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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

“There’s nothing new under the sun”.

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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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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
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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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Geoffrey25
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December 22, 2020 - 8:11 pm
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Hi all just getting ready to put Monobloc carb back on and was wondering which method people thought was best way to go,is it best gasket and o ring or plain o ring on carb to barrel many thanks Joe.

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stubaker58
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December 22, 2020 - 8:57 pm
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Hi Geoff,

between the carb and the barrel there should be a thermal spacer. The sequence is then paper gasket, thermal spacer, ‘O’ ring, carburettor. Remember the nuts do not have be very tight or you run the risk  of warping the face of the carb and even distorting the body.  It’s worth checking the flatness of the carb flange before you start and gentle lapping it flat if there’s a problem.

Regards

“There’s nothing new under the sun”.

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sunny
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December 23, 2020 - 1:57 am
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its usely   the  wrong  0 ring  that   gets fited   that  macks the  carb face bend  so  correct  any  bend  and use   a  gasket   

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cocorico
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December 23, 2020 - 7:14 am
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stubaker58 said
...Remember the nuts do not have be very tight or you run the risk  of warping the face of the carb...

What stu said - remember, for any joint made with an O ring (as long as both mating faces are flat) you only need slight compression of the ring to make the seal. Anything tighter risks damaging the ring.

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sunny
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December 23, 2020 - 7:25 am
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yes  Bob  thats  wot  i was  trying  to say   in   post   3       as   [  serplyers  sell    the   thick   0 rings    ther  too   BIG    ]  as  ther  cheper  than  the  rite   ones   

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Geoffrey25
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December 23, 2020 - 1:29 pm
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Thanks for all the replies I do have a thick cardboard spacer and a slightly thinner bakerlite one so I guess I have a choice merry Christmas to all regards Joe.

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Ringting
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December 26, 2020 - 9:39 am
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Hi Geoff. Absolutely as above. It's very easy to bend the carb flange and if sufficiently distorted it can pinch the throttle slide so be careful. The thermal spacer needs to be hard so that it wont squash under the pressure of the fasteners and allow the flange to bow.

Also, what is not well understood about O rings. Rubber does not compress. It distorts. So, as Coco says, only slight protrusion of the o ring above the mating face is needed. The O ring should have enough space in its groove to completely press in to. The usual recommendation is for the cross sectional area of the O ring to be about 80% of the cross sectional area of the groove.

Alan.

D1, D14/4, Guzzi LeMans, Triumph Trident 900, Maserati 160 t4. Mk1 Mini Traveller, Berkeley T60.

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Geoffrey25
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December 27, 2020 - 5:39 pm
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Hi all rebuilt top end on d7 today I wanted it back to standard it did have a d10barrel and piston but I wanted to put it back as it came out the factory. The original barrel was rebored to a +40 piston and new rings I swapped over to small end bearing fitted it all back with 375carb and chockslide. At first the carb kept flooding but once sorted it started after 2 kicks but it was gutless and not responding great to the throttle then it would not start at all I am now thinking is it the carb ,I can try my one off my c15 but I didn't reset the timing when I swapped things over any ideas please many thanks Joe.

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