Poor Running

Hi All I am a new XJ750 owner When I got the bike it would hardly run now it runs as long as you keep it above 3k rpm. Starts ok no choke needed. slow tick over sluggish till 3k rpm then takes of. give some hard welly splutters at 6k then settles. come to a stop ticks over at 1k for a while then stops. will restart but need to blip the throttle. work done so far, carbs striped cleaned rebuilt, mixture set, balanced, new coils and plugs. MOTed taxed. all electrical connection's remade so good. Air leak test done and none found. I must be missing something any ideas?

 

If you read through the forum you'll find that pretty much every other thread on every page includes the question: did you check your valve clearances?

That's one thing you might be missing.

Other things that would be nice to know are how long the bike's been doing this (since you got it? any better since the carb work?), and whether you'd checked compression.

 

Hi Sorry should have said The bike is running like a dream compared to when I bought it. Just not smooth at lower revs Yes I have checked the valves and they are within spec. Compression was done and all cylinders have the same reading. The carbs have been checked several times but I haven't found anything to cause concern. The diaframs look good and no sign of holes or splits. It just feels like im missing something silly or my skills arnt good enough to see the problem.

 

Two things for you to consider;

Sputtering is a symptom of Fuel which is insufficiently atomized.
Erratic performance is a sign that the Diaphragm Piston may be encumbered by oxidation and not totally free moving.

The Emulsion Tubes (Nozzles) require inspection.
There are 16 to 20 Air Holes which need to be clean.

The Diaphragm Piston Bores should be refinished and Polished.
Their age, alone, requires it.

 

Thanks for your feed back. Have read your posts on the carbs and had another look I will be striping them back again at weekend to polish the slides. That bit i missed thought they were ok but after reading your post not so sure will update once done polish

 

???

 

You might want to check them again, and post your actual measurements so bigfitz can be SURE you mean what you said. :wink:

Listen to what he tells you; he's one of the wizards around here.

 

If the valves are in spec; cool.

The fact that no "choke" (enrichment circuit) is needed to start the bike is a major clue.

What method was used to set the float levels?

 

Hi The float levels were set all the same although the haynes manual says in the specs to set the floats 17.5mm but does give how to make the measurement. the other way is with the carbs on the bike using a tube. if there is a way to measure the float heights on the bench please enlighten me.
Will go and check fuel height tonight and will post result.

 

You can use the "clear tube method" on the bench, utilizing an external fuel source.

You NEED to "wet-set" them; the dry setting is only a starting point. Only by using the clear tube method can you really tell at what level they're actually stopping flow.

It's actually easier to do them on the bench, as it's much easier to get the rack perfectly horizontal in all directions than it is by propping up the bike.

 

Hi have just checked using the clear tube method and all of them are sitting about half way up the bowl. but when the bowels were removed the fuel in them was almost to the top.

 

Were the carbs level front-to-back AND side-to-side? If you did them on the bike, it's kind of hard to achieve this.

The "clear tube method" gives you a "peek" at what's going on inside; as you remove a float bowl, it WILL fill up some more during the removal process (if fuel is still supplied) because as soon as you begin to remove the bowl, the float lowers and the valve opens.

Here's a good overview: http://mok.fileave.com/setting%20fuel%20levels.pdf

Here's a thread devoted to the trials and tribulations; look at page 3, IIRC.
http://xjbikes.com/Forums/viewtopic/t=3 ... art=0.html

 

Hi Guys Im back. Have striped the carbs given a good clean and rebuilt. The slides drop like lead balloons until the spring and diafram are fitted then they slow up a bit but still drop to give a clunk. rebalanced the carbs. Checked the valve clearances with the following readings inlet 1= 0.11mm 2= 0.11 3= 0.11 4 =0.11. Exhaust 1=0.15 2=0.15 3= 0.15 4=0.20 , Compression test done with the following 1=130psi 2=128psi 3=120psi 4=120psi. Took the bike for a run slugesh up till 3k then goes like hell. hangs at 2k when i stop for about 2mins then dies. Still cant find what is wrong. Plug colours after run 1&2 light gray. 3& 4 dark gray bordering on black. Please confirm the mixture screws it is in (clockwise) to lean the mixture. and the one other thing i have picked up on is the inlet boots dont have any clips on them would this account for the problems as they are before the carbs? Any ideas would be helpful .

 

I would get clips at Home Depot for cheap as insurance.
It doesnt say you Synced the carbs or colortuned them. I would do that as well.
Also it doesnt say that you checked the floats! Whats your float height? Use the clear tube method. (EDIT just saw you did check them once) Make sure they are just below the float bowl gasket on each. Carbs need to be level to do this

After installing the clips use an unlit propane torch and run it along the boots and see if the idle rpms change. If yes, you have a leak and need to replace boots.

Mixture use are (as a good base point for tuning, USUALLY) 2 1/2 turns out from bottoming.

More people will chime in I'm sure

 

Ummm...

Valve clearance specs are .11~.15mm intake; .16~.20mm exhaust.

You've got three tight exhaust valves (1, 2 and 3.) That's why those cylinders have lower compression numbers than #1.

You're currently wasting your time, you need to get the valves in spec FIRST:

 

Hi all
Im Back The valves have now been reset Float levels all the same 3mm below gasket height set on bench so carbs were level on all planes. Cant find any signs of air leaks. Clips fitted to intake boots. Can get all set nice then take it for a run under load at low revs runs rough above 3k runs as sweet as a nut keep it on the lower revs ie in town the more miles i do the worst it gets check plugs on return 1 and 2 plugs a good colour 3 and 4 plugs black sooty. clean them and all good again for a short time. Any other ideas.

 

Rudie; Please don't quote me if it no longer (apparently) applies, as in this case. Honestly, no offense but until you have a running bike, please don't quote me; I can speak for myself, really.

So now, speaking for myself: "valves have now been reset." You re-shimmed all the tight exhaust valves? We discussed this previously, you said they were in spec when in fact they weren't, so I gotta ask: What are your clearances now?

Have you run a compression test now that the valves are in spec? Results?

Were the float levels set using clear tubing and actual gasoline, or ???

Did you bench sync before putting the carbs back on?

Where are the pilot screws set? Where were they set before you pulled them for the cleaning? New o-rings on the pilot needles?

What method was used for the running vac sync? YICS blocked?

I have a feeling you might be getting close; but the process cannot be "shortcut." The biggest issue most people have when doing their vacuum sync is "over-adjusting" they simply crank the screws too much on each "tweak."

Give us some honest answers on the above, and we'll see where it gets us.

 

Hi I have just striped the bike and double checked all settings as follows
Exchust 1 = 15-20, 2 = 15-20, 3= 15-20, 4=15-20, Inlet 1=10-15, 2=10-15,3=10-15,4=10-15. Compression test results 1=135psi, 2=130psi, 3=140psi,4=140psi. Float levels were set using petrol feed from a funnel on the bench all carbs were showing 3.5mm below gasket line. Did not bench sync carbs. Pilot screw settings as follow 1=2.5turns out, 2=2.25turns out, 3=1turn out, 4=0.25turn out. The bike was taken for a 60mile run and all plugs are the same colour Dark gray. Vac sync was redone but with problems. YICS not blocked as been unalble to get a tool to do it with. Can sync carbs 1&2, Can sync carbs 3&4, but from left to right carbs strat engine left is pulling harder stop engine restart then right is pulling harder. am having great trouble to balance side to side. hope this gives a clue with your experience. No new o rings were not fitted as they looked in good order.

 

What do you mean "15-20" for the exhaust valves? "10-15" for intakes? In your initial post, you gave us valve clearances that were too tight. Unless you actually replaced the shims on the tight valves, nothing there has changed.

If any exhausts are at .15mm they are too tight.
If any intakes are at .10mm they are too tight.

What are your clearances exactly? Just suddenly declaring your valves to be in spec when they weren't before doesn't actually bring them into spec.

Why didn't you bench sync?

I would record your pilot screw settings, then set them all at 2.5 turns out.

Then vacuum sync, with the YICS blocked. You don't NEED a special tool to block the YICS; you can soak an old tee-shirt in motor oil and then twist it into a rope and pack the passage.

But any and all of the above is based on you actually getting your valves in spec not just changing how you "report" your measurements. They're either in spec or they're not; if you haven't actually adjusted them since your initial post then you are in fact wasting your time (and ours) and will continue to be until you actually COMPLETE this important first step. You can't "deny away" the need; JUST DO IT.

It's not something you can "fudge." Or you will not be able to get the carbs adjusted correctly. You're beginning to find that out, how long you go on beating your head against the wall is up to you.

 

Right lets get this right feeler gauges marked up .05,.10,.15,.20,.30,.40,.50 ect. Then if .15 fits but .20 doesn't Then it is in-between there fore in spec? or am I missing something?

 

Back on Oct 22:

Again: spec is .16~.20mm. If those are the actual clearances on exhausts 1, 2 and 3 then they are in fact too tight. If they're based on your "go-no go" method, then you need a more accurate feeler gauge. Just because a .15 fits doesn't make them in spec, if they're AT .15 then they're NOT.

With three tight exhaust valves and one at the loose end of the spec, you're going to have quite a bit of imbalance between #4 and the rest of the gang. Especially with all four of your intakes sitting right on the tight edge too.

You asked what you need to do so you can get your motor dialed in. This is the first something you need to do. If not, your carbs efforts will continue to fail. That's just the way it works, honest. Believe what you want.

One other question though: If your feeler gauge set is only graduated .10, .15, .20, etc., where did you get .11 for the intakes?

 

Hi Some more work done on the bike
New feeler gauges bought checked valves Ordered new shims fitted shims. new readings , Exhaust 1=.16, 2=.18, 3=.18, 4=.16. Inlet 1=.14, 2=.15, 3=.15, 4=.13. Compression test re done 1=150, 2=150, 3=150, 4=140.
Reset all mixture screws to 2.5 turns out, Balanced the carbs, Have now got a tickover Seems to run ok until under load then sluggish until 3k anything over that runs well and smooth, Under 3k still plays up.

 

Under 3K is all pilots and mixtures will play a role. Unless you have a ColorTune it's time to read your plugs, "tweak" the mixtures accordingly, ride, read, and tweak some more. And by "tweak" I mean 1/16 of a turn or less.

You did block the YICS when you did the running vacuum sync, right?

 

Hi Im Back again still at same point under load under 3k rpm poor running above 3k runs like a dream. Tried tweeking mixture no joy obtained colour tune reset all mixtures so all ok with colour tune. blocked yics system ballanced carbs. all to no avail have got a steady tickover engine does not hang anywhere open up throttle when it gets to 3krpm takes off will run steady and nice at any speed above 40mph below runs very poorly.

 

Have you fit a new set of spark plugs? And what state is the battery in? I'm wondering if you're not getting a good strong spark until the charging system is fully "kicked in." Have you tested the alternator output?

 

Hi Yess new plugs and coils were fitted. Battry in good condition 12.8v stored 13.2v on tick over rising to 14.5v at 2k rpm

 

New coils? New-used Yamaha coils or aftermarket? Have you checked inside your plug caps to be sure the resistor cores aren't loose?

If it's not ignition related, your valves are now in spec, compression within spec, the only thing it can be is the pilot circuits/pilot jets but you'd think that would show up when using the ColorTune.

That or you've got a binding float causing oddball fuel levels.

 

New coils were bought from wimito have checked plug caps and all seem ok and tight. had a friend round who has worked on severely bikes and rechecked all settings and all seem to be ok redone colour tune and balance still running the same. he asked about the exhaust as it is a 4into 1 system would this have any bearing on the problems?

 

It sure could. You may need to do some rejetting to accomodate it.

Any intake mods you might have failed to mention?

 

All the jets are standard sizes according to the Haynes manual so if the need changing because of the exhaust how can I find out what ones to fit?
The air box is still standard

 

go to the top of the screen, click on the XJ4EVER. There is a "formula" to determine a "starting" point. some one with a similar mod chime in?
Don't take it for gospel but I think if you up 2 times on main and 1 on pilot youll be around a "fair starting point"
rejetting these carbs is not a science, its hit and miss (more miss) experimenting.
try the search feature and select in the xj mods section.
good luck, (i got mixture issues too)

 

The rejetting recommendations can also be found in "The Church of Clean" here: http://xjbikes.com/Forums/viewtopic/t=14692.html nearly at the bottom of the page, below all of the stock jetting for every XJ ever made.

According to the "guidelines" it's up 4 sizes on the mains and 1 size on the pilots for a 4-1.

Do a few plug chops at over 6K RPM on full throttle, and see what the mains are doing. If the motor isn't "falling off" at higher RPMs, then up 2 on the mains and 1 on the pilots might be the ticket.