Carb Tuning Questions

[drawkward]

After changing the oil and installing an oil filter (ya, one of the previous owners skipped that step ) I started on syncing the carbs.

I made my own homemade carb tuner that is hooked up to the vacuum nipples in front of the carbs that are normally all blocked off with caps with the exception of one:



I guess I have some basic questions.

1) Do you need to let the bike warm up first before you hook up the tuner?
- I hooked it up on a cold engine and one of the tubes almost had the fluid sucked into the throttle body. I seem to remember reading that you need to warm the bike up first.
- Also, when the bike first kicks on it really causes the fluid (tranny fluid for color and viscosity) to froth up and generally be useless. Will warming the bike up before hand help this?

2) I've read elsewhere on the forum that a good starting position to have the air/fuel mixture screw set at is 2.5 turns from fully seated. Is this recommended?
- I haven't yet messed with the mixture screw to see where they are right now, but I can only imagine that they are all over the place because of the previous owner.

Any other tips you could throw my way would be greatly appreciated.

And to give you an idea as to why I am tuning:

The bike runs great in the higher RPMS (over 3500-4000) but has barely any power below that, to the point where it almost will stall out when first letting the clutch out. I have cleaned the carbs with both carb cleaner and a compressor and am confident that they are in good working order, so I thought that tuning the carbs was my last step. Will a detuned set of carbs cause the bike to act this way?

Thanks for the help!

[racerman_27410]

you might need to put some kind of restrictors in those balancer lines to reduce the amount of vacume and as a result the frothing problem.

always warm the bike up to operating temp before trying to tune the carbs.

2.5 turns on the mix screws is a good starting point but the real tell is the "blip test"

how low do you have the idle set?..... it should be somewhere around 1000 RPM

KOokaloo! 

[drawkward]

I won't lie, I haven't a clue how to set the idle on this particular bike. It seems to idle below 1000RPM though, that I know.

[MyFirstNameIsPaul]

Idle spec is 950-1050 RPM.  I usually use a tach that clips on the plug wire since I have no trust of bike instruments.  The idle adjust knob is actually on the bottom of the carb assembly in the middle towards the front.


I know that my Morgan Carbtune http://www.carbtune.com/ has restricters on every line.  I've been using mine for about 10 years now.  The bike has to be completely warmed up to synchronize properly.

I'm no carb expert, but I doubt doubt synchronizing the carb vacuum will solve your problems.  Do you have a jet kit or after market exhaust or air filter installed?  You may need to replace one of the jets.  I'm not sure which one does low RPM.  I installed a Dynojet kit and it has the same problems.  In fact, between 1500-2000 RPM the bike can't even function, I just have to blast through that range. I've gotten used to just winding the bike up to go from a stop and slip the clutch lots in low speed maneuvers.

That procedure is similar to the one a old-timer told me on setting the mixture screws.  He said to start at 3 turns and warm the bike up in the driveway.  Next turn each screw a quarter turn in until the engine is just about to die, then turn each screw a quarter turn out until the engine idles normally, then go half turn beyond that.  I did that about 30,000 miles ago so far no problems.  However, if you screw it up, you'll burn a hole in a piston.  I have a friend who has done that on two bikes.

[drawkward]

So I've warmed the bike up and one cylinder is still pulling more than the other and the fluid is still frothing even with restrictors in the line.

However, I've played around with each individual mixture screw so that the bike idles (per the stock tach) right around 1000RPM and it responds pretty much like it should doing the "blip" test. I still think it needs to be sync'd, but I'm not sure I'll be able to do it with my homebrew kit. I'll try some more, but without having the fork springs here I can't do a sufficient road test to see how the bike is performing.

I'm gonna keep my fingers crossed.

[racerman_27410]

a proper carb synch is critical to the low speed performance of the engine.... if all the carbs arent drawing equally its gonna vibrate and run like crap.

[Ratchet_72]

I've found that I get the best carb sync when my bikes idling well ABOVE 1k.  I usually sync around 1500-1700.  When you're riding who cares if they're sync'd at 1000 rpm?  Sync'd carbs at closer to running speed just work best for ME.  The Fj is of course air cooled so have a fan blasting the front of your bike while you do this.  It doesnt take long to do so you really won't overheat.  Remember that carbs 1, 2 and 4 sync off of 3 so your bench idle set correctly on 3 is important if you wanna do the whole process quickly.
Keep in mind that this works best for MY bike(s).

[drawkward]

Remember that carbs 1, 2 and 4 sync off of 3 so your bench idle set correctly on 3 is important if you wanna do the whole process quickly.

I'm not sure I understand what you mean by this.

[racerrad8]

It means the syncing of the carbs is not done by adjusting the idle mixture screws. The sync is based on the vacuum found behind each carb butterfly at a idle or almost closed position; not the idle fuel mixture. The idle mixture screws should be adjust the same on all carbs. The syncing is completed by adjusting the screws on the carbs at the throttle shafts. #3 is the carb that is set to the main idle adjusting screw so that is your "base" carb as far as vacuum is concerned.

One issue that I see that you need to address is that the tool you made needs to have an atmospheric vent in order to work properly. I would add another tee in the middle of the two lines by your clamp and run another line up at act as the vent. Without the vent your reading will be hard to adjust because there is no vent. You will need to put small restrictor in all four lines. They all need to be identical in hole size. My set has a .030" hole and is pulling the heavy liquid mercury. You might have to have an even smaller restrictor is you are using water or oil as your liquid.

Good luck,
             Randy - RPM

[Marsh White]

First off...lose the 4 hose setup and make a simple 2 hose unit (yours looks WAY too complicated):

http://www.powerchutes.com/manometer.asp

Just use two of the tubes and sync 1 to 2, 3 to 4, and 1 to 3.

I made one of those a while back and it works great! I modified the design slightly by using a 4ft 1"x4" board as a backing plate. I then used little staples to attach the yardstick and tubing to the board. I just screwed the thing to the wall of my garage and lengthened the tubing by about 12 feet on each side so that I can easily take the tubes over the either bike while the device hangs vibration free on the wall. I also used the lightest weight ATF fluid instead of oil. The lighter weight of fluid in it = the more accurate it is. It is many times more accurate than mercury or Morgan sets. Also, it is very easy to see from afar with the bright red ATF fluid. Works great! I think I spent something like $8.00 on mine...mainly because I put some vacuum dampeners inline to help reduce the pulsing.

I would set all of the 4 screws (the "Idle Mixture Screws") to the same setting...even though they didn't come that way from the factory. I would set them all to 2 turns out. Then get a good sync. Then adjust your main idle screw (underneath the carbs) so the bike idles around 1000 RPM. Then you can do the blip test on the Idle Mixture Screws to get them setup fairly correctly. Basically, you want to blip the throttle (say to 3 or 4K) and have the tack needle fall smoothly and exactly back to the 1000 RPM mark. You don't want it to fall back down too low and slowly raise up, or fall down fast (to say 1500 RPM) and slowly fall back to the 1K mark. Adjust accordingly. When you get good at doing the above...you can use your ear to tell when the carbs are perfectly synced while using the oil manometer for the general ballpark area. It is amazing how LITTLE you have to turn each screw to really fine tune it.

Sometimes it helps to balance the whole setup twice. The other thing I have learned over the years is that just warming up the bike in the garage and then balancing the carbs will result in a fairly crappy sync (the carbs body has to actually be HOT like it would be when you are riding it). I highly suggest actually riding the bike for 30 minutes, and then syncing the carbs....trust me, it can make a large difference when out riding.

Oh, one more thing, open up the throttle a bit while you sync.  You will find that you may (I usually do) get two different readings.  One when the throttle is closed, and one when it is open a bit.  Sync to the reading when it is open a bit.  The bike runs a hell of a lot better properly sync'd that way. 

[andyb]

I read someplace that you want to synch the motor at about half the rpm of peak torque.  That leaves it smoothest on average.  It also works out to leave it smoothest at cruise rpm on an FJ.  I set my idle up to 3500-4k or so when I do the synch, the homemade tools do work and are quite sensitive, but using the "real" tool is decidedly easier and faster.

You say the bike doesn't have much power under 4k or so.  What's been done to it?  With the typical stage3 mods, I know mine is pretty soft under 2krpm, though it'll wheelie at 3k with some effort.  Also, don't expect the idle mixture screws to really do all that much, they run out of adjustment pretty quickly; generally you'll need to change the pilot jets to get an appreciable change.

If you're noticing this mostly from takeoff or otherwise when you're rolling into the gas at very low rpm, I'd wager that your pilot/idle circuit and synch are not at fault, your fuel/float height is likely to blame instead.  What's been said about the blip test is correct and will help get those circuts right, but I'm  not sure that they're where your issue really is.

[drawkward]

I guess I should explain what the bike does from 1000 to 2500rpm to avoid any more confusion.

It essentially wants to bog down and shut off. Almost feels like a fuel starved condition. However, after that initial period right at take off it screams like a bat out of hell and is very responsive anywhere in the RPM range. It could be better, but the condition is essentially gone.

The only aftermarket modifications that I know about are a different set of cans (the PO said they were Viper?) I can only assume that maybe the jets were done at some point but I have no idea how I would know that for sure. I have the stock airbox on the bike when I notice the problem.

After the bike heats up a bit I noticed that the bogging problem lessens a bit but is still there if you mash the throttle or do a really quick blip. I've cleaned the hell out of the carbs but I suppose I could do it some more. I'm just at a loss at this point...it's not as easy as a Harley motor!!

I was told I could "bench" sync the carbs to get them close to proper sync by shining a light through the front of the carbs and under the butterfly. I guess what you essentially do is try your hardest to get the light output from under the butterfly to be the same across all of the carbs by adjusting the sync screws (or whatever they're called). This makes sense to me I suppose. When I first shined the light through, the butterflys were all in different positions, which I think means they're out of sync. So I eyeballed them all into the same relative position.

I haven't put the carbs back on the bike just yet, but I am hoping to get my springs tonight, so I may have the bike up and running by wednesday. I don't think the problem I am having is really a big one, but I'd love to have the bike running the way it's supposed to. I may take a video of the condition sometime this week...I'll post it up in this thread so you guys can hear what it's doing....who knows, that may help!!

Jon

[SlowOldGuy]

How do the plugs look?  If they are black then your fuel level may be too high.  A high fuel level will let unmetered fuel in to the intake and cause the engine to respond badly during off-idle acceleration.

If the description is anything like this:

Idle will be okay, but when you try to accelerate the engine bogs badly but if you keep rolling the throttle, it clears out and it's like flipping a switch as the motor comes to life. 

Then you need to look at the fuel level.  It's really hard to screw the jetting up badly enough to cause this, that's why it's usually a bad float(s) or a level problem.

DavidR.

[racerrad8]

I have a "Sioux" brand vacuum testers used for checking cylinder heads and valves seat sealing. I use that tool to "bench" sync the carbs to get them in the ballpark. They still need to be sync when installed on the engine due to the difference in the valve settings and ring seal that creates the vacuum behind the throttle plate(s).

The bogging could be caused by with either to rich or too lean of an idle mixture. If too rich it can't burn all of the fuel and too lean it is starving.

I think it might be time to have the carbs cleaned and inspected to verify what you are actually working with. You might try and adjust until you are frustrated and still not get the results due to the improper part being installed.

Let me know if I can be of further assistance.

Randy - RPM
rpm@swaybar.com

[drawkward]

So, I got my idle up to 1000RPMs but it kept on dropping below. After it dropped, when I blip the throttle, there's a hesitation present.

I did some research and found that I needed to do the carb cleaner/starter fluid test to check for any leaks between the carbs and the intake manifold. Well, lo and behold, when I sprayed the carb boots the idle increased to a little over 1000RPMs and the hesitation was gone.

So, if my estimation is correct, my problem is that I have an air/vacuum leak between the carbs and the intake manifold manifesting itself in the rubber boots that connect the two.

Sound about right??