Main jets?

[Anonymous]

OKay, could they be too big. I have an 81 550 w/ MAC 4-1, K&N pod filters, ACCEL coils an stock NGK at .35 gap as recommended by ACCEL. I have the idle screw set at 3 turns from bottom and needles at 4 from the top w/ a recent sync. She idles fine and pulls like the dickens thru 3/4 throttle, at which point I am having a crazy bog, loss of power. I removed the inline gas filter, tried running w/ the gas tank cap off & petcock in prime, no help. I did a full throttle chop after a few moments of the full throttle hesitation and the plugs read rich. I am running the 150 mains that came w/ the DynaJet stg 3 I installed. Does this seem right? Any help & ideas would be helpful!!!

Thx

[Carter Turk]

I'd say for one the pipe is mostly to blame. I put one on my wifes 82 550 Kat and it had a giant flat spot. No pods.

If I recall the stock 550 had 92.5 mains so 150 is way too big as your plugs show. My 1150 with pods and Gisxr 36mm semi flats run only 147.5.

I'd start around 120ish and go up from there. Carbs are easy to remove with pods. I have many jets if you need some cheap. Carter

[KEITH KRAUSE]

About the main jets. Don't confuse Mikuni jets with DJ jets. They are sized by different methods. Mikuni jets are rated by how many cc's of fuel can pass in one minute. DJ's are rated literally by the size of the jets hole.
So a 150 Mikuni is not the same as a 150 DJ. There are charts on the internet that compare the jet sizes. I forget right now the exact numbers, but a 150 DJ jet is about the same size as 140/or 137.5 Mikuni jet.(?) Something like that.
Also, larger engine size doesn't always mean the main must be larger. For example, a stock '79 GS750 uses 102.5 mains, but a '79 GS1000 uses 95's. Both use VM 26mm carbs.

[Anonymous]

Keith, I thought that was the case. Why does DJ specify a smaller jet for stage one? Maybe I'm not understanding this correctly. GS1100Gk, think stock is 115, but DJ gives you a 113 or 114 to replace it (one of those, not sure at the moment), for stage one. My GKs only have the center of the stock exaust pushed through, K&N elements in stock air box, and HP coils. Greater velocity through the DJ? that's what I've heard.

[robinjo]

My stage 3 1100 uses DJ 136's, it doesn't make sense that yours are bigger.

[edit] Aren't those the airscrews? My kit had 160's in it. [/edit]

[Carter Turk]

Keith,

Even knowing that the DJ is different than Mikuni, wouldn't you say 137 ish is still way to big for a 550? Carter

[Anonymous]

Nah, the air screws are 150 as well. These are definitely the main jets. The stage three kit came w/ the 155 & 150's. I think FirstTimer is running the 155's. Dunno. I've been all over w/ the bike. Great spark, carbs are super clean, new o-rings (thanks Robert), floats perfect. She starts perfect w/ the touch of the starter button, and pulls good thru 3/4 thottle. It only makes sense that the mains are too blame. Suggestions on sizes? 120 or 130 Mikunis?

Thx for all the help!!!

[Anonymous]

Well 120 is too small. 135-140?

[KEITH KRAUSE]

Keith, I thought that was the case. Why does DJ specify a smaller jet for stage one? Maybe I'm not understanding this correctly. GS1100Gk, think stock is 115, but DJ gives you a 113 or 114 to replace it (one of those, not sure at the moment), for stage one. My GKs only have the center of the stock exaust pushed through, K&N elements in stock air box, and HP coils. Greater velocity through the DJ? that's what I've heard.

Well, I don't know. You'd have to ask Dynojet. I've never heard of their jets having greater velocity, but ?
Sounds like you're not sure of your stock main jet size. If it really is a 115, then the equivalent DJ main (numerically) would be about 124? If your stock mains are 115's and DJ supplies a 114 main (equivalent to about a 105 Mikuni) then something's not right, I'd say. Maybe they made an error?
With your mod's, a 120 Mikuni main or 130 DJ seems right.

[KEITH KRAUSE]

Keith,

Even knowing that the DJ is different than Mikuni, wouldn't you say 137 ish is still way to big for a 550? Carter

I would say a 137.5 Mikuni is too big for a 550. I have to wonder if DJ made a mistake with his jet kit?
robinjo, I understand your thinking but you can't just figure a bigger engine has to have a larger main. It generally works that way, but there are exceptions out there.
As I said, a VM equipped GS750 uses larger mains than a 1000, using the same 26mm carbs. DJ supplies my '79 1000 with 138/142 mains in the stage 3 kit. Your 1100 uses 136's. So mine are a little bigger than yours.
The factory jets the carbs not just by displacement but also by that engines designed characteristics. Some engines will have more HP, some more torque, different powerbands...
Also, I think for emissions reasons the larger bore bikes are given even leaner jetting than the smaller bore bikes. Not a lot, just a little. The smaller bikes typically get better gas mileage just because they're smaller. The larger bikes get typically worse mileage so I think the factory jets them even leaner.

[KEITH KRAUSE]

Well 120 is too small. 135-140?

The 150 DJ's are about the same as 140 Mikuni's. Close enough.
If the 120's are too small, then I guess 130 Mikuni's are right in between.
All you can do is test.
Have you synched the carbs with a vacuum tool? You simply can't re-jet accurately until the carbs are vacuum synched. Before synching, the valve clearances and ignition timing must also be good.
Have you marked your throttle? Are you sure the bog starts at that point?
Can you VERY GRADUALLY get up to 3/4 throttle position in top gear with no bog at all and then the bog hits as you fully open her up?
Or are you cruising in top gear, say 60 mph, then ROLLING the throttle fully open and get the bog? On a roll on like this, the jet needle has a lot of overlap effect on the main.
If you mark your throttle at 1/3 to 1/2 and chop test, I wonder what your plug reads would be?

[Anonymous]

Yes sir, vaccum sync, timing good, valves w/in specs, plugs clean etc... I have not marked the throttle for a more exat bog point, but I feel pretty confident about it. I can cruise and the the bike pulls up to 68-69-70ish before it begins to bog. I have tried the roll-on test and it begins to bog at WOT, then when I back it off, she picks up again.

[KEITH KRAUSE]

I have not marked the throttle for a more exat bog point, but I feel pretty confident about it. I can cruise and the the bike pulls up to 68-69-70ish before it begins to bog. I have tried the roll-on test and it begins to bog at WOT, then when I back it off, she picks up again.

Your 550, going nearly 70 mph in top gear will be SOLIDLY on the jet needle. The main simply has no effect at the 1/4 or so throttle opening your bike is at. It would be good to have a plug read at this speed. Your first post says the bike pulls hard to 3/4 throttle but your last post says cruising at 70 starts the bog? I'm not sure what you're saying.
A "bog" can be a rich or lean condition. Your previous posts suggest rich. My experience has always been that if the bike bogs or lags at full throttle but picks up if you back off some, then the engine can't burn the fuel in the mixture. It's rich.
A lot of people will disagree with me but I have less trouble by getting the jet needle set right first, before sizing the main. I get the needle circuit performing right and the plug color always agrees.
THEN I install the largest main that won't create a bog or lag during the above mentioned roll-ons. I also try to install the main that results in the highest top speed. Sometimes it's a bit of a compromise. Given the choice, I always go with a better performing needle circuit. That's where you'll spend the most time riding. I may give up a couple of mph's at top end but to have a good performing needle and a good performing overlap of the needle/main during roll-ons is way more important. Some bikes, with some jet kits, just won't allow perfect carburetion. Some variables just create problems. Even the jet kit makers warn you about "driveability issues".
I would mark the throttle at 1/3, test and get a positive needle read with new plugs first. Depending on that read/performance test I would set the needles first and then decide what to do with the mains.
You say the carbs have been completely gone through, that's why I'm focusing on the needle setting.
PS: you did remove the two floatbowl vent lines to help the bowls vent better?
ANY possibility you didn't replace the jet needle washers/spacers in correct order?