Finding vacuum/air leaks

[azr]

I'm working on my 79 GS850. Valves are done, Carbs dipped, cleaned and synched, Airbox rebuilt and sealed. New O-rings for intake. Basically I have a bit of a rough idle and she is stalling from idle when I give the gas a bit. I want to rule out any vacuum leaks. I know I can check my intake boots and anything around the carbs with WD-40 or Carb starter but how do I check to make sure my airbox is sealed tight?

[Nessism]

There is no test for the airbox - and that WD-40 test for intake leaks is unreliable as well. Just inspect the various joints and make sure everything is sealed properly.

Regarding your bike, do you have the fuel screw set to 3/4 to 1 turn out, and the air screw set at 1.5 - 2 turns? Sync the carbs also.

Good luck.

[azr]

I tried setting the carbs to factory specs but she ran awful (would backfire in the carb). Most of the fuel screws are 1/2 out and the airs are anywhere from 3/4 to 1 1/4. I found it odd when doing the best idle method that with some screws there was no difference when turning them.

[alke46]

I found it odd when doing the best idle method that with some screws there was no difference when turning them.[/quote]

I found the same thing to be true on my 79 GS1000. The air screws made no noticeable increase in the rpms unless I screwed them in and then it would bog down a bit. Does this mean that they are set correctly?
Anyone can answer this please .

[Nessism]

If the engine doesn't respond to the screws at idle just set them to provide the best running. As mentioned before, set them to 3/4 and 1.5 and try there. Adjust as necessary.

[alke46]

Thanks Nessism, mine were set at 1 1/4 so that is where I left them when I could not get the rpms to change.

[bwringer]

The WD-40 or carb cleaner tests (or ether, or water, or whatever) will only uncover the very worst intake leaks.

I've seen intake leaks that made the bike run like poo that gave no response to the "foreign substance" tests.

As with the airbox, you have to physically take things apart, look at the seals or lack of seals for yourself, and think -- "is this airtight?" It's the only way to be sure.

Don't neglect the boots between the airbox and carbs, too. They're usually rattling around and need to be replaced.

[Pacman]

I tried setting the carbs to factory specs but she ran awful (would backfire in the carb). Most of the fuel screws are 1/2 out and the airs are anywhere from 3/4 to 1 1/4. I found it odd when doing the best idle method that with some screws there was no difference when turning them.

The '79 850's I've worked on, and own, are about 3/4 turn out on the idle air screws. After setting those in a bit I would take the idle mixture screws out about another 1/8-1/4 turn.

Also, I saw in your other thread that you had mixture screws broken off and had to drive out the tips. If you enlarged the hole those tips were in even the slightest bit then those screws will need to be turned in some compared to the others. (I had one broken off in one of my carbs and the hole got very slightly enlarged getting the tip out. That one runs the mixture screw almost closed, maybe 1/8 turn out).

Chris

[azr]

That's good to know, that specific carb is set at 1/4 turn out if that on the mixture screw and I thought I was crazy at keeping at that, good to know it worked for someone else.