Personal experience of a BSA B175 Bantam

I had been riding MZ's for 10 years before I decided for a change.

MZ to BSA

At this time I owned a 1991 MZ ETZ 125 (12v, Disc Brakes, Oil Injection, 5 Speed) and an 1976 MZ TS250/1 Supa 5 (6v, 5 Speed).

I found both machines very quiet, smooth (rubber mounted engine) and great to ride, especially over long distances. The light weight of the machines is also an advantage.

But whilst in Skegness one summer I was tempted by a change to the BSA Bantam. Every day I passed one on display in a High Street window, and two others were regularly riding around town. I was impressed by the looks of the machine, the apparent ease to ride, the great sound and that it was British.

I decided that I would substitute my MZ125 for a Bantam. The logic I had at the time was that as the capacity was slightly larger it would have a bit more power whilst still being very lightweight to handle. And so started a new journey......

Buying a B175

I found a machine for sale in a neighboring town and went to have a look. The bike was basically sound with all parts present. The chrome on the nuts, exhaust, headlight and wheels were in poor cosmetic condition but otherwise ok. I was reassured that the engine was rebored (receipts) and that the owner had bought several items to improve the looks of the machine. The engine started and I had a quick ride up and down the drive. The machine had not been MOTed for many years so I knew I was taking a bit of a gamble. We negotiated a good price and it was trailered home.

Year 1 : 2007

As I bought the machine late in the year I had the entire winter to tinker.

The first issue to address was the poor running of the engine. We cleaned the carburetor out, new air filter, spark plug etc, but this had limited effect. We then noticed the lighting was very poor with dim lights. We checked everything out only to discover that our 6v bike was fitted with 12v bulbs! This should have been a warning about the problems to come.

Over the winter the rectifier, switch gear, speedometer cable and lever rubbers were all changed, ready for the spring ahead.

I also discovered at this time that the choke worked the opposite to what I expected (pull on for normal running, release for choke) which improved my 'starting' chances.

All seamed well for the year ahead.

YouTube video of Bantam restoration    
Direct YouTube link https://youtu.be/6cGlpkxBgsM

Year 2 : 2008

Spring came and the machine was presented for it's MOT. It rode very well on the 30mph roads, with plenty of power and good exhaust sound. After passing it's test I was riding the couple of miles home when a huge smoke cloud started billowing from the rear. At home I discovered the gearbox oil was empty and knew instantly that the engine seals had failed.

Pulling the engine apart revealed major problems. All of the bearings and seals were finished. It would appear that a layer of water had formed on top of the gear oil which had rusted the bearings. With loose bearings the seals soon failed. Also the big end pin was worn through its hardening so this would need changing also.

The new parts were easy to get mail order. We bought a new set of bearings, seals, conrod kit and primary drive chain.

The biggest problem was getting the crank shaft pulled and the conrod changed. We found a local company and the job was done in a couple of hours. However we then had great difficulty getting the crank back into the engine. It appeared the crank was too wide to fit. We got it back together but the crank was stiff to rotate, so we trimmed the seal mounting slightly and tried again. (DON'T DO THIS!)

Out on the road it was clear I had a massive vibration problem, but I persevered not knowing what a Bantam should ride like. On it's first major run I powered through the vibration to 50mph when the machine seized. Luckily the clutch wasn't up to much and the bikes wheels did not lockup. After it cooled down it freed up and I took it back home.

I tried to ride the bike a few more times that summer, but found the vibration unbearable above 40mph. Then the oil level in the gearbox started dropping and I knew I had major problems.

Year 3 : 2009

Over the winter I discovered that the two flywheels were not parallel in the crankcase. It could have been that when I had trouble with the width of the crank assembly I had applied too much force and distorted it. It was also clear the seals were completely destroyed with slivers of rubber seal in the crank case.

I had the crank realigned and fitted new seals. Again the assembly was too wide so needed to trim the seals to fit.

Whilst the vibration was now much less the bike only lasted two rides before the seals again blew.

Year 4 : 2010

With the engine apart once more I noticed that the packing plates were slightly distorted. It was these plates that were causing the crank to be too wide. In addition the rough edge could be responsible for cutting the seals. After a quick phone call to a Bantam parts supplier it was suggested to remove the packing plates as they only provide marginal power increase. Once this was done and the engine reassembled the transformation was immense.

Now the engine was smooth (up to 50mph anyway), and no more oil problems.

Engine starting problems again arose, and I had great difficulty setting the ignition points. I found out that although the contacts themselves were good, the pivot of the points was very worn. A new set of points and all problems were resolved.

But now the clutch started slipping. I bought a new set of clutch plates and pressure springs and the clutch was fixed.

I now have a bike that I feel confident taking on longer runs (60 miles plus) and the bike for the first time got some serious mileage.

Late in the year however one the front forks sprung a leak.

Year 5 : 2011

It turns out that both forks were corroded beyond repair. Two new forks were easily ordered and a two night job to replace. Also took opportunity to change final drive set.

This year I am using this as my first bike. It is great around town, and capable of longer runs. Very reliable, good fuel economy, free road tax, nice to ride and gets good looks. I am now glad I bought Bantam, just a shame I had to learn all the lessons the hard way.

On the “to do list” are fixing the centre stand, getting a side stand, replacing some of the chromed nuts and maybe a new battery before the end of the year.

 

UPDATE : 4/June/2011

Today the Bantam's engine again seized! I had just climbed a steep hill at full throttle and was on the level when without warning the engine stopped suddenly. I coasted for 1/2 mile to let the engine cool down, but it then would not restart. It appeared to have lost most of it's compression. After pushing it towards home for a mile or so I was at the top of a incline so I thought I would see if it would bump start. After some coaxing it did finally cough to life.

I can only assume that the rings had stuck in and that they eventually released. Doesn't appear to have suffered any lasting damage.

UPDATE : 6/June/2011

Bantam doesn't appear to have any lasting damage and runs fine. I may have traced the fault that led to the seizure. I have noticed that the exhaust was slightly loose on the cylinder. I remember reading somewhere that exhaust leaks on a two stroke cause weak mixture. Weak mixture causes overheating and therefore possible seizure. All I can do is just run the bike and see if it happens again. This fault could also explain the slight runaway it has had this last few weeks when coming onto tick over.

UPDATE : October/2011

The bantam ran well to the end of the year.

 

Bantam : The Good

• Lightweight.
• It's British!
• Looks Good.
• Sounds great (if you like the 2 stroke sound).
• Classic pedal layout.
• Engine pulls good from low revs (for a two stroke).
• Free Road Tax.
• 91 MPG.
• Simple to work on.
• Nice ride up to 50mph. Can be rode at 60mph for short periods.
• Pulls up inclines without changing gears.
• Good spares availability.
• No indicators : Yes I like this....

Bantam : The Bad

• Only 25:1 oil ratio with the oil getting ever more expensive.
• Don't think it's suitable for long distance touring, although other people seem to do this.
• Not as smooth running as the MZ's, or the top power of the MZ 125. (But better low down power).

The bike is now everything I wanted, and if it stays reliable it will stay as my number one bike.