running rough

[Tapartacus]

Hi all, anyone know what would make my bike run rough at 1500 rpm doing about 25 miles an hour? It runs great at anything above that and also idles totally fine. Now I know a lot of you will say well what are you doing running your fj at that speed? Well where I live there is a boulevard by the sea where you can only do 30 km and at that speed the engine just doesn't like it. I have an 89 that purrs at all rpms. Just redid the carbs with Randy's full kit (didn't replace the seals as I didn't separate the bodies). Had them synced at local dealer. It was doing this before the rebuilt, that's why I thought rebuild would fix the problem. It's quite pronounce when gearing down just before first gear (around 1500 rpm) It's very bothersome especially knowing how smooth I know the fj can run. Could it be something other than carburetion? Valves? Float bowl setting? Plugs? I'm at a loss here. Help

Andrew

[ribbert]

anyone know what would make my bike run rough at 1500 rpm . Now I know a lot of you will say well what are you doing running your fj at that speed?

You're right.  Change down a gear!  At 1500 rpm, you're just above idle and a hesitation, stutter, roughness, whatever just off idle under light load I would punt on idle mixture.
There will no doubt be other suggestions to follow.
Having your carbs synced by the local shop does not instill confidence in everything being spot on and is it possible they didn't do the mixtures? ( as you didn't mention it ).
There's plenty of info here on setting mixtures.
Noel

[SlowOldGuy]

In neutral, if you slowly give it slight throttle, do the revs begin to come up to about 1500 to 1700, then it stumbles a bit and the revs fall back down to about 1200, then with slightly more throtle it revs past that point and appears fine?

If so, then you've got one or more lean idle circuits (probably still some crud in there).  If you go back in, make the switch to #40 idle jets (from Randy of course).

DavidR.

[andyb]

While you're at it, a worn chain will cause a noticable roughness at low continued speeds.

[Tapartacus]

Thanks guys it idles perfectly, blip the throttle and it returns normally. Slight stumble when cold but when at operating temp absolutely no probs

[Tapartacus]

By the way if I went to the pod filters would I need to adjust mixture? And also what holds up the carbs when the stock air box is removed? As far as I can tell the pods would make adjusting and cleaning the carbs a heck of a lot easier. How ofter do you need to change them or can you clean them.
Thanks
Andrew

[Arnie]

Changing from airbox to foam dualpods will probably require raising the needle 1/2 to 1 groove, 1-2 mainjet sizes, and tinkering with the idle mixture screws. The carbs are held in position by the intake manifolds that are bolted to the head and clamped around the carb throat.  The foam filters can be washed, dried, and re-oiled with foam filter oil about as often as you'd clean or replace the paper filter in the airbox.  You'll likely never have to replace them.

You'll find that the carbs are much easier to get at for tuning and cleaning, and you get a nifty extra storage spot where the airbox was.
You do lose a bit of low end torque, but gain a wonderful intake howl when you open the throttle, and get a bit more top end power from them.

Arnie

[fj11.5]

+1 on storage space , somewhere to hide the airhorn, alarm system, and still plenty off room to rip the carbs out for tinkering 

[RichBaker]

I was getting lean surging at just off-idle, normal for any big-bore Japanese bike because of the lean idle and off-idle jetting for emissions... Fixed it by going with smaller idle air jets (when I installed the DJ kit, 1995) and bigger pilots last time I had the carbs apart(2008).

[Tapartacus]

Hi Rich thanks for the reply. Can you give me the numbers of those jets? I bought the 88-95 fj kit from Randy, supposed to be the right ones for a 92.
Thanks
Andrew

[fj11.5]

Sorry to highjack the thread, ,having a problem when backing off the throttle 5k and up,  bike seems to surge, like its trying to accelerate   , will check the sync tomorrow,  thinking i may need to use 117 main jets, 115 at the moment, needles set at 2 nd last slot from top,  had been very rich,  po had drilled out the jets to at least 150 size, maybe she likes a drink still

[racerrad8]

...I bought the 88-95 fj kit from Randy, supposed to be the right ones for a 92.
Thanks
Andrew

Andrew,

The jets in the kits I sell are the O.E. sizes that were original spec from Yamaha. You might need to change jets to get the best performance. But, even with those jets you should not have a rough running condition unless things are not properly set-up & adjusted.

What did you set the main needle clip?

How many turns out are the pilot screws?

Are the all even?

The reason I ask if they are even is some people, who don't work on FJ's, adjust the idle mixture screws to sync the carbs not the throttle plates. the carbs are then either rich or lean to compensate for the other spectrum.

Don't forget, the jets Rich are talking about are DJ specs and you have Mikuni specs now, that is like comparing apples and plums.

Randy - RPM

[Tapartacus]

Randy can I call you? It would be better if I can explain it.
Andrew

[racerrad8]

Sure, I am just heading out for lunch and will be back in a hours or so. Then I will be around most of next week.

Randy - RPM

[RichBaker]

The idle air corrector jets are 144 Mikunis , the pilot jets are 40 Mikunis, DJ doesn't supply pilot jets... I'm pretty sure the IAC jets from DJ were Mikuni.  You'd probably get the same results with just Mikuni 42.5 pilots...