Author Topic: BSA M33 500 1955 Engine looses Power after 20 min  (Read 354 times)

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Barrieh

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BSA M33 500 1955 Engine looses Power after 20 min
« on: 17 September, 2021, 19:37:59 »
Hi,

Bike has had a new engine rebuild, magneto rebuild, fitted with Amal concentric with 260 main jet (middle position needle) 19cc Idle jet 1 1/4 turned back. Oil getting to top of engine and back down on return pipe. Running Mineral SAE40 oil. Sits between advanced and retard even warm, doesnt like to advanced or too retarded. Carb has 10mm of heat protection with heat insulting plate... mildly warm to the touch after 20 min, not in any shape or form hot. Choke is fully open from the start and doesnt need any at all.

After 20 minutes of riding - getting up to 50mph, it slowly and surely starts to lose power to the point I have the throttle wide open it chugs along... sounds more throaty. Still idles well, but when hot gets slow, loss of power. When cooled it back to normal and will run quickly.

Any advice gratefully recieved.

Hamish

cdsdorset

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Re: BSA M33 500 1955 Engine looses Power after 20 min
« Reply #1 on: 17 September, 2021, 20:13:05 »
You say newly rebuilt. How long ago? Rebored?

Calum

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Re: BSA M33 500 1955 Engine looses Power after 20 min
« Reply #2 on: 17 September, 2021, 20:19:00 »
Valves not closing properly when hot?

Mike40M

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Re: BSA M33 500 1955 Engine looses Power after 20 min
« Reply #3 on: 17 September, 2021, 21:35:39 »
Could be no tank breathing. Check if vacuum by removing tank cap when it looses power. Sometimes it can be that simple.

Barrieh

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Re: BSA M33 500 1955 Engine looses Power after 20 min
« Reply #4 on: 17 September, 2021, 21:58:24 »
Hi,

Engine was professionally rebuilt by Chris Odling along with a fully reburb'd mag dyno. It was fitted with a new liner - new old Piston, the cylinder head had hardened valve seats etc. Was restored to a very high degree of reburb. The engine was rebuilt about 2 years ago and sat in my office while I rebuilt the rest of the bike, It was a total rebuild... this bike was a BSA shipped to Burma and used and abused until 1997... every part of this bike was worn out.

How would I know if the valves are not closing properly? I would welcome advice on this as I am not very familiar with how best to adjust them?

I will check my fuel tank cap to see if that could be the problem... I like simple reasons for the issue!!!

This is _so_ frustrating so really appreciate the advice!





chaz

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Re: BSA M33 500 1955 Engine looses Power after 20 min
« Reply #5 on: 18 September, 2021, 09:05:47 »
firstly as said, the blocked fuel cap vent a favourite especially on bikes used on dirt.

secondly, Its a problem I worry about, Ive rebuilt a customers DR400 engine, the frame is still not ready for the motor thats nearly a year on the floor, no lubrication going around the bearings although I have used plenty of oil in the rebuild. the latest B40 motor rebuilt a year ago and not near starting yet, but I have tried to keep it turning.
I had a can-am bombardier that nipped up on a long ride , waited until it cooled down and it ran again enough to get me where I was going and back home,
maybe a clearance issue, something may have closed up due to heat

Pete C

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Re: BSA M33 500 1955 Engine looses Power after 20 min
« Reply #6 on: 18 September, 2021, 09:39:03 »
You could try setting giving the tappets a couple of thou extra clearance to rule out the possibility of the valves not closing when hot. It will rattle a bit but should help rule out that problem.
The requirement to have 3 hands to adjust the tappets on these bikes makes it a chore I hate.  :'(

royblackburn1@btinternet.

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Re: BSA M33 500 1955 Engine looses Power after 20 min
« Reply #7 on: 18 September, 2021, 20:29:09 »
Could it be fuel starvation when reaching 50 but when slowing down theres enough to let it tick over and when cold its replenished itself happened with me once the filter in the carb was part blocked I would start looking at simple things first just a thought Roy

Barrieh

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Re: BSA M33 500 1955 Engine looses Power after 20 min
« Reply #8 on: 15 October, 2021, 17:12:27 »
TAPPETS!

Been busy at work hence no response... apologies.

It was the tappet, specifically the Inlet Tappet that was wrong. For some unknown reason the tappet lock nut had loosed off.

What is truly amazing is it was off by about 1/2 cm and yet still managed to run and function for the first 20 min consistently.

Thanks
Hamish