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D3 gearing for today’s roads
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stubaker58
Sunny Felixstowe
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November 16, 2021 - 9:48 pm
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Hi,

that looks great Bill.  

I’ve been following your process and I’m impressed with your speeds, my D7 just about gets past fifty (although it is still less than 1000 miles into a rebore). I can see the principles behind the slant to the head and understand the need for a squish band.  What I don’t follow is how you have actually been machining the head!

Any chance of a bit more detail for the slow of understanding? I do have a head to experiment on ready for my engine rebuild for my trail project (or I might just fit the B175 head I’ve got!)

Thanks

“There’s nothing new under the sun”.

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BillShas
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November 16, 2021 - 10:48 pm
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Hi Stu

I’ve basically used an orbital sander, and belt sander to change the compression ratio, remove metal from the base of the head and a tungsten carbide burr in a battery drill to create the swish band, wet and dry - various grades on a flat surface to ensure the head remained flat.

15E40EC9-C807-4AB0-BA01-AD1E60A60919.png
I used the foam pad from a headlight polishing disk in the battery drill with one of the fine sandpaper discs to smooth off the swish band edges.

Woodwork tools are quite commonly used on aluminium, quite easy to control how much materiel is being removed and using marker pens and spare head gaskets helps keep the shape.

the key to maintaining the speed is the 26/21 gear cluster from the late D7 / D10 - first is higher than std but perfectly ok for pulling away from a standstill on hills at junctions. 2nd is now really the go to gear for most scenarios outside of stretches of road where speeds warrant top gear

D3s rev better than d7s the bore and stroke being almost 1 ;1 ratio, optimum for 2 strokes, and they are lighter, there isn’t really that much between them on the road.

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stubaker58
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November 17, 2021 - 7:09 am
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👍

Thanks Bill,

That’s really helpful. 

Is the syringe for measuring the volume of the combustion chamber?

“There’s nothing new under the sun”.

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BillShas
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November 17, 2021 - 8:00 am
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Hi Stu

Yes syringe ml / cc same thing, the heads seem to vary a lot, there were lots of different castings.

Bill

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GlenAnderson
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November 17, 2021 - 3:13 pm
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BillShas said

20 vs 27 cc 

 

Making the assumptions that your engine has the same parts as mine, although the variations are negligible really, each overbore would make the resultand ratio slightly higher for the same combustion chamber volume:

 

0.020" overbore, flat topped piston, 0.9mm thick head gasket with a bore of 59mm...

 

27cc volume in the head equates to 6.1:1

 

20cc volume is 7.7:1

 

19cc would be 8:1

 

17.5cc would be 8.5:1

 

16.2cc would be 9:1

 

15.2cc would be 9.5:1

 

14.2cc would be 10:1

 

I have yet to measure my combustion chamber size properly in my Todd head, but given the results you've achieved, I would think that anywhere between 8 and 9 to 1 would be about right. You are in the situation of trying to make the space smaller without taking too much off, and I am approaching from the other end and having to make it bigger without taking too much off. 

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BillShas
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November 17, 2021 - 5:06 pm
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Glen

excellent, mine is +30 so I worked that out as 151.24cc plus 2.35 for gasket and 27 vs 20 

Old 6.15 : 1

New 7.76 : 1

Bike has noticeably more low end power now, can pull up our steep drive in first at tick over and will accelerate cleanly

To test whether the bike will benefit from a larger main jet, i dropped the needle / raised the clip, to the no 2 setting on the 523 from 3 which Amal state as the correct setting and the bike surges during initial acceleration a bit like there is a momentary power band, so i assume it is running lean in this condition - I’ll try the 95 main when it comes and see whether that helps across the entire throttle opening range - 

seem to be different settings for the 523 in the bsa manuals, Amal settings and pitman books for the D3 regarding needle clip, main jet and slide part number

I would trade some of the low down torque for more top end / read top gear speed for which I would need to retard the ignition, not sure how much by, I did move the stator in the slots to the max anti clockwise and it appeared to help but it was only a couple of degrees of movement if that.

really I’m happy, I can ride round here locally safely and maintain 35 - 40 up hills with ease and 48 - 50+ on the flat which is adequate,  I just feel that it could rev more in top with more fuel.

also I’ve checked the speedo against the local 30 and 40mph speed checkers in the villages and it’s accurate at these speeds.

thanks for the help

Bill

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GlenAnderson
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November 17, 2021 - 6:36 pm
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No problem. Happy to help. It’s a good coincidence that we are doing the same thing at the same time, it makes sense to share the info. 

My D1’s speedo is absolutely bang on in comparison to the gps on my phone. In favourable conditions it’ll sail right round to 53, although getting it beyond that involves gravity not engine characteristics. 

Given the somewhat flexible nature of the chassis of the rigid and plunger framed bikes, I think that a well set up 150 suits them very well. I ran a stock D7 engine for several years and that would happily take the bike off the clock, but things do get a bit unpleasantly lively as you get north of 50. laugh

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cocorico
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November 25, 2021 - 10:10 am
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Looking at this again (after looking at another topic on the forum), it strikes me that a little more performance could also be gained by opening out the 1"od inlet stub. On a D1 it has a 5/8" bore, on a D3 it is upped to 11/16", which leaves what seems an unnecessarily thick wall IMO. If this was opened out to 3/4", a 19mm carb could be fitted without much problem, eg a Dellorto, Mikuni or clone. I don't know what difference it would make, but it's one of the first things the moped tuners over here do.

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GlenAnderson
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November 25, 2021 - 1:10 pm
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I’ve measured the capacity of the combustion chamber in my Todd head. It was originally machined to give 11:1 on a 52mm bore, so way too small for a 58 mm bore 150. I machined it out to suit, and now have 17cc, which gives 8.75:1, which is pretty much ideal I think. 

I’m at the point now where everything on the new engine is done, and it’s ready to fit. All I have to do is find the time to do so. I’ll report back as soon as I have got it in and running. 

C2121A61-BAB2-4D2C-84BE-56F4A34179DB.jpeg1C0AC66B-26A4-4666-997D-7F12DF4A0F45.jpegD1CEA9C8-71AA-4F6A-8EB4-C1FFBDA790D1.jpeg

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BillShas
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November 25, 2021 - 11:44 pm
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Looking good Glen, I’m interested in seeing how much better the centre hole head is.

Cocorico, the french and Italian scooter crowd seem to favour variations of the 19mm Dellorto copy PHGB - I think one could just radius the intake stub on the barrel to meet the carb outlet rather than drill the stub

 

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Complete 19mm Malsossi PHBG carburettor kit with all necessary mounting parts. Professional solution for fitting a tuned carb without any alterations. Carb kit is suitable for all standard 17mm intake manifolds, as well as 20mm open versions.

 
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cocorico
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November 26, 2021 - 6:38 am
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That's true, Bill, but surely you would get a better flow from an inlet matched to the carb venturi? The PHBGs are an easy fit over the cylinder stub whatever your choice.

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BillShas
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November 26, 2021 - 1:52 pm
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If my limited knowledge of engines is correct, Having a slightly restrictive inlet stub will help speed up the air fuel mix going into the cylinder at low to medium rpm, giving better low down torque / performance / acceleration, obviously that means it won’t be quite so efficient at high rpms as it will limit the amount of air that can be admitted, since I’m not really looking for outright speed in top gear it should work to improve performance for the D1 / D3, which doesn’t Rev that high compared with modern engines

testing is what is required

i agree the Phobos are a great mod for the bantam with all parts easily obtainable.

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cocorico
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November 26, 2021 - 3:10 pm
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If you buy one, make sure it has the correct  needle jet for 2 stroke use (AU I think).

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BillShas
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November 26, 2021 - 3:48 pm
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BillShas said
If my limited knowledge of engines is correct, Having a slightly restrictive inlet stub will help speed up the air fuel mix going into the cylinder at low to medium rpm, giving better low down torque / performance / acceleration, obviously that means it won’t be quite so efficient at high rpms as it will limit the amount of air that can be admitted, since I’m not really looking for outright speed in top gear it should work to improve performance for the D1 / D3, which doesn’t Rev that high compared with modern engines

testing is what is required

i agree the Phobos are a great mod for the bantam with all parts easily obtainable.  

PHGB not Phobos, - predictive text

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