1985 XJ 700 BREAKING UP UNDER LOAD

I'm not sure if I'm posting or doing this correctly but I need some help.

I have an 85 xj 700. It idles fine revs to redline in netrol fine sounds good. In gear riding it runs and rides good untik 5k rpms under load. 1/4 to mid throttle it runs strong and pulls good. 3/4 to WOT past 5k rpms it sputters coughs and breaks up horrible.

I have new coils new plugs I have had the carbs off and rebuilt the carbs and pulled them off and cleaned them 3 separate tims tbining i missed somthing. I have had them all the way torn down but did not brake them apart. Put in all new rubber new needles in with springs and wahsers adjusted them to 2.5 turns out I balanced all the carbs.

It has an after market UNI filter. I removed the filter completely took it down the road and it made no difference to the breaking up.

I tried pulling in the choke while it is breaking up and it makes no difference.

Compression is low but even on all cylinders around 100 psi. Not great but I don't think it would run good but not rev out?

I had an inline fuel filter removed it thinking its restricting fuel and it made no difference.

I ran the bike with the petcock on prime and it makes no difference.

The plugs are reading lean.

The only thing I can think to adjust further is the float height. I set them all to 14mm above the gasket base. Would a out of spec float height cause this to run like this?

Haven't checked the valves but I figured if it starts good and idles well bad vavles wouldn't just cause issue under load after 5k rpms?

I am pretty mechanically enclined and very familiar with carburators but I cannot figure this bike out and why it won't rev out under load.

Any suggestions of things to try? Thanks in advance.

 

Two things jump out at me - new needles - these are what regulate the amount of fuel in the range your having an issue. Are they the same as original? Why did you change them? When did you fit them? Before or after you discovered the issue? Many aftermarket "kits" are not correct.
Secondly, the air filter. This could drop the upstream pressure in front of the carbs, affecting the lift of the slides (they will lift higher sooner).
So, carbs off again, check everything non original against the originals. If correct, buy a filter.
Lastly, it has been known for the tci to cause such issues, but my guess is your carbs.

 

Thank you much for the reply. When i said needles i meant the air fuel mixture needles for the idle circuit I set these to 2.5 turns out. I had an original filter and it was not exactly clean so I thought it was restricting air flow causing this breaking up problem so I ordered a UNI and it made no difference. What is this tci issue you mentioned?

 

I found one on Ebay and I am tempted to purchase it just because you mentioned it and the thought my troubles might be fixed with plugging something in.

Is it common for these issue I'm having relating to the TCI?

Would this only effect when the bike is under load? It will rev freely the entire rev range in neutral.

Is there any way to test this theory or do i just have the bite the bullet and buy it.

Would this cause the plugs to look lean? My plugs look slightly lean. I assume if this TCI is bad it would be struggling to send spark, wouldn't that make the plugs show black and rich if we are loosing spark?

 

I wouldn't rush out to buy a tci -as far as I know you can't get a new one, so it would be a lottery.
So you didn't change the needles, just the idle screws - this removes this as a possible issue.
Are you sure the stumble was there with the dirty air filter?
Also, did you make sure all your air jets went back in the right place under the top hats (common mistake to swap the air jet for the idle jet).
Did you check carefully all the diaphragms are good?
Lastly, the fuel levels - these should all be 3mm below the float chamber join if I recal - use a clear tube held to the side of the carbs to check, i do it with engine idling, but be careful. This could indeed cause your issue. This is fuel level, not float level...
Oh, and before someone else beats me to it - stop double posting, it will only confuse respondents.

 

Yes I am sure it broke up with the original filter installed which is why i purchased and installed the new one.
As far as the air jets i put them back where I got them can anyone confirm if they guy before me had them in the right spot? I have attached a picture below.
I checked the diaphragms but not carefully, I will look them over again.
I will also check the fuel level again I had it close before but I will check again.
I believe the fuel height for an 85 Xj700 is 1 mm below or above not 3mm correct? From the image below to me it might look like the fuel is 2mm or so to low? (refence on height below from Xj forever's reverence sheet for an xj700)
My apologies for double posting i tried to delete the old thread I could not figure out how to change the title of the post so i double posted like an idiot.

 

The jets are in the right places.

 

Thank you for confirming

 

Good pictures.
So, fuel levels (I assume all of them similar) are good (my manual says -3mm - no matter, they are close enough not to make it stumble).
Needles haven't been changed - (by you - pull one out and check the number).
You'd have seen a hole in the diaphragm of any of them, so we have to assume all good.
Someone check those air jets are correct - not trusting my manual.
So that leaves the main jets and or emulsion tubes, after that its tci...
The point being anything after a quarter throttle, engine on load is fed from the main jet, emulsion tube, needle. So if these are all correct and fuel level is there or thereabouts it should run cleanly.

 

Sounds good.

What makes me question it being the TCI is it revs just fine to redline in neutral. When the bike is under load this would not require *more spark* or electrical draw than it does in neutral so I don't know how it could effect under load but not neutral revving.

Let tear down process begin again (for the 5th time).

I will double check all the fuel levels to be sure they are the same and be sure the main jets and emulsion tubes are perfect condition. I have 3 spare carb kits and they have new emulsion tubes but i was worried they would be different but maybe ill try throwing a set of new ones in. I wire wheeled the original ones and blasted through all the holes with break clean but maybe its lined on the inside with junk.

I will check back with my findings.

 

Any idea what the number should be on the needles?

 

Just verify that all the little holes in the emulsion tubes are clear. Shouldn’t need new ones unless they look damaged

 

My 85 XJ700 had a UNI filter when I bought it. No matter what I did it never ran right until I replaced it with a stock paper filter. The UNI filter flows a lot more air than the paper filter and may be leaning out so much that you are getting an extremely lean mixture at high rpm under load. In any case the stock filter will help with your tuning.

 

You can check for reliable and correct spark/ignition timing with a 30 dollar strobe timing light, although some will not work reliably at the revs you struggle. A spark tester may be more reliable. And generally cheaper.

If it is a TCI issue, it must mean that there is spark power issue, meaning spark fail under high load conditions. Not sure if a TCI control voltage output or not, electrons are not my force. But a spark tester should verify - I find it hard to believe that a strong spark at idle will falter at higher rpm.

 

Noah i think you and i have the same problem i have a 81 xj650 my thread is under intake boots ..well thats how it started, i get a surge feeling at 7k .. accelerating from stop hard i get a bop... bop bop feels like spark is getting cut off

 

Stock XJ700 airhead needles are Y-20

This is a great reference:
https://www.xjbikes.com/forums/threads/xj-parts-catalog-section-c-fuel-system.44642/