1981 XJ750 Seca, starts then dies

Hello all,

So to start of I own a 81' XJ750 Seca, as with all old motorcycles I went to the forums and read the first things to do. I read the information overload hour thread, a d have done the following:

• Rebuilt the entire brake system
  • Replaced old hidden MC with a new handle mounted one from a cbr 600 (same bore)
  • Stainless steel lines
  • And new seals and fluid
• repaired any botched wiring
• Fresh fluids and filters
• New batteries
• Rebuilt carbs
  • Didn't do it myself do to a time constraint at the time. But have it to a well respected and reputed virago rebuilder.
• Checked and did the valve clearances and triple checked they were to spec
• New spark plugs
• New starter relay

Now I haven't changed the plug wires coils or the ignitor module. Nor have I checked the resistance yet. (Bit iffy on where I need to check). I did the test of taking out the plugs on two of the cylinders one for each of the ignitor to see if it was making a spark and it was.

As far as my issue goes, the bike starts on full choke, i wait about a minute take it off of choke, and it revs really high (no idea on rpms as there is no tach) or it dies. If it stays revving high I blip the throttle then it dies.
I know it is getting fuel as I can see fuel moving through the lines.

I haven't tried the propane into the intake to see a vacuum leak.

This has gone from a fun project to a huge money pit. All I want to do is ride it once before winter comes. Any ideas?

 

Did your rebuilder replace the throttle shaft seals?
Are there any cracks in the intake boots?
Is the vaccum line for the petcock new?

Methinks you have a vacuum leak.

 

Just replaced my intake boots and gaskets to get rid of similar problem on my Xj750rl

 

Yes he did replace the throttle shaft seals and bench synced. As for the boots are you referring to filter to carbs? They were a pain to reinstall so potentially yes. As for the petcock vacuum line yes it is brand new

I know vacuum leaks cause high idle but would it cause the bike to die with throttle being applied?

Where did you purchase them?

 

My ones for the Xj750rl are the same as the Xj700n. Yours are the same as my xj650rjc
@chacal has them at xj4ever. Icon on top right of home page. Carb to engine. Ones for the air box are available also but not for my Xj750rl. My ones are still in good shape.
Doing intake manifolds and carbs on a kz650 today.

 

I just want to establish a baseline of where things are at before making any diagnosis.
Let it warm up longer.
After starting the bike (enrichment lever on full), wait unitl the revs begin to rise on their own.
That is when you can begin to think about reducing the amount of enrichment you give her.
An XJ takes a while to warm up, and the stallling is indicative of that.
The racing is indicatave of a vacuum leak.
The carb to head boots are the ones that would cause the problem. Be careful, as the bolts that hold them on are prone to corrosion, and can snap off easily when removal is attempted.
If you don't mind a bit of ugly, you can thoroughly degrease the boots in-situ, and cover them with oil-resistant Black RTV. Rickcomatic has a post or three about that method, and it's well worth searching for. It used to be what we all had to do before replacement boots became available again.

 

ohming out ignition.
ohm from spark plug cap 1 to 4 then ohm from 2 to 3.
you will also want to unscrew spark plug caps and test them.
to do primary on coils you can ohm out from tci plug. remove it from tci then test from gray to red/white and orange to same red white.
if test for primary is out of spec test at connector to coils under tank.
for the pick up coils test from other connector gray to black and orange to same black wire


clean all connectors with electronic contact spray and grease with dielectric grease. do the tci connections too.

a quick test you can do is test header pipes for heat make sure you do not have any cold(er ) pipes. this will help you tell if your problem is a particular cylinder.

specs for this test can be found in this link
The Ultimate Relay, Switch, Sensor, and Diodes Guide

 

So on average how long should I wait? With it on full enrichment it sounds like it's on redline from the beginning with the occasional backfire (forgot to mention that). Also before I forget it has a 4-to-1 exhaust. With that being said I revert to my time question, where you dont hear the revs fo up.

Will definitely try this today prior to purchasing new ones.

Thanks, I'll definitely check it out but I'll try the DIY first to be sure that's the issue. Good luck on the kz!

 

did you hook the choke to throttle on carbs and throttle to enrichment circuit?

 

+1. Either the cables are hooked up wrong, or you definitely have a vacuum leak.

Normally the engine would be at the normal idle speed (~1,100 RPM), or a bit below that on startup from cold (enrichment on full), then after 2-3 minutes the idle will slowly increase as the engine warms (up to around 2,000 - 2,500 RPM) at which point you'd back off the enrichment to either pull away a bit early, or continue warming until you can close the enrichment circuits completely without her dying. Total warmup time is around 5-7 minutes (10 in cooler weather).

 

Nope, triple checked that on install the enrichment/choke cable goes to the cross bar at the top of the carbs.

I'll try this, I'll start the bike spray some carb cleaner over each of the boots see which one causes the idle to change slightly (might be slightly difficult as the engine is going at it). Wait 5 minutes for it to warm up then I'll take it off of the enrichment circuit.

Now if it dies after that and I didn't hear any idle changes what's the next step?

 

The next step: If she's still idling high, adjust the idle speed down. Then run the engine speed up to about 5K with the throttle, and see if she drops back down to idle quickly. If she's still racing, or the idle hangs, then you have the type of lean condition that's usually caused by a vacuum leak.

 

Just so I am clear and don't screw it up

XJ750 air-cooled models:

Pick-up coils:
650 ohms +/- 20% = 520 ohms to 780 ohms acceptable range
Great to black and orange to black

Ignition Coils:

Primary side (input from main wiring harness):
2.5 ohms +/- 10% = 2.25 ohms - 2.75 ohms acceptable range gray to red white orange to redwhite if it fails at both connectors I need to replace/rewire that circuit?

Secondary side (spark plug wires, without their end caps):
11K ohms +/- 20% = 8,800 ohms - 13,200 ohms acceptable range testing the spark plug wires 1-4 and 2-3.

 

Primary side is checking the low voltage side of the coils. If it fails at a connector, you have a bad coil and that coil would need to be replaced.

 

Well it failed as per the pictures.
So how do I know which one is bad? Also would this attribute to the occasional issues starting?

Everything else checked out fine in terms of resistance

Did have the battery disconnected for safety.

 

your meter is set to high for primary go to the lowest setting 200 ohms

if you leave the spark plug caps on add 10 k for a total of 21k ohms

may be easier to probe from back side of connector where wire goes in.

if primary fails at tci connector go up to ignition coils and check at that connector and clean the connector for each coil (2) you do have the tank off.

 

Awkward. I honestly thought it said 2.5k ohms. Well it passed now

 

So I tested for vacuum leaks, using carb cleaner it didn't raise the rpms in any way. I did however see that on "fuel" my pet cock stops delivering fuel now. So will definitely need to rebuild that.

But after 5 minutes the rpms on full enrichment sounded like it was stable so I took it off enrichment. Then I bliped the throttle just a little and it went berserk. Stayed way beyond "Red-lime" as the bike started to shake on the center stand. Shut it off after 30 seconds to see if it would normalize. It didn't. Now my head to exhaust is smoking.

Buy and old bike they said. It'll be fun they said.

 

OK. You do have a vacuum leak. Since the petcock isn't delivering fuel in the ON position while the engine is running I would focus on the vacuum leak being related to the petcock vacuum line or the petcock vacuum diaphram. It is possible to have more than one vacuum leak, but tackle the petcock first so that can be eliminated as a cause.

The smoke you see is likely just accumulated dust and oil burning off. I'd not worry about it unless it does not clear up once the engine has run at temperature for a good while.

 

So, someone else bench synced your carbs? You know what's coming next - take them off, strip the tops and bottoms off them, check the diapghragms, needles, jets, float heights (unless you've previously wet checked and are happy with these). Finally, bench synch them to the closed edge of the first hole - that is each butterfly edge sits just on the closed edge of the hole. This will get you running - in fact if all else is good this will produce a very rideable bike, with the minimum of adjustment of the central idle screw to get around 1200 rpm.
Oh, set mixture screws to factory recommended settings, if you haven't allready as a starting point.
I don't disagree with kmoe about the vacuum leak, but really, you should be able to see any cracks etc, and trusting anyone else with these delicate instruments is asking for trouble.