1974 GT750 CARB PROBLEMS

[nobaffle]

Hello,
This is my first post...
After about 6 times of taking carbs on and off I'm stumped.
Bike will not idle, throttle idle knob cranked tight.
Appears to be running rich and very boggy to 3000 rpm and no power under 3000 rpm. Great power and acceleration above 3500 rpm all the way to redline. My Sense is that it is between 0 to 1/4 throttle where it is worst. Once rpms come up to 3000 it seems like it can cope with the amount of fuel it is getting.
Stock bike, stock pipes and air filter.
Float levels set to 28mm.
Idle circuit is clear on all carbs. Fuel/air screw set to 1/4 turn
These carbs were in a terrible gummed up state and were full of nasty varnish and I soaked them for a couple days on my first cleaning attempt.

Could needle jet be allowing excess fuel in at idle? That seems to be my last resort to further inspect them...

Any ideas?

Thanks!

[tz375]

Could be a few different things. I would start by fitting all new genuine Mikuni jets. If the old ones are still in there, they could have been damaged by previous owners.

Next I would check all three choke circuits. The latest bike I built had one choke plunger that was hard and "worn" and didn't seal well. You can test the sealing with a mityvac quite easily.

After that I would set the FUEL level. To do that you need to modify a drain screw to insert a tube and slide a clear tube over that. Level the carbs on the bench above a large baking tray to catch fuel and then fill the carbs and see where the fuel level is. It should be 3mm below the gasket surface +/- 1mm. Repeat on the other two.

BS40 carbs have some very strange tiny drillings for fuel and air in the body and in the bowl. I like to start with one - say air for the slow jet and blast carb cleaner or WD40 through that circuit. Repeat on the other two and see which one is less flow that the others and clean it again. Repeat for every hole you see and blast it both ways and compare all three carbs. Then move on to the next circuit and so on.

Good luck.

[Warehouse1001]

Could be a few different things. I would start by fitting all new genuine Mikuni jets. If the old ones are still in there, they could have been damaged by previous owners.

Next I would check all three choke circuits. The latest bike I built had one choke plunger that was hard and "worn" and didn't seal well. You can test the sealing with a mityvac quite easily.

After that I would set the FUEL level. To do that you need to modify a drain screw to insert a tube and slide a clear tube over that. Level the carbs on the bench above a large baking tray to catch fuel and then fill the carbs and see where the fuel level is. It should be 3mm below the gasket surface +/- 1mm. Repeat on the other two.

BS40 carbs have some very strange tiny drillings for fuel and air in the body and in the bowl. I like to start with one - say air for the slow jet and blast carb cleaner or WD40 through that circuit. Repeat on the other two and see which one is less flow that the others and clean it again. Repeat for every hole you see and blast it both ways and compare all three carbs. Then move on to the next circuit and so on.

Good luck.

TZ, I have been searching for the fuel height spec you mentioned. Mind sharing where you found it? Thanks

[nobaffle]

Thanks for your input. Checked more things and found very little "thin" fuel smelling oil in the gearbox. Refilled up to check screw and rode bike for an hour...slobbering the oil out the pipes and lots of smoke, but then lessened toward end of my 1 hour ride. Rechecked oil level and it burned a quart of gearbox oil...bad news guess the crank seals are bad...that would explain the gutless feeling it has up to 3000 rpm.
Guess it had been sitting way too long to expect it would be ok...like 25 yrs.

Wishful thinking it was just a carb problem!


Who out there does crank seal jobs?

Thanks

[Warehouse1001]

Bill Bune in Minnesota does really nice work and is reasonably priced. Chuck

[garyr]

Leaking crank seals can also mimic your problem. maybe someone here can tell you how to test them with a vacc gauge..I don't know how.

[rngdng]

There are two types of Mikuni pilot jets that will fit in the GT carbs, and they are numbered the same. Trouble is, one one type works. I don't have any stock carbs, so I hope someone will be kind enough to post pictures of teh proper jets, and (better yet) give you the Sudco part number for the correct ones.

When our friend Coyote built his 750, the carbs were killing him, so I had him send them to me, and he had two of the wrong kind of pilots. I made up a document and the time and posted it, but can't find it. It was years ago. I know I just gave MGMark a bunch of stock GT carb parts, so maybe he can help.


Lane

[tz375]

When you get the crank back and start to rebuild the carbs and motor, you could check http://pinkpossum.com/GT750/carb/BS40carbs.htm" onclick="window.open(this.href);return false; for all the details you need.

[Alan H]

I have a GT750 that had symptoms to that last year.
The problem was a cracked piston, so it's time for a stripdown for yours too I reckon.


Lucky escape methinx!!

[Suzsmokeyallan]

When crank seals go out the engine behaves differently to which seal has failed and how its been destroyed. When the left inner one on mine went out, the bike was rideable but had about 1/3 the power and no low end at all.
On removal of the crank, that seal had no centre section just the metal frame its bonded onto, so the left and centre cylinder volumes were now joined at the lower case.
A simple test is to remove the carbs and place your palm over each carb intake rubber on the barrel then turn over the engine. You should feel a slight suction effect on your hand, if you feel nothing on one or any cylinders, the seals are definitely blown.
It also makes sense to have the rods balanced on a crank rebuild, I've found most are off by a measurable amount.

[Warehouse1001]

+1 on the rod balancing. Had Bill Bune do that at as well. Chuck

[Suzsmokeyallan]

I'm glad to see someones followed up on rod balancing as well. Rotational mass such as crank wheels need to be accurate, but reciprocating mass such as rods and pistons also need to be closely matched in their weights on multi cyclinder engines. A case in point, on one GT750 engine of mine it had a weird above average vibration which seemed to be more noticable on the left side. When that crank was removed for new seals the left rod was heavier by nearly 25 grams compared to the lightest one.
On all the Buffalo cranks I've had rebuilt, two rods always need to be skimmed accordingly to obtain matching weights against the lightest one.
Bill Bune does excellent work and the guys in his shop are pleasant and easy to work with, highly recommended.