Tuning 1979 GS550 Straight Pipes and Pods

[Haett0325]

When I purchased the motorcycle the person I bought it from converted the original 1979 GS550 to a cafe racer style build. He removed the original intake box and converted to pods but never re-jetted the carbs. While I was researching the jetting changes i decided to go straight piped. I re-tuned the carbs and did some more research on what i would need to change to go to open headers and bought the jets to go with.
I cut the original pipes off just before they merged into the run back to the muffler and I love the sound. I am in the process of tuning the carbs and I'm a "rideable" state but I'm somewhere between to rich and to lean with the fuel to air mix and i cant seem to get it right. I bought a gunoson color tune kit to help me get it dialed but i have some other questions.

1) When riding in any gear my tachometer doesn't go above 4k rpm and adding throttle doesn't change RPM or rate of acceleration. Is this an exhaust back pressure issue or a tuning issue? Or something else entirely?
2) Do i need to lengthen the exhaust back to OEM length to regain the RPM range or does the open header reduce the rpm range that drastically?
3)On deceleration the engine is backfiring but seems to be localized to a single cylinder, is this indicative of a different issue or would tuning resolve that?
4)I resynced the carbs when i put everything back together but i cant seem to find a good resource that shows what the vacuum pressure is supposed to read for my year bike is there a specification noted somewhere or is it just balance what the gauge says between all 4 and move on?

Apologies for the long post I'm new to modifying motorcycles, especially vintage motors and such. Any advise would be greatly appreciated.

[rphillips]

Welcome, hope others can help you more than me. Let me poke a little fun at you. If you're between too rich and too lean, you've got it about as close as it gets. The exhaust doesn't effect running near as much as carbs. Good luck.

[unfocused]

Search " air correctors " if using VM24's round slides. If not let us know. I installed these on my VM26's when I had serious accelleration issues after modifying my air box and exhaust installed on my GS1000 engine. It affects all VM series carbs with exhaust/air box mods. Some worse than others depending on mods.

Any reduced length of exhaust pipe after the mid-pipes will cause loss of low end/mid-range period. However, your top end will come alive if you can get to it. The length of the exhaust moves your power range around. Don't worry about this until you solve the above issue.

Don't worry about anything else until you fix the top two. I just spent some time with both of these issues on 2 Suzuki engines.
I have from a 77 GS550, a complete exhaust system and air box ( i think ) that you're welcome to have - yes for free - if you pay the shipping costs. I can assist with shipping.
It came from a almost complete motorcycle so if you're in need of something, let me know. I'm not sure what my plans are for this...
This would solve your issues without needing the air correctors...

Spent 25 years in NW ARK / SW MO. Miss the beauty, the fishing and the riding but not the cold LOL... We had our own Tail of the Dragon coming out of Blue Eye, MO. in to Northen Ark.

[Nessism]

Straight pipes are a fools play. Please have some social responsibility. Loud bikes create ill will in the population at large, which translates into hate for all bikers. Not a good thing.

[zed1015]

Search " air correctors " if using VM24's round slides. If not let us know.

More likely to be VM22's on the 550.
Either way i make the air correctors and provide jetting recommendations for all VM22,24,26,28 and 32mm 4 cylinder carb banks as well as for the Mikuni CV's.

[unfocused]

Thanks for the correction Zed !

[Haett0325]

Hey All,
Thanks for the advice i need to get a local fabricator to bend up some exhaust pipe to lengthen the overall tube length to help with the RPM range. I think the carbs i have are a VM26 smoothbore? i have attached photos of the bike and the carbs to help identify the model. Still a bit of work to do tuning, i think i'm going to start with rechecking the synch then using the color tune to get the mix right.
1979 GS 550 Overall.jpg

[zed1015]

I think the carbs i have are a VM26 smoothbore?

There's no such thing as VM26 smoothbores.
Only 29's and 33's of which yours are certainly not.
Yours are either VM22's or 24's which are identified by the three horizontal ribs on the slide body and most likely yours will be the 22mm version which are stock on the GS550.
The 22's which yours should be are identified by having the kidney shaped casting as arrowed in the attached pic.
24's do not have that.

download (1).jpg

[Nessism]

GS bikes never used VM24's. The early 550's used 22's.

[KiwiAlfa156]

Interesting questions, here's my 2 cents worth


1) When riding in any gear my tachometer doesn't go above 4k rpm and adding throttle doesn't change RPM or rate of acceleration. Is this an exhaust back pressure issue or a tuning issue? Or something else entirely?

If you engine won't rev beyond 4,000 under load, it can only be one of three things. Ignition, lack of air, or incorrect air/fuel ratio.

Ignition? First thing to check before you pull the carbs off. Make sure the plugs, coils and leads meet spec and are working properly. Check the timing with a timing light. Check that the Automatic Timing Unit (ATU) is advancing so that the fully advanced timing mark aligns with the reference mark at around 2,750 rpm. This the more important mark than the idle mark and your setting of the timing should focus on this.

Air? Make sure the pods are clean and oiled. A clogged air filter reduces airflow. Check the flanges of the pods aren't blocking the small round/kidney shaped air inlets on the outer edge of the carb bell mouths as shown in Zed's photo above. Some cheap, crappy pods have a step molded into them which block these inlets. One of these inlets is the air jet that bleeds to the main circuit emulsion tube and if blocked or partially obscured will screw both your needle and main jetting -making it rich and reducing reducing atomization.

Fuel? Is enough fuel coming out of the tank to top up the float bowls? Cap vent? Petcock? If yes then start looking at the jetting.

Exhaust back pressure being necessary on a normally aspirated four-stroke is a myth. Performance is enhanced by the negative exhaust pressure creating a vacuum in the cylinder (scavenging), which increases the velocity and momentum of fresh charge entering the combustion chamber. This is why classical long valve over-lap engines (exhaust and intake valves spending some time open at the same time) make the most peak power. The high momentum of the fresh charge caused by the exhaust vacuum is rammed into and effectively 'super-charges' the cylinder. The reason loss of exhaust back pressure can have a detrimental effect is that the carburation isn't jetted to meet the the combustion conditions changed by removing the back-pressure.


2) Do i need to lengthen the exhaust back to OEM length to regain the RPM range or does the open header reduce the rpm range that drastically?

Open individual exhaust pipes are like organ pipes and reflect a negative pressure wave back up the pipe at the speed of sound. When the exhaust valve opens it sends a shock wave (pulse) through the exhaust gas in the pipe. When that pulse hits the open end of the pipe the pulse is partially reflected and inverted as a negative pressure pulse back up the pipe. When the negative pressure pulse hits the open exhaust valve it assists scavenging. Because the length of the pipe is fixed, the rpm at which the pulse wave arrives back at the exhaust valve at the correct time to be effective is also fixed. This is the tuned length of the exhaust pipe/header and is only effective over a relatively narrow rpm band of may be 2,000 rpm. Above and below this the pipe is just a drain pipe.

Where this band is set is -like a organ pipe- determined by its length. Short tuned length, higher frequency, effective at higher rpm. Longer tuned length, low frequency, lower rpm. All other configurations of exhaust systems, 4 into 1, 4 into 2 into 1, and all the various butterfly exhaust power valves, all try to broaden the rpm range where the exhaust tuning is effective at improving power.

There is math to work out pipe lengths considering pipe ID and target rpm, and for better or worse the internet is awash with online exhaust pipe calculators. The rule of thumb is to attempt to match the pipe to maximum torque rpm for street, and to maximum horsepower rpm for competition.


3)On deceleration the engine is backfiring but seems to be localized to a single cylinder, is this indicative of a different issue or would tuning resolve that?

Could be a number of things, but if it is localized to one cylinder, I start by looking at the the exhaust gasket on that pipe. An air leak can cause back firing on deceleration. If not it might be fueling.


4)I resynced the carbs when i put everything back together but i cant seem to find a good resource that shows what the vacuum pressure is supposed to read for my year bike is there a specification noted somewhere or is it just balance what the gauge says between all 4 and move on?

Pressure measurement isn't important. Its balance between the carbs. I use a genuine Suzuki ball and glass tube synchronizer and there's no way to read pressure on that.


If you want to tune your motorcycle carburetors.

Read this before you begin.

https://www.cycleworld.com/story/bikes/kevin-cameron-explains-how-to-tune-carburetors/

Then read it again. And then read it again.

[KiwiAlfa156]

Serendipity?

https://youtu.be/NWgbXU8thec?list=PLmxTBAJjAAmU8b-IzoSIW9A4DHjnfXl1N&t=996

[Rob S.]

Kevin Cameron is a treasure! I wish Cycle World were still available as a monthly hard copy. Sigh...