean condition that is caused by the engine not being able to breathe since the PV is closed when it should be open at higher RPMs. Too little pre-load and the valve will open prematurely, loosing bottom end power. Too little pre-load will also allow the valve to open and close by it’s self as the bike bounces around the track. These conditions appear to be jetting issues and you can end up chasing them to no avail, trust me.

Keep in mind that the PV is used to get the best of both worlds, low and top end power. Adjusting the pre-load can help you customize the power delivery but the motor is still limited by the port size and port timing. If you still don’t get it to run the way you need by adjusting the PV pre-load, consider sending the cylinder to Eric for some massaging.

Good luck

 

[Vic]

Thanks, guys. I'm mostly convinced that it would be okay to run one of the regular plugs, as I normally run plugs for less time than it takes to read an average Truespode post.:confused: (slight exageration)

I'm still confused about the heat range, though. A BR8** is normally specified for 250s, while the BR9** is for 125s. It's possible that the RM 125 would want a hotter plug than the others, but I tend to doubt it.

 

[Rich Rohrich]

Originally posted by Vic
BR9** is for 125s. It's possible that the RM 125 would want a hotter plug than the others, but I tend to doubt it.

Vic - Unless you are riding in some really wide open areas it will be pretty tough to get enough heat going to require a BR9xx plug. Mortals get by quite nicely running BR8xx in the majority of cases.

 

[Old CR goat]

I remember reading (I think on Eric's site) an article on plug reading, that said depending on riding styles a hotter plug may be needed. I do run a 9 in my sons KX125 because an 8 (what was in it when we bought it used) burned off too white, the 9 reads fine. He revs that thing a lot more than me old butt does.:)

 

[Vic]

I feel pretty safe running a BR8xx, but I'd still like to know if it has the same range as the R6918B-8 .

 

[Junkie]

NGK site

Click the link, I knew I saved it for something.

http://www.ngksparkplugs.com/techinfo/spark_plugs/

 

[TM-Frank]

Originally posted by RM_guy
...I never found the ND website but if someone does, please post it here...

and here we go: www.nippondenso.com

Sparkplugs for motorcycles, karts, personal watercraft etc. are listed behind the "Iridium Power" button.

 

[Vic]

I have seen the NGK site and, unfortunately, it doesn't help me, here.:(

ND does not list a plug for my application or even a direct cross for my stock plug, but based on the plugs that are listed, it does look like the NGK plugs with numbers ending in ' -8 ' do, in fact, have the same heat range as the BR8xx plugs.

 

[cr250john]

Technical Information on NGK plugs

This link requires the free Adobe Acrobat Reader but has a lot of information on plug designations.
Click HERE

 

[nate_z]

RM plug

Your stock spark plug is a heat range of 8. The -8 in the plug number denotes the heat range. There is also a R6918-9 and KX 85's and 100's come with a R6525K-105, which is a number 10 heat range. It isn't abnormal to run an 8 in a 125. In 1998 and 1999 KTM's 125 SX has the exact same plug as your RM. I would probably run a BR8EVX because those are around $8 and the bike seems to run a little better than with a BR8ES. Definetely keep an eye on your plug readings though.