XJ750 with NO low end power and stalling

Hey. I'm new to this group and my 81 XJ750 Seca. Bought it as a "project " bike. Now certified and on the road after replacing brakes, all fluids, seals and some repainting. Changed bars to clubmans and huge difference in comfort and handling. I'd recommend this to any with same bike. I have a couple of issues. (lots, actually, but only a couple that are bike related...)
1. I have no low end power under 2000rpm. After the bike warms up, I can barely keep it running at idle. Once she dies, it won't restart until cooling off. I've checked pilot and they're clear. choke doesn't seem to be sticking and when I open the choke at low idle once it's warm, it makes NO difference in keeping the bike running. Fuel petcock works fine at both ON and PRIME positions so I'm getting fuel. I also changed the plugs. Made a minor improvement for a couple rides, then back to same issue.
Almost seems like I'm flooding it, but there's no gas smell.
Any similar experience or suggestions ?
Also, front brakes very spongy. I've bled them a couple times, changed pads and even cracked the banjo bolts under positive pressure to get rid of all potential air pockets. Maybe these brakes were never very firm..
Thanks

BTW - I'm in South Mississauga (Clarkson)

 

Welcome. First of all, keep in mind that the bike is over 30 years old. Things need done.
Check valve clearances. Zestfully clean and rebuild carbs. Set floats both dry and wet. Check for vacuum leaks. Adjust and sync carbs.
As for the brakes, rebuild master cylinder and caliper. Replace brake lines, preferably stainless.
You have a lot of work ahead of you. Believe me, it's worth it. Take your time and be patient. That way it's done right the first time.
There is oodles of information to found here, so don't be afraid to ask questions. Everyone is here to help.

 

Welcome ... First thing are you running stock airbox or the dreaded Pods? If you are running pods your jets will need to be changed up, 120 are stock main/ pilot 40 jump up to 124 main 42 pilot . Yes carbs will need to be removed to change . Your Brake hoses are they original ? look for a MFG date if they are from 82 they need replaced , recommend upgrade to steel braided brake lines ( xj4ever sells them look at right upper corner click it will take you to a happy place all the parts you need) DO NOT buy used brake lines from ebay etc .!!!! Your safety and life depend on this!!!! Good luck

 

Thanks. Running stock air filter. Syncing carbs is a good idea. Do you know what pressure I should see ? I dod suspect it's either carbs or combination that and valve settings. Valves are fairly noisy and probably way off.

 

Actually noisy valves are happy valves....I would do a compression test with all plugs out and throttle wide open while cranking see what reading you get . Autozone can rent you a tester unless you have one good investment (Hint buy one) then I would check valve clearance. Get a good metric feeler gauge ,after you pull tank remove valve cover. Remove cover on right side this will give you ability to turn over engine,since plugs are out makes it easy to turn over. Take a piece of paper write down Intake/exhaust and cylinder number. Put lobe of the cam on #1 cyclinder all the way down, take a medium size zip tie bend the locking tab at a 90 degree angle . Put it in the spark plug hole towards the intake side(Carb side) you want to put in towards the valve ,turn over the cam away from bucket, pull lightly on the zip tie if you did it right it wont come out. Remove shim see what number is on the bottom go to the chart in manual if any valve is out ( say you have a 280 you might need a 275 ) it all depends on your measurement .....There are several posts on adjusting valves....Hogfiddles,kmoe ,Polock and several others will be able to guide you along ...good luck

 

You need to go to The Church of Clean. Doesn't matter what was done before, or by who. you have crud in there and it must be removed.
Wet set fuel levels.
Bench synch carbs.
Adjust valves pt 1.
Adjust valves pt 2.
Almost everything else.
Brakes must be rebuilt top-to-bottom in order to be safe. I'll wager that the brake lines are original, which is where much of the spongy feel is coming from. Rebuild the master cylinder, calipers, and get new brake lines (braided stainless is preferred) along with new pads and shoes. Being able to go does you no good if you can't stop (or if a wheel locks up in the case of the old rear shoes).
The anti-dive units in the forks can get gunked up with old brake fluid, so here is an exploded view of the inards so they can be cleaned out and put back into working order http://www.xjbikes.com/forums/posts/368361/ and a how-to http://www.xjbikes.com/forums/posts/159261/. lots of parts, but not terribly complicated.

 

Thanks for this. I wish I had those fork assembly pictures BEFORE I replaced my fork seals and cleaned the forks. lol. THe service manual is almost useless for illustrations.

I checked my valves (cold) yesterday and ALL (surprisingly) well within tolerance. I was sure this was the issue. Bike will barely start now. I was expecting to find tight exhaust valves.

Next is a thorough carb cleaning. Thanks for the carb cleaning article. I already checked pilots and diaphragm (can check mine by removing top) and all looks good, but maybe gunk in basic causing problems... I'd rather be riding...

I'm starting to wonder if my CDI is weak. I'm told that's not very common, but possible.

YES to changing the brake lines. That's high on my to-do list. Already did a good caliper cleaning and fresh DOT 4 fluid. One thing that helped was craking the banjo bolt connections under slight pressure. Seems that small air bubbles like to hide there.

Any good links or articles on making my own carb sync tool (robber hose and a pressure gauge ) ? What pressures should I be seeing and how do I adjust to sync them ?

 

Impossible. You have a TCI

 

No matter what you do it will never have any power below 2K.

 

What he means by that is you won't be pulling stumps with an inline 4.

 

The rubber brake lines are the spongy source, mine are the same. Regarding your running issue, it sounds very much like the float level is too high, and raw gas is getting dumped into the cylinder(s). That would explain (in my mind at least) why the fresh plugs helped for a short period- they were clean and not fuel fouled yet.

 

I'm back.. and frustrated.
I removed and cleaned the carbs. Removed all jets, carb cleaner, blah blah.
Found idle enrichment passages plugged and some varnish on the main jet screens. Got the idle passages clean enough to pass carb cleaner spray, but not completely clear.
Liquid checked float levels and all good.
Checked valve clearances and "bang on".
No obstructions in gas line.
Choke seats fine when off (not sticking open)
It does run somewhat better .
Bike starts fine cold with no choke. Over 2000 rpm decent power.
Little missing at 6000-8000 rpm, but not bad.

BIG hesitation from 1000-2000rpm that gets worse as bike warms.
After riding for half hour, The idle jumps from 600 to 3000 and back and forth.
It's very difficult to get rolling as it wants to stall until I run high revs.
Cruising along at 2000 or higher rpm is fine, but if I stall it, she won't start for an hour.
Then barely starts when I crank with wide open throttle.

It seems I am running too rich, especially at low rpm. I removed the air filter and that seemed to help a little. Removed some air flow resistance and negative pressure in the carb, thus drawing less gas ?
engine was rebuilt before I bought it and air filter is stock.
Is there a transitional jet or metering orifice in the carbs that manage the low idle to mid range that I need to adjust ? Why does it get worse when engine gets hot ?

I am fairly mechanically inclined, but can't see the fix on this one. Maybe carb sync ?
My mechanic says "nothing to adjust on the bike that will help". I'm thinking XJ Forum AND find a new mechanic ??

 

Your mechanic is a (insert epithet of choice).
Did the idle air screws get uncovered, removed, passages cleaned, o-rings replaced as needed, o-rings installed in the correct order, and set to 2 1/2 turns out from a soft-seat?
Were the needle jets (emulsion tubes) removed and cleaned?
Were the air jets put back exactly where they came from (the image in the service manual is mislabeled)?
Have you used carb cleaner or propane to check for vacuum leaks?

 

I hate to even ask...YOU need to do FULL carb rebuild take each carb off the Rails.Fully tear the carbs down IE butterfly shafts removed soak the carb (Berryman makes a gallon with a basket in container) soak for 1 to 2 hrs ...All butterfly seals need replaced(xj4ever has all seals ,gaskets.) After you put carbs back together , do a bench synch with a buisness card under the butterfly , mixture screws out 2.5 turns. Everthing I have said is in the church of clean. Using a can of spray carb cleaner just won't do as well as soaking. Leaking shaft seals can make for problems. I just did this to mine after this runs much better.

 

Hallelluia brothers and sisters of the XJ Forum!! I have attended the Church of Clean and I have seen the light !!! Praise the folks who put their heavenly efforts to this forum to save the rest of us wandering souls.

Full carb teardown, 24 hour soak and thorough clearing of all ports, jets, holes and anything else that dared to hold back what needed to flow.
She starts with minimal effort, runs well and has taken me at least 1000kms of easy riding since then.
Still planning on a carb sync. Pics attached very shortly (if I can make that work).
Many thanks to all who responded.

 

All right!!!!!!!