Putting ‘78 gs750 back to stock and can’t find definitive specs on 45020 carbs

[mattd1205]

Hi,

ive been wanting to put my ‘78 gs750c back to stock and finally have the stock exhaust to complete this project. I have the 45020 carbs and can’t find any definitive specifications in the spreadsheet or any of the other tables floating around on this particular carb setup. I took my carbs apart as far as I could and found that it has the 0.6 needle jet, 5f21-3 needle, 1.5 throttle valve cutaway, 15 idle jet, the air jets are unknown due to the fact they don’t appear to be removable. I’m not sure which main jet to go with because there’s conflicting information on whether they came with a 100 jet or a 105 jet. Anybody know which ones these came with?

thanks
Matt

[Steve]

My chart shows 100 main jets, but that's for the 45030 carbs.

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[mattd1205]

My chart shows 100 main jets, but that's for the 45030 carbs.

.

Its very confusing. I guess those were on the 750ec’s. I don’t know why the carbs were different considering the differences between the c and ec were cosmetic.

matt

[hannibal]

Its very confusing. I guess those were on the 750ec’s. I don’t know why the carbs were different considering the differences between the c and ec were cosmetic.

matt

So maybe they were the same.

[Steve]

Just so you know, the "C" indicates the '78 model year.

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[Sam 78 GS750]

I'm going through something similar with the same bike, and have just learned most of this myself....

It seems there were carb jetting changes midway through the model year on the 78 750 because of EPA regulations. There are two fiches for the carbs, based on engine #. One is labeled "Emission Style" and is for the later engine numbers. They switched from the 5F21 needle with O-6 needle jet in the original jetting to the 5DL36 and O-4 for the "emission style" jetting.

From my thread where I'm digging into the same question, but going the other way (jetting to pods and 4-1). Chuck explained the reasoning behind the changes.

The 5DL36 needles are slightly thicker at the base area, but thinner as the taper is more aggressive for more performance at larger throttle openings. But a slightly cleaner idle to satisfy the stricter fuel emissions standards.

So if you want to go back to the original jetting for the bike, before the emission changes, that would be this fiche here --->https://www.partzilla.com/catalog/su...o-gs750e-19995

5F21-3 needles, O-6 Needle Jets, 15 Pilots, and 100 mains.

The fiche lists various size mains as "optional" on the fiche, ranging from 92.5 up to 107.5, I'm assuming to account for elevation at different locations.

Like I said, I'm just now beginning to piece this info together myself in trying to answer the same question... But that's my understanding so far of the reason for the conflicting info that's out there on what "stock" jetting for the 78 750 was....