X7 Gamma delivers on the dyno
The engine runs. Now to get it running at the optimum air/fuel ratio across the rev range. And discuss the niceties of Jis-spec screwdrivers
OWER CORRUPTS, we’re told. If that’s true, we’re certainly corruptible. Not that we expect massive numbers from the Suzuki Mk3 RG250 engine powering Project X7 Gamma but we’ll take everything we can get, much like those in exalted positions. Suzuki claimed an optimistic 49bhp for the AEC powervalve-equipped Gamma, so we’ll see anything over 40bhp at the rear wheel as a result.
So we book an appointment in the dyno room of RTR Motorcycles in Bingham near Nottingham to see what we can get from the little Gamma motor. RTR were suggested to us by our old mate Steve Panter, proprietor of carb specialists Allens Performance. We’d asked Steve to help us
Pout with the jetting of the Mikuni VM28SS carbs as we are running pod filters; a stock airbox is no longer an option on our special. By happy coincidence, RTR and Allens have their units just a few doors apart, so it should be pretty much a one-stop shop.
Apart from his massive supply of carbs and parts and extensive expertise, Steve has an additional qualification pertinent to Project X7 Gamma. “I used to race a Mk3 in production events,” says Steve. “By the time I was done I was getting 44.5bhp at the wheel out of it. Mind you, it was hard work against FZ600S. I started getting results once I got one of those.” Being no strangers to the concept of Gammas and hard work ourselves, we’re happy to tap into Steve’s hard-won knowledge.
Equally knowledgeable is the proprietor of RTR Motorcycles. Dan Hegarty is a top road racer and was the leading privateer in last year’s TT races. A 128.431mph fastest lap makes him 25th fastest man ever around the Mountain Course. Bike set-up is a big part of Dan’s business and one that he’s keen to expand. His growing reputation means he’s in demand and we have only a few hours to see what we can do with X7 Gamma before he has to work on a customer’s classic Ducati racers.
Before we put the bike on the dyno, Steve and I first check the operation and adjustment of the powervalves. Satisfied that they’re doing what they should, we pour a generous helping of 30:1 super unleaded/two-stroke oil premix into the tank and roll X7 Gamma onto the dyno where Dan straps the bike down.
Rightly dubious of the provenance of X7 Gamma’s tank and the cleanliness of its interior, Dan fits an inline fuel filter before firing the bike up. His decision is quickly vindicated. The left carb starts leaking fuel and when we drop the bowl to investigate, there’s a fair bit of rust in there from when we fired the bike up back at the PS workshop. Steve’s expert eye also spots that the float height is wrong. “People often try to cheat leaks by messing with the float heights instead of sorting things out properly,” says Steve. “The knock-on effect of that is lean running because the jets can no longer pick the fuel up from the correct level.” Supposing that if the left carb’s float height is wrong, then the right-side level will be out too, Steve drops the other float bowl to correct that carb as well.
He wearily notes that a couple of the screws have been chewed and slots cut to compensate necessitating the use of a crosshead and a flat screwdriver just to do this simple task. “Why can’t people just buy some JIS screwdrivers if they want to work on their Japanese bikes?” opines Steve. Finally, with that done, we’re ready to see what things look like on the dyno with standard jetting.
Because the engine is a fresh build, Dan runs it up gently for a while on the rolling road before stopping the engine, letting it
cool, then restarting the bike and warming it through so that the dyno runs can commence in earnest.
Predictably, the first run on stock jetting sees the bike struggling to pick up revs, it barely troubles the 40bhp mark and shows that the bike is running way too lean, with a stoichiometric ratio of 17:1 air to fuel, a long way from the 13.5:1 where we should see best power and even further from the 12.8:1 that would be safest for general road use.
A logical first step is to go from the stock 160 main to a 180. Steve raids a box of jets he’s brought from his unit to Dan’s dyno. This time we get 40bhp but the stoichiometric ratio is still dangerously lean at 15.4:1. One of the exhaust header nuts has loosened itself off too and that isn’t helping our cause. The kickstart is also attempting to detach itself. Editor Jim did say he wanted a shakedown of X7 Gamma this month. By doing that on the rolling road we can at least find all of the departing parts easily.
“The bike is running way too lean with a stoichiometric ratio of 17:1 air to fuel, a long way from the 13.5:1 for best power”
Dan wants to try 200 main jets to see if that offers improvement on the air/fuel front. Even these, a long way from the stock 160s, only richen things up in the order of 14.5:1 – still way too lean. “She likes running lean,” says Dan. “We got a bit more power there but we can’t risk it. If the engine was running lower compression then it might just about be OK.”
Steve and Dan debate whether to go to 220 or 230 mains for the next run. Finally plumping for 230s, that does yield a couple more horses but now it’s a little too rich at the top-end while still a little lean in the critical 6500-9000rpm midrange. Steve and Dan decide to have a play with the air-screws to see if they can help to balance things out. Unusually, the single screw on each carb affects both both pilot and main circuits. Something of a compromise. Steve turns them out from the standard 1.5 to 2.5 turns out. Eventually we settle on two turns out for these and we’re getting a consistent 40bhp. The stoichiometric ratio is now a reasonable 13.5:1 on part throttle. “Provided a two-stroke isn’t highly tuned, you can run a little lean without problems,” says Dan.
There’s still one more thing to try – the carbs Steve ran on his Gamma race bike. Steve lifted the lid on the airbox on his race bike to flow more air, so the effect would be similar to if less extreme than our pod filters. These carbs have 165 mains, different needles, needle jets and pilot jets. Immediately we see 43bhp and with 180 main jets fitted, we get the same power with a better stoichiometric ratio. The decision to spec our carbs like Steve’s is a no-brainer, so we make a shopping list for 180 mains, 37.5 pilots, 5DP5 needles (on second groove) and 169/O/0 needle jets. These needle jets let the air in after the restriction where the stock bleed-type ones it comes in before.
Looks like we’re sorted thanks to the top efforts of Dan and Steve. Now we just have a handful of detail jobs to sort out and X7 Gamma is finally done.
X7/gamma makes pleasing power while Alan ponders the wisdom of the sleeveless look
Demo of the merits of the JIS screwdriver
Get the pilots wrong and he’ll get the needle
Airscrews on these affect both pilot and main circuits
“Where are you Alan?” “Just putting me sleeves on Dan” Manky 160 mains were swapped for new 180s Steve’s proddie race spec set-up was the way to go
That’s right: make Alan do the smelly, fuelly jobs Alan tries to rub the smell of petrol off with flex. That won’t work Big fuel filter to avoid debris-related disruption