750 engine tune.

Maney cam right?

Should be: inlet 99deg ATDC exhaust 104 BTDC.

At your figures, I would leave it alone.

Thanks Nigel. Are those figures from Steve? If they are it seems my cam is a fair way out. As it stands at the moment I might pull the motor & rebuild it back to standard, as I think I've been spoilt with the 920 which is just soooo much better in every respect. I could even build an 850 motor out of parts I have but would really like to keep the original engine for now.

Yes, from Steve.

Your cam is 3 degrees retarded vs ideal (Inlet). Personally, I don’t think that’s a huge amount.

But if you advance it 5 degrees it’ll ‘only’ be 2 degree advanced vs ideal. Probably a better overall setting.

IMHO, I doubt it’ll make much difference. But it might be worth a try to advance it before you decide to strip the whole motor.

I‘d still favour bumping up the CR myself matey.

Matchless said:

Not sure what you mean Jim. Please elaborate.

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Symmetrical cams have the same lobes on intake and exhaust. A straight up split is when the centerlines of the intake and exhaust lobe both have the same lift at TDC (both lobe centerlines are the same distance from TDC). An advanced cam has more intake lift at TDC than the exhaust.

A Maney cam is similar to a 4S cam and is not symmetrical. But you can still calculate the lobe centerlines and see the distance of each lobe centerline from TDC. Its better to use the lobe centerlines as measurments rather than the opening and closing ramps.


Straight up split shown below.

Just to update you chaps on my 750 motor. It is now in bits & I've built & fitted an 850 motor which is in standard tune with the exception of a PW3 cam. I fitted this because I didn't have a cam lying about, so had to purchase one, & I know they work well in an 850 having used one before. The bike is now a delight to ride.
As for the 750 engine, well I was lucky enough to find a good bare unmolested head for £200. I will be building a standard 750 motor with this over the next few months & might then sell the Maney head.
Just before I pulled the 750 apart I measured the CR & it was exactly 9:1. So I was misleading you all at 8.5:1 & it just goes to show how poor my memory is!

Martyn.

Well, Unless it's a Combat head. I'd still shave .040 off that stock 750 head to get to 10:1 and find a set of '74 32mm carbs to 30mm head manifolds if it's not a Combat head. Much better flow going into 30mm ports. Cam choice could be as simple as stock or 2S or similar lift/ duration . Higher lift/ longer duration just makes it a high end motor. That's all I did and was very happy with results on the street. And oh yes, a 19 tooth sprocket really is nice in a 750

Deckard said:

Well, Unless it's a Combat head. I'd still shave .040 off that stock 750 head to get to 10:1 and find a set of '74 32mm carbs to 30mm head manifolds if it's not a Combat head. Much better flow going into 30mm ports. Cam choice could be as simple as stock or 2S or similar lift/ duration . Higher lift/ longer duration just makes it a high end motor. That's all I did and was very happy with results on the street. And oh yes, a 19 tooth sprocket really is nice in a 750

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The head I've aquired is an earlier one with the 1.125" (28.5mm) inlet port dia. & has not been skimmed, so it's staying that way. If I want more performance I'll take the 920 out for a spin.