Welcome back folks,
I think I mentioned in my previous post that I had a lot of content from the build process with little time to do anything with. See below the first instalment of the “trying to remember what the hell I have been doing over the past 4 months”.
After what seemed like an eternity of staring at a half built engine and forgetting how it goes back together (I sense a theme here), I got my ass in gear and started to get it rebuilt.
I discovered the hard way all of the intricacies of the Yamaha XT600/TT600 range (there’s a lot…) and ended up sourcing the required parts from what seemed like every single country in Europe . The parts were back from the vapour blaster (for a second time…) and I was finally in a place to start getting this bag of bits back together.
My carbs looked a little tired, and ever since I rode the bike I thought about cleaning these as it felt a little sluggish. I’m sure it was the carbs and nothing to do the head gasket having a massive hole in it. Yamaha created the “Yamaha Dual Intake System” (YDIS) carb for this bike, which means it is a combination of both manually operated carb and Constant Vacuum carb (CV). I have written a detailed post about how carburettors work previously.
In other words they have combined the two main ways of functioning a carb (mechanically lifting the slide with the throttle cable, or via differential pressure and a diaphragm, CV). In the picture below, the carb on the right is the mechanical one, which operates up to a set RPM. This causes the airflow to the engine to increase enough to function the secondary CV carb (or so the nerds on the forums tell me). All this means more power and better efficiency at lower speeds. And a total pain in the ass to tune and find parts for. Cool.
Knowing the complexity of these things, It’s a good idea to be organised when dismantling and keep the two parts separate:
Stripping them down was fairly pain free. I made sure not to disassemble so far that I would mess with the existing settings…the bike ran OK before and I didn’t need another complexity to deal with when my engine will inevitably not start once assembled.
To clean them up. I made use of my trusty “motorcycle dishwasher” to get any old tarnished fuel out. Remember what I said about keeping those two carburettors separate? Well that went completely out the window when I bunged them all in the Ultrasound bath and they got mixed up….woops!
They came up looking pretty nice so the next step was to reassemble. I am so glad I took a video of the dissassembly process because this took a little longer than I would like to admit (AGES!), Huxley thought he would lend a hand which was…nice?
One of the main issues during reassembly was the additional spring I found! I assembled everything as per the manual and had a tiny spring left over. My heart sank a little and I started to strip the carb back down but everything was as per the instructions.
I figured out that the carb rebuild kit I bought also comes with new springs, and one had slipped out the bag into the box with the rest of the parts…OBVIOUSLY (FFS). So I rebuilt it again.
The carbs sat ready to go on it was now time to focus on the cylinder head.
The engine had been through various iterations of parts sourced from all over. When my cylinder head arrived, I wanted to be sure it mated up with everything before sending it away to be vapour blasted:
Firstly, I wanted to replace the engine bolts, as I had a newly painted engine and vapour blasted parts, I didn’t want the shoddy bolts to let it down. I decoded the Yamaha numbering system of their bolts to figure out the sizes I needed.
I compiled a big spreadsheet relating to the parts and diagrams (I can share if anyone wants) and ordered all the correct crank case bolts from Grampian Fasteners (thanks Terry). For anyone doing one of these builds, if you figure out what size you need, purchasing this way can be orders of magnitude cheaper than getting the bolts from the dealer (or should that be Stealer am I right?! hahaha oh god i’m so alone). Not all bolts can be ordered this way though, specialist engine bolts are best to come from the manufacturer.
So the engine was reassembled over a couple of evenings, see footage below:
The worst/most nerve-wracking part of the build was fitting the new cylinder head bolts (around 25 mins on video). This was the main reason that this bike became the project bike. Having sourced parts from all over Europe it was a relief to see it come together and to finally get those bolts at the correct torque!
I think it looks ace all together.
I had a mad panic once I realised that the rocker cover arms weren’t lining up with the top of the valves. The arms are shaped and have a corresponding locating number, I appeared to find numbers difficult that day. I also had a picture to show you…but it is lost it…trust me it was an easy mistake!
Anyway, the engine went back in the bike and started up OK. I will do a full post detailing the tuning of the engine required to get it to run to the 3D printed velocity stacks soon. My next post will sum up my Bike Shed London 2019 experience.