1980 GS750 Massively underperforming

[Big T]

I'm thinking I should start by cleaning the carbs (though I already ran seafoam about a month ago) and then sync them. Possibly a valve tuning?
Uh, I'm pretty lost here, boys. lol

The members here have found that people who decide to "Bob" or "caf?" an old bike generally lack the mechanical expertise to make the bike perform decently. They cut it up, take off the airbox and find it runs like crap. Then, they sell it to cheap

Not critical of your bike or styling, it just seems to be a fact

So, you need to do all of the maintenance, because you can be sure that the PO never did any, and ,if he did, he probably did it wrong

Go here

http://www.thegsresources.com/_forum/showthread.php?t=196338

and go to his website. Download a factory manual for your bike and check out the CV carb rebuild tutorial. Check the Top 10 Newbie mistakes. Yeah, you're not a bike newbie but it's what you need to do when you buy a 30+ year old bike

Order up some O rings from cycleorings.com, get a valve cover gasket and a suitable feeler gauge

Then, adjust your valves and clean your carbs and install the new O rings. It seems you a canuckian, go back to your profile page and put your location and bike model in your profile to help us help you. We have lots of members in the CA and one nearby may offer you some help.

Now, you have a bench line from which to work. Best of luck solving what the PO did to you

[Crankthat]

The valve adjustment is really easy.
Here is a link to a members website.
You will need to find the 16 valve adjustment guide on the website.
Just scroll down the page and it is in the column on the left.
When removing and installing the bolts for the cover it is best to loosen them and then upon installation tightening them a part turn on this one and then a partial turn on the next one and so on and so on.
You do it that way to keep from pinching down any one bolt and therefore your gasket at a time.
When you have the pressure off all the bolts you can start to turn them normally.
Some people go a crisscross pattern on the bolts also but if your turns are small enough there is not a need.
You may find you need a new gasket but you may be lucky and able to reuse the old one.
I prefer two sets of feeler gauges as the u shaped holders like to rock when tightening.
You will understand when you are doing the adjustment.
If not just PM me.
You will need to tackle this before the carbs as when you go to synch the carbs this needs to be done.

You will also find the correct way to clean/rebuild your carbs on the same site.
The information will be in the left hand section of guides the top two If memory serves.
Though running cleaner will sometimes help with a bike that has been sitting for a few months A older bike like your GS will usually have other containment's plugging up the small passages.
These need to be removed.
While you are in there you will pull your jets and be able to determine what sizes you are running now.
After doing that search the forum or ask some more advice on where to start.
You should search some threads where others have rebuilt there carbs as it may keep you from difficulties others have already experienced.
You will most likely need to buy a o-ring kit for them also.
Someone may chime in on the member to contact.

[Steve]

I am concerned about the K & N pods because they're quite small and I notice they have inner threading.

Can you actually see the letters "K&N" on the end of the pods or are you just calling them because they are 'pods'?

They don't look like K&Ns to me.





If they are K&N filters, you need to get some oil on there quickly.

.

[Guest]

K&N has a lot of small round tapered filters, it looks like K&N construction to me. The size is all wrong, would most likely be for a small ATV. I would recommend going with a stock air box and rebuilding the carburetors completely per the tutorials to establish a base line before fooling with pods. Spitting out of the carburetors is a symptom of being too lean and can cause damage. I wouldn't even try to run it before you do the required maintenance.

[Crankthat]

1980 gs 750 stock Mikuni jet sizes for United States. LT LX and EX

Pilot jet-42.5
Main jet-112.5

And a chart found by a different user so you can compare different manufactures main jets.

Keihin, Dynojet, Mikuni Jet Size Cross Reference

All Hole Diameters Are in Inches

Hole Dia?---Keihin # -- DynoJet # -- Mikuni #
0.0350---- 92.5--------- 92----------- 86.3
0.0360---- 95----------- 94----------- 88.1
0.0370---- 97.5--------- 96----------- 90
0.0380---- 100---------- 98----------- 91.9
0.0390---- 102.5------- 100---------- 93.8
0.0400---- 105--------- 102---------- 95.6
0.0410---- 107.5------- 104---------- 97.5
0.0420---- 110--------- 106---------- 99.4
0.0430---- 112.5------- 108--------- 101.3
0.0440---- 115--------- 110--------- 103.1
0.0450---- 117.5------- 112--------- 105
0.0460---- 120--------- 114--------- 106.9
0.0470---- 122.5------- 116--------- 108.8
0.0480---- 125--------- 118--------- 110.6
0.0490---- 127.5------- 120--------- 112.5
0.0500---- 130--------- 122--------- 114.4
0.0510---- 132.5------- 124--------- 116.3
0.0520---- 135--------- 126--------- 118.1
0.0530---- 137.5------- 128--------- 120
0.0540---- 140--------- 130--------- 121.9
0.0550---- 142.5------- 132--------- 123.8
0.0560---- 145--------- 134--------- 125.6
0.0570---- 147.5------- 136--------- 127.5
0.0580---- 150--------- 138--------- 129.4
0.0590---- 152.5------- 140--------- 131.3
0.0600---- 155--------- 142--------- 133.1
0.0610---- 157.5------- 144--------- 135
0.0620---- 160--------- 146--------- 136.9
0.0630---- 162.5------- 148--------- 138.8
0.0640---- 165--------- 150--------- 140.6
0.0650---- 167.5------- 152--------- 142.5
0.0660---- 170--------- 154--------- 144.4
0.0670---- 172.5------- 156--------- 146.3
0.0680---- 175--------- 158--------- 148.1
0.0690---- 177.5------- 160--------- 150
0.0700---- 180--------- 162--------- 151.9
0.0710---- 182.5------- 164--------- 153.8
0.0720---- 185--------- 166--------- 155.6
0.0730---- 187.5------- 168--------- 1575
0.0740---- 190--------- 170--------- 159.4
0.0750---- 192.5------- 172--------- 161.3
0.0760---- 195--------- 174--------- 163.1
0.0770---- 197.5------- 176--------- 165
0.0780---- 200--------- 178--------- 166.9
0.0790---- 202.5------- 180--------- 168.8
0.0800---- 205--------- 182--------- 170.6
0.0810---- 207.5------- 184--------- 172.5
0.0820---- 210--------- 186--------- 174.4
0.0830---- 212.5------- 188--------- 176.3
0.0840---- 215--------- 190--------- 178.1
0.0850---- 217.5------- 192--------- 180

[sokocanuck]

Hey guys.
As an update, I synced the carbs...they were waaaaay off and that helped a little but I think you were right about the lean/rich issue and rejetting. A couple people have told me that rejetting that bike is a nightmare with pods, so I'm looking into getting a used air cleaner set up.
Does anyone know if multiple models would work on the 1980 gs750? I found a guy in my area that has one for a 1980 gs1100 but I doubt it would fit...

Edit: When I got the bike safetied, the guy thought that the pods were too small for the bike.

http://www.bikebandit.com/aftermarket-parts/motorcycle-fuel-and-air/air-filters-universal/k-n-universal-round-tapered-chrome-air-filters?m=21462

A lot of people mentioned running lean but would the symptoms I mentioned possibly be from running rich? Will my spark-plugs solve this mystery?

I plan on taking the carbs apart and check the number of the jets this week. Hopefully I can get this worked out.

Also, I realize how annoying all these posts must be but I really appreciate all your patience and advice.

[tom203]

"Also, I realize how annoying all these posts must be but I really appreciate all your patience and advice."

not to worry, this is what these guys do on here! But I find the "massively underperforming " title disturbing!