Moto Guzzi V7 Classic Streetbike Surgery – Motorcyclist Magazine

14 Jun 2015 | Author: | Comments Off on Moto Guzzi V7 Classic Streetbike Surgery – Motorcyclist Magazine
Moto Guzzi 850 T

The Littlest Goose Learns to Fly

Moto Guzzi’s V7 Classic is a predictable handler that proves you don’t need 100 horsepower to have fun on backroads. Still, a little more power is always welcome!

Moto Guzzi’s V7 Classic is a predictable handler that proves you don’t need 100 horsepower

“My dirtbike has more power,” said Daniel, our “A” technician. He wasn’t kidding: His old Kawasaki KLX650 churned out 45 rear-wheel horsepower on our shop’s Factory Pro dynamometer. Our bone-stock Moto Guzzi V7 Classic, by comparison, made a paltry 38 horses in spite of its additional cylinder and 100cc displacement advantage. Shame is a powerful motivator.

At that moment, I decided to light a fire under the little Goose’s tail feathers.

Moto Guzzi’s success with the V7 Classic (and the recently released V7 Racer, see page 54) was a long time coming. The Lino Tonti-designed small-block V-twin first appeared in 1976 with the 346cc V35 and 490cc V50. Both were lighter and more compact than previous big-block Guzzis.

Displacement grew to 650cc, then 750cc in the ’80s when engine development halted.

A current V7 Classic tips the scales at just over 400 lbs. with all vital fluids, its svelte form invoking the iconic V7 Sport and 850 Le Mans. Power output, however, is underwhelming. The manufacturer claims 48 horses, but by the time they’ve galloped to the rear wheel, 10 have gone missing.

The last thing I rode with that little output ran on hay! Luckily, I had a few tricks in mind to put some pep in this pony.

I have some experience with small-block Guzzis, having won AHRMA Vintage Superbike races at Daytona the past four years on a V65. My racebike weighs 320 lbs. wet and makes 55 bhp. It’s a hoot to toss around racetracks like Barber and Willow Springs.

It’s proven dependable, too, finishing its last 100 races. But its 13:1 compression requires 110-octane leaded race gas, and there’s no headlight, sidestand, charging system or electric starter. That’s great for a six-lap sprint race, but not practical (or legal) on the street.

First, off with its heads! Thirty minutes after getting the V7 home, both cylinder heads sat on my bench. The 750 uses Heron-style heads, wherein the combustion chambers are machined into the tops of the pistons; the heads themselves are as flat as a Karaoke singer. Valves have a zero-degree included angle, moving straight up and down.

It’s a fairly obscure arrangement (Moto Morini is the only other familiar brand using the design), but hot-rodding Herons is common practice. We started with the same basic porting and valve package employed on my racer. Tuning guru Bruce Meyers fitted oversized Kibblewhite valves cut from blanks.

We went up 2mm on the intakes and 1mm on the exhausts for better respiration. Porting was unsophisticated: just clean up the head castings.

Next? Measure and measure again: I checked squish clearance by placing solder on the pistons, reassembling the engine, and then turning it over to compress the solder. The stock 2mm squish screamed for attention.

Moto Guzzi 850 T

So I pulled out my burette and measured the combustion chamber volume: around 30cc for the piston with another 10cc from the squish and head-gasket areas. The heads themselves hold zero volume, but the valves are dished to hold 1cc. Every bit counts when you’re calculating compression—set it too high and those pistons will self-destruct!

One nice feature of the V7 engine is the prospect of increasing compression by decreasing combustion chamber volume. No need for aftermarket pistons; just machine the stock ones. After trimming 2mm off the tops of both pistons and cylinders, compression soared from a lawnmower-like 9:1 to 11.5:1, which works fine with high-octane pump gas.

Each piston ended up just a few grams lighter, so the crankshaft didn’t have to be rebalanced.

The V7 pistons have a wide outer squish band. As the pistons approach the head on compression, intake charge moves away from the cylinder walls toward each piston’s center. That compact charge produces good power and resists detonation.

Tight squish bands are a good way to build power in Heron-headed engines. But not too tight, or pistons kiss heads, causing catastrophic failures. I set squish to just under 1mm via small cuts in the cylinder decks.

Raising compression decreases other clearances, and requires checking for valve-to-piston interference. I measured mine by placing clay in the combustion chamber, assembling the engine and turning it over by hand. The valves came close to kissing each piston, so out came the Dremel tool with a ball-end steel bit.

I slowly cut away the offending piston casting to create a good 2.5mm vertical and 1mm lateral clearance between valve and piston. Then I smoothed and polished both piston crowns with a de-burring disc on my die-grinder until they shone.

After double-checking my top-end work, a baseline run on the Werkstatt Racing and Repair dyno told us the stock airbox was restricting further progress. Letting the engine inhale through open throttle bodies added 2 bhp at 7000 rpm! Since dust never sleeps, KN pod filters keep the dirt out without undue restriction.

Moto Guzzi 850 T
Moto Guzzi 850 T
Moto Guzzi 850 T
Moto Guzzi 850 T
Moto Guzzi 850 T

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