84 FJ600 - Clogged Idle Circuit for Cylinders 1 and 2?

[mosedocta]

Observations: Recently got a colortune and noticed that I am not getting consistent combustion (read this as mainly no combustion with a sporadic flame here and there) for cylinders 1 and 2 during idle. I can still get the engine to run and have decent responsiveness when opening the throttle. When I do open the throttle then I get consistent/continuous combustion in the cylinder (yellow/orange flame). I also experience a rev/rpm hang until I blip the throttle and the rpms will drop back down to idle.

Thoughts on problem/solution: So my go-to thought on what's wrong is that the idle circuit for cylinders 1 and 2 are partially or almost clogged and therefore not receiving enough fuel to combust consistently. Since I can get the engine running, I've set the idle speed to ~1200 rpm based solely on cylinders 3 and 4. Once I get the rpms up, the main circuits on 1 and 2 kick in and give the engine extra power. Since the idle was set most likely high due to being based on only 2 cylinders, it wants to stay high now that it has all 4 cylinders combusting. Some how blipping the throttle interrupts the rpms enough for the main circuits of 1 and 2 to cut and then rely on the idle circuit and going back to almost no combustion, ie engine running on only cylinders 3 and 4.

Known: I know there is good spark in cylinders 1 and 2 because I can see it with the colortune. Diaphragm slide bores in the carbs have been polished and they "drop like a bag of rocks". Carbs have been synced with gauges. Valve clearances fix to within spec.

Would love to hear yalls thoughts on if I am on the right track to fixing this problem or not. I'm going to do a deep clean of the carbs no matter what but thoughts on other possible causes would be much appreciated. Thanks.

[Pat Conlon]

Welcome to our campfire...shall I call you Mose? We are on a first name basis here.
I love the little FJ600's although I grew up the the Honda CB500's. That little Yamaha FJ would eat the Honda for lunch.

I wouldn't change the idle to mask the problem. Half measures will avail you nothing. You want a smooth delivery of power...no surprises, no sudden surges.
Do an ultrasound cleaning on your carbs and then see what you have with your Colortune.

Clogged idle circuits are common, especially on cylinders #1 and #2 when the bike was stored on its side stand.

Cheers

Pat

[mosedocta]

Hey Pat, yeah 1st step is to do a thorough cleaning of the carbs. Don't have an ultrasonic cleaner but might invest in one, was going to do a carb cleaner soak. I wouldn't say I'm changing the idle to "mask the problem" just that I'm setting the idle to its current position so I can keep the bike idling with whatever is going on in cylinders 1 and 2 and still troubleshoot/check other systems. I also need to check the jetting because I've just been rolling with what the previous owner had in there and I'm not sure if they compensated for the 4-1 exhaust, pod filters (bought it with the 4-1 and pod filters) or Colorado elevation.

[Pat Conlon]

You may want to send those carbs to Robert at RPM (Oakdale, CA.) He does excellent work on ultrasound cleaning and repairing/rebuilding/rejetting our BS36 FJ carbs.... If you do, call him first to see if he can accommodate the smaller carbs used on the 600.
Robert is highly recommended by many forum members.

Cheers

[twood1972]

Hey Mose. I've got a 600 also. Love the bike. Had it 10ish years and it's my go to around the neighborhood and local back roads. I've just installed pods and a stage 1 jet kit. Winter lasts for a long time here and I did the pods in December so I haven't road tested it yet. It does start well and blips decent as well as rev out. I never see any others on the road and always get questions at the gas station or at stop lights. Not a bolt I haven't touched on it. Hopefully I can be helpful in the future if you have any questions or issues!
Cheers!

[Old Rider]

I also had a fj600 about 35 years ago great bike!  6 speed gearbox and 73 bhp if i remember right  Have you tested to adjust the mixture screws ? i think maybe the cylinders that is not firing at idle is running too lean try to turn the
screws out a quarter at time and see if they fire up at idle. I remember that my fj600 was hard to start when it had been washed but back then i was not so experienced  so did not think of using some dielectric grease inside ignition connector  also should have used wd-40  or dielectric grease into sparkplug caps

[mosedocta]

Twood - Appreciate it, I'll definitely look your way if I have any other issues or questions while working on it.

Old Rider - Havent really messed with the idle mixture screws much, have them all set at 2.5 turns out as a baseline right now. Will see if turning them out more has any effect.

[twood1972]

Mose. I should mention, my FJ600 likes squeaky clean carbs. Have had to adjust float height multiple times also. Fuel line routing needs to be spot on "no kinks or tight radius". Petcock is problematic, plug's just need to be replaced every season for some reason and I feel the ignition system is inherently weak. Are you getting good spark at idol? Mine also likes a strong battery. Any of the issues mentioned can affect idle.

[mosedocta]

Twood, thanks for the advice. My fuel lines aren't kinked and I put in a brand new petcock, plugs and battery. I'm getting a good spark as I can see it through the colortune. I'm gonna do a thorough cleaning of the carbs and check for air leaks at the boots and throttle shaft seals.

[Old Rider]

I found this tread made from a member  Paul G may be useful https://fjowners.com/index.php?topic=17039.0

[mosedocta]

Got the carbs completely taken apart and soaking. The previous owner had size 115 main jets and the service manual calls for 105 for 1/2 and 102.5 for 3/4. Ordered new main jets, pilot jets and pilot screws. All other jets looked within spec and very clean.