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Engine hits a brick wall around 5k

  • Thread starter Thread starter kfb
  • Start date Start date
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kfb

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I have been sorting out my new to me 1978 GS750. Changed the spark plugs and adjusted clutch plus new chain and adjusted alignment. The problem is that around 5k rpms the bike doesn't want to go any higher revs. The bike is quick up till that point but doesn't want to go higher. If I twist the throttle wide open I can force it up to 7k+ but it takes time. Once it gets to 7K it is really hauling. Almost like a really flat spot between 5-7k. The PO said that he rebuilt the carbs. Is this an electrical issue or carb related? The bike idles okay any runs smooth. Without a load in neutral it will go smoothly up to 7k+. With a load it is difficult.
 
Carburetor problem. VM carbs I think. For yours I would imagine its a needle problem? You probably have to adjust the needle to make the mixture a little more rich.
Number 22 below. Not sure if it has a clip to adjust it but that needle needs adjusting is my guess?
78750.gif
 
kfb said:
I have been sorting out my new to me 1978 GS750. Changed the spark plugs and adjusted clutch plus new chain and adjusted alignment. The problem is that around 5k rpms the bike doesn't want to go any higher revs. The bike is quick up till that point but doesn't want to go higher. If I twist the throttle wide open I can force it up to 7k+ but it takes time. Once it gets to 7K it is really hauling. Almost like a really flat spot between 5-7k. The PO said that he rebuilt the carbs. Is this an electrical issue or carb related? The bike idles okay any runs smooth. Without a load in neutral it will go smoothly up to 7k+. With a load it is difficult.
Click to expand...

It's worth mentioning that there is a difference in air flow with pod filters and/or aftermarket exhaust. That will change the way it runs and the jets that are needed to run correctly. You didn't mention if your bike is stock.

http://www.mikuni.com/tg_main_jet_size.html
 
brick wall

brick wall

yes the bike is 100% stock. I clean out the air filter and sealed the box. However it is interesting to note that I can see daylight through the bottom holes of the air box. I was surprised because I thought that the air would only be coming through the intake.

I will take it out tomorrow without the air filter to see how it rev's at higher rpm's. That may narrow the issue.
 
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Don't take the filter out. You can simply just take the lid off the airbox to increase airflow.

Or you can do it the right way. Take some tape and put it on your throttle handle. Mark the locations of 25%, 50%, 75%, and 100% throttle.

Go out and drive at 50% throttle for a few miles. Come home at 50% throttle and hit the kill switch and the clutch at the same time. Stop and pull your plugs. The color of the plugs will let you know your problem.
 
Here is what the tape job looks like. My marks are: idle, 1/8, 1/4, 1/2, full
You can find more details on how to do plug chops here.
IMG_3646.jpg
 
Main jets.

Mine was doing the very same thing. I tried adjusting the needles, with no good effect. Then went one size, and two sizes down on the main jets (stock size was 110, went down to 105, then 102.5).

Now it pulls like a champ all the way to redline, at any load.

Also adjusted the idle screws after to get good idle, but the main jets were the problem.

Next is the regulator conversion to get 12 volts to the coils. Good fun.
 
rkubik said:
Main jets.

Mine was doing the very same thing. I tried adjusting the needles, with no good effect. Then went one size, and two sizes down on the main jets (stock size was 110, went down to 105, then 102.5).

Now it pulls like a champ all the way to redline, at any load.

Also adjusted the idle screws after to get good idle, but the main jets were the problem.

Next is the regulator conversion to get 12 volts to the coils. Good fun.
Click to expand...
So you were way too rich and it was only showing up in the middle of the throttle? What kind of carb do you have out of curiosity? :)
 
Not to add confusion but are you still running the stock point ignition?
 
THE STOCK main size for that bike and carbs (provided it is indeed left with a stock intake and exhaust) is a 95. However, they WERE lean from the factory, so if you REALLY want it to punch, id drop one more size in the main, maybe two, down to 97.5 and call it a day.

Those carbs are VM26SS (gen2) on the 78 750.
 
brick wall

brick wall

Thanks for the advice. I will do the throttle marking and see what the plugs look like..
 
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