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Discussion Starter · #1 ·
Hi Everyone, I have a 1985 CB700SC with low miles. It sat for a few years. So I rebuild to carbs and had the carb bodies professionally "boiled out". Now all put back together , it idles fine, which it didn't do before. And revs up fine passed 1500 RPMs. But the problem is...getting it to rev off of an idle, it wants to stall. I've tried opening the pilot screws from 2 turns but to 4 turns out but still the stalling trying tom rev off an idle. Any ideas? Thanks! Kevin
 

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The pilot screws make no difference with the throttles opened, even just a little. You need to find out if it is becoming too rich or too lean, as the mixture transfers to the main/needle jets. Put in fresh plugs, then raise the rpm as far as you can without it stalling, and hold it there. You may want to aim a fan at the engine to keep it from overheating. After a few minutes, shut down with the run/stop switch (do not return to idle), and look at the plugs. The should indicate rich vs. lean.
 

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Discussion Starter · #3 ·
Thanks WintrSol, I'll try that. But I"m confused on "raise the rpm as far as you can without it stalling".
Raise it by the throttle stop screw?
 

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I believe he means to just use the throttle like you are pulling away from a stop. Just get the rpm's up just short of that stalling point and hold it there. And be sure you have a window type fan or larger blowing on high pointed at the engine to keep it cooler but similar to riding. Got to run it like that long enough to get a good reading on the plugs. If you have a throttle lock it would certainly help. So if you don't you might want to figure out a way to hold it in that critical zone. You might get tired holding it there.
 

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Did you check/adjust the float height?
Did you reconfirm by checking the fuel level? (clear tube method)
Did you check/adjust the sync?
 

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Did you check/adjust the float height?
Did you reconfirm by checking the fuel level? (clear tube method)
Did you check/adjust the sync?
Yes, all those things; the spark plug reading is the easiest, to me, then follow that they say.
 

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Discussion Starter · #7 ·
What is the correct/factory size pilot jet in my 1985 CB700SC
I think they might have sent me the wrong jets. In the bike now are Keihin N424-26 #35
Are these right?
Yes, I synced the carbs and checked the float level.
 

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Again, the pilot circuit has little to no effect on open throttle running. Also, #35 is in the range for the pilot jet for that bike.
 

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Discussion Starter · #9 ·
3 questions then...
1. What does the pilot jet do?
2. If the spark plugs show lean what do I do?
3. If the spark plugs show rich what do I do?
 

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The pilot jet (and pilot air jet) mixes fuel and air to provide the needed mixture to run when the throttle is closed to idle, and the amount is controlled by the pilot screw. As the throttle begins to open, there are small transition ports, also fed by the plot fuel and air jets, on the upstream side of the throttle to provide some fuel/air until the needle jet takes over; it doesn't take much throttle to pass this transition, maybe less than 1/8 full travel of the throttle. From there, the needle jet regulates the fuel/air mix coming from the main jets, until the air flow is high enough to pull the CV piston nearly all the way up, and then regulation is only set by the main jets.

If lean, make sure the fuel levels are high enough; if rich, fuel levels may be too high. If one is rich, the other lean, you probably have a bad sync problem, on top of a possible fuel level problem. Rich could also indicate a float valve sticking open during operation, lean the opposite.
This is a CB450 carb, but the principles all apply:
 

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Also aside from carburetion issues, other factors can cause a rich or lean condition. Vacuum leaks, or air intake leaks at the intake manifold can cause a lean condition. I even had a vacuum leak in my Pair valve that caused a running rich condition.
 

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Also aside from carburetion issues, other factors can cause a rich or lean condition. Vacuum leaks, or air intake leaks at the intake manifold can cause a lean condition. I even had a vacuum leak in my Pair valve that caused a running rich condition.
My XS400 was having idle problems, then started reving too high at idle. My early suspects were blockages in the low speed circuits. But when the high idle problem appeared I changed my thinking, and suspected an air leak. With the engine running at high revs at idle, I blasted the left hand carb and boot with WD40. Then the RHS. The engine picked up revs when the lower part of the boot was sprayed. I checked the boot and it looks new. I checked the lower mount bolt, and it was loose. After tightening the lower bolt, the overall performance has improved, and the idle is fine. Tach cable broke, so I set it by ear.

I think the moral is: It is a good idea to check for air leaks, especially on an older bike.

Study how the float level is set. Some bikes use different ways to measure. The special tool for my XS400, is a piece of fuel line.

UK
 

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Air leaks can show up in a lot of places. The left carb on my CB450 developed a leak around the felt seal on the throttle shaft. Made it impossible to get a reliable sync at idle and open throttle. The standard fix (soaking it in 90 weight) wasn't enough, so I came up with a way to repack it.

Are you a farmer or rancher on the side? That's thinking outside the box. Way to go. Another way to state that would be, never give up. There's a solution to almost every problem. But care to give us your secret fix? I'm curious as I bet others are as well.
 

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Could the jet needle/clip possibly be installed incorrectly, loose (and able to "float"), defective or parts missing?
 

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Are you a farmer or rancher on the side? That's thinking outside the box. Way to go. Another way to state that would be, never give up. There's a solution to almost every problem. But care to give us your secret fix? I'm curious as I bet others are as well.
Just a really old school, self taught, shade-tree mechanic, working small engines since I was 9yo. I used some of the packing 'string' used on faucets to fill the gaps, pressed in with a small bit of metal tubing, then oiled or waxed (forget which) the throttle shaft before smoothing it over with fuel-tolerant silicone sealant. After several years, it still runs well.
 

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Well that is very clever. Certainly thinking outside the box. That should earn you a farmer merit badge. Or the very least a red neck one. :grin:
 

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You need to find out if it is becoming too rich or too lean, as the mixture transfers to the main/needle jets. Put in fresh plugs, then raise the rpm as far as you can without it stalling, and hold it there. You may want to aim a fan at the engine to keep it from overheating. After a few minutes, shut down with the run/stop switch (do not return to idle), and look at the plugs. The should indicate rich vs. lean.
Excuse the ignorance, I am also trying to learn. I understand what I have to do to my bike to test if the mixture is too rich or too lean by following the steps above.
What I do not know is what I'm looking for when I remove the plugs and I'm physically looking at them.

Sorry for the dumb question
 

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There are plenty of pictorial guides out on the Internet to teach you how to read your spark plugs. Here is one.
 

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Note: that chart was from the days of lead in gasoline, which is what makes for the tan color, but the overall ideas are sound. Plain gasoline, without ethanol, leaves gray to black carbon soot deposits on the insulator, away from the hot tip, and some soot on the metal shell, because it is cooler. The closer the soot is to the tip, and the thicker it appears, the more rich the mixture is. Add ethanol to the fuel, and the fuel tends to clean the plug more, removing some of the soot, so the deposits will be relatively thinner, and farther down the insulator, at the same mixture. If, after the operation I described above, there is a deposit close to the tip, it is very rich, but if it looks almost like a new plug, it is lean. A short run should leave some amount of soot, but not much; there should also be no oil deposits, as the pictures show, nor any sign of burning of the tip.
 

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I never considered how much ethanol changes reading plugs. One more variable trying to make things more difficult. When you see as many as you wrenches see I guess it's easier.?
 
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