81 GS450E Misfiring left cylinder only...

[scooterfox]

A little background. I bought this beast a hair over a year ago (July 06) with 8K miles on it, and it ran like a top. I have made no modifications to stock, and have just performed routine maintenance on it. It now has approx 24K miles on it and this problem just started in the last 1k or so (which have been put on in the last week and half - i just moved to a place that makes my commute a bit longer.)

Previous to this problem, I had trucked it up to Chicago and rode it back to Austin, TX in two days, never really going more than 70-75 at any time..

About the only thing it picked up on the trip was a slightly noisier valve train, which I'm really thinking that at almost 25k miles, i really need to check em out.

Currently, I can FEEL it sputtering at all RPMs, but it seems to do it less under hard acceleration. It is ONLY the left cylinder that is the problem, when I pulled the plugs, the left was black (not oily) and sooty, indicating a rich mixture - somehow - and the right was perfect cocoa.

I cleaned both plugs, hooked em back up to the leads, and ran the starter to check the spark, strong and consistent on both sides (the left arced every once in a while, but i think it was because the way it was laying on the cylinder -- but im including this detail just in case it is significant).

Assuming it wasnt spark, I went to the carbs. Using the clymer manual as a guide, I disassembled the carbs, cleaned every jet and passage, and checked all the needles... the needles looked good, the choke seemed fine, and I put it all back on after making sure everything was clean. Being an 81, there is no adjustment option for air fuel mix.

No vaccum hoses are cracked or loose... I cleaned up the plugs on both ends and put em in, and fired it up. It appeared to run slightly better, but after a few blocks, the carbon started to build up, because the misfiring 'hesitation' started to come back.

if i set the idle below 3k, it will stay at that idle for a good 10-15 seconds then drop to about 1500 for short time, then die... at that point its difficult to start, but letting it sit for about 2 mintues, it will start up.

At this point, i havent checked the compression... is there anything else I should be looking for? Could this be due to needing the valves adjusted?

Any help appreciated, as this is my daily driver, and really, at this time, my only transportation! (call me crazy) -- Chad

 

[scooterfox]

update.

update.

I have swapped the coils (which is a trick, had to do a little re-wiring) and im going to test the bike on the way to work, if the opposite cylinder causes a fouled plug, i know my culprit. If not...

 

[Nessism]

First off, welcome to the GSR. GS450 is a fine bike. Please post a photo of yours when you get a chance.

The mixture screws are located on the top front of your carbs, facing downward. They come sealed under a small cap, which can easily be removed by screwing a sheet metal screw in the center of the cap (don’t go deep) and prying the cover off. Once you get the cover off I recommend you tear the carbs down again and this time replace the various o-rings on the inside – try cycleorings.com and see if they have a kit that will work for your bike. Replace the carb boot O-rings as well since they are almost certain to leak if they are the originals.

Lastly, ADJUST THOSE VALVES. You may have already burned some of them due to neglect but the GS engines are pretty stout so maybe you will be okay.

Good luck.

 

[scooterfox]

Coil swap result

Coil swap result

no change. im having a local mechanic look at it to get his professional opinion. I hesitate to assume its the mixture screw on the carb, but it can be a possibility. im my experience, mixtrure screws regulate air / fuel mix only a low rpms or idle, and once the throttle opens, it affects the mix very little...

 

[scooterfox]

next step--

next step--

well.. this is my latest update: Compression test. approx 40 cycles with compression tester brought right cylinder to approx 145 psi, same amount on left side maxxed out at 120 psi

this would probably point alot more towards valves than carb, am i right?

also could be rings, but im more inclined to think valves..

 

[scooterfox]

oh.. and

oh.. and

here's a few photos of it

 

[Nessism]

well.. this is my latest update: Compression test. approx 40 cycles with compression tester brought right cylinder to approx 145 psi, same amount on left side maxxed out at 120 psi

this would probably point alot more towards valves than carb, am i right?

also could be rings, but im more inclined to think valves..

Adjust the valves and then try the compression test again. That said, It's not likely the cause of the problem you describe since the symptoms sound like carb problems.

Good luck.

PS: nice photos. :-D

 

[scooterfox]

so i got a pro to look at it, and after $160 nothing has changed. He says the carb body on the left is bad, and possibly with compression issues, its a combination...

the compression is still close to within spec, so its hard to tell

the other thing he noticed is it fires intermittently on the left ...
no matter which coil is used.

im looking at maybe some used carbs to see if that makes a difference..

 

[nabrams]

Let's take bets. I'll bet it's the vaccum hose (which goes to the left carb) that's sucking fuel (it should NOT) - which indicates a bad petcock. Disconnect the vaccum hose from the carb and put the petcock on "Prime". If your petcock doesn't have a "Prime" setting the do a search for various methods/options for testing this vacuum hose/petcock problem.

 

[Nessism]

so i got a pro to look at it, and after $160 nothing has changed. He says the carb body on the left is bad, and possibly with compression issues, its a combination...

the compression is still close to within spec, so its hard to tell

the other thing he noticed is it fires intermittently on the left ...
no matter which coil is used.

im looking at maybe some used carbs to see if that makes a difference..

Your bike is old, and needs maintanance. The o-rings in the carbs are hard and not sealing properly and the valve clearances need to be tended to. Purchasing some used carbs will be a major gamble unless the o-rings have been changed.

As you found out by your recent experience with taking your bike to the shop, it costs a lot of money for very little benifit. You best learn how to do some work yourself or you will go broke.

 

[scooterfox]

he did a complete carb rebuild (he spent 5 hours on the bench with em - lucky i know the guy and he didnt ream me for the labor..)

it just occurred to me, can i swap the carbs left and right? with a few parts swaps ...

 

[nabrams]

He could've re-built your carbs fifty times - still won't fix a bad petcock that lets fuel into the vacuum hose. Checking this will take minutes. The symptoms you describe certainly point to this possibility.

Also, regardless of what a mechanic says, most of them do more harm than good to our older bikes - see the many horror stories on this site.

 

[IanFrancisco]

He could've re-built your carbs fifty times - still won't fix a bad petcock that lets fuel into the vacuum hose. Checking this will take minutes. The symptoms you describe certainly point to this possibility.

Also, regardless of what a mechanic says, most of them do more harm than good to our older bikes - see the many horror stories on this site.

I have to ditto Mr. nabrams. I had exactly the same problem with the petcock leaking gas into the left cylinder. And, the GS450 petcock is weaker than others in that it only uses a single diaphragm. I tried two aftermarket petcock rebuild kits in which the diaphragm was crap and failed in a short time. Didn't get the problem resolved until I went with a 1979 GS1000 petcock which uses two diaphragms and is much heavier duty. The GS450 makes enough vaccuum to make the GS1000 petcock flow plenty of gas. I can run at 80+ mph as long as I like without fuel starvation issues.

Dealers in my area won't even work on bikes older than ten years, and mom n pop shops have to be able to provide references, ie, names and numbers of satisfied customers before they get any of my dough. I've been able to repair all my problems myself, charging system, valve adjust, carb clean, caliper rebuild, fork seals, steering bearings, etc. The only things I've paid for recently are having tires mounted and having a helicoil put in the spark plug hole of my old XL175. And I don't have a bitchen shop in my garage, just a few tools and some service manuals.

 

[scooterfox]

i can definitely see the possibility of that fuel issue..

how can i test it, really?

as far as the mechanic, he is a friend of several of my friends, and he works exclusively on older bikes, he even said that my GS was a little newer than he normally takes in..

i trust his judgement, and he did say there were some issued with the left carb in general, and he did his best to fix em..

but .. how do i check it?

 

[nabrams]

ScooterFox,

You need to stop "sitting-on-the-fence" - get a good manual and learn your carbs thoroughly.
I'm trying to help by giving you a push in the right direction. I was intimidated at first too, but once I dived in, I was fine. Since then I've restored at least 5 bikes from non-running condition and fixed several others with various problems.
I don't intend to be obnoxious here, but it's clear from your last post that you don't even know what I mean when I said "a bad petcock that lets fuel into the vacuum hose". Have you verified this???
The GS450 is a GREAT bike (I've restored two from non-running condition - mine and a friend's), but they're not bullet-proof. I had one for seven years, so I ran into all the typical problems.
Your bike is getting to the point (in mileage) where all sorts of things will start popping up (misadjusted valves, oil leaks, brake issues, air/exhaust leaks, etc.). They are expected and will require attention. If you take it to a mechanic every time, you'll find yourself broke in short order (unless you're wealthy - in which case you should get yourself a classic Triumph).
Also, I gave you some very specific advice: "Disconnect the vaccum hose from the carb and put the petcock on 'Prime'. If your petcock doesn't have a 'Prime' setting the do a search for various methods/options for testing this vacuum hose/petcock problem." - This has been discussed MANY times on this forum. Using the search feature of this forum effectively (in tandem with a decent manual) will save you TONS of time/effort/money.

OK, I'm done with my rant. Let us know what you find.

 

[scooterfox]

A) i have a manual...

B) ive already rebuilt them myself which ****es me off that i paid someone else to do it by second guessing myself... im not NEW to rebuilding carbs... just not as confident with this bike as i am with my vespa

C) its not the petcock - i tested it the old fashioned way... sucking on the damn hose... lovely

D) Ive narrowed it down to carbs (most likely) or valves (less likely, but probably contributing..) and have a set of carbs on the way i got pretty cheap.

 

[matt_gs450]

I didn't see anyone mention the diaphragm within the left carburetor. I assume you and your mechanic inspected them carefully for any leaks. I rebuilt my carbs earlier this year and found that to be the culprit. Once I'd swapped out the bad diaphragm the bike ran much better.

Good luck,
Matt

 

[nabrams]

Yes, once we start talking about the carbs themselves we can write pages and pages of advice about things to check (like the diaphragm, sticking slide, float needle valve that doesn't seal fully - all these can lead to your symptom) - but it's all been mapped out in detail a million times on this site - do a search.

I can say (as others have) that things I've checked the first time and swore were OK, turned out to be bad on the second or third inspection - usually after ANOTHER full disassembly of the carbs. It's just a reality you deal with if you've been doing this stuff long enough.