XJ750 won't start unless primed.

I am restoring an '82 XJ750 Seca after it sat indoors for 2 years. I cleaned the carbs and it started and ran with no problems except gas was pouring out of the #3 intake. I fixed that problem and again, it started and ran with no problem. Then I put it in first to give it a test ride around the yard and it died. Now it won't start unless I prime the carbs. After that it will run all day as long as I keep the RPM's up over 2 grand. The moment it drops below 2 grand it dies and although it sounds like she wants to start, she won't without another prime. Is this a problem with idle jets? Settings?

 

test the pet cock.

 

Okay I lied a little. The bike will run indefinitely as long as I keep the RPM's over 1500. Also, if it sets for a few minutes after running I can usually get it started first try but after it dies, it won't start again unless I wait a while.

 

Wiz, there is fuel coming out of the pet c**k. What else am I looking for?

 

The only other thing it could be is the idle circuit is clogged, try putting some Gumout in your tank, if that doesn't work within a week, you may have to pull the carbs to do a more intense cleaning. Sometimes the varnish build up is so hard, you may need to rod out your jets with a bristle from a stiff brush.

 

You really have to start with clean carbs & put the valves in spec.

 

I would imagine pilot jets/enrichment circuit (the jet pressed into the bowls).

Best to just clean the whole system so you know for sure, while they're off anyways.

 

Fintip, FYI, the idle(pilot) circuit is separate from the enrichment circuit. The passage in the bottom of the bowl is where the "choke" gets it's fuel supply, while the idle jet screws in next to the main jet.

 

Gas pouring out of the intake = stuck float needle. Just cause you don't see gas anymore doesn't mean it's fixed. Sniff the oil fill hole for a gasoline smell. If there's any gas in your oil, fix the float needle, then change the oil. Did you wet set your floats (a stuck needle will be obvious if you did this)?

Dies if revs are under 1500 rpm = need to clean pilot circuit/sync/

Only runs when primed = I'm assuming you mean CHOKE, and not PRI on the petcock. Probably an air leak. Something is likely leaning out the mixture, either lack of fuel in the pilot circuit, or too much air. That's why it only runs on choke.

If you mean PRI on the petcock, then it's not functioning properly. These petcocks are vacuum operated, and ON and RES only flow fuel when the engine is running (if it's functioning properly).

Wizard is right, it's been said many times on the forum before. Fully clean the rebuild the carbs right, check and spec the valves, THEN start troubleshooting.

You mentioned that you cleaned the carbs. What EXACTLY did you do. We can help you cover any bases that you might have missed.

 

Is it possible it is getting flooded?

I could be wrong, but maybe its something to check.

"Aftermarket ON-OFF Fuel Valves:

fp17) Aftermarket complete ON-OFF-RESERVE FUEL VALVE KITS. Not really a "petcock" in the XJ sense of the word. Yamaha designed their petcocks to be a very clever "automatic" fuel supply valve, meaning it has to have engine vacuum for it to function. Great idea (when it works) because it allows us to basically "set it and forget it". Bad news is that when the seals go bad, or as age wears the internal components, it can become a really bad idea, since it will leak fuel into the carbs when it's not supposed to, leading to flooded carbs, fuel in the crankcase, spun bearings, etc.

A true fuel VALVE is either ON (in either the standard "ON" or the "RES"erve positions) or it's OFF. And it doesn't depend on engine vacuum, just gravity. So it won't ever leak or pass fuel in the OFF position. That's the good news. The bad news is that you have to remember to turn it on or off manually (as in every time you start your bike up or shut it down) or, instead of blaming a faulty petcock for your resultant problems, you'll have to blame yourself!"

Maybe pull the fuel line after the valve to see if its leaking.
Good luck

 

I disassembled the pet c**k and am confident that it is functioning properly.
I cleaned the carbs as instructed by Evan Fell on his site cycles.evanfell.com "Hoe to clean a motorcycle carburator the right way."

There are a couple of small cracks in the rubber boots connecting the carbs to the cylinders but I originally didn't think they were sucking any air. I will remove them and take a closer look. Has anyone had any luck repairing these or do I need to replace them?

Also, a previous owner removed the airbox and installed individual filters to the carb intakes. I've heard that this may lean out the mixture. What can I do to compensate for that?

I have never fooled with valves on a bike motor. Where can I find instructions and specs? Do I need any special tooling?

Thank you all for the help. It is much appreciated.

 

Oh yah, ManBot13- when I say the bike won't start unless I prime it I mean that I have to squirt a little gas directly into the intake of a couple of the carbs. Choking it doesn't get it started. Sorry for the confusion.

Once started the bike runs fine with the choke off as long as I keep it over 1500 RPM's

 

Does it flow gas when ON? If you suck on the vacuum line, does it flow gas then? Did you replace any of the internals or just clean it out?

That is a decent How-To, and has been referenced before on this site. He does miss a couple of things too:
1. He doesn't break the rack. It's the only way to replace throttle shaft seals, and those are notorious for leaking air into the intake as they age.
2. He doesn't go into detail about cleaning the enrichment plungers. Hopefully you cleaned out that part
3. He mentions the enrichement well in the bowl, but improperly labels it the idle jet. The idle jet and pilot jet are the same thing on these carbs. You should be able to shoot carb cleaner through the enrichment well like a fountain (were goggles)
4. He doesn't bench sync the carbs or wet set the floats.

Yes, you can slather them with RTV, working it into the cracks. Someone recently posted about this. I followed that up with a bicycle inner tube sleeve over the RTV, to make sure it seals well.

DING DING DING - we found the air leak! :lol: Search the site for tuning with pods. The BEST way to compensate...get a new air box. And unfortunately, (although there may be someone out there who claims to have done it) you can't get an airbox into the frame without pulling the engine on a 750 Seca

BigFitz52 wrote up an excellent How-To here: http://xjbikes.com/Forums/viewtopic/t=14827.html. Checking the valve clearances doesn't involve anything special other than a set of feeler gauges. Get a manual if you don't already have one. Removing the shims if any are out of spec will require a special tool.

This further points to your enrichment circuit not be cleaned properly, as it actually squirts a little gas in the intake, just like what you are manually doing.

As you can see, you've got several issues with the carbs, and you'll need to tackle them all.

 

Thank you so much ManBot13!!! A site like this would be sorely lacking without the generosity of experienced individuals such as yourself.