Overall Performance restoration - 1980 gsx750

[CincinnatiKid]

I love this bike. it's a 16 valve 1980 gs750.
I got it for cheap from a friend of a friend, and have put a good deal of work into making it my reliable daily commuter in Chicago. I rode it through the winter last year, and intend to do it again this year.

One gripe I have with it, though, is that it seems to lack the proper power, and I don't quite know where to go to restore it's factory power. I tested compression recently and it was about 135 in all cylinders, within spec. For the most part, the engine and bike is stock. Stock airbox, exhaust, carburetors, engine, ignition, drive sprockets. My issue with power is 2-fold:

1) The bike has a top speed of about 90 mph. About 30mph less than the spec listed for the bike in stock form.

2) I have stalled out on the highway more than once while trying to test max speed. It seems like the bowls drain empty, but I haven't been able to empirically prove that yet. I will open to full throttle from my 80mph cruising speed, with little to no power increase. Then the engine will surge with power once or twice sporadically, pulling hard, and will eventually start to lose all power, and the engine will slowly die. Pulling off to the side of the highway, I wait a few minutes and try to start the bike with some choke and it fires up and we're back off like normal. Very curious. Living inside the city of chicago, there aren't many locations to test full-throttle runs, especially when they inevitably end in a highway shoulder intermission.

tank is fine/clean. petcock is unobstructed. using 5/16" fuel line, plenty of flow from the petcock to the carbs. at one point, I found that the vacuum line controlling the petcock had a small leak, and while replacing that line helped, the problem didn't go away entirely.

Engine runs well enough in the low range, and pulls nicely from 4k onward. It's an excellent bike for commuting through the city, but I want to have the full power that this engine can afford.

I'm making this thread partially as a sounding board for ideas, and partially as a captain's log.

I think my first plan of action is to verify the jet sizes in my carbs, and then follow the tuning steps outlined with this guide (http://www.factorypro.com/tech/carbtun.html). basically, work backwards from the main jet. Any suggestions or directions to work in would be greatly appreciated.

thanks!
here's a picture of the beauty with her October passenger...!

[alke46]

Before going too crazy with the fuel issue I have to ask if you have checked the resistance of the spark plug caps. These guys don't last forever. Also if you have not done it, trim off about a sixteenth of an inch from the end of the plug wires and then replace with new caps. Just trying to eliminate the electrical trouble possibilities.

[tom203]

Yes, check simple stuff first.. have you tried running petcock in PR spot during one of these high speed test runs?

edit... The dreaded 1980 leverless petcock, oh no!

[mmattockx]

Is the tank vent partially plugged? You should have no problems getting well over 100mph on that bike, something is definitely not right. Have the carbs been cleaned? You know for sure the jetting is stock?


Mark

[CincinnatiKid]

i don't know that the jetting is stock. i cleaned the carbs well a year+ ago when i got the bike, and my memory doesn't serve me as to the jet sizes. I can get the carbs out pretty quickly, so i will check them to be sure.

i've tried doing one of these high speed runs with the gas cap opened, it still starved out on me.

Also, interestingly, the fuel starvation thing doesn't happen all the time. usually on the interstate when i've already been cruising for a while and then want to try WOT for a bit. A few weeks back, I was riding with some friends in western Wisconsin and on the back country roads I was able to hold WOT for a few miles consistently without starving out. but then on the interstate back home, I could tell I had to dance that line again of the throttle position that's just on the safe side of not-starving.

haven't measured the caps, but for 15 bucks, I just went ahead and ordered some new ones. simple peace of mind.


thanks

[CincinnatiKid]

carb main jet 112.5, pilot jet 42.4

cant find specs listed in my manual (clymer)

[TxGSrider]

You could grab a spark plug wrench and gloves and when it lays down like that for a few seconds, pull in the clutch and kill the motor and coast to the side. Jump off, pull out a plug or two and see how they look. If they are wet, it may be an electrical problem or very rich mixture. If they are bleach white it's running really lean and is a fuel problem. If you pull a plug and it looks ok, keep pulling them and noting their condition. If you find just 1 that's lean or wet or looks allot different than the others, it may just be that 1 cylinder that has a problem and you can narrow your search.

[posplayr]

I had a 1981 GS750EX which is most the exact same bike. It was remarkably reliable. The jetting is stock and shoudl not need any changes, as some others have mentioned I would check electrical. Start with the charging system (Quick test) and then work your way to what the coil voltages are.

On the way you should run into several things that probably need cleaning like the fuse box and ignition switch. My first guess is you have really not gone over this as mine would start with even a slow dragging starter.

I put in an 1100 intake cam and bumped the main jet up including a K&N filter element and this added a noticeable improvement as well.

http://www.thegsresources.com/_forum/showthread.php?237763-Recommended-Products-for-Preparing-Electrical-Connenctions&highlight=electrical+cleaners

This is what your fuse box will look like when you pop the back cover off. You want to thoroughly clean those crimps with something like naval Jelly then use some flux to flow solder into the crimps. Those crimps will oxidize and the heat up with the current flowing through them. The more they heat up the more then oxidize and the worse it gets.





I'm pretty sure both the 80 and 81 both has electronic ignition but you should pull a plug wire and check to see fi you have a blue spark. (use a screwdriver) and see if it will arch 1/4"-1/2" to the head with a blue flame. yellow is bad.

[mmattockx]

carb main jet 112.5, pilot jet 42.4

cant find specs listed in my manual (clymer)

Go here and download a factory service manual for your bike: http://members.dslextreme.com/users/bikecliff/


Mark

[CincinnatiKid]

i have the factory service manual donloaded, thanks for the link. i was using a different computer today in the shop, so i was chained to my hard copy clymer's.

as far as electrical goes, I have been over the bike pretty well. i've rebuilt the charging system, it's charging very well. put a new fuse block in. got around to adding the coil relay a month or so ago. Ive got a tiny little voltmeter reader that i could patch into the coil lead so I could monitor it while riding. though with the coil relay mod, I suspect that they're getting sufficient voltage.

In my suzuki manual it looks like the jet sizes are edited?? see the attachment. If somebody can confirm that a 117.5 main and a 37.5 pilot are indeed the stock sizes, i'll get some in there as a baseline.

[posplayr]

i have the factory service manual donloaded, thanks for the link. i was using a different computer today in the shop, so i was chained to my hard copy clymer's.

as far as electrical goes, I have been over the bike pretty well. i've rebuilt the charging system, it's charging very well. put a new fuse block in. got around to adding the coil relay a month or so ago. Ive got a tiny little voltmeter reader that i could patch into the coil lead so I could monitor it while riding. though with the coil relay mod, I suspect that they're getting sufficient voltage.

In my suzuki manual it looks like the jet sizes are edited?? see the attachment. If somebody can confirm that a 117.5 main and a 37.5 pilot are indeed the stock sizes, i'll get some in there as a baseline.

You already have the stock jetting in the bike.

carb main jet 112.5, pilot jet 42.4

[CincinnatiKid]

okay great thanks and also dangit

[CincinnatiKid]

is there a good empirical test for checking on the spark under load, aside from swapping known good parts in?

[CincinnatiKid]

Installed new spark plug boots. all are 5 kOhm. the old ones tested 9.3, 9.6, 9.7 and 11.2 kOhm. that 11 seems off. any idea what the plug cap should be?

Did some voltage testing to confirm proper electrical continuity and health:
Battery @ Rest (keyswitch on, lights off) - 13.63v
Battery @ Idle (lights off) - 13.5
Motor running @ 5k - 14.7v

Measured voltage drop between the battery positive and the coil side of each coil's plug to the wiring harness
Coil voltage drop @ idle - .11v
Coil voltage drop @ 5k - .04v
Both coils gave the same reading, as expected.

So unless something occurs under load that isn't happening in the shop, the ignition seems to be in good health.

[CincinnatiKid]

Before going too crazy with the fuel issue I have to ask if you have checked the resistance of the spark plug caps. These guys don't last forever. Also if you have not done it, trim off about a sixteenth of an inch from the end of the plug wires and then replace with new caps. Just trying to eliminate the electrical trouble possibilities.

Larry, I have to say, I was skeptical, but I installed those new NGK caps, and from initial riding yesterday, the responsiveness of the engine is on a whole new level. There don't seem to be power band "zones" anymore to the same degree... it just pulls through the RPM range. Weather sucks today and I need to wait until next week to install my new tires and get my brakes back in order, but I'm optimistic that this next run will see that speedometer blast right past the 100 mark.

TBD
thanks for the opinions, folks.