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Born in the USSR
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Discussion Starter · #1 · (Edited)
Hi,

Here is another question:)

As I wrote in my previous topic I faced some trouble with my GL1200A at high speed (rpm) only. The bike starts perfect and run smooth at low and medium rpm. But at high rpm the ignition timing seems to become "late" (no speed, vibration, shots in muffles). As far as I know the ignition timing depends on vacuum capacity and corrects by vacuum also. If there is some vacuum leakage ignition becomes "late" and effects the problems I always meet with:). This problem always appears exactly at 120 km/h speed (70 miles/hour). So the question is: is there some kind of valve in vacuum line that works incorrect or it's just a small hole somewhere in the line that can cause incorrect timing at high rpm.

I will highly appreciate any ideas, advise or help.

Regards,

Sergey

P.S. Spark plugs, all the filters, ignition wires are new. Carburetors were rebuilded an synchronized
 

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I really doubt you have a timing issue. It sounds like you have a bad coil. Mine had the exact same problems. A new coil fixed it.

Hope this helps.
 

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Born in the USSR
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Discussion Starter · #3 ·
I'm not sure it's a coil (coils). I put new plugs in and run about 100 km in different modes (slow, medium and high). When the bike started f**cking my brains I stopped it by kill switch and put the spark plugs off. They were covered by white-grey mixture (mostly white). That means:
1. Overheating (not in my case)
2. Wrong spark plugs (not in my case)
3. Incorrect ignition timing (seems to be my case). But in my case ignition timing is Ok at low and middle rpm. It's incorrect at high rpm only. And ignition timing is corrected by vacuum and depends on ride mode and rpm
 

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1985 GL1200 A
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953 Posts
Are your carbs properly synchronized? The reason I ask is carb synch'ing will show vacuum leaks as inability to stabilize the vacuum level, which is what your checking/equalizing when synch'ing.

Are the large 0-rings at the bottom of the intake tubes, where they connect to the heads, new/fresh and tight? That's the first place I look for vacuum leaks. Easy test is with motor warmed up spray some Gumout or similar around the joints, one cylinder at a time, with the engine revved to 2,000-2,500 rpm and if it revs up at any time, that's where there is a leak.
 

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Born in the USSR
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Discussion Starter · #5 ·
Hi Cornpone,

1. The carbs are properly synchronized for sure with special professional tools.
2. The intake O-rings are new
3. I did the "spray test" with my friend but found no leaks. But we did not run engine till 2 000-2 500 rpm. We tested it on idle only. Maybe that was our mistake.

Thanks! I will try another "spray test"
 

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To answer your question, the cdi controls spark advance. It does it by "monitoring" PORTED vacuum off the #4 carb, NOT the one on the intake runner. The vacuum hose runs up the left side of the air box to a tee. I believe it's just a test fitting. The vacuum then goes to the cdi.
The way I've always understood it to work was; When the cdi sees the ported vacuum signal (off idle) AND it sees an input from the the gear position switch (neutral switch) for 4th or 5th gear, it alters timing.
 

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Thanks glhonda!

But what if ported vacuum signal is incorrect?
I don't think the cdi is that sophisticated.
This is just a "read only".
If "A", then "B".
If I see a ported vacuum signal, I know I'm off idle.
It isn't monitoring the amount of vacuum, only the presence of vacuum.
 

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Born in the USSR
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Discussion Starter · #9 ·
Ok! I see...If it's not vacuum problem so I don't know what exactly problem I faced:) I think I will start tracing step by step: if spark plugs, filters, ignition wires are new and the carbs are synchronized so I will check coils, cdi and pulse generator. There are no more items to check.
 

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so I will check coils, cdi and pulse generator. There are no more items to check.
It isn't going to be a pulse generator if only one cylinder is affected.
Not the cdi either.
The pulse generator send a signal to the cdi to fire the coil at the right time. This is a wasted spark system (two plugs firing off the same coil at the same time). The pg's and the cdi are doing their job correctly, since it doesn't affect the other cylinder fired by that coil.
 

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I'm troubled by the "white grey" mixture on the plugs. That to me is an over lean condition. Which would also explain why you can't get any top end speed. It is starving for fuel.

I would do the fuel delivery test in the Factory Service Manual to make sure your pump is putting out enough fuel. I would also change the fuel filter to make sure you don't have a clogged filter.
 

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Born in the USSR
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Discussion Starter · #12 ·
The pulse generator send a signal to the cdi to fire the coil at the right time. This is a wasted spark system (two plugs firing off the same coil at the same time). The pg's and the cdi are doing their job correctly, since it doesn't affect the other cylinder fired by that coil.
Let's imagine (in theory) that "some bug" makes ignition timing "late" at high rpm only. What kind of bug could it be?
 

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Discussion Starter · #13 ·
I'm troubled by the "white grey" mixture on the plugs. That to me is an over lean condition. Which would also explain why you can't get any top end speed. It is starving for fuel.

I would do the fuel delivery test in the Factory Service Manual to make sure your pump is putting out enough fuel. I would also change the fuel filter to make sure you don't have a clogged filter.
Hi,

Thanks! But I did fuel pump test. It puts out exactly 0.5L per minute as it must with regards to Service Manual. The fuel filter is also brand new.

By the way if I'm not mistaken with lean fuel mixture the bike runs bad on every gear. In my case on 1st, 2nd, 3d and 4th gear the dynamic speed and torque are excellent but when I reach 120 km/h on 5th gear it seams that somebody or something changes ignition timing in a moment.
 

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Do you watch the tachometer? The problem should occur at about the same RPM in any gear, I would expect; if not, and it is truly a 5th gear issue, then engine "load" is likely a contributing factor.

There was one member (Broke Winger) that switched his actual vacuum advance unit to one that had a "weaker" feel to it, and it cured his high speed "bogging" if my memory serves. (I don't remember if the 1200s HAVE a mechanical advance, or if they are all electronic)

Some other things that might be considered as possible, could be problems in the carburetor slides and/or the diaphragms, faulty spark plug wires, or even a head gasket issue. :waving:
 

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Discussion Starter · #15 ·
Thanks for ideas FenderHead. Yes, the problem occurs at the same rpm in any gear at about 4000 rpm. But I can't ride for a long time in 1st gear for example with 4000 rpm.:) and in 2d too:) So let us say this problem appears at high rpm (doesn't matter what gear is switched on).

P.S. Carburator sliders and diaphragms are Ok. Anyway I like your idea to switched vacuum advance unit to another one.
 
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