Vintage Dirt Bike Q & A

I need to know the plug type and gas/oil ratio for my '74 MT250. The oil injection has been removed, so I have to pre-mix the gas.

Thanks!

The stock plug is an NGK B7ES, but I'd strongly recommend a BP7EVX, Yamaha P/N NGK-BP7EV-X0-00:

http://www.boats.net/parts/detail/yamah ... 00-00.html

In my opinion it's worth the $10 just to see the difference a plug can make, and you may never have to change it again.

I run 36:1 Bel-Ray H1R, which would also work well in your bike.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

sorry but stock plug is a B8ES.

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80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

Hi rayiver,
I have a 1979 cr125r, stock plug NGK B9EV. Is there a better plug that I could be running?

thanks,
nola_fiddy

you can use
NGK
B9ES
B9EV
B9EG
B9EIX
or what i prefer
Denso
IW27

but there´s no need to change, B9EV is a good plug.

_________________
80 CR250Ra
80 CR250Ra
78 CR250R
80 CR80
82 Bultaco Sherpa 340
78 Bultaco Pursang 250 MK12
76 Bultaco Frontera 370 MK10
76 Bultaco Alpina 250
73 Bultaco Lobito 125 MK6
76 Suzuki RM125A
90 Kawasaki KX250

NF,

If you can find it, you might want to try an NGK BP8EVX or BP9EVX plug. Failing that, BPR8EIX or BPR9EIX are pretty much the same plugs in resistor format, and easily available. There's also the BP8ES and BP9ES non-Iridium plugs, which are really inexpensive and would make great try-out plugs. I never have plug-overheating or detonation problems so I'd probably end up using 7-series plugs, but that's just me.

I used the B9EV in my CR125M for years. In fact, I have a well-used one from over 30 years ago that's still working fine.

But, to answer your question... in my experience, the projected-nose plugs (BPxxx, BPRxxx, etc.) give slightly better low end and throttle response than conventional ones, and even start a little easier too. Although I knew different heat-range plugs made a difference in resistance to detonation and fouling, I never thought they made all that much difference in actual motor performance until I tried projected-nose plugs; now I won't use anything else. You really have to try them one-after-the-other to notice the difference, but it's there.

I also feel that Honda tended to supply the Elsinores with pretty cold plugs. Back in the day I was scared to try anything hotter than a 9, but these days I'm much more fearless and went down to a BP6ES for testing in the MR at one point (there was really no difference from the 7, so I went back).

I also prefer precious-metal plugs, as it seems they just last forever. Other than repeated fouling from way-too-rich jetting, the only thing I've found that kills them is ultra-leaded Sunoco MO2X race gas; they develop a high-rpm/high-load misfire and must be replaced. I switched to unleaded race fuel, problem solved.

Lastly, I don't think vintage ignitions really appreciate 5,000 ohm resistors in the spark plug circuit, although local TV/radio reception probably benefits a bit - so I don't use resistor plugs in my vintage motors. EVO and modern ignitions seem to handle them just fine, though.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric

Ray and b-joe, thanks for all the great information. I checked my plug and it is an NGK-B9EG that I got from a local honda dealer. I haven't had any problems fouling plugs. I replaced the carb with a vm 32mm mikuni and painfully rejetted the bike till it was right. Just looking for that something extra. I will definitly experiment with your sudgestions.

definitly printing this page and putting it in my elsinore manual.

thanks,
nola_fiddy

have a b9ev in my 73' cr250, b8es in my 74' mt125. the b9ev is a hotter plug i believe. whatever you end up getting, get the platinum or higher end plug. you wont regret it. better life and power. you wont be sorry.