Hi
I recently acquired a 1956 Bantam major D3 (150cc) as an oily rag project. I managed to get it started, and when stationery, it revved up. However, as soon as it was under stress it lost power and would stall.
I cleaned and reset the points to 10 degrees BTDC, stripped the carbs and gave them a though clean and reset the needle in various locations on the slide, but the problem persisted.
I thought it might be compression so I strip the head and found a slight gas leak evident on the head gasket, this I replaced along with the lower cylinder barrel gasket; During this I discovered a broken piston ring which fortunately had not damaged the cylinder bore so I ordered a new set of rings for D3 major, this is where the story gets interesting!!!
The rings arrived but where to big (being 57mm) for the piston attached. On examining the piston it is marked D1 and the existing rings being 51mm in size!!
Therefore, I can only assume that the previous owner replaced the original D3 cylinder barrel with a D1 barrel and changed to a smaller D1 piston...
My Questions are as follows;
1) are the D1 and D3 cylinder barrels interchangeable
2) If so do I need to change the con-rod to reduce the pistons travel
3) Is there any way of identifying weather the head is off a D1 or D3 and will that need changing as well
4) Despite all the above am I barking up the wrong tree regarding the original problem of loss of power and is what I have described above likely to be the root problem.
The bike is in fare condition and worth the effort to keep on the road so I'm hoping some of enthusiasts find this as interesting, and frustrating as I have and can point me in the right direction.
Many Thanks and happy Christmas
1. They have different bore sizes. You can put a 1955 on D1 barrel on a 1955/56 D3 and the other way around.
2. The crank assembly for 1955/56 D1 and D3 are the same. But not for 1954. So is your engine actually 1956?
3. The attached service bulletin may give you some information about the cylinder heads. Same head fitted to both D1 and D3 from 1955.
4. If the bike has been laid up for a long time the crank seals may be worn or gone hard, they need to be in good condition for the engine to work properly, try varying the timing by rotating the generator on the elongated mounting holes.
Sometimes they will not start until the points gap is reduced. See the second attached service bulletin.
thanks that's Great advise. I will start by trying to age the various components when it's light, eyesight not as good these days :)
curiosity got the best of me. It would appear that the engine is a 1954 150cc bantam Major engine number BD3B 3550 but fitted to the 1956 frame.
Therefore, if I understand correctly from what your are saying, and the information you have kindly provided, is that I will need need to replace the existing D1 cylinder etc with a 1954 Cylinder 150cc barrel, head and larger 57mm piston?
would you agree with that conclusion please?
Kind Regards
Following on from our discussion I would appreciate some knowledge please :)
Is a 1956 D3 150cc bantam major Cylinder barrel, head and piston (57Mmm) compatible with my 1954 150cc bantam major engine please as I have an opportunity to acquire these parts.
Kind Regards
The D3 barrel was the same part all years. With 55mm stud centres. So I would think the parts would fit OK given the correct head for the piston used.
I agree with Julian, those parts should be fine.
A lot of parts on Bantam engines changed little over the years, one of mine has a D7 crank in D14 cases. So long as the barrel stud centres are the same the barrel will fit. The extra 25cc makes a difference to performance too (apparently!)
Regards
That's great advise Thanks. I will up date when the parts arrive :)