79 GS550 Carb Tuning Issues (K&N Pods + Mac 4-1)

[drudgeons]

Recently re-jetted my new-to-me 1979 GS550.

I bought the bike in March for cheap on Craigslist and have been riding it since, but initial repairs included having to replace exhaust. PO decided somewhere along the line to fix exhaust leaks/holes where the original mufflers met the original pipes by welding the mufflers on. That was a short term fix, and by the time the bike got to me, the holes and rust were abundant. So I went with a Mac 4-1 which was fine for everything but highway riding; it really hampered top speed/acceleration at 3/4-full throttle due to the extra airflow.

I decided to rebuild the VM22 carbs and while I was at it, replace the air box with K&N pods. I also re-jet it due to the new much-increased airflow and based on a lot of the reading I've been doing on this forum, and went with the following. As far as I know, everything in the carbs was stock but *according to PO* it had been rebuilt/cleaned at the beginning of last riding season:

Needle: up 2 notches
Main: 110 (from 80)
Pilot: 17.5 (from 15)
Fuel screw: out 2 turns (1 is factory?)
Air screw: out 1 turn (factory)

I did both a bench sync as well as a vacuum sync with a manometer and everything looks okay as far as sync. Moving the needles up helped with idle and now the choke doesn't rev the rpms way up like it used to (would go from 1,200 to 5 or 6k). But it still appears to be running incredibly lean.

I have little power/acceleration at all, and the bike just straight up dies if I rev straight to 1/2 throttle or more from idle. Here's a video:

https://www.youtube.com/watch?v=fpD1Oo90D_Q

I can't currently get the bike above 45mph in top gear and and it's incredibly slow to decelerate when I roll off the throttle. It's also backfiring a fair amount (lots of small pops, not really any large ones). I'm located in Madison, WI so elevation really shouldn't be an issue.

Do I need bigger mains and/or pilots? Should/can the fuel screw come out more? Any words of wisdom/advice/etc? And let me know if you need more info.

 

[tkent02]

The fuel screws should be less than one turn out, 5/8 or 7/8 or so, the air screws about twice the fuel screw for starters. You don't mention your elevation or even location, so it's hard to guess exactly.

 

[drudgeons]

I'm located in Madison, WI

Fixed.

Question: If the air screws are that far out and the fuel screws are in even further, wouldn't that make it run even leaner?

 

[tkent02]

They are mostly for idle and very low throttle openings. They don't do much at half throttle or more. I am running bigger mains on mine, and I ride at 6,000 feet, so you probably need bigger too.

My pipe is a Kerker though, so it might flow more.

You needle should be up two notches, not down. Or did you mean the needle clip was down?

 

[Steve]

Yes, it would, but mainly at low throttle openings.

Are you sure you are running lean? Do some "plug chops" to look at your spark plugs to see what color they are before making any changes.

.

 

[drudgeons]

the needle clip was down

Yep, that. I moved the clip down to the lowest notch.

 

[drudgeons]

Are you sure you are running lean? Do some "plug chops" to look at your spark plugs to see what color they are before making any changes.

Poor acceleration or it doesn't seem to rev out, sometimes if I back off the throttle slightly the revs will pick up.
Power feels better when I put on the choke.
Popping or spitting back through the carb.

I'll try a chop test tonight to see if it's truly lean. Do I need brand new spark plugs for that? I know it's checking to see if it's brown/black/white-ish, and brown is a good mix, correct?

 

[Steve]

New plugs are a bonus, but probably not necessary in your case. If they are as lean as you think they are, there will be no color on them to start.

Do several tests, I am not sure if order is important or not.
1. Pilot circuit. Mark your throttle so you can see the actual setting, then hold about 1/8 throttle as you mosey around town for a bit. Don't be afraid to get to 4th or 5th gear, but hold that throttle opening. Cut the throttle, hit the kill switch, pull the clutch, coast to a safe stopping area where you can inspect the plugs.
2. Transition to needle. Hold 1/4 throttle for about a minute, if you can. Low speed in a higher gear and up a hill will keep you from going too fast.
3. Half throttle. Fully on the needle. Won't need to hold this quite as long.
4. Full throttle. Third or fourth gear up to redline once and you should be good.

.

 

[drudgeons]

One quick question before the chop test tonight - after some reading it seems as though an extremely low top speed (in my case 45mph) and little to no power at around 2k-6k rpm is indicative of a rich condition. Does that sound right?

 

[tkent02]

Dunno, are the plugs black or are the plugs white? More likely it's a clogged exhaust.

Did MAC leve any packing material in there?

 

[drudgeons]

Dunno, are the plugs black or are the plugs white? More likely it's a clogged exhaust.

Did MAC leve any packing material in there?

I've been riding it for 2 months and before the carb rebuild/re-jet and adding the pods, the only issue I've had is it would bog between 3/4-full throttle in top gear.

 

[HanfordTec]

Sounds more like its getting too much gas and fouling the plugs killing spark. Close the fuel screws to 1/2 to 7/8 from bottom seatted position. air screws should be 1 to 1/2 out

 

[Guest]

Sounds more like its getting too much gas and fouling the plugs killing spark. Close the fuel screws to 1/2 to 7/8 from bottom seatted position. air screws should be 1 to 1/2 out

Your answering a question that is over 2 years old. Welcome to the site.

 

[HanfordTec]

Your answering a question that is over 2 years old. Welcome to the site.

Just got a 79' 550e a few months ago and have been slowly working on it. Just from my mechanical experience that's what it sounds like to me. I'm always up for helping others when I can

 

[pedrobighead]

what you need to get your bike running smoothly

what you need to get your bike running smoothly

Hi I am a VRRA racer with a 1979-80 gs500e (these bikes are many flavoured and have different setups depending in where you bought it and they often have features from a year below and above. sorta hybrids)
mine has mag rims and a single disc up front.... kinda odd, most single caliper front brakes on the gs550 came with spoke rims. the mags all had duals except mine.... WTF

I run a mac header (it's the only one I can find to fit this bike) and no filters or vel stacks whatsoever. (open throats for racing)

this is the setup I used this season.

first thing is to change out the main jet to a 102.5 (a 120 is for a vintage honda 750 and waaay too large for this bike)

replace the pilot jet with a 42.5

You should not have to move the needle up or down any slots until you have road tested the bike for flat spots in your complete throttle up and down.
The only time you change slide positions is when you feel a dull or flat spot in the middle running range.

You're slow and pilot jet take care of idle and the initial 1/3 of the throttle.
Then there is a small overlap of the slow/pilot and the needle. (mid range acceleration)
the needle affects the second or middle third of your acceleration. If there's no flat spot up or down or dogging: the needle should be left alone, if there is a flat spot you do one needle position up (move the clip down so the needle sits up higher in the slide) until the bike runs smoothly in the mid range

the last 1/3: your throttle from 3/4 to full open is controlled entirely by the main jet which feeds mixture from 2/3 to full throttle.
when your bike is tuned properly you should be able to grab throttle from idle to full open without any hitches or pops and no lagging. The front wheel is 19 inches and the fork rake is rather severe on this bike so the front end should stay on the ground even with your new jetting.


you will increase the horsepower a few points but more importantly you can grab it all the way thru the range and put some serious power to the back wheel.
the stock top end is 112 mph, this will increase your top speed over 120 mph.

now the second point, you need to change your gearing to support the new capabilities: a ratio of between 2.00-3.00 is recommended.

short and fast requires sprockets mated in this range, bike will accelerate quickly but lose top end speed

13 front, 39 rear (3.00).... not common
14 front mated with 41-42 rear. (2.94-3.00)
15 front 43 rear (2.87)

some of these teeth configs don't exist so you have to mate them accordingly.

to go faster at the top end,

15, 44 (2.93)
16, 45 (2.81)
17, 49-50 (2.88-2.94)

but this makes the bike take longer to get there, and wont be noticeable until the end of 5th gear and all of 6th gear.

Recently re-jetted my new-to-me 1979 GS550.

I bought the bike in March for cheap on Craigslist and have been riding it since, but initial repairs included having to replace exhaust. PO decided somewhere along the line to fix exhaust leaks/holes where the original mufflers met the original pipes by welding the mufflers on. That was a short term fix, and by the time the bike got to me, the holes and rust were abundant. So I went with a Mac 4-1 which was fine for everything but highway riding; it really hampered top speed/acceleration at 3/4-full throttle due to the extra airflow.

I decided to rebuild the VM22 carbs and while I was at it, replace the air box with K&N pods. I also re-jet it due to the new much-increased airflow and based on a lot of the reading I've been doing on this forum, and went with the following. As far as I know, everything in the carbs was stock but *according to PO* it had been rebuilt/cleaned at the beginning of last riding season:

Needle: up 2 notches
Main: 110 (from 80) (WAY TOO LARGE, the jet should only be 102.5)
Pilot: 17.5 (from 15)???? (this should be a 42.5 pilot)
Fuel screw: out 2 turns (1 is factory?) (too much fuel, bike will run fat and like a lazy dog or sloth)
Air screw: out 1 turn (factory)

I did both a bench sync as well as a vacuum sync with a manometer and everything looks okay as far as sync. Moving the needles up helped with idle and now the choke doesn't rev the rpms way up like it used to (would go from 1,200 to 5 or 6k). But it still appears to be running incredibly lean. (sorry but moving the needle doesn't help your idle at all, it has nothing to do with it)

bench syn, useless waste of time)

a real time sync makes each carb suck the same amount of air and will not help the bike run faster or better unless one carb is waaaaay out, it will make the plugs show the same colour which should be brownish grey at full throttle shut down. the sync should be done in the bike after the carbs are installed and the bike put back together with everything hooked up and the bike brought up to running temp.

I have little power/acceleration at all, and the bike just straight up dies if I rev straight to 1/2 throttle or more from idle. Here's a video:

https://www.youtube.com/watch?v=fpD1Oo90D_Q

I can't currently get the bike above 45mph in top gear and and it's incredibly slow to decelerate when I roll off the throttle. It's also backfiring a fair amount (lots of small pops, not really any large ones). I'm located in Madison, WI so elevation really shouldn't be an issue.

Do I need bigger mains and/or pilots? Should/can the fuel screw come out more? Any words of wisdom/advice/etc? And let me know if you need more info.

please look at the notes above your jetting is way out and your main is waaaaaaaaaaaaaaaaaaay tooo large for these carbs, your basically flooding the bike every time you try to increase the throttle.

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