Which spark plug?

[Active Rascal]

Hi all,

Yes, I have done a search on here, and also on google, topped with talking to friends in the knowledge, so i think i have found the answer, but...

Basically my plug keep fouling (wet/shiny black)....

Now this isn't a simple set up, however the answer is probably simple, and I think I know it, just want confimation. The engine mods are as follows:

CR250 air filter and airbox
Mugen Head
Mugen ECU
Mugen jet needle
Main jet 170 or 175
pilot jet 45
ports etc smoothened
Aktive dual stage reeds
Pro Circuit front pipe
DEP rear pipe.

Current plug used NGK B9EGV (supplied with Mugen head).

Always been ran on super unleaded and Motul fully synthetic oil (have always ran fully synthetic, and want to carry on using that really).

I have advanced the oil pump slightly to compensate for the much bigger main jet. The manual does state I think that going past the line/marker a couple of mm is OK.

I used ball park figures off the tuning guide off here with regards to jetting and then had a play once all running. I ended up dropping the pilot from a standard 148 to a 145, and going up on the main from I think 165 to whatever is in it now, have a feeling it's 170.

Anyway, as already mentioned the spark plug is always fouled, sometimes to the point it mis fires and won't run right. The symptons become worse after a day's riding at an enduro track whereby not opening the throttle much or revving too high. If trail riding it's usually OK.

I think the answer is to use a hotter plug (8) as a compromise. I didn't realise the BR9 supplied is for racing, designed for flat out use. Apart from road work between trails, I rarely hold the throttle open for long periods.

Will a hotter plug be OK for road use with my engine mods? (I assume so)?

Which plugs do people recommend?

I seem to remember running a B8EGV in my old standard AR with no problems and never fouled.

There's also the BR8EG, or BR8EIX (iridium) I have ran in my old bike. I seem to remember the latter being much more expensive, and not even that good.

Think I'll try a B8EGV, but I'm just interested in other people's experience/advice and oppinions.

Cheers,

Shaun.

[knackeredMk1]

I would try a new plug (8) as you suggest also try dropping the needle in the carb by one to begin with, or possibly two, stops and see how that goes.

I've found Iridium plugs foul more easily but I bought a few when I rebuilt my Mk1 so I still use them .

[Active Rascal]

Bugger I forgot to mention:

Usually the bike revs out high, clean and crisp, no bogging really or anything when riding (unless plug fouled).

Also have had it dyno'd to check the jetting, curves on the graph seemed to be OK, and the chap doing the dyno run said it's near enough bob on and he wouldn't change anything, wouldn't get the jetting much closer at all.

Hence me thinking it's the wrong heat range plug for my riding.

I wonder if with having a amaller pilot jet, and advancing the oil pump could be causing the plug to foul at low revs, however in saying that the pilot is only idle to 1/8 throttle off the top of my head, so unlikely!!

[knackeredMk1]

I stick to what I've said above.

[Active Rascal]

I'll try just the plug first, but you're thinking it's too rich around half throttle?? I'll see how it goes as it's a big job to take the carb out, entire back end off unfortunately!!

[Active Rascal]

Another thing in comparrison to my old CRM while we're here:

The gearing on this one is 14:51; which is similar to what I used to run; 13:48, infact my new bike is actually geared slightly higher.

My old and fairly standard AR would comfortabley do 80mph+, however the new one seems to scream at 65mph, and getting near 70 sounds like it's dying to change gear.

My old AR gearing did about 90mph geared 13:45 before I changed it to 13:45.

Can anyone explain about the new one? I know nothing can really be done. I would expect top speed to have been higher than the old standard AR? It seems as if it only has 5 gears, but I think I did actually count 6 up once to check haha. Maybe this should be moved to a new topic?

Shaun.

[knackeredMk1]

Two things that strike me as possibilities :

1). All the enigne modifications you have done might have made your engine more of a 'mid range' kind of power . Particularly your addition of length to the first part of the expansion chamber. Not sure of how much you added or what degree of effect this would have but I know that lengthening this part does limit top end and over rev.

2). Somebody put a Mk1 gear box in it . Is the change up from 5th to 6th really only like an overdrive

[xander77]

Oh yes on the longer front section, I only know this as my mate bought RD250 expansion chambers for a RD400 back in the heady 2 stroke days, he had to cut the down pipes down to make them fit, bike well revved out high and made more top end power, after looking this up longer front makes more low end, shorter front more top end, maybe that's why trail bikes like the CRM have that big curve on the down pipe section!!

[helisupp]

6 up, must be seven speed close ratio box.

[Active Rascal]

You got me there Mark!! You know what I meant!!

Interesting Steve and xander!! I wasn't sure what this would do. Think I had to have a collar made 10mm longer so that the exhaust would fit. Somebody did say to me it wouldn't make much difference, but I didn't have a choice. I've heard also the taper angle of the outlet also makes a difference to the power in terms of low/mid/top end range, as uzushio do performance collars for the CRM exhaust, and you have to specify which area you would like to increase your power.

It's not really a massive problem, and nothing I can change, but this could be the reason for it!! It's only on fast roads I notice anyway, just seems to be revving extremely high.

Not sure about the mk1 g/box theory, the engine was fully rebuilt by LT, would have thought they would have noticed?

Cheers.

[buggsywabbit]

to be such a dramatic change in speed and gearing , sounds like it has been tamperd with for track use , or some other reason ! normal :- nutral, 1st d , 2nd u ,3rd u, 4th u, 5th u , 6th u

[xander77]

Yeh your right the front taper does affect it as well, I only know about the longer front pipe, but when you get into it the whole exhaust length and shape will affect the power, way more than a 4 banger! the change on my mates RD400 was quite noticeable, as in his was way faster top end than normal OEM or after market pipes because of the shorter down pipe, not much difference in the first part of a flat out race but when he got into the higher gears his would pull away!! with more power top end, and that was the only tuning he had done, no porting or high compression.

My CRM is standard and I have run 8`s and 9`s both worked really well, though the 8`s were standard plugs and the 9`s have all been iridium ones! when I got it someone had drilled the side plate of the air box, I filled those in, (glad I did when the water in a "small puddle" came up to the seat!!) but if anything mine lacks top end power, at this age could be the tail section is blocked! or just age, yet to properly investigate this this yet! every time I take the exhaust off I forget to check manually how far the power valve opens!I might try another tail section just to see, But I do like the quietness of the standard end can!

[pudzie83]

i run denso wr24s in my crm for some reason my crm loves them starts easy never miss fires tryed ngk 8s,9s and it didn't agree....pig to start didn't run as crisp

[Mudfacetony]

i run denso wr24s in my crm for some reason my crm loves them starts easy never miss fires tryed ngk 8s,9s and it didn't agree....pig to start didn't run as crisp

What is the rest of the code for your WR24? FSU4038, ESRU, ESU-4030, FSR? I would be interested in trying an ND plug as I've only ever stuck to NGK

[pudzie83]

WR24 ESR-U think there around £2.50 each