750 engine tune.
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[QUOTE="acotrel, post: 494165, member:
Re ; 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. [/QUOTE]
You are slighty wrong on the first count.
I've had an ex police Atlas with a single carb, it will just pull top gear from 15mph.
On enlarged inlet ports, I agree with you.
Norton put split tubes in each port to INCREASE the gas flow at low engine revs.
Making the inlet ports in the cylinder head bigger has the opposite effect.
Re ; 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. [/QUOTE]
You are slighty wrong on the first count.
I've had an ex police Atlas with a single carb, it will just pull top gear from 15mph.
On enlarged inlet ports, I agree with you.
Norton put split tubes in each port to INCREASE the gas flow at low engine revs.
Making the inlet ports in the cylinder head bigger has the opposite effect.
OK I stand corrected.Yes, but the weren't "Saint" They were the Interplod
baz
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That's not what I said
I said do you mean the triumph Saint?
I know Norton made police bikes but that's not what I asked is it?
baz
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That's also not the question I asked
Go back and read the actual question
baz
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Apologies accepted
Matchless
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Gentlemen,
Thank you for all your replies. I think you have all come to the same conclusion as myself, that is the head & cam are not suitable for a 750 road bike. I did think about turning up a pair of sleeves as fitted to my '62 650 ss, but this would not reduce the size of the inlet valves. Might still be worth a try though along with bumping up the compression ratio to 9:1. I was wondering whether altering the cam timing a few degrees would improve matters also.
Thank you for all your replies. I think you have all come to the same conclusion as myself, that is the head & cam are not suitable for a 750 road bike. I did think about turning up a pair of sleeves as fitted to my '62 650 ss, but this would not reduce the size of the inlet valves. Might still be worth a try though along with bumping up the compression ratio to 9:1. I was wondering whether altering the cam timing a few degrees would improve matters also.
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the 4s cam was reputed to have more midrange than the 2s
You misunderstood the reason for putting a port reducing collar inside the inlet port. You leave the valve size as standard, don't touch them at all, the Atlas had a measly 7.5 :1 compression, you can leave the pistons you have in now, but reduce the inlet port size about 1mm below what it was as standard. Try that first.
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I would check the valve timing and make sure the 4s is dialed in as spec. If it is retarded then low revs willsuffer . In anycase would try a couple of degrees advance . But if you change valve timing do checkyouhave notgot valve piston contact issues.. And as I haev already suggested you might find the bikewill run better with ign retarded a degree or so.
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If you have the possibility to have spare cam followers, you can radius them to get less duration , bumping cr is good as you will keep the same lift, and reducing size of inlet to improve velocity will help , plus advancing cam timing ..........IHMO !
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34mm Porting and hot cam will give you a top end motor that won't pull until 4500 RPM or higher. You can accept it as a hot cafe bike and ride it that way or back off to a stock type cam and sleeved intake runners. The large valves are OK but the best option is to raise the port floor and match the intake manifolds (not easy to do). If you sleeve the intake ports make them eccentric to raise the port. No smaller than 30mm ID. The problem is raising the port floor radius leading up to the valve seat. I have seen aluminum runners bolted through the port floor. Or you can drill through from the bottom and spot weld the runner in place. Very time consuming.
Or relocate 4 bolt holes and use that head on your 920
My unfinished project below.
Or relocate 4 bolt holes and use that head on your 920
My unfinished project below.
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Matchless
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The 920 has a Comstock modified Fullauto head along with your rods & pistons and lots of other nice bits. It really is my perfect Commando engine, but it would also be nice to have a 750 I loved as much, just for different reasons. I may well try turning up a pair of eccentric push fit port liners & backing off the ignition timing a degree or two as a start. The valve timing if I remember checked out to Steves figures.
Fast Eddie
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I‘d start by bumping up the CR Martyn. I’d have thought you could run 750s (assuming flat topped pistons?) with a tight squish and still have ‘sensible’ CR?
Seems sacrilege to me to start sleeving a Maney head...
Seems sacrilege to me to start sleeving a Maney head...
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A CR bump up is easily reversed for a stock head.
If it doesn't work, sell/trade the Maney head for a FA 750 head. I think there a a couple about.
If it doesn't work, sell/trade the Maney head for a FA 750 head. I think there a a couple about.
Lots of advice already given on getting power back below 3000.
I have a question, does anyone ride below 3000? When having fun and really using the bike, my 850 never sees revs below 3000.
This is where things get smooth so it spends very little time below 3000, only in town putting around, doddling along because of slow traffic ahead, 30 mph speed limit etc. There is no huge power requirement here, a moped output would suffice. Even in those situations I often shift down to bring revs back to 3000 for smooth operation, as advised in the riders manual. So should the need to accelerate rapidly present itself, that is available.
Even though it's a stock 850 with lots of low end, for me low end acceleration pull starts at 3000.
You mentioned that your bike pulls well from 3000 up, sounds ideal.
Now if you said it was gutless between 3000 and 4500 then took off like a rocket, that would be the description of a really annoying roadbike, like some I've ridden. A much hopped up 790 Hinckley Triumph Bonneville ( owner spent $7k on performance mods) comes to mind.
Glen
I have a question, does anyone ride below 3000? When having fun and really using the bike, my 850 never sees revs below 3000.
This is where things get smooth so it spends very little time below 3000, only in town putting around, doddling along because of slow traffic ahead, 30 mph speed limit etc. There is no huge power requirement here, a moped output would suffice. Even in those situations I often shift down to bring revs back to 3000 for smooth operation, as advised in the riders manual. So should the need to accelerate rapidly present itself, that is available.
Even though it's a stock 850 with lots of low end, for me low end acceleration pull starts at 3000.
You mentioned that your bike pulls well from 3000 up, sounds ideal.
Now if you said it was gutless between 3000 and 4500 then took off like a rocket, that would be the description of a really annoying roadbike, like some I've ridden. A much hopped up 790 Hinckley Triumph Bonneville ( owner spent $7k on performance mods) comes to mind.
Glen
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ashman
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Under 3,000 RPMs you are starting to labor the motor, not good for any motors, as Glen has said 3,000 RPMs is just about right for for a Commando motor, my 850 is built to stage one motor for street riding (from the performance section in my Norton workshop manual) the stock cam was built up to 2S cam specs with lots of port work but still using stock valves, crank balanced at 72% for the Featherbed frame, I can ride it at 40 MPH around town in top gear but under that I have to drop it to 3rd, its quite smooth at 40 MPH but its on the verge of laboring the motor if I go under that speed, but its after 4,000 RPMs when the cam kicks in and it just wants to pull and when it pulls it just keeps going and will rev freely way past over reving it so you got to watch your throttle hand as it will keep reving till it goes BANG, I built this motor back in the early 80s when I convered my 850 to the Featherbed frame and to this day still running the same set up with the same built up cam and head with still the original valves but running different carbs and Joe Hunt maggie.
With this hot motor it has been very reliable, is very quick reving and have clocked up a lot of miles as it was my everyday ride till just over 6 years ago when I brought my new Triumph Thruxton so its semi retired just like it owner but I still love taking it out every so often as its a great joy to ride and gets up and goes when I give it to it, it surprises a lot of modern bikes and I have a lot of fun on it.
So really you shouldn't run your motor under 3,000 RPMs as its not good for any Norton motor whether stock or a hottie.
Ashley
With this hot motor it has been very reliable, is very quick reving and have clocked up a lot of miles as it was my everyday ride till just over 6 years ago when I brought my new Triumph Thruxton so its semi retired just like it owner but I still love taking it out every so often as its a great joy to ride and gets up and goes when I give it to it, it surprises a lot of modern bikes and I have a lot of fun on it.
So really you shouldn't run your motor under 3,000 RPMs as its not good for any Norton motor whether stock or a hottie.
Ashley
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