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D7 lacks top end power.
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Geoffrey25
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November 21, 2020 - 10:12 am
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Hi all my 1966 d7 still lacks top end power after years of trying different things.last year it had a full engine rebuild and there was some improvement ,it's a 66 model with a concentric carb with a 170 jet and pancake air filter.I have also just had the flywheel 're magnetized but still no top end performance.I rebuilt the magneto with new coils and oil lite bush and put in a new points can and set points to 15 thou.It's really frustrating it's starts well pulls well in 1st and 2nd up to 30mph but in 3rd gets to about 35mph and won't respond to full throttle .I have checked exhaust for blockage that's fine the only thing I can see is where the down pipe meets t he silencer  some blue smoke is escaping surely it's not that I can't seem to seal it any better.I really don't know what to try any more any advice would be much appreciated many thanks Joe.

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sunny
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November 21, 2020 - 10:29 am
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hi  Joe       check  for     carb   air  leaks  &   was  the  crankshaft  in  good  condition   the  oilite  bush   has  a maxanin   gap of  1/2  a  thou    ,.,. oil the  pad    ,   points  & oilite  bush   correctly   ,.  check   all  gap  sizes     &    concentric   carb  fits  the  inlet  port size   &  its  a  2 stoke  carb  &not  4  stroke

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stubaker58
Sunny Felixstowe
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November 21, 2020 - 2:39 pm
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Hi Joe,

Just a thought ... I run what is basically a D7 engine and had some running issues.  I think I’m right to say that the D7 originally had a Monobloc not a Concentric carb (although like you I ran a Concentric).  When I had to rebuild the engine I looked more carefully at the carb and the bore was considerably bigger than the D7’s inlet port, not good for gas flow so I’ve reverted to a Monobloc of the correct size (is it 15/16ths?).  I have also had issues with the air filter strangling the engine.  Taking the filter off completely was a revelation, I am now trying to source a free flowing alternative so that I can add a filter back in, some (e.g. Sponge) have opted to use the big, remote filter fitted to the D14/B175 but I don’t have any side panels fitted so another solution is needed.

All that’s a long way of saying check the carburation!

Keep us up to date with how you get on.

Regards

D7/14 hybrid (4 speed with D7 crank etc.) on the road, D10 Bushman awaiting rebuilding.

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Geoffrey25
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November 21, 2020 - 5:14 pm
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Hi Stubaker  thanks for your reply it sounds similar to problems I am having earlier last year I had my original d7barrel rebored to a +40 with matching piston and rings.so I fitted them to bike and it was hard to start and when running had no at all and this was with the original monobloc carb too. So I changed it all back to the barrel and concentric carb all was ok I thought until a day later while riding it that the oil lite bush giveway and at the same time damaged the magneto coils as well.This when I decided to strip and rebuild the engine with new bearings and seals gaskets etc  and at the same time I had the crank shaft checked over by f and d motor cycles .When I was at the point of rebuild I had to make the choice stick with concentric barrel and carb or go for the original rebored barrel and Monobloc carb I chose the concentric which is how it is at the moment.I am tempted to put it all back to standard but I don't know after all the effort will it be any different or worse,as you are aware it's a pain to keep changing small end bearings but I would like to think with the engine now rebuilt it could perform as it should with all the original parts back on .

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Swing42
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November 22, 2020 - 4:51 am
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I'm having these troubles as well Geoffrey ..

Two things that came to light were ...

Yes,the new muffler (from Armours) I believe was 'over-baffled '.I removed the innards and drilled some 5/8" holes in the discs within.... and it breathed more freely.

And what I believe was the real culprit for lacking top end power ,was the fitting of a 16 tooth gearbox sprocket.Knowledgeable Bantam blokes have said that a 150cc D3 (mine) just doesn't have the horsepower to cope with the 16 t sprocket.Another bloke in this group said even his 175cc couldn't handle it..I'm going to fit a 15 tooth sprocket in the hope it will fix this issue..

Good luck with it mate ..

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Number6
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November 22, 2020 - 5:50 pm
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Bantam blokes have said that a 150cc D3 (mine) just doesn't have the horsepower to cope with the 16 t sprocket

I was one of those blokes! laughYes I did it on my D3, but only 'cause I couldn't get a 15-tooth at the time, not good, definitely struggled, and the gap between 2nd and 3rd was sometimes unbridgeable, if a gale was blowing or a steep hill.

Mike H --

Murphy's 4th law of motion states that any small object that is accidentally dropped will immediately hide itself under a larger object.

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Geoffrey25
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November 22, 2020 - 6:39 pm
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Ok I might try that where are you hopeing to get a 15 tooth sprocket from.

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Swing42
NSW Australia
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November 22, 2020 - 11:00 pm
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I'm getting the 15t sprocket from Oz Bantams here in OZ ... Draganfly may have them ? ..

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Sponge
Lancashire (A chip shop somewhere near Preston)
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November 23, 2020 - 5:22 am
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C&D Autos have 15T sprox.

Swing42 said
I'm getting the 15t sprocket from Oz Bantams here in OZ ... Draganfly may have them ? ..  

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Number6
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November 23, 2020 - 11:04 pm
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To avoid any confusion, the D3 I'm talking about I sold a long time ago, 1993 I think laugh

Sorry about that

laugh

Mike H --

Murphy's 4th law of motion states that any small object that is accidentally dropped will immediately hide itself under a larger object.

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