Advice on overhauling a 1971 750.

T95 · Oct 22, 2010

What are the typical issues that need to be addressed with the 1971 750cc bikes?

I am trying to do some research prior to ripping into my project. I would appreciate hearing about the common internal issues of the 1971 750cc bikes. I know my question might be a bit vague, but I need to focus my search somewhere and I need help. If you can just point me in the right direction I will research the items individually.

I am not really looking for upgrades at this point. I more concerned with the systems that should not be overlooked or are known to be problematic.

Thanks, Gary

kommando · Oct 22, 2010

The 72's had the major internal issues especially on the combat, for 72 they dropped the sump filter and move the oil pickup to the wrong place, these changes were reversed for 73 so are not applicable for 71's so you can breathe easy. Main one for a 71 is to add an oil filter to the oil return pipe.

dave M · Oct 22, 2010

If you are splitting the cases do split the crank shaft and clean out the sludge trap. Before you do this mark the two crank sides in relation to the flywheel to keep it in balance. Depending on the condition of the crank studs, nuts and bolts when you dismantle it you may want to order up a new set which are available as a kit. Use 'Superblend main bearings when you reasemble the bottom end and check the camshaft to make sure there is no wear or loss of surface hardening on the lobes. Measure the con rod bolts for stretch and fit new nuts on the con rods. All this is applicable to any Commando bottom end. If you have the timed breather on the end of the cam shaft make sure that it is located properly and turns the breather disc, also that the slotted disc that it bears up against in the cases does not rotate, I have recently come across one that was loose and needed to be re-pinned in place. I support Kommando's advice on fitting an external oil filter.

ILLF8ED · Oct 22, 2010

Hi Gary,

Some additional:
'71 side stand lug or bolt design was prone to breaking...changed for '72. Will let others comment on that. Maybe just needs a grade 8 bolt to prevent breaking.
If you get into the gearbox replace the layshaft ball bearing with a roller bearing...search this site for that info.
Check the swing arm spindle for movement...common for these to go dry and seize. Lube is not grease, needs to have gear lube (oil) even though there is a zerk fitting. Check this site for swing arm modification as the tube in the rear engine plate that the spindle goes through gets out of round.
Check all wiring connections - clean the bullets and make sure the female connectors don't have broken metal inserts - replace those as needed.

Ride it and have fun

T95 · Oct 22, 2010

kommando said:
The 72's had the major internal issues especially on the combat, for 72 they dropped the sump filter and move the oil pickup to the wrong place, these changes were reversed for 73 so are not applicable for 71's so you can breathe easy. Main one for a 71 is to add an oil filter to the oil return pipe.

Did the 71 suffer from crankcase breathing issues or other issues that will require any machining to the cases?

I just read the recent post on locating the oil filter at the electric start boss! I'll do some research on alternate locations.

T95 · Oct 22, 2010

dave M said:
If you are splitting the cases do split the crank shaft and clean out the sludge trap. Before you do this mark the two crank sides in relation to the flywheel to keep it in balance. Depending on the condition of the crank studs, nuts and bolts when you dismantle it you may want to order up a new set which are available as a kit. Use 'Superblend main bearings when you reasemble the bottom end and check the camshaft to make sure there is no wear or loss of surface hardening on the lobes. Measure the con rod bolts for stretch and fit new nuts on the con rods. All this is applicable to any Commando bottom end. If you have the timed breather on the end of the cam shaft make sure that it is located properly and turns the breather disc, also that the slotted disc that it bears up against in the cases does not rotate, I have recently come across one that was loose and needed to be re-pinned in place. I support Kommando's advice on fitting an external oil filter.

I am keeping a journal so I will have all these tips at hand!

This is the first time I have seen a reference to the timed breather. Not sure what its all about yet, but I will look into it!

Great information, thanks.

DogT · Oct 22, 2010

If your engine is actually a 71, you may have a timed breather. It's on the drive side, and is an angled fitting that comes off the end of the cam shaft and a rubber hose is attached going to the oil tank. My rotary breather was cracked and I replaced it. From what I read, it was not enough breathing to take care of the volume of air to be expelled and the later bikes had breathers off the old magneto fitting or newer ones had it off the bottom rear of the crank area. For now I have left mine in stock configuration, but there are plenty of threads about improving the breathing of the crank which involves drilling holes in the timing side crank case.

Dave
69S

dave M · Oct 22, 2010

DogT's explanation on identifying whether your engine has a timed breather or not is clear, however if your engine has the breather at the back of the crankcase near the bottom, then there are some modifications to the cases and breather system that you might consider. Some would recommend these modifications and others might not consider them necessary, I myself modify the cases when I am rebuilding a set with this configuration, but I observe that others have experienced absolutely no issues with leaving the cases as standard. There is a good article with photos on the 'oldbrits' web site which explains the issue and the appropriate fix, although again there have been subsequent developments including the Colorado Norton Works built-in reed valve breather which offers an elegant, if relatively expensive, option. I suggest you identify what you have and then trawl through some old threads related to engine-breathing/crankcase ventilation.

T95 · Oct 23, 2010

DogT said:
If your engine is actually a 71,

Dave,
From what I understand my bike is a 71. Also, the breather appears as you described a 71 should. Anyway, I included a couple of pictures for verification.

I have looked through my manual and read several forums, I have yet to find a picture of the timed breather and its components. Some threads stated that the breather issues were more of a problem from 72 on and especially while revving in the mid 4000's.

Lots of information on breathers to wade through.

dave M said:
DogT's explanation on identifying whether your engine has a timed breather or not is clear, however if your engine has the breather at the back of the crankcase near the bottom, then there are some modifications to the cases and breather system that you might consider.

Would it be safe to intrepid this as a favorable endorsement for the 71 style breather and a modification to the system is not necessary? Man I hope you say yes!

Thanks Dave and Dave!

EGN# 20M3S/145099

DogT · Oct 23, 2010

145099 puts the engine between Jan 71 and March of 71 manufacture date when they started producing the SS. So according to my list it is possibly a Roadster MKII. Does the plate on the steering head match, and on top of the gearbox too? Just curious.

Yes, that is a timed breather you have. If you take that fitting off, you can look inside it and see 2 fan shaped cut-outs, and you should be able to see the simular-ly shaped rotary breather behind it that rotates on the end of the cam shaft. If you turn the engine over, you should see it open and close.

I'm not convinced it was an especially good breathing system. Effective but not enough air flow. Mick Hemmings blocks off the hole with a bolt and puts a one way breather on the timing case after drilling more holes from the timing side into the crank case. He also suggests venting the breather into the air, not the oil tank, which ever method you use. But it will drip oil eventually. I have heard of people that have no problems with the timed breather though. I'm going to try it for a year and see how it goes.

If you look here http://www.oldbritts.com/1971_g1.html, you will see the timed breather at the end of the cam shaft, item #46.

I want to see this bike running.

Dave
69S

T95 · Oct 23, 2010

DogT said:
145099 puts the engine between Jan 71 and March of 71 manufacture date when they started producing the SS. So according to my list it is possibly a Roadster MKII. Does the plate on the steering head match, and on top of the gearbox too? Just curious.

Yes, that is a timed breather you have. If you take that fitting off, you can look inside it and see 2 fan shaped cut-outs, and you should be able to see the simular-ly shaped rotary breather behind it that rotates on the end of the cam shaft. If you turn the engine over, you should see it open and close.

I'm not convinced it was an especially good breathing system. Effective but not enough air flow. Mick Hemmings blocks off the hole with a bolt and puts a one way breather on the timing case after drilling more holes from the timing side into the crank case. He also suggests venting the breather into the air, not the oil tank, which ever method you use. But it will drip oil eventually. I have heard of people that have no problems with the timed breather though. I'm going to try it for a year and see how it goes.

If you look here http://www.oldbritts.com/1971_g1.html, you will see the timed breather at the end of the cam shaft, item #46.

I want to see this bike running. 69S

Dave,
The numbers match on the frame and motor. I got some help last month confirming that I did indeed have a 71, thanks again boys!

So now it confirmed I do have the timed breather, and thanks for the link to the picture at oldbrits. Now I can drive myself crazy researching this topic, exactly why I asked for help on this thread!

Oh, I'll get it running but your first up! Maybe by this time next year?

69S[/quote]

DogT · Oct 23, 2010

I'm hoping you can get it done in a year. If you get at it, you can. Actually if you didn't have to fettle with lots of stuff, and had all the parts, you could put one of these bikes together in 2-3 days from zip. I'm just waiting for a clutch cable, dead in the water. It's running, I just haven't got it on the road yet. There is still time this Fall.

Don't rush it though.

Dave
69S

dave M · Oct 23, 2010

I built a cafe racer with a high compression ration engine, hot cam and various other mods using a set of standard cases with a timed breather and had absolutely no issues with the crank case ventilation.

T95 · Oct 23, 2010

DogT said:
'm hoping you can get it done in a year. If you get at it, you can. Actually if you didn't have to fettle with lots of stuff, and had all the parts, you could put one of these bikes together in 2-3 days from zip. I'm just waiting for a clutch cable, dead in the water. It's running, I just haven't got it on the road yet. There is still time this Fall.

Don't rush it though.

A year is a loose goal. She will be 41 next year and having her up and running would be a nice way to celebrate it! One of the big

DogT said:
I'm hoping you can get it done in a year. If you get at it, you can. Actually if you didn't have to fettle with lots of stuff, and had all the parts, you could put one of these bikes together in 2-3 days from zip. I'm just waiting for a clutch cable, dead in the water. It's running, I just haven't got it on the road yet. There is still time this Fall.

Don't rush it though.

dave M said:

I built a cafe racer with a high compression ration engine, hot cam and various other mods using a set of standard cases with a timed breather and had absolutely no issues with the crank case ventilation.

Click to expand...

Now that is promising news!

Was that a 71 motor and was timed breather discontinued in 71?

T95 · Oct 23, 2010

Ok, I know the timed breather was an issue. I would like to be able to research these items while I have time, any other issues I should be focusing on?

DogT · Oct 23, 2010

From the OB parts lists, it looks like in 72 the timed breather was discontinued. Other issues are the layshaft bearing for the gearbox, superblends for the crank shaft, real seals for the gearbox instead of o-rings, o-ring seal for the tach drive, improved mounting of the swing arm spindle, improved head steady if you want to go there, improving the Amal carb slides, upgrading the clutch pack to 5 friction plates or shimming the clutch pack, see old britts tech article on the clutch, I'm still using 4 plates and the adjustment is very critical. If you can reduce the play between the diaphragm and the circlip that holds it, you can have a real nice clutch pull and it breaks over like a compound bow. Then there's electronics, I won't go there since I'm running the AAU, points, and original stator, rotor, rectifier, condensers and coils. But I never had a kick back and when it starts, the kickstarter goes through like it's helping you. It's a pleasure to start. See my starting video at the Phoenix thread. I hardly had to push it. Cool.

Dave
69S

ClutchOut · Oct 24, 2010

Thanks to T95 for asking this question. I am in the research process for my '71 and want to make sure I have the knowledge before jumping in and missing an opportunity to improve or correct the original design.

Taking notes here.

T95 · Oct 24, 2010

ClutchOut said:
Thanks to T95 for asking this question. I am in the research process for my '71 and want to make sure I have the knowledge before jumping in and missing an opportunity to improve or correct the original design.

Taking notes here.

Exactly!

I would like to get her running, mechanically sound and looking good. I have already found a large volume of information within this forum on upgrades. For us the only ones that currently matter are those that apply to the 71 750cc.

Then we need to sort through the immediate need and nice to have items to find what our pocket books can handle. Of course, it would be ashamed to miss an opportunity to attend to some of these things while they are easily accusable. I sure don’t want to make splitting the cases routine maintenance!

Let me know if you find anything pertinent to our projects!
Gary

gjr · Oct 24, 2010

I have the same breather on my '70 Commando and it is just fine. The engine does not leak oil, most of it comes out the the air filter.

A few more things that I would look at are: The camshaft, I had a soft lobe on mine, Lapping the oil pump to help with wet sumping, and Fitting 850 valve guides.

Greg

DogT · Oct 24, 2010

Yes, I had Memphis Motor install bronze valve guides and the valve seal that is not in the early bikes. Not cheap, but I don't have the tools to do head work.

Oil filter would be nice if you don't have one. RGM has them cheaper than even fleabay if you buy other items and share the shipping costs.

Dave
69S

Advice on overhauling a 1971 750.

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