Hi all, at the moment I am riding a 250 Mar in comps. Its a great bike, does everything it should but i think its lacking that something extra in the engine department. Its on its first oversize with new piston and rings, 26 mm Mikuni and thinner head gasket. The engine pulls from nothing, but just seems to lack that sharp response when power is needed. So my first question would be, is this a characteristic of the engine? Secondly I have a 250 gripper engine sat in the corner of my shed and was wondering would I get much benefit fitting this in my Mar chassis, and would it cause me much hassle?

Cheers HIGGO.

Yes, the MAR is a lazy responding engine. You can sharpen it up by fitting an enduro flywheel which is about 1.5lbs lighter, or skim a spare MAR flywheel, fit a thin copper head gasket of maybe start with a 1mm gasket instead of the usual 3mm thick spacer. The engine will pick up quicker but the trade off is that it may stall easier at lower revs.

The MK1 MAR picked up a bit quicker than the MK2 onwards as it had a slightly smaller crank assembly, hence lighter.

The Gripper engine is the same bottom end but with better selector shaft and layshaft bearings. The cylinder porting was different from the MAR and was more motocross orientated for quicker response and more revs, although the MAR will rev out pretty high too. The problem you'll have fitting the Gripper engine is what to do for an exhaust. It uses the motocross type expansion exhaust and no way will you get that onto the MAR chassis. It may work with a conventional exhaust but with the taller fins on the head and the high inlet port, you may have trouble designing an exhaust to fit

First thing I'd try is lightening a MAR flywheel, fitting the thin head gasket and making sure the carb is in good order.

Higgo,

I have an alloy cylinder head gasket i made out of sheet about 1mm thick and softened it. Also i drilled out the clutch basket to lighten the flywheel effect too and the gripper 250 has a flywheel over the cushdrive spring which i have had turned down so it is about the same as your MAR. All this has made my gripper more revy but if i go any more i will loose its bottom end softness which i feel is its advantage when its slippy.

As Woody states you would have a problem with the front exhaust from either bike. If i was you i would get hold of a second flywheel and mod that. Also make sure the timing is spot on and crank seal on flywheel side is good- easy to replace when flywheel is off.

Steve.

Higgo - your advert for a MAR flywheel

You need to specify which one as there are two types. Although they all look the same, the timing pick up is in two different places. On the early bikes, the hole that the timing pin uses to locate in the stator to set the timing is at about 8 o'clock at TDC. On the later flywheels, around 1974 onwards, that hole is around 5 o'clock as the stator is different. They aren't interchangeable, you have to have matching stator / flywheel

Hi Woody, excuse my ignorance the flywheel i am after is the one where the hole, for the timing pin locates with the stator at 5 o'clock. I will amend my wanted advert. Thanks for your help. And you Steve.