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80 gs750 won't top out

  • Thread starter Thread starter p_trice
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p_trice

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I have a 1980 gs750e. It has a 4 into 1 exhaust. It has the stock airbox and filter. it has a dynojet stage 1 kit with DJ116 main jets and the needles are at the 3rd groove from the top. It runs great, sounds great, and accelerates great. It won't quite top out though. In 5th gear the highest rpm i have hit so far is 7,500. It just seems like it should have a bit more top speed too it. Am i maybe running too lean or something near the top? My plugs r a nice brownish tan color near the bottom, but the electrode is whitish grey. Also, it seems like it pulls a bit harder near the top with the airbox lid off. Any thoughts?
 
Maybe someone changed the sprocket ratios, some folks don't like to listen to the engine sing and they change to a lower ratio.

What are you sprocket sizes, and what are the stock sprocket sizes?

Also something like a worn chain can slow you down that much.

You could be too rich too, what do the plug chops tell you?
 
my rear sprocket is a 41 tooth. I believe that to be the same as stock. I'll have to get back with ya about the front sprocket. i haven't got to it yet. My chain is newer and is in perfect conditon. My brakes don't drag at all, they r all new. It's kind of hard to read the plugs, by the time i get back to my garage and shut it off to pull plugs, they r white on the tips from running lean at idle. If I rev the bike to 4-5 rpm in neutral, hold it for a bit, then kill the bike. the plugs have a wonderful tan color all over them, so i would say the midrange is about perfect. I'm just having trouble near the top of the rpm range. It raps out great in town, in like 2 or 3rd gear. its just when i'm trying to top it out in 5th. It seems like in 5th at high rpm, when i go from 3/4 throttle to full throttle it doesn't pull any harder, it's about the same.
 
If you're doing plug chops, find a place where you can hammer it, shut if off and coast to a stop and check plugs on the side of the road. A long country road with no traffic sounds good.

I'm no expert mechanic, but wouldn't an engine wrap out regardless of what size sprocket is on it? The first thing that came to my mind was the spark advance, but then again, I know nothing about rejetting so maybe you're on the right track already.
 
I drove at full throttle in top gear for about a mile, pulled the clutch, and killed the bike. I pulled over and checked the plugs. The tip of the plugs were whitish gray, the base of the plugs were a bit sooty, and the electrode was a nice whitish brown color. Am i too lean at full throttle?
 
p_trice said:
.... it has a dynojet stage 1 kit with DJ116 main jets ... Any thoughts?
Click to expand...
I think it's running LEAN. :eek:

Let's just put it this way: stock Mikuni jetting in an '80 750 is 112.5. That is the same as a DJ 120. You are running 116s, which are two sizes leaner than stock.
icon_shrug.gif


You should be running Mikuni 120 or DJ 128.

.
 
Steve said:
I think it's running LEAN. :eek:

Let's just put it this way: stock Mikuni jetting in an '80 750 is 112.5. That is the same as a DJ 120. You are running 116s, which are two sizes leaner than stock.
icon_shrug.gif


You should be running Mikuni 120 or DJ 128.

.
Click to expand...
Yes, but the 750 kit comes with a main air jet corrector which changes things. If it was installed that is.
 
Gonzo said:
I'm no expert mechanic, but wouldn't an engine wrap out regardless of what size sprocket is on it?
Click to expand...

No, it takes a certain amount of power to move the bike that fast. If the ratio is too tall, it ain't gonna get there.
 
my stage 1 kit didn't come with air correctors. I believe the stage 3 does though. I will look into getting some bigger main jets. thanks for the advice everyone.
 
I have the same bike and almost the same set up as you. Though when I added my pipe and K&N air filter all I did was step up the main jets. I also run ACCEL coils. Far as I know this bike won't top out in 5th gear, at least I never got mine too. My plugs run a med tan so I know my jets are just right for my altitude here. I'm at 4,000ft. Matter of fact my main jets are Mikuni 115s. Where are you located p_trice?
The big draw back on this bike are the cams. Just not enough cam to let it breath at the high end in 5th. I was once clocked at 114 in 4th :lol:

p_trice said:
I have a 1980 gs750e. It has a 4 into 1 exhaust. It has the stock airbox and filter. it has a dynojet stage 1 kit with DJ116 main jets and the needles are at the 3rd groove from the top. It runs great, sounds great, and accelerates great. It won't quite top out though. In 5th gear the highest rpm i have hit so far is 7,500. It just seems like it should have a bit more top speed too it. Am i maybe running too lean or something near the top? My plugs r a nice brownish tan color near the bottom, but the electrode is whitish grey. Also, it seems like it pulls a bit harder near the top with the airbox lid off. Any thoughts?
Click to expand...
 
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I am located in Illinois. at sea level. I have some 117.5 main jets on the way. they should do the trick for me. I plan on adding a k&n air filter too.
 
p_trice said:
I am located in Illinois. at sea level. I have some 117.5 main jets on the way. they should do the trick for me. I plan on adding a k&n air filter too.
Click to expand...


where in ill?
its too lean, maybe drill those out with a baby bit, if your getting bigger jets, my bike 750ed came with 117.5,and the 700 comes with 122.5 mains
so with your loss of back pressure youll need bigger jets,always better richer than leaner
it will save 25 dollars!, your air box lid ,and k&n filter will change things as well
 
thanks for the advice. I'm located in central Illinois, like right in the middle of the state. My new main jets will be here today. I'm ready to tear up some pavement.
 
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Think I would leave the air box lid on especially when you get the K&N air filter. I've tried running my bike without the air box lid once and it just didn't work good.
 
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