GS450L Running Rough

[r98]

Since I'm new here, I guess an introduction would be nice: I'm a new rider from SC, United States that knows a good bit about older cars but not that much about motorcycles, and I got a GS450L so I can learn to ride and start a new hobby. I'm looking forward to learning about how these machines work.

Anyway, I recently bought an '81 GS450L with just under 15,000 miles. It's in decent shape - compression was ~150 or more on both cylinders (got it up to 160 and 170), and the filter cartridge was clean, save for a few tiny metal specs, so I decided it was a decent, fixable bike for the right price. It ran when I bought it, but it didn't take throttle well - it bogged down and coughed out the carbs. Looking at the spark plugs, it seems to be running rich.

So, I took the carbs apart, cleaned them, and reassembled. They were actually pretty clean - I've seen much worse on vehicles that still ran. So, I re-installed them and continued wrenching. I also threw in a new oil filter for good measure.

But, the bike is extremely hard to start, even with choke, and still coughs through the carbs. When it does start, it sometimes coughs and threatens to stall, but keeps running. On a positive note, the engine runs smoothly - no knocks or noises that would be a red flag.

I had taken the carbs off and decided to check compression again. I looked in the intake ports after cranking and there was a small amount of bubbling around both intake valves since there was a small amount of excess fuel in there. So, I think leaky valves might explain the coughing and the bubbling around the valve seats, but I wanted to ask here first. However, the compression is good, so I'm not sure if this is an issue or if it's normal.

Also, I've been doing some electrical work on it, and there are a few wires that could be hooked up wrong from my work or the previous owners' work - could improper wiring or installation of the signal generator cause the misfiring and poor throttle response?

Any help is appreciated. I look forward to learning about motorcycles.

 

[Spyder]

Welcome to the site. Although you have posted in the wrong place, you have come to the rite site. You will get more responses to your inquiries, when they are posted in the correct spot. ANYWAY, if you have fuel "sitting" on to of a valve, I would have to say that you are running ridiculously rich. I'm not even certain the idle and enrichener circuit (choke) can even get that much fuel in there. Where did you set your air/fuel mixture screws? (how many turns out from seated?) My bet would be, that either your floats are set too high, or the needle and/or seat are bad. One way or the other, there is a lot too much fuel.

Again, if you post this in the carburetor section you will get more results. there are some great carburetor rebuild tutorials available as well...

 

[Gorminrider]

Yes, I'm not sure of this but as the original poster, you might be able to move the thread into Carburetors section.

otherwise,^^second Spyder as the first to check. Overflow of fuel from float needles is fairly common as are failed petcocks leaking gas through their vacuum ports into the intake and worst case-thereby into the oil.

could improper wiring or installation of the signal generator cause the misfiring and poor throttle response?

"Improper" is unlikely to run at all well anytime but a bad connections at the bullet connectors or frayed wiring can cause intermittent firing. Look especially at where the signal generator wires go into the case. Also, the Black box ("ignitor") ground wire. Then, you could move on to cleaning up resistors inside plug caps, spark plugs themselves(which by the way- are they blackened by richness?) etc etc.
I'd start with fuel then goto ignition

 

[r98]

Thank you for the advice. The thread has been moved - the liquid on the valves may have been a mixture of starting fluid and fuel, etc. I'm less worried about the fuel/air scenario since this can be resolved by turning screws and bending tabs - my real concern is the health of the engine, bubbles coming through the valves, and the engine coughing back through the carbs. I tried setting the idle mixture screws about 2 turns out to begin with, and cranked them all the way in and a little further out while testing a few things.

I guess I couldn't go wrong with checking valve clearances.

My petcock isn't hooked up to the vacuum port, which I temporarily capped off, so getting fuel in that way wouldn't be an issue.

 

[Steve]

One of the first things to do is to check valve clearances. Probably not too far off, considering your compression numbers, but clearance is VERY small on these bikes, so it does not take much of a reduction in clearance to be in dangerous territory.

Also very high on the list is to THROW AWAY THE STARTER FLUID. It is way too easy to over-use it, which can cause BIG problems.

Next, you need to rebuild the carbs. You said you "cleaned" them, but you need to be aware that the parts that get 'dirty' are parts that can not be seen, so they can't be pronounced 'good' just by looking at them. It is also important to change the o-rings in the carbs. One of our members has o-ring kits available at his website: cycleorings.com.

"Hard to start, even using the choke" might simply be TECHNIQUE. :-k
You need to understand that these bikes don't have a choke, they have a "choke". It's actually an "enrichment circuit" that feeds air and fuel through different passages. When you pull the "choke" knob, do NOT touch the throttle when starting. If you have problems starting the bike using the "choke", it's quite likely that those circuits still need to be cleaned.

.

 

[Gorminrider]

petcock isn't hooked up to the vacuum port, which I temporarily capped off, so getting fuel in that way wouldn't be an issue.

trusting you also blocked the port on the carb throat!!

bubbles coming through the valves

yes that's odd but given your compression test was "good" it could be the excess fuel mixed with something else viscuous like oil?and just created as the valves slam shut...but it's odd alright.
I don't think your problem as described can be resolved by "bending tabs"unless someone else has already been attacking the float level and adjusting the idle mix isn't going to affect starting-it's a fine adjustment made from the manual's bench setting that's made when the bike is already running decently.

I took the carbs apart, cleaned them, and reassembled

I too-often trust this has been done per the shop manual and also that all passages cleaned,...including, specifically per your starting- the enricher tube. And of course floats and needles checked for level and sealing and butterflys bench-synched etc etc....