Motoscrubs.com

Hi all, I need some advice. I've again been reading through the forum for a few days, trying to figure out where to go with my top end, and now my own head is.....again spinning. That's a lot of info.
My motor is a 250 n/c, which has a cyl head with a 25.5mm inlet port with casting marks (meaning no die grinder has been into it). Intake valve 36mm ex valve 33mm. I ran a cam with a grind for a 350 Sebring. The piston is an aftermarket Asso (distance from center of wrist pin to crown is 36mm). There is the 1.5mm spacer under the barrel, screw adjusters on the rockers and a Mikuni 28mm carb.
I accumulated 600 miles with this set up! It was horrible, and I don't recommend it....I would be surprised if the motor made 10 horsepower.
Well how's that for full disclosure? What an ass you're probably thinking, and you'd be right. But I call it a learning curve, those were the parts I had, that was the place I was in....so I went with it.
Fast forward 6 or 7 years, now I'm in a place where I can try again.
So I'm looking to make it a Sunday rider that can be in traffic as well as out in the countryside. I don't want something that needs to be wound up tight to access the power, rather something that is entertaining and can pull away from a stop without cranking it up.
If this is a Monza head that has the wrong sized intake valve, I can remedy that. Is it a Monza head?
If I buy a Monza piston, does removing the 1.5mm spacer from under the barrel mess things up? Clearance issue?
Is there an aftermarket combination that works well with the Monza head, or does one simply have to remain within the parameters of Monza spec?
Pete.

Pics are comin' just trying to figure that part out.....

I dont think there is anything much wrong with your port,valve and carb sizes , these are nearly the same as the best overall performance 250 , the American Dianna. This would give a 650 a fright with its acceleration and top speed. You need to aim for a properly measured compression ratio of between 9 and 10 to one and a more sporty cam . Its also likely that your silencer is too restrictive. If you could get the head /piston working to give a squish effect , even better. Some mild Inlet port work up to 28 mm that does not weaken the inlet guide support too much will help. A strong spark essential. Any modifications aimed at increasing the power at over 8500 revs will not be helpfull as the mid range and big end life will suffer. A red or grey cam is about as far as I would go for a road bike thats going to get lots of use and not be an ornament. Forget talk of 27 hp (sales talk) if you can get 23/24 you will have a 90 mph bike. I once did 104mph with this level of tune.

Hi Pete

what piston are you using ? is it the low compression 4 ring cast Asso piston as used on a 250 Monza ?

BTW Sebring camshaft is the same as a Monza cam .

Eldert

Hi Pete

you have a low compression piston fitted . the 10 to 1 Mach1 type piston is 41 mm centre writpin to top of dome

Eldert

Thanks you guys for sharing your knowledge.
Did all the 250 n/c heads start life with the same volume combustion chamber? I ask because I have another head (ebay special) which is 32mm across the inlet port at the carb flange, has 36 & 33mm valves, and measures 74mm. Possibly its the better candidate from the porting point of view. That said, the head which measures 25.5mm across the inlet port at the flange received new valves, seats & guides only 600 miles ago. It's just that I doubt I should be attempting to get hopeful with the die grinder.
Here comes a pic of the Asso piston which I ran....

Your spare head with the 36mm inlet valve is already bigger than is usefull for an alround rider. Big carbs and ports will slow down the air flow at the revs you will be using most of the time and make starting,slow running and fuel atomisation poorer than they could be . Its a 250 .

Thats what I suspected. Thank you!
Now onwards to source a piston/cam combo, and an experienced person with a flow bench & the die grinder
Pete