Poor performance puzzling me..

[Guest]

Heres the long and short of it...

1981 Suzuki GS 550T
Stock exhaust (except PO drilled out some of the baffles which is annoying, but does give the bike a unique sound)
Stock intake (though with a drop in K&N filter OR stock filter holder with UNI foam oiled with K&N oil)
Carburetors Dipped and cleaned thoroughly.

Starts and idles fine, runs good up to about 4k to 5k rpm's (depending on how I get there) then feels like hitting a rev limiter. On one ride to check if throttle position changed anything, I opened to full throttle from 3k (where idle was sitting during that ride) and at full open it pulled strong all the way up to 4~5k. Before I only noticed it at that range and 1/2+ throttle, so had been thinking it was throttle related, but with a full throttle open working fine up to the problem area, I'm not sure it's related to the throttle opening. This limits the top speed of the bike to be at about 55mph with absolutely no power to pass.

The Bike/engine is basically freshly built over using the bottom end and the bits and pieces from the original and two other spare engines. We did put new rings in, and he honed the cylinders.

Done so far:

• Put stock air filter holder in place with uni foam (oiled) (had started with K&N in from the build). No improvement
• Checked air intake system visually for anywhere that could be leaking. Found the connection between plenum and airbox wasn't sealed correctly due to bracket being mounted in wrong order. Corrected that, but now that I think about it didn't visually check to make sure that the rubber seal was properly in place. No improvement
• Changed air filter back to K&N drop in replacement. No improvement
• Checked Jets in Carburetors stock as listed for the 550T model (for some reason it is different from other 550's). No improvement (duh, no change)
• Forum member suggested that cam timing was off by a tooth, checked and it was (depending on your source of information), corrected. No improvement (link to thread on forum http://www.thegsresources.com/_forum...d.php?t=156312 )
• Forum member suggested trying to limit air flowing into airbox, put workshop rag in intake of air box (not sure how much affect it really would have as it wasn't a good fit). No improvement.
• Forum member suggested making sure advancer was working. Visually verified it was, while doing this noticed the plate the signal generators are mounted on can be rotated by a couple of degrees, rotated it as far to the right as it would go. See next section.

*just this morning*

• Played with signal generator, moving from full retard to full advance in three locations (rule retard, mid way, then full advance). No improvement
• Measured resistance of signal generator found it to be a bit out of spec (approx 83 Ohm's spec is between 60 and 80). Replaced with signal generator that measured about 69 Ohms. No improvement

Any thoughts? I'm not sure I'm describing the symptoms all that well it is just a very odd feeling it keeps running but the engine feels completely different and won't accelerate any further.

Thanks for your time and thoughts.

Here are a few of things I have yet to try (in the order I'm probably going to try them as it goes from easiest to hardest to try), but don't know that they will make any difference.

1. Loosen air filter element to allow freer air flow to see if maybe filters are over-oiled and not allowing enough air flow for engine
2. Put foam into air intake system to limit air flow there to see if it has any affect (maybe air filters are flowing too much air and vacuum isn't enough to keep CV's open?)
3. Triple check the connection between the airbox and plenum to ensure there isn't an air leak there

Amos

[SqDancerLynn1]

No mention? What do the plugs look like ??? The drilled exhaust may require you to rejet the carbs

[rustybronco]

1981 Suzuki GS 550T
Stock exhaust (except PO drilled out some of the baffles which is annoying, but does give the bike a unique sound)

Amos

The drilled exhaust may require you to rejet the carbs

That's what I have been thinking also. and in addition to those plug pictures, how about a shot or two of the "drilled out baffles"?

[TheBigRed]

I'll get pics of the exhaust and the plugs sometime this weekend.


I'm thinking a re-jet shouldn't be needed as the exhaust had these holes in it before I started the project, oh almost 8 years ago, and I did not have any issues with the performance of the bike then, except for the transmission problem. That is why I went and bought the uni foam to put the stock air filter back in place to make sure that it wasn't the K&N making the difference.

I should be able to get the pics of the exhaust up tonight, but I'll have to wait till tomorrow for the plugs. I forgot to mention I wanted to check the gap on them as well to make sure that is correct.

Amos

[rustybronco]

then I will ask you how long has it been since the carbs were done?
possible holes in the diaphragm(s)?
how is the fuel flow out of the petcock?
is the tank properly venting?

***edit*** do you have a timing light to check if the spark is correctly timed and properly advancing

[TheBigRed]

then I will ask you how long has it been since the carbs were done?

Funny story, these carbs had been running my 650 (with 650 jets etc) for the past 5 years till last spring when the 550 came back together. So, that would be last spring, but I did open them in the past month or so to check what jets are in there and there wasn't any varnish.

possible holes in the diaphragm(s)?

See above, as these were in the 650 running fine I doubt there are any holes, that does remind me though I do want to check the index tab to make sure I didn't get that out of place allowing for vacuum to not work right.

how is the fuel flow out of the petcock?

I believe it's fine. Last time I primed (after it sat a while) I heard fuel flowing quickly (I know prime isn't the same). And when we were doing a vacuum sync we tested and it seemed to flow well.

is the tank properly venting?

Last year when I first started tracking this issue down (or trying to in the limited time I had to work on it) I took the gas cap appart to make sure it wasn't blocked and it appeared clear and fine.

[TheBigRed]

***edit*** do you have a timing light to check if the spark is correctly timed and properly advancing

No, don't have a timing light.

Amos

[rustybronco]

that does remind me though I do want to check the index tab to make sure I didn't get that out of place allowing for vacuum to not work right.

That was one of my thought's also.

get a container, flip it on prime and let the fuel run a while. do the same with the fuel cap open.

have you ran the bike on prime also, trying to eliminate fuel flow as a cause?

[TheBigRed]

have you ran the bike on prime also, trying to eliminate fuel flow as a cause?

Now that you mention it I have actually done that, it was one of the rides last year, but I do remember doing a ride with it on prime to check that.

[TheBigRed]

Here are some pictures of the exhaust. BTW Thinking this might have been causing the problem I did get some rubber stoppers (my dad was a chemistry teacher) and plugged the drilled holes out last year for a ride and putting stoppers in made no difference either. (click the pics for larger views)

[TheCafeKid]

Check your diaphrams for pinholes. Put a light behind the diaphram and look for pin holes. Dont assume. A year of sitting dry is pleanty of time to start the process of the rubber coat cracking (ive seen it happen) Especially if they sat somewhere hot. Like a garage. You cant assume just because parts were working fine last year that they're good now, that includes your petcock. If your timing is right, your jetting is right (which BTW, honestly, you should probably install a jet kit if one is available. Stage one. At very least, a bump in the main and shimming the stock needle is probably going to be required. Those drilled baffles are like running an aftermarked pipe at this point...and the KN drop in, or UNI drop in usually requires carb tweaks.) and your getting good spark, there is no where else to look except for a malfunction of necssary parts. Motors only need three thing to run, air, fuel, and fire. Its not getting enough of one of those up top...

[TheBigRed]

Check your diaphrams for pinholes. Put a light behind the diaphram and look for pin holes. Dont assume. A year of sitting dry is pleanty of time to start the process of the rubber coat cracking (ive seen it happen) Especially if they sat somewhere hot. Like a garage. You cant assume just because parts were working fine last year that they're good now, that includes your petcock. If your timing is right, your jetting is right (which BTW, honestly, you should probably install a jet kit if one is available. Stage one. At very least, a bump in the main and shimming the stock needle is probably going to be required. Those drilled baffles are like running an aftermarked pipe at this point...and the KN drop in, or UNI drop in usually requires carb tweaks.) and your getting good spark, there is no where else to look except for a malfunction of necssary parts. Motors only need three thing to run, air, fuel, and fire. Its not getting enough of one of those up top...

Hey, I'd tend to agree, however the bike had these symptoms last year right after the build, life was just too busy and other priorities kept me from addressing in earnest till now.

I'll have to see about tracking down a kit, but I really don't have much to spend. BTW the 650 handled the k&n drop in without any jetting changes. and the uni is just their foam in the stock air filter element holder, do you really think that is that much different from the stock foam in same holder?

When I pull the covers to check the indexing I'll also double check for pin-holes.

Amos

[TheBigRed]

No jet kits available for the 550t, and the closest is a stage 3 kit for the 82 550mz (mini-kat)

However..

When I was using the 550's carbs in the 650 it was with a k&l kit. Well, when I re-built the 650 carbs for it (the 650 cleaned etc) I put the k&l kit in there, and kept the original jets from the 650 carbs meaning.. I have a set of 110 mains (the 550T uses 92.5 mains) and 160 Pilot Air Jets (the 550T uses 150 and most other 550's use 120)

So, is that too much of a jump to try out?

Amos

[TheCafeKid]

Hey, I'd tend to agree, however the bike had these symptoms last year right after the build, life was just too busy and other priorities kept me from addressing in earnest till now.

I'll have to see about tracking down a kit, but I really don't have much to spend. BTW the 650 handled the k&n drop in without any jetting changes. and the uni is just their foam in the stock air filter element holder, do you really think that is that much different from the stock foam in same holder?

When I pull the covers to check the indexing I'll also double check for pin-holes.

Amos

ACtually, re-reading your post RE: throttle position. Ive done this before, many times, before I really understood the relationship of the carbs and throttle position rather than relative to speed and or RPM. The carbs dont care how fast you are going persey, everything functions around throttle opening. It sounds as if you've gone through A LOT of different possible causes, and possibly doing or trying more than one thing at a time? This can get tedious, and isnt conducive to finding the problem. Beleive, Ive been there. You need to start out at ONE thing, and completely eliminate it from the equation.

So what do we know?

We know that the exhaust is modified, and that the intake can/will/has been modified. I'd suggest, for these trials, sticking with the UNI filter, as you are correct, its CLOSEST to factory flow. Starting at the airbox, what can we look at?
1) are you running the lid on the box?
2) if so, is the airbox SEALED properly?
3) are the airbox boots seated and sealing well? (ive had them LOOK as if they are, but move the box and Ta Da! they slip right off the carbs.. which is the same as running NO filter restriction, and will do exactly what you are talking about. If they are hard, they've usually shrunken back toward the box, and really arent doing their job)

After assuring that is all good, lets move on to the carbs themselves

What do we know?
Stock jetting? If so, you night need a change. The 650 those previously ran on had a larger displacement (they are actually larger than 650CC) so you may be bogging out from being too rich.. Establish your jet sizes, and needle position, this will help all who are trying to pin this down with you, as WE know what you are working with.

Once you have that, we an address possible jetting changes if needed. But in the meantime, lets make sure the diaphrams are good as well, eliminate it from the equation.

Youve said you will check the index on them, so that covers that.
Now, YOU know what happens to the bike, but what you need to tell us is, not WHERE in the RPM range it happens, or at what speed, because that has no real bearing on anything (assuming here, your problem ISNT timing, but we arent going to work on THAT now, because thats doing to many things at once) But RATHER, what THROTTLE POSITION this starts to occur. 0-1/4 throttle could be pilot or pilot needle transition, 1/4 -3/4 throttle is likely a needle issue, and beyond that is WOT, or all main jet. So at what throttle position does it start to break up? or just not respond anymore? It may help to mark off your throttle in increments as above with tape so you can visualize exactly where the problem starts to occur..


I am not assuming you're stupid, or you dont know this stuff already. I know this stuff, and sometimes, in the heat of battling with a bike that isnt cooperating, I start into attempting more than one fix, and then you never know what worked or what didnt. It helps (me anyway) to take things down to basic components or such. May help you too. Otherwise I end up running around trying this and that hoping for a silver bullet fix..

[TheCafeKid]

No jet kits available for the 550t, and the closest is a stage 3 kit for the 82 550mz (mini-kat)

However..

When I was using the 550's carbs in the 650 it was with a k&l kit. Well, when I re-built the 650 carbs for it (the 650 cleaned etc) I put the k&l kit in there, and kept the original jets from the 650 carbs meaning.. I have a set of 110 mains (the 550T uses 92.5 mains) and 160 Pilot Air Jets (the 550T uses 150 and most other 550's use 120)

So, is that too much of a jump to try out?

Amos

Ok, THIS info is good..

110 main is pretty big jump.. that what 7? 8? sizes there? Thats drowning probably. What do your plugs look like?