'77 carbs okay on '78 engine?


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El Taco
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'77 carbs okay on '78 engine?

Postby El Taco » Wed Oct 24, 2012 7:34 pm



I was wondering about the air cut valve on my '78 carb rack, and noticed that my '77 didn't have the valve, or any of its associated hoses or passages. Would these carbs run well on the newer engine?



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feetup
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Re: '77 carbs okay on '78 engine?

Postby feetup » Thu Oct 25, 2012 9:49 am

I'm pretty sure the 78 engine uses the same cam, and heads as the 77, and the ignition system is the same so I could give you a qualified answer and say that the 78 will run just as well as the 77, if fitted with the 77 carbs. The air cut valve was put there to help with emissions so you would negate that. It is questionable whether the 78 ran any better than the 77 because of the updated carbs, and if you have issues with the air cut valve it is likely it doesn't run as well, so...

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El Taco
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Re: '77 carbs okay on '78 engine?

Postby El Taco » Thu Oct 25, 2012 7:02 pm

I have no issue as of yet. Just noticed the differences, I have three separate years of carbs set aside, and I just happen to be building with the '78 engine. Everything appears to flow well, although I haven't made it to the valve yet.

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Fred Camper
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Re: '77 carbs okay on '78 engine?

Postby Fred Camper » Thu Oct 25, 2012 8:15 pm

1978 has difference cams and carbs, but the heads are the same. The 1978 carbs are known for about 1 mm smaller throats for higher torque at lower rpm. Not sure anyone found that to be true, but it did cut top end power. Most likely was done to help low rpm emissions. 1977 had some emission compromise too, but they are easier to correct.

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Re: '77 carbs okay on '78 engine?

Postby dingdong » Fri Oct 26, 2012 7:07 am

If by "air cut valve" you are referring to the air cutoff valve then the 77 should have one just like all other gl1000s. It's purpose is to prevent back firing on decel. By increasing the air/fuel ratio. Has nothing to do with emissions.
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El Taco
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Re: '77 carbs okay on '78 engine?

Postby El Taco » Fri Oct 26, 2012 2:12 pm

I see what you mean. On the 77, the cutoff valve was integrated into each carb individually. On the 78, the valve was singular, external, and plumbed through the manifold. I wonder now if the carbs that came off of the 77 were from an 1100, as one has an accelerator pump.

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terryt
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Re: '77 carbs okay on '78 engine?

Postby terryt » Fri Oct 26, 2012 3:47 pm

What is the carb code number. it is stamped on the flat part of the carb near the air intake chamber. all four should have the same number.
1977 764A
1978 769A or 771A
they also have different primary main jet. idle jet and needle valve
and as said earler the cams are different

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Re: '77 carbs okay on '78 engine?

Postby scotterichmond » Fri Oct 26, 2012 6:01 pm

cut off valves on all four carbs came out with the 80 GL 1100 model

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Re: '77 carbs okay on '78 engine?

Postby Missourimike » Sat Oct 27, 2012 7:21 am

True, the heads on the '77 and '78 will interchange also, but right again, the cams have a different lift. Timing on the '77 is set at 5 degrees while the '78 is set at 10. All the carbs, for all years, will interchange on the 1000s and 1100s. I'm running '81 carbs on my '79. Makes for a slower bike, but it's a tourer, not a hot rod like my '75, which is running a Weber 2-barrel.

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Re: '77 carbs okay on '78 engine?

Postby dingdong » Sat Oct 27, 2012 7:29 am

On Gl1000s all have single valves. You must have Gl1100 carbs on that 77.
Tom

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Re: '77 carbs okay on '78 engine?

Postby Missourimike » Sat Oct 27, 2012 8:33 am

If you can't see the numbers on the carbs, the caps will indicate which size of engine they are from. The 1000s carb tops have a white plastic button on top of them. The 1100s do not and are smooth.

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El Taco
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Re: '77 carbs okay on '78 engine?

Postby El Taco » Sun Oct 28, 2012 1:39 am

[quote="Missourimike"]If you can't see the numbers on the carbs, the caps will indicate which size of engine they are from. The 1000s carb tops have a white plastic button on top of them. The 1100s do not and are smooth.[/quote]


Could I use the smooth caps from the later models on my 769A carbs?

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Re: '77 carbs okay on '78 engine?

Postby Missourimike » Sun Oct 28, 2012 7:05 am

The caps and the pistons inside are matched at the factory, for fit. All the books highly recommend that you NOT switch the caps around or get them mixed up with a rebuild. Stick with what you got, glue beer bottle caps or coins on top of the white buttons if yours look crappy.

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Fred Camper
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Re: '77 carbs okay on '78 engine?

Postby Fred Camper » Sun Oct 28, 2012 6:49 pm

Exactly, you cannot move the sliding pucks from the caps, they are a matched set and will not typically slide perfectly if mixed up.

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Re: '77 carbs okay on '78 engine?

Postby sfruechte » Sun Dec 02, 2012 12:43 am

It sounds like you have 1100 carbs. I have two 77's, one I bought new, they both have a single air cut valve forward of all four carbs. The 77's also do not have accelerator pumps, they did not come out until 1980.




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