t500 carb ?

[argo1974]

According to official data, early T500 had EX:170, TR:120 and IN:163 degrees of duration. Looking at plain numbers, the intake duration seems to long already for street use.

[Vintageman]

According to official data, early T500 had EX:170, TR:120 and IN:163 degrees of duration. Looking at plain numbers, the intake duration seems to long already for street use.

Does seem like a lot, Is this taken from a Suz manual about a stock bike? I know there is the official Suz performance bulletin that says take 5mm off Intake and 5mm off Exhaust on early engine cyl.

I had two sets of each. One of those earlier set is now on my 75 T500. And, I still have one set each (early and later) not in bikes. There is at least 2.5mm difference between all my earlier and later. That is a lot too. Another gent or two one this board about a year ago here verified as well when I only had a later set.

I have another engine I plan to hot up a bit more. In hindsight I could have just shaved skirt, but not as affective I hear as changing port. Also the later has the full intake divider but, they did widen intake port too. I can't remember if the windows widened or just the bridge widened (e.g. GT250 versus GT380 intake). I’ll have to check this.

I understand it’s something you have to see yourself to believe, like UFOs, Big Foot, Loch Ness, or the world is round.

I’ll try to take some pics. Waite... i've seen pics of UFOs, Loch Ness…

[argo1974]

I have that data from french bulletins:
http://www.bilder-hochladen.net/files/b ... 0-fa83.jpg" onclick="window.open(this.href);return false;
http://www.bilder-hochladen.net/files/b ... 1-02a3.jpg" onclick="window.open(this.href);return false;
Those numbers have been published elsewhere as well.

[Vintageman]

argo1974,

Thanks for posting that.

Per A. Graham Bell book, 163 degrees is tuned for 8000 rpm (wonderful!)

This is strange for what I have seen in 1973 K/L/M/A/B power dropped 45/44hp and peak went down a full 1000 rpm from 7000 RPM to 6000 RPM. What is pictured in that period paper is actually the later one K (detuned) but, it still shows 47hp@7000 RPM.

It must be hard to believe when I say intake port timing was detuned: after all these years I am the only one I know stating this to be the case. Can't believe I am the only person to have compared port dimension.

[tz375]

163 degrees is more or less 106mm down which is the same as an MAB GT750. Earlier GT750 were 101mm or 145 degrees of duration. According to our analysis, 106 is too long on a 750 and that's what hurts low end on those models - not lack of pipe couplers or the extended Exhaust duration.

That's why I recommend JKL Barrels for the street - even on a tuned motor and why adding a spacer is a great way to grab some of that excess duration back.

I'm sure that there are people out there who have emasured early and late ports and have theoir own ideas as to what works and why.

[Vintageman]

. According to our analysis, 106 is too long

Don't agree. My experience it works real well on early T500, gives more spirited running, while still retaining great street bike manners.

Staying within scope of this forum. I'll counter your GT750 early late changes with Suz GT250 pre 1976 versus 1976 and up. Same complaints about how it lost too much of its sweet street manners.

How does Ex timing compare T500 with early/late GT750?

[argo1974]

Vintageman, I have untouched T500 cylinders with 40mm, 40.5mm, 41mm and 41.5mm exhaust height on the shelf.
E. g. those with 41.5mm came off a 1977 GT500. My 68" Cobra has exactly 40mm.
I wonder if you considered different exhaust heights as well early/late bikes may have? And shorter intake ducts?

Richard, what would be ideal transfer and intake port durations for early GT750 cylinders with say 42mm EX height?

[tz375]

. According to our analysis, 106 is too long

Don't agree. My experience it works real well on early T500, gives more spirited running .....

That means you do agree.

More intake duration cuts low end power and potentially adds some at the peak, but if it isn't running very high revs, you lose more at the bottom than you gain at the top. On a GT750 most riders want a big soft bike that just rolls forward without a lot of revs, so long duration is typically "bad". On a lighter 500 a rider may prefer the more spirited style i.e. less low end and a little more above 5,000.

Argo, with a stock JKL 42mm exhaust port, I'd leave stock inlet and transfer timing. For a little more fun, raise the exhaust port and for a lighter bike, raise the barrels and widen the ports. We know of a 750 that was making 127 at the rear wheel and it isn't very radical.

[Vintageman]

On piston ported engines if you increase intake timing it can increase over rev rpms too, so you gain top end. Yes, it depends on the state of other ports how it turns out. Good luck so far with T500 and T3xx. But, it did not help my GT550 over rev but, did not hurt bottom either. Per good advice here and our comparing to RD400 I have raised Ex 2mm on GT550 and will soon know.

The early T500 set I swapped for with on my 75 had same Trans. and Ex. height but, I would still estimate my paper trace technique has at least +/- 1/2 mm of error. I am not 100% sure what years the early was but, T500.

I have checked various other cylinders from Suz and seems like either Suz either plays with +/- 1mm year by year or that's tolerance of manufacturing. Yes, if you have a cylinder that was 1mm lower and another 1mm higher for a 2mm difference that you should feel.

It would be nice to see if others see the 2.5mm diff on intake port. There is a post here where another person or two saw the same, then I spent money saw the same but, more samples would be better proof of a detune.

I have another early engine and curious about exhaust height too (can we tell year by serial #?)

Yes, I did consider the shorter intake length and acquired everything from early T500.

The later T500 carb I had just tuned well for my 75 engine with JEMCO's. It took me a while to get all throttle positions acceptable and to reduce changing too many things at once I tried just early cyls only first. It was sufficient improvement and have not tried other changes.

Intake length carb to cyl. I did consider that important as others have as well. I had one point compared length from carb to cyl divider. The later is longer but, not that much % overall.I did not feel that was as significant for the RPM we are talking and not motivated to try. I could be wrong.

And my 75 would not look original. I also did not like the kink the early boot made when mating into cylinder and to me it looks like the later via the aluminum piece smooths that kink out gradually if that helps anything.

For me just cyl swap worked well T500 and the only diff I say was Intake porst lower and no full length intake divider: Sample of one

[tz375]

On piston ported engines if you increase intake timing it can increase over rev rpms too, so you gain top end.

Almost. Unless the motor is intake restricted, changing inlet timing won't automatically add power or revs to the top end. Think of it as rocking the power curve around a point at say 5500 revs. With a motor that doesn't rev high, the leverage point is roughly 2/3 of the way up, so as the curve rocks, it will tend to lose more at the bottom than at the top.

In the real world, at lower revs, we are rarely using full throttle, so any loss can be compensated by a few more revs or a touch more throttle. A heavy bike will reveal any loss of pull at low speed more than a lighter bike, but with longer intake time, the motor will feel more sporty because it makes less power at the bottom and has to be ridden slightly more aggressively to actually make the same performance, but our "Butt Dyno" is not so finely calibrated as we might believe.

But we all like a different feeling from a bike. Some of us like a top end screamer and some want smooth pulling power from low revs and some of us want both It all comes down to objectives and as Steve (Vintageman) suggests manufacturing tolerances appear to be pretty poor at the time, so some bikes were probably much better than others straight out of the box.

Just as an aside, some years ago I was involved with a team racing Aprilia Cup bikes and one off season, I took three motors plus spares and measured every critical part and the motor with all the "best" bits regularly made more power than the others and I mean considerably more - and that was a modern motor, made to modern, tight manufacturing tolerances.

At 65HP they made more power than 95+% of all GT750s and were so fun to ride. Such is progress.