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D14 top end on a D7 bottom end??
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stubaker58
Sunny Felixstowe
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June 9, 2021 - 7:52 pm
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Hi all,

I’m gradually putting together a mixture of parts into something that will resemble a 60s trail bike.  It’s got to be cheap(!) so I’m using what I’ve got, and what I’ve got is a D7 bottom end plus small bore high level pipe and (knackered) small bore silencer.

I sourced a D14 barrel and used +.020 piston (very reasonable at £45) plus I have a B175 head.  I’m planning to marry this to the D7 bottom end.  I’ve managed to get the large bore adapter off the barrel (lots of diesel and heat) so I can use it with the small bore pipe and I’ve got the right size Concentric to fit.

So the question is can anyone see any fundamental problems?

Thanks.

“There’s nothing new under the sun”.

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Sponge
Lancashire (A chip shop somewhere near Preston)
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June 9, 2021 - 10:39 pm
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Hi Stu, I can see a fundamental with converting down to a small bore pipe whilst running a D14/B175 top end. I tried it once with a B175 Bushman. The result was a bit more torque at low revs but severe overheating on a run. 

Messing around with carbs and jets might help but the easiest way ahead to my mind would be to source a D10 barrel which will be quite happy with the small bore exhaust. Or source a big bore pipe and silencer.

Also ....all that extra power and the std 3 plate clutch might induce slipping but not worth upgrading the clutch until you know how it runs. 

Other than that .....with the right carb settings and ignition timing set for a D14/D10 it should run fine.

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stubaker58
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June 10, 2021 - 9:39 am
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Thanks Sponge.  That’s just the sort of thing I need to know.

My thinking was that the D7 bottom end would make the engine less revvy and calm down some of the peakiness of the D14??

I’ll probably try it with the small bore and see how it goes, although I’m a long way from that as I haven’t got a rolling frame yet!

Regards.

“There’s nothing new under the sun”.

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Sponge
Lancashire (A chip shop somewhere near Preston)
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June 10, 2021 - 10:03 am
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I guess you could run it in extra carefully with the small bore exhaust and see how it runs whilst keeping an eye out for a large bore pipe and silencer. 

Keeping the D7 flywheel will definately smooth the engine down but be careful to match the weight of your new piston to that of the original D7 piston as close as you can....this will help to avoid undue vibration. 

A worthwhile project.....best wishes and looking forward to aeeing it when completed.

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GlenAnderson
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June 10, 2021 - 2:04 pm
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I ran a D7 engine with a D14/4 top end, small bore high level pipe and silencer for a while, years ago. It went ok, the extra power made the big gap between 2nd and top less of an obstacle. I think that I came down one main jet size from stock (D14/4) but that was all. The clutch was ok for a few thousand miles, but I fitted the later type when I rebuilt the engine. The heavy Wipac flywheel definitely dulled the throttle response compared to my mate’s stock D14, and I remember Andy at T&G advising me not to rev the engine hard for extended periods as the Wipac flywheels had a tendency to let go; he had a rogues gallery of exploded ones on the counter of the shop in Thornton Heath. 

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stubaker58
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June 10, 2021 - 3:47 pm
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Thanks guys, that’s all very helpful.

I’ll keep you up to date as things get going.

“There’s nothing new under the sun”.

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