750 engine tune.

Matchless · Aug 27, 2020

I have recently dug my 1968 Commando out of it's long hibernation & want to re jig the engine. After riding my 920 which I love more than life itself, the 750 is no fun. The problem is that it's gutless below 3,000 revs, above which it's fine. The spec is as follows:
Maney stage one head (34mm inlet ports & 3mm oversize inl valves
Maney camshaft which I believe is very similar to a 4S type
Maney crank
Maney barrels
8.5:1 cr
Boyer mk3
Original silencers (Atlas type)
I have tried various carbs, 34mm MK 2s, 32mm MK1s, & even an SU but I think the problem is either the cam or head or both.
Do any of you clever fellows have any good ideas as to what makes the most usable 750 engine?

Martyn.

peter12 · Aug 27, 2020

Not an expert at all but the compression ratio seems very low.

ashman · Aug 27, 2020

Most hot cams don't begin to work till they reach a good working RPM so under 3,000 RPM your bike will be a bit sluggish and lazy and you say its fine after 3,000 RPM and for a hot motor your compression might be a bit low at 8.5:1 that's around a stock compression for a 750.

Ashley

t ingermanson · Aug 27, 2020

Higher CR and a 5 speed to keep the revs where your parts are designed to sing?

jan nelder · Aug 27, 2020

Matchless said:
I have recently dug my 1968 Commando out of it's long hibernation & want to re jig the engine. After riding my 920 which I love more than life itself, the 750 is no fun. The problem is that it's gutless below 3,000 revs, above which it's fine. The spec is as follows:
Maney stage one head (34mm inlet ports & 3mm oversize inl valves
Maney camshaft which I believe is very similar to a 4S type
Maney crank
Maney barrels
8.5:1 cr
Boyer mk3
Original silencers (Atlas type)
I have tried various carbs, 34mm MK 2s, 32mm MK1s, & even an SU but I think the problem is either the cam or head or both.
Do any of you clever fellows have any good ideas as to what makes the most usable 750 engine?

Martyn.

Large inlet ports won’t help at low revs, and may even make things worse, AFAIK. It was probably set up for high revs power, which is what it is doing.

Matchless · Aug 27, 2020

I am aware that the compression ratio is on the low side, but sometimes only 95 octane is available & I wanted to avoid pinking, especially on warmer days. The fact that it goes so well over 3.000 revs surely means it can't be too low or does it? Unfortunately I don't have a standard head to try but I keep thinking the ports are on the large size.
As for a five speed box, it is a close ratio four speed which helps to keep it reving. but more low down grunt would be nice.

bill · Aug 27, 2020

the head and cam selection you have all pionts to a top end motor build not a good all round street build. I would look for a stock head and std camshaft and go down to 30MM carbs but if you want more low end a single 34 carb or maybe a pair of 28s

baz · Aug 27, 2020

I agree with Bill on this
A stock camshaft and maybe just sleeve the inlet ports on the head you have ?
Leaving the compression where it is
Cheers

Bernhard · Aug 27, 2020

There is a general rule, the bigger the bang the bigger the power, you have tuned this engine everywhere except where you have fitted lower compression pistons than a standard Commando comes with, which is 9:1. WTF????
Your engine tuning is both innovative and lacks innovation.

kerinorton · Aug 27, 2020

Back in the very early 70"s my 750 was a rocket from standing start [ 19 tooth sprocket ]. Still is in a way, though that's not my interest anymore.

RoadScholar · Aug 27, 2020

bill said:
the head and cam selection you have all pionts to a top end motor build not a good all round street build. I would look for a stock head and std camshaft and go down to 30MM carbs but if you want more low end a single 34 carb or maybe a pair of 28s

I agree with one caveat: Depends on your riding behaviors and expectations. For what you have spent on the parts you mentioned I would think you could have purchased a low mileage crotch rocket that would outperform the 920 in all respects or a GT bike for touring. So, as Bill mentioned, a sedate 750 for when you want to relax and smell-the-roses, not trying to join them, makes sense to me; again depends on the envelope you expect to ride it in.

A simple fact that more power means less reliability is worth considering. The Norton I am currently building is tuned for the roads and speeds (40 to 80) I ride at in New England. I have a blueprinted stock 750 engine w/single Mikuni. I have set the suspension Consentino/Ikons/NYC Norton) to be a bit stiff below that envelope and a bit "loose" above that. I am looking for the motorcycle to take me for a ride in that envelope and let me know when I have to stop smell-the-roses and become its partner.

Tell us what you expect your riding envelop to look like for the 750.

Best.

Bernhard · Aug 27, 2020

Matchless said:
I have tried various carbs, 34mm MK 2s, 32mm MK1s, & even an SU but I think the problem is either the cam or head or both.
Do any of you clever fellows have any good ideas as to what makes the most usable 750 engine?
Martyn.

Norton solved the low down power with a single Amal carb by inserting split inlet port inserts on the Saint versions. This gave a top gear range from 15 to 100 mph.

jan nelder · Aug 28, 2020

Bernhard said:
Norton solved the low down power with a single Amal carb by inserting split inlet port inserts on the Saint versions. This gave a top gear range from 15 to 100 mph.

Never heard of this: how is split inlet different from a typical 2-1 manifold please?

baz · Aug 28, 2020

Bernhard said:
Norton solved the low down power with a single Amal carb by inserting split inlet port inserts on the Saint versions. This gave a top gear range from 15 to 100 mph.

"Saint" version??
Do you mean the triumph Saint?

Triton Thrasher · Aug 28, 2020

Enlarged inlet ports and a racing cam: you went to trouble and expense to get rid of engine flexibility.

Any road up, an old bike bodge to get some low rpm power back is to increase the inlet valve clearances.

acotrel · Aug 28, 2020

If you want pull from lower down in the rev range, a 2 into 1 exhaust system will help any cam give you that. Changing the inlet configuration from twin to single carb won't change where the power band starts. If you have enlarged the inlet ports, you are probably stuffed. The gas flow becomes too slow at low revs.

baz · Aug 28, 2020

Has anyone tried the inlet sleeves on a commando like the Manxman had?

gortnipper · Aug 28, 2020

With higher CR it will probably spin up more quickly and you wont notice the reduced low end torque so much. That is what i would do first with the motor.

Chris · Aug 28, 2020

Hi Martyn
My 750 race engine. Is an original Maney engine from when he first went to Daytona. So an older build. Spec is stage one head.
Steel flywheel. Alloy plate welded to strengthen the cases. It was supposed to have have Steves Coswoth pistons! However I got stitched & found They had been switched. His cam ie similar to a 4s. I ran 34mm mk2 smoothbores. Alloy barrels. Belt drive etc.
As you know I love the big girl, all 960cc it is just a joy to ride. Pulls from everywhere.
The 750 revs. A few points, ie feel when riding it. As a racer compared to your road bike, one is weight. Second its lower, so I would expect it to feel better stronger than your road bike. The bike ticks over! Trickles along to the holding area, pulls cleanly of the start & revs to the point where I feel uncomfortable. The only difference I see is compression ratio. When I the big engine went, I put the 750 straight in. I enjoyed the ride before the bogus pistons cried enough but I also found the exhaust was much louder harsher. It's a very different ride.
I will dig out the head details for you.
Hope this helps Chris

oldmikew · Aug 28, 2020

Matchless said:
I am aware that the compression ratio is on the low side, but sometimes only 95 octane is available & I wanted to avoid pinking, especially on warmer days. The fact that it goes so well over 3.000 revs surely means it can't be too low or does it? Unfortunately I don't have a standard head to try but I keep thinking the ports are on the large size.
As for a five speed box, it is a close ratio four speed which helps to keep it reving. but more low down grunt would be nice.

Does pulling the ignition back a tad help matters? Easy enough to do . And why not straight through pea shooters? . I think the engine would tolerate a higher compression even on 95 octane. And there is always witches brew , though not too keen on making repeated applications for smallish tanks . It is a serious health hazard even with skin contact.

750 engine tune.

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