problem, need suggestions

chainsaw · Post by **chainsaw** » Fri Jul 17, 2009 9:06 pm

1976 GT 750, pods, stock exhaust, carbs cleaned out well(no trash), gas tank cleaned out well (no rust), new inline filter between tank and carbs.

Problem: Got her running today. putted around neighborhood a few times just to make sure all functions properly. Brakes work, idles decent, shifts OK. There is a very loud throttle noise when accelerating (not the problem) just loud due to pods.

I got on a main road after my neighborhood runs to run it through the gears. (Had her up to 95mph in 4th

) Got on the highway and it ran great on very hard acceleration through 4th gear. When I shifted into 5th and slowed down to 75 or 80mph, it developed a bog, BAD. Had to pull off onto the shoulder and fiddle with fuel petcock. Turned it to prime and it went back to proper operation on idle and the lower gear runs.

I didnt get to run it up to 80 again so I dont know if it would have still done that or not. I didnt want to take a chance of holing a piston so I kinda took it easy and went back home.

So, I dont really know if I have a fuel starvation issue with the new filter, new fuel line or a semi-seizing problem.

Is this enough info for anyone to gander a guess as to what may be happening?

Post by **Suzsmokeyallan** » Fri Jul 17, 2009 10:38 pm

You may have one of those 'faulty' petcocks on the bike. You will have to remove it to make sure someone in the past didnt refit an old style one just to be sure.
There was a suzuki service bulletin about it regarding how the hole into the diaphram area is shaped, this was fixed in the bulletin by chamfering the holes edge into the petcock body.
The problem was that at high speed cruising the vacuum would become restricted to the diaphram thereby closing the diaphram and fuel delivery would reduce and cause a fuel starvation issue.
At prime the fuel flow is direct so it wont do it on that setting of the tap lever.

chainsaw · Post by **chainsaw** » Sat Jul 18, 2009 9:07 pm

ok, so what exactly do I have to do to fix it?

also, can I just run it on prime for now until I get more time and money to fix it?

thanks Allan

Post by **Suzsmokeyallan** » Sun Jul 19, 2009 8:08 am

Sam you carefully have to open the diaphragm section by the four screws.
Use an exacto knife to CAREFULLY lift the edge of the diaphragm once the four screws are out as it can stick to one or both sides of the cover and body.
If you see the hole from the vacuum line fitting just enters into the inner section plainly it needs modifying.
It should have a wide chamfered edge so it flares out into the diaphrams chamber.
Later versions of petcock had a cross cut and a chamfered end at the casting of the hole so once you see this style your problem lies elsewhere as its not the petcock.
If you run the bike on prime just remember you have NO RESERVE so be careful or you'll be stranded.
Time you have when you can get it, it wont cost you anything financially to fix, just some time.

tz375 · Post by **tz375** » Sun Jul 19, 2009 10:16 am

I wouldn't have though of that one. Another possibility is that a partial blockage in the filler cap which as to let air in as teh fuel goes out. Did you check it and clean it out?

Barry S. · Post by **Barry S.** » Sun Jul 19, 2009 11:20 am

http://www.3cyl.com/bulletins/gt21-1.jpg
http://www.3cyl.com/bulletins/gt21-2.jpg

chainsaw · Post by **chainsaw** » Sun Jul 19, 2009 7:54 pm

Wow, thanks barry & Allan!!!!

Post by **Suzsmokeyallan** » Sun Jul 19, 2009 8:55 pm

Glad to help anytime Sam and its great that Barry sent the links to the service bulletins for you. Let us know what you find once you open the petcock.

rngdng · Post by **rngdng** » Mon Jul 20, 2009 12:48 am

I had that problem on Blueboy. I happened to have a 75 model petcock, so I used it.

Lane