XJ 400 - A Few Questions

1: How to Reduce Heat on a Yamaha XJ400 (YICS) Motor? Any Possible means to reduce as much as possible plz.

2: I Observed all 4 NGK D8EA Plugs installed on my machine & it seem's my bikes running pretty Cold atm, Was Wondering if there any easy methods on increasing the fuel intake on it? Considering it Pauses at approximately 120 KM/H or 55 M/H, No matter what gear it's on, The throttle isnt being utilzed to the full too, just above half i figure, On Old CD70, CG125, CD125, CC175, CD175, CD200, I Used to increase/decrease by adjusting the Locks located on the Slide Pin on top of their carbs, So wondering how to go about it on this particular XJ400 of mine.

Appreciate all thought's, Ty

 

Ty:

You might need a bigger bike! That, or you'll have to do some experimentation with Main Jetting and Needle Valve height at Wide Open Throttle.

(If I understand you're Post to mean you aren't getting enough fuel for the power you need for highway speed.)

 

I had an old XS400 - 120-130kmh (if you got down nice and low with a tail wind) was as fast as it would go. More fuel, better fuel, nothing would make it go much faster. 400cc is just not meant for racing

 

That depends on what sort of 400 you're talking about. The OP's bike is a 4-cylinder, not a twin.

I weigh 250# dressed to ride, and have been known to disappear on smaller riders with larger bikes when I'm on an FZR 400.

A decent stock one will do about 130 mph.

Modified ones are even faster

 

Mine is a XJ400 YICS Machine, thing is all 4 Brandnew NGK D8EA Plugs are as white as they could get, The bike seem's to be be consuming less amount of gasoline too, So im assuming the previous owner had got the carbs adjusted to give as much as mileage possible in less fuel, ofcourse sacrificed the performance along.

Now apparently i need that performance back by re-adjusting those mixture screws & slide pin locks inside the carb's, Any Suggestions/Tips regarding the issue would be much appreciated. Thanks

 

Exactly, The Plugs appear to be running very cold, Nor the bike is anywhere close to the true performance potential it has, Simply fails to go anywhere beyond 100 KMH. I assume It's the Carbs which need to be adjusted to increase that Fuel intake.

Btw, The Engine is as sound as it can be, In a Pretty healthy state as well, i don't see a reason why it cant do over 140/150KMH, Considering The Cams, Cam sprocket/Timing chain/Roker or Tappids/Pistons. Simply all are in perfect conditon.

 

Well, then:

Now, we need to know ...

Present setting of Mixture Screws Out
Actual size of Main Jets presently installed.
Relative position of the Vacuum Needles and where they are set.

I'm thinking:

Pilot Screws OUT to --> Between <-- 2.75 and 3.0 Turns OUT
(Not 3 O'clock -- Not 12 O'clock ... 1 O'clock with any err on the 12 side.)

Stock Main Jets replaced with the next larger sized Jet.
Raise Needle (If adjustable) --> 3/4 of a turn from factory.

See how that goes down the road.

 

White plug insulators aren't necessarily a problem, and there is more to reading plugs than simply looking at the color. Try using 9s if you like, although I think they'll be a little cold for street use.

BTW- your plug's heat range will have zero effect on the engines' operating temperature. Calling a plug hotter or colder only refers to how quickly the plug can reject heat that comes from the engine itself. The spark plug only initiates the combustion event- it does not add significant heat of its' own.

How to read spark plugs

The fact it has weak top-end indicates that it may be lean, but there's no point in doing any adjusting until you know what jetting you have in there now.

Is the rest of the bike stock, and what flavor of carburetors does it have?.