The Sensible Twin

[Sketchy_Racer]

Well it appears I have been entrusted with getting the Sensible twin to the track where it belongs. I figure I get it up and running, see what little issues pop up and go from there.

I spent a bit of time trying to get the NSR flat slides to work, but no matter how much fettling with the float levels it was either flooding or no fuel at all. So I've come up with a plan to get the bike on track as close to the current setup as possible but hopefully still show its potential. I have a set of 3LN carbs here, so trimmed them down to 2 cylinders and put them on, bike fired straight up and idles nicely. It seemed to lean bog as it got onto the main jet so went up a few jet sizes and runs pretty well possibly still a bit lean, but really need to put it under load.

Time to address the oil leak, I popped the cylinder end plate off and impressively the plugs in the end of the cams to block oil had rotated sideways and were acting as oil jets against the end plate, Probably lucky it did leak as would most likely be low on oil pressure. I think I'll turn up some solid plugs and tap them in with some retaining compound.

With a bit of luck I will have it back together tomorrow for a first road test.

The long term plan to really make the most of this great engine is to fuel inject it. I'm curious as to what would be the optimal throttle body size for it.

A short video here: https://www.youtube.com/shorts/Mc0NQ4Q4zKA

[Grumph]

Well, that was unexpected. What happened to the order for downdraft Yoshi carbs ?

The 28 FCR downdrafts I ordered and paid for some years back still haven't arrived
and the supplier has been silent since the money arrived.

I'd suspect the midrange stumble with the CV's can be tuned out by enlarging the
vacuum sensing holes in the bottom of the slides - a little at a time.
It's worked for me many times previously.

[husaberg]

Well, that was unexpected. What happened to the order for downdraft Yoshi carbs ?

The 28 FCR downdrafts I ordered and paid for some years back still haven't arrived
and the supplier has been silent since the money arrived.

I'd suspect the midrange stumble with the CV's can be tuned out by enlarging the
vacuum sensing holes in the bottom of the slides - a little at a time.
It's worked for me many times previously.

all those dynojet and similar kits used to have the drill I remember they had stiffer springs as well
I remember Bob saying he used to preload the std springs with washers for good results.
I would make sure I had some form of airbox.

[speedpro]

As we've discussed, this might be of interest to you for the other project. https://youtu.be/6u-tYvooY60?si=S90sydwgDTtJfPdY

[quallman1234]

As we've discussed, this might be of interest to you for the other project. https://youtu.be/6u-tYvooY60?si=S90sydwgDTtJfPdY

I'm working with Sketchy on this project. Previously, we used an NO2C board with a Mega microcontroller (NO2C v0.2.x) in a setup that handled up to 12k RPM on batched injection. Based on that experience, we're confident a mega based 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.
Currently we have one ready to test on the "other" project to verify this. If not we will go to stm32 or teensy based speeduino like the dropbear you suggested :)

The sensible twin has a head gasket leak, so we'll pull it apart tonight, and go over it.
Since efi components are coming this week, so we will go straight to efi. Will post updates as we go :).

[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.

355114

[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.
:innocent:

[Grumph]

Re gearing. It was built for the big tracks down here. Because i had the sprockets it's got stock 250 gearing fitted.
Once it was up and running I'd have changed it to make 6th usable at Ruapuna or Levels.

[quallman1234]

Re gearing. It was built for the big tracks down here. Because i had the sprockets it's got stock 250 gearing fitted.
Once it was up and running I'd have changed it to make 6th usable at Ruapuna or Levels.

Got a 13t and 14t. Probably chuck the 13t and will track test it from there :). Will need to be quite a bit shorter for the kart track here in Wellington.
Will run extra cables for a crank trigger just incase we need it.

[speedpro]

I made a 60T back sprocket for mine. With a 13T front it's useable at Tokoroa but could still use lower gearing. I figure it's geared for about 130kmh in 6th

[husaberg]

Whilst not ideal for chain or swingarm life. JT do a 11 and 12T option
It cross references with a YZ80 so should be find able
https://www.jtsprockets.com/catalogue/model/s3130/

[quallman1234]

Whilst not ideal for chain or swingarm life. JT do a 11 and 12T option
It cross references with a YZ80 so should be find able
https://www.jtsprockets.com/catalogue/model/s3130/

Yep would have to figure a way to mount it as the 11 and 12 tooth versions do not feature bolt holes for the mounting plate. But always an option!

[husaberg]

Yep would have to figure a way to mount it as the 11 and 12 tooth versions do not feature bolt holes for the mounting plate. But always an option!

We just used to use circlips and a spacer washer in place of a screw in tab on the MB's as it was faster to change.
Most gearbox's have gears secured the same way. So seems legit.

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTut0CtT9CEWar1AZsRSlPfA_HuZ4y2c zAa8g&s
here is a "factory" made one for a tlr250
count those teeth

There are also other options.:innocent:
the cig method

[Grumph]

That rolling chassis had been Rob's original beast with a 2T in it. It had something like a 52T rear fitted when it arrived.

Pretty sure that went with the scrap I cashed up when I moved.

The 2T would have peaked at a lower RPM I suspect. Plus it had tiny Avalon aboard.

[speedpro]

After today I'd suggest a 60T as a starting point for bigger tracks. If you go for something like 65T you will have useful gearing for the shorter kart tracks. the gearbox is pretty good, especially the higher gears, but 1st to 2nd is a bigger jump than you really want to be using. I had a 13T on the front

[quallman1234]

Alright a quick update.

We bought all the injection parts, then got distracted :). But we are back at it.
Glen's done an awesome job of making our throttle bodies we bought for it fit + build throttle cabes + get the tps positioning sorted and a whole bunch of stuff needed!
We built a wiring loom for it over a few nights, and got her running for the first time in about 5 minutes once everything was connected!

We need to sort out a few more sensors + fit the electric water pump and it should be ready for the dyno to tune up.
Hopefully if she's still in one piece after that we can get to the track very soon!

I'll take some more pics next time im up.

Here's a quick vid of her running https://www.youtube.com/shorts/OT08NgrsLsA


355845355846355847

[quallman1234]

Dyno in the next week or so.

A little tease. https://www.youtube.com/shorts/l4UdqRRYDCg

[quallman1234]

24.6hp :). Good start. No pictures or video as we forgot. A few things we can improve but i need some track time. So will focus on getting it on the track and doing some riding. Will update then.

[quallman1234]

Track next week hopefully.

A small celebration https://youtube.com/shorts/FJcWNWYtJbY

[F5 Dave]

Came out to track last week hoping you guys would be there.

[quallman1234]

Finally on track for the first time.
She ran well, didnt have any issues with her for the 4 sessions we did.

First Run
https://youtube.com/shorts/ZHabVJWYiWM

A couple of corners of it in some anger
https://youtube.com/shorts/S16b1lMUJ3o

Next up paint and a few small tweaks but otherwise shes pretty much race ready!

[F5 Dave]

Well, it shed a bolt or two in a self lightening effort;)

Aesthetics huh? What's the world coming to?