Due to hot weather baking my shed and family stuff my "two day" project dragged on for a month.
But now it is finally up and running with valve springs and cams from a NT650 Hawk GT.
It now finally feels like a 750 did when I was a kid, it feels like a different engine.
I have not swapped out the exhaust yet, but I'm sure it is a bottleneck in the performance at this point.
I put Uni pods on the carbs, and followed along with stretched and heat treated slide springs to stop the slide flutter and fuel spray that is usually the nightmare of such installations. Slow jets are 42, main jets are 140 front and 145 rear and the Dynojet jet needle is on the second notch. The plugs look OK, but I'm going to have to go richer still with an exhaust change. My rev limiter is set to 6800 rpm that it hits readily in second and third gear, didn't want to cover ground faster than that while sorting it out.
Things I have learned:
I changed the valve springs without pulling the heads, making your own valve spring compressor is a pain, making 3 for different locations is worse.
I would probably pull the heads if I was doing it again even though I had good compression, despite what the manual says it can be done without pulling the engine.
Just for entertainment, stock intake springs you can remove with your thumbs, just grab the keepers with a magnet!
I almost forgot the most important thing: If you use a 4mm wrench when setting your valves tie a couple of feet of string to it. Those things are tiny and if they have a long tail it is harder for them to disappear. "Ah ha I've got you", is way better than "WTF did you go".
NT650 valve covers fit the VT750 although you lose the place for the secondary air cleaner and the crankcase vent is just a hose connection of the rear one and is noisy.
As the bike is now it only takes unplugging the spark plugs and four bolts to remove the covers to check the valve clearances.
I have attached a picture of how it looks with no added trim.
View attachment 282334
Just a bit of an update, since I had it out for a couple of hours of backroad riding today after some in city holiday stop and go.
All has turned out as I hoped. The jetting isn't quite right, it almost looks like I should swap the front and rear mains, but nothing too drastic plug colorwise and I will leave it as it is 'til I change the exhaust and to less restrictive carb boots, hopefully this week.
The downside to this mod other than needing an aftermarket ignition box to take advantage of it, is the loss of some low end flexibility, more like a motorcycle and less like a tractor. It cares more which more about the gear it is in when you roll on the throttle and it requires more throttle when you are starting out, I don't have a tach on it yet, but I would guess the engine likes it above 3000rpm and comes on strong where the stock setup is switching to running out of breath mode above 5500 rpm.
I'm still running with the previous 6800 rev limit, I'm not too worried about pushing it to a 7500 red line and will probably set the limiter to 8000 rpm. The valve train can keep up, the NT650 redlines at 8500 and is rev limited to 9300 rpm. The other more sporting versions of this engine design all seem to be able to routinely make it past 100,000 miles with routine maintenance despite considerable higher rev. limits than the 52 degree VT750. The other versions all have offset cranks though and also have shorter strokes and therefore less piston speed. The pistons are what I fear are the weak link in VT750 abuse.
If/when I finally blow it up I'll let you all know. (-;
Did the camshaft change make the bike sound any different? Did it give it a “choppy” idle?