1984 750F Idle issues, help needed

Hey, I've already posted similar things in the 1st gen forums but am looking for more input.
Engine cold with the choke closed, the bike will start fine and idle fine. However, as the engine warms up, there seem to be issues. The bike's idle has two phases. A normal 1k idle that decreases as the engine warms up, and a much higher idle after a small amount of throttle, maybe and 1/16 open. After giving it throttle, the engine begins to race to 3-4k rpm and seems to idle there. Eventually it will come down while I'm downshifting to a stop occassionally to a normal idle, stumble and die. I can offset the dying idle by adjusting the idle, but then when you give the slightest throttle, it races even higher than before and dies eventually. The engine has trouble cranking on startup if it is still hot. Sounds like the battery is very weak, but will start in an instant cold. Not a California model. Things already done include: Cleaned carbs, adjusted valve heights, new ignition coils, new R/R new starter motor, choke and throttle cables adjusted, carb mixture adjusted, new spark plugs. It sounds like a possible vacuum leak. Where else could there be a vacuum leak besides the carb boots and head gaskets? Also, could this somehow be an electrical issue? Please Helps, I need a bike ride that lasts more than 15 minutes!!

 

Still think it's a carb sync issue. One carb is too rich and when conditions are right, it pulls the other three along with it. One is too lean and when bottoms out, drags the other three along with it.

Did you do a bench sync with a 3/32" slip of paper? Are the throttle shaft springs in place (large horizontal springs).

 

Throttle shaft springs were all missing when I pulled the carbs. I replaced them and synced the carbs as best they can be. The 1 and 3 carbs, rear bank, need to be opened a bit more, but the adjuster is at its limit it seems. And I am a bit confused on the bench sync you are talking about. Do you mean to slip a thin slice of 3/32" paper between the throttle plate and throttle bore with the carbs on the bike? Also, is 3/32" paper regular printer paper?

 

Look to see if some linkage is installed on the wrong bank of carbs or maybe they are bent - it happens. You need to get this fixed before you attempt to sync the carbs. To give you an idea of how sensitive they are - when you perform a carb sync on the bike with gauges, you turn the sync screws in 1/4 increments.

The bench sync with the slip of paper is exactly that - you sync the carbs on your workbench. Attach the carbs to the aluminum airbox plenum and sync using the slip of paper as a feeler gauge. Using the #4 carb as a reference, you adjust carbs 3, 2, 1 in order so that you get the same pull on the paper for all 4 carbs. After each adjustment, snap the throttle to "set" the adjustment and measure that one again.

The thickness of paper is not that relevant. You are going for the same pull on all 4 carbs. But if you need to know, 20 lb paper is what I use and it is normal copy paper. The ream or carton will tell you what weight the paper is.

On my bike, one carb could not be adjusted any more because the synch screw spring was completely compressed. I looked for a bent linkage, but could not find one. Since I only needed a little more adjustment, I took a grinding wheel to the spring to grind it down just a little bit - with 1 complete turn tighter allowed as adjustment.

This is not recommended as it just masks the problem - but it seems to work for me.

 

Alright, so I'm going to do a more accurate syncing of the carbs tomorrow. I am still puzzled over how the engine performs great at low temps and idles fine. Why would the idle gradually go down as the temp. increases. If the idle cold was 1000-1200, by the time the fan comes on, it is sputtering to its death. It only takes the bike about 10 minutes to reach this temp, is that too fast? Temperature outside is 70-80 btw. And thanks for your help Mason, you're a great help. Also bear in mind its a 1st gen without on oil mod, could low oil pressure be a cause, I know the pressure drops as the engine warms up.

 

With the choke on, the bike should idle somewhere around 2K. Once the bike heats up, the mixture becomes too rich and the idle will start to come down and eventually die.

Adjust the idle with the bike warm. It should be around 1100 +- 100.

Are your brass choke valves completely closed with the choke off? If you can move them even a little bit, then they are not fully closed. The choke cable takes more slack than the throttle cable.

Since you're going to take the carbs off, remove the brass valves and clean with WD-40 and 0000 steel wool.

Are your pilot jets complete with spring -> metal washer -> o-ring and set at stock setting?

Many times I've seen them missing/torn or too many because the originals are still present.

 

I'm actually trying to get the carbs synced with them on the bike as I don't want to pull the carbs again. The choke is operating correctly, they close all the way. I can't be too sure of the pilot jets. If I understand this, too rich of a mixture can cause the engine to die on idle? The exhaust isn't noticeably smoky or anything, so I wouldn't think it was that rich. I also readjusted the pilots last week. If the bike starts and idles cold with the choke closed, is there a problem? Would that indicate too rich a mixture right there?

 

You are 100% correct. You have too rich a mixture at idle if the bike will start and idle when cold. Of course if it's hot outside, it may start but idle badly with no choke.

The only way to sync the carbs on the bike is to use a carb synchronizer and adapters. Synchronizing the carbs is the last step when everything else carb related has been completed.

The pilot jets affect off-throttle to mid-throttle. After mid-throttle their input is small.

If not sure, I would remove the pilot jets, spring washer and o-ring and inspect. Easy to do with the carbs off the bike - and can be done on the bike if you are very careful.

This is how I do it.

(assuming the EPA caps have been removed from the pilot jets)
1. Using a good screwdriver, remove the pilot jet and spring. Sometimes the washer will come out on the jet.

2. Look to see if the washer and/or o-ring are still in the pilot jet bore. Make sure the washer and o-ring are horizontal in the bore. Take a deck screw and GENTLY screw in until it bottoms out. Unscrew 1/4 turn and gently pull. If it doesn't come out, turn 1/4 turn more. The washer/o-ring should be on the threads of the screw. You don't want the screw to mark the bore.
(disclaimer: professionals usually use a dental pick or some other device as you have no chance of messing up the bore. Since I do not have a dental pick and the deck screw is faster - it's what I've used for years)

3. Look inside the bore and you should see aluminum metal at the bottom of the bore with a hole in the center. Make sure there is not another washer or o-ring still inside - common occurrence on carbs built by a novice.

4. Once inspected and cleaned, replace in this order
-o-ring at bottom
- metal washer
- spring
- pilot jet
All parts should fit on the pilot jet. Be sure not to booger up the threads.

5. Screw pilot jet in GENTLY until it bottoms out. Then turn out to stock setting.

 

Well Mason, it turns out that the carbs were way out of sync and that seems to have been the problem. I can't express how much help you have been. I would like to share how I got them synced with everyone else because of how accurate a tool it was and how inexpensive a tool it was to build. If anyone is ever in need of a tool to compare the vacuums on the gauges, here is a link to a great article explaining how to build the tool and how to read it. Super cheap, super accurate. The only other thing you might need is the adapters for the vacuum hole which you can find from a motorcycle depot of some kind. I got mine from Flanders in Pasadena, CA. Anyways, here is the link, it might take a minute to load... http://www.airheads.org/content/view/183/56/
Again, thank you very much Mason.

 

Just glad I could help...