Carb Jetting GS550

[Theodore]

Hi everyone.

This is my first (of many) posts here, Hope you will be able to give some insight and advice.

I recently bought a "1979" GS550 here in Cape Town, South Africa. the bike runs well but it needs some TLC.

Every time I start the bike it runs on 3 cylinders (1,2,4) and as soon as I drive about 200 meters the number 3 cylinder starts firing and then all is well. I have changed the plugs and removed and cleaned out the carbs (which were extremely dirty) but still the problem persists.

Any ideas on what might be causing the number 3 cylinder to not fire for the first while?

While taking off and cleaning out the carbs I decided that I want to replace the jets to run small pod filters instead of the stock airbox, simply because it is a huge pain to re-install the carbs because of the space being extremely tight between the airbox and the intake boots, I'm sure you know what I mean.

so my other question for you is, what size jets would I have to buy to run the pods?

Thanks in advance
Regards
Theodore

[Theodore]

ahh I almost forgot, here's a photo of my new (old) bike


IMG_7706.jpg

[Big T]

#1 cause for #3 cylinder not firing is petcock failure.
Fuel runs down the vacuum line into #3, flooding it

To test, pull the vacuum hose off the petcock and plug the hose with a bolt or something.

Run the bike on prime and see if the problem disappears.
If that test solves the problem, buy a new petcock.

[Big T]

As to the jetting, you bike has a non stock exhaust. What jets are in it now?

[Theodore]

Thanks for the advice on the petcock, will give it a try tomorrow and report back!

When I cleaned the carbs I couldn't make out the jets sizes, even tried with a magnifying glass. So I have no idea that's why I'm looking for some advice.

[Steve]

Hello, welcome and all that. I have a few questions of my own.

Every time I start the bike it runs on 3 cylinders (1,2,4) ...

Let's first make sure you are numbering the cylinders correctly. #1 is under your clutch hand and #4 is under your throttle hand.

... I want to replace the jets to run small pod filters instead of the stock airbox, simply because it is a huge pain to re-install the carbs because of the space being extremely tight between the airbox and the intake boots, ...

How many times do you plan on removing and re-installing the carbs?

I can pretty much guarantee that you will do it several more times in the process of getting the jetting correct than you would if you left the airbox in place.

It has been noted that you already have an aftermarket exhaust. If it jetted properly for that and seems to run well, I would leave it as-is.

While you have the carbs out, have you cleaned them and replaced the o-rings? Highly recommended, easy and cheap, especially if you get your o-rings from cycleorings.com. Would also be a good time to check the condition of the o-rings on the intake tubes, they are a very common failure point.

.

[Theodore]

Hi Steve. Yes cylinder 1 on left and 4 on right when sitting on the bike.

Well I removed the vacuum line and plugged it to see if it might be the petcock but still cylinder 3 isn't firing when the bike starts from cold, took her around the block and cylinder 3 started firing and all was well again. It's not a huge pain in the ass but I would like to get it sorted.

Now....about the Carbs, when I cleaned them I couldn't find any good quality o-rings so I re-used the old ones knowing that I will remove the carbs again when I get the o-rings, I now have the new o-rings.


Maybe the cylinder 3 not firing will solve itself if I replace the o-rings and "re-jet" the carbs with the correct jets for pods and the exhaust?

Regarding the jets and pod filters, The main reason I want to have the pods is not only because of how difficult it is to re-install the carbs but I quite like the look of pod filters as well. When I removed the plugs today I started thinking that it might still be the stock jet sizes because the plugs are indicating that the engine is running rich, so I think the previous owner just slapped that exhaust on there without changing the jets. Could this be the reason the bike is running rich?

[Steve]

Changing the exhaust to an aftermarket very seldom makes it run richer.
It usually runs leaner.

As for jetting suggestions, it will all depend on the pods that you get and the exhaust that you have. One clue to whether the exhaust might be leaning the mixture is to listen. How loud is it? A louder exhaust system is usually less-restrictive, which means it can flow more air. More air requires more gas (larger jets). The stock Suzuki exhaust is remarkably free-flowing, along with being rather quiet, so your exhaust would have to be obnoxiously loud to be considerably less-restrictive enough to require much change in jetting.

Pods, on the other hand, replace a rather restrictive air box and require considerable changes. Then again, there are differences in pods. Some of the cheapest units are just some wavy paper in front of the opening. Not much restriction, but not much filtering, either. Better pods will be fabric, usually with a light oil film added to catch the dirt. Some pods have a LOT of filter media (paper, fabric or foam), so don't allow much of an increase in airflow.

As a basic principle, you want to avoid the EMGO and other cheap Chinese pods (I hesitate to call them "filters" ops) that cost about $25-40. UNI has a nice foam filter. K&N are generally regarded at the top of the heap, with APE pods right below them. APE's advantage is that they cost considerably less than K&Ns.

Let us know what you get, we can make suggestions from there.

.

[Big T]

How many times did you restart the bike once you plugged the vacuum line? I'd recommend 3-4 times,to get any fuel out of the line

On the issue of pods, you're just getting into more pulling the carbs to get it tuned

What settings are you using for the fuel and air screws?

[Big T]

https://www.thegsresources.com/_foru...ers-CLICK-HERE!!

If you haven't read thru the above thread, you should. Follow all of the links and the tutorials, clean your VM carbs according to that tutorial

[Theodore]

I started the bike 3 times after plugging the vacuum line but still nothing.

Some recent fiddling with the air screws sorted out the problem I had with the bike running too rich, the air screws were all set differently, some 1.5 turns out and some 2. I set all of them to 2.5 turns out, took a long ride with the bike (about 10 miles or so) and checked the plugs and now they aren't black anymore, so that's one problem sorted (for now)

Other thing I've realised is that when I start the bike with the choke on, the third cylinder isn't firing but as soon as I pull the choke off it sounds like it starts firing, any ideas?

also having Idle problems bike will idle fine between 1200-1500rpm and as soon as I start driving when I stop the idle sits at about 2500+rpm :/


Thanks everyone for your input I really do appreciate it! can't wait to get this bike running properly

[Steve]

With the air screws out at 2.5 turns, I have to wonder where the fuel screws are set.
If your carbs are CLEAN and everything else is doing well, they should be between 3/4 and 7/8 turn out from lightly seated. The air screw setting will be up to double that setting, or 1 1/2 to 1 3/4, but might need to start on the richer side to make sure all cylinders are firing well first, then do the fine-tuning.

With the idle speed going up as the bike warms up, you are showing the classic sign of an intake leak. The most-likely culprit would be the o-rings on the intake tubes. They were mentioned in post #6.

.

[Theodore]

Strange, as the bike was idling "better" before I did the carbs and I didn't take the intake boots off so didn't disturb the o-rings there.

I tried setting the fuel screws yesterday but it is a mission to get to them with the carbs on the bike, they are probably way out then. Will have to take the carbs off soon I guess.

Planning on doing a head gasket repair in January, so will have to removing everything any way.

Any thoughts on the choke influencing the 3rd cylinder on startup?

Any tips on getting the carbs back in without a lot of swearing involved?

[Big T]

New boots and O rings are the best solution for an easy carb installation.

Let us know what you find when you tear it down next month.

[Steve]

Strange, as the bike was idling "better" before I did the carbs and I didn't take the intake boots off so didn't disturb the o-rings there.

Rest assured, you don't have to disturb them for them to fail. 40 years of time and constantly-changing temperatures will do a number on them. I'll bet that you find one or more with a serious gap in them.

Regarding the "choke" problem, ... it's possible that the pickup tube is not completely clean. I prefer to back-flush when possible. Use some carb cleaner spray down the back hole in the "choke" plunger area to flush any crud back through the pickup tube. Follow that with a blast of air. If you don't have an extended tip that will straddle the small hole, you will have to get creative and block the port that leads to the intake tube on the engine and the port at the airbox side, then blast air into the plunger area.

Sorry, no tips for removal/replacement without some creative vocabulary, except a reminder that new, pliable boots on both sides of the carbs make the difficult process a LOT easier.

.