82 Maxin 750 XJ Engine Racing

I recently removed, cleaned, and reinstalled the carburetors. The engine now runs smoothly BUT when I apply some throttle, the engine revs WAY WAY UP, so much so that I have to shut the bike off. If I adjust the idle down, I can get it back to about 1100 rpm, but add a bit of throttle and off it goes again. Anybody have any ideas?

 

Did you bench synch the carbs? How?

Does the throttle move smoothly? Cable free and not binding?

Choke and throttle cables not mixed up?

You may also have an air leak but let's get the easy stuff out of the way.

 

I did not sync the carbs: I don't have the equipment or experience. Both the throttle and choke (which I'm not using) operate smoothly, and the cables aren't reversed. Could I have accidentally put a hole in one of the rubber hoses from the air cleaner to the carbs or the carbs to the block?

 

Try building a manometer and syncing the carbs first.
This could help you get started.
http://www.xjbikes.com/Forums/viewtopic ... meter.html

 

it's possible to have put a hole in the boots or manifolds. Let's still address the easy things first until they're out of the picture.

You need to bench synch the carbs. The only tools required are a strip of 3x5 index card and a screw driver.

Each of the throttle blades should be closed to where the index card strip drags on them.

That should help. A vacuum synch is easy as well. You can make the tool for about $4 and all you need to do is level the fluid line.

Also, have your valve clearances been checked?

 

The carbs are back on the bike and it starts.
Thats why you bench sync to get it to run. Its runs so lets do a running sync. Why pull them off again???

BTW... if you do pull them off. Make sure to polish the slide bores with 1500 grit sand paper or some kind of aluminum friendly polish.
I did mine with the polish to touch them up AFTER I had honed them with the sand paper.

 

It starts but is racing. It's possible to get the fluid from the manometer sucked into the engine. That will atleast make some smoke depending on what fluid was in your manometer.

Could do it either way and will ultimately need a running synch.

My thought was that for someone with no experiance at synching them, the piece of paper with carbs in hand is easier than trying to settle down a racing engine with sync tubes hooked up.

 

Lets see if he polished those bores out too...

 

When I had the carbs out I actually did "sync" them in the manner you suggested, and the were all good to go. If you can tell me how to construct the vacuum sync, I'll do it. As for the valves ... and frankly the rest of the engine ... I've done nothing. By way of background, the bike had been sitting unused for a few years when I bought it. It has not been tuned up or adjusted since I got it.

 

BillB's link above has the tool options as well as links to the instructions.

If you need restrictions in your vacuum line you can get fish tank air valves at your local pet store for about $3 for 5 of them, you only need 2 for the two bottle or U tube tools.

After the running synch if it still will not come down to idle it's time to check for air leaks.

You have alot of work ahead. Based on the background you gave I would:

1) check the rear brakes for delamination
2) check the tire date codes to see how old they are and if they need replaced
3) check brake line date codes for the same reason
4) valve clearances
5) compression test just so you know where you're at
6) oil change
7) spark plugs
8) check the fuse box, if original glass tube fuses consider replacement of the clips or the whole box.

There's write ups and info for all of this on the site.

 

When you say "air leaks," do you mean something as simple as a hole in one of the rubber "boots"?

 

yes. in the boots or the vacuum caps on the nipples or the throttle shaft seals. But you most likely won't see it with just your eye.

The test is to spray a flammable liquid at the suspect area and watch for an engine rpm change as it gets more fuel.

an unlit propane torch works very well and will not harm any rubber bits.

 

Let me start by thanking all who have responded with advice: it is great! If you'll allow, another post/update. After putting in a new battery and properly gapping the plugs, the bike is running much better. I checked for leaks using propane, but found none. I still have a question/issue. If I'm idling in the driveway, it's solid at about 1000 RPM. But if I go out riding, it's a different story. When I release the throttle to shift, the RPMs does not drop immediately: is gradually winds down. The throttle cable does not appear to be sticking, and the throttle valves seem to "snap back" immediately. Any ideas would be most appreciated. Is is time for a tune-up?

 

You need to hone out the slide bores inside the carbs.
Use some 1500 grit sand paper and maybe polish them out to where they shine. (tear off a couple inch wide strip and work your fingers back and forth in each bore). That is how you make your carbs go clunk AND it should make it return to idle really quick.

 

Bill,
I'm sorry to be a dope, but what are the slide bores? Are they the chambers that the throttle valves are in?

 

See them shiny brassy things? Those are the slides, which have the needle at the lower end, and diaphragms at the other. They are pulled up by vacuum as the throttle opens the butterflies.

A spring pushes 'em back down, but if the spring is weak and/or the slide bores are dirty or gummy, the slides won't snap down as quickly as they should.

I'd be reluctant to use even very fine emery paper on the bores, but some decent metal polish/cleaner should smooth things out.

hth

bd

 

I agree that using abrasive in the slide bore is not a great idea. If the slides drop after you have lifted them up and released them, they should "clunck" right back down. If they don't FIRST clean them and their bores. If that doesn't get it, THEN think about polishing the bores, I use a fine grade of scotchbrite pad chucked in a dremel.
But this is beside the point, a "floating" idle usually means a rich fuel mixture. What color are your plugs? Did you get the sync done and what vaccum readings did you see?

 

Re: 82 Maxin 750 XJ "Floating" Idle

I think the plugs look normal. I'd like to do a vacuum reading. Somewhere I saw a post about building your own vacuum "gauge." It that doable?