On my mc21 fitted with a tyga 300 kit I am having issues with the front tyre hitting the spark plug cap. Now I don't believe the problem to be solely the centre position of the spark plug on the nsr150 heads as when I bought the bike it had a fairly well destroyed spark plug cap and was still only a standard 250cc back then. However, fitting the 300 kit could not have made things better.

I do not want to go to an (ultra short) RS plug as they are quite expensive and I read they don't last particularly long, especially on a road bike that is not raced. This may be a solution, though, but I wonder why it was hitting originally (as a 250)?

Could the tyre be too high a profile or the springs be sagging to much, for example? (I don't think either of those are reasonable explanations, btw.) The forks are up about 1.5mm in the triple clamps, I could make them flush with the caps though 1.5mm shouldn't make too much difference, right?

Ideas?
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MC21 Honda NSR250. TYGA 300 conversion.


Last edited by anonymous.shyster on Thu Jun 09, 2011 12:58 pm; edited 1 time in total

Are you sure they are nsr forks and not vfr ? Are you using the br9ecm spark plug , put a cable tie around the fork tube and go for a ride to check the fork travel . If you are wheeling , the front could be bottoming out as the wheels slams back down .

Are you sure they are nsr forks and not vfr ?

How to check?
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MC21 Honda NSR250. TYGA 300 conversion.

If the 150 heads are like the 250 heads, you need to fit them so the plug angle is offset sideways, not straight downwards.
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No, the 150 heads I have are like the HRC head conversion, with the spark plug dead centre.
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MC21 Honda NSR250. TYGA 300 conversion.

The 400 forks use a 298mm disc and the stanchion is shorter too . I think you may have to use the rs plug , just trim your rubber cap shoter to use the rs plug .

anonymous.shyster wrote:I do not want to go to a short plug

That's your problem. The short seat height makes all the difference.

I think he means he doesn't want to go to the RS plug? But yes, double check you have the ECM plug.

If he meant the RS plug he should have said so.
When I made the mistake of using a NON "CM" (short seat) plug, I also got the front wheel rubbing on my plug cap. (Using Tyga 300 heads) Changed to the correct plug, problem solved.
Naturally I assumed from his comments he has made a similar mistake.

Right, yes. I did mean I do not want to go to the RS plug, as they are expensive.
Running BR9ECM plugs currently, and it is hitting. Makes me think suspension/geometry?
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MC21 Honda NSR250. TYGA 300 conversion.

Well, the bad news is that all stock NSRs are basically rooted. Stock **** suspension, old saggy springs, and a setup aimed at little Jap teenagers rather than Aussies who've had a bit much beer (not the designs fault, you see how much your bits sag in 20 years )

There is hope though! Try and set your rider sag (plenty of guides all over the net) and see what that tells you. It'll probably tell you that it's basically sacked out and needs a little suspension love. This is good though! Springs are cheap and easy to fit (well maybe not easy on the back but you'll manage).

Won't make up for the deficiencies in the dampening mind, but one problem at a time. Don't even both dicking about with the geometry without fixing those fundamental flaws first.

Thanks, but I have the sag set pretty well. I am pretty certain the last owner put in stiffer springs too.
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MC21 Honda NSR250. TYGA 300 conversion.

Well then, you might have a problem! What is the front and rear sag at currently? Perhaps we can compare it with Phils for you? No fat in the caps that could be trimmed by switching to another type? Have you tried Neal's suggestion with the cable tie?

I've been through the very same issue myself so here's what i have to add:

I'm using an RVF400 (upside down fork) front end which has less offset than standard so that brings the wheel closer to the spark plug. The 150 cylinders are taller than standard and the big BR9ECM plug poking out the center makes things tight. I did however get away with it whilst i had a 120/60 Michelin Pilot Race on the front wheel. It was only when i put my wets on (120/70) that i started touching the plug cap when on the brakes.

The short RS plug and cap solve the problem immediately but as you say they're very dear (i get mine: Denso IAE01-32 from R S cycles in the US). I now run 120/70 size front tyres for both wet and dry.

You could try a lower profile tyre if you have a 110/70 (i assume you have the standard, narrower wheels?), but i don't know how easy it is to get a 110/60.
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Neal wrote:I think you may have to use the rs plug , just trim your rubber cap shoter to use the rs plug .

Can you do this? I was under the impression that I needed to buy a $60 spark plug cap to use the RS plug.
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MC21 Honda NSR250. TYGA 300 conversion.