Any Advice To Get This Engine Started?

[luckydevil]

I'm attempting to bring my 1977 XS750 triple back to life. It started to run poorly (inconsistent idling and bogging down when rolling throttle), and then sat for a couple years. I was able to get it to fire up, but it was backfiring, stalling, and missing cylinder 2. I broke the carbs down and can't believe it even fired up...clogged pilot jets, stuck float needles, stuck slides, varnish galore. I disassembled the carbs and scrubbed everything down...lots of carb cleaner and compressed air. Replaced jets and needles, checked float height, and bench tested float valves. Everything looked happy. I then checked valve clearances and replaced shims, nothing too far out of spec (0.10mm or 0.05mm tight). I also installed clutch plates and the crankcase looked to be in good shape. I gave it some new oil and a filter but i can't get it to fire. Spark looks good from all 3 wires (visual inspection grounding against block). I checked timing with a timing light and it's right on the mark. It has an electronic ignition without mechanical contact breakers. Compression tested low (about 90psi on all 3 cylinders), but I'm not sure if testing conditions (cold engine with plugs removed, turning with electronic ignition) are fit to give an accurate measurement, or if compression that low would prevent the engine from firing at all.

I'm stumped, but I'm also not a mechanic. Any advice on how to troubleshoot from here? I'm happy to upload photos or clarify if I described anything inaccurately. Thanks in advance!

 

[jetmechmarty]

You need spark on time. You say you have it. You need fuel. Are you sure fuel is feeding? You need compression. I would expect it to run at 90PSI.

Put some gasoline in a spray bottle and spray some into the carburetor throats. See if it lights off.

The XS750 has a battery excited alternator AFAIK. The battery needs to be good and strong to crank and light off.

 

[Max Midnight]

Did you check the compression with the throttle open?
Do the plugs show signs of dampness which would indicate you are getting fuel?

 

[gggGary]

spray carb cleaner even easier than gasoline to get a motor to fire off. A good shot in each carb throat and if that don't light it off your prob ain't the carbs.
Also agree; 90 PSI should be enough to run.

 

[arcticXS]

2+ years old gas, or fresh new gas? Do you get fuel into the float chambers?

 

[gggGary]

Um, petcock on Prime?
Remember to take back off prime before you walk away.

 

[JJames]

You need spark on time. You say you have it. You need fuel. Are you sure fuel is feeding? You need compression. I would expect it to run at 90PSI.

Put some gasoline in a spray bottle and spray some into the carburetor throats. See if it lights off.

The XS750 has a battery excited alternator AFAIK. The battery needs to be good and strong to crank and light .

It sounds like you gave the carburetors a carefully cleaning. It is important to have a healthy, well-fed battery in these bikes. If you're running the one that sat in it. Could be your problem.

 

[Wirenutt]

My apologies if you have already checked this, but are the right plug wires on the right cylinders? Are you sure the carbs are getting fuel and filling their float bowls? Definitely try gggGary's suggestion to spray carb cleaner into the carb throats when cranking. I mean, it doesn't take much to get an engine to at least fire on a cylinder or two. It sounds like something is way off, hence my question about the plug wires. Compression, fuel, and spark. 90 psi is nothing to write home about, but it's enough to get pops and bangs, at least, and probably run half decent. You say you have spark. And the carbs are rebuilt. So, if you bypass the carb issue with carb cleaner, if you have spark at close to the right time, it should fire.

 

[gggGary]

I'll add correct resistance in the secondary ie 5K ohms, spark plug OR caps, should have 5K resistance.
A strong blue spark inside a cylinder isn't as easy as out in free air.

 

[luckydevil]

Did you check the compression with the throttle open?
Do the plugs show signs of dampness which would indicate you are getting fuel?

I checked compression with throttle closed. They're squeezing about 140psi with throttle wide open...thank you very much. The plugs are damp when I pull them out.

 

[luckydevil]

Um, petcock on Prime?
Remember to take back off prime before you walk away.

PO put manual petcocks on this tank, but I've been forgetful in the past. Now that you mention petckocks I bet air is getting through these cracked old caps over the petcock vacuum barbs. Thanks!

 

[luckydevil]

2+ years old gas, or fresh new gas? Do you get fuel into the float chambers?

I have fresh, ethanol-free gas, and I'm getting gas into my float bowls.

 

[luckydevil]

My apologies if you have already checked this, but are the right plug wires on the right cylinders? Are you sure the carbs are getting fuel and filling their float bowls? Definitely try gggGary's suggestion to spray carb cleaner into the carb throats when cranking. I mean, it doesn't take much to get an engine to at least fire on a cylinder or two. It sounds like something is way off, hence my question about the plug wires. Compression, fuel, and spark. 90 psi is nothing to write home about, but it's enough to get pops and bangs, at least, and probably run half decent. You say you have spark. And the carbs are rebuilt. So, if you bypass the carb issue with carb cleaner, if you have spark at close to the right time, it should fire.

Holy cow! I had 2 of my plug wires criss-crossed...bonehead move. I got hung up on small details, missed a big one. Sprayed a little carb clearer and boom, fired away. I can't quit get it to idle, but it fires so long as I mash the starter down. Gives me something to work with. Thank You!!

 

[luckydevil]

I'll add correct resistance in the secondary ie 5K ohms, spark plug OR caps, should have 5K resistance.
A strong blue spark inside a cylinder isn't as easy as out in free air.

I think I'm following you. I'm reading 13.9 ohms on all 3 coils. Spec says 7.9k ohms (+/- 20%), plus 5k = 12.9k. So I'm a little high, but within spec? Or am I all mixed up?

 

[Grimly]

I checked compression with throttle closed. They're squeezing about 140psi with throttle wide open...thank you very much. The plugs are damp when I pull them out.

Much betterer.

 

[Wirenutt]

Holy cow! I had 2 of my plug wires criss-crossed...bonehead move. I got hung up on small details, missed a big one. Sprayed a little carb clearer and boom, fired away. I can't quit get it to idle, but it fires so long as I mash the starter down. Gives me something to work with. Thank You!!

That's awesome. Glad I could help!

 

[Max Midnight]

Holy cow! I had 2 of my plug wires criss-crossed...bonehead move. I got hung up on small details, missed a big one.

First rule of diagnosis: Check the basics first.

Thanks for the update.

 

[luckydevil]

Thanks to everyone for the input! It helped get me out of the mud.

Any tips for idle speed screw setting just to get it idling? I'm using factory settings for jets and air/fuel mixture screw (backed out 1 1/4 turns). Also, does anyone know the tach gear ratio for a replacement? It looks like a 1:5 ratio but I've found conflicting info. The exact model is 1977 XS750-E.

Thanks for putting up with my xs750 questions, I have a 1978 xs650 basketcase up next.

 

[jetmechmarty]

Most likely, if you have a cable driven tach, it’s going to be for a Yamaha. I can’t understand why the wouldn’t have made them all the same. Maybe someone on here knows for sure, but …