1981 GS450L - Video; Having trouble with low throttle openings

[Guest]

Was wondering if anyone has dealt with this before as I'm not seeming to make much progress. The bike will rev well when given 1/4 turn or more of throttle, but when slowly rev'd or the throttle is held at low RPMs it will start to stumble. To me this sounds like it's going rich. With the current filter/exhaust set-up I would think I'd have to bump up the pilot jets a notch or two from the 17.5 stock set. In reading Pete's GS450 build that's similar to mine it sounded like he was just fine with the stock 17.5 set yet others report success with pilot jets in the 40's. I've been trying three different pilot jets from 17.5 to 22.5 as well as float height adjustments and mix screw settings but can't seem to get rid of the partial throttle stumble. Starting to wonder if it's something else, or perhaps I need to get some pilots in the 40's? Lots of details and a video below if anyone has any suggestions on where to go from here.

Current Settings:
Mixture screw - 2.25 turns out
Pilot jet - 20
Float height - 23.5 mm
Jet needle - plastic spacer removed, one 0.5 mm washer in its place
Main jet - 145.2
Notes - Still stumbling... see video.

Bike specs:
1981 GS450L
13,500 miles
Filters - K&N ovals with Outerwears pre-filters
Exhaust - MAC 2-1 with shorty muffler
Carbs - BS34SS; 24 hr chem dip; hex bolts; balanced; new o-rings including intake boots.

Settings Tried:
Mixture screw - 3.25 turns out
Pilot jet - 17.5 (stock)
Float height - 23.5 mm
Jet needle - plastic spacer removed, one 0.5 mm washer in its place
Main jet - 145.2
Notes - Bad at small throttle openings, dies quick when given any throttle.

Mixture screw - 2 turns out
Pilot jet - 20
Float height - 26.3 mm
Jet needle - plastic spacer removed, one 0.5 mm washer in its place
Main jet - 145.2
Notes - Much better at low throttle, not dying. Still a bit of rev hang (lean) so turning mix screw out.

Mixture screw - 3.75 turns out
Pilot jet - 20
Float height - 26.3 mm
Jet needle - plastic spacer removed, one 0.5 mm washer in its place
Main jet - 145.2
Notes - Best setting so far, idles at 1200. No rev hang. Still partial throttle stumble.

Mixture screw - 3.75 turns out
Pilot jet - 20
Float height - 23.5 mm
Jet needle - plastic spacer removed, one 0.5 mm washer in its place
Main jet - 145.2
Notes - Idling well, still stumbling at part throttle. Perhaps slightly worse than previous settings.

Mixture screw - 3.25 turns out
Pilot jet - 20
Float height - 23.5 mm
Jet needle - plastic spacer removed, one 0.5 mm washer in its place
Main jet - 145.2
Notes - Maybe slightly better on part throttle, hard to tell.

Mixture screw - 2.25 turns out
Pilot jet - 20
Float height - 23.5 mm
Jet needle - plastic spacer removed, one 0.5 mm washer in its place
Main jet - 145.2
Notes - Still stumbling... see video.


20160826_181139.jpg

[chuck hahn]

Have you backlit the diaphrams with a flashlight inspecting for pinholes??? Do the slides drop smoothly if you lift them and let go??? Take the air compressor nozzle and squirt air into the hotdog shape at the top to see if the slides rise with the introduction of moving air. Got the correct needles in the slides??

[pete]

I no longer have the 450 carbs on mine, I'm running 2001 GS500 carbs now and can highly recommend them.

But aside from that, I never got that low throttle perfectly sorted... always seem too rich or too lean. If the issue is with the throttle cracked at all, it won't be the pilot jets, it'll be the needle.

The problem with the needle is that you cannot get a generic Mikuni needle that is comparable, so the only option in that direction is a Dynojet kit. I never tried that as they're waaaaaay too expensive here, and the only ones available in the US are stage 1 whereas I think I needed a stage 3.

The end result of my 450 carbs was holes in the diaphragms, and that ended the story for them, so definitely check out what Chuck has said above.

[Northish]

But aside from that, I never got that low throttle perfectly sorted... always seem too rich or too lean. If the issue is with the throttle cracked at all, it won't be the pilot jets, it'll be the needle.

Have you backlit the diaphrams with a flashlight inspecting for pinholes??? Do the slides drop smoothly if you lift them and let go??? Take the air compressor nozzle and squirt air into the hotdog shape at the top to see if the slides rise with the introduction of moving air. Got the correct needles in the slides??

Thank you Chuck and Pete for your thoughts; I greatly appreciate it. I have some success to share with you.

I've broken down the carbs again tonight after work and gone through everything again. Per each of your thoughts I focused on the slide/jet needle/diaphragm parts. The diaphragms showed no holes when backlit. The slides were buttery smooth upon lifting and dropping and lifted when presented with air. The jet needles were genuine Mikuni non-adjustable 4C2's (somewhat unfortunately) and showing no wear.

The issue was completely my fault and ended up being the C clips in the slides not being seated correctly. I thought I had gotten them below the ridges but they were slightly above. This caused the jet needles to be too high up which was flooding the carbs with gas at partial throttle when the transition off the idle circuit was occurring.

I'm happy to report that the low throttle stumble is gone and I took the bike on the longest ride I've ever had with it (which was about 1 mile.) It seemed to be a hair lean, with a little rev hang when blipped, but I am happy to have a base I can work with.

20160902_200655.jpg

[pete]

Good news! Pretty neat effort there too, I like it!

[Boba]

Wait that's it? No intro? No Hi my name is Dave?

hmph! Fine! No compliments on the motorcycle then!

[Wingsconsin]

Wait that's it? No intro? No Hi my name is Dave?

hmph! Fine! No compliments on the motorcycle then!

At least he posted pictures....

[Steve]

With the current filter/exhaust set-up I would think I'd have to bump up the pilot jets a notch or two from the 17.5 stock set. ... I've been trying three different pilot jets from 17.5 to 22.5 as well as float height adjustments and mix screw settings but can't seem to get rid of the partial throttle stumble.

I don't know why anyone thinks the pilot jets need to be bumped when you make filter/exhaust changes to a bike.

Let's look at the physics of the whole situation. The bike is jetted based on available airflow. Yes, the stock airbox and exhaust will have some restriction in high-flow situations (especially the airbox), but below about 3/4 throttle, they are no restriction at all. So, ... if you remove those devices, there is not really any increase in airflow AT IDLE AND LOW-THROTTLE. Since there is no increase in airflow, why do you need bigger jets?

Go for bigger mains. Raise the needle by shimming it. Definitely. Those help with the mid-range and full throttle, where you now have additional airflow, but low-throttle is unchanged.

Or is there something happening that I simply don't know about?

.

[Northish]

Wait that's it? No intro? No Hi my name is Dave?

hmph! Fine! No compliments on the motorcycle then!

Haha - Hi Boba! All you have to do is ask! My name is Northish and my 1981 GS450L is my first motorcycle. I got it for free as a Xmas gift from my brother because it was in poor shape and had a few dangerous issues to be fixed. I started working on it in Jan. 2016 and have now more or less wrapped up a "scrambler" build of it. I am a young whipper-snapper and the whole '80s cruiser vibe wasn't working for me. Plus, it would be spending part of its life on gravel.
I've put a lot of time into documenting the build over on DoTheTon and as a matter of fact my bike has been nominated for "Bike of the Month" over there. It isn't going to win as it's up against two really quality builds, but please check it out! It's been a fun first motorcycle project for me.

Here's the build link: http://www.dotheton.com/forum/index.php?topic=70280.0

[Northish]

I don't know why anyone thinks the pilot jets need to be bumped when you make filter/exhaust changes to a bike.
.

Well Steve, I cannot argue with your logic there. All I can say is that when I left the stock 17.5 jets in I had a lot of trouble starting the bike. I had the mix screw out close to 4 turns and had to leave the choke on partially to keep it turning. Ultimately, going to a 22.5 pilot jet has my mix screw at a much more reasonable 2.75 turns out and it doesn't need the choke left on once warmed a bit. Wish I could explain why the pods and new headers/exhaust affect the airflow at idle as agree with your thoughts, but it does seem to make a difference in my case.

[Steve]

Wish I could explain why the pods and new headers/exhaust affect the airflow at idle as agree with your thoughts, but it does seem to make a difference in my case.

Several people here have noted that some pod designs block the intake ports at the mouth of the carbs. If those ports are blocked, strange things will, indeed happen. I don't remember if there was a particular brand involved or any other details, only that there was a lip inside the pod that overlapped the intake for the air jets.

.

[chuck hahn]

I have a 81 450L and there are just the two side mixture screws that you adjust from each side of the bike on mine. What I would do is set them both at 1 1/2 out ( this is where mine were when i broke down the carbs for rebuilding). Im pretty sure thats where they were from the factory as the bike only has 3,703 original miles and nothing looked disturbed on the carbs when I took them off the bike.

Then take a small screwdriver with you and ride it at the 1 1/2 out setting. If its acting up then hop off and turn the mixture screws out 1/4 turn and take off again. if that doesnt help ...or makes it worse...turn it back in the added 1/4 turn so your back the the 1 1/2 setting. Turn them in 1/4 from there and try again. Youll find the sweet spot. My hearing real bad so doing the "fastest idle " method doesnt do well for me.

[Boba]

Several people here have noted that some pod designs block the intake ports at the mouth of the carbs. If those ports are blocked, strange things will, indeed happen. I don't remember if there was a particular brand involved or any other details, only that there was a lip inside the pod that overlapped the intake for the air jets.

.

A lot of them do and that lip also tends to scre up the air velocity going into the carburetor. When pods are purhased, they should be inspected to make sure they don't have a huge lip that will block the carburetor inlet whatsoever, especially those ports at the top.

The first thing I did when I bought my gs450s was checking those pods.

I believe K&N have a small bead that doesn't interfere. I know that at least some of the ebay emjo or emgo brand (spelling?) have a huge lip to it that restricts air. I suggest the trash bin or a razor blade. Or both. Sometimes paying extra is worth it.

[pete]

The K&N's are ok, but my throttle response has never been better than when I swapped to velocity stacks. I have a dual stage foam filter over the top to keep the crap out but I won't be going back to pods...