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Air-Fuel mix problem?

  • Thread starter Thread starter Bob Gibson
  • Start date Start date
B

Bob Gibson

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Here?s what I?ve done to my 1981 GS850GT since our last write-up few months ago when the bike did not have enough capability to go on the freeway. Valve adjustment (all chambers now at 120-135psia). Rebuilt carbs (not real dirty and nothing damaged) and replaced main jets back to stock. All other jets have stock numbers except pilot air jet has the number 780 or 180 (hard to tell first number on any of the 4) and I?m not sure if that?s close enough to factory (it?s a .070 inch diameter orifice). Turned Pilot screws to 2.5 turns out. Checked igniter box and coils and they meet factory specs under diagnostics. New plugs and a new tank of premium gasoline. Here?s the symptoms:
With factory air box and K&N filter = Engine rpms will not go higher than 5,000 rpms. It pulls up to 5,000 rpm quickly and easily, but then seems to hit an rpm wall. If I shift bike to next gear as I get to 4,500 rpm the bike runs fine through all gears.
With old air pods installed = Engine rpms will not go higher than 6,000 rpms.
Without any air filter or pods = I can get engine rpms OVER 9,000 rpms! That is, at full throttle I?ll be over 65 mph as I?m shifting into 3rd gear.
Note that the bike starts instantly whether cold (choke pulled out) or warm (no choke), and I do mean instantly (starter button depressed for LESS than one second).
So, could the pilot air jets be the culprit? Or maybe pilot screws at 2.5 turns? Or something else? Anyone got any ideas or suggestions?
 
The pilot air jets are reading 180. I'm not sure of your precise set up as they varied this a bit on the 850 depending on year / market but standard as far as I know is either 170, 180 or 190. So yours are probably standard.

I doubt that they are causing you much problem at WOT though as the pilot circuit is swamped out of it.

It sounds as if you're running rich. First check, is your choke mechanism working ok - ie off when you want it off?
 
Bob Gibson said:
Here?s the symptoms:
With factory air box and K&N filter = Engine rpms will not go higher than 5,000 rpms. ....
With old air pods installed = Engine rpms will not go higher than 6,000 rpms......
Without any air filter or pods = I can get engine rpms OVER 9,000 rpms!
Click to expand...

My SAWG is that your filters a restricted!
 
Probably not the cause of your problems, but get rid of that "premium" gas. :eek:

There is nothing in there that warrants the designation of "premium", and, unless your engine is built quite a bit over stock, it will actually give you less power than "regular" gas.

"Premium" gas does not give you more power. In fact, it has fewer BTUs than "regular" gas. It is actually a band-aid to compensate for what does give you the power, compression. More compression raises the heat inside the combustion chamber, possibly allowing the mixture to ignite before the spark lights it. Higher octane ("premium") gas resists this pre-ignition because it is harder to light and also burns slower. Because it burns slower, you also have to advance the ignition timing to have combustion completed in time to do its work on the piston.

Unless you have a higher-compression engine, merely adding "premium" gas means that peak combustion pressures will happen considerably after the point where they will do the most good, typically a few degrees after TDC. If you advance your timing to compensate, you still won't have as much power, due to the lower number of BTUs.

.
 
Sounds like you are running rich, or you over-oiled your K&N. The only way to see where your problem is is to first check that filter, then do plug chops. Read the plugs at minimum throttle, half throttle, and full throttle after about a mile or so.

The pilot screw only effects the performance at idle and minumum throttle, and just a little in transition to the needle. RPM's are not what is important during the testing...throttle position is.
 
Reading the information on this resource site also states the air pilot jet affects idling and very low speeds, so like many of you, I didn?t suspect they were the problem and that?s why I didn?t buy replacements. Thanks for the reinforcement. As for the choke operation, when the bike is warm, pulling the choke kills the bike. When cold, the bike won?t start unless the choke is pulled out, so I think it?s OK. Is there any other diagnostics I should do?
As for the Dynojet kit, I don?t think there is one, but I?ll research this more. I?m not familiar with these. The jet needles, cylinders and diaphragms appear to be stock and I don?t see any adjustments/notches on the shafts. The cylinders slid real easy after carb rebuild. Am I missing something on this topic other than Dynojet research?
I will also clean out the K&N oil in my new K&N filter to see if that helps, but note that the air pods also bog the bike rpms and they?re not oiled at all (cleaned during carb build). As many of you have stated, it seems as if I?m pumping too much gas or starving the air, but being that it runs real nice without any air restriction (filtering) leads me to the starving air. Maybe it?s time to get one of these Dynojet kits?
Thanks for the incite on the premium gasoline. I really added this to compensate for the old gasoline in the tank I did not remove. However, for now on, regular gasoline it is.
I heard of the plug chop test in some other comments to other members (during my research). I?ll try this and get back to you. This weekend I?m out of town, so upon my return.
Thanks again for all of your ideas. At least I can now climb onto the freeway with the old air pods.
 
If you can pop off the K&N and it runs good, then the K&N is over-oiled. The K&N filter is very non-restrictive...it should be like running no filter at all. The pods you speak of (if they weren't K&N) ARE restrictive.
 
Thanks for this note. The test where I was able to get to 9000 rpms was with the air box case installed on the bike, but the K&N filter and both chrome side covers were removed. The air pods I mentioned were not K&N, but they are a name brand. Cleaning the air filter is an easy fix. Maybe tonight.
 
You were right on my K&N air filter being over oiled. I cleaned it out over the weekend, did not add any oil to the filter and left the chrome air box covers off, and now I can get the bike nearly to 8,000 rpm if I go at a normal-to-full acceleration. Going full throttle occasionally gets up to 8,000 rpm, but normally I run into that rpm wall around 7,000 rpm. It's quite sporadic. At least I can now ride the bike on the freeway.

Still, any idea why I'm still getting that rpm limit? It seems as if there's still something not quite right. It's almost as if it's running rough above 5,000 rpm. Is that just fine tuning? Carb float adjustment? Not quite the right main jets? It sure starts quick.
We're getting close. Thanks for any additional ideas/suggestions
 
If you are hitting a wall at wot and run better with no air filter your main jet is too big.
Or it is ignition related or a fuel flow problem.
I'd start with the main jet.
 
Ya gotta do those plug chops. They will tell you alot. I would agree that you are probably too rich still.
 
I'll research and perform the plug chops this weekend.

Main jets were #130 and now I'm down to factory (#115). Obviously it runs much better, but was that due to the carb rebuild/cleaned, jets now back to factory, air intake o-rings, getting rid of air pods, or all of these.

I know, plug chops first and then you'll be able to help more ...

Thanks again for your help. By-the-way, I forgot to add that I can run the bike at 7,000 rpm (once I get it there) for a fair distance without it pulsing or dying, so I think the fuel flow is OK. I know, plug chop test!
 
You are running 115 mains with pods? :eek:

Or is that with an aftermarket insert in the stock airbox? :-k

You kept mentioning pods and chrome air box covers, so I got confuzzled.

You also did not mention whether your exhaust is stock. That will also affect your jetting choice.

.
 
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