1981 GS450 - hard to start

[ssco156]

For the last few months, my GS450 has been very difficult to start if it had been sitting there for more than a week or ten days. Before that, using the priming circuit usually did the trick but now I just about have to flatten the battery to get it to start. It works ok once it has started. My petcock is one of those with two positions and a screw for priming. If the screw is let out too much fuel starts to leak out, so to my mind that indicates that the priming part of the petcock is functioning. Is there another way to confirm if the primer is working? The "choke" mechanism seems ok too as turning it off too quickly before the engine has warmed up brings the revs down. Thanks in advance for any suggestions.

[tom203]

I’d start by ditching the petcock with screw for priming....and replacing it with the simpler vacuum operated unit like in eBay link below.
Of course, mounting holes need to be similar and petcock output fittings need to clear obstacles.
these critters need a extra dose of fuel when cold starting ....your current petcock seems iffy.

https://www.ebay.com/itm/Petcock-GS450-GS750-GS1100-GS1150-18-4359-new-DIRECT-REPLACEMENT-/362001826516?_trksid=p2349624.m46890.l49286

[Gorminrider]

...Is there another way to confirm if the primer is working?

Disconnect the fuel line and run he tap into a jar. You don't usually need to turn the screw out very much for enough flow- certainly not all the way. If your bike is running fine otherwise, it's most likely the problem is immediately at the screw's passages only but...
Other things relevant:
-while you're at it, attach a bit of hose and suck on the vacuum port of the tap to test ON and RESERVE functions too.
-if flow is poor, the vicinity of the prime screw AND/OR the filter screen in the tank AND/OR in the carbs' screen at the carb fuel inlets might be choked with debris and rust.
-if you have the skills to get at and clean the carb's screens, give the float needles and their seats a flush too.

ADD: I have this tap but I can't remember if theirs an o-ring involved at the screw...anybody? If so, apart from just wear, sometimes these get twisted ,broken and pushed into the wrong place. (The carburetor idlemix screws are a similar example)

[ssco156]

I’d start by ditching the petcock with screw for priming....and replacing it with the simpler vacuum operated unit like in eBay link below.
Of course, mounting holes need to be similar and petcock output fittings need to clear obstacles.
these critters need a extra dose of fuel when cold starting ....your current petcock seems iffy.

https://www.ebay.com/itm/Petcock-GS4....m46890.l49286

Thanks for your quick reply. I will try to get a better petcock, the only problem being that with postage and taxes it will cost me double of what it would cost in the States. Since the bike runs fine once it starts, I can rule out a blocked fuel hose. The only other thing I can think of is a blocked vent in the fuel cap, though that's not very likely.

[ssco156]

Disconnect the fuel line and run he tap into a jar. You don't usually need to turn the screw out very much for enough flow- certainly not all the way. If your bike is running fine otherwise, it's most likely the problem is immediately at the screw's passages only but...
Other things relevant:
-while you're at it, attach a bit of hose and suck on the vacuum port of the tap to test ON and RESERVE functions too.
-if flow is poor, the vicinity of the prime screw AND/OR the filter screen in the tank AND/OR in the carbs' screen at the carb fuel inlets might be choked with debris and rust.
-if you have the skills to get at and clean the carb's screens, give the float needles and their seats a flush too.

ADD: I have this tap but I can't remember if theirs an o-ring involved at the screw...anybody? If so, apart from just wear, sometimes these get twisted ,broken and pushed into the wrong place. (The carburetor idlemix screws are a similar example)

Many thanks for your suggestions. I'll try to check the flow out of the petcock and go on from there. I have a phobia about tinkering with the carbs, they're way beyond my level of expertise.

[LAB3]

Many thanks for your suggestions. I'll try to check the flow out of the petcock and go on from there. I have a phobia about tinkering with the carbs, they're way beyond my level of expertise.

From what you're describing my best bet is the problem lies in the petcock and if it runs well after it starts and you're getting the kind of mileage you'd expect then there's no reason to tear into the carbs. If it where my bike I'd drain the tank, pull out the petcock and give it a good cleaning including taking it apart to give everything a good look. Many "problems" where solved on my 450 when I replaced it and being a cheap bast4rd I waited too long to do it!

[Gorminrider]

Yes, in your circumstance, start at the minimum. DO really try to be sure you have identified the problem before spending a lot of money...if it's looking like the simplest isn't working move to the next ( as LAB3 describes)..because you'll have to take the petcock out anyways to put in a new one so you might as well see if you can find the trouble there before you buy-and-wait for a new one.

[pete]

Are you sure it's related to lack of fuel? Hard starting has other causes such as low compression and valve clearances out of spec. Have you checked these at all? There's also spark plug gaps to check as well.

The petcock you describe sounds like the stock one for the 80 to 82 GS450E and S models, two positions for "On" and "Reserve", with the brass screw as the priming screw. You only need to undo the priming screw a turn or two to prime the carbs. If you open it up too much it will definitely leak fuel which doesn't indicate anything's wrong with it.

Many years ago when I first got my 450 it became harder and harder to start, and it turned out to be a combination of valves not in spec and the spark plug gaps were way too big. I was none the wiser at the time, but after sorting those things out it started first time, every time.

Good luck!

[ssco156]

Are you sure it's related to lack of fuel? Hard starting has other causes such as low compression and valve clearances out of spec. Have you checked these at all? There's also spark plug gaps to check as well.

The petcock you describe sounds like the stock one for the 80 to 82 GS450E and S models, two positions for "On" and "Reserve", with the brass screw as the priming screw. You only need to undo the priming screw a turn or two to prime the carbs. If you open it up too much it will definitely leak fuel which doesn't indicate anything's wrong with it.

Many years ago when I first got my 450 it became harder and harder to start, and it turned out to be a combination of valves not in spec and the spark plug gaps were way too big. I was none the wiser at the time, but after sorting those things out it started first time, every time.

Good luck!

You might well be right and the problem is not a lack of fuel. I checked the flow from the petcock and I actually was surprised at the amount of fuel that came out in a few seconds. So that rules out a petcock problem. I haven't checked the compression for a while, the last time I did both cylinders were around 150psi but that was a couple of years ago. Come to think of it, the problem could have started when I replaced the plugs and caps. The original setup was resistive plugs and zero resistance caps, but now I'ver got resistive caps and zero plugs. Does that make a difference? The valves I've never looked at so that's another thing to do. What puzzles me is how the bike starts fine if it hasn't been standing for too long. Would the plugs/caps, valves or compression only be a problem if the bike hasn't been started for a while? Thanks for your interest.

[LAB3]

The original setup was resistive plugs and zero resistance caps, but now I'ver got resistive caps and zero plugs. Does that make a difference? The valves I've never looked at so that's another thing to do. What puzzles me is how the bike starts fine if it hasn't been standing for too long. Would the plugs/caps, valves or compression only be a problem if the bike hasn't been started for a while? Thanks for your interest.

On my 82 gs450t the stock set up was a resistor cap and non resistor plug. As long as you have the same resistance in there you're gonna be OK.

[ssco156]

On my 82 gs450t the stock set up was a resistor cap and non resistor plug. As long as you have the same resistance in there you're gonna be OK.

Thanks for your help.

[ssco156]

Are you sure it's related to lack of fuel? Hard starting has other causes such as low compression and valve clearances out of spec. Have you checked these at all? There's also spark plug gaps to check as well.

The petcock you describe sounds like the stock one for the 80 to 82 GS450E and S models, two positions for "On" and "Reserve", with the brass screw as the priming screw. You only need to undo the priming screw a turn or two to prime the carbs. If you open it up too much it will definitely leak fuel which doesn't indicate anything's wrong with it.

Many years ago when I first got my 450 it became harder and harder to start, and it turned out to be a combination of valves not in spec and the spark plug gaps were way too big. I was none the wiser at the time, but after sorting those things out it started first time, every time.

Good luck!

I did a compression test and got 147/159psi. Not perfect, but good enough I think. The plug gaps were actually too small (less than 0.6mm) so I increased those to 0.7mm. What I'll do next time that the bike has been standing for a while is to use another battery to start it up, that will eliminate the possibility that the battery was not providing enough juice for a decent spark.

[ssco156]

Well the bike had been standing for three or four weeks and this morning same thing happened, wouldn't start. So I hooked up a fully charged car battery and started right off. Looks like the battery is on its way out and losing its charge quicker than it should when left standing for a while. So in the end, it's an easy fix.