Roper's Motomoda racer - smallvalve?

[rolf j]

Hi Pete R!
I seem to remember you mentioned way back in time that you built this race bike with rather small valves, right? What cam did you match with this setup? And will it swallow a pair of PHM40 carbs?

In the long run, say 2-3 years, I'll build a completely new engine from a square fin 1100, but meanwhile I'll have to use what pieces of ol' roundfin stuff I have in the shed to make something that can work reasonably on the track until then. A cost saving and pretty homespun effort to say the least!

I intend to use 78 crank, 88 bores and the smallvalve heads that I got available in order to keep costs down. I have a number of pistons in stock now, ranging from the flat Sp1000 up to pretty high domes (that probably will need to rest for now). The trial/error excercise now will be to find out if my Megacycle 620x8 that I have lying about will make the valves knock in the pistons by such a margin that a dremel can't fix it. The main question is really whether such a cam will have an effect in this otherwise very standard engine compared to the milder ones I have, such as the standard cam or the V7Sport?? Sinking the valves and reshaping the chamber is probably out of economical bounds for now, as well a change to bigger valves. The next issue is that I only have PHM40 carbs available. Can the small valve heads utilise such a throaty big carb? (Will of course grind the intakes to fit smoothly).
Any point making this effort at all, or perhaps better off with letting the Sp1000 mill work in standard setup until the bigger engine building project takes off?

 

[Penis Rotor]

In it's last itteration it was running mid valves front and rear. With hindsight I would prefer to re-seat the exhaust side and use the 1mm smaller valves from a smallvalver due to their better head back-shape.

You can build something pretty good out of a smallvalver. I'd suggest that while you'll be able to get the 40's to work it will be difficult to get them to work everywhere! There will be a lot of metal requiring removal from the inlet and exhaust ports too but it's all doable and worth it for shit and giggles

As for the 620X8? Not sure. You'll have to dummy it up and see if the valves tangle on overlap.

Pete

 

[rolf j]

Thanks Pete! Planning continues! (for shit and giggles... I love that!! :lol: :lol: )

 

[jr1967]

Note that with your 78mm crank even the V7sport cam will touch the inner conrod bolts at high revs due to the camshaft flexing, even though on the workbench you seem to have enough clearance. There are pics all over the web of the results of people that weren't aware off this fact. Just grind an angle on the bolt heads, and take some metal away, it's no big deal.
My midvalve LMII with 90mm pistons, 1:10.5 compression, 1mm squish, a V7sport camshaft and 40mm Dello's with velocity stacks went up from 57 horse to 70 rwhp, with torque between 70 and 80Nm in the 3000-7500rpm range. Very nice. With K&N's the same bike did 66rwhp but tapered off at 7000rpm.
I don't know what that Megacycle cam would do though. Good luck, JR

 

[rolf j]

Note that with your 78mm crank even the V7sport cam will touch the inner conrod bolts at high revs due to the camshaft flexing, even though on the workbench you seem to have enough clearance. There are pics all over the web of the results of people that weren't aware off this fact. Just grind an angle on the bolt heads, and take some metal away, it's no big deal.
My midvalve LMII with 90mm pistons, 1:10.5 compression, 1mm squish, a V7sport camshaft and 40mm Dello's with velocity stacks went up from 57 horse to 70 rwhp, with torque between 70 and 80Nm in the 3000-7500rpm range. Very nice. With K&N's the same bike did 66rwhp but tapered off at 7000rpm.
I don't know what that Megacycle cam would do though. Good luck, JR

Thanks so much for the tip! I'll definately check this!
Before I had my top-end disaster by the end of the season the engine was somewhat similar to yours: midvalve heads fitted home made by the look of it and a domed psiton to make a comp somewhere in the region of yours, 88 bore, 78 crank, V7Sport cam and I set the squish at a somewhat risky "less than" 1 mm. The PO had fitted PHF36s but I ported inn a set of PHM40s which turned out to have ONLY advantages at all revs! Never tried it with KNs, but used 70mm stacks. The bike ran very well through the entire register, but seeing I need to replace so many parts now, and I in the mean time have aquired a Megacycle x8 cam, the idea of building a smallvalver came up. I have by now heard at least two racing-guzzisti having had positive experience with such a setup. Curious to see how it works!