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.

“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

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

X7/gamma makes pleas­ing power while Alan pon­ders the wis­dom of the sleeve­less look

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

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