Engine bogging down while cruising

[MrZig]

Bike/engine are in signature.

Sofar I've replaced the intake o-rings, replaced the airbox-to-carb boots, cleaned the carbs thoroughly, done a valve adjustment, got new spark plugs, and got a new petcock.

I just now got in my first few days of real riding and it's a lot of fun. The only thing is, my bike has a major missfire/bogging down/flat spot problem. Here's the gist of it:

At 50 KPH in third gear holding a steady cruise, I can feel the bike misfire now and then. When it does this, it slows down and then speeds up quickly. Very annoying.

Then at 80 KPH at around 4500 RPM, the bike will bog down and slow down if I give it gas, but after a couple seconds it will pick the RPM's up, slowly. If I go full throttle it doesn't bog, but it doesn't accelerate as fast as it should.

What's it sound like? Fuel delivery? Ignition? I checked the plugs and they looked perfect - no pitting or bright white or anything, just peachy. It feels like it's running out of gas. If I do a 0-100 KPH run, it'll do it just fine. Accelerating around town is fine. It's just higher speeds and cruising. It idles really well and with no load (IE neutral) it revs just fine.

Fuel cap creating too much vacuum?
Carbs plugged even though I cleaned them out twice?
Ignition?
Air leak? (doubtful, I don't see where it could leak from to cause a problem this dramatic)

 

[BassCliff]

Hi Mr. MrZig,

Sometimes it's just a process of elimination. To your list I would add checking the float height. I've heard of these symptoms being caused by intermittent coils or igniter, or even bad valve timing. But I'm just guessing at some possibilities.


Thank you for your indulgence,

BassCliff

 

[MrZig]

When I cleaned the carbs I adjusted the float height to spec..

Here's what I did today:

Cleaned petcock filter - it wasn't bad at all, although there was some rust at the bottom.
Added a better ground from battery to R/R.
Made fuel line more direct and more pointed down.
Squeezed excess air oil out of air filter.

And there was no change. Still bogs and missfires like crazy. I'm thinking the carb's might have gotten plugged again.
I tested the spark of the plugs, and they came out light blue/violet.

Also tested the resistance of the spark plug elbows and one came to 8.82k ohms, the other came to 9.62k ohms.
The coils each read 3.9 ohms on primary. The secondaries with the caps attached are 21.2K ohms, and 22.1k ohms.
I've also run it in prime with no change.

Here's a couple pictures I took of the spark plugs:

 

[Guest]

a couple of ideas...

a couple of ideas...

what were the carb diaphragms like?
and, have you confirmed that you have the right size jets for your intake/exhaust setup?

 

[MrZig]

what were the carb diaphragms like?
and, have you confirmed that you have the right size jets for your intake/exhaust setup?

Carb diaphrams were perfect.

I havn't confirmed the jets, but I imagine they would be stock as everything else on the bike is stock.

 

[Guest]

Plugs look good. I'd have a look at the igniter.
If you say you have rust in the tank that could be the issue also.

 

[MrZig]

Plugs look good. I'd have a look at the igniter.
If you say you have rust in the tank that could be the issue also.

If the igniter failed wouldnt it just shut the bike off?

And yea the tank has some rust so im going to tear the carbs apart again..

 

[Guest]

If the igniter failed wouldnt it just shut the bike off?

And yea the tank has some rust so im going to tear the carbs apart again..

No, the spark becomes irregular and causes situations like yours. I've seen it and chased the jetting issue to no avail.

 

[MrZig]

No, the spark becomes irregular and causes situations like yours. I've seen it and chased the jetting issue to no avail.

How do I test it?

 

[Nessism]

The GS400 had points, not an ignitor. Of course, your bike has a different engine so not sure what's up with the ignition now. Sounds like time to pull off the ignition cover and have a look. If it has points, make sure everything is hunky dory in there.

 

[KEITH KRAUSE]

I'm guessing fuel delivery. You mention rust and that the problem seems more pronounced at a constant throttle position. Most flow/venting problems are more noticable at a constant throttle. I also think the plugs look on the lean side, though it depends on how long they've been in use. The ceramic insulator looks too white to me (keeping in mind plug pics are notorious for looking different in person).
All rust has to be removed and the cause fixed. Jets cleaned. Gas tank vent needs to be cleaned if any doubt. If you have floatbowl vent lines, are they clear and not pinched and breathing free to atmosphere?
I also wonder about the air filter. You say you squeezed excess oil out of it? An over-oiled filter should darken the plugs but I wonder what would happen if you ran without it just for testing purposes.

 

[MrZig]

The GSX400 in my GS400 has electronic ignition. Positive about that.

I think the rust isn't a big issue. What happend was I emptied the tank out, and washed it out with water. Flash rust occured, and I ended up swashing some phosphoric acid around in there. When I took the petcock out, there was only a little bit of rust on the very bottom. Not lots. Probably won't be an issue anymore.

Where is this mysterious gas tank vent? I don't have any other lines coming out of it except for the fuel/vacuum. Float bowl vent line is good.

I'm pretty sure it's a carburetor problem.. If it still persists after another cleaning then I don't know wtf.

 

[mustangflyr]

Bogging

Bogging

I had a similar situation on my 80 GS1000E a slight hesitation like I had backed off the throttle just a bit. It would pick back up but it was disconcerting at best. My problem was a terminal?! terminal on the connector going to the fuse box. Unplugged it and replaced the offending parts crimped and soldered them and all is well now, hope that is some help.

 

[MrZig]

I had a similar situation on my 80 GS1000E a slight hesitation like I had backed off the throttle just a bit. It would pick back up but it was disconcerting at best. My problem was a terminal?! terminal on the connector going to the fuse box. Unplugged it and replaced the offending parts crimped and soldered them and all is well now, hope that is some help.

hmmm interesting. You might've had an ignition cut out by the wiring. I'm not so sure that's what my problem is, but you never know. I don't have a fuse box but I do have a lot of old connectors

 

[MrZig]

I ripped the carbs apart and they looked pretty good. The main jet and needle jet, and the jet needle, all look perfectly fine. I havn't taken the pilot jets out yet. They're a pain.

The float heights are as follows:

Left side carb: 23mm
Right side carb: 22mm


Clymer says for all models except the GS450, it should be between 25.6mm and 27.6mm. The GS450 should have between 21.4mm and 23.4mm.

I'm basing it off the GS450 model, as I think they're the same carbs as mine, and by doing so, my float level is correct.

I'm kind of at a loss..

 

[TheCafeKid]

I ripped the carbs apart and they looked pretty good. The main jet and needle jet, and the jet needle, all look perfectly fine. I havn't taken the pilot jets out yet. They're a pain.

The float heights are as follows:

Left side carb: 23mm
Right side carb: 22mm


Clymer says for all models except the GS450, it should be between 25.6mm and 27.6mm. The GS450 should have between 21.4mm and 23.4mm.

I'm basing it off the GS450 model, as I think they're the same carbs as mine, and by doing so, my float level is correct.

I'm kind of at a loss..

Go with the 22.4 +/- 1mm ALL the GSs im aware of use the same hight stock. set em at 22mm and call it good. AND, make sure they're BOTH the same hight exactly, or as close as humanly possible. Being a twin, loading up on one cylinder is likely to be more noticeable than four. NOW, WHERE are you measureing that from? dont forget to take the gasket out, or subtract 1mm if you leave it on. Also, you made no mention as to whether or not the adjustment screws onthe carbs had ever been fiddled with (if they havent they're under a cap and youre not likely to see them) If they HAVE, they're the screws on top of the carbs, motor side. Those SHOULD be set to about two turns out from lightly seated as a good starting point, and then adjust to highest idle speed from there. If you havent PULLED the pilot jets and checked em out, they use MICRO TINY holes thru the shaft of it as their own lil emulsion set up. Rust WILLL get in there, and cause your issue. Im having a similar issue of my own, but im rich for a reason ive yet to track down... Take a look at those things, and see where you're at from there...


PS: MUSTANGFLYR...WHICH connection were you having an issue with? I have a bit of a twitchy wire going to my fuse block that will suddenly kill the bike dead cold once in a blue moon. I havent been able to track which it is yet, perhaps your info could help me out.

 

[MrZig]

Here's updated carb specs:

Float height
Stock 22.4mm +/- 1mm
Left 23mm
Right 22mm

Jet Needle P-1 - STOCK: P-1
Needle Clip Center - STOCK: Center
Needle Jet 5D69 - STOCK: 5D69
Main Jet 110 - STOCK: 117.5
Pilot Jet 42.5 - STOCK: 42.5

As you can see, everything is fine except the main jet is 7.5mm smaller. This shouldn't be causing the symptoms I'm having, right? Just WOT.

The carb is completley ripped apart now. All jets and needles were clean, pilot needle included, and no jets were clogged. Passages unknown, I'll spray them down with solvent. I'm pretty sure this might not be a carb problem after all.

 

[mustangflyr]

Connector

Connector

In reply to TheCafeKld it was the connector between the fuse box and the main harness, to be exact the orange wire with a white tracer. I went ahead and replaced all the terminals (8) and the connector block. Fixed it. As I noted I crimped and soldered them. Again hope it helps.

 

[MrZig]

Oh sorry Cafe Kid I didn't see your reply there

I measured from the gasket surface (without the gasket) to the highest point on the float, but not the step up. IE This image explains it perfectly of what I did:

The idle needle/pilot needle has been messed with by me personally. It has a new o-ring, and I believe each one was between 2 and 2.5 turns out. When I took it out an hour ago the tip was perfectly fine. And the pilot jets were also perfectly fine.

 

[TheCafeKid]

In reply to TheCafeKld it was the connector between the fuse box and the main harness, to be exact the orange wire with a white tracer. I went ahead and replaced all the terminals (8) and the connector block. Fixed it. As I noted I crimped and soldered them. Again hope it helps.

That does help. Thank you sir. That Orange wire with the White tracer actually feeds your ignition, just for future reference. I am sort of glad you said that because I believe THAT very wire is the one that is at fault on my ES. This could also be the source of my senseless rich problem when i SHOULD be lean by all rights, as I may not be feeding enough voltage to the coils.