'81 XJ650 Maxim won't start

Hi all, I bought a non-running 1981 XJ650 Maxim to learn more about motorcycles (I've only ever worked on cars before) and give myself something new to do with my spare time. My ego and pride told me I could get it up and running on my own, but I'm finally at a point where I think I need external help.

So far, nothing I've tried has worked and I'm looking for more experienced opinions on what I should try next.

Using the Haynes manual, I outlined a plan of things to check to try and get the bike to start.

Step 0.5. Changed the oil, just to have some fresh stuff in there.

1. The very first thing I tested for was to make sure I had spark, air, and fuel. I pushed the start button and nothing happened. Tested the battery, it was dead. I tried charging it and it wouldn't hold a charge so I got rid of the old one and replaced with a new one. Tried starting again and got spark at each spark plug (they didn't look too good, so I replaced those too). Still won't start.

2. Second thing was air. I popped open the air filter box and everything looked fine visually. I couldn't find any leaks or anything from the air box through the carbs to the cylinders, so I called it good there. Still wouldn't start.

3. Third thing was fuel. I unscrewed the plugs on the bottom of each bowl and fuel came out, but it was old and smelly. The previous owner left me a carb rebuild kit, so I figured I might as well rebuild them and get rid of any gross gummy fuel that was left in there. I took out the carbs and got to work cleaning them (not a full Church of Clean job, but they didn't turn out to be too bad to begin with) and replacing all the parts included with the carb kit. Still wouldn't start. I did notice that the carb kit he left me wasn't a full kit, it didn't have new floats and a few other things, but it was free, so I thought I'd give it a shot. Got some coughing and sputtering from the engine, but still wouldn't start.

4. Purchased a full carb kit and rebuilt them again replacing the floats this time. It didn't start right up, but I started to get some coughing and sputtering. I tried it a few times and it finally started but it ran terribly and if I gave it any extra gas, it would die immediately. It wouldn't start every time, and usually required quite a bit of cranking.

5. At this point, I was out of ideas from the Haynes manual, so I took to the forum and read a bunch of threads looking for other things I could try and came across some threads about compression, so I did a compression test.
My first compression test numbers were as follows (in PSI): Dry test: Cyl 1:94 2:89 3:80 4:85 Wet test: Cyl 1:100 2:97 3:100 4:100. This was in February of this year and the temperature was probably right around freezing. I think I remember reading that ideally, you want to do this test in a warmer environment, so I'm not sure how much that affected my numbers.
I started looking into the valve clearances since my compression was so low. Clearances on Feb 6, 2022 are as follows: Intake 1: 0.11mm 2: 0.10mm 3: 0.04mm 4: 0.05mm Exhaust 1: 0.12mm 2: 0.12mm 3: 0.10mm 4: 0.15mm I moved shims around and ordered some replacements to get the right sizes (took a few tries) and ended up with all my valves coming in within the specs outlined in the Haynes manual (Intake 0.11 - 0.15mm / Exhaust 0.16 - 0.20mm).
Still won't start. Actually, I think I got it to start once after this, but it died pretty quick afterward and wouldn't start again.

6. I also read about a leakdown test, so I bought a leakdown tester and tried it out. Not really sure how to represent the numbers here, so I'll just put a percentage of difference between the two gauges.
Numbers are as follows: Cyl 1: 93% 2: 96% 3: 98% 4: 97.5%
Based on all my reading, those numbers were still pretty solid, so I figured I didn't have any problems with bad cylinder rings or leaky valves.
Right around this time, I also tried the acetone + ATF trick to see if it was just sticky piston rings. When cranking the engine after this, it sounded a little closer to starting, but still won't start.

7. Throughout the different steps, I've worked on a couple other things that weren't working on the bike and wanted to mention them here in case I inadvertantly broke or disconnected something that might affect the bike from starting. a)The front brake caliper is frozen and I can't get any use out of it. b)The lights weren't working, so I played with the wiring and took apart the switch on the handlebars. There was some corrosion in there that was preventing the pickups from getting a clean connection. Once I cleaned it, the lights mostly worked. Rear light turns on, but doesn't get brighter when I apply the brakes.

8. That's about where I am now. I've tried everything I can think of and am totally stumped.

Apologies for the massive post, but I wanted to be as descriptive about what I've tried and give as much context as possible in case anyone can offer some help. Any and all thoughts would be appreciated. I'd love to get this thing up and running for the remainder of the summer!

 

You have checked a lot of things and the more information the better. Did you clean the tiny jets at the bottom of the float bowls. They are at the bottom of the hole in the float bowl the metal tube on the carburettor body fits into. This jet one per bowl allows fuel to go up that tube for cold starting.

 

It's been a long time since I had the carbs out for the cleaning. This whole process has been over a year for me as I mostly work on it in my spare time. I vaguely remember cleaning them, but I'm sure I could pull them out and do a more thorough job. I assume the carbs are probably the next thing I should check again? I hear carbs are very particular and have to be just right.

 

Yes you want to make sure float bowl jets are clear. My 900 would not start and once I found out about these tiny jets and cleaned them it started immediately. Most times with our bikes assuming the valves clearances are right, compression is good and the ignition system is working properly starting and running problems are carburettor related. Have you tried engine start spray?

 

I haven't tried start spray. Don't think I have any in the garage, so I'll have to run to the store to grab some. I assume if it works with start spray that essentially confirms that the issue is fuel related.

 

Yes if the float bowl jets are blocked mixture will be too lean to start the engine. I think it is definately a carburettor problem. Try the starter spray.

 

When I cleaned my carbs I tore them all down and put them in an ultrasonic cleaner with a mix of water and Simple Green Extreme Aircraft and Precision Cleaner. I also bought some Welding Torch Nozzle Tip Cleaners and Carb Carbon Dirt Cleaner Tool Kit that was basically fancy pipe cleaners. Went through all the jets and made sure they were super clean. It was worth it, bike cranked right up and I immediately had a celebratory beer.

Nozzle tip cleaners: https://www.amazon.com/gp/product/B014D1M62G/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Fancy carb pipe cleaners: https://www.amazon.com/gp/product/B07RZ6V4VJ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

I ended up buying both of the above after seeing someone else mention them on the forums.

 

Starter spray worked! Sounds like I'm on to carb rebuild #3

 

One step I skipped and then had to go back and do was get the fuel levels in the bowls within spec. I didn’t notice if you said you did this step. I made this carb stand from ABS to hold them level. (An idea suggested by another member here). I had to open up the holes on the 90s and cut some slots in them to make collets out of them.

 

Quick report back after cleaning the carbs. It starts! Many thanks to those who added their input here, I'm thrilled that it finally runs!

Once running, the choke makes it shoot up to like 5k RPM almost immediately, so I still need to figure that part out. Not really sure what normal behavior is though so I'll be reading more there.

Initially, I got it to run with an aux fuel tank, and it seemed to idle at a somewhat normal RPM (~750 - 1000) but once I installed the regular bike fuel tank, without choke applied, the RPM seems to creep slowly up and doesn't drop to a regular idle. When I hit the throttle, it jumps up and drops a bit, but then starts to creep slowly up again and it never goes down to a normal idle number, it usually bottoms out around 2000 rpm which seems high to me. I'll be reading what I can find about high idle too. Any pointers or good threads to read through would be helpful.

Thanks again for the help!

 

Have you done a bench sync? If it runs bad with the fuel tank attached, do you have an inline fuel filter? Do you have a vaccum petcock, is it supplying enough fuel to supply the engine? What does it do if you put the fuel tap on prime? When the idle creeps up open the tank cap slightly to see if it goes down again.

https://xjbikes.com/forums/threads/how-to-bench-synch-your-carbs.6366/

 

I did a rough bench sync, but didn't follow these steps from the link you included. I can pull the carbs back off and do a much better job, I'm sure. I'll try that again.

PO didn't have a fuel filter and I haven't added a new one yet. Should that help?

I believe I have a vacuum petcock. It's got a vacuum line between the boot between carbs and head on cyl 3 connected, but I'm not sure how to tell if it's supplying enough fuel or not.

Fuel tap on PRI doesn't seem to change much, but I had a facepalm moment when I read that. I didn't put much gas in the tank initially because it's a pain to drain. I swapped the petcock to RES and it seemed to start much better, so I think I just didn't have enough fuel for PRI/ON. But perhaps I'm misinterpreting the meaning of those three settings?

I don't have a 2nd key yet, so I can't open the tank and keep the bike on at the same time, but I'll try this as soon as I'm able.

Thanks!

 

I asked about the filter in case you had one on it and it could be restricting fuel supply so that is not the case. Follow the steps in the link and let us know what happens. You do have a vaccum petcock, leave that just now until you deal with the carburettors as it may not be the issue. Prime is to fill the float bowls initially, after that the vacuum setup on the petcock is operational whether on reserve or main tank. I had an old bike years ago and it was spluttering to a stop after a little while. What happened is that air could not get into the fuel tank because the hole in the tank cap was blocked and it created a partial vaccum in the tank above the fuel stopping the fuel flowing to the carburettor, hence why I asked about your bikes tank cap.

 

Unlock the cap, pull the key out and start the bike.

 

First, it's my understanding it's not a choke in the traditional sense, but extra fuel enrichment for cold starts. When my XJ750 is in good tune, I seldom used unless it was like 50s 'cold,' IIRC. So, try NOT using it and see how that goes first.

Second, when I had idle creep it was due to a stuck float. I remember rapping on each bowl in succession with a wood-handled screwdriver until it was remedied. Don't think I had problems after that, just when initially getting the carbs back on, but that was ages ago. My thread: https://www.xjbikes.com/forums/threads/carb-slide-sticking.1259/

 

Yes choke reduces the air supply whereas an enrichment circuit increases the fuel supply l think. The XJs have enrichment circuits.

 

Bench sync done and the bike runs much better! Idles well, right around the 1000 - 1500 RPM zone, revs nicely and comes back down with the throttle closed. The throttle doesn't snap back to 0% very well, but if I make sure it's back to 0%, it idles perfectly fine. Not sure if there's a common method to lube that up or tighten the return springs or something.

Moved on to working on the brake caliper, which was very stuck. With a lot of penetrating fluid and compressed air, I finally got the piston out. The seal had gotten stuck to the inside of the caliper, so I got a rebuild kit (just the new seal inside and boot outside) and will re-assemble when that gets here.

Last thing I'm working on before I'm (hopefully) roadworthy is the lighting. The headlight doesn't seem to be getting power. Consulting the wiring diagram, it looks like there's a relay that should be sending power to the left handlebar switch, which then sends it to the light. If I'm correct, it's the relay on the left side of the bike, just under the side panel with the badges (which my bike doesn't have sadly). I'm getting power going into that relay on the red/yellow wire, but nothing coming out on any of the wires, but I think it should be the blue/black wire. Are those relays a common fail point? Is the best way to test that just to get a new one and swap it out? I also could be totally wrong about which relay that's going through.

Also, turn signals are turning on, but not flashing, so I'm also looking into that.

 

There is a white wire to the relay from the alternator that activates the relay, it's only hot when the engine is running. Ergo the headlight should only be on when the engine is running.

 

Well that's easy, tested the headlight with the engine on and it works. Now I wish I hadn't spent hours trying to figure out why the headlight wouldn't turn on with the bike off.

 

Have you got vacuum gauges to balance the carburettors? Bench sync will get it near but gauges are required to get it right.

 

Just a regular bench sync, no vacuum gauges yet. Sounds like I’m getting a new tool!

 

Going to "swear in the church" here, go out on a limb and say that in my experience, as long as an engine idles fine and revs raise and drop as they should, carbs are more than well enough in sync. No carb set will ever be in sync from idle to redline, and at higher throttle openings carb balance hardly matter. YMMV.

 

@hogfiddles I cannot remember the name of the carburettor balancer he uses at the carb clinic but it is worth considering.

 

No carb set will ever be in sync from idle to redline, and at higher throttle openings carb balance hardly matter. YMMV.[/QUOTE] Faffi That must be because of low vacuum in the inlet tracks when the throttle plates are wide open? Is that correct?

 

You're certainly not the first person that's bitten. At least you're up an running. Now to just dial her in.

 

Carbtune. Or... make your own cheap manometer.

 

More specifically, the "Morgan Carbtune Pro" is probably what you will be looking for if you plan to buy one.

 

I use the Morgan Carbtune Pro

 

I am going to get one of these, found a video on the forum.

https://xjbikes.com/forums/threads/running-carb-sync.61677/

 

Frank, the learning curve is literally only a few minutes long. It’s EASY. Plus, you’ll have the unit for future tunings, and you can sync all your friends bikes, too..... and anything else that has 2-4 cyl

 

I finally bought one as well after several times of dinking around with frustrating homemade solutions in the past. Can't wait.

 

I'm still having trouble with the front brakes. I rebuilt my caliper to refresh the decades old seal in there and bled the system, but when I pull the lever with the bleed nipple open, fluid barely comes out. With the bleed nipple locked, the pads don't even move. I don't see any air bubbles coming out with the fluid anymore, but there's not enough pressure to move the pads. Would super old brake lines do that or is the master cylinder the problem here?

 

Well, if you rebuilt the Caliper, then more than likely that is not the problem. That leaves you with either a clogged/collapsed brake line, or a master cylinder that needs to be rebuilt.

 

Or air getting past the bleed valve threads, I put a little PTFE tape on the threads which worked .

 

Could be either in my understanding. I'll be rebuilding my master and putting on stainless steel lines myself.