The Sensible Twin

[Grumph]

That motor has a .020in thick copper head gasket. It was with one of the engines I got from Husa.
It was annealed prior to use.

Any leak may not be the gasket. I relinered it to suit the bigger pistons.
There's a possibility a liner's sunk. The block was decked to give about .035in squish.
When I inserted the new liners I let it cool with a weight on top so every precaution
was taken.

Let us know what you find please.

And mark the cams for reinstallation please. I took some pains to get them on 103/104 lobe centers

[Sketchy_Racer]

I got the the stock CV slide carbs working pretty well, enough to give the bike a button up and take for a road test. Initial reaction is that it pulls much stronger lower in the revs than I expected, but as soon as I really started to build power and revs, the fueling went way out and the motor spluttered. It would rev out to 16k cleanly at part throttle. On the way back on the test ride I started getting pressure washed by the coolant overflow.

Initially I though it might be pump, thermostat or radiator cap but after ruling them all out, it kept pushing water under engine load.

So tonight Kyle and I dropped the motor out of the chassis and pulled the head. Pretty clear on the gasket that it's been leaking around the thinned section of the top hat on the sleeve.

Personally I've never had much luck with copper gaskets, especially in an open deck without a fire ring, so after some measuring up both the cylinders and head are flat, it does not appear that the sleeves have moved at all. We decided since I have a couple spare head gaskets floating around to use a standard gasket opened out to suit the bore. It just stays within the pressed deformed detail in the stock head gasket.

Also noted was the pistons were kissing the head ever so slightly.

As you mention Grumph, the copper gasket was .020 (0.5mm) and the stock head gasket is 0.2mm. So in order to resolve the thinner gasket and kissing the head, I machined the squish are of the pistons down a total of 0.4mm. This will also give a slight increase in comp, which recall in this thread was somewhere around 12:1?

Moving the cylinder deck height 0.3mm will move cam timing so I will get the degree wheel out and make sure we get it back to the 103/104 you originally had it set at.

Next step will be set cams up, measure squish, valve to piston clearance (I didn't make the piston pockets any deeper so will now be approximately 0.3mm closer) and finally see what comp it has ended up at.

[Grumph]

Good work. Yeah, original com I think was 11.8.

Tad surprised pistons have hit - but 16 grand I suppose and they're a bit heavier than OE.
I widened the valve pockets by hand with a die grinder which appears to have worked OK.

Carry on.

[quallman1234]

EFI Bits and bobs are here.

We've gone with 30mm bore throttle bodies per cylinder, as the OD is the same between a 28mm bore and 30mm bore for this type of throttle body.
Taking in mind that we do not need to atomoize the fuel charge with injection.
Should almost go straight into the inlets, maybe a slight tickle up.
Injector's are 125cc/min. Which from our expirence will be in the ballpark.

We have the ecu/o2 controller already, so just waiting on the fuel pump, but can make a start on the wiring loom.
We will also be building a dashboard. Which enables remote tuning and datalogging (really!)
Heres an example of the CBR100T setup we've just done. https://www.youtube.com/shorts/tH05Ufmyez4 (its in launch mode in the video)
Planning on a nicer mounting system for the sensible twin. I 3d scanned the front of the bike last night when Sketchy was turning the spanners. So we will model something up to be nice and clean.
Will include some sort of reference to "The sensible twin" on it.

[Grumph]

Calling it the sensible twin was a gentle poke at Mike's version.
I felt the half a four concept was a good one - but simply making it
bigger and unblown was the sensible way to go.
And easier engineering.

Realising that early GSXR400 pistons would work - and finding some - helped a lot.

I've known a few sets of twins - and there's usually a wild one and a sensible one.

[speedpro]

speeduino can handle 18k RPM in sequential mode as long as we use a low tooth count on the cam input to avoid overwhelming the microcontroller.

I have a single tooth on the cam trigger. This is used solely to identify where the engine is in the 4-stroke cycle. Link suggested 6 teeth on the crank which I did originally but I needed electricity so put the rotor back on and reduced the long tooth to match the other 3. You need a crank trigger to accurately measure crank position. You might get away with the single trigger on the cam but it's not going to be as accurate.

[F5 Dave]

I have a single tooth on the cam trigger. This is used solely to identify where the engine is . . . .

It's the hot metal part below you.
No problem.