'81 GS550 Cafe Build

[cowboyup3371]

Actually it's been stated here that having the tank too high will cause issues like that. I found by hooking it on the left side handlebar it works just fine and you won't see any problems.

[D-Mac]

Actually it's been stated here that having the tank too high will cause issues like that. I found by hooking it on the left side handlebar it works just fine and you won't see any problems.

Great to know! Will do.

[psyguy]

lol on the "moron alert"... but we've all done things like that...

yes, the pressure from the fuel supply being that high would likely overcome the (minute) pressure from the carb bowl floats onto the bowl valves... hell, a faulty petcock can be enough for the carb valves to leak...

[D-Mac]

Yup. I'm hopeful this latest issue can be resolved easily enough. If I'm still getting evidence of flooding after lowering the aux tank and fitting new plugs, I'll pull the carbs again and re-test them on the bench for flooding and re-check float heights. The fact that fuel remains in the line right over top of the carbs when the engine is off seems to suggest the float needles are doing something right anyway.

The petcock is OEM and new, and at least it works great (tank is currently sitting on the bench 'on' and not leaking a drop - with a little vacuum pressure it opens and flows well, and it always flows freely on 'prime' as it should).

[D-Mac]

Note: I'm cross-posting a similar message with photos and instructions to a thread in the carb section (more people check that one, but I want a record here as well since it's an important part of my build).

So where were we.....?

After fixing the level of my aux fuel source, I still experienced the same problems with the engine missing at idle and even under partial throttle (popping and fuel being blown back through the carbs into the air filters - especially on Carbs 1&2). I checked for ignition issues (swapped ignitor boxes with a spare I have; still need to check timing though, although it's not a points ignition). Admittedly, I started the bike on the side stand (no center stand anymore), so if the float levels were set too high, that might explain why the problem was worse on carbs 1 and 2). A leaky float seat(s) could also explain it. When I pulled the carbs, the was a lot of gas in the boots. Some plugs looked wet with gas. So it was either flooding, or ignition timing (recall that cam timing and valve clearances have been set and triple-checked; bike shows no other symptoms of either of those being wrong).

I pulled the carbs again and left them overnight with my fuel source hooked up and open. No evidence of any float seat leaks. Good.

Next, I measured a couple of float levels to confirm that I had set them right (the measurement made inside the carb between the float and the carb body - gasket removed per instructions; no pics). The measurements I checked were dead on (within 0.05mm of the center of spec = 22.4mm). Yes, I checked on both sides of each float.

After checking the floats, I rigged up a setup to check the fuel levels of the carbs (what really matters to the engine). I set the carbs in a vice on a 7-degree angle (to match the angle they are in the bike) and rigged up my fuel cell to it. I used some vacuum connectors screwed into the float bowl drains to set up the 1/4" hoses to check the fuel levels within the carbs.

Single carb hooked up.


Shows how you take a measurement (note: the photo is approximate - in the case the actual measurement is a little below what is shown - I just held the camera up to give you an idea of what I'm talking about).


Shot of how I took my measurements. Note how carbs 2 and 3 have the highest fluid levels (smaller measurements). You'll see this is my measurements below.


Shot of my testing setup.


The spec is 5.0mm +/- 1.0mm according to the table in my Suzuki manual, although the detailed procedure suggests 5.0mm +/- 0.5mm.

I took an average of 5 measurements made from the edge of the bowl to the bottom of the meniscus (there's my science-geek coming out). I'll spare you the standard deviations.....

Carb 1: 4.54
Carb 2:3.60
Carb 3: 3.75
Carb 4: 4.67

Hmmm..... looks like my floats ARE set too high - #2 and #3 especially. Consistent with rich condition and perhaps flooding? I'm going to tinker around with them sometime soon and see if I can get them close to 5.0mm. At least there is hope!

Could it be the replacement seats/float needles I bought, or maybe the old floats aren't quite as buoyant anymore?

I gotta take a break from the fumes. Pretty awful!

[tkent02]

Hmmm..... looks like my floats ARE set too high - #2 and #3 especially. Consistent with rich condition and perhaps flooding? I'm going to tinker around with them sometime soon and see if I can get them close to 5.0mm. At least there is hope!

Could it be the replacement seats/float needles I bought, or maybe the old floats aren't quite as buoyant anymore?

I gotta take a break from the fumes. Pretty awful!

Slightly too high perhaps but not enough to explain the HUGE mixture issues you are having.

The test would be better if it was done with the engine running.

Did you check those plugs between the float bowls and the venturis?

[D-Mac]

Slightly too high perhaps but not enough to explain the HUGE mixture issues you are having.

The test would be better if it was done with the engine running.

Did you check those plugs between the float bowls and the venturis?

Yeah. I would tend to agree (although my experience with tuning bikes is limited).

I did check and the plugs seem to be ok. I'll post a pic later to confirm we're talking about the same ones.

[Nightcrawler]

Wow, just wow.

The attention to detail, the time and effort. A superb bike man!

[Sci85]

D-mac, I'm no master mechanic but I agree with tkent02. Float bowls are designed as a "reserve"area for the fuel. They do not meter flow through the carb (that's the job of the jets and needle). Case in point, shut the fuel flow off while the bike is running and it will run just the same until the bowls are almost emptied.

I have not seen a situation where fuel is spit back through the carb on a bike before but HAVE seen this many times on car engines and in each and every instance, it was timing causing the issue (either electrical or mechanical due to improper cam timing setup). The plugs being wet may not be a "fuel" issue at all but the fact that the plugs are not firing properly leaving them wet when you pull them.

If it were me, I would go through the ignition and cam timing with a fine tooth comb. Suspect every part in the loop Until it checks out good. I've even seen bad plug wires create some of the strangest issues. If you had the cams out, I would pull the cover and check the cam timing as well.

Good luck. I know how frustrating these gremlins can seem.

[mash35231]

Carb parts

Were did you source the new jet needle and pilot jet rubber plugs from? I've been searching the web but I can't find them.

[cowboyup3371]

Were did you source the new jet needle and pilot jet rubber plugs from? I've been searching the web but I can't find them.

Depending on what bike you have (HINT - Put it in your signature block), you can still find them from Suzuki. Look at the dealer sites like www.gssuzuki.com or www.boulevardsuzuki.com.

[DannyMotor]

Just throwing this out there, as it happened to me....

How much oil in on that KnN filter?? I over-oiled mine and the bike ran TERRIBLY rich until I completely washed it out, dried it, and then BARELY misted the filter on the OUTSIDE only with oil.. just barely enough to turn it pink... now I can actually tune it.

If it's over saturated, it's not going to run right.

[mash35231]

Jet needle

The reason I asked about the jet needle is the stock GS550LZ needle has no adjustment...only 1 c-clip groove...but the needle D.Mac had pictured is a adjustable needle with multiple c-clip grooves. I have a K&N air filter with stock exhaust and I may need to adjust the needle a little bit.

[D-Mac]

The reason I asked about the jet needle is the stock GS550LZ needle has no adjustment...only 1 c-clip groove...but the needle D.Mac had pictured is a adjustable needle with multiple c-clip grooves. I have a K&N air filter with stock exhaust and I may need to adjust the needle a little bit.

Adjustable needles came with the DynoJet kit I bought.

I got the plugs through Z1enterprises I think (or they might be OEM).

[D-Mac]

Thanks guys! Even when I disagree with y'all (which isn't often), it still makes me think, which almost always leads to better ideas.

I haven't had a chance to get back to the bike in a few days, and with my 'day job' starting up tomorrow it'll be at least a few more before I do.

I agree that float level settings being off just a bit wouldn't cause this (unless they are set too high and flooding - seems unlikely though). That was a case of wishful thinking. I will adjust them a bit and keep the bike more level on next start-up, just to rule out any flooding issues. I have seen both cam and ignition timing cause these kinds of problems, so they both make sense - more than what I think could be carbs alone.

I'm setting aside cam timing for now given the fact that it idled well when first started, it always starts easily, leak-down at TDC is excellent, and I did (10? 20?) re-checks I did on cam timing during the latter part of rebuild, plus photos indicate it's fine. If I do end up checking cam timing again, I promise to do it in front of at least 20 mechanics so that they can beat me silly.

!!!New thought (also leads me to think ignition timing might be the culprit). Now that I think about it (and I can't believe I didn't before), this bike did the EXACT same thing - backfiring through carbs - when I first got it and managed to start it up (I even have a video around here somewhere of it "running" back then). I had assumed it was just clogged carbs at the time, although in retrospect it didn't show any other symptoms of being clogged, and the guy I bought it from (he's an experienced vintage motorcycle mechanic I know well) had actually cleaned them before I got it. So maybe it IS an ignition problem..... If it's the same issue as when I bought the bike, that would rule out coils (now new), ignitor box (I've swapped two in and out with no difference), spark plug wires (now new). That leaves the setting of the ignition pickup/timing plate, the ignition pickup itself, and wiring to the coils (now only triggering a relay, but still potentially important).

I think I should probably check the timing with a timing gun (maybe the previous owner moved the plate too far?) and I'll probably swap in a new pick-up unit and try that too (I have a spare from my parts bike). The ignition switch is new from my rebuild, but that still leaves the ignition/kill switches and lots of wiring. Although the coils now run straight to the battery, I am still using that old wiring from to trigger my coil relay, so an issue in that wiring could still cause a fault I suppose. I could just bypass my relay completely (run coils straight to the battery with no relay) to test that out, right?

What's killing me is the fact that I don't know for certain that the carbs are OK, so I keep leaning in that direction. Your assessments are helping me see other possibilities.

Stay tuned. I might have more info by Sunday.