88 FJ1200 - Engine dying when throttling up very quickly

[chaunceyw]

What I have done to the bike:

Bike was working fine but needed a valve adjustment
I adjusted the valves
In the time waiting for some shims, I took the carbs off and cleaned them
Synced the carbs
Put on unipod filters

Now this problem started: https://www.youtube.com/watch?v=ZE-svo598qA

There are also backfires in the bike when throttle is held around 2k RPM - I can't reproduce them but they happen and I'm not sure where they are happening.

I thought it might be the unipod filters, so I put the stock air box on but the problem persisted.

I took the carbs off again, cleaned them and sprayed them again - even got the floats/needles/seats taken out. The original owner had teflon tape there, so I put other seats in with o-rings (http://www.fjowners.com/index.php?topic=12277.0).

I inspected the CV boots and there were no holes in them. Floats all seemed to be within spec from what I could tell.

Can someone give me a laundry list of items that would cause this symptom so I can recheck things? This is really frustrating - I thought I was doing good with my bike by giving it a nice tune-up and turns out I made it worse. Taking the carbs apart twice and having the bike disabled for over 2 weeks is the worst

[chaunceyw]

Tooks the carbs off and adjusted the floats to within spec, issue still hasn't gone away.

The bike rides fine, pulls hard. Seems to stutter while around 2k RPM just like where it backfires when sitting still in neutral. I don't get backfiring while riding, though. I assume the issues are linked but I am at a loss as to where to look/what to replace.

[Arnie]

The '88 model year FJ did not go to the US.  So, the knowledge base on that year is a bit thinner than for other years.

Does your bike have a fuel pump or is it still the gravity feed system?
This is important because the float needle jets changed along with the addition of the fuel pump.
They went from 2.3mm in the gravity system to 1.5mm for the pumped setup.

When you changed these jets, did you possibly install the wrong ones?

[chaunceyw]

Hey Arnie,

It has a fuel pump.

The jets that are in the carb are stock for the bike, and I didn't switch any of the numbers around.

I've adjusted the pilot screws a bit - the manual I have was useless in telling me the factory setting. I have them set at 3 turns out. I'm not sure if this makes it dangerously leaner, but that seems to have toned down the problem a bit.

I had the carbs apart again today and verified the floats which are all good. Not sure where else I might be losing power on the transition, but before I could stall out the bike completely just by cranking fast, now it doesn't even come close... maybe I'm just seeing something that isn't there at this point?

[yamaha fj rider]

What size pilot jets are you running?

Kurt

[chaunceyw]

Stock 42.5 pilot jets

[yamaha fj rider]

Sounds to me like you still have some plugged passages in the carbs. Remove the jets and spray out the passages again, both ways. You may want to consider the RPM screw and o ring kit. Hope this helps.

Kurt

[chaunceyw]

I've opened up the carbs again, cleaned everything again, sprayed all the passages and jets. The 3 holes in the throat seem to be free and clear, pilots look good, floats are to spec.

I took apart the slides and needles and set it one notch higher for the unipods.

Issue seems to mostly persist.

I was messing with the pilot screw - at 2.5 turns out it's definitely happening. When I'm around 4.5-5.5 turns out the idle seems to be peaking (at 10 turns out the bike barely ran), and the throttle up issue is mostly gone. It's still slightly there, but barely. I'd have to double check to make sure I counted properly but.. does this mean I should get larger pilot jets?

I have been over these carbs 4 times and none of the times has the issue gone away.

I recently adjusted my valves - could out of spec valve shims cause this problem? I measured and all were good but never know... I could have messed something up?

[racerrad8]

I took apart the slides and needles and set it one notch higher for the unipods.


I was messing with the pilot screw - at 2.5 turns out it's definitely happening. When I'm around 4.5-5.5 turns out the idle seems to be peaking (at 10 turns out the bike barely ran), and the throttle up issue is mostly gone. It's still slightly there, but barely. I'd have to double check to make sure I counted properly but.. does this mean I should get larger pilot jets?

You set what one notch higher; the clip or the needle.

If you have adjustable needles, do you know what needles you have?

If you are getting that large on the idle mixture screws with 42.5 pilots then that tells me the idle/pilot circuit is still plugged/restricted.

How did you sync the carbs, because if they are way off, it will truly only be running on the carb(s) that are open enough to draw from the pilot circuit.

and finally, the idle seems low to me from listening to the video, it should be at 1100.

Randy - RPM

[chaunceyw]

Thanks for the reply!

I set the clip one notch down to raise the needle up.

I tend to agree with the plugged but I can't for the life of me find any dirt or plugged passages or jets.

I synced the carbs using a pair of vacuum synced bottles with water. Made sure the vacuum was equal in both for each for 1/2, then 3/4, then 1/4.

Perhaps I should take the carbs off and bench sync them.

Since the video I've put the idle up properly but it doesn't correct the issue.

[FJ_Hooligan]

Did you remove the idle mixture screws when you cleaned the carbs?

[chaunceyw]

Yep I removed the screws, springs, and washers. Soaked the metal bits and scrubbed then put back in.

[Pat Conlon]

Did you clean out all the little holes in your nozzle jets? aka emulsion tubes.

Sounds like your transition from pilots to needle jets to mains are suspect. I had a problem like that years ago, kept cleaning the carbs only to have them re-contaminated by crud left in my tank.

You have a fuel pump and a real fuel filter so I don't think that should be the problem.

[chaunceyw]

Those tubes are so clean. All jets are free and clear both when I put it together and when I take it apart again.

I checked the pilot holes near the butterfly valve as well and they seemed ok.

[racerrad8]

Yep I removed the screws, springs, and washers. Soaked the metal bits and scrubbed then put back in.

Did you put new o-rings on them?

If not you will have an air leak leaning out the pilot circuit.

Randy - RPM