GT750 Porting

[pull the wire]

I believe I have read every GT750 porting technique available and I'm still a bit confused. I'm starting with a rare set of mildly ported Ocelot cylinders and also have the Ocelot chambers that were originally installed when this bike was built up in 1973. The bike originally belonged to the owners son of the local Suzuki dealership. I also have Mikuni TM 34MM carbs that were on it.

From my reading, it sounds like the safest way to get port timing in a greater state of tune was to add a spacer plate under the cylinder. I ordered a 2mm spacer and a matching 72mm head gasket from Cometic. The base gaskets i'm using are .5mm thick. So I should have a total rise of around 3mm. Currently my exhaust port opens at 38mm and the transfers open at 50.5mm. According to my degree wheel I have 29 degrees of blow down. Also if my math is correct adding the spacer and 2 gaskets will put my exhaust opening at 35mm and my transfers opening at 47.5mm. Soooo, from waht I understand is that will get me very close to most of the optimum ports timing numbers I want. Of course I am going to machine off the top of cylinders for a 0 deck height.

I measured the bore of a stock cylinder head at the machined in groove and came up with 70 mm. I am bored to 72mm with a .043 thickness 72mm bore head gasket. The difference in these bore sizes concerns me a bit.

So my first question is; when using the spacer, do I need to lower the exhaust and tranfer port floors so they are open that extra 3 mm or leave it alone?

Do I need to lower the intake openings too?

Do I need to machine 3mm off the bottom of the intake side of my very expensive and rare Wiseco forged pistons or leave it alone?

Should I mill .020" off the head and have a 72mm groove machined into it? The head that was on it was only milled leaving no discernable "edge", and resembled a gentle curve starting at the head gasket fire ring. I think I would prefer a noticeable groove or edge to make sure I get maximum seal at the fire ring. I have 4 heads so I don't mind using 1 as a sacrificial.

Does anyone have the cylinder spacing centerlines so I can give that spec to my machinist if I do modify my head??? I might as well go all the way now. HAHAHA

I have dumped a bucket of money just on the engine. Crank is going to Bill Bune for an overhaul with everything execpt the actual crank webs. New trans bearings, new clutch parts, polished covers, endless odd and ends and chassis parts. She's gonna look good and hopefully 80 ponies on the dyno.

Anyone with answers or comments are welcome please

[tz375]

Ocelot made some nice GT motors back in the day with their specially manufactured WISECO pistons. If those are old style WISECOs grab a set of Wossners to replace them.

Just as a heads up, You don't need to go that far to get to 80HP. A set of JEMCo pipes and mild port job will get you pretty close.

In theory with a spacer plate, you should lower the floors of all the ports but to be honest the transfers and exhaust don't flow much on the floor so the effect will be small.

Do not mess with the piston skirts.

Measure the floor of the intakes and determine where it is and where you want power to be. If the barrels are J/K/L then you can lower the floor by 3mm to clean them up. If the barrels are M/A/B then just clean the floors up to remove any steps. Lower that stock on a MAB motor and you lose power below 5500 and only really gain anything above 6500.

That exhaust port is already considerably higher than stock and with a spacer will be getting close to TR750 timing.

Bore spacing appears to be a little inconsistent, so take you barrels to the machine shop and have them pinned in place and he can machine the combustion chambers concentric with the bores. You don't need that counterbore but you do need to measure the compression to avoid major meltdowns and you need a head gasket that matches the bores and bore spacing. Stock GT gaskets are way too large and cause gasket failure and detonation.

34mm carbs are OK up to crazy HP levels.

With decent pipes and that tune, you should be looking at 100-110 at the rear wheel so get a new clutch center bearing and good friction plates. I just removed a set of plates with 4 cracked and broken plates in one set.

You will want a decent ignition system to light the fire and jet rich to stay safe.

Now where are the pictures?

[pull the wire]

This is a couple pics of the barrels.

I would like to see if anyone can idetify the chambers I have. The pics will be restoration "Can you identify these cambers".

[ja-moo]

With decent pipes and that tune, you should be looking at 100-110 at the rear wheel

Are there any dyno graphs posted anywhere?

[tz375]

John McCann has an Ocelot motor that he rebuilt and posted a dyno chart somewhere showing 118 at the rear wheel IIRC.

[pull the wire]

Hmmmm. That should P--- off a lot of my harley buddy's with 110 CID and maybe 90-100 ponies. This will be good enough HAHAHA A few more months and we will see what the dyno says. Cyinder spacer should be in this week and then to the machine shop to chop off the top. Taking tz's advice I am gonna pin the spacer to the jug for sure.

Thanks for your help

[tz375]

It's the head and barrels that need to be pinned. The spacer should be close enough.

[ja-moo]

Does John McCann have a screen name?

[oldjapanesebikes]

It's the head and barrels that need to be pinned.

This is just to ensure the head gasket and head properly aligns with the bores ?

[Suzukidave]

It's the head and barrels that need to be pinned. The spacer should be close enough.

TZ375's site with the missmatch between heads and blocks explained http://www.pinkpossum.com/GT750/GT750head.htm" onclick="window.open(this.href);return false;

[pull the wire]

Holy two stroke smoke batman! I have some spare dowels that will work perfectly for this. I have 4 heads so I'll pick the best one and skim it off to make sure its flat and correct as needed. I thought I read somewhere to pin the spacer because of gasket squish and movement, but if ya'll say its ok I'm just gonna pin the head.

Thanks to all