need FJ1100 carb tuning help

[Antonn3]

Hey guys, I'm about to get the 1100 on the road and am doing the final carb setup.
After adjusting shims..balancing carbs.. the bike starts fine using the choke then idles fine once warmed up ..
steady at about 1100rpm idle...but.. when I rev it a bit I get a  "popping" sound at around 2200rpm.  Im assuming backfire?
The idle mixture screws are turned out about 3 1/4 turns.
What could be causing the back fire?

Thanks,
Tony

[movenon]

Hi Tony.  Sounds like your pilots need cleaning.  Forgive my questions but what is the history?  Did you rebuild the carbs?  What is your pilot/air jet combo?  Have you checked the valve clearances?  As a suggestion you might go ahead and open up the fuel/air screws more in 1/2 turn steps to see if the popping diminishes.  It would give you a clue that the problem is indeed in the carbs.  How is it reacting when you do a blip test?  I assume that at 2000 RPM you are running less than 1/4 throttle opening.

Mine will "pop" slightly at 2 turns out but go's away as I add more air/fuel mixture.  No sign of it at 3 turns out. Mine is a 1200 and is set at 3 1/4 turns out.

George

[Flynt]

What could be causing the back fire?

Are you sure your carbs are balanced?  Did you balance at idle AND at ~2,500rpm?  Sounds like classic indicator of carbs out of synch to me...

Frank

[Antonn3]

Hi George, the carbs have been cleaned thoroughly several times and rebuilt with a RPM kit. With Randy's suggestion I am running 112.5 mains  with 37.5 pilot (elevation about 1600ft). Randy did state I might have to shim the needles (which I have not done yet).

Hi Frank, I re-checked the sync and they are dead even at about 2500rpm. Just for clarification, with you stating to sync at idle and at 2500rpm? How can you do both as one setting changes the other?

Today I did notice that when I was checking the sync at about 2500 (while holding throttle steady) the rpms would rapidly increase  and I would have to back off on the throttle.
Also noticed some gas had dripped under the bike from the floats.

Tony

[racerrad8]

Tony,

Call me or Robert next week.

Randy - RPM

[Flynt]

Just for clarification, with you stating to sync at idle and at 2500rpm? How can you do both as one setting changes the other?

They should be close at the two points and I usually split the difference as a compromise...  If they're way off, you have an issue with linkage most likely and need to get that sorted.  The runaway RPM at constant throttle is concerning too and also needs to be sorted before the bike will run right.  Unstable RPM at a given throttle position is a mixture problem typically and can be induced by a vacuum leak.  Easiest check there is to spray starting fluid around the intake between carbs and engine...  you'll hear engine rev if you find a leaky spot.

I'd go back to basics if I were you and mechanically inspect to make sure everything is working correctly...  choke plungers seating, throttle linkage tight and butterflies all operate simultaneously, slides all move easily and are held open when the vent (upper port on the intake side) is plugged, etc...  the leaking fuel also indicates you may have leaking needle/seat in the offending carb, so I think you're still fighting basic stuff.

My 1100 has stock carbs, uni pods, and RPM exhaust.  I live at sea level basically, but routinely ride to 2500ft.  I have 42.5 pilots (160 air), 117.5 mains, needles out 1/2 slot, and float level lowered by .7mm (lower fuel level in bowl)...  you sound pretty lean compared to where I am.

Frank

PS - the right answer is to call RPM...

[ribbert]

Today I did notice that when I was checking the sync at about 2500 (while holding throttle steady) the rpms would rapidly increase  and I would have to back off on the throttle.

Tony

Tony, it is possible you are describing a normal FJ attribute. When increasing the throttle slowly from idle in neutral, the engine will labour until about 2500 rpm, at which point, with virtually no more throttle, the revs will shoot right up. The 2500 you were aiming for just happens to be right where this takes place.

This is by design, presumably to tame throttle response at low revs.

This is different of course to the revs changing significantly as a direct result of synching.

Noel

[movenon]

Mine has done the same under no load.  Much like a two stroke when it hits its "pipe".  I just think it is where the advance kicks in but I could be wrong.

I balance my carbs pretty much like Frank.  I adjust the fuel/air screws then balance at idle 1150-1200 RPM, then I put tension on the cables to check the balance. I observe the tubes to see how they are pulling and will tweak if needed.  Then I repeat the process readjusting the fuel/air screws and re-balance.

No matter what I do I will have a vibration point somewhere around 3-3500 RPM but I think that is because of the inherent design of an in-line 4 (crankshaft pulses?).  Just the way it is.  At 5000 RPM my FJ is real smooth.

On the fuel air screws I set them at a base line (for me 2 1/2 turns) do a blip test.  On mine it will have a slight popping out the exhaust at 2 1/2 turns so I keep adjusting them out (giving it more fuel/air mixture) until the popping just stops then I give it another 1/4 to 1/2 turn. At present I am 3 1/4 turns out with free flowing exhaust, UniPods, #117 Main Jet, 155 Air pilot, 42.5 Fuel pilots. adjustable needle set in the mid position with no shim, Vance 4 degree advancer, 180 rear tire, 17/38 for sprockets.

Until the Black Hills Rally I had never checked fuel economy.  But as someone asked I did on two refuels and I am getting in the mid 40 MPG range.  Now I am intrigued and will do more testing using the GPS etc..

George

[Antonn3]

Hey guys, well after dealing with other issues, I am almost finished with the rebuild of this '85.
Down to the final carb adjustments, I have now installed 115 mains and 40 pilot.
Bike starts and revs fine after sync adjustments, but one thing I notice is I'm only about one turn out with the mixture screws? Needle adjustment?
Blib  test is good.


TIA
Tony

[Arnie]

Well, if it all works well and your 'blip' tests confirm that the mixture is good, why worry?
If you want an explaination..... Here's my guess
By putting the 40 pilot jets in, you've richened the idle mixture enough that it doesn't need the mixture screws to be wound out to 2 1/2 or so.  Especially if you're using OEM exhausts and air box the richer pilots may not have been needed.

Now go ride that thing :-)