VT750 ACE Camshaft Change

7114 Views 71 Replies 12 Participants Last post by Troy Jollimore, 8 d ago

CSRoad

Discussion Starter · Aug 31, 2019

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.

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CSRoad

Discussion Starter · #51 · 8 mo ago

I've played around with my stock carbs (36mm on my '99 ACE, IIRC
they went to 34mm at some time in '01) and the 36.5mm Hawk carbs.

I think my Hawk GT carbs came from a flood bike and were priced commensurately with that condition. (-:

The Hawk carbs are more open in the area before the slide, obviously a larger bore and definitely do make more power, but mine are still a work in progress and I switched back to the stock carbs as they are better behaved at slow speeds in stop and go traffic, it is all a matter of tuning. I'm hopeful I will get them dialed in and smoother next year.

For the boots I use OEM NT650 boots with both carb sets as they have less restriction and I use Norma 9mm "Euro style" hose clamps to hold it together and leak free.
The "Choke" plunger is different with the Hawk GT carbs, uses the plunger with the needle like the VT1100's, it swapped over to the stock cable fine, but I haven't really used it for more than a few seconds at a time.

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CSRoad

Discussion Starter · #53 · 8 mo ago

LAsfinest_SS750 said:
Hopefully, I can do a one for one swap with internals. Seller did advertise that it sat in outside and it is also coming from Florida, so hopefully it isn’t seize at all from Hurricane Ian but I’ll give a rebuild when it gets here.

The floats are the same, but the float height is different IIRC 9.5 mm instead of 7 mm and you will need to bump up the main jet sizes.
I use 42 pilots, but I don't know what your combination is going to want for mains, going to be a bit of trial and error.

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CSRoad

Discussion Starter · #55 · 8 mo ago

I'm waiting for your report back with the Revere cam and the tales of the smile on your face. (-:
The Revere cam is probably what I should have used actually.

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CSRoad

Discussion Starter · #58 · 8 mo ago (Edited)

I did the "4 degree" mod when I replaced the clutch soon after I got the bike (2014), but backed it off when I switched to the Ignitech box (2015}.
The "4 degree" mod simply bumps the initial timing which then effectively shifts the whole advance curve contained in the stock ICM.

If you look at the Dyna3000 advance curves on the chart and imagine the stock curve moved up by 4 degrees + or - then you see the appeal of the cheap fix and its limitations.

Colorized chart thanks to Drive_Bye. Drive_Bye's Lead Sled

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CSRoad

Discussion Starter · #61 · 7 mo ago

I used the Honda OEM NT650 boots they just slipped in no problem.
I put one boot on the cylinder and the other on the carb it makes fitting it in a little easier.
They were a slightly loose on the stock carbs and tight on the NT650 carbs, but no leaks when the Norma clamps were snugged up.

What is happening different for you?

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CSRoad

Discussion Starter · #66 · 6 mo ago

Before you start everything should be in perfect running condition.

Well the way I look at it you need:
The inner intake and exhaust valve springs to allow higher rpm, Honda parts, as discussed above.
The cams, careful you don't get the cams for a horsepower restricted bike.
An ignition box to allow the higher rpm, I use an Ignitech TCIP4 Electronic for motorbikes - IgniTech Přelouč

Things that you will definitely benefit from:
A less restrictive exhaust.
A less restrictive intake, a Bean box mod to the stock intake is probably the cheapest way and one of the best approaches.
I would also add a Factory Pro Stage 3 kit to the carbs as a starting point.
If 36 mm carbs prodh35.html, Honda vt750 c Shadow ace / aero - Data by EC997a Eddy Current Dynamometer
If 34 mm carbs Honda, vt750, Shadow, H39 Series
Final tuning is likely going to mean trial and error plug reading and/or paying for some dynometer time.

There is nothing written in stone there multiple paths and choices once you head down the rabbit hole.
If you are having to pay full retail or worse for the cams, you might consider getting your stock cams welded up by someone like Web Cam or Megacycle Cams and using their valve spring recommendations.

Good luck and enjoy the fun. (-:

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CSRoad

Discussion Starter · #68 · 9 d ago (Edited)

My cams came from a 1990 NT650, but other than the reduced power versions for some markets (beware of imports), they are all the same for the full run of NT650's from '88 to '91.

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CSRoad

Discussion Starter · #71 · 9 d ago (Edited)

Yeah, but no.
Two different engines.
The 52 degree engine was based on the VT500 design, but evolved into a mess of variations, a couple of which they detuned to become new cruisers including the VT750, there is some parts interchange you can turn the detune around, you've still got the paint shaker Harleyesqe crank timing though.
The Hawk GT has an established performance history, it was the inspiration for Suzuki's SV650 which had better market timing.
Race replica liter bike eats Harley? Noooo! (-;

Edit: A little perspective: A VT750 RC44 engine is never going to be a sport bike engine, it will make 50+ hp at the back wheel with mods up from 36 hp stock.
A stock NT650 makes 47 to 50 hp at the back wheel (Honda claimed 68 at 7500) it has a 9.5:1 compression ratio vs 9:1 for the ACE which has the same bore and 100 cc more displacement.
The ports and valves are the same. A NT650 adds 4-5 horsepower with exhaust and a couple more with rejetting and UNI pod filters. With the same treatment it is not inconceivable that the VT750 would respond similarly. FWIW race prepped a NT650 can make 80-90hp. For the original SOHC CB750 Four Honda claimed 58 hp and it was earth shattering in 1968.

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