I'm a newbie here, just picking up a '85 XJ 700 in really good condition for $700 with a carb problem. Seems that when the bike is first started the idle is about 1K... As soon as you blip the throttle it immediately rev's to about 4K and stays there... I checked the cable to see if it's stuck and looked around the intake manifold to see if there is any vacuum lines off.... Does anyone have anything else I can try?


Thanks,
Curt

Hi Curt,

Don't know if this will be much help, but I'll take a shot. Sounds like maybe the PO had the carbs apart and didn't get everything back together correctly. It'd be worth your time to pull the carb rack and check everything over and give it a thorough cleaning. Don't use carb cleaner, though unless you're rebuilding the carbs with new seals... trust me. Stuff eats the rubber seals <sigh>.

Does the idle hang up when the bike is hot, or will it do it cold too? Mine used to when hot at exactly 4k after touching the throttle. Had leaky throttle shaft seals in the carbs.

When you pull the carbs, have a really good look at your intake manifolds to make sure you haven't got a leak there, either. Little air leaks are sometimes hard to spot.

GG

Air getting in from somewhere. ONe trick is to use an unlit propane torch. Place the unlit gas flow around the intakes and listen for an increase in rpm. That is where your air leak is. Could be throttle shaft seals or carb boots too. All can be checked with the torch.

Thanks for responding. When the bike is cold and choke on, the idle stays pretty low. But as soon as it warms up, BAM right up to 4K and hangs there.. I will look at the throttle shaft seals, but I will try the propane just to eliminate that possiblity before taking the carb's out. Any place I can get an exploded view of the carbs?


Thanks,
Curt

Don't say "propane torch" and "exploded" in the same post... bad luck!

For a good look at the carbs and a how-to, go to http://www.xjowners.com/ Master of the carb, Dwayne Verhey, put together a great step-by-step a few years ago.

The fact that the idle takes off after the engine warms up is a good indication you've got a vacuum leak. Now, the trick is to find it! I've heard the "propane torch" suggestion a number of times, and I'm sure it works great, but what it won't tell you is WHERE the leak is. Once you start shooting gaseous propane at the boots, it can just as easily be sucked in at the throttle shaft seals, and than you really haven't proved anything.

Despite the posts in other threads that recommend cheap fixes like covering the intake boots with a sealer, I strongly recommend against it. Those sealers will SOMETIMES work well on rubber car parts, but the sealer really doesn't get exposed to the weather under the hood of a car like it can on a bike. Stuff weathers and peels off pretty fast. And worse yet, if you take your carbs off, the sealer will flake off when you muscle the carbs around, and you're right back to square one. Besides, the stuff stinks pretty bad when the engine gets hot and heats the carb boots. I put up with it for one summer, regretting the decision with every ride. Any decent bike shop can order you a new set of intakes for around $120 and you'll never have to worry about them again.

If you're ripping into the carbs to replace the throttle shaft seals, it's a good idea to pick up a carb rebuild kit. Your carbs MIGHT be in okay shape, but these bikes are over 20 years old and it only makes sense to replace the rubber bits, gaskets, and float valves in one shot. After all, you wouldn't want to replace the seals only to have to take the whole thing apart next year when a new problem surfaces!

This advice comes from hard experience. My Seca was laid up for 3 summers while I chased vacuum leaks and tried half measures to fix them. For less than $250, you'll have eliminated your carb and intake woes and your bike will ride like new. Not to mention the pride you'll feel having done it all yourself.

Have you tryed adjusting the idle control knob between number 2 and 3 carb yet. I had the same problem and took it to a repair shop and they adjusted the knob until it came back to the right idle. I have not had a problem out of it since.

Looks like I found the problem. The last owner, bent the tip of the needle in one of the last carb I disassembled and the top diaphragm is starting to go bad. Also, it sounds like these two parts are VERY expensive and not available anymore from my local dealer. Does anyone know for a place that might have something like this?



thanks,
Curt

Curt, it will take a little leg work, but www.bikebandit.com has the diaphragms for the 1982-1983 XJ 650-750 available. It seems the 1985 XJ is no longer available, and the bikes made in 1984 & 1986 used a different kind of dipraghm. I've ordered from these folks several times and they are good to deal with.

From the schematics on bikebandit, it appears the 82-83 and your 85 might have used the same diaphragm. Yamaha tended to use the same parts in many different bikes. You might check with your local dealer to see if they can match up part numbers between those years. And yes, they are pricey. About $122 per diaphragm.

Otherwise you may be relagated to ebay or your local bike salvage yard. :(

good luck. Cary.

I had the same issue. But , I also added MAC Exhaust. If you are planning to make any mods such as a performance exhaust or a clamp on air filter you will need a jet kit. See this link:

CarbJet Kit


There are 4 needles included in this kit for $120 along with stock & modified main jets. (No diaphragms though) It is worth it to get the kit for just that and you have the ability to move to a stage 1 or 3 if needed in the future. Yamarobber wanted $60 per needle, I said see ya....!

I'll need to look in to the MAC exhaust. Wanted to find some new pipes that will take the exhaust a little further back than stopping at the foot pegs. Does anybody have any pictures of the exhaust install on a bike?

The stage one kit might be the way to go, just need to find a used diagraphm at the local bike salvage yard.


Thanks,
Curt

Have look at this baby!

My Bike

My Bike

That exhaust is perfect. Now just need to start looking on ebay for a good price.

Thanks,
Curt

Sounds like the "other guy" did have the carbs appart and forgot to count the pilot fuel mixture screw adjustment. Not like any of us have ever done that. Any way turn them all the way in don't force. Then turn them out three full turns this should get you going.