81 GS450S - Engine knocks when low on fuel

[Anonymous]

Hi, I've been dreading posting this question for fear of what answers I will get, but here goes...When the gas level in the tank starts to get low the engine begins to knock a bit. As soon as I fill the tank the knocking goes away(or seems to). Anyone have any ideas as to what would cause this?

Cheers!

[DMPLATT]

How Low?
I just ran out the other day and Mine knocks, pings rattles and I was freaking til I realized I was running out. Switched over to reserve and ran great to the gas station. Never a problem since. I just like to know what my fuel window is.... 8)
This is just a sign of fuel starvation. THis happened with my '85 honda as well.
Now, if you have considerably more gas, then it may be more.

Have you ever run out of gas in a car? It bucks and knocks and all kind of stuff til it quits. Same principle.

Dave

[earlfor]

Re: 81 GS450S - Engine knocks when low on fuel

If you mean that when the tank burns down to where you are running out of fuel and need to switch to reserve, then as rpms drop below about 800 and continue decreasing, the engine surges and runs too slowly and unevenly.
It is not uncommon to hear the cam chain and some chatter. You didnt say what rpm you hear the noise at. Is it an actual banging knock, or more a rattle/chatter knock? :-) What part of the engine does the sound seem to come from?

Earl

Hi, I've been dreading posting this question for fear of what answers I will get, but here goes...When the gas level in the tank starts to get low the engine begins to knock a bit. As soon as I fill the tank the knocking goes away(or seems to). Anyone have any ideas as to what would cause this?

Cheers!

[Anonymous]

It seems to happen before I need to switch to the reserve, how close I really can't say, everytime it happens I freak out and get gas. It seems to be more pronouced at lower rpms, the bike usually idles arround 1100. The knocking seems to be comming from front right and upper part of the engine (can you tell I don't know my engine terms very well? ) and I really not sure I know the difference between a banging knock and a rattle/chatter knock, but if I were forced to choose is seems more like a banging.

My theory was that for some reason when the fuel started running low the carbs may not be getting equal amounts of gas and this is causing the knocking which definetly seems to be from the right side of the engine. I really don't know what I am talking about, but it sounds good to me.

[DMPLATT]

It seems to happen before I need to switch to the reserve,

If it runs fine when you switch to reserve, I feel that this is normal.

I wouldn't worry. Now that you know your fuel window (as I just learned on my GS).

[bwringer]

I'm guessing that a fuel tank without much fuel amplifies and transmits much more engine noise than a full tank.

Assuming the bike is running normally, this is the probable explanation, but without hearing the bike, who knows.

To eliminate the amplifying effects of the empty tank, you could try placing the bike on the centerstand and listening from a few feet away in both the full and nearly empty states.

[earlfor]

If the engine sound normal as long as it is getting fuel, I dont think it is a mechanical problem. If it was mechanical, it would knock all the time.
My best guess is that your carbs need synchronizing and your knocking cylinder is running just a tad lean in normal operation. When you run low on fuel, it becomes even more lean, the cylinder may be just hot enough to cause a slight preignition.......just a guess. I would synch the carbs and set them/have them set, just a fuzz on the rich side. I think that will solve your problem.

Earl

My theory was that for some reason when the fuel started running low the carbs may not be getting equal amounts of gas and this is causing the knocking which definetly seems to be from the right side of the engine. I really don't know what I am talking about, but it sounds good to me.

[Anonymous]

Thanks for all the advice and info. I guess I will have to look into syncing the carbs. I've never done that. Is is hard to do? Does it require special tools? How much would a shop charge for something like that?

[earlfor]

Its pretty simple to do, but it does require (for a 450) a pair of vacuum or mercury guages. It is easy enough, that the cost of taking it to a shop should be minimal. A mercury carb stix will cost you about $40 if youre inclined to do the work yourself. You will need to remove the gas tank to be able to access the adjustment for the throttle plates. A couple of wrenches, screwdriver and vacuum guages is all you will need. If doing your own mechanical work interests you, go for it. If you dont enjoy that sort of thing, its a better choice to just take it to a shop.

Earl

Thanks for all the advice and info. I guess I will have to look into syncing the carbs. I've never done that. Is is hard to do? Does it require special tools? How much would a shop charge for something like that?

[Anonymous]

I definetly enjoy wrenching on my bike. I just don't have a lot of experience. I do have a Clymer manual and can follow instructions pretty well though. If it is something that is fairly easy then I may give it a try. I do have a complete spare set of carbs I pulled off my parts bike. Maybe I should try and rebuild these. The spare carbs look to be in good condition, at least in as good of condition as the carbs I am running. I've never had to do any real engine work before so maybe I should start here.

Any advice on what I would or wouldn't need to buy to rebuild these spare carbs. I have done some searching and have found different post on this subject (obviously this is hot topic ) but some say order parts A,B, and C and others only say A and B are needed. (Replace A, B, and C with different carb parts )

Again, thanks for the advice. I really appreciate it. Maybe this will be my first step into real wrenching.

Cheers!

[earlfor]

Usually, on the CV carbs just replacing the "O" rings is sufficient. I imagine you can get them from Robert Barr. They arent expensive.

Earl

Any advice on what I would or wouldn't need to buy to rebuild these spare carbs. I have done some searching and have found different post on this subject (obviously this is hot topic ) but some say order parts A,B, and C and others only say A and B are needed. (Replace A, B, and C with different carb parts )

Again, thanks for the advice. I really appreciate it. Maybe this will be my first step into real wrenching.

Cheers!

[Anonymous]

I have been doing more reading on carb tunning and found a thread where the second poster describes a way of syncing the carbs on a GS450 without guages.

Any thoughts on this anyone?

[Anonymous]

1. It works.

2. Do it.

That was probably my thread you read. It helped me. Give it a shot. It can't hurt (as long as you follow the directions ).

Michael

[Anonymous]

earl, it might be that he would need a valve adjustment as well.

[Anonymous]

mopolopo, yup it was your thread I stumbled on. :-)

Uh oh...valve adjustment sounds scary. Bike was free to me along with a parts bike that used to be a race bike. Right now I have the race engine pulled. My friend who gave me the bikes said he was going to rebuild the race engine, but when he checked the valves and whatever you check to see if your engine is in spec everything checked out very close to factory. So I could always try to swap engines if I had to.

I've already sent an email to Robert Barr about o-rings and I am still interested in trying to rebuild and then sync the spare carbs.

Thanks for all the good info.