1980 GS750L Air Filter Box Question

[almarconi]

Some backround information:

Bought bike about a year ago, it had been sitting since 2001. I disassembled the carbs, replace o-rings, reassembled per GSR manual.
Replaced petcock, adjusted the valves, coil mod, new plugs,wires and caps, replaced throttle, choke and clutch cables, new battery,replaced air filter box because it was missing. I am trying to get the bike to the point where it will at least start and run.

I have been having cold start issues with the bike. Usually the only way to get the bike started is to puff on the carb breather hoses and apply choke . Once I do this the bike will start. I have not done a vacuum sync yet, at this point I just want to be able to get the engine to start,idle, and rev to red line.

I purchased two air filter housings off ebay and both have holes cut out of the underside of the box. On one, there are two round holes the other has a jagged square hole. I can't tell if they are factory made holes or done by the PO of the parts. Got engine started today after I adjusted the valves and engine is running like crap. When I adjusted the valves, some were tight and others were loose. Valve clearance for the 16v engine is .003-.005 so I set then to the .005 spec. Anyway, engine will not idle, pipes 1&2 are warm (3&4 are hot) , and throttle response is very lumpy.

As luck would have it, I put my hand over the cutout hole in the underside of the box and the engine smoothed out and it maintained a steady idle. Removed my hand and engine runs like crap. I can hear a shhh sound coming from the filter housing . I thought to myself, what the f**k is going on here. I removed the air filter housing and put some tape over the holes. Reinstalled the housing and started the engine. The engine ran better than it ever has. I adjusted the mixture screws on the carbs and pipe 1&2 heated up. I set the idle around 1500 rpms and the engine will now rev to the red line..no lumps.

I will try to start the bike when it is cold to see if it will start.

Here is my question for you guys that have a 750L is there supposed to be two holes in the underside of the air filter housing??

Any help would be appreciated.

 

[salty_monk]

I very much doubt it... I don't know for sure but most stock airboxes use a snorkel. Your tape trick seems to prove you are lean too.

I would block those holes & synch the carbs & go from there...

Dan

 

[Headphase]

I've got a 1980 gs750L (a 16 valve one)

On the bottom of the airbox, on the front right as you are facing forward looking down, there are 2 round holes , about 1.5" to 2" in diameter, I haven't measured.
If you shine a torch down through them, you can see the top of the rear brake master cylinder.

Hope that helps, just nipped up the garage and had a quick look.
I can take a photo tomorrow if it is any help.

 

[GSexpress]

There is supposed to be a removeble lid on the airbox to get the filter in and out. With that on the only way air gets in that box and through the filter is through those holes. If its running better with those holes blocked off, it proves one thing. Your manifold "O" rings are not sealing right, or the manifolds themselves are letting air in. It would normally kill the enging if you block off those holes. so the air has to be getting in where its not supposed to. This would also make it run too lean with the holes unblocked. Take the carbs back off and check the manifolds. There may also be vacuum plugs on those manifolds or next to them on the head, make sure they are sead as well.(the rubber boots that bolt to the back of the head) If you want to make doubley sure that that is it, spray some carb cleaner around them while trying to start or run the engine.

 

[almarconi]

Headphase- that is helpful, now I know that one of the boxes was cut by the PO. The other one has the two holes.

That sort of puts me back to square one. I will have to remove the tape from the holes and see how it runs. Will try to start the engine when cold.

I don't know what else to try. I could go through the carbs again and check the choke circuit. I was very careful to clean out all the passages and jets, even sprayed them with carb cleaner and blew everything out with air after soaking.

I searched the forum and others have used the carb breather tube trick to get their bikes started but I can't really find a definitive cure for this problem. Its seems like my engine is not able to pull enough fuel on its own to fire the engine when cold, that why the blowing on the breather tubes works.

If you've got some suggestions, I'm all ears.

 

[Headphase]

I hope this isn't stating the obvious but...

put it on prime, leave it for a wee minute , then try it a few times.
Check there isn't any kink in the fuel line or vacuum line to the petrol tap.

Mine never starts from cold without me using prime, if it has sat for more than a day or so.

 

[almarconi]

Your manifold "O" rings are not sealing right, or the manifolds themselves are letting air in.

GSexpress

The 1980 750 engine does not have O rings under the manifolds like some of the other GS bikes. I will check the sync plugs to make sure they are tight. I will also double check the boots between the intake and the front of the carbs to make sure they are on correctly. Will spray boots with carb cleaner to double check. You made an excellent point about it running lean I need to try and find out where air is leaking.

What about float height? Would floats that are too high or too low cause starting issues??

put it on prime, leave it for a wee minute , then try it a few times.

and

Mine never starts from cold without me using prime

Headphase

I did not try to start it with the petcock on prime. My KZ starts cold with the choke on and petcock in the ON position. Is this a peculiarity to the GS models?

 

[gearhead13]

With the petcock in the 'on' position, fuel will only flow when there is vacuum present, if your bowls are empty it will have to crank for a while for enough fuel to get to the carbs. putting it on prime will bypass the possible problem of a messed up petcock

 

[Suzuki_Don]

Make sure there are no other air leaks into the air box. Make sure the lid is sealed with weather stripping and has a good seal. Extra air going into the air box will upset fuel delivery mixture.

 

[salty_monk]

There is no way a bike needs to be started on prime after only a day or so... I started one of mine today after two weeks... second hit of the starter button it was away.

You could have a weak spark or dodgy petcock too... don't rule those out yet! Did you set your float levels right?

Dan

 

[almarconi]

You could have a weak spark or dodgy petcock too... don't rule those out yet! Did you set your float levels right?

I did the coil mod, battery voltage at the coils and the spark looks good.
The petcock is new from Z1, my engine exhibits these problems on a test tank so prime is really not an issue at this time.

I did not check the float levels. I will try and see if I can hook a piece of tubing up to the drain plug hole and check the float levels this way. The manual has a procedure for this. If I can't I'll have to pull the carbs and check the float levels.

I am going to check for air leaks because the engine should not run if the air filter box is sealed, where is the engine getting air from??

 

[Headphase]

nobody checking it with prime on then, cos that would guarantee that there is fuel getting to the plugs.
Just that it is the easiest thing to do.

 

[Headphase]

sorry, just got your last post.

 

[Larry D]

My air filter box

My air filter box

This from an 81 GS750L.
The gray square is just a foam pad. There are no holes, except for the snorkel and the connection to the forward air box.

 

[almarconi]

Larry,
Does the hole on the bottom of the box(snorkel) connect to any thing or is it just open to the air?
The one I have looks the same except there are two holes in the part where you have the snorkel.

 

[Larry D]

Larry,
Does the hole on the bottom of the box(snorkel) connect to any thing or is it just open to the air?
The one I have looks the same except there are two holes in the part where you have the snorkel.

Doesn't connect to anything, it's just the air intake. The ONLY air intake. How does your lid look? I put some foam strip on the lid and it's inset slightly to fit onto/around the lip of the box. Makes sense? You want these airboxes tight, no leaks. Except, of course, for the snorkel.

Edit: Wanted to add that the float heights make a huge difference. Too much gas/not enough gas. Mine were 5mm off. I sorta forget, what they're supposed to be. But I do remember they were waay off. Also the hissing sound may be the lid leaking, it should be sealed, as mentioned above. The original foam that was installed 28 yrs ago is probably long gone.

 

[almarconi]

I had about an hour this morning to tinker with the bike. I tried starting it cold and it was a no go. Choke position made no difference. I noticed that 2 of the rubber carb boots on the front of the carb were backwards. Switched them around but still no start cold. I puffed on the breather tubes and the bike started up. I removed the airbox, sealed the lid and removed the tape from the holes. Reinstalled the box and the engine started. When I had the engine running I sprayed everything with carb cleaner and did not notice any change in RPM's.

I called a friend of mine who owns a motorcycle shop. From the conditions that I described, he seems to think that the engine may not be developing enough vacuum to get the fuel into the engine on initial startup. He said it could possibly be a stuck piston ring or valve train issue. I guess the next step would be to do a compression test and/or leakdown test to see if I have an internal engine problem. I'm going to drop the bike off to him on Saturday and see if he can get it squared away. I need to have a state inspection done anyway so I'll just pay him to finish the bike up.

Thanks for all your help. I'll give an update after the mechanic looks at it.

 

[dneomax]

I'm having the exact problem as you are having. I did the same thing, I rebuilt and cleaned the carbs thoroughly, and haven't sync it yet. I can only get it to start by puffing into the vent hoses. I'm looking forward on hearing what the problem is from your update.

 

[almarconi]

I dropped the bike off this morning at the shop. I had the bike running yesterday and noticed that gas is coming out of my airbox. This makes me believe that I've got either a stuck float, bad needle valve or the float height is not correct. The game plan is to get the bike running, check the compression to rule out any engine problems. If the compression checks out the mechanic will go ahead and work on the fuel system. Since the bike runs once its started I'm thinking that my problem is carb related. Hopefully he'll get it all sorted out in the next couple of weeks. In the meantime, I'll be riding my KZ.

 

[almarconi]

Wanted to let you guys know that the GS is alive and running well. Turns out the carbs were still dirty..they had some crud in the needle seats. The mechanic charged me $480 to do the following:

Remove and Clean carbs
Vacuum sync carbs
Md State Inspection
Install new chain and front/rear sprockets
Bleed front and rear brakes
Adjust choke,throttle and clutch cable
Install mirrors
Replace brake lever
Repair Oil leak(oil pan gasket)
Change oil and filter

I supplied him with the chain, sprockets, brake lever, mirrors and brake fluid. All that I need to do now is go get insurance and tags.

Once I get the bike on the road, I'll need to do some shake out runs and put some miles on it to see if any other issues creep up.