FJ1200 carb spitting

[km4hr]

Before I cleaned the carbs my FJ1200 engine was hard to start and ran poorly. Now it starts and idles fine but it spits back through the carbs when running above idle. The problem is most obvious on two of the four cylinders. The problem may have existed before I cleaned the carbs but it ran so bad overall that I didn't notice.

My problem is that I don't understand the spitting problem. It is more likely a carb problem or could the valves need adjustment? Can ignition problems cause carb spitting?

I've never worked on an engine with multiple carbs before. Could spitting be caused by incorrectly balanced carb adjustments?

Any thoughts/comments are appreciated.

Thanks!

[racerman_27410]

sounds to me like you still have some blockages in the carbs.


did you clean all the little pin holes in the sides of the emulsion tubes ?


Nancy K was having similar problems with her FJ at the spring rally and once i got in there thats what was clogged up.

ran right as rain after a thorough cleaning.... you might even have to find a small piece of brass wire to physically clear the holes.


KOokaloo!

[SlowOldGuy]

Clogged idle jets or an otherwise lean idle curcuit will cause the carbs to spit back through the intake.  A bad (or cracked) spark plug can also cause spitting.

DavidR.

[km4hr]

Thanks for your responses!

Since my original post I have cleaned the carbs two more times. They're so clean they could be approved by the FDA for food preparation. But the engine still spits back through all four the carbs and backfires out the exhaust. It starts and idles fine however.

When I say I "cleaned" the carbs here's what I mean. I removed all the bronze jets including the pilot jet that's located way down in a deep hole. I sprayed an entire can of carb cleaner through all the passages and made sure the tiny little holes in the throat of the carb and jets are perfectly clean. I originally thought the problem was the carbs but now I think it must be something else.

Since the engine idles ok I wonder if the spark advance is working. What happens if you rev up an engine and the advance is not working? The ignition is controlled by a TCI ignitor. But there's also another electronic box with a vacuum line that connects to the number two cylinder. That box apparently provides a signal to the TCI that is used to advance the timing. Could a lack of spark advance cause timing problems that result in spitting and backfiring?

What happens if the carbs are out of sync? Can that cause spitting?

Here's my game plan. I'm going to try to sync the carbs (I've never done it before). How critical is carb sync anyway? I thought syncing was just for fine tuning. Can it be off enough to cause spitting and backfiring?

I'm also going to troubleshoot the vacuum advance box. I have a diagram that shows the voltages that the box is supposed to produce with changes in pressure.

I'm also going to do a compression check. If the pressures are good can I be sure the valves are not leaking? Would the engine idle ok if the valves were leaking?

Hmm ... I just had another thought. I wonder if the fuel filter could be plugging up. Maybe it's allowing enough flow to keep the engine idling but not enough for higher speeds. The engine does run like it might be running out of gas. I'll put that on the list of things to check as well.

Any other thoughts or ideas are welcome.

If you've read this far, thanks for thinking about my problem!

[Flynt]

Thanks for your responses!

Since my original post I have cleaned the carbs two more times. They're so clean they could be approved by the FDA for food preparation. But the engine still spits back through all four the carbs and backfires out the exhaust. It starts and idles fine however.

When I say I "cleaned" the carbs here's what I mean. I removed all the bronze jets including the pilot jet that's located way down in a deep hole. I sprayed an entire can of carb cleaner through all the passages and made sure the tiny little holes in the throat of the carb and jets are perfectly clean. I originally thought the problem was the carbs but now I think it must be something else.

Since the engine idles ok I wonder if the spark advance is working. What happens if you rev up an engine and the advance is not working? The ignition is controlled by a TCI ignitor. But there's also another electronic box with a vacuum line that connects to the number two cylinder. That box apparently provides a signal to the TCI that is used to advance the timing. Could a lack of spark advance cause timing problems that result in spitting and backfiring?

What happens if the carbs are out of sync? Can that cause spitting?

Here's my game plan. I'm going to try to sync the carbs (I've never done it before). How critical is carb sync anyway? I thought syncing was just for fine tuning. Can it be off enough to cause spitting and backfiring?

I'm also going to troubleshoot the vacuum advance box. I have a diagram that shows the voltages that the box is supposed to produce with changes in pressure.

I'm also going to do a compression check. If the pressures are good can I be sure the valves are not leaking? Would the engine idle ok if the valves were leaking?

Hmm ... I just had another thought. I wonder if the fuel filter could be plugging up. Maybe it's allowing enough flow to keep the engine idling but not enough for higher speeds. The engine does run like it might be running out of gas. I'll put that on the list of things to check as well.

Any other thoughts or ideas are welcome.

If you've read this far, thanks for thinking about my problem!

I would say carb synch is most likely your problem...  multi-carb systems feeding different mixtures into their individual cylinders will cause the spitting/popping/backfiring/etc you describe.  This behavior will also be different at different RPM levels and will be hard to track down.  In my experience you need to be within a couple of cfm across the carbs to get even reasonable running behavior... then you focus on tuning your fuel delivery circuits.

Frank

[FJ111200]

I would say carb synch is most likely your problem...  multi-carb systems feeding different mixtures into their individual cylinders will cause the spitting/popping/backfiring/etc you describe.  This behavior will also be different at different RPM levels and will be hard to track down.  In my experience you need to be within a couple of cfm across the carbs to get even reasonable running behavior... then you focus on tuning your fuel delivery circuits.

Frank

+1 on that Frank . Same with my experiences.

[yosemite]

Hi
for some reason the bike is running lean, this is usually the cause of spitting  back  if you are confident that all the jets are perfectly clear then check that there are no air leaks anywhere  if your bike has rubber covers on the little stubs you would use to connect the carb balance gauges to check these are OK also check there are no leaks on the vaccum pipes and check that the carb mounting rubbers are not cracked/ leaking this can be done by spraying them with wd40 when the engine is idling a change in engine speed indicates a leak 
  as  racerman said you may need to use a strand of copper wire to clear the jets carb cleaner will take off mos crud but if the blockage is solid you may have to use a physical method
  incidentally if you do a compression test  and find that compression is down then put a small amount of oil,about 5cc in the affected bore and run the test again if the pressure rises its a problem with the piston/rings/bore if it doesn't then the problem is with the valves

[RichBaker]

Did you also remove the enricheners (chokes) and mixture screws and clean all those passages good?  An air compressor is one of the best tools for carb cleaners you can own, they do great job of making sure all the passages are clear..... Canned carb cleaner is a distant 2nd.

[andyb]

^ I'd take an ultrasonic cleaner first

Sounds like an air leak someplace to me, but it's not gonna hurt to do a synch.  There's a chance it's got slipped cam timing or something also, I suppose it depends on if it was running well recently or not, etc.

[km4hr]

Hello everyone, thanks for your replies!

Since my last post I've tried most of your suggestions. So far no improvement. Carbs still spit and exhaust still backfires.

Like several people suggested I feel the carbs are too lean. But I don't know how to make them richer. The pilot mixture screws are currently turned out three full turns. I'm going to open them some more later today. I thought three turns should be plenty.  Anybody know where theirs are set?

I cleaned the emulsion tube pin holes like Racerman suggested. But they already looked perfectly clean.

Should I try raising the jet needles in the sliders. Would that make it run richer? The spitting begins as soon as I crack open the throttle. I don't think the sliders have even moved by time the spitting begins.

Since my last post I synced the carbs. It was the first time I've ever done that. It smoothed out the idle but didn't help the spitting/backfiring problem. I got the manometer levels almost perfectly even on all cylinders with the throttles closed. But that raises a question, do you sync carbs with throttles completely closed or should the engine be idled up a bit?

I haven't removed/cleaned the enrichers (chokes) as Richbaker suggested. I've avoided separating the carbs from each other so far. I've been cleaning each one individually to avoid mixing the parts. Also, separating the carbs looks like it might create other problems. Maybe I should do it just for the sake of completeness.

BTW, what is the second vent tube for? Each carb has a bowl vent tube and a another vent tube. What does the second vent tube do?

I looked for air leaks but didn't find anything obvious. The rubber intake boots that the carbs plug into seem to be in good shape.

I also haven't tried using compressed air to clean the carbs as Richbaker suggested. I'll try that next. Carb cleaner sure flows through the passages just fine however.

Thanks again! Really appreciate your suggestions.

[SlowOldGuy]

Is it just 2 cylinders spitting?  Which cylinders are spitting?

Did you have the air pilot jet, idle jet and mixture screw out when you cleaned the idle circuit?  Still sounds like a lean/clogged idle passage.

How old are the plugs?

DavidR.

[weymouth399]

Simple check if it'sthe pilot jets is to pull the choke out a LITTLE, and see if that stops the spitting.
It will raise your idle a little but it's a quick check.

Bob W

[ddlewis]

...
Like several people suggested I feel the carbs are too lean. But I don't know how to make them richer. ...

Did you check the float height on the spitters?  angle of the dangle and all that..

[km4hr]

Is it just 2 cylinders spitting?  Which cylinders are spitting?

Did you have the air pilot jet, idle jet and mixture screw out when you cleaned the idle circuit?  Still sounds like a lean/clogged idle passage.

How old are the plugs?

DavidR.

Number 1 & 2 cylinders spit more than 3 & 4. But I believe they all do it some. I can't tell which cylinders are backfiring however.

The plugs don't have many running hours on them. I'd say less than 20 hours.

[km4hr]

Simple check if it'sthe pilot jets is to pull the choke out a LITTLE, and see if that stops the spitting.
It will raise your idle a little but it's a quick check.

Bob W

The spitting is reduced but not eliminated with the choke pulled out slightly. In fact that's about the only way the engine will even accelerate. Without any choke the engine idles beautifully and carbs sync perfectly. But open the throttles and the engine shuts off.  A small amount of choke is required to make the engine speed up without a lot of pampering of the throttle.

Perhaps one way to make the carbs richer is to put more money into them. But that makes me poorer.