putting 998 a plus head on an 1100 - problems

[wil_h]

Making the following assumptions:
Head cc, 24
Piston cc, 6
Head gasket cc, 2.8cc
Piston flush to deck

Then you have a CR of 9.4:1

10:1 would be better. But you can't assume that 25thou off the head will do it. You need about 21.5cc in the head chambers. To find out how much of a skim this is you need to put the head on a flat surface and fill them with 21.5cc of fluid, then measure from the head face to the liquid level. This is how much will need skimming off. It could well be a couple of mm.

[mk1rally]

I wouldn't have thought that the difference in cr you have from th eideal will make a huge amount of difference, though a increase would help.

9.4: is quite acceptable but that has been created from a generalisation as Will has orrectly done.

To my mind you have 2 options:-

Take the head off, measure the chamber volume in the head, the dish in the pistons and also if they finish flush with the block at TDC (where I suspect you may well find it may not). Then calculate and machine the head (if there is enough meat on it) to make the CR up to reaonable figure. Then get it on the rollers to check everything out. Is the dizzy ok and correct (well as close as it can be)

The second option is to take it to the rollers and see if you can get to the problem that way (presuming everything else is in line).

If it was me, I'd whip off the head.

[InimiaD]

12g202 heads have 26.1 cc chamber volume.
12G206 and 12g295 have 28.3 cc chamber volume.

[Vegard]

An A+ head is alos inferior to the standard 202. Both inlet valve sizes and port shapes.

[Lord Croker]

I may be stating the obvious here, but before you have any metal removed from the head surface, check the depth of the rocker feed oilway, which is beneath no 1 rocker pedestal, with a depth gauge with a probe ground to a point & then compare the measurement with the external depth of the head from the top surface (where the rocker cover gasket fits) & the lower head gasket surface, you will then know how much metal is available before breaking into the oilway, on a good head this can be 100 thou (.100"), in which case you can safely remove 60 (.060"). You may not need to remove that much, but it pays to know how much metal you have there to play with. I agree with you that the HS2 is a fine carburettor, but I think an HS4 would help you get the performance you are looking for. I hope this is helpful to you & good luck with your project.

[Vegard]

Or if you fancy this....

[mk1]

As has already been said apart from the unleaded advantage, the standard 998 A plus head is inferior to the one you have removed, using this head unskimmed with a single HS2 carb that is presumably on a standard manifold & exhaust will (I reckon) make no improvement to performance, and with the lack of a tune after the change will most likely run pretty much as you describe.

An 1100 engine with an 1100 carb & needle will almost certainly be better for performance.

[Spitz]

I would simply try fitting an HS4 carb and manifold. ( or HIF4 )