new xj owner, bike bogs down badly, wont run w/o choke

Hey everyone, just traded my truck for a 1982 XJ750 Maxim , and I've been having a few problems with it. The throttle response is fairly lacking, and the bike wont idle under 1500 or it dies. I dumped some seafoam in the tank, changed (and gapped) the plugs, and had a reputable mechanic sync the carbs and replace the air filter. Everything on the bike is stock or follows OEM specs. It starts right up, no problem - but it won't run without the choke at least halfway on, and it bogs down badly when I twist the throttle. The more throttle, the worse it gets, till it recovers and finally revs up. The bike got tipped over the other day, and ran like a champ after that. Crisp throttle response, no bogging down, and idled at 1k without the choke. It ran like this for an hour or so, then went back to its old ways. Could this possibly be a clog in the fuel system somewhere? I thought perhaps the seafoam might have knocked a good amount of gunk loose all at once. Any help appreciated.

 

Sounds like the carbs need cleaning. Running with the choke on indicates the carbs need fuel. When the bike was tipped over I suspect a float became stuck and allowed fuel to flow in. With sufficient fuel the bike ran.
The best way we have found to use Seafoam for a condition like this is to run the tank about empty. Add half a can of Seafoam to 1 gallon of fuel. Start the bike and run it until the Seafoam is in the carbs. You will know this by the smoke coming out of the pipes. Let it sit over night and in the morning fill the tank with fuel to dilute the Seafoam and ride for a while. If this doesn't open up the carbs then they need to be pulled and cleaned.
Another thing to do before taking the carb plunge is to replace any inline fuel filter that may be on the fuel line. They clog after time and could also cause these symptoms. Since it will run well that cancels the coils and electrical problems and leaves us with fuel starvation.

 

I was afraid carb cleaning would come up ;-). I have an old beater '79 corvette that I've done massive resto work to, and part of that was rebuilding the carb. If it's not too different, I suppose I could pull it off on the bike's carbs too. I've read a few threads here about the carb cleaning process, so I may be able to pull it off incident. 8)

 

Welcome to you Castellorizon, just follow the bouncing ball that Rick-O-Matic has so thoughtfully outlined in his posts and you should breeze through this. No worries mate, your a pro and don't know it yet.

 

You may also want to check the fuel screen connected to the patcock for foreign material. Could be starving on fuel caused from a blockage in this area. The bike falling over may have dislodged the block and running the bike and causing fuel flow caused it to get clogged again. Give it a try. Easy to do. just not on a full tank. If you find junk in it it is time to clean the tank.

 

I have the same problem. I did clean the carbs (twice, just in case I screwed up the first time). The pilot screw is 2.5 turns out. But the carbs are not synced.
Could them not being synced cause this problem?

 

Thanks for the help guys, I appreciate it. Despite its problems, I really love the bike and I'll be hanging around this forum for quite a while :twisted: . I'm gonna go ahead and pull the tank tomorrow and see if there is any rust/foreign debris mucking things up. I can't imagine the PO took the incentive to do any actual maintenance to the bike, so this might just be the first step to getting the bike up and running again. I was cruising the xj forums days before I took the bike home, and (impatiently) ordered a new set of handlebars, mirrors, and grips in hopes of slapping them on when I got the bike. After I get this idle problem sorted out, I'm sure I'll have quite a few threads full of pictures in the xj mods section. 8)
I'll post the results of the tank/petcock check sometime tomorrow.

 

If you fild stuff in the tank and petcock screen then the next step will be to clean the petcock. May as well clean it while it is off anyway. Make sure it is is the run position also and not the pri or prime position. Hopefully the petcock diaphram is in one piece. Be careful when taking it appart as no not damage it. A small pinhole will cause you nightmears. Hold it up to the light. If you see light through it then a new one is in order.

 

If you have to choke it, that sounds like fuel starvation alright but if the PO didn't do any maint to speak of and you do pull the carbs to set floats and whatnot. I suggest you pull the jet under the slide needle and check for stuff plugging the holes in it.
But maybe you should start witht the most obvious, like pinched fuel lines and clogged filter and petcock.
Just hang in there you'll get pretty quick!
>

 

My first guess as to your problem is that one or more of your carbs are lean. So for you I would colortune and then sync. I have had 2 or 3 bikes come to our carb clinic with the carbs set by ear. Both have had carbs so lean they were firing intermittently. I backed out the screws and viola, engine roars to life. Then I turn the idle screw down and the bike idles normally and all cylinders hit. I then sync and the bike is sssmmooootthhhh!
So I would back out your pilot mixture screws 1/4 of a turn and listen for a change in idle then ride the bike and see if that shows improvement. I the mean time look for a member in your area who might have the colortune.

 

Hi guys I guess I am the new kid on the block. I think that you should check the rubber intake neck between the carb and the motor. You may find out they are dry rotted and cracked creating a vacume leak (your lean situation). When you knocked your bike over you probably hung your float open, and the additional gas that you needed due to the lean condition. Once you ran the bike for a while and the gsa overage got used the float settled back in causing the needle and seat to function again, and returning you to your lean condition. Two very simple ways to check are:
1- spray some starting fluid around these necks, if the idle increases you have leaks
2- wrap some duct tape around the necks and see if some responce returns. If so just replace the necks (intake necks).

 

Alright guys, this is getting nuts. I took the bike out about a half hour ago to warm it up and see if it would scoot around the yard- not only would it run just fine, it was idling around 3k. Any choke would kill it. If I put it in first, and back to neutral, it would drop the idle below 1500, but if I touched the throttle again, it would rev up and settle back at 3k. So far, I've established that the carb boots aren't leaking, the cables aren't binding, and the petcock diaphragm was in alright shape. As soon as I shut the bike off, I heard a nice *phoot* come from the tailpipes. Sort of like a...half-assed backfire. Gremlins, I tell you! 8O

 

Well that is getting nuts! I must ammend my previous advise and say a sync is in order first. That's the only other main source for a high idle. The backfire you heard is from unburned fuel in the headers. Usually indicates a rich mixture. either way it sounds like all you need is a tuneup. Maybe the Seafoam did work and with fuel flowing the ill-sync is now showing up. Well......who wants to guess what the bike will do next.....Robert ,Rick, Hired Goon, Hvnbnd, Samsr?

 

I'm sorry but I'm not the carb meister you fellows are so I'm going to defer. This is undoubtedly an over rich mixture. Is the airfilter in good shape? Fuel float levels within specs? I haven't heard that you took the carbs off yet. That would be my first suggestion. I know, that is the last thing you want to do but you must. I'm betting your problems are lurking inside a mucked up carb bank.

 

Haven't taken the carb off yet- waiting till the weekend for 2 days of freetime in the garage. The mech that synched the carbs also changed out the air filter, but I really should be doing this all myself- so I know what really gets done and what doesn't. I may have to attend one of these world famous carb clinics I keep reading about. :idea:

 

Beware following my advise if you're not already really comfortable around carburetors. My guess is that your pilot jets need adjusted. Your description matches exactly what both my XJ550 and XJ700X were doing before I attacked the pilot screws.

Here's what I think is happening:

Pilot jets are gummed up and/or pilot screws are totally out of adjustment. Because of this, engine won't run on idle circuit. Engine runs with choke on because choke circuit provides gas and thus idle circuit doesn't have to. Engine runs aright in normal range because main jet provides fuel and idle circuit doesn't have to. Backfire is because choke being on produces a rich condition.

Your petcock leaks a little bit and you have at least one float that isn't sealing. When you let the bike sit, the level of fuel in the bowl went above what it should be. When you fired the bike up, this provided a rich condition much like if you had the choke on. Putting the choke on at that time makes a condition so rich that the engine will no longer run. General rich condition causes backfire and gremlins. If you would run it around for a while like this, eventually it would burn off the excess fuel and you would be back to your original symptoms.

How you fix it:

I tore off the carbs and bench synced just for good measure. I did not really clean any of the jets. Next, I drilled out the plugs blocking access to the pilot screws. I put the carbs back on the bike and followed the process that I learned working on Edelbrocks and Rochesters:

1. Set the idle as close to where you want it as possible. (1100, in this case)
2. Pick a pilot screw and turn it in until you hear the idle begin to lower.
3. Turn the same screw out until the idle doesn't raise any higher.
4. Turn the same screw in 1/2 turn.
5. Reset the idle.
6. Repeat 2-5 for each remaining pilot screw.

After this process, both XJ's are purring like kittens.

Replace drilled-out metal caps with press-in plastic caps to keep muck from getting in and making it impossible to readjust the pilot screws.

Caveat:

It is entirely possible that in this process that you are compensating for a gummed up pilot jet by opening up the adjusting screw further. This is not the right way of doing things, but it does get the bike running well without pulling off the carb bank. The adjustment may need to be repeated as the gunk erodes thanks to the new fuel flow.

Best of luck --

 

Thanks for the in depth description, now I have a pretty good idea of what may be going on here. I went to check out the pilot screws, and found that someone had already removed the brass caps blocking them. The kicker is, they used some sort of silicone sealant slathered all over the holes to block any access to them. After scraping out most of the gunk on carbs 1 and 4, I decided I'd have to take the tank off to get to 2 and 3. I'll take it off tomorrow, and see what comes of it. Hopefully, since the carbs may have been adjusted before, it is just out of whack. I'll have to get a colortune to really set things right, but for now, I'm sure old schooling it will get it running much better than it is.

 

Something to watch for when playing with the pilots. It may be obvious, but just in case:

If you come upon a pilot screw that you can move through its entire range of motion without it having any effect on the idle, then you've got a completely blocked pilot jet. You don't have much choice but to take off the carbs and clean it right.

Let me know how you come out with the colortune. I've considered getting one, but I've gotten pretty good at old-schooling it and don't want to go through the hassle of getting one and learning all new methodology (not to mention all the wrenching in and out of plugs) if the result is the same.

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