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PostPosted: Sun Jun 09, 2019 10:42 pm 
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Still in the Driveway

Joined: Wed Dec 24, 2008 12:32 pm
Posts: 6
Location: Portland, OR
I have a 75 GT750 with an older set of denco chambers and I would like to fit a new set of mikuni VM carbs. What options are out there that manage the 3-way throttle cable along with the oil pump actuation? Are there bolt on kits available?


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PostPosted: Mon Jun 10, 2019 5:19 pm 
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Moto GP
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Joined: Mon Nov 03, 2008 10:47 am
Posts: 6073
Location: Illinois
How big do you want to go? 34's are easy, 36 or 38 will work and 40's are great on a drag race motor. :P

Stock 72-72 cable will work to lift the slides on 34s and you will need to create a cable stop and end fitting for the oil pump. The right carb will rest on the clutch cover, so the usual fix for that is a GS1150 inlet rubber to tilt the carbs up.


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PostPosted: Tue Jun 11, 2019 1:12 am 
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World Superbike
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Joined: Thu Feb 16, 2012 11:50 am
Posts: 2233
Location: The Republic of South Yorkshire
Country: England
Bikes owned: 2 x GT550s GT750, T200, TC200
VM carbs cause surging. Better with the originals.

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PostPosted: Tue Jun 11, 2019 11:29 pm 
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Still in the Driveway

Joined: Wed Dec 24, 2008 12:32 pm
Posts: 6
Location: Portland, OR
Thanks for the replies! I should probably cut to the issue that led to this inquiry. I purchased a 75 GT750 with bad crank seals. Had the seals replaced and rebuilt the motor with new gaskets and seals, water pump etc, but did not do the top end yet. New electronic ignition. Compression tests were around 120-125. It has some old chambers, Denco’s I think. Pod K&N filters, may go to uni’s though. I put on GS1100 wheels, swing arm, shocks and brakes. Carb settings are 122 mains, 45 pilots and 4DN18 needles at bottom clip (5th down from top). Fuel screw around 3/4 turn out. These settings seemed in the ballpark from what I have researched so far and how the bike runs. The bike runs pretty well, revs fast, hardly surges, alot of fun really!!! The problem is that it runs hot and wants to overheat when just riding around town. Could these jetting settings be far enough off to cause the overheating (lean) issue or is there some other fundamental problem? Maybe I should just jet super rich and work my way lean....?? Any thoughts?


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PostPosted: Wed Jun 12, 2019 1:35 am 
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World Superbike
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Joined: Thu Feb 16, 2012 11:50 am
Posts: 2233
Location: The Republic of South Yorkshire
Country: England
Bikes owned: 2 x GT550s GT750, T200, TC200
You need to do plug/combustion check to see what's happening inside. When you say running hot, do you mean coolant (suggesting partly blocked radiator/pump not working properly or dodgy thermostat) or combustion temps.
Try a colder plug - B8ES - which might help.
If you have a laser temp gauge, check the exhausts as close to the barrels as possible.
Pod filters will make you run weak too.
Couldn't tell you what the jetting is on my Hippo, but I run standard air filter, original Gibson expansion chambers (from 1978!), an aftermarket alloy radiator, Mobray wheels & a Dresda swing arm. It does lots of mileage too, just back from a 790 mile tour of Ireland two up with luggage. The +1mm overbore helps as it makes the engine a 760cc lump now.

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PostPosted: Wed Jun 12, 2019 9:51 am 
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Expert racer

Joined: Wed Jul 10, 2013 12:04 pm
Posts: 1003
Country: England, UK
Bikes owned: FZ50, GP100, RG125 Gamma, GT380, Bandit 1200S
Speedisking wrote:
The problem is that it runs hot and wants to overheat when just riding around town. Could these jetting settings be far enough off to cause the overheating (lean) issue or is there some other fundamental problem? Maybe I should just jet super rich and work my way lean....?? Any thoughts?

I am guessing you mean the temperature of the coolant is high when riding around town? If so, I agree with Alan, that you might have a different problem. Your radiator could be a bit inefficient due to corrosion or clogged internally or externally (once had similar symptoms on a Kubota tractor. It turned out to be that someone had fitted the wrong length drain plug in the radiator, thus impeding flow).
Or your radiator fan may not be working (either faulty wiring, faulty switch or faulty fan).


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PostPosted: Wed Jun 12, 2019 4:34 pm 
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World Superbike
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Joined: Thu Feb 16, 2012 11:50 am
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Location: The Republic of South Yorkshire
Country: England
Bikes owned: 2 x GT550s GT750, T200, TC200
Or faulty temp gauge. There's an issue with the earth on them, but I can't remember what it was.

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PostPosted: Thu Jun 13, 2019 12:51 am 
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Still in the Driveway

Joined: Wed Dec 24, 2008 12:32 pm
Posts: 6
Location: Portland, OR
Some good advice here. When I refer to overheating, my temp gauge slowly, but steadily rises and I can feel the heat from the engine is definitely moving to the excessive range. The thermostat is new. The radiator looked good, but you never know. I think I’ll try giving the carbs a though cleaning, jet a little richer, run it for a bit and use an IR camera to check temps all around including the radiator and pipes and see if there are any thermal anomalies. I’ll report my findings.


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PostPosted: Thu Jun 13, 2019 11:02 pm 
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Moto GP
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Joined: Mon Nov 03, 2008 10:47 am
Posts: 6073
Location: Illinois
Assuming that we are talking stock BS40 carbs, the mains are probably too large, but safe. I'd go up to 47.5 or even 50 pilot jets.

What did you set the ignition timing at? You may need to retard a little.

Clogged radiator would certainly cause issues and quite common on our old bikes that have sat for years.

I read that the wrong type of thermostat can cause issues. It is designed to shut off the bypass passage when it's fully open.


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PostPosted: Mon Jun 24, 2019 2:43 pm 
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Around the block

Joined: Tue Jul 03, 2018 4:32 pm
Posts: 84
Country: Usa
Bikes owned: '74 GT250
I never liked the stock carbs vacuum carbs on a 2 stroke is counter productive in my opinion. A set of Mikuni TM flatslides is my preferred choice or Keihin PWK's those are great carbs.


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PostPosted: Mon Jun 24, 2019 6:21 pm 
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Moto GP
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Joined: Mon Nov 03, 2008 10:47 am
Posts: 6073
Location: Illinois
And yet hundreds of thousands of GT750s run quite well with stock BS40 carbs. My own bike has done 13 second quarter miles at 117 or so MPH and it runs clean as a whistle.

Maybe people don't ride hard enough....... :shock:

It's usually possible to get a set of stock carbs to work well enough, but sometimes they can be a PIA. Flat slides are nice and flow well, but there are so few people who can jet them properly that it becomes another expensive crap shoot.

This bike apparently runs just fine but tends to run hot and that suggests that carbs are OK, or maybe a touch lean at low revs.

Probable solutions:
- Flush radiator out.
- Check timing with a dial gauge
- Make sure the plugs are not rated too hot. I run D8 Iridiums in mine.
- Go up a size on pilot jets

That should get him back into the cool zone


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PostPosted: Sat Jul 06, 2019 11:09 pm 
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Still in the Driveway

Joined: Wed Dec 24, 2008 12:32 pm
Posts: 6
Location: Portland, OR
Great recommendations. I’m on it!


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