Simmy's Naked Turbo Project

I had the exact same thought after I wrote that.
On the other hand it would be so much cheaper to just swap in the new block and pistons to the original motor. I like the idea of having the original 650T motor unmolested but it just becomes a heavy lump I have to find a corner to store it in.
I figured if I ever sold the bike it would be worth more if it included the numbers matching motor in original state. And I could always sell the 772 separately if it was a complete motor.
Also provides the option of going back if for any reason the 772 doesn’t work out.

 

My 772 is closer to reality.
The block is vapour blasted and machined shorter, 3 sleeves shortened the same and pressed back into the block.
All according to plan until my engine guy fumbled the last sleeve and a big chuck broke off when it hit the concrete floor.
Shit happens. Now waiting for another 31A (67mm) cylinder block from Germany just to salvage another sleeve.
He suggested machining a replacement but the blank is 8 weeks delivery.
I'd rather go ebay Germany route so I know all 4 will be the same.
Once all 4 sleeves are in it will get a fresh bore from 67 to 68.5.
The 68.5 pistons are vapour blasted and look new.

I ordered a set of piston rings from the UK, not many vendors out there with standard rings for the 58L.
UK Yamaha dealers all show rings no longer available.

 

I received another 31A cylinder block and dropped it off with the shop.
I have in my hands new ring set for the 58L
Shouldn’t be long until the block is done.
He actually had the tap for the knock sensor so that is complete.
So other than paint it will be ready to bolt in.

I’ve decided to just pull the top end off my original motor.
I will have the block, pistons, rings, wrist pin ready to bolt in. Those 4 items are the only parts I’m changing and the block is the only modified Yamaha part.
Once I get the block back I’ll start tearing my bike down again.This makes so much more sense than rebuilding and changing out another motor. A lot cheaper to.
Same head and cam chain. I could have this ready for the spring.
63 year old rider (soon) 42 year old bike and I’m just as excited to ride it now as ever.

 

900 block tapped for the knock sensor
Nice of Yamaha to cast in the boss for this

 

The vapour blasting looks pretty good in the pic he sent me.
Should be good to accept paint.

 

Forgive me, I don't recall what head you'r running with this. If it's a 650/750 I would be looking at getting my die grinder out to open up the exhaust ports, and then maybe looking at some of the same on the inlets - effectively a free-er breathing top end makes more power from the same boost, things run cooler, win win.
Good to hear there is progress..

 

I like vapor blasted finish better then paint, I'd leave it as is, but that's just me. BTW, I made and installed
aluminum motor mount bushings to help tighten up the chassis on my 750. Does anyone else here do that?
With the engine out it an easy modification and it works, I've done it on two on my bikes XJ750 and XS1100
Cheers, 50gary

 

my head is still the completely stock original.
The only mod I'm making is a bigger bore.
My biggest complaint with the OEM Seca Turbo was they were a complete dog below 6,000 rpm
Mine is already much lighter, I want 750 like power before boost.
I don't know enough to start hollowing out the ports.
I can't see that it would make much difference anyway.
When this thing hits boost I have no complaints.
A turbo becomes a matter of time.
How long are you going to hold the throttle open?
120 MPH comes up real fast.
Anyone else with a Seca Turbo knows the shot out of a cannon feeling.
When I was riding it at 16 psi it scared me that it was making so much power.
It felt like something had to break.
@Minimutly you mentioned earlier the turbo spinning up faster is a really good point.
If boost were to come on 1,000 rpm sooner then I would assume the taper on the main needle would need to be different.
What do you think of modifying tapers on main needles?
Sandpaper and a drill?
Thinking this would be easier than finding a Mikuni p/n that would work.

We need some pics @50gary
Did you drill your mounter mounts to accept bushings?

 

Hmm, porting a head takes some skill and knowledge, needs specialist tools, and is easy to do wrongly, and scrap a head.
That said, adding a turbo, and therefore more airflow to the 650 head (especially exhaust ports) is likely to restrict its efficiency to lower levels than the original engine - sort of three steps forward, one back.
Maybe it should be a job for further along the learning curve, you might find you feel it's not required...
As for modifying four needles identically, I used to do it with a file on one side of the needle, that way you can see the amount removed by the width of the flat created, but I've never done four of them. you likely will need bigger main jets too? Weak running turbo engines have a penchant for eating pistons, in about 6 seconds in my experience.

 

Simmy,

On the subject of Mikuni needles, having the original ones to compare to this chart may be helpful in determining what your options may be:
https://www.jetsrus.com/a_jets_by_carburetor_type/needle_mikuni.html

I will be a challenge to dial in the changes in jetting once you get it running and back out on the road. I would start with going up with the mains and that will hopefully keep you from leaning out at higher rpms and potentially burning a piston or two. Also, with the pilot jets, going up at least one size from stock may add a bit of insurance too.

I'm with you as far as the heads go, and feel porting could turn out to be way more of a risk than it's worth.

Tony

 

I scratch made aluminum bushings (gotta have a lathe) the size of the rubber ones with a slight shoulder to keep them from migrating.
I have the dimensions written down somewhere. Keep clearances tight for the most effect. They are simple parts and cheap to do. I'll look at them tomorrow.
Cheers, 50gary

 

That's the proper way of doing it; the only thing that would worry me is that fitting those hard bushings into the mounting ear on the engine block.....which is very thin on the diameter.....might result in a fracture on the casting (the rubber bushings absorb a lot of the stress/flex which the alum bushings won't, and thus transfer the stress to those thin "ears"). On bikes (Seca models, mainly) that don't have rubber-bushing mounting for the engine to the frame, the mounting ear is basically solid, with just a small "hole" (passage-bolt diameter, either M8 or M10) and that style mounting lug is a lot beefier than on the bushing-mounted bikes......

 

@chacal, what are your thoughts on using something like urethane such as the truck guys use for suspension bushings for the engine mounting bushings? Or maybe skate board wheels for their material? I have a 750 engine going into a 650RJ frame.
Sorry for the thread hijack @Simmy. I am enjoying your build thread greatly, thank you.

 

OK now I understand, stiffening up the Maxim rubber mounts.
Seca already solid bolts.

 

nice info, I have some reading now thanks.
Too bad the BS carbs are not in the listing.

 

Not to hijack the thread but here picture of the alu. motor mount bushing, click the thumbnail to enlarge.
Look closely.
Cheers, 50gary

 

I believe the #5 series is the group you will need to look at as far as choices that will be close to what you have stock.
The overall lengths do vary quite a bit though, and will need yours out to measure the existing ones as a baseline.

As backwards as it may seem, the upper rpm fueling should start with dialing in the mains first, then needles will be able to be dialed in for the midrange.

Tony

 

As backwards as it may seem, the upper rpm fueling should start with dialing in the mains first, then needles will be able to be dialed in for the midrange.

Tony[/QUOTE]
Indeed, correct. The main jet must be big enough to give you your maximum required fuel, you could faff around with needles in the mid range, get it right, then find it falling flat on its face at the top end - cue bigger mains and starting again at the needles.
Question is, how big do the mains need to be? I would be doing a lot of research....
And then guessing.

 

here's my 772 big bore kit.
This block will make the motor 1 mm shorter.
Unfortunately this adds 2 mm slack to the cam chain, fingers crossed that this can be taken up by the tensioner.
New 68.5 mm rings, fresh bore, he did skim the top surface to level out all the sleeves.
I had him shorten the sleeves 2 mm just to be sure the won't foul anything with the crank.
The material was removed from the bottom so he also grooved the edge around the sleeves for the big o-rings.

block drilled and tapped for the knock sensor.
If I was more anal I'd grind smooth the 853 and stamp in 772.

 

Nah... keep it on as a sleeper