rejetting xj 750 carbs

how do I know if I need to rejet my carbs?

 

Stock jetting is based on stock intake, air filter, and exhaust, and running at sea level. If you're bike doesn't match one or more of these, then you may need to re-jet.

Other than that, assuming carbs are properly cleaned and in good condition, with no vacuum leaks, plug chops, plug reads, and dyno testing with a wide band O2 to check AFR will tell you.

 

SQLGuy,

after the fuel injection upgrade you did on your bike can you tell us what was the benefit?

Carvall

 

Main benefits are:

1. Just push the starter button to start, whatever the temperature, however long the bike's been sitting. No choke, and not really any need to warm up before riding. Warmup enrichement, and idle air, are actively controlled so that the engine is always getting just what it needs. Last winter I was doing some work on the bike and it was about 0 Fahrenheit in my garage. The bike still started with one push of the button and idled right up.

2. A pretty flat torque curve and some nice low-end pull... though I didn't dyno the bike "before", so I don't know what original looked like.

3. Less to worry about during storage (no float bowls to drain).

4. Better mileage, I think. I was averaging about 52mpg during a 250 mile day trip up in the mountains I took with some friends from work back in September.

5. Better handling of the kinds of altitude changes that are easy to get around here. I live at about 6500 feet, but an hour's ride puts me in Cripple Creek, which is over 10000 feet. With the separate baro sensor I added to my setup, the ECU compensates in real time for such changes in ambient pressure.

6. Since I'm using Microsqurt for both fuel and ignition, I have no TCI to worry about, and I can use off-the-shelf coils (in my case, GM Saturn coils) with replaceable ignition wires. I also have full control over timing advance based on load, engine temp, RPM, and a few other factors.

Down sides are, obviously, the cost of parts and time to build, and that I'm still in tinkering mode, playing around with new tunes and options as new firmware updates become available. Over the summer the configuration was pretty stable, and I didn't do much besides minor tweaking - and a lot of riding . However, a month ago, or so, I bought a wideband O2 sensor kit and upgraded to the most recent experimental firmware. The new firmware had some problems and, rather than swap everything back, I've been riding my other bike and waiting for a new update. That new update came out a few days ago, and I'll probably install it today and see how things look from there.

Cheers,
Paul

 

First a question.
Have upgraded your exhaust or intake?

If you have then take a spark plug out.
If it’s a light milk chocolate color then you’re OK.
If it’s white then you need to re-jet.

If you haven’t upgraded and your plugs are WHITE or BLACK then SEARCH is your friend, you probably have carburetor problems.

ab

 

I think the vast majority of our Membership would be very happy to have their Bikes tuned to a Moderate Degree of Fine Tuning.
Especially those who abandoned their Airbox and installed Pods.

Getting their Bikes to Start right-up, Idle, get-out of the hole and just Cruise without burping and farting is what most guys are shooting for.

There might be a few who'd like to get to a Stage of Tuning where Dyno Testing is needed to maximize performance.

But, there are simply not enough of the Membership with Bikes so well maintained and tuned that would require Dyno Tuning to improve performance without having spent a good deal of money and plenty of time prepping an Engine to the point where needing a Dyno Run to extract additional horsepower through Ultra-Fine Tuning is needed.

I suspect there are Members who might be just getting-by.
Very happy to have their Bike running just the way they've got it.

Probably one or two that a Redline Run-up on a Dyno would cause the Bike to have some catastrophic failure and have the Engine quit before the test was complete.

 

Hi Rick,

Sounds like you and I have different ideas about what dyno tuning is for.

For me, it's not about peak power at 9000 RPM. It's about making sure that the engine is getting the right amount of fuel at multiple ranges of RPM and load. Having the right setting of idle screws to give you a nice paper bag color on your plugs when sitting on the center stand is, first of all, probably a bit rich, and, secondly, tells you nothing about your mixture when you're in 4th gear doing 50 on the highway, or in 3rd, near WOT, climbing up a hill.

If you have a stock bike, at or near sea level, and in good condition, no tuning should be needed... the factory did it for you when they designed the bike. If you don't, then plug chops and color tune are going to give you some information, but not nearly as much or as accurate as you'll get by actually loading the bike and seeing what AFR looks like under all conditions. From there you can really see whether your needles, or your mains, are well chosen to work with the rest of the bike.

That said, engines will run within a wide range of non-optimal tuning. You can work with all sorts of mismatched jets, exhaust, air filters, altitude, etc, and still ride, and still have good longevity to the engine, as long as you're not too lean under load. The compromise usually means harder starts, less power, and worse fuel economy. Some people don't mind these, and some people do.

My .02, anyway.

Paul

 

Yes.
We differ.
My idea of what happens when a Bike gets Dyno Tuned is that the Tuner makes changes to the Software to reprogram the Timing and Injector Pulse Rate to a point on their curves that gives the Engine its maximum
Horsepower at the rear wheel.

Guys with Sporty Bikes crowd-around the Dyno Machine on Bike night to either have their Sport Bike adjusted and hearing damaged.
They pay for two runs.
Or, just have the Dyno guy run-up their Bike flat-out and print-out the Bikes Stats for those who care to wager their Bike got more balls than the next guys.
Also damaging their hearing and those nearby.

Running an XJ Bike on a Dyno is akin to spending dough for a Plug Chop.
The only thing you can do is turn the Mix Screws In or Out.

 

This is the thing I'm pretty sure I disagree the most with you on. The mixture screws have very little impact on mixture when you're running over 30% of max power.

There's a pretty good synopsis of data from the Mikuni CV carburetor tuning manual here: http://www.zrxoa.org/webpages/techinfo/ ... uning.html

I don't have a Yamaha factory manual, but I do have one for my GPz. It provides a very nice explanation about the different air and fuel circuits in the Mikuni CV's and which parts are providing the bulk of the fuel under different operating conditions. It's a good read, and much more in-depth than anything the Clymer or Haynes manuals get into.

So, yes, if you think the only thing to tuning an XJ is adjusting the idle mixture, then I could see why you don't think a dyno is helpful. I think it's much more important to get the needle, needle jet, and main jet right, since that's where you'd be most at risk due to too lean a mixture.

 

Now now you two. Lol!

 

"The only thing you can do is turn the Mix Screws In or Out."
gosh, i wish that was true

 

"If you no longer wish to watch this topic you can either click the "Stop watching this topic link" found at the bottom of the topic above, or by clicking the following link:"

click

 

Greetings
I hope you guys have not clicked this topic off yet. The discussion is good.

I wish I had fuel injection, but I have the stock CV carbs and I have a low rpm problem. Let me briefly describe and ask for feedback:
-1982 XJ750 seca. MAC 2x2 exhaust. Stock air box and paper element.
-Running sea level, CA. Weather is dry and 60F usually, summer 100F
-Plugs are lean.
-Main fuel jet is #120. Pilot fuel Jet is #43.
-Compression is 150ish in #1,2,&4. #3 is 145ish.
-Running vacuum sync is at about 7 inches on all 4.
-Color tune and the pilot mix screw will change color in 1,2,4.
*** #3 was dark, no flame. I can see the spark arcing. Then every 5 seconds or so the flame appears and is rich. Screw in mix screw and every 5 sec or so the color is blue.
*** #3 on the running vac gauge will lose vacuum to 4-5 inches when #3 shows a flame.
*** At high rpm(2000+) #3 flame goes rich for a second then tuns blue and stays blue. BUT, I know there is still a miss going on because at steady 4000rpm there is wavering in the exhaust note.

When I add a shot of carb cleaner to #3 vacuum port engine idling:
-Generally it speeds up.
-Once it died.
-Once the flame came up through the vacuum tube

The carbs are clean. The pilot mixture screws are clean.
I did not set the valves...this was my impatience at work. I will order parts from Chacal after this post to do the valve clearences.

What am I missing here: Valves? Timing-have not looked at this? Other?
Thanks, Greg

 

Gregu, I kind of think you should have started your own thread on this, as we now have parts of this issue spread across two other threads.

Anyway, per your question on the "runs on three cylinders..." thread: first of all you need to check (and possibly adjust) valve lash, then you can proceed with more work if needed for #3.

At that point, if you still have problems, you can pull the carbs and the intake boot, and you should be able to see the intake valve pretty well. From there, probably the ideal thing would be to get a lamp with a white LED on a stalk that you can put down into the spark plug hole. With the intake valve theoretically closed (cam lobe up), you should be able to look in the intake port and see whether there is light leaking through the valve edge. If there is, then you have sealing problem, and, by opening the valve (cam lobe a bit down), you'll probably be able to see whether it's carbon buildup that's causing it. If it is, then you can probably get in there with a bit of pegwood or something to clean it up. (Notice the extensive use of "probably" here... good luck.)

I've also heard of people using walnut shell blasting to clean out combustion chambers of assembled engines, but I'm not really sure how that works... probably set the intake valve open and blast back and forth between the intake port and the spark plug hole.

 

I agree!! I thing this disccusion is good. I don't know why people gets scare about it. As soon as it is respectful and I don't see why not to share their own opinion and still desagree at the end.

Thank you SQLGuy and RickoMatico for your input and passion about motorcycles. Let's do this las vegas style "Anything said here stays here" Out there we still riders and that is what counts even if your bike is running on just one cilinder!
Have a great Sunday!!

Carlos

 

Hey guys...thanks for replies.
Yes...could have started a new thread but hopefully my situation will help the others looking at a cylinder misfire.

I will dig into valve clearances. I get the impression there are ALOT of maybe's involved with valve cleaning in place. That said, if there is indication of a leaking valve, debris etc, I will be pulling the head to fix it correctly.

Besides, I can clean up the pistons and bores at the same time!

 

back to my topic. How do I know if I need to rejet my carbs.
I have a 1983 xj 750. I pulled the "snorkel" out of the airbox
and put sportster pipes on. It runs better now. It's got more get up and go
and runs easier at highway speeds and intown speeds. A little more get up and go also. Does not hesitate when I give it gas and does not backfire.
I have not check the plugs yet. I have read where owners ahve needed to rejet so I was wondering how do I know if I need to.

 

From your description ... you're pretty proud of how you've got it running.

If you can toe the line and take-off fast without a stumble or delay and run the tach right over to right on every shift without having the bike seem like your towing a piano ... you win a prize.

 

Thanks.
The bike does scream. No stumbles at all. By the time I hit 3rd gear,
which is pretty quick, It is pass 80 mph. It is surprising how fast the xj 750 is. Never thought that bike would be that quick when I bought it. I don't crank on it too much, this is my daily rider, work bike. My wife has a pickup and I have the bike. Got to keep it running.

 

Hey RickCoMatic.
Where did you get the windshield for your xj.

 

SlipStreamer.

It's a SlipStreamer Hellfire SS-24-V
(V indicates Round Headlamp Bucket)