73 gt750 bucking/surge

cabz · Post by **cabz** » Sun Sep 27, 2009 6:56 pm

I finaly got the carbs back together on the old girl and went for a little ride

I still have a surge / bucking it happens when at steady throttle below 3000 rpm and when the throttle is closed at speeds. other than that it is realy crisp and running well any idea on what I can do to make it smoother when the throttle is closed ?

Wayne Meuir · Post by **Wayne Meuir** » Sun Sep 27, 2009 7:11 pm

There is an air jet mod that needs to be done. It was a Suzuki Service bulliten.

It involves tapping the air jet orifice in the front of the carb and screwing in a smaller air jet so that it won't run so lean on decel.

Someone with all the correct info you need on how to do it will chime in I'm sure.

Wayne

Post by **Suzsmokeyallan** » Sun Sep 27, 2009 9:34 pm

Wayne that was for the BS model of CV carbs of the 74-77 models not the 73 which had conventional slide carbs

Post by **oldjapanesebikes** » Sun Sep 27, 2009 9:42 pm

I think you are looking for service bulletin GT-36 from March 1977 - you can find a copy here.

cabz · Post by **cabz** » Sun Sep 27, 2009 9:56 pm

oldjapanesebikes wrote:I think you are looking for service bulletin GT-36 from March 1977 - you can find a copy here.

unfortunatly that bulletin only covers the BS carbs , I have the VM's

Post by **Suzsmokeyallan** » Sun Sep 27, 2009 10:43 pm

Id go over the carbs again, however if the bike is stock make sure a few things are to spec.
Are the float levels correct.
Are the choke plunger rubbers and their springs good.
The 73 carbs carried an o-ring at the bottom of the air screws, make sure you have new ones in there.
Start with all carb jetting and settings at stock if your bike is stock and work from there.
Make sure its got in the correct needles etc as well and not keyster ones.

Post by **oldjapanesebikes** » Sun Sep 27, 2009 11:17 pm

cabz wrote:
oldjapanesebikes wrote:I think you are looking for service bulletin GT-36 from March 1977 - you can find a copy here.
unfortunatly that bulletin only covers the BS carbs , I have the VM's

Correct - I'd vote with Allan on this then, and add to also check for possible air leaks.

cabz · Post by **cabz** » Mon Sep 28, 2009 5:31 am

Suzsmokeyallan wrote:Id go over the carbs again, however if the bike is stock make sure a few things are to spec.
Are the float levels correct.
Are the choke plunger rubbers and their springs good.
The 73 carbs carried an o-ring at the bottom of the air screws, make sure you have new ones in there.
Start with all carb jetting and settings at stock if your bike is stock and work from there.
Make sure its got in the correct needles etc as well and not keyster ones.

Hmm I will check on the orings , I might have missed that . as far as the keyster needles how can I tell the diffrence?.
everything else is in good shape , floats and levels jets are nos new carbs have been cleaned 10 times it idles perfect as well. I'll check the orings and needles and the sync again for giggles

Post by **Suzsmokeyallan** » Mon Sep 28, 2009 5:52 am

From what youre saying theres a slight hiccup somewhere to cause the issue. It may not be the carbs are dirty since youve cleaned them 10 times but there are so many variables to consider without actually seeing the bike working.
The needles should have a 5F 16-3 printed into the shaft near to the clip so look for that number, also what clip position are you using???
Are the intake tubes from the carbs to the cylinders soft? old ones tend to play the fool with sealing issues causing leaks because they shrink and get hard.
It sounds like something simple, youve just got to find what is causing it.

tz375 · Post by **tz375** » Mon Sep 28, 2009 8:42 am

Surging at close or almost closed throttle may be rich or lean, but it's almost always a mismatch between the slow jets and air jets for those slow jets.

If it isn't a missing O ring, I'd pull the carbs and focus on the idle circuits - especially the air circuit.

cabz · Post by **cabz** » Mon Sep 28, 2009 3:50 pm

Suzsmokeyallan wrote:From what youre saying theres a slight hiccup somewhere to cause the issue. It may not be the carbs are dirty since youve cleaned them 10 times but there are so many variables to consider without actually seeing the bike working.
The needles should have a 5F 16-3 printed into the shaft near to the clip so look for that number, also what clip position are you using???
Are the intake tubes from the carbs to the cylinders soft? old ones tend to play the fool with sealing issues causing leaks because they shrink and get hard.
It sounds like something simple, youve just got to find what is causing it.

well the needle s are the right ones , I am useing the 4th slot from the top, I tried to compensate for the new air filters. I ended up putting new mains in ( one size up) to make the wide open throttle better.

the bucking/surge has not changed any sinec the main swap . the rubber conectors are not soft but not beyond useable .

I am going to recheck the sync next and see what I find.

tz375 · Post by **tz375** » Mon Sep 28, 2009 4:08 pm

Focus on the pilot jet circuits. That's all that's operating at closed throttle or close to it. Needles don't start to make their mark until 1/4 or more. The risk is that it will end up rich or lean all the way through to try to compensate for a problem at the bottom end.

Poor filters or an incorrect fuel level could also be factors here.

cabz · Post by **cabz** » Mon Sep 28, 2009 7:17 pm

tz375 wrote:Focus on the pilot jet circuits. That's all that's operating at closed throttle or close to it. Needles don't start to make their mark until 1/4 or more. The risk is that it will end up rich or lean all the way through to try to compensate for a problem at the bottom end.

Poor filters or an incorrect fuel level could also be factors here.

this is where I was thinking the issue is ...........but I have cleaned the life out of the carbs and replaced all the jets with new ones . as far as filters go they are brand new uni's and the floatlevels are dead on.

I did have a couple ideas though,
1 .I have two bad choke cable rubber boots.
2. There does not seem to be any kind of seal at the silde caps are there supposed to be?

other than that I have no clue . I checked the air mixture needle's and there are no orings on them and the books dont show one for them either .
and I checked the cable/slide window dots and they are dead on.

and the head scratching begins

Post by **Suzsmokeyallan** » Mon Sep 28, 2009 7:39 pm

Cabz you missed it, the 73 carbs with the tall caps carry o rings on the base of the airscrews under the spring and the parts diagrams i have show it clearly.
The part number you want is three o-rings, 13295-33010 specific for the K model carbs only.

tz375 · Post by **tz375** » Mon Sep 28, 2009 8:25 pm

Cabz,

It's possible that they are still down in the hole or maybe a PO lost them. Either way, it's a 73 thing. Not used on other GT750 models.