1974 750 GT carb question

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gt94stang
Cucciolo - the Lil Pup
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Joined: Fri Dec 28, 2018 4:53 am

1974 750 GT carb question

Post by gt94stang » Sun Dec 30, 2018 4:17 am

Good Morning,

In getting my newly acquired 750 GT back on the road in running condition, I have a carb question. I cleaned (ultrasonically), rebuilt, and hand synched the carbs. Bike has some type of electronic ignition although I can't figure out which one yet. I also replaced the exhaust gaskets. Bike starts right up nice and easy and idles well. Take off on a ride, and it runs great, good power through out the rpm range. Problem is once it gets warmed up good, when coming to a stop, the idle hangs up high and slowly comes back down. What could be causing this? I checked for air leaks around the carbs and no air leaks present. Should note, that I believe someone had upgraded the carbs on this bike at sometime in it's past to PHF32's as I believe the GT's came iwth PHF30's.

Thanks for the help.

Rob

wdietz186
888
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Joined: Sun Sep 10, 2006 5:40 pm

Re: 1974 750 GT carb question

Post by wdietz186 » Sun Dec 30, 2018 8:10 am

The ignition is likely a Dyna unit, a very reliable setup. You could have a sticking/gummed up advance mechanism, see if the points cam moves freely. With the Dyna you will have a black cap on the shaft retained by a set screw. On my 750 I've found that it wants to be on the rich side with the idle mixture. The rubber insulators between the manifold and head can look fine but seep depending on how old and hard they are, they are cheap and easy to replace so I would start there.

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Craig in France
Cagiva Gran Canyon
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Re: 1974 750 GT carb question

Post by Craig in France » Mon Dec 31, 2018 6:46 am

Hanging idles can be a right PITA to chase down - lots of variables :( . But to add to what Bill says ...

Start with the simple:
Throttle cable
Make sure you have some free play (1-2mm) in the throttle cable. Then check the cable is running freely within its sheath - ideally take it off, pull the inner cable out of the outer, check it for any broken strands, lube it and put it all back. Now check it's not snagging somewhere on the frame or under the tank etc.

Ideally do the same with the choke (actually, 'enricher') cables.

Carb slides
Make sure the carb slides are descending smoothly and bottoming together. If one is staying up higher/closing after the other = fast idle. If so, you need to reset and rebalance the carbs.

Other
Make sure your exhaust nuts are done up tight - use the proper spanner - and their gaskets are good. Same goes for all the joints in the exhaust system, in fact.

Good Luck and please let us know how it pans out ...

P.s You're right: 74 GTs originally came with PHF 30 carbs. But changing to PHF 32s shouldn't affect much.

Craig

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geodoc
750 GT
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Re: 1974 750 GT carb question

Post by geodoc » Mon Dec 31, 2018 11:52 am

I had a sticking slides problem that turned out to have an off the wall cause. I concocted a Guzzi boy-racer unit with an aftermarket fiberglass tank and one day the throttle stuck (exciting!). I had the (new dellorto) carbs apart probably half a dozen times looking for problems, but it kept happening. It turned out that the resin used in fabrication of the tank was slowly being dissolved by the fuel and leaving a residue on the slides which would cause them to stick. I cleaned the tank out and sealed with Caswell. Problem solved.

Per chance, are you using a fiberglass tank?




.

Vince
750 Sport [BEVEL]
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Joined: Tue Nov 09, 2004 12:28 am

Re: 1974 750 GT carb question

Post by Vince » Mon Dec 31, 2018 5:12 pm

The exact same thing happened to my Pantah with a glass tank when a mate owned it, that became obvious when the bottom of the tank came off in his garage one night.

gt94stang
Cucciolo - the Lil Pup
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Re: 1974 750 GT carb question

Post by gt94stang » Tue Jan 01, 2019 5:23 am

Thanks all for the help so far. Throttle slides are not sticking, they go up and down fine and both close at the same time. Steel tank, so now worries about fiberglass getting in the way. Proper freeplay on cables and cables aren't sticking. Brand new exhaust gaskets just installed which did not fix the problem. Will have to look at intakes.

Rob

ducadini
750 Sport [BEVEL]
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Location: Antwerpen

Re: 1974 750 GT carb question

Post by ducadini » Wed Jan 02, 2019 4:17 am

It could be a leak in the intake between carb and manifold/head.
Didn't You mention cracked rubbers between manifold and head ?
Or on some of the newer type manifolds : the rubber ring between manifold and head.
The plastic bushing between carb and manifold that is split and doens't seal properly.

ciao
ducadini
Nego i ricordi peggiori
Richiamo i migliori pensieri

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Craig in France
Cagiva Gran Canyon
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Joined: Tue Mar 23, 2004 3:58 pm
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Re: 1974 750 GT carb question

Post by Craig in France » Wed Jan 02, 2019 8:38 am

Hi Rob,

Bill's original diagnosis (and now Ducadini's) is looking increasingly promising ... :)

Steve has new insulators - and they're still made by the OEM!
https://store.bevelheaven.com/Carb-Fuel ... vel-Twins/

And he has the 'modern' version too:
https://store.bevelheaven.com/Carb-Fuel ... vel-Twins/

Steve also has the plastic bushing thing that Ducadini mentions in his last line:
https://store.bevelheaven.com/Carb-Fuel ... lorto-PHF/

One further thought, FWIW. I remember having a Laverda SF2 that had a tendency to hung up on idle before settling down. I'm pretty sure that turned out to be uneven float heights (but please don't quote me :) . It was all a long time ago). But it might be worth checking.

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