Starting problems with 86 xj700

Yes, this bike has pod filters, but it's not mine... it's for a friend and he asked me to try and get it started for him. I told him to go with the airbox, but it's past that point ... so I told him I'd give it a shot.

So this is an 86 xj700, that has been hard-tailed/bobbed (there might not be room for an air box anymore anyway). It has pod filters, and chopped and open exhaust.

At first I couldn't get it to fire, so we cleaned the carbs. It had the original jets (107 main, 36.5 secondary), so we swapped them out for 116 main, and 40 secondary just to compensate for the pods and exhaust (thanks to xj4ever's formula).

Now I can get it to start, but it seems to be having a vacuum problem as it immediatly jumps to about 5-6000 rpm, and with any twist of the throttle it hangs and drops very slowly. Occasionally it'll drop down to a regular idle, but it eventually dies - then takes lots of cranks to get it to fire again.

The bike seems to have been sitting for quite a while, and oil buildup on the exhaust makes it smoke like crazy. Today I also had a stream of white smoke coming out of carb #4. Spark plugs seem to be colored just right (although we haven't had much run time other than just here in the garage, so it may not be enough to tell).

There are a couple tiny cracks on the intake boots, which is what I believe to be the cause of the vacuum leak and the high rpms/slow rmp drop. But they're minimal. I've put a new hose on the vacuum line from boot #2 to the petcock, as I assumed the loose hose was causing some vacuum leaks.

I'm in Utah, so I'm at 4500 feet above sea level.

Is there anything you could suggest to me as a next step? I'd love any help!!

Thanks!

 

How is the bench sync? For that high rpm, your mains are engaged. Something isI holding them open. Also sounds like your pilot circuit is clogged. Check valve shims first.

 

Oh, almost forgot... I tested compression and all four cylinders are at about 90psi. I've also got spark on all four cylinders.

 

On other xj's i've worked on, there's a pilot adjust screw at the top of the carb body, near the intake boots. Should these carbs have that also? These ones have what appear to be brass plugs.

Time to pull the valve cover off I guess

 

Ti Fine Tune those Carbs, ... you have to Pull those Plugs hiding the Pilot AIR Screws.
Drill a Pilot Hole into the Center of the Plug.
About the width of a Nickel.
DO NOT allow the Drill Bit to crash into the Slot Wings on the Pilot AIR Screw which is VERY CLOSE to the bottom of the Plug.

Cut the Point of the end of a Dry Wall Nail.
Start the Nail into the Drilled Pilot Hole and get a good bite.
With Vice-Grips,, ... grab the Nail and work the Plug loose.

 

90 psi seems very low. My 85 XJ700 has 135 as the lowest.

If you still have the original jets in good condition, I might be interested. Let me know at tabaka45@gmail.com.

 

So I just pulled the valve covers off this bike to check clearances. First problem, the little pointer plate to align the "t" with... it's missing! So I try to align my "T" mark up as best as I can with the bolt hole for the pointer plate, and the intake cam is not in the right place at all.

I check into the cam installation, and I find they were installed incorrectly. The "T" mark is aligned with the grove of the plastic cover of the top point, not to the missing pointer plate.

Here's how it's installed:


The "T" is located there, when the camchain mark is aligned at the top like:


So, time to fix that! Any thoughts you have about this?

I know I can find TDC without the plate, but I want to find a new one anyway for future ease. I've checked oem drawings, and I can't find it anywhere. Not in the pickup coil governer drawing (which is where I expected it to be), or crankcase, electrical, or anything else I can think of. Anybody know where I can pick one up?

Thanks!

 

Awesome, thanks for the tip on this!!

Yup, I believe once I get the timing correct again it should either improve, or point me to another problem. Thanks!!

 

So, apparently these bikes don't have a timing plate, but time with the points.

Okay. So I checked shim gaps, and they are off a little bit. Spec for intake is .11-.15mm, and exhaust is .16-.20mm. Here's what I get:

Cylinder: #1 #2 #3 #4
Exhaust: .178 .178 .127 .203
Intake: .102 .102 .127 .076

So, I'm going to replace the following shims:
E3, E4, I1, I2, & I4 (but I don't know with what yet because I don't know what the current shims are), but I apparently need the YM01245 tool first.

So, would the symptoms I described above be from these valves needing this adjustment? What should I test next while I'm ordering this tool to then find out what shims I have.

 

i just went through this...over the winter i threw in a stage 3 dynojet kit, pod filters and 4 to 1 exhaust. you probably dont wanna hear it, but when i put the stock airbox back on, bike ran perfect. the carbs need the certain air pressure that the stock box provides. i even tried cutting the back of the box off (where the filter is mounted), and that still caused the situation you are describing.

the cracks in the intake boots ARE causing you a problem. if any air at all, even the slightest amount, is getting through, it will cause this condition.

make sure your butterflys are all synced, use a feeler gauge. you wanna set these at about the width of a piece of paper, then use the idle adjuster to bring them open to hold a good idle. remember: of the 3 sync bolts. the left one syncs carbs 1 and 2, the right will sync 3 and 4, the middle will sync the pairs together.

its also good to check the choke adjustments, make sure theyre all the same and when the choke lever is off, the chokes on the carbs are also completely off. if one or two choke circuits are plugged, this will also give you trouble.

make sure you have no vacuum leaks (like the intake boots and hose from intake to petcock), or anything plugged up in the carbs. jets and etc obviously, but also there are many small passage ways throughout the carb bodies, use a small light and make sure these are all clear.

now, check the mixture screws to make sure these are all at the same level. if you changed jets, you will have to change the air/fuel ratio. to get these even closer, buy/borrow/rent a carb synchronizer tool so that you can hook up all 4 carbs at the same time and sync them perfectly to each other.

i kind of got lucky with mine, i started doing all kinds of things and at some point fixed the problem. im sure all this is nothing new to you, but i hope i helped. my bike is an 82 xj650.

 

So, I just got done putting in new shims, and I've got all my clearances within spec at this point.

New compression values:
#1 - 105
#2 - 110
#3 - 90
#4 - 115

I'm having a difficult time with this bike, it's driving me nuts! Right now, the main symptom I'm worried about is all the white smoke (when i can acutally get it to fire), it's almost like a fog machine! It's not a liquid cooled engine, but it's burning a lot of something. I know oil is usually blue, gas is black... ??

Going through the basics again... battery is okay, should be changed, but there's enough voltage to start the bike. I'm getting a bright blue spark on each plug. Fuel & air are getting into the cylinders. Boots should be changed/sealed, but I'd like to get it to start first . Carbs are bench sync'd, recently cleaned (by me), etc.... With the compression where it's at, I don't think there's a head gasket blown... but I still can't explain all the smoke.

I'd love any new thoughts.... thanks for all the help so far

 

With all the valves in spec, those are still marginal to downright crappy compression numbers. Cylinder #3 especially has issues.

They should all be within 14psi or less of each other;

and I believe that 100psi is "minimum" for you motor, with 120 being considered "normal." (Check your manual.)

Next step: do a "wet" compression test. Add a teaspoon or so of oil to each cylinder and re-test. (Not too much you don't want to hydro-lock it.) If the numbers jump way up with oil in the cylinders, it generally indicates a piston/ring issue. If they change very little or not at all, it's probably valves (burnt, bent, rusted, or otherwise not sealing properly.)

Also, borrow or rent another compression tester and get a "second opinion." You don't want to start pulling down a motor on the word of one compression gauge.

Something is amiss.

 

So, how much of an improvement should I see? I did this test to a 2 stroke scooter the other day that was showing 60psi, and it went up to 120psi with the oil.

I just did it on this xj700 and these are the oil in cylinder compression values:
#1 - 125
#2 - 120
#3 - 110
#4 - 120

So, I've obviously got some ring problems. But would that explain my starting issues (I know I need 100+psi to start), and the white smoke?

Thanks bigfitz52, and everyone who has helped!

 

Sure looks that way, especially in regard to #3. A cracked ring can push all kinds of oil into the chamber.

BUT:

You can't compare the results between this motor and a 2-stroke in any respect. But it sounds like the scooter needs rings too.