GS1000 Electronic Ignition/CarbQuestion

[Deegz]

Hi folks. It’s been a while since posting anything about the gs1000.


Last week I sent the bike out to a mechanic to check the valve clearances and to install a dyna s ignition system. I elected not to have him tune the carburetor as he said it would be a full days work and I didn’t want to spend too much money at the time. When he did the valve clearance and electronic ignition the bike had k&n pod filters. I got the bike back and it wasn’t running 100%. It was running about 75% of its full potential. Small things like slight popping, idle would hang a little high then come down, if I tried twisting the throttle too fast it would want to stall. Small stuff.

I thought maybe putting the original air box was the solution. I may have been wrong because the bike seems to be running worse. I didn’t have as much time as I wanted to play around with the carbs but it didn’t sound that great when it started.

Sorry for the long post but I’ll get to my question….. if the mechanic installed the dyna s ignition with pod filters would me switching back to the original air box throw everything out of wack? Or can I go back and forth between pod filters and original air box without a worry?

Settings with pod filters: 130 Main Jet, 17.5 Pilot Jet. 2.5 turns out for both air and fuel mixture screw, tried both 4th and 5th clip position on needle and 5th clip position was more responsive

Settings with Airbox: 95 Main, 17.5Pilot, 7/8 turns out on fuel screw, 1.5turns out air screw, 3rd clip position on needle

Any help is appreciated!!

Best,

Carlos

[Nessism]

Those airbox jet speces look right

I recommend a good carb cleaning with new O-rings

[Deegz]

Appreciate the response Nessism. I did change the o rings and made sure all the passages were clear. Do you think the timing for the Dyna S might have something to do with it?

[zed1015]

The ignition has no affect on carburation and whatever filtration you use on the intake won't affect the ignition.
The mechanic has likely made an error when fitting and timing the Dyna.
Also that 130 main jet is miles too big and should be more in the area of 115 - 117.5..

[KiwiAlfa156]

Just to echo Zed above, accurate ignition timing makes a huge difference and given that this has been changed...

You said "Settings with Airbox: 95 Main, 17.5 Pilot, 7/8 turns out on fuel screw, 1.5 turns out air screw, 3rd clip position on needle" So, both fuel and air screws, so I assume we are looking VM26. Checking this against the Bwinger Carb Spec Chart, the pilot jet is up one size from 15 at 17.5

https://gsarchive.bwringer.com/bikecliff/images/carbspec_float_height.html

Assuming the ignition timing (especially at full advance) is correctly indexed and the carbs are clean, if it were me, I'd go back to the stock 15 on the pilot, as it was probably upped to try to compensate for the pods, and they are relatively easy to swap. If your pilot jet is rich, the air screw will have be backed out more than the standard the number of turns compensate for the extra fuel and to give the best pilot mix AFR.

In the VM pilot circuit, the fuel from the float bowl is metered through the fixed pilot jet and the fuel-air ratio of the pilot mixture is set by the air screw which either adds or subtracts air to that fuel flow. This then flows to the pilot orifice (metered by the fuel screw) and by-pass orifice. As mentioned above the air screw setting affects the AFR of the mixture of the pilot circuit, but adjusting the fuel screw doesn't change the AFR as its downstream and can't add either fuel or air to the pilot mixture.

What the the fuel screw does is add or subtract this pre-set pilot mixture to the air flowing under the slides when the throttle is shut. The pilot orifice is experiencing high engine vacuum and is supplying almost all the fuel and air, but air is still passing under the slide -which is why the idle speed knob works- but not enough to activate fuel flow in the main (needle/main) circuit. So at idle is where the fuel screw has the most effect, varying the overall combined AFR. This diagram below has an air jet instead of a air screw, but you get the idea


As the slides lift they uncover by-pass orifice (from memory, the VMs only have one, BS Mikunis have 3 or 4 by-passes) and it too becomes exposed to engine vacuum and higher air flow under the slide. The by-pass flows pilot mixture in addition to the fuel mix flowing pass the fuel screw. This is what you running on from just off idle until the slides are high enough for the needle/needle jet to take over. Its usually at this transition that worn oval needle jets will make their presence known.

I observed these effects using a colortune, looking at the changes of combustion flame colour on adjustment both fuel and air screw at normal idle and at about 3,500 rpm. At the higher rpm the fuel screw had no visible effect. But adjusting the air screw did. This made sense, as at higher rpm in neutral (no load) the engine was running on the pilot mixture, but most of it wasn't being metered by the fuel screw. So this is how I ended up tuning these particular VMs, setting the air screw first (pilot mixture) and then the fuel screws (idle mixture). They carbureted well off idle

Generally, assuming that there are no air leaks, light quick blipping of the throttle on a warmed up engine, just enough to lift the rpm a couple of thousand rpm, can tell you if you are rich or lean in the pilots. A slow drop or hanging is lean, revs dropping or rising/quickly dropping back but under-shooting idle rpm, is rich. But being slide carbs suddenly whacking the slides wide open from idle, will make even perfectly set up slide carbs baulk.

[unfocused]

Several things to add:

Make sure the individual pick-ups on the Dyna are gapped correctly - I set mine around .028/.030. If set wrong, this can affect timing on that coil/ 2 cylinders.
Also, make sure the timing is checked at idle and at 3000rpm and your mechanical advancer is working.

Running pods with VM26 carbs you need the air correctors installed. No questions about this. However, I'm not sure which carbs you are running.
Your jet sizes if running VM26's are way out of range. Stock engine with pipes and filters doesn't need much change, if any.
My 78 GS1000 with pods and pipes ran 102 mains (stock for my 77 GS750) and stock pilots. But I've installed the air correctors.
If running the CV carbs I won't be able to help because I haven't tuned a set to the 8V engines.

Settings with air box from factory I believe was 100 main, 15 idle and 3/4 and 1.5...

[Big T]

First, check the ignition timing as above. If it's good, open the pilot air screws 1/4 turn at a time to see if that improves the off idle response

[Deegz]

Thank you guys a ton for the responses. I really appreciate the help!

I tried going back to the stock air box with all the stock jet settings and the bike did not run well at all! I tried playing with the fuel screw and air screw and it didn’t make enough of a difference to have the bike running well. (Settings 95main, 15pilot, 3rd clip position for needle, 7/8 turns out on fuel screw,1.5 turns out for air screw)

The changes I made after trying the factory air box was back to the k&n pod filters, 115main, 17.5pilot, 3rd clip position, 2out fuel screw and 2 out air screw. Ran better than stock but still seemed to be starving for gas especially when you open throttle quickly.


Next change was k&n pods, 130main, 17.5pilot,4th clip position, 2.5 turns out on fuel screw,2 turns out on air screw, also new spark plugs(b8es)NOW this made all the difference in the world! Bike idles perfectly and I took it for a quick spin and the bike is night and day! I don’t want to jinx anything as I still have to test the bike when it’s cruising and I’d also like to check the spark plugs but the bike is running the best it ever has and pulls so hard now. Almost a completely different bike.


I’m going to do more testing these couple of days and keep you folks in case anyone needs this information in the future.



Thanks again folks!





Carlos

[rphillips]

Went back to stock air box and stock jets and settings and bike did not run well at all. If all back to OEM., and not running right there's got to be an electrical or fueling part not functioning right. Fiddling with fueling changes things will point to carb. problem. Thinking probably a restricted passage some place.

[Deegz]

UPDATE - I had the bike at my work shop for the last couple of days testing it in the area, taking it out for a spin to make sure it ran well. It was running perfectly! I decided I would bring it home and enjoy it this weekend. It was about a 35min ride and half way through the ride the bike just started running like garbage.

it started idling too low and did not have any power whatsoever. It stalled a couple of times and was running “rough”. I thought maybe the fuel was running low so I pulled over at a gas station and the tank was about 75% full.

I guess I can pull the carburetor off again and pull it apart to make sure I don’t have any clogged passages. If it’s something to do with the dyna ignition that’s kind of out of my wheelhouse as I don’t feel confident messing around with that. If it was running great for the last two days would the dyna s ignition be a possible issue?

this was so disappointing because I spent so much time last weekend “dialing” it in and it was running incredible.

any ideas why this may have happened? What would be the next step you folks would take? Any help is much appreciated ♂️

[rphillips]

Dang that's disappointing. I don't know. The easiest thing to check, try it on PRI. If it fixes problem you know it's the petcock, vacuum line, or fuel line. Maybe try playing with "choke" if that changes anything, would tell not ign. related...

[Deegz]

One change that I made recently was remove the in line fuel filter. When I got the bike it had been sitting with gas for so long the fuel rusted pin holes through the tank and we repaired the tank the best we could and installed a pingle fuel valve. Removed the vacuum. Could tge fact that i removed the filter be causing the issue? I might pull the carbs apart again and install an in line fuel filter and give that a shot. Hopefully that’s the issue.

thank you

[Deegz]

Still having issues with the bike and for some reason I think it may be an issue with the dyna ignition. I’m going to look into it this weekend. I did pull the carburetors apart again and clean all the passages very very good. They have o rings from last year but I might change them again just to be on the safe side. I also made sure the petcock and fuel filter was working properly and it’s still running like garbage. Weird thing is I had it running sooo good about a week ago!!

Anyways I’m the meantime I added some Ohlins rear shocks that I got for a screaming deal and things are coming together nicely. Can’t wait to get her running nice again.

[Deegz]

[Rijko]

How do the new spark plugs look, any clues there ?