85 Maxim XJ700N High Idle

Hi all,

Brand-new member, although I've been visiting here often over the last couple of weeks and soaking up info like a sponge. I have a 1985 xj700N that I just bought. No previous experience owning or working on bikes, so forgive me if I seem like a dunce.

My problem is a high-idle. When I first rode it over to my house from the PO's garage where it had sat for two months, it started right up and ran great for 20 minutes, then sometimes the idle began to creep up when I came to stoplights. same thing for the next couple of rides: after 20 minutes (about the time it takes for everything to warm-up, right?) when I came to a stop the bike tach would climb to anywhere from 2 to 3.5K. As soon as I let the clutch out into the friction zone with the bike in gear, it would drop back down to 1.5, then, when I pulled the clutch back in, it would hold for a second or two, and then climb back up around 3K. The throttle cable isn't stuck; I played with it, and I don't think it's an idle adjustment problem because it was intermittent at first (now happens all of the time) and the idle speed varied by about 2K. When I rode it last week it started doing it within a minute and when I put it in gear and drove down the street, even though I wasn't giving it gas, the tach would climb to 3.5K and stay there. (The air was unusually cold, last week, if that has anything to do with it...)

I took it to a part-time mechanic (I don't have a garage or a covered area to work on it at my new place, and it rains all the time here in the Pacific Northwest) and we checked the rubber by spraying it with ether while the bike was idling on the centerstand. Nothing happened. I did notice, however that the airbox-to-carb boots were worn where they joined to the carbs. The clamps were so loose they spun around the hose when I pushed them even though they were screwed all the way. I could even pop off one of the hoses with my finger. No holes or cracks in the rubber that I could see, but the boots are "frilly" at the end where they join to the carbs and seem a bit short, though they're the originals.

The fellow thought that this might be a part of the problem, but said he couldn't be sure. It seems to me that all this does is circumvent the airbox and allow the carbs to draw air straight from the atmosphere; it wouldn't affect the mixture, right? He says it doesn't sound to him like a carb problem, but can you tell at all by listening? The carb-to-engine manifolds look pretty good and -again- didn't yield up any cracks or holes during the ether test. Will replacing the airbox-to-carb boots help the high idle, or should I have him start taking the carbs apart?

I don't know how to get any of the discontinued rubber parts. Does anyone make new ones? Probably not good to trust 20-year old boots off a parts-bike, right?

If anyone has any comments or insight, it would be greatly appreciated. All this stuff is brand-new to me, so I'm kinda lost...

Thanks,
PJ

 

I'd start right off with a can of seafoam. I'd check the routing of the throttle cable, then I'd check the air filter. To make doubly sure your carb to engine boots are not cracked, with the bike running-spray wd-40 on them to see if the idle increases.

I have this exact same issue on at least one of my bikes. I'd be interested in seeing what others have to say.

 

An air leak is the most common reason for a high idle. The mech was right to suspect the airbox boots. Another way to test them for leaking is to use an unlit propane torch. The leak can pull the gas in much easier than a liquid. When it gets in it will alter the idle either up or down. Doesn't matter which way just that if it can do it then it is getting in and therefore so can air. Tighten your clamps, check the seating of the boots and see if this brings the idle down. It is also a good idea to check the choke and make sure it is off all the way. Open the choke while looking at the carbs and you'll see the plungers rise. Turn it off and push down on the plungers to see if they are all the way seated. The choke is actually an enrichment circuit which means it let's in more fuel rather than restricting air. Don't know how far it is for you to the bay area but I would encourage you to plan a trip there in the spring for Peter's annual carb clinic. You will learn more about your carbs there than anywhere. It is a hands on clinic with all the special tools provided. This will cut out that mech who will be expensive!

 

I would definity get the carbs balanced. It could very well be that one or two of them are out of whack, and the butterfly valve isn't fully closing when you release the throttle.

I had the exact same problem. A wise board member (Rick something ) told me, 3500 rpm is not idling, it's racing. IOW, you can't get to that rpm with pilot jets only, you need some main jets.

 

All vacuum leaks aside, if you find nothing there (but check first!) you might have a sticking slide in one of the carbs. This is the exact problem I found on a Maxim X last fall. I was able to pull the carb domes one at a time and clean the slides/bores with the carbs in place. After cleaning, I lubed the slides and bores with silicone spray and reassembled. Voila-problem solved!
I've also heard of this problem being caused by leaking float needles or improper float height. The reason it doesn't show up as fuel running out of the carb throats is due to the petc0ck shutting off fuel flow when you kill the engine. Run it in the prime position, shut it down and see if you begin hemorraging fuel out of the throats; if so, that's your culprit. Unfortunately, either problem will require you to yank the carbs. The good news is, those are some of the easiest carbs to pull there are!