74 GT750 cylinder VS 76 cylinder

[h2okettle]

My 74L had much better low end and mid range pull than my 76.. I know the porting was changed in the later years for better top end pull.. All of my riding is mid range and miss the roll on power.. I understand the gearing was also changed from 15/47 to 15/43 on the sprockets.. I do a lot of distance riding and my 74 had 16/43 and ran fine loaded down and two up..
SO the question here show do you tell the difference between the 74 cylinder and the later ones ? I am thinking about swapping the cylinders to a 74 model.. Also, for the motor heads, do you think this is a good idea ?

My rpm is 4500 at 70 mph, which is probably why my fuel mileage is less than 30mpg..
Thinking by just changing the front sprocket to a 16, will only make it lazier, and have to gear down to accelerate..

[rngdng]

There is a service bulletin that explains the 74-75 porting changes. If I remember correctly they raised the exhaust port about 5mm. A 74 cylinder should work fine on it, and I don't think they changed the head in 75, but not positive on that one.


Lane

[jabcb]

They also changed the exhausts.

[rngdng]

Here is the bulletin. It was 2.5 mm on the exhaust, lowered intake, skimmed head, and thinner gaskets. Look at GT-7.


http://www.oldjapanesebikes.com/mraxl_G ... ins/page-2


Lane

[tz375]

There is a belief that the removal of exhaust crossovers hurt low end power, but I have not seen a comparison of one exhaust to the other.

Analysis we did suggested that the later (taller) exhaust added top end but at very little cost at the bottom end. The intake is different though. It appears that the later (low) intake port hurts low end performance disproportionately. For a generally more torquey motor I would fit a set of 74 barrels if you can find a set or a set of 73 with the same ports but without recesses for the water cooled badges.

[h2okettle]

There is a belief that the removal of exhaust crossovers hurt low end power, but I have not seen a comparison of one exhaust to the other.

Analysis we did suggested that the later (taller) exhaust added top end but at very little cost at the bottom end. The intake is different though. It appears that the later (low) intake port hurts low end performance disproportionately. For a generally more torquey motor I would fit a set of 74 barrels if you can find a set or a set of 73 with the same ports but without recesses for the water cooled badges.

I put a set of 75 exhaust on my 74 GT750, top get rid of the crossovers as it almost never failed that they would begin to leak some on my rides... Which usually had between 4K-7K miles per time.. Anyway, I did not notice any power difference in the pipes without the crossovers..

So the question is, other than having to get the ports measured, is there any marking to tell if the cylinder is a 74 or later ?? I don't really want a 72-73 jug, because I want it to still look stock..

Thanks..

Allen..... Rides without adult supervision !!

[Frank T]

I just pulled out my spare GT750 cylinder and measured it. It is the rare and extremely desirable '74 cylinder! Only $500, what a bargain!

[tz375]

An L barrel is externally identical to an MAB. Unlike early 72 compared to late 72 and 73 which were all different, a 74 looks the same as a 75. The only way to check is to measure the ports. They are significantly different and easy to differentiate.

I checked my records and don't have a set of 74 barrels available. You could probably take your later barrels and raise the intake floor with Devcon F to shorten the intake timing.

You could also mill 3mm off the top and slip in a spacer plate to raise the intake port floors. It would bring the inlet back a little and the extra transfer and exhaust time should add more back at the top end. I like higher compression for low to mid rang grunt, but that requires a good ignition system to avoid pinging/detonation at lowish revs and big throttle openings ie high load.

[h2okettle]

I just pulled out my spare GT750 cylinder and measured it. It is the rare and extremely desirable '74 cylinder! Only $500, what a bargain!

Well kind sir, I will purchase said item... I will send you the funds via Western Union from my personal Nigerian account, that my "future" Ex Father-inlaw has set up for me.. I will send you much more than you require, in return you send me through paypal your account information, the remainder of the excess money.. Also a current bank deposit slip with said amount of money in account, so that I can also empty your account while I am online... This of course is all legal in my country, so not to worry..

Allen.... Still wasting CCI !!!

[Frank T]

I just pulled out my spare GT750 cylinder and measured it. It is the rare and extremely desirable '74 cylinder! Only $500, what a bargain!

Well kind sir, I will purchase said item... I will send you the funds via Western Union from my personal Nigerian account, that my "future" Ex Father-inlaw has set up for me.. I will send you much more than you require, in return you send me through paypal your account information, the remainder of the excess money.. Also a current bank deposit slip with said amount of money in account, so that I can also empty your account while I am online... This of course is all legal in my country, so not to worry..

Allen.... Still wasting CCI !!!

I guess that sounds legit.....ok I will do it.

[rngdng]

I have a standard-bore 74 cylinder as well. I don't think I touched the ports on that one. Not sure about the head....


Lane