'82 Maxim runs better with choke on, even after warmed up

Howdy all,

I'm a new owner of an '82 XJ650J Maxim, and I'm having an issue with my bike. Everything is stock, including jets.

Upon purchase, the bike was struggling to start, and would take a very long time to warm up, relying on the enrichment on med-full about ten minutes into a ride. I got into the carbs, gave them a good clean, and reassembled, but no change. Next, I checked my valves and found that two of my intake valves were way too tight (reading .06-.07mm), so I replaced with the appropriate shims.

Now the bike starts much easier and runs better overall. It is happy without choke after about five minutes of running (seems acceptable to me?), but I have noticed that when the throttle is open, anywhere from 1/4 to full, applying the choke to 3/4 gives a noticeable boost in power. Keeping the throttle steady and closing the choke, the power disappears. It won't idle with the choke on after it's warmed up.

Am I correct in thinking that this indicates too lean a running condition on the main jet? My next course of action would be to raise the needles a click to see if that helps, or possibly go a size up on the main jet. What do you all think?

Also, I'm noticing that the bike is especially limp under 5kRPM. To get going from a start, I need to rev up to 4k before letting out the clutch to avoid stalling. Is this normal?

Thanks in advance for your time and help. I've already learned so much about this machine from reading this forum, and I'm really grateful that this community exists.

Nico

 

Since posting, I got into the carb tops to find that there is no provision for needle adjustment. I guess my options are to upjet or to replace the stock needles, assuming my hypothesis of lean running is correct.

 

Try checking for vacuum leaks by spraying carb cleaner at each of the throttle shaft ends and around the intake boots, whenever carbs are acting squirrelly that's a good place to start. Then confirm the idle mixture screws are set correctly and sync the carbs.

 

I’ve done the spray down test and haven’t found any leaks. Idle screws are set to 2-1/2 turns from bottom. Haven’t done the synch yet. Was hoping to get to the bottom of this mystery first.

 

Assuming all your fuel passages in your carbs were clear - did you squirt carb cleaner out of each of them to check?
Next place to look would be to synch them, an imbalance could cause low speed issues.
Then I would have a look at your air filter - is it a paper type or some aftermarket last forever jobby? I'm told the Hitachis are very fussy on air filter choice, more air than stock caused a weak mixture - you have a weak mixture. If the carbs are the correct ones, jets and needles correct etc, I wouldn't change any jets or needles.

 

Thanks for the help! Yes, I got carb clean into all jets and orifices and chased with compressed air. They were very clean by the end.

That’s another vote for synching my carbs. That will be the next thing I look at.

I checked out the air filter when I got the carbs off. Looks like an OEM paper filter. It was pretty clean, so I just hit it with some compressed air and reinstalled.

It occurred to me that I’ve only done the leakdown test at an idle. Since I’m experiencing issues at higher RPMs, perhaps I need to spray down my carbs/intakes/etc while revving. I’ll add that to my checklist as well.

 

Might want to think about colortune too. Maybe the pilot screws need a little more than 2 1/2 turns

 

Thanks for the advice! Do you know if out is richer or leaner on the pilot screws?

 

Out is richer. I think 2 1/2 is usually a good starting point, but without a colortune, examining the plugs, or an exhaust gas analyzer or something you don't know for sure.

On my XJ650J, I have to go a lot farther than 2 1/2 to get the colortune consistent blue. But there is also a good possibility that those carbs are long overdue for an overhaul, so I wouldn't use mine as a reference.

Punch line is with the tools most of us can afford, the usual process would be to colortune - synch - colortune - synch etc. until you are happy with it.

 

Good suggestion. The way I was taught was to adjust the air-idle mix screw while the bike is idling until that cylinder reaches its maximum rpm, then adjust the idle screw to spec. That works really well for one and two cylinder bikes, but the four cylinders seems to add too many variables. This is the first four cylinder engine I’ve worked on.

I just tried to synch my carbs, but forgot about the YICS tool. I had a go for it anyway, but the results was slightly worse running than before. I guess I order the tool, possibly colortune as well, then try again.

 

I had poor results carb syncing w/o the YICS tool.

I have a colortune but turning the idle mixture out until I had peaked on idle rpm kept me in the blue. turning it lean, then opening it up and Listening for peak RPM put me within 1/4 turn of using color tune by turning it out till it showed rich (yellow) then turning back in 1/8 turn from when it first turns blue. I prefer a slightly rich idle.
With the slightly rich idle, idling the bike a long time then checking plugs is obviously moot. I have to ride my bike like 30 mins, then check plugs to know my mixture is good. I have all stock jets in my bike. I recall reading someone said stock jets are on the lean side for fuel economy? compliance?
When rebuilding my carbs, I did inspect all the jets and pulled out main jets that were bigger than spec.
Did you look at all your jet numbers? Did the needle on your carbs have a # on them?
Carb specs (find your bike/year)
http://xjbikes.wikidot.com/carb-specs

 

I would start it up and feel the pipes to make sure they are all heating up even. If one of your cylinders isnt firing right, you get the same symptoms. Tough start, easy to stall, lack of power. My X will run on 3 cylinders, so I'd wager your 82 xj will too. Pretty easy to check so I'd think worth doing just to see.

 

Thanks for the tip! Good to know that I can tune by ear on a four and get pretty close. I also prefer a rich idle, and yes I believe that most bikes are tuned slightly lean at all rpm’s for economy and epa standards. I used to work in a moped and scooter shop and we’d constantly be upjetting just to get the things to make any power.

My Maxim has what I believe to be the stock jetting configuration - #40 idle jets, #110 main jets. I just had the needle in my hand but didn’t write down the number. I believe it was Y10.

 

Good thinking! I’ve been checking this along the way and they all feel pretty even.

 

UPDATE:

After my foray with synching without the YICS tool, I’ve made several realizations.

1. I don’t think I was operating my vacuum gages properly. This was my first attempt using them, and I believe that I over-restricted them using the tuning knobs. The clue here was that when I turned the bike off, several of them stayed in position. I believe that I may have effectively disabled my gages and was turning screws blindly hoping to get the needles to change. I think with a more delicate hand, I can get a better synch even without the YICS tool, which I’ve learned that many consider optional.

2. I think that I do have a vacuum leak. When removing and replacing the intake vacuum port caps, I noticed that they were pretty stiff. They took a good pull to get off and a good push to get back on, they each rotated fairly freely once in place. My vacuum line to the petcock has about the same amount of staying power. This is low-hanging fruit, so I ordered some fresh line, caps, and spring clips. I’m going to tighten up this system, then proceed with tuning.

Thanks all for the information and suggestions!

 

All cylinders chugging away at the same temp means they are all affected by the same issues. At this point, a lot of the variables point to running lean, but you could always pull the plugs and read them to absolutely confirm you are indeed running lean. Seeing as you worked on the mopeds and whatnot, you probably already know how to do it, and if not, its a good skill to learn for these old bikes. From there, did you only spray the throttle shaft, or did you also spray all the intake boots, both to and from the carb as well? Doing it while the engine is cold is a good idea, because the rubbers expand when they get hot, but you are also having issues when the engine is warm, so I'd do both.

 

Only turn in the restrictors until the needles stop wavering rapidly. At that point the gauges are still seeing sufficient vacuum to give a good reading, but also respond rapidly to changes.

 

one thing pointed out a lot is if you pulled jets out to look at them, do you know the Haynes service manual is wrong? it has the pilot and main AIR jets backwards in the pictures. Pilot AIR jet is big hole, Main AIR jet is small hole. Main Fuel JET is big, Pilot Fuel JET is small.

 

Good to know! I didn’t pull the air jets because they’re pretty large so I just blasted them in place. I pulled the main and idle only and am confident they went back in correctly.

 

Hey all, thanks for the replies and info! I’m so impressed with the quality and attention of the responses I’ve gotten from this group. Thank you!

I’ve took a brief break from this investigation while waiting for some parts to come in and have been wrapped up in some minor front-end work. The thing was due for a tire change, and the front break pads had encountered some grease, so they’re getting changed. When I get back on the running issue, I’ll post an update. Thanks again.

 

Okay, got that front tire on and new pads on so I’m back on the running issue. Decided against purchasing the YICS tool after reading many folks on this forum reporting that they were able to do the synch without it. Had another go and got some good results. All the needles at about the same range and the engine sounds great. I went ahead and adjusted pilot screws by ear and it went well.

I put on new vacuum port caps and replaced the vacuum line to the petcock while I was at it. Got it on the road, and it falls flat like it has been between idle and 4k. Put the choke to 1/2 and it nearly does a wheelie from the additional power. Leakdown test (everywhere - intakes, airbox rubbers, shaft seals, vacuum plugs, petcock, vacuum line, carb tops) reveals no obvious vacuum leak.

Looks like I’m getting back into the carbs?

 

You got the wrong jets in there....

 

Okay, revised update. I did the spray down test again today with a clearer head and found that the engine RPMs dropped significantly when I sprayed the ends of the throttle shaft.

I have read that replacing these seals is recommended, but is a bit of a job. Anyone able to point me in the direction of a useful instructional on this? Also, replacement parts?

Out of curiosity, I went ahead and took a closer look at one side of the throttle shaft. I noticed that the washer retained by the clip was loose and would rotate freely, even with the clip in place. I assume this is normal, as I don’t see any way for that clip to secure the washer more positively.

I took off the clip and the washer and inspected the shaft seal. It appears to be in fine condition (no tears, wearing, or irregularity), but noticed that is sat fairly loosely in its hole. How snugly ought these shaft seals fit?

Thanks in advance,
Nico

 

Sorry, but throttle shafts will affect idle speed and synch smoothness - they will not give you the symptoms you are seeing higher up in the rpm range. I would be looking elsewhere.

 

But, did the po get these wrong?

 

Good to know!

 

It’s very possible. I will check now that the carbs are off.

 

UPDATE:

So I DID have the wrong jets in there. The needle jets! When I first took the carbs off for a clean, I was equipped with a pretty cheesy Amazon carb rebuild kit. The only item I used from it was the needle jets, which I replaced on two of the carbs because I noticed that the threads were damaged on the originals.

On closer inspection, I found that the side holes in the emulsion tubes of these parts were noticeable smaller than those of the original. I also checked the location of the air jets (they were correct) but found one of the main jets was completely obstructed.

I put the stock ones back and the issue is gone!

I’m now suffering from a high idle - about 2.5 - 3kRPM with the main idle screw backed all the way out (I’ve since turned it in a couple rotations). I haven’t yet re-synched the carbs. I also replaced one of the shaft seals on the butterfly valve rail with an o-ring that seemed to fit well, but possibly not well enough. Maybe I replace the original part and re-synch the carbs.

The only other variable is that one of the butterfly valves came off while I was inspecting the assembly. I put it back on with care as to the direction and alignment, but I’m aware that this could be an issue as well.

Any thoughts on this high idle?

 

One of the butterflies came off? You realise if this happened whilst running your engine would be wrecked?
Now sync your carbs, job done.

 

Apologies for use of the passive voice. The butterfly valve did not simply drop off, I removed it and replaced it with a bit of thread lock and adequate torque.

 

No problem. Further up you did say your throttle spindle seals were leaking - given you’re in the States and @chacal can supply the right ones at a decent price you should replace all of them. However this is a big job for a newly, involving stripping each carb completely, and it is easy to cock up. It would be good to know how badly yours idles, how well it sits at the correct idle speed over and over again without changing them?
Ps I would never recommend buying kits from Amazon, quite simply the sellers don’t know or care if you have the right ones.

 

Only useful parts I found from amazon/ebay carb kits are:

• Bowl gasket (if not solid thick rubber kind)
• bowl needle (if it has the rubber tip)
• idle screw O ring
• bowl drain screws (if yours were messed up and not usable)

All the other parts in your carb, you should clean up the brass and re use. Even the carb filter screens can be reused if not torn IMO.
The needle valve housing (if that's what you call it) can be polished so any residual build up of rubber does not interfere with operation. 10% Ethanol gas attacks the rubber and can leave bits of it on the brass.
Many of the carb kits don't come with screens or the screen filter mesh is not tight enough to really filter out like the original screens. External filters I've learned can cause problems and reading many threads, people have fixed issues (like me) by removing external filters.

Good Luck.

 

Okay, I’ve replaced the original shaft seal as my idle was actively rising, which seemed a sure sign of a vacuum leak. Original shaft seal back in and the idle stabilized. Did the synch (no YICS tool) and the idle’s settled down to about 1250 RPM with the main adjustment screw at a reasonable range (it will go lower and idle out if I back it out).

This is workable for me. What are your thoughts? I’m not opposed to breaking the rack to replace the seals, as I know that they’re a source of air incursion (at least the outer two). I’m new to this bike, but used to be a mechanic and trust myself to do meticulous work if necessary.

Also, I’m still wondering if I’m getting enough power at low RPMs. To take off from a stop, I need to rev the bike to 5k before engaging the clutch, or else the engine lugs. This seems excessive to me (at least in terms of volume), but my only reference point is my other motorcycle, a 350 two-stroke twin, which will happily move from a stop at 3k. Does this sound like normal operation for this bike?

 

Well, it depends how heavy you are....
But seriously though, you probably have a mixture/synch issue still. The point is high enough idle rpm can be maintained by 2 pots, but you'll have half the power - this is the issue you have now I believe. Note that at just off idle the fuel is all coming from the idle and first progression holes, so the throttle plates positions must be identical. The potential isdue here is that if you have substantial vacuum leaks, and have compensated by adjusting synch and mixture, they will not be in the same relative positions.
So check idle mixtures (hot) again, and the vacuum synch and see if it improves. If not its rack out and new seals time - @chacal s your man. Or live with it?

 

@Minimutly, you are correct again. I tested your hypothesis by starting the bike cold and feeling the header pipes as they warmed up. I’d previously only tested them after a ride and couldn’t sense any difference (i.e. between scalding and only very hot), but in the cold start test, I noticed two pipes warning very very slowly compared to the others.

Thought this could be an idle-mix screw issue, but the bike was unaffected by any adjustment on those cylinders. Finally checked for spark and there was nothing on those cylinders. Replaced those two plugs and boom, stuff’s good again.

The bike will pull me gladly from a stop at just over 2kRPM (I only weigh 160lbs btw, so I figured that wasn’t the issue) and power is much improved at all rpm’s. Thanks for the help!

 

I was content with where this landed, except that I was concerned that my plugs had burnt out so quickly. I replaced them about a month and less than 100 miles ago.

Later I realized that the two plugs that had died were in the cylinders corresponding to carbs with the shitty jet needles from Amazon. It’s now my theory that the lean running condition on those cylinders caused the premature failure of those plugs.