1980 GS750 Massively underperforming

[sokocanuck]

Hey guys. Awesome forum, I can't wait to learn all about the GS (just bought my first one).
That being said, my 750 is massively under-performing. I was hoping to get some diagnosis help.
Basically, the bike seems to run nice, but can't break 90kms/hr...crazy right? The guy I bought it from was trying to make it into a cafe racer or something and he had pods on the carbs instead of the airbox. When you give it some throttle, these pods even flex, like it's starving for air.
Am I correct to assume that maybe he just threw these pods on without rejetting the carbs and the bike is just being starved of oxygen? Would that cause a 200km/hr bike to be a 90km/hr bike?
I really appreciate any information I can get on this issue, as I love the bike and want to be able to take it out of the city. lol

 

[theresthatguy]

This may help. http://www.thegsresources.com/_forum/showthread.php?t=199275&page=6

 

[sokocanuck]

This may help. http://www.thegsresources.com/_forum/showthread.php?t=199275&page=6

Great, thanks for the link. If it is rejetting, I'll definitely poke around that link some more. Unfortunately, I'm not sure if that's the issue.

 

[theresthatguy]

Yes, you'll have to do some digging to find out. I don't what condition your bike is in but perhaps it needs a valve adjustment and carbs cleaned as well.

 

[duaneage]

Sounds like the slides are not going up. How are the intake boots and clamps?
Might be a fuel supply issue too, have you tried it on PRI instead of On or RES?

 

[sokocanuck]

Sounds like the slides are not going up. How are the intake boots and clamps?
Might be a fuel supply issue too, have you tried it on PRI instead of On or RES?

The boots are all just clamped on instead of screwed on like I've seen in the manuals but the condition of the rubber is good. I am concerned about the K & N pods because they're quite small and I notice they have inner threading...yet it isn't being used. The boots are just clamped over them. I tried to check the product number on them and as far as I've found so far they don't exist...

Good point about the prime. I'll give that a try tomorrow!

 

[sokocanuck]

Sounds like the slides are not going up. How are the intake boots and clamps?
Might be a fuel supply issue too, have you tried it on PRI instead of On or RES?

Opps, didn't notice the slides part.
Probably no easy way for me to check this, is there? Is that related to the amount of air coming in? They are sitting 3/4s the way down when I just took a peek.

 

[Jcgonzal]

Very easy to check since you have pods. Just take all the pods off and look into the carbs as you rev the engine. If everything is working correctly and the carbs are synched, all 4 slides should move up at the same time/same distance.

 

[Guest]

Too much air will kill your power big time and that is what it sounds like, very lean conditions. does your bike have a 4-1 pipe? Last time the valves have been adjusted? Gonna have to take the carbs off and see if the main jets have been changed out. Lots of things to look at here.

If you were to take the stock air box off one of these bike and try to throttle up the engine with out it, you would be hard pressed to see 4,000 rpm. You gotta have some air restriction (and the right jetting) for these CV carbs to operate properly.
A few pics would be nice.

 

[sokocanuck]

Very easy to check since you have pods. Just take all the pods off and look into the carbs as you rev the engine. If everything is working correctly and the carbs are synched, all 4 slides should move up at the same time/same distance.

I checked it out and they seemed to move up at the same rate when i hit the throttle. They do a little stutter and move about halfway up.
I did notice that one of them was spitting some gas back out. Any idea about that?

 

[sokocanuck]

Too much air will kill your power big time and that is what it sounds like, very lean conditions. does your bike have a 4-1 pipe? Last time the valves have been adjusted? Gonna have to take the carbs off and see if the main jets have been changed out. Lots of things to look at here.

If you were to take the stock air box off one of these bike and try to throttle up the engine with out it, you would be hard pressed to see 4,000 rpm. You gotta have some air restriction (and the right jetting) for these CV carbs to operate properly.
A few pics would be nice.

Here are the pics, Mr.Bill.
Also, the pipes are 4-2

 

[sokocanuck]

More pics

More pics

Here is the whole bike.

 

[sokocanuck]

More pics 2

More pics 2

Here is the K & M filter next to a 12 mm wrench. Seems too small, no?

 

[ShadowFocus603]

I'd ditch those pods and get a stage 3 dynojet kit from z1 as well as a set of ape super pro pods from pingel. Otherwise you are going to rip your hair out chasing your tail trying to dial those in. An alternative is get a stock air box and take her back to factory. Option 1 is about $160. Option 2 depends on whether you can find a use able air box and for how much. Either way I wouldn't ride it til its sorted or you risk overheating it from being massively lean.

 

[sokocanuck]

I'd ditch those pods and get a stage 3 dynojet kit from z1 as well as a set of ape super pro pods from pingel. Otherwise you are going to rip your hair out chasing your tail trying to dial those in. An alternative is get a stock air box and take her back to factory. Option 1 is about $160. Option 2 depends on whether you can find a use able air box and for how much. Either way I wouldn't ride it til its sorted or you risk overheating it from being massively lean.

Ugh, it just rained for a week and it's going to be nice for a week. Terrible timing to put it away.
You could be right about it being lean. I really don't know and am just learning. It seems to run and idle fine, doesn't stall or sputter or anything though. I do notice a "tick, tick, tick, tick" noise when I'm driving though.

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

 

[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. :-k





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.

[FONT=&quot]Keihin, Dynojet, Mikuni Jet Size Cross Reference[/FONT]

[FONT=&quot]All Hole Diameters Are in Inches[/FONT]

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