Author Topic: Cb650 won't start after top end rebuild (Read 6258 times)

1SpeedD · « **on:** June 12, 2016, 08:30:02 PM »

Just like the title says, I rebuilt it, it won't start. But let's get some details in here for ya'll...

The bike wasn't perfect, but it started and ran strong before my ill-fated rebuild. After a road test it developed a significant leak in the cylinder head/base gaskets on the #1 side of the engine. No one in town wanted to touch the bike, so I took it upon myself to get dirty. I pulled the engine all apart, cleaned as well as I could, replaced gaskets, any reachable o-rings, and put it all back together. Biggest pain was only having one hand due to a post-surgery wrist.

I got the bike all back together, hit the starter and it sounds like just the starter is going and that's it. No inclination towards firing. I pulled the cylinder head cover off once more, realizing I got the timing off. I meticulously reset all that. For a fraction of a second it sounded like it would fire up, and then...just the sound of the starter and nothing else.

I only disconnected the battery and the left, 1-4, ignition coil during the rebuild. The spark seems a bit weaker than before, but doesn't change when a new plug is swapped. The battery is fully charged.

SO...here are my thoughts on potential causes:
1. Witchcraft
2. Timing still off somehow
3. Cam chain somehow not engaging entirely at the base of the engine.
4. Cam chain tensioner not in it's proper notch down at the base
5. A bum like me rebuilding this engine
6. Random decrease in performance of starting system

I'm open and willing to hear any and all suggestions. Thanks!!

WV_750 · « **Reply #1 on:** June 12, 2016, 08:41:24 PM »

Are you getting ~12V at the coils? How about at the points (or electronic ignition module)? If you are and you're confident in your carbs and fuel source i would suspect point gap and/or ignition timing.

harisuluv · « **Reply #2 on:** June 12, 2016, 08:57:16 PM »

Check your kill switch.

After that it's pretty straightforward:

1. Fuel/Air
2. Spark
3. Compression

There is your plan.

Art · « **Reply #3 on:** June 12, 2016, 11:42:49 PM »

I just rebuilt my top end on my cb750f and had the same issue. I had to resort to kick starting the engine because the electric start just wasn't working. I kept kicking it over and it just was not starting. I checked spark and I had a very weak spark, I changed the coils and ta-da!!! the engine started up again. Check the 3 things that someone mentioned and double check everything. You would not believe just one little thing off would do it.

Even if the timing is off, the bike should start so I doubt it's the timing for now.

1SpeedD · « **Reply #4 on:** June 13, 2016, 08:44:14 AM »

Thanks for all the suggestions! I have to go out of town, but I'll update at the end of the week when I get a chance to work on the bike again.

sinister902 · « **Reply #5 on:** June 13, 2016, 11:06:07 AM »

Quote from: calj737 on June 13, 2016, 05:14:47 AM

I would confirm that you have the valves adjusted properly now that you finally set the ignition timing correct. Of course, you need to confirm voltage at the coils.

If you pull the plugs, what do they look like? If you have spark and fuel, they should be showing signs of that mixture. Are they wet? Dry? Sooty? Fouled? Spotless?

this sounds familiar. LOL.

1SpeedD · « **Reply #6 on:** June 16, 2016, 05:37:54 PM »

Well I had a bit of time to run through the bike today with not much luck.

I borrowed my friend's multimeter, a Fluke T600, and can't seem to get much for readings. I'm not the best at electrical testing so I could be using it incorrectly. How should I go about testing my ignition coils to make sure they're getting 12V? The coils are new so I can't imagine they died during the rebuild, but maybe...

I pulled the carbs and ran through those again, making sure fuel was going in.

The spark plugs are always dry when I hit the starter. Oh, I also swapped my kill switch assembly with a spare to see if that was the issue, but nothing changed.

I'm still sitting at square one with the starter going, but nothing else happening. I only unplugged two things during the rebuild, how did it get so messed up?

b52bombardier1 · « **Reply #7 on:** June 16, 2016, 05:55:27 PM »

I'm never one to recommend much use of starting fluid / ether on engines because excessive use can wash oil from the cylinder walls. But, in certain cases like this, it can be a very good diagnostic tool. Spray a little into each carburetor and see if it wants to start a little better. If there's spark, this stuff will find it. If it starts for a few seconds, you can zero in on it being a carburetor(s) problem.

Rick

1SpeedD · « **Reply #8 on:** June 16, 2016, 08:32:39 PM »

Cool, I hadn't thought of starting fluid. I picked up a can just before the hardware store closed. I'll give it a shot tonight once the battery is charged

Bodi · « **Reply #9 on:** June 17, 2016, 05:35:52 AM »

This Fluke is a decent meter and easy to use. Turn the knob to V. Touch one probe tip to "ground" metal - the head of a bolt holding something to the frame is good - and the other tip to what you want to measure, with bullet connectors you can slide the point under the insulating cover and touch the metal connector. Probe colour doesn't matter... unless you're afraid the battery is in backwards (?). If you have the black probe to ground the display will have a "+", you see a "-" the other way around - but the volts reading is still the same.
You must have battery voltage (somewhere around 12VDC) at the black/white coil wires with key and kill switch ON.
Assuming points ignition: you must read 0V OR battery voltage at the blue or yellow coil wires to the points, this must alternate as you slowly turn the engine over to open/close the points: the points close to connect ground to the coloured wire, that completes the circuit powering the coil, then the points open to disconnect coil power - the coil make the spark when the points open.

As said a few times, you need three things for a gas engine to fire:
1) Fuel/Air mixture that will burn: that's the carburetor. If it roars to life for a second then dies when a shot of ether starting fluid is given ... carb trouble.
2) Compression: piston and valve sealing, cam timing, tappet adjustment. You can try a compression gauge, but screw a spark plug in about 2 turns and have someone kick it over - or use the starter with kill switch OFF - and you can hear compression leaking out if it's actually compressing.
3) Spark: The above will tell if you have power to the coils and that the points are working. You need to confirm that the ignition timing is roughly correct - static timing adjustment - and that the 1-4 coil is connected to the 1-4 points (and 2-3 to 2-3). Reversing these is common and usually causes fiery backfires through the carbs (not always) but definitely the engine won't start.

WaleFab · « **Reply #10 on:** June 17, 2016, 03:35:53 PM »

did you check compression after you buttoned things up? are you at full torque spec ?

1SpeedD · « **Reply #11 on:** June 17, 2016, 09:44:47 PM »

Alright, so I gave the starting fluid a try with no success. I tested the ignition coils as per your instruction Bodi and calj737. I'm getting 12V at both coils.

I picked up a compression gauge so I'll try that tomorrow. I've done a finger test with my thumb over the hole and I can feel a poof of air when I crank it.

It occurred to me last night that cam chain tension could be a potential culprit. Shot in the dark here. I can't find any documentation about proper tension, and what that may be. Could a loose, or tight cam chain cause an issue like this? Either the starter is overloaded with a tight chain, or just pulling slack with a loose chain?

Thanks for all the help, I really appreciate everyone on this forum a ton at times like this!

harisuluv · « **Reply #12 on:** June 17, 2016, 10:02:43 PM »

1SpeedD,

I know you're new here and that's no problem. It's ok if you're a novice and don't know how to do everything that's ok too--we'll teach you. But you're going to have to LISTEN to what people are telling you. I replied in the 3rd post of this thread with:

Quote from: harisuluv on June 12, 2016, 08:57:16 PM

After that it's pretty straightforward:

1. Fuel/Air
2. Spark
3. Compression

There is your plan.

Now in the meantime here we are going on about coils and cam chains and we don't even know if we have spark or compression. I also have no idea what your methodology was for using the starter fluid so I don't even know that.

The absolutely most basic of troubleshooting a non start we are at 0/3.

How about some pictures? I don't know what else to tell you right now, we have very little information, no pictures, and we're still questioning fuel, spark, and compression.

1SpeedD · « **Reply #13 on:** June 17, 2016, 11:12:45 PM »

Quote from: harisuluv on June 17, 2016, 10:02:43 PM

1SpeedD,

I know you're new here and that's no problem. It's ok if you're a novice and don't know how to do everything that's ok too--we'll teach you. But you're going to have to LISTEN to what people are telling you. I replied in the 3rd post of this thread with:

Quote from: harisuluv on June 12, 2016, 08:57:16 PM
After that it's pretty straightforward:

1. Fuel/Air
2. Spark
3. Compression

There is your plan.

Now in the meantime here we are going on about coils and cam chains and we don't even know if we have spark or compression. I also have no idea what your methodology was for using the starter fluid so I don't even know that.

The absolutely most basic of troubleshooting a non start we are at 0/3.

How about some pictures? I don't know what else to tell you right now, we have very little information, no pictures, and we're still questioning fuel, spark, and compression.

Fair enough, I'll be the first to admit I can jump around some times, or pretty often. I don't mean to devalue the help you all have provided. I'm a hell of a bicycle mechanic, and a s**t motorcycle mechanic. I realize now more than ever I'm over my head, and that it might be best to hand this bike off to someone more capable. I'm pretty close to bowing out and selling the bike if I can't figure this out in the next month.

The hardest part for me is to throw out the fact it was a 100% running bike before I replaced some gaskets, and now it's further from starting than it ever has been.

So let's start with the procedure here:
#1--Fuel. The carbs fill up, float levels are set, idle circuit is exceptionally clean, main jets are clean. All throttle cables are attached properly, same with choke, springs are all in working order. It seems they should be delivering fuel just fine, but the spark plugs don't seem wet when I try and start the bike. This might be an/the issue, but I'm not sure what can remedy it.

#1.5--Air. The air box is in good working order, new filter installed. No leaks that I can see, clamps are tight around the carb intakes. What other things should I be checking here to ensure air is not part of the issue?

#2--Spark. I'm getting spark, but potentially not as much as I'd like. Battery is new, reads 12V. I've tested ignition advance unit, it's got power. Ignition coils are getting 12V and are also brand new. The old ones were cracked. New spark plug caps/wires. All those things were replaced before this most recent issue.

#3--Compression. Tested with thumb over plug hole, passes that test. Haven't tested with actual gauge yet.

As for pictures, I'm honestly not sure what may help in that regard, but I'll take pictures of whatever you want if you think it'll help any diagnosis or guessing.

1SpeedD · « **Reply #14 on:** June 18, 2016, 05:03:39 PM »

So I posted a few rounds of pictures but none of the posts went through. Is there some sort of cool-off/approval period for posts with pictures?

Scott S · « **Reply #15 on:** June 18, 2016, 06:01:40 PM »

You should be able to post pics.

I'm surprised no one's mentioned cam timing. What method did you use to I stall the cam and cam gear ? How are you determining when #1 cylinder is at TDC for the valve adjustment?

1SpeedD · « **Reply #16 on:** June 18, 2016, 06:20:38 PM »

Ok, hopefully this works this time around. Like I mentioned, I posted a bunch of pictures and the posts didn't go through. Bummer.

So here are the ignition coils. Left coil goes to 1-4, right goes to 2-3. Wiring and/or plug wires could be off, that's why I'm giving ya'll pictures.

Also I have the battery voltage reading. First picture is battery with key and ignition OFF. Next picture is key ON, ignition OFF. Last picture is key ON ignition ON. All are reading 12V as far as I can tell.

1SpeedD · « **Reply #17 on:** June 18, 2016, 06:23:28 PM »

Well this is a mess, only one picture at a time works. Here's the 2-3 Ignition coil

1SpeedD · « **Reply #18 on:** June 18, 2016, 06:24:33 PM »

Battery reads 12V-13V with everything OFF, key ON ignition OFF, and key ON ignition ON

1SpeedD · « **Reply #19 on:** June 18, 2016, 06:27:31 PM »

Here's the ignition advance unit. Set to first mark at 1-4 F. lines drawn for easy re-assembly when the little pin kept coming out. It's securely in place now.

1SpeedD · « **Reply #20 on:** June 18, 2016, 06:28:20 PM »

Another picture of it.

1SpeedD · « **Reply #21 on:** June 18, 2016, 06:35:05 PM »

Bear with me here. Here are some pictures of the carbs. I pulled the air box as far back as possible. Here's carb #1 with the choke completely off.

If I may interject my own ill-equipped opinion here, I have a feeling the carbs may be a large part of the issue. When i put the gas tank on last night, and again this morning, gas did not flow freely when the petcock was opened. I had to open a drain plug to get the carbs to start filling up. After that they all filled. Maybe something is keeping fuel from going fully into the carbs? Flame away if my guesses are miles off point.

Believe me, at this point I'd pay to have someone check this bike out some, but no one in town wants to touch it.

1SpeedD · « **Reply #22 on:** June 18, 2016, 06:36:42 PM »

Here's carb 4 without choke as well. Hopefully you'll believe me that carbs 2/3 are the same as 1/4 in this regard. They're a bit tougher to photograph with the carbs still on the bike.

1SpeedD · « **Reply #23 on:** June 18, 2016, 06:37:29 PM »

Two more pictures. Carbs with choke ON. Here's #1

1SpeedD · « **Reply #24 on:** June 18, 2016, 06:40:06 PM »

And last one for now. Carb #4 with choke ON. I'll post pictures of float levels as soon as I can get help putting the bike on the center stand. Recently installed screws in my wrist make it a bit tougher of a task than usual.

Once again, with all humility, thank you for the help and for bearing with my novice abilities.

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Author Topic: Cb650 won't start after top end rebuild (Read 6258 times)