Another GS550 carb issue, stutters at 3000rpm

[Guest]

Hi guys I am at my witts end with this one. I have been working on this GS550 for awhile now and I though I had it licked but no it was not to be.

I have had some carb/fuel issues with this bike for awhile. It runs well at idle and unloaded throttle response was great but under load it bogged down once it heated up consistently. Once it was hot it didnt want to start, it is a 80 or a 79 with a headlight switch and I noticed that with the headlight off it ran fine but then I turned on the headlight it drug down the idle speed. The voltage also dropped to the battery with the headlight on. So I printed out the papers to check the ignitor box and started working through the eletrical. I found that the outer 2 spark plug caps had way different resistance so I replaced those with 2 that has 5k ohms and are equal with the inners. I also found that 1 of them was not screwed on to the lead well so I cut the lead and screwed the cap into fresh wire, first prolem solved. Next I found tht all the stator connectors were getting quite hot quickly so off they came and I soldered all new connectors in problem 2 fixed. I then went on to the signal generator connector and even after repeated cleaning it still had a inconsistent contact so I cut it out and soldered in new individual connectors, problem 3 fixed. I will still be going through the connections in the headlight bucket but all the other connections seem to be happy now. They still get warm but not hot and the voltage does not drop when I turn on the headlight.

Moving on, I decided to clean the carbs out again. I totally dissasembeled them and ultrasonically cleaned them again, reassembeled them and reinstalled them on the bike. I synced them and color tuned them so now the bike starts with a blip of the starter and idles great. throttle response is great. These carbs got all new parts installed when I cleaned them including butterfly valve shaft seals, new cycle orings and I did not find any pin holes in the vac slide diaphrams. It has stock jetting and stock jet needle. I have also checked the valves clearance and they are all in spec. A new vac operated petcock has also been installed in the tank and it seems to be functioning properly. The tank vent line is clear at least its letting air through when I blew on it. I put new vac hose from the carb to the petcock when I re installed the carbs.

On the first ride i did notice that I still had the same stutter at 3000 rpm. It is not as noticable when I slowly roll on the throttle and is more pronounced when I throttle hard. Once past the stutter it goes great with brisk acceleration and under it the response seems descent. I have installed new intake boots and orings on this bike and it also got new airbox boots. It has a new airfilter and new weather stripping on the airbox cover. I have looked for any leaks in the intake with starting fluid spray and no matter where I spray it the idle does not change so I am confident I dont have any intake leaks.

So after reading my essay can any of the 550 gurus please give me some guidance with what I can try to get rid of this stumble. I am pulling my hair out and I am nearly out of hair. I thank you for any help you can send my way.

[tom203]

What year is this 550- does it have mechanical advance?
Have you tried just backing out the idle/air mixture screws to richen things up a bit? To see if you get a better transition as idle circuits hand off to mains.

with all the electrical stuff you did, did you do posplayr's quick test to check your charging to confirm it's behaving normally?

[bomber737]

I can richen it up a bit but the idle mixture screw are already out about 3 turns. Il try a little more at 1/4 incriments. I downloaded the ignition box check papers from basscliffs site. Working through those pages is where I found the spark plug cap issues and signal generator connector problems. The bike is an 1980 so there is no mechanical advance. It has an electronic ignition box.

[DimitriT]

Generally speaking a poor idle when the bike gets up to temp indicates a rich mixture.

Of course if the bike overheats you may find it difficult to start under any circumstance.

You mentioned that you color-tuned the carbs but was that while the motor was up to temp?

When you rebuilt the carbs did you check the float pins for wear? Did you measure fuel level?

What sort of voltage are you getting at the battery terminals when its idling? With headlights on?

[bomber737]

I did check the float pins when I cleaned the carbs and I did set the float levels to spec. I downloaded the carb spec spread sheet from bassclifs site and I also checked the idle jet and main to make sure they were stock.

I colortuned the carbs at temp and I synced them at temp. Or at least as close to normal temp as you can get with a fan blowing air at the engine. The bike idles great, the problem comes in when I am riding it, the stumble happens consistently at/around 3000 rpm under load.

Before I changed out the connectors the voltage droped from 13.8 to 13.4 or .5 and there was a noticable decrease in the engine idle speed. Now that the connectors are new its holding right at 13.8 to 14.0 volts at the battery and no drop in idle speed when you turn on the headlight. I changed out the hottest connector when I was eliminating electrical issues and then the other ones got hotter so I just changed them all out. I also change the one out in the smaller heavy red wire since I was doing all the others.

[tom203]

Me confused- I thought the 80, 81' and 82 had mechanical advance behind signal generator plate. This stuff....

"stumble happens consistently at/around 3000 rpm under load. " makes me wonder about a sticky advance cuz this is point where timing needs to almost fully advanced.
do you have a non stock ignition system?

[bomber737]

I dont know that. it is a good question though. How would I determine if it does? I have a 80 GS1000 and it is the first year of the electronic ignition on that bike and the signal generator on the 550 looks just like the one on my 1000. That would certainly answer the question though. Does anyone have a picture? I guess I was guessing the electronic ignition did not need a spark advance since its handeled in the ignitor box. Am I wrong here? The bike is an 80

[tkent02]

Part number 5 on the signal generator page is a mechanical spark advance.

https://www.rockymountainatvmc.com/oem-schematic/7

[bomber737]

Ok I see where that is. So I am guessing I will have to pull the signal generator off. How will I tell if the thing is sticking?

[chuck hahn]

Twist it and see if the weights swing freely and return to the retracted positions.

Take a sharpie and make a line across the timing plate and the engine case so you can line it right back up after taking it off to get to the advancer.

[bomber737]

I checked the spark advance and it is free and moves smoothly. I did put a little handy oil on the pivots since I had it apart. I put it back together and the bike fired right up. I happened to touch one of the spark plug caps and I got a shock from it. The bike also started running poorly. So I toched the other 3 and I did not get shocked from any of the other ones. So I think that one might be bad or going bad. Since they arent that expensive I will just relace them with new.

I am about out of options at this point, is there any one on the forum from western wi or the minneapolis/st paul area who might be willing to stop out at the house and put another set of eyes on this bike?

[tom203]

Yes, try new plug caps-cut back plug wires about 1/4 inch to expose fresh copper. This should rule out ignition issue, so get back to carbs. You did some stator work, have you done the Quick test to make sure your charging system is behaving normally?

[bomber737]

I have not checked the stator leads because it is putting out 13.8 to 14.2 at the battery with the headlight on and off. When I started working on the bike it was not charging at all so I replaced the regulator and then it charged normally.

[bomber737]

Well I got the new spark plug caps on the bike and it seems to have solved the engine wanting to die off and on when ideling. I am not sure if lubing the spark advance or the new caps or the combination fixed things but it no longer stutters at 3000 prml and goes down the road as well as you can expect a 550 to.