GS550 (and others) have different intake sizes for each cylinder - what to do

[Tiger_Oscar]

I googled and searched, I couldn't seem to find anything about this, but surely someone has noticed this.

So I've owned a 1982 GS550L for 3 years, had the carbs off and on a dozen times or more like most old GS owners. Stock airbox and exhaust, I know better than to change them. I am considering a 4-1 exhaust though simply because the stock exhaust has gone black and ugly. Anyway, the stock main jets per the manual were #95 1 and 4, and #105 2 and 3. I always wondered why the main jets were different, since all the carbs are the same, and all the other jets are even the same.

Anyway, I may have figured it out. I replaced the carb boots this weekend (the airbox to carb ones, not the carb to engine ones) since the stock ones were getting crackly, and I noticed cylinders 2 and 3 have a major intake size reduction in the boot, seen below;



So what's the deal with this? If anything this intake reduction would choke those two carbs, leading to a richer mixture, right? but the two inner carbs are supposed to have BIGGER main jets than the outer, leading to an even more richer mixture? What the heck?

Now, as it is stock, this bike ran way too hot and too lean. 80's emissions controls had something to do with this I guess. Before I found this intake size difference out, years ago, I upped the pilot jets from 40's to 45's, and now all mains are #110's. This has been working pretty good - engine runs and pulls well into full throttle, great power. I have done chop and reads of the plugs, but I only ever pulled the outer plugs, and they were always nice and tan like coffee, which is good. But now I wonder, how do those inner plugs look? The two inner cylinders must be running differently, right? I've always felt like the bike could run a little better. It's good as it is, but it seems like it could run a little smoother.

I might just cut the reduction section of the inner two carb boots out. But I want to know why they are like this to begin with. Does anyone know why Suzuki did this? Do I need to tune my inner two carbs differently? I kinda want all 4 cylinders to run the same - that just makes sense. So should I cut the reduction bell out of the inner boots to make them as open as the outer boots?


Thanks for reading and thinking.

 

[LAB3]

I googled and searched, I couldn a't seem to find anything about this, but surely someone has noticed this.

So I've owned a 1982 GS550L for 3 years, had the carbs off and on a dozen times or more like most old GS owners. Stock airbox and exhaust, I know better than to change them. I am considering a 4-1 exhaust though simply because the stock exhaust has gone black and ugly. Anyway, the stock main jets per the manual were #95 1 and 4, and #105 2 and 3. I always wondered why the main jets were different, since all the carbs are the same, and all the other jets are even the same.

Anyway, I may have figured it out. I replaced the carb boots this weekend (the airbox to carb ones, not the carb to engine ones) since the stock ones were getting crackly, and I noticed cylinders 2 and 3 have a major intake size reduction in the boot, seen below;

View attachment 58128

So what's the deal with this? If anything this intake reduction would choke those two carbs, leading to a richer mixture, right? but the two inner carbs are supposed to have BIGGER main jets than the outer, leading to an even more richer mixture? What the heck?

Now, as it is stock, this bike ran way too hot and too lean. 80's emissions controls had something to do with this I guess. Before I found this intake size difference out, years ago, I upped the pilot jets from 40's to 45's, and now all mains are #110's. This has been working pretty good - engine runs and pulls well into full throttle, great power. I have done chop and reads of the plugs, but I only ever pulled the outer plugs, and they were always nice and tan like coffee, which is good. But now I wonder, how do those inner plugs look? The two inner cylinders must be running differently, right? I've always felt like the bike could run a little better. It's good as it is, but it seems like it could run a little smoother.

I might just cut the reduction section of the inner two carb boots out. But I want to know why they are like this to begin with. Does anyone know why Suzuki did this? Do I need to tune my inner two carbs differently? I kinda want all 4 cylinders to run the same - that just makes sense. So should I cut the reduction bell out of the inner boots to make them as open as the outer boots?


Thanks for reading and thinking.

Just for grins you might want to check if there's a difference in part numbers on those intake boots, you might actually be seeing a sin of a previous owner! As to jetting, a richer mix contributes to cooling as well and the two inboard cylinders are going to be running hotter than the outboard ones.

 

[cowboyup3371]

Part numbers on the boots are only different for the right and left sides NOT each individual cylinder. I have a feeling someone may have done something to that bike because it doesn't matter which boot goes to which cylinder except that the ones marked with the L go on the left side and those marked with an R go on the right side of the engine.

 

[Guest]

Just for grins you might want to check if there's a difference in part numbers on those intake boots, you might actually be seeing a sin of a previous owner! As to jetting, a richer mix contributes to cooling as well and the two inboard cylinders are going to be running hotter than the outboard ones.

What he said.

 

[Tiger_Oscar]

All right, the cooling thing I like. I should probably just leave it as is then, since my air-cooled engine always seemed to run pretty hot as it is, I'm fine if the inner two cylinders are running cooler and richer. Probably not that big a deal that all 4 carbs are running the same main jet now, since the inner two cylinders still have that intake reduction they should still theoretically be running richer than the outer two.

Not sure if everyone is getting which boots I'm talking about - NOT the carb to engine ones. The airbox to carb ones. The inner two boots are identical, the outer two boots are identical, but the inner two boots are not the same as the outer two boots. The inner two boots are the same part # and the outer two boots are the same part #. I just ordered them from BikeBandit about a week ago, they are about $15 each (so yeah I spent $60 on rubber boots...)

Also, the stock jetting figures I am quoting from the shop manual, not the previous owners setup. I don't remember what was on the bike when I got it, but I do remember I've gone up a couple sizes but not by much.

Was hoping to get the bike going this weekend, but now the new rear sprocket I ordered turns out is too large on the bolt pattern, so it looks like I will have to wait another week :topsy_turvy:

 

[Guest]

The outer two boots are further away from the filter outlet, so are a little larger (on their intake side) to compensate for the fact that they don't draw air as easily as the inner two. Obviously the ends that fit the carbs should be the same.

I'm curious as to which manual you're using? I've checked three sources. and they all say 92.5 mains for all four on a 550LZ.

And just to be certain, you have the 8-valve engine, with four distinct carbs? As opposed to the 16-valve engine, with two pairs of two carbs?

 

[Steve]

Looking at the parts fiche, there are different numbers for inner and outer boots, but there is no information on actual sizes.

The carb fiche shows that the main jets are all 92.5, the pilot jets are all 40.

I can understand upping the main jets for increased airflow, but you did not mention that you had removed the airbox and installed pods. Unless you install a straight header or one with a competition baffle, jetting changes for an exhaust system are minimal. Either way, there is never any need to increase the pilot jet size, especially going up TWO sizes.

Unless you are the original owner and have done ALL the wrenching on it, you never know whether anything was "stock" or, like LAB3 said, "the sins of a previous owner".

.

 

[bwringer]

Another factor to consider: these boots shrink and harden an astonishing amount over time and with heat (many people have ordered new airbox/carb boots and been upset when the new much larger and softer pieces arrive). The inner two boots may shrink more than the outers because they get hotter.

As far as different shapes between inner and outer, that's not really unusual either. If you can still get replacements, order up a set.

 

[roeme]

As for running hot: my fuel-injected-but-still-air-cooled-2014-guzzi runs WAY hotter than my 1978/9-GS550's (or at least seems to. Also, granted, the latter might be set more on the rich side to be safe).

Can't recall different settings for in/outboard cylinders from the manual. While it seems to make sense on first glance; I think the differences are too miniscule to require them.

Mind though that these vehicles/engines have not been designed with traffic jams in mind. On a stationary vehicle, the inner cylinders will heat up much more.