750cc Atlas motor

Previously wrote:

Hi, I am building a cafe racer and have found a 750 Atlas motor that has been completely rebuilt by a person that was going to race it. The thing I am not sure about is how it will behave when used as a regular ride, the engine has been balanced at 72% still on standard with heavy duty rods and 4s cam.

The head has been ported and flowed and now the inlets are at 32mm, can anybody give me an idea on how this engine might behave on the road.

I have read some info that the 4S cam is not so great??, if I have too I will put another cam which may be more suitable.

Thanks in advance for any advice.

Cheers

Andy

Hello Yes the S4 cam suppose to give you good power in mid to top end range . what the bottom end range is I do know this one. you may need to get the motor rebalanced as these motor do have a tendency to vibrate . and break engine mounts ,like the front frame lugs break in the welds .but it will be a powerful motor , the next thing the engine trys to do its pull the clutch off . the gearbox shaft. Newby Belt drive would help here. yours anna j

Atlas engines are not quite as bad as Anna implies. 32mm is pretty big for inlets - I would have thought that 30mm was quite big enough. 928 or 1 1/8 monoblocs suit the standard engines very well. A 4S cam will move the power band up the rev range. Fine for high rpm use but maybe a bit of loss of flexibility in normal use. If your engine is in bits, I would suggest a standard SS type cam might be better. If your engine is ready to fit, just ride it and see what it's like. My Altas engine hasn't broken the front engine mounts nor have I heard of one that has and neither have I heard of one pulling the clutch off the gearbox shaft. Urban myth perhaps? Gordon.

Dunstall recommended 30mm for silenced exhausts but Steve Robinson in his tuning notes writes 32mm or 34mm on open pipes. Like Gordon, for a road bike I think tractability is more useful than top end power. What do you mean by heavy duty rods?

Previously wrote:

Hi, The motor was built for racing and never used and now I have purchased it at a good price I will have to wait for it to arrive to see what has actually been done. The motor has beefed up conrods not sure what brand but as I said before it is set up to race, the engine head and barrel was taken of to show the condition in photos. I think it might be best if I purchase a more suitable cam as I want the bike to be a smooth ride not a rocket.

I am not sure about the affects of the porting of the head, I need to purchase a new set of 32mm concentrics but I was hoping to find out what sort of jet set up would be best for my needs. Would appreciate and info on set up timing etc and an idea where I can the standard SS cam mentioned above.

Cheers Andy

Hello Andy I would of thought that 30mm would big enough for a 750cc Atlas .you may have problems with flooding. if so drop the main jet size .and needle size . The Atlas Should have Monobloc 389/88and 389/87 carburettors up to 1966 from then they were going on to concentric carburettors Mk1 930L &930R with a 350 mail jet size 3 cutaway slide,size 20 pilot jet ,needle size .106.5 all it would be a advantage to fit a engine sprocket with one tooth bigger like 22 tooth sprocket As you may find that you engine is doing a lot of revs on A and B roads . I have Fitted a 22 sprocket to my 650 Manxman .it works nicely now. as I found the 21 tooth to be on the low geared side ,On A And B roads hope this helps yours Anna J

The Commando Combat had 32mm carbs and a 2S cam. The one I had was tractable and took off over4000rpm.

I replaced the 2S cam with an SS cam as I was to tour all round Europe and thought the base level SS would give improved fuel consumption. There was no change below 4000rpm but the kick above this haddisappeared. It also had no effect on the fuel consumption.

My advise would be to leave it alone, fit 32mm carbs and filters and see what its like. If the 4S is tooviciousthen fit a 2S.

Anna's main jet info of 350 on a pair of 30mm concentrics is ridiculous. 220 or 230 is the standard Commando setting. Flooding carbs is a function of badly seating float valves and not size of carb.

You may consider a PW3 cam as the price is about the same. You will need to pay close attention to cam timing with the PW3 as just fitting it with the Commando timing marks is not accurate enough.

In addition have a good look at the pistons fitted and avoid any with the slotted oil control ring groove. While I have never had a problem with these pistons in 50,000 miles, there are those who reckon they can loose the skirt. Later pistons have radial holes under the oil ring to remove the oil.

Previously wrote:

Anna's main jet info of 350 on a pair of 30mm concentrics is ridiculous. 220 or 230 is the standard Commando setting. Flooding carbs is a function of badly seating float valves and not size of carb.

You may consider a PW3 cam as the price is about the same. You will need to pay close attention to cam timing with the PW3 as just fitting it with the Commando timing marks is not accurate enough.

In addition have a good look at the pistons fitted and avoid any with the slotted oil control ring groove. While I have never had a problem with these pistons in 50,000 miles, there are those who reckon they can loose the skirt. Later pistons have radial holes under the oil ring to remove the oil.

Hello Yes You are right This should of been the sizes for the Monobloc Carburettor. As the Concentric Carburettors are smaller sizes yours anna j

Hi, nice motor. I would suggest if you are worried about vibration to Loctite all engine and silencer nuts and bolts, use cycle threads and EN16T studs, bolts, as they stay tight longer. Also rubber mount the carburetors, and most important, Viton tipped float needles to stop the mixture varying when the needle bounces at certain revs with metal ones. If the engine is no good to you, I have a standard one for a swop, just joking. Have fun, Paul

The thing I would worry about most on a "Racing" Atlas is the compression ratio. Use of Commando pistons in the Atlas, which had low compression dished pistons as standard, is reputed to make vibration a major issue.

I think the vibration will depend mainly on the crank balance. My Atlas cafe racer vibrated no more than most of the Norton twins I have ridden.

When I had an original dished piston split on my Atlas, I had no option but to use Commando pistons. I knocked back the ignition timing a couple of degrees and all was well - no noticable difference in vibration. I picked up an old Classic Motorcycle magazine which had a road test of a freshly restored (by a Big Name restorer) 88 Deluxe. The tester couldn't ride it for any time above 50 mph on account of the dreadful vibration. Something wrong there surely!

Broken or loose head steady?

Colin.

Previously wrote:

When I had an original dished piston split on my Atlas, I had no option but to use Commando pistons. I knocked back the ignition timing a couple of degrees and all was well - no noticable difference in vibration. I picked up an old Classic Motorcycle magazine which had a road test of a freshly restored (by a Big Name restorer) 88 Deluxe. The tester couldn't ride it for any time above 50 mph on account of the dreadful vibration. Something wrong there surely!

Sounds like it. My first ride on my cheap very second hand 99 back in 1969 was the most dreadful vibro-massage. It shook the plates off the new battery. The helpful RAC man said "it's yer battery mate" and b******* off. I did not renew my membership. Later I discovered what the odd bit of metal was left over when I first assembled the partly dismantled heap of junk. So much smoother with its head steady fitted! And it still is to this day. Smoother and a heap of junk. Gordon.