81 GS850G Won't Rev over 4000 RPM

[49er]

I doubt that your valve clearances are too loose. They are really noisy when the engine is run that way. They are shimmed buckets and the clearances get tighter with use. Suzuki recommends they are checked every 6000 kms (4000 miles).

Have you fitted a snorkel to that inlet window yet? If not, that will be causing some leanness at mid range to WOT.

Try partially blocking the window by placing masking tape over 80% of the inlet area. If your performance increases, you'll have confirmed the need for a snorkel.

Are you running an extra inline fuel filter? Some filters don't flow well and if there is excessive fuel line looped too high, the carbs will get starved of fuel when under load. Are the carb butterflies fully opening when the throttle is wide open?

No major flow restrictions in the exhaust system?

Just some other areas to check.

[GS850GFan]

I doubt that your valve clearances are too loose. They are really noisy when the engine is run that way. They are shimmed buckets and the clearances get tighter with use. Suzuki recommends they are checked every 6000 kms (4000 miles).

Have you fitted a snorkel to that inlet window yet? If not, that will be causing some leanness at mid range to WOT.

Try partially blocking the window by placing masking tape over 80% of the inlet area. If your performance increases, you'll have confirmed the need for a snorkel.

Are you running an extra inline fuel filter? Some filters don't flow well and if there is excessive fuel line looped too high, the carbs will get starved of fuel when under load. Are the carb butterflies fully opening when the throttle is wide open?

No major flow restrictions in the exhaust system?

Just some other areas to check.

The valves are not overly noisy, but will check anyways since someone had mentioned checking the cam timing. Might as well check it off the list.

I have not fitted it with a snorkel and you're right, I do need to cover that opening. The guy at the junkyard said the same thing, forgot to do that.

No extra fuel line filter. I did rebuild the fuel petcock and cleaned the fuel filter (it was already clean).

Fuel line is short, no loops, straight from the tank to the carbs.

Butterflies open all the way.

I pulled exhaust system, inspected and reinstalled with all new gaskets.

I will go test the snorkel first, since it is the simplest and post back!

AS ALWAYS, THANKS EVERYONE FOR YOUR IN PUT!

[salty_monk]

check the ignitor... That can act sort of like a rev limiter when it goes. I chased jetting for ages on the 1000G until I figured out the ignitor was shot.

[GS850GFan]

check the ignitor... That can act sort of like a rev limiter when it goes. I chased jetting for ages on the 1000G until I figured out the ignitor was shot.

Another item on my list to check.

A bit of an update:

I haven't been able to test the bike because two other issues came up.

1) I have had an on going issue with fuel overflowing the carbs. Thought it was the float levels, pulled the carbs a few times, readjusted them a few times. Turns out there was a faulty part in the petcock rebuild kit I used. There is a diaphragm with a small disc in the kit. It regulates the flow of gas from the tank to the carbs and shuts off the gas when the engine is not running using the vacuum off the carbs. But I kept coming out to find gas on the ground. A new part has been reordered. Will be here in a week.

2) It turns out the previous owner had the wrong brake handle and master cylinder. It was at the wrong angle which caused brake fluid to leak, which got on the little window to see how much fluid is inside. I went to wipe it clean to check the fluid and the window broke! Also the top screws to open the lid were stripped. LAME. So I spent a few days tracking down a suitable replacement which will keep the fluid at the right angle. Also had to rewire the brake light switch.

Currently, there is a pair of vice grips on the gas line from the tank to the petcock. Once I can resolve that issue, I can start working on getting back to checking things off my list to find why this engine won't rev.

More to come...

[Steve]

1) Your best bet will be to order a new petcock, not replacement bits and pieces for the inside.
The success rate for petcock rebuilds is very low.

2) I have always been under the impression that BOTH sides of the view window would be impervious to brake fluid.
Evidently not. You can use a piece of Lexan that is cut and shaped to fit the hole then epoxied into place to replace the broken window, but evidently you just had the wrong master cylinder anyway.

.

[DanTheMan]

check the ignitor... That can act sort of like a rev limiter when it goes. I chased jetting for ages on the 1000G until I figured out the ignitor was shot.

I second what Dan said. I chased the carbs and fuel guesses for a good six months on my '83 GS1100G, and it turned out to be the ignitor. Luckily I had an older ignitor from the '81 GS850G to replace it with. I had to put it in a different spot, and make a jumper cable to attach it all. New ignitors cost around $450.00. I'm glad I had one that would work.

[GS850GFan]

OK, Now I have to ask (noob) what is the ignitor?

[Steve]

The "ignitor" is the little box that has a few transistors in it that takes the place of points in the ignition system.

It is a poor choice of names, as it does not "ignite" anything, but we are pretty much stuck with what Suzuki calls it.

You will find it on the left side of the battery box, right there with the starter solenoid and the fuse panel.

EDIT: it is the larger black box on the left.



.

[GS850GFan]

The "ignitor" is the little box that has a few transistors in it that takes the place of points in the ignition system.

It is a poor choice of names, as it does not "ignite" anything, but we are pretty much stuck with what Suzuki calls it.

You will find it on the left side of the battery box, right there with the starter solenoid and the fuse panel.

.

OK, and how do I test it?

[Steve]

There is a procedure in the service manual that is rather easy, but must be followed EXACTLY, or you can blow it up.

It involves connecting a flashlight battery (D-cell) to the input leads, but it must be done with the correct polarity. Remove the spark plugs from the engine, plug them into their wires and lay them on the engine so they can ground out. When you connect the battery, one coil (two plugs) will fire. When you disconnect the battery, the other coil will fire. Consult your service manual for details.

.

[GS850GFan]

Will do, Thanks

[gustovh]

What ever you do, don't chech the valve timing. 20 pins between the arrows.

V

[GS850GFan]

2 steps forward!

Got the piece to fix my gas leak. That's handled, leak fixed.

Now, back to the issue of not revving beyond 4000 rpm. I rode the bike today after fixing the leak. It still did the same thing, not revving

I stopped by a local bike mechanic not far from where I live who works out of his house. I've never been over there before. I told him my problem and he suggested I pull one side of the air box off to see if it makes a difference and it did! The engine revved to 4,000 paused, when I back off a little it revved beyond. I called him up, told him what happened and he thinks that the needle jets are warn out and that's it's running too rich.

More Air = Revving higher.

What do you guys think?

[Brendan W]

You are getting 12.5 V at the battery at all engine speeds? If so you have a problem. You should be getting over 14V at 5000rpm. You have either a wiring problem between the r/r or problems with the r/r itself and/or the stator. Until this is sorted you cannot be sure the ignitor is seeing the correct voltage. In my experience fixing electrical issues like this sorted a lot of other symptoms. For example poor connections to the coils themselves can be responsible for bad firing at high revs.
The motor is basically an air pump. If it won't rev out the first suspect is valve timing. If the chain is out by a tooth the motor suffocates at high revs. You can rev to the red line in neutral which would discount this a little but it still needs checking.
Motor bogging under load can also be too much air or too little fuel. The most critical place for air leaks is between the carbs and the head. Have you replaced the o-rings on that side? If you spray a bit of wd-40 in there at idle the speed can change if there is a leak.

[GS850GFan]

UPDATE:

Had to take a short break from this project as I was getting frustrated.
Returned to a low battery, charged it the last few days.

There is definitely as issue with the carbs and the mixture.

The best I can get it to run requires the mixture screws to be at 3-3.5 turns out. Everything I have read says 2.25 or 2.5 MAX. If set at 2.25 the bike will barely start and I can hear it detonating telling me it is LEAN.

So, 3.5 turns out, it runs, but not well, this means it has to be getting MORE air from somewhere to require 3.5 turns out to make the mixture the correct amount.

Riding the bike, with 3.5 turns out and the sides off the airbox fixes the revving issue and it will redline. 2-2.5 turns is super LEAN because it's getting air from some place (remember, intakes are new, airbox rubber rings and air filter are sealed)

SO, 3.5 turns out, airbox open is the ideal mixture.

A local mechanic suggested the Needle Jets may be worn and leaking.

Before I spend the money, what do you think?