FJ600 Jetting Dilemma

I have an '85 FJ600 and looking at the jets am a bit confused. The bike is stock except for a K&N air cleaner in the stock air box. It came with the following jets, which are stock as per the factory service manual, in two different locations in the book, so not a typo:

Mains: #105 (Cyl 1&2), 102.5 (Cyl 3&4)
Needles: 4CP4 (Cyl 1,3&4), 4CP6 (Cyl 2)

My question is why have larger main jets in the left side of the engine than the right? And why a different needle only in Cylinder 2?

If anything, I would think the middle cylinders could benefit from a richer main jet due to running a little hotter.

Any thoughts?

 

Possibly due to airflow...... some of the 400’s required mixed jets. The Turbo, does, too, I believe.

 

That would make sense if it was the center cylinders with different carb-to-box boots and position relative to the box geometry, but not left side vs. right side as they are pretty much reverse images of each other.

 

I suspect a typo, probably should have read 1/4 and 2/3. Never heard of different needles though

 

After looking at this some more, I think hogfiddles was on the right track. The back half of the airbox with air filter is offset to the left side of the bike with the battery on the right side. Even though there is a large full-width chamber past the air filter, it is being supplied by a port on the left side, so incoming air has a much straighter path to the left cylinders.

I think Yamaha was trying to maximum performance out of this bike due to competitive pressure from the Honda and Kawasaki and the FJ600 was popular on racetracks in a nearly stock format due to how Yamaha engineered it.

I am going to play with my jetting some and see what I can learn. I want to go a little richer on the mains and needles to see if I can get it running better. Finding mains is not a problem, but needles are almost impossible to get. After hours of searching on the internet, I finally found a source for 4CP7 needles, which are quite similar to the 4CP6 needle in cylinder 2, but with 5 grooves for a clip. With the clip in the 2nd groove, the needle is leaner than the 4CP6 and it the 3rd groove, it is richer. The taper is the same, so with a 0.6mm shim, it would perform the same. I found these needles at:

moto-lab 4CP7 needle

My plan is to go up one size on the mains (107.5 in cyl 1&2, 105 in cyl 3&4) and run the 4CP7 needles with the clip in position 3 for cyl 2 and position 2 for the remaining cylinders. From there, I will tune, see what works the best and report back with results.

Chacal has been a huge help and I will be sending him an order for parts once I finish going through my carbs and figuring out what needs to be replaced.

 

Just a quick data point. I now have the 4CP7 needles in hand and they are identical to within 0.001" to my USED 4CP6 needle that has 5K miles on it and has been cleaned in a carburetor dip and an ultrasonic cleaner. I also bought another 4CP6 needle off eBay and will compare against it when it arrives.

The middle clip position on the 4CP7 is the same as the single clip position of the 4CP6 needle.

At this point, I feel like the 4CP7 needle from moto-lab is an excellent replacement for the 4CP6 needle that chacal recommends on his fuel system pages.

NOTE: This info supercedes what I said in the previous post that I can no longer edit. That data was from needle info sheets and I feel that my actual measurements are more relevant.

 

Interesting stagger on the jets.

On a side note, the Fazer 700 has a unique airbox inlet tube set up that requires 102.5 mains on 1 & 4 and uses 105's on the 2 and 3 cylinders. Also, the pilot air jets are 130 on 1 & 4 and 135's on 2 and 3.

I agree with you that the center cylinders will run hotter temps, whether they're air or water cooled, and need a richer mixture to help with cooling. On my Maxim X, I run 115 on 1 & 4, 117.5 on 2 and 3. Plus some other tricks too.
They have Mikuni BS33's, so very similar to your's.

Also, nice find on the "adjustable" needle jets, and I'm sure you'll have fun with tuning. My guess is the stock jetting that Yamaha set them up with has gotta be close. Remember to make only one change at a time to see what improvement you make, or don't.

If you haven't done them already, valve clearances should be checked, and any vacuum leaks need to be addressed. Don't forget that your ignition system needs to be in tip-top shape too.

Tony

 

After many trials and tribulations bringing this bike back from a lack of adequate care and then 10 years sitting in my garage, it is now running and I can begin fine tuning the carbs.

So far, I have rebuilt the carbs, front forks, replaced or rebuilt everything in the brakes, adjusted the timing chain, checked the valve adjustment, replaced the tires, repaired and cleaned the K&N air cleaner and a bunch of smaller tasks as well as a whole lot of cleaning.

It seems to be running and riding like it should, so now for the fun of making sure I got the jetting choices correct.

 

I am coming back to this thread because I have run into a new issue. I have been quite happy with how the bike has been running with the jetting described above.

Unfortunately, I have a new problem that has appeared over the last month. I first noticed it on an early morning ride where once the engine was started, taking the choke off became difficult because the engine would take longer than usual to support running without the choke. A later warm start was much better, but not quite as easy as usual. I didn't ride the bike for a couple of weeks, and the next time I tried to start it, whenever I started to remove the choke, it would not run. If I revved the throttle, it would run at 3,000 RPM or higher. It finally cleared up enough to try and ride the bike, but it was always quite flat coming off idle with significant hesitation until the revs got up a bit.

About a week later, I tried again and it was even worse, to the point that I was never able to get it to run without choke unless over about 3,000 RPM.

Thinking it was likely to be a carb problem, I removed the carbs to take a look at them. I pulled a float bowl to check the enrichment jet in the float bowl. The bowl was clean and the gas looked fine with no sediment at all. The float needle looked good and I did not see anything that was a potential problem. I also pulled one of the diaphragm caps to look at the diaphragm, which was fine. I then did a drop test on the pistons with the carbs fully assembled. They were all equal and took 1-2 seconds to fall.

So, at this point, I am not sure what might be causing this. It seems like maybe a float level problem, so I am going to bench test that next. They were correctly set using a clear hose when rebuilding the carbs a couple of months ago, so I am not expecting to see a problem there.

A few months ago, I did take apart my petcock, clean it and reassemble with some spare parts I had on hand. I have noticed a slight dripping when switching between on and prime, so perhaps there might be an issue there that is restricting fuel flow. Since this bike had not run in 10 years prior to my partial restoration, I added a fuel filter to the gas line and it was not showing any dirt. I plan on removing it when putting things back together again, especially since I have never been able to get all the air out of the clear filter.

Any suggestions for things I should look into are most welcome!

 

You should look inside the little well in the bottom of the bowl with a strong flashlight, to make sure the bottom of the well isn't clogged (this well is where the brass choke tube drops into the carb bowl). Same with the brass tube.....you'll need a thin wire to probe it (them) and make sure they are clear, as well as a tiny hole in the SIDE of the brass tube(s)....an air vent.

P.S. how do you know when your starter jet is really, truly, zestfully clean? One of two ways:

a) shine a strong penlight or mini flashlight into the bottom of the bowl, where this jet passage "intake" is located. Look through the top of the bowl down into the jet passage "outflow" passage (this is the passage that the brass suction tube in the bottom of the carb body actually fits down into). Focus your eye carefully on the jet opening and make sure it's clean. P.S. it helps to do all this while in a darkened area.........see the picture "starter-jet.jpg" on page 2 (post #23) of this forum thread topic for a great image of what you want to see!:

http://xjbikes.com/forums/index.php?threads/4050

and post #5 on this thread, the starter jets is quite visible on the lower left side of the image:

www.xjbikes.com/forums/threads/i-think-i-know-why-the-sequel.116988

 

Len, that was all done when rebuilding the carbs and checked on the float bowl I removed.

Do you think the symptoms point to an idle circuit problem more than a float level one?

One thing I did not do when rebuilding the carbs was to replace the enrichment plungers.

 

Don't worry about air in the filter, that is normal. Keep it on for safety if it is the sintered brass type.

 

I would check the jet in all the bowls first. Make sure something didn't happen.

 

Done on all four carbs as per Len's procedure above.

I think this was mostly unnecessary as the carbs were immaculate a couple of gas tanks ago and never run without a fuel filter.

It seems to me that this problem is affecting all four cylinders, not just one or two.

One reason I think I need to recheck float levels is that we have E10 gas here and the new float needles I used were NOS Yamaha ones that are from pre ethanol days.

 

The fuel filter I used is a paper one with a good flow rate.

 

Well, the fuel levels would have to be REALLY low to cause the symptoms you are describing, unless it's really cold outside choke should basically not be needed, at all.........the bike runs off the idle circuit. If the choke tube and bowl wells are clear, then the idle circuit may be presenting some type of issue. Although if you just went thru the carbs recently, and it was running okay before, it's hard to understand how they may have gotten blocked so quickly, but things happen.

One easy test is to remove the inline fuel filter and try again, who knows, if the choke is getting fuel then the idle circuit should begetting fuel, too.

Petcock: leave it in the PRIME setting, see if fuel flows and flows (remove hose to carbs), and if so, try the bike with it in PRIME setting, see if that makes any difference.

Pilot circuits clog up really easily since they are so tiny (internally), maybe you got some bad gas somewhere............

 

Over the weekend I had some more time to work on this. While I had the carbs on the bench, I checked the wet float levels and all were at 3mm as per the factory service manual. After checking the in-tank filter screens, I removed the inline filter, clamped off the vacuum line (in case there was a leak at the petcock) and set the petcock to prime. None of this made any difference.

I think it is valuable to go back to the basics at this point:

1. The bike went from running properly to not being ridable over two short rides a few weeks apart. During this time, there was no gas added and no work done to the bike. The temperature dropped a bit, but not significantly.
2. The engine will not idle and needs the choke on to run at 2-3K RPM with no load. If you give it throttle, it will rev cleanly once over 4-5K RPM, even if the choke is removed. It is not able to run at all with a load.
3. The engine revs drop cleanly when letting off the throttle. I am not sure if this is because the mixture is OK, or if it is so far off that combustion can not be supported.
4. Once the engine has run for a few minutes but not long enough to really warm up, it will idle for a couple of seconds when first started, then stall.
5. The engine will sometimes idle for 10-15 seconds at no throttle, but at very low RPM and then stall.
6. The carb bowls are not showing anything other than clean fuel, the enrichment tubes and starter jets in the bowls are clean. I did not verify the holes by the butterflies in the throats, but based on the clean bowls and having a decent fuel filter, I cannot see any real likelihood of this being a problem.
7. It seems like either all the cylinders are firing with the choke on, or none are firing at all. It also feels like this is happening at 0-1/4 throttle position below 4K RPMs.

My gut feeling is that this problem is across all cylinders and not carb related. Perhaps I have an intake vacuum leak?

FWIW, I am the original owner of this bike and it has never been easy to start, always needs the choke for a couple of minutes and I have usually needed to begin my ride with the choke half on. Once it warms up a bit, it has run quite well though I always thought the jets were a touch small, which led to my minor changes outlined above.

 

I put this on a back burner once the weather got cold and recently started working on it again. I am having trouble blaming this on the carbs as they were properly rebuilt and running correctly with little chance of any new fuel contamination. Before doing anything else on the carbs, I decided to check the ignition system. Here is what I found:

1. Primary and secondary circuits on the coils were within spec.
2. The TCI pickups are within spec.
3. Only one of the plug caps is within spec and the rest of them are flaky. These are the caps with removable resistors and it looks like there has been some internal arcing going on with at least one of them. The resistors are not within spec when removed, so no amount of cleaning will fix this and I will be replacing them. That said, none of the resistance values were so bad that I think they would have kept the bike from running better than it has been.

I also notice that the #1 cylinder intake manifold did not look like it was properly sealing against the cylinder. I was able to slip a .002" feeler gage though the gap with a little resistance, so it looks like the intakes will have to come off and be resealed.

Once I get all this done, I will follow up with what I have learned.

 

Just wondering if you checked your valve clearances?

 

Yes, they were already correct and I adjusted my timing chain at the same time.