loss of power

Hey guys,

I went for a long ride today (1982 XJ750 J 17.5K miles, carbs need work, idles with a little choke) and I noticed a loss of power in comparison to previous rides this season. No modifications were made, however this is the first real ride after an oil change. The engine revved up just a quickly as it always has, but the acceleration from a dead stop was significantly slower. Acceleration in top gear (like from 50 to 60 MPH while passing) was also noticeably less. No other problems were noticed, just a general loss of "balls". Anything immediately jump to mind?

Thanks,

-AM

 

sounds like your in no hurry to clean the carbs, so put a half can of seafoam in a full tank of gas and go for a ride
when you get close to home with a quarter tank put the rest in
let it set overnight, fill it up, cross your fingers and see if it helped
significant loss of power might be one cylinder not firing
one pipe will not be as hot as the others

 

Thank you, sir. Any other thoughts?

 

using that choke so much probably fouled a plug, pick up a set when you get the seafoam
whats this worst roads stuff, you either live in PA, or have never been here

 

The roads in PA might be bad, but the combination of worst roads and highest taxes definitely belongs to New Jersey. If only because we have the highest property taxes in the country. In reference to the plugs, they don't get fouled because adding a little bit of choke seems to correct some "lean" condition I'm experiencing. It's not actually running rich. I know I need to work over the carbs. I'm just happy I can ride the damn thing now, and I'm afraid that If I start messing with it I won't be able to ride at all.

 

Throw a set of new Champion Plugs in there and take it somewhere where you can run the bike over 45 MPH and then hit the Kill Switch without being a traffic hazard.

After you roll to a stop ... let the bike cool a bit and then pull those Plugs and "Read" them.

I'm going to hazard a guess, here, and predict that they are going to be pretty dark in color.
If they are ... you have to Lean-Out the Mixture a little bit at a time to get the Plugs looking "Light Brown"

Don't mix-up the Plugs.
One Cylinder may need LESS of and adjustment than the others.

Adjust the Mixture Screws IN ... the width of a Nickel ... not more than
2 or 3 Degrees. Replace the Plugs and ride some more.
After you do this exercise a few times you'll manage to get your Mixture Dialed-in to where the Plugs are very-light Brown or Tan.

The Bike will be running real good about the time you get Dieled-IN to where the Plugs are Tan to Light Brown.
Tan to Light Brown is GOOD.

Clean is NOT Good. Clean is deadly to your Engine.
The Bike will run GREAT at some point where the Plugs are LEAN.
But the Air-Fuel Mix will be so explosively HOT that it could melt through the top of the aluminum piston. (Among other things)

Become a Tweaker.
Tweak your Bike until she's running GREAT.
Just remember one thing ...

The screwdriver you use on the Pilot Mixture Screws better FIT the Slot with NO side-to-side end-play.
The wings on the Screws will bend right-around to fill the gap without the body of the screw moving at all if the screwdriver doesn;t fit the slot with GREAT precision!

Always add a drop of oil or two to the Pilot Mixturue Screw Holes to keep them lubed for Tweaking.

 

Thanks a lot Rick. That sounds like a pretty straightforward procedure. However, I've always been told never to remove spark plugs from a hot engine...something about how the head could crack around the empty hole as the engine cools. I guess this isn't an issue with the XJs?

-AM

 

Uh,
"After you roll to a stop ... let the bike cool a bit and then pull those Plugs and "Read" them."
Rick did mention the need for cooling. Removing the plugs out of any hot aluminum head is asking for trouble. Still, the need to let the engine cool down a bit cannot be over emphasized. Ease your way through all of this with a dab of anti-seize on the threads, it will help with plug removal down the road (literally). Best of luck hunting down the gremlin!

 

i see a carb rebuild comming

 

There is a list of things that will eventually bite ALL these bike. Fuse Panels, Alternator Brushes, Starter Problems ... and the list goes on. We read about them in these Forums every night.

The Carbs are on the list. If you have been years without a tear-down and careful refurbishing; the Carbs will cause trouble.

Micro-particulate small enough to pass through the Float Valve Body Beenie Screens and collect in the bottom of the Fuel Bowls needs cleaning out as do the Pilot Jets and Fuel and Air Passages that are introduced to the stuff.

Beyond that is the inescapable passage of time that causes Aluma-oxidation.
The Bores that the Diaphragm Pistons rise and fall in shrink due to the Bores being affected by the oxidation process. When this occurs; the Pistons begin to drag, hesitate or in some cases bind causing the Bike to not respond to Throttle.

The only way to correct the problem is to resurface the Bores. Using a ScotchBrite Pad or Ultra-fine Finishing Papers; the oxidation needs to be removed to allow full and unimpeded travel by the Diaphragm Pistons.

I used to sand the Bores by hand until I got the "Wand" attachment for my Dremel Tool.
Now, I make mini-discs of ScotchBrite and mount them on a Dremel Sanding Bit and scrub-off the thin layer of oxidation until the Bore is CLEAN and the Piston will rise and rall with a clunk ... ("Clunk-Test).

If you finger the Piston movement and there is any drag to them ... the Bore needs to be Cleaned-out.

Taking it one more step for those who send me Carbs to Cleann and overhaul ... I Polish that surface to a mirror finish; guarantying that the Piston is going to immediately respond to any differential in pressure caused by the Intake Air rushing below the Piston.

The resultant factor of Polishing the Bore (along with Fine-Tuning the Pilot Mixtures) is a highly responsive Engine that will accelerate with the best of them and offer the rider the utmost in Throttle Control.

It's a Race-Prep Step.
But, if you have the Tools and the patience ... you can get the Carbs to perform just about as good as if the Bike were Fuel Injected!

Just about.
Close.
Real close.

 

I had a similar problem, but there was a significant loss of power. Full throttle fourth gear would only go 60mph. Wont idle. Idle's with full choke at 1200 or so rpms. Died on the highway (while full throttle fourth gear at 60mph.) Wouldnt restart. Grabbed trailer to bring her home. Going to start trouble shooting tomorrow. Judging by this thread, the carbs are a good place to start. Then I will try the spark plug work as mentioned earlier in the thread. Wish me luck, Im a virgin!!

 

It's a big mistake to think that the Carbs will perform like new after more than 20 Years on-the-job without tending to the Bores.

Even Bikes that have never spent a day not started and run can have Bores that have the sticking problem.

Because the Bike manages to run pretty well without the smooth Bore, some will act and behave in some pretty strange manners.

Bikes that suddenly run Rich.
Bikes that suddenly have no pull.
Serging forward and gaining revolutions unexpectedly are the symptoms.

That the problem sometime goes away when the Bike gets warmed-up good yet rides like a stranger to you before getting up to temp is another sign that you might have your Diaphragm Pistons sticking ... or "Shuttering" up and down.

It's something that you can feel as a performance problem that mimics a Cylinder misfire ... but, the Plugs look good because the Piston eventually allows the Cylinder to fire as it normally does.

 

Would it make a difference if the above situation happened only in the rain?