1985 Yamaha XJ 700 (Airhead) won't idle without choke

Hello,

I have a 1985 Yamaha XJ 700 air cooled bike. This bike has had a huge overhaul and I got it from not running at all to finally able to idle with choke or if I keep on the throttle.

Previously, the original OEM carbs were a nightmare! I decided to just replace the carbs with an after market 2 into 4 performance carb conversion kit provided by Murray's Carbs (Mikuni VM34). These carbs were tuned and pre-jetted for the bike specifically as well as for the shorty exhaust with no baffles installed.

I've checked valve clearances, compression and confirmed both coils are producing spark on all four cylinders. New boots and gaskets for intake manifold have been installed as well and I upgraded the plugs to the recommended iridium with resistors.

Cylinders 1 & 4 are firing correctly and seem to be the only cylinders producing heat as they should. 2 & 3 are not cool to the touch but are significantly cooler than 1 & 4. I can't be sure, but when I rev the engine its sounds as if it is running on all four cylinders but when returned to an idle or RPM around 1,000 it sounds like it is only running on two cylinders.

I had to do a decent amount of work to the wiring harness to fix damaged and exposed wiring behind the headlight and under the seat. During that process I made sure all ground connections are clean with good contact. All diodes, coils, ignitor, etc.. are OEM but I removed and cleaned every contact point for all electrical components.

I am at a loss and need direction so I appreciate any help that can be offered. I imagine I will need to break out the volt meter to do some more technical diagnosing but, I'm not sure where to start.

Repairs made so far;

Wiring harness repairs
Starter motor rebuild
Performance carb kit installed
Performance Clutch installed
Shorty Exhaust
New plugs
New hand controls and switches
New signals
Complete brake caliper rebuild and rear drum rebuild
Fork rebuild
New battery
New plug caps (NGK 1k resistor cap. with standard terminal - cut 1/4" off of coil wire and installed new caps)
New starter solenoid
New fuses
Numerous cosmetic repairs

 

The thing cyls 2 and 3 have in common is the ignition coil. I would dig out the old multimeter and check that first. Maybe it is good enough to show spark without compression and run when RPMs are higher, but not quite work at low RPM. That's my knee-jerk guess.

 

Thanks for the info - I just tested and I’m getting around 2.7 - 2.9 ohms from both the coil and harness. Do you know if that is in spec?

 

A little reading suggest that I have the wrong resistor caps at 1k and need non-resistor caps if I have plugs with resistors - can anyone verify that or confirm it can cause the issues I’m having?

 

If you are testing primary side of the coil (the "input" side), then that is within spec. Next, you need to test the secondary side (the "output").

If you have 5k resistor caps then you have to figure that into the equation. The most reliable thing to do is to remove the plug caps so as not have any additional variables in the mix. Test from one plug wire to the other, should be 12K Ohm +/-. If you have the resistor caps on, then add 10K Ohm to the expected result (2 x 5K Ohm).

Reference: http://xj4ever.com/catalog/d-6-ignition.html

XJ700 air-cooled models:

Ignition Coils:
Primary side (input from main wiring harness):
2.7 ohms +/- 10% = 2.43 ohms - 2.97 ohms acceptable range

Secondary side (spark plug wires, without their end caps):
12K ohms +/- 20% = 9,600 ohms - 14,400 ohms acceptable range

 

True, if you have resistor plugs, you don't need resistor caps. If the coil is marginal, maybe adding a little more resistance would be enough to cause your problem at low RPM- at least it would make sense to me.

 

Okay, thank you. I’ll put some new caps on this week and see if there is any difference.

Previously, I had non resistor plugs in with the 1k caps and it would start without choke and idle no problem and seemed to fire intermittently on cylinders 2&3. After adding the iridium plugs with resistors it won’t start unless fully choked. Seems to line up that the added resistance is at the very least, not helping anything. Hopefully this is the underlying cause but I’m not too optimistic. Haha. I will post an update later this week!

 

Test that secondary side of the coil while you're in there!

 

I am getting 12.89k ohm on the secondary side with caps removed - so the coil appears to be working fine..

The PO stated that when he got it to start it sounded like it was misfiring. He said he wasn’t much of a motorcycle guy and didn’t want to find time to diagnose it. I’m trying to void that from my memory considering the condition the bike was in when I purchased it. It could have been a number of things causing it to misfire at that time.

The first time I tried to start it was after I had removed the OEM caps and put on the NGK 1k caps. Hopefully the misfire he mentioned isn’t related because that means my issue is still more technical than I want it to be. Hah. Fingers crossed.

 

Quick update - Still waiting on the non-resistor caps to come in. I went ahead and ordered a new battery after having mine tested. Tech said it is showing signs of failure.. not surprised since it was a $50 cheap buy meant to be used during the rebuild process only.

I tested resistance on both the pick up coils and alternator stator - both within spec! I'll update later this week when caps and battery get in. With a faulty battery and added resistance to my spark on coils that are already considered week off the line.. I'm feeling confident my problems should be resolved.

 

Fingers crossed... My 700 airhead ran like crap with a weak battery - but there was a laundry list of other issues too. I didn't have the patience to fix them one by one, so I parked the bike and sent the carb rack to @hogfiddles. While the rack was vacationing in beautiful upstate NY, I worked through some of the other issues, including replacing the battery and plug caps.

Mine would die pulling up to stop signs and things like that - when the RPM dropped. So if somebody told me that the root cause of both our problems was a bad battery, I would believe it. In your case, turn off the choke, idle goes down, bike dies. My case is similar - pull up to stop sign, idles down, dies.

 

Well I finally replaced the caps and coil wire - both 0 ohm high performance yada yada.. and replaced the battery. It will run better.. at full choke.. and is firing on cylinders 1, 3, 4. It will fire on cylinder three more frequently now, but cylinder 2 is almost useless. I tested spark on all cylinders once again and can confirm they are getting spark. I also rechecked compression on all cylinders and they are all in spec. Since I didn't mention this previously, cylinder four has lower compression but is still within spec and I know for a fact cylinder 4 is firing as it should. 1 and 4 have been the only reliable cylinders since this project started.

I checked cylinder two plug after I let the bike run for about a minute on choke and it was drenched in fuel, cylinder four plug was black as night and 1, 3 were perfect. RPMs with choke would average around 2200-3100K and an idle without choke would sit at 650-600 and then die within seconds. With choke off and at 1/4 throttle, I could keep the bike running but it would die once the throttle was released or eased down.

I am wondering if I need to veer away from the electrical side of things and see about the carburetors being tuned properly.. These were aftermarket performance carbs and all pre-jet work was done at factory for year model, engine and exhaust type. I would really like to think they aren't the issue.. but..

Thoughts?

 

When I start to "refresh" a bike that has been sitting for some time. I always take the carbs off and spend a lot of quality time on them. I have learned in the past if I was to do a quick job on them, I will not be happy with them. The carbs, gas tank and fuel lines need to be very clean.

While you have the bike running, see if you can hold your hand over the intake portion of the cylinders that are not running. Just enough to get some more suction to see if the fuel will then start to make that cylinder run or fire.

You really don't even know if the carbs are synchronized. I am wondering what the aftermarket carbs are that you have. If they are CV of flat slide carbs? If would be nice to see some pics to help us with the trouble shooting for you.

Good luck, keep testing and trouble shooting and let us know how it goes.

 

I’ll upload some photos here. They are Mikuni VM34 carbs with a 2 into 4 setup.

These are said to be pre-jetted and synced to my bikes specs. Check out the website:

https://murrayscarbs.com/product/yamaha-maxim-xj700-carburetor-kit-mikuni-vm34/

 

I just re-read the thread...you have changed so much at one time. Have you switched the coils to see if the issues follows the coil?

When the bike is running with the choke on, can you rev the engine up with the throttle? Will it rev up the RPM ladder with ease?

No vacuum leaks that you might have missed? Good fuel flow from the gas tank? The petcock is working well? I can't tell from the pics if that petcock has a vacuum control feature or not. Looks to be an aftermarket.

 

Yes I switched the coils to see if the problem changes and it stays the same, unfortunately. I’ve tested resistance on the coils and they are both in spec as well.

The bike will rev while fully choked and it will rev up no problem. If the choke is off the bike will struggle to rev from idle which is being controlled by 1/4 throttle - as soon as the throttle is released rpm’s drop and it dies.

The fuel petcock is not OEM but was suggested by a Yamaha representative and seems to be working well. It has the vacuum line to cylinder 2 attached. Fuel line is all brand new, fuel filter is performance grade and has good flow with 1/4” inlet/outlet and fuel tank had deep clean prior to install and is free of contaminates.

I’m going to test for vacuum leaks again when I get home today. I have a phone call with Murrays Carbs this afternoon to go over troubleshooting.

I’ll post an update later!

 

You’re not going to like this bit of advice.
Bin the aftermarket carb kit. Send the original carbs to your local friendly carb rebuilder.
Fit and ride.
Once you’ve stopped smiling do a carb sync, smile some more.
Easy.

 

UPDATE***UPDATE**


Keep it simple stupid is what I should have been saying in my head over and over.

I had the throttle cable incorrectly routed. I had the cable routed in the same way the OEM cable was routed but that is incorrect for these carbs and will cause poor performance. Rerouted to carb manufacture recommendation and the bike will fire on all four cylinders now. I broke the splitter housing where the 2 throttle cables join to one so I’ll need a new throttle cable. 2 months of guess work for the problem to be right in front of my face.

 

Wow a cable routing issue fixed this? I am happy for you but still confused on how that would fix this issue. So the bottom line is was fuel issue with the carbs. They were not in correct position because of the cable routing?

Either way I am glad it worked out for you and that you have your bike back. Get that new cable in there and enjoy the bike.

 

You are correct! I was just as surprised. I didn’t think to check the cable because it didn’t even occur to me that it could be the issue.

I spoke with the owner/manufacturer of the carbs and he said that the carbs are balanced and synced through the two throttle cables that run from the top of each carb. These two cables control the internal slides. The two cables are then joined together in the splitter (just an inline plastic cylinder housing with a plastic molded cable joiner on the lower portion and rubber boots covering both ends) and from the top of the splitter the now one cable runs out and to the throttle tube.

He said that having the cable incorrectly routed can cause poor performance secondary to the possibility of the two main throttle cables having tension where they shouldn’t, causing the internal slides to not be where they should be to maintain idle and optimal performance.

Unfortunately for me it looks like I have damage to the molded plastic joiner inside of the splitter and it can’t be repaired without syncing/rebalancing the carbs.

I send them off for repair tomorrow and I’ll post an update once I get them back and put back on.

 

Thanks for the response... I get the idea of what the manufacturer is telling you. What I can't wrap my head around is why each carb does not have a good running idle set screw that will allow the engine to run even if the cables were not attached?

I guess the "Y" cable same thing that is on my Suzuki VT1400 intruder, does the synch. I would guess that both of those carbs have a idle screw? Or like the VT1400 only the rear carb has the idle screw and then you synch the front to the rear.

Bottom line, at idle all 4 of the cylinders should be running. You should not have been getting the intermediate firing of your cylinders that are attached to the same carb. If you have spark to all plugs and 1 and 2 are running the same off the same carb, same for 3 and 4. The coil banks are 1 &4 then 2&3, the carbs are feeding 1&2 then 3&4, still scratching my head why your bike will idle well without throttle cable action? There should be a "base setting" on the idle screws to keep the bike running like a well oiled engine without input from the throttle cable.

As long as you are happy with what the manufacturer is telling you and the bike is running the way it should be, that is great!

 

I understand your confusion.. I wish I knew more about these carbs. I’m sure they have all the bells and whistles a carb should have, but the manufacturer insisted no adjustments be made. These are Mikuni carbs but deeper reading online says that they are modified from OEM by Murray with “trade secrets”.

Thats enough info for me to be perfectly fine with sending them back for repair/tuning instead of handling it myself.

I don’t understand how 1 and four can be firing normally but 2 and 3 have issues when the left carb feeds 1 and 2 and the right carb feeds 3 and 4, like you were saying. I’d expect issues with every cylinder or just no start at all. Maybe if the carbs weren’t tuned properly to begin with or tuned for the incorrect make/model/engine, I could be experiencing these issues..

I pulled the plugs after a short run with choke and then a short run with just throttle control to keep the bike at idle and plugs 1 and 3 were perfect, 2 was wet and four was black as night.

Sounds like carb issues to me considering I’ve tested just about everything else that could be a problem but it’s kind of all up in the air until I get these carbs back and put on.

They promised a one day turn around so I’ll update with either good news or bad news on their return. Hah

 

i also dont get, how the carbs can be "sent off" to be tuned,?
surely the carbs are tuned to the engine , if one, or more ,of your cylinders is pulling more "suction" than another , then the carbs are synched to the cylinder, to make them even again , how can they do that "off the bike"

 

They have XJ test bikes and motors that the carbs are tested on before shipping out.

If all of your cylinders are in an ideal state and everything is within spec, then I don’t think there would be any issues with a tune on a test bike verifying the carbs will work for your bike. People should probably verify everything is in spec before making the purchase anyway since they aren’t cheap. I didn’t do that but I got lucky..

The owner said he keeps open communication so that anyone can call in and receive instructions to adjust carbs if they aren’t performing as they should. I called and we determined I had a bad splitter effecting the carbs slides and for safety and reassurance he wanted the carbs to be sent back for new cabling and splitter to be installed. Then they will be tested on a XJ test bike, tuned/recalibrated and sent back.

 

Every bike has different compression, valve clearances, etc that affect how carbs are tuned/synced. How do you do that on a different bike?

 

Thats where the phone number for instructions on adjustments to be made at home comes in handy.

 

I really hope this turns out well for you. Hope it is a plug and play for you when the carbs come back. Enjoy the bike and be safe out there.

 

My XS was fitted with a pair of VM34s by a PO.
It's not exactly rocket science to sync them.

 

Any update on this?

 

Carbs came back and I reinstalled with no change in performance. Home adjustments with manufacture recommendations over the phone helped a little but didn’t solve the problem. I identified a few vacuum leaks around the intake boots. No surprise the leaks are on the two cylinders running cold. It’s been a bit cold out so I haven’t been too motivated to work on it. I doused the old boots in carb cleaner while running with choke and it would rev up and fire on all four. As of right now, vacuum leaks are the new suspicion. I’ll post another update once I get the boots installed!

 

So I guess my question is, are you happy with the carb purchase?

 

Yes. I don’t regret the purchase. Granted I could have spent the same amount of money having the originals professionally rebuilt and tuned but I wanted the added performance. I’ve talked to a few people who have purchased the same carbs for the same bike and their bikes are running like a fine tuned machine with no issues. I just need to find my underlying problem.

 

**Update** Intake leaks from the carb boots was the issue after all. I do still have to use choke to start the bike if it sits long enough to cool down but after about a minute of running with choke, it will idle well. I think once the piston rings reseat then the inability to start without choke, will go away. Time will tell.

Took the bike for inspection today with no issues. I’ll post an update in the next 100 miles and see where we are at.

Here are couple photos of the bike finished and the carbs that have been much of the discussion.

Also, if anyone wants my oem carbs - you pay for shipping and you can have them. I’m fine with letting them go instead of them sitting in a box in the garage. They’ll need rebuilt.

 

Good deal. If I was keeping my airhead I would be interested in an extra set. @hogfiddles?

 

I’ll take them!!

 

@Shelby Biggs can you post a picture of the correct throttle cable routing? I'm installing the Murray's carb set on mine right now.

 

Shelby hasn’t been seen since March 28... two days after his post....

 

Well, I suppose that figures.

 

Cable routing is simple though ....

-Left hand enricher cable goes into the tunnel from the left side of the headtube

-left hand clutch cable goes into the tunnel from the left side of the headtube

-right hand throttle cable goes into the tunnel from the right side of the headtube

That way you don’t get cables being pulled tight by wrapping around the headtube as you turn the handle bars.

 

What a load of twaddle - fitting two carbs instead of four adds performance?
Sure, in the land of bullshit....

 

Ooooh, get two pods, too!!