Crank assy, pistons

The crank was rebuilt and rebalanced by Darcy, with RZ (or was it Banshee) rods. I run stock TZ single-ring pistons and put over 4000 miles on my first set, and they are still within spec for clearance. TZ pistons are FAR heavier than RG pistons and the crank had to be rebalanced to suit. The extra thickness comes in mostly at the piston crown, which provides a wide heat path for heat to exit the top of the piston. Anyhow, 4000 miles is probably some kind of record for TZ pistons being run in a TZ barrel. As for the RZ350 rods, we had looked at getting custom crankpins made to allow us to use OEM TZ rods, but it looked like new crank wheels would be needed, and that was introducing custom items as service parts.. bad idea.

One item of note, the TZ barrels have very short fingers coming down from the bore on ONE SIDE ONLY, to support the piston’s side thrust. When adapted to the RG engine, two TZ cylinders are running in their normal direction of rotation, and two are running ‘backwards’ , and so the piston is not supported as well for side thrust on the backwards cylinders. This was a concern at first, however, it has not turned into any sort of wear or distress that I can measure. I have a redline of 12,000 RPM and so far have not seen any problems with the pistons. At first we considered inatalling iron wear plates in the case, to support the piston skirt at BDC. But, we decided to try it without.. so far so good.

Heads

The OEM TZ heads were a problem. First, they are very high compression, I believe about 16: 1. Second, they have fixed water outlets, and when mounted on the cylinders, I had outlets running smack into the back of opposing cylinder heads. Third, they are butt-ugly.

Fortunately for me, Darcy’s friend Pete Mills stepped up and offered to help. Pete was a CNC lathe operator and was quite capable of machining new heads from billet. Since we had room to play, we decided to go with removable combustion chamber inserts, so the engine could be easily configured to a variety of setups and fuels. A bit of conversation and we settled on two inserts, one providing 14:1 mechanical CR that would work on pump gas, one at 16:1 that would work more like a stock TZ head, with race fuel. We kept MSV on the low and safe side, but this is one thing I’d like to change on the next set of inserts- increasing MSV in to the 20 m/s range, more like a MX engine. Riding the bike on the track, I’m usually pitted against 600s and 750s, and I can use more upper-midrange than peak rpm power. I don’t plan to run Bonneville, or do any extended top end runs, so high MSV shouldn’t be too much of a concern. The bike has proven to be extremely sensitive to jetting, and that is one hallmark of too-low MSV. So, I’d like the power to come in as soon as possible, even if it’s at the expense of a few peak HP, or even if it means I can’t run WOT for a mile straight. The finished heads look like this: deltaheads.jpg

Juggling combustion chamber volume and squishband width will give you the compression you want, and the squish velocity you need to have good turbulence/ performance but not invite detonation. So, the heads were made. I adjusted squish clearance by varying the thickness of the base gasket, but we got clearances right in the 0.032” range we were shooting for. Base gaskets were cut from Cometic gasket stock.

Cooling

We weren't sure if the coolant flow pattern we wound up with would be accaptable to the engine, so I added a temp sensor to one head on each bank. Pete built a nifty manifold that collected all the coolant exiting the motor and allowed me to install a thermostat, before routing it back to the radiator. The stock RG base gasket is quite a complicated piece, with some passages open, and some blocked. Apparently this is done to fine tune just where all the coolant goes.

I was hoping that even with no base gasket restrictions, our engine would cool properly. But, who knows? We paid attention to burping the coolant system before running, to eliminate air pockets, and were rewarded with an engine that cooled both banks equally. Whew!!

I run a 55 degree C thermostat. In race use, my coolant temp rarely exceeds 60 degrees celsius. No problems there. My motor definitely runs best when it’s cool.

Head gaskets

The heads use standard TZ250 O-rings. I’ve had the heads off at least 20 times and am still on my first set of O-rings. It was almost worth it, just for this!!

Intake system

Airbox- filters

Shortly before this swap, I changed bodywork, and my side-mounted airboxes no longer fit the new body. I had to shelve them. Greg Lewis came up with a set of one-off Unifilters that fit my TMXs and fit inside my bodywork, and so once again I was an individual filter guy. I fully intend to return to side-mounted airboxes as soon as I can, since they have every advantage over open filters and will also allow ram-air to function.

Carbs/ carburetion

I mentioned above that metering signal was affected by the larger case volume. Another thing that I had not considered was this: The weaked carb signal doesn’t just apply to low rpm situations. The engine sucks more weakly at the carb in all situations. All other things being equal, I can’t see why this engine should fill the case as well as an engine with a higher case CR. Less air in the case means less air available to move into the engine. A high CR setup will push harder on that air as it blows up into the transfer ducts. A low CR setup will not generate as much case pressure, either positive or negative. The large-case setup really depends on the pipe do do a lot of the work in moving mixture up into the cylinder.

Since I have less vacuum at the inlet tract, I enlarged the size of the inlet passage- by boring the carbs from 35mm to 36.5mm. This was worth about 4 HP, so I suspect there is more to come from larger carbs. A TZ comes with 38mm carbs and I think now I see why. In the meantime, I’ll try shortening the inlet tract as much as possible (if I even can) to reduce inlet restriction. My current fantasy is to install a low pressure, wastegate modulated turbocharger that would provide a constant 1 or 2 psi to the engine. it would ovecome the (theoretical) case-filling problems and probably light up this motor.

As far as carburetion is concerned, I have problems with the slope of the fuel curve as supplied by the fixed main air bleed. What works at WOT and 8000 is far too rich at WOT and 6000 rpm. I need to steepen the fuel delivery curve, by installing smaller main air bleeds and compensating for mixture from there. This will require that I drill and tap the main air bleed system to accept Mikuni air jets. But, it ought to fix a large bugaboo I’ve been fighting with this setup.

Disc valve

One way of getting more air into the cases is by going to longer disc timings. Unfortunately, it makes for crappy low-end, and MOTA shows significant reversion thru the disc port at low revs-- perhaps I can retard the opening point of the discs with little performance penalty? Only testing will tell. I have tried extra-long disc timings on the dyno, but with no increase in high rpm power, and a significant reduction in lower end power. No dice.

My disc timing is about as rad as it’s going to get.

Gearbox

The bike was built around a GHN close-ratio tranny, but it blew up a year ago and I haven’t been able to afford repairs. However, if we do get 150 Hp, I suspect immediate carnage with a stock tranny. I have to do something.

Chassis

The chassis was upgraded with the addition of huge, carbon-fiber braces on either side. The resulting increase in stiffness is apparent even on a casual ride, and even moreso on a racetrack. I had to fuss a bit to get the braces to fit around everything, and the upper carbs’ cables go through the braces, but installation is actually pretty straightforward. I considered a whole lot of options before doing this, including welding on extra height to the side spars, but this was the only thing that I could completely remove and enable full access to the engine, etc. The short term program had me removing the tank to change plugs, etc. Eventually, I’ll have a new, lower fuel tank installed, with an R1 fuel pump, the coils will be at the rear of the bike, and I’ll use a prop stand to flip the tank up and access plugs and the top of the engine. I’d like to swap to a 5.5 rear tire but we’ll see what the winter brings.

spring. We plan to swap crank wheels from side-to-side, and cut a new notch in one of the currently virginal crank wheels.

On to page 4