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The added weight of the stronger big-end pin, the heavier duty conrod and the higher-compression piston all added up to a significant increase in weight on the pin side of the crank. Of course this weight needed counter-balancing the non-pin side of the crank so that the balance factor was the same.

We did this by weighting the crank with Tungsten. This was not idea however as the crank now had significantly more mass than the original 750 set-up and this affected pick-up and power.

JBS Racing chose Arrow Precision (http://www.arrowprecision.co.uk/one-piece.html) a world renowned conrod manufacturer to make our new rods for us. The conrods are forged, fully machined, heat-treated, shot-peed and magna-flux tested. The conrod was a little heavier than the OEM item but much stronger.

After the problem with the OEM conrod snapping we had 2 options – 1) Lighten the piston so that the OEM rod coped with it. We were worried that the power broke the conrod rather that the increased piston mass so this option was out, for now. 2) Increase the strength of the conrod, this would add to the conrod mass and possibly cause other problems but we felt at this stage that we would stick with the other components of the 750 kit that had not failed and focus on the one that had. So we worked on the conrod.

In September 2005 the new 06 Husaberg “628” crank was released which had a 35mm pin the 2000-2005 cranks had a 32mm pin as did our 86.0mm stroke crank. We decided that as we were going to have custom made conrods produced we would increase the pin size too. Due to our increased stroke room in our crank was limited so we decided that we would go with a bigger pin and a plain babbit-bearing for the big-end. After all 99% of the engines in existence run with this type of big-end and main-bearing.

We even tried a Metal-Matrix-Composite rod that the factory Husaberg enduro team were running. Everything seized, broke-up or wore faster than we could tolerate. So we changed back to a roller bearing but this meant we had to scrap the very expensive custom conrods we had and have another set made to use a 32mm pin. We also had our own pins made even though we were now back at the OEM 32mm diameter we felt that we needed a stronger pin so we had one made, it was a little heavier of course!

Head sealing problems were experienced with the engine that was tested in Latvia. A fix came from Sergis mechanics who installed studs with the OEM course thread on the bottom and a finer thread on top with a nut holding the head down. I hindsight this was the obvious thing to do.

JBS Racing produced a production version of this system which we still use on “700” kits. Further to this improvement we changed to a copper head gasket and machined a very small ridge on the top of the liner that impales itself into the copper to create a high-pressure seal around the combustion chamber.

Sergis Tests Engine – In June 2005 Kristers Sergis was struggling with his 2-stroke engine situation and was looking for another option. BSU and Brouwer Motors had heard of the JBS Racing 750 kit and through BSU a test was arranged with Sergis.

JBS Racing put a 750 kit in an engine supplied by Brouwer Motors and it was sent to Latvia for testing. Initially the testing went well and in a phone call to JBS Racing from the track Sergis said he was impressed with the power. Later that day we got a call to say that the conrod had snapped and punched a hole in the engine.

The engine Sergis tested was running a high-compression version of our 105mm piston that had not been run before the compression was over 12:1 and the increased dome thickness made it 30 gms heavier as well. This combined with the fact that a 5-times world champion was riding it caused the OEM rod to fail.

Sergis remained in contact with JBS Racing right up until January 2006 and was interested in the engine until he had to make a decision for the 2006 season in the spring. Fixing the conrod problem caused further problems with other engine components further down the line and the time got away from us.

Development and testing continued through 2004 and into 2005 leaks were sporadic when it worked it worked well and made loads of power, occasionally with out warning we got milk in the gearbox again. DCR tried a JBS Racing 750cc kit in 2005 we did not have any direct involvement as we were focusing on Sidecars but we supplied the parts and some technical support. It was reported back to us that on initial instillation in Andy Mitchell’s bike the slipper-clutch slipped badly even when just running the engine in, proving that the JBS Racing 750 made a lot more torque than the factory “650” engine which did not cause the clutch to slip. A stock clutch was put in replacing the slipper unit this held the torque as it did in our bikes.

The old leak problem reared its ugly head again DCR’s gearbox filled with milk! DCR eventually made their own “694” kit based on a 103.0mm KTM piston, the early OEM 82.0mm crank and an OEM head gasket which is 100.0mm bore!

Gaskets, More Leaks & More Development – We didn’t really understand why the head leaked after half-an-hour. It sealed well to start with and remained sealed while the engine was running in. Looking back the problem is obvious, the added power of the 750 pulls the bolts loose, but at the time it was a mystery.

We tried Aluminium gaskets, Copper gaskets, drilling and tapping the bolt holes in the cases deeper and longer bolts.

Eventually we found that if we lapped the head and liner-packer assembly together with lapping paste, thoroughly annealed the gasket and put high-strength Loctite on the bolts the head would usually hold down OK but the problem hadn’t really gone away. 

First Engine Test – Finally the very exciting day came when we were able to run the 750 in a bike for the very first time. We were running with a light-weight big-end pin, OEM conrod and for safety a compression reduced to 9:1. We had a 105mm bore and an 86mm stroke giving 744.6cc.

We tested the engine in a flat field with a data logger attached to the bike measuring RPM, 3-axis acceleration, wheel-slip, water temperature, oil temperature and oil pressure. We ran the engine in gently for an hour, changed the oil and ran it again this time at full power.

The engine produced a lot of power especially considering the reduced compression ratio. We could lift the front wheel in 4^th gear and even at the low compression ratio the data logger, which estimates BHP from its RPM and Accelerometer data, showed well over 70 BHP.

After half-an-hour of power test runs we noticed the oil in the sight-glass was “milky” we had a leak! We changed the oil and water and tightened up the 4 head bolts ran the engine again and the same thing happened – Milk in the gearbox after half-an-hour.

Assemble First 750 – Some of the parts that we ordered for the 750 needed additional machining and some were damaged in transit. After what seemed like an eternity of work we finally had a 750 kit that we installed for the very first time in Jan 2004. Work on other engine projects also took up much of our time in 2003.

Cranks, Liners & Packers – The 2003 engine that we were working with at this very early stage had the original OEM 82.0mm stroke circular crank design. This crank was known to flex badly and was a source of many of the problems that the early engines had when they were tried in Sidecar. JBS Racing decided that for our engine we would have a billet crank made from 4340 steel and remove the counter-balancer to try and stiffen the arrangement up.

We also decided that as we were making a billet crank we would make the stroke 86.0mm as this would fit in the cases with the stock conrod and would keep the bore-to-stroke ratio the same as a “650”.

JBS Racing spoke to several liner suppliers about our options for the wet-liner and decided on spun iron for strength as the wall thickness is significantly thinner than the OEM liner and spun iron is much stronger than steel.

We also had to put a packer under the head, which could be seen in our early engines as the thick black anodised packer with www.jbsracing.co.uk on it under the heads.