fuel delivery?

[Guest]

Hi everyone,

First off let me say thanks to those involved with the website-its been an absolute lifesaver.

Ive got a 1979 GS 850 with a 1981 1100 engine in it and with many things my timing is incredibly bad and started a project and unfortunately am running out of summer. The bike ran fairly well before I started the work but there appeared to be an intake leak and a leaky petcock.

I've done the carb rebuild , new o rings, new petcock and left the carb boots since they were still squishy and appeared to have been replaced prior to me getting the bike. I bench synched the carbs and put it back together. The first day I had everything put back together, the bike idled and ran great when blipping the throttle. The next day I decided to take it out for a quick run, started out running well till I hit a hill and it bogged down and died. It started and I was able to make it home.

Long story short (or short story long) when I replaced the petcock, the new outlet faces towards the rear of the bike and the old one faced forwards. I ran a longer length of flexible fuel line to the filter and noticed that when it was running an airlock appeared and disappeared in the fuel line (not sure if this normal and the carbs draws additional fuel when needed). This was accompanied with the bike idling at 3k and then going higher when the bike was lifted up right from the kickstand.

The bike hasn't ran the same for the last two days so I'm trying to sort this out.

I have limited knowledge when it comes to mechanics so please excuse these potentially obvious questions.

Is the extra length of fuel line too much for the vacuum to pull the gas into the carbs or is it more gravity fed?

Would insufficient amount of fuel cause the rpms to jump around as they did?

Would a proper carb synch correct these symptoms if the fuel delivery is sorted out?

Any advice is greatly appreciated,
codder

[Dogma]

Only gravity pulls fuel down to the carbs. Adding a filter can interfere with this. The extra filter is unnecessary unless you have rust or some other problem in your tank. The petcock filter screen should be enough. I'd recommend eliminating the filter unless you need it. In that case, make sure you have a high-flow filter designed for gravity fed systems. Something for a lawnmower, for example.

If you have a fuel flow problem you can definitely starve the carbs on a hill climb or long run at highway speed. A gas cap that is not venting properly can also cause a vacuum in the tank which will impede fuel flow. Something else to check.

Strange that the idle would come up when you lifted it up off the kickstand. Was it sudden or gradual? It could be that the fuel level in the carbs was low enough to cause difficulty in picking the fuel up from the float bowls, and standing the bike up made it easier.

Do you have the carbs from the '81 1100 motor on it?

[codder]

The carbs are 33mm Mikunis. I will keep an eye on the gas cap and pop it off while its running just to rule that out and will eliminate the additional filter. I didn't change anything internally with the carbs during the rebuild since it ran great until the air leak pressured me to start looking into things on the bike.

Thanks for the help!

[codder]

Thanks for the advice Dogma! I eliminated the additional filter and shortened up the fuel line and it started right up with the choke on. It warmed up and I turned the choke off, dialed in the idle screw and its running good.

Looks like I will be on 2 wheels for a bit before the snow flies.

codder

[Steve]

Nice to see that you have your problem resolved, but I have a couple questions that are based on these statements:

Ive got a 1979 GS 850 with a 1981 1100 engine in it ...

and

The carbs are 33mm Mikunis.

First, what '81 1100 engine do you have in there? The only 1100 engine that I know of that was available in '81 was a chain-drive and the 850 is a shaft-drive.
Did you do a shaft-to-chain coversion or is the engine not an '81?


Second, where did you get the 33 mm carbs? All of the GS1100s came with 34 mm carbs.

.

[codder]

That was the way the bike was when I purchased it. The engine could be an 82 but I thought the PO said it was 81. I triple checked the carbs when I ordered the o-ring set and they are 33mm. The PO mentioned to not mess around with the carbs too much as they aren't too common. I took everything at face value and am really pleased with the bike....This is my first bike that I've owned (ridden several for years) and have enjoyed working on it...especially now that its working.

[7981GS]

Can you post pictures of the bike, engine and carbs?

Eric

[codder]

So theres the bike. On the lower pic you can kinda make out the VM A 33 stamped on the left of the carb. My camera died and I couldn't snap a pic of the engine.

[Steve]

Well, it's definitely an early frame (79-81).

The engine appears to be an '83, due to the black paint, but would have to be verified with an engine number.

The carbs are DEFINITELY not stock.

.