Missing at high speeds/rpm

[Scudder]

I have a 1978 gs750. Everything is stock except for 4 to 1 exhaust and the main jets which are at 102.5. 100 mains are stock for my bike. The airbox is on.

The bike will run fine (maybe not perfectly) at lower speeds and then when I hit higher speeds 60-70mph and rpms, then it starts missing. Then I will limp home missing all the way, even at low speeds. I assume this is from the fouled plug which are black and gassy when I pull them.

I've... replaced the plugs and caps, replaced the points and gapped them, and basically eliminated the possibility of an electrical issue (I think). All 4 cylinders are firing all the time now, which didn't always happen before the plug caps and points.

Do you think it could be the main jets? I have heard that if you go to a 4 to 1 exhaust you should go up one size in mains to accomodate the added air flow. The bike had 102.5's in it when I got it. It didn't run well for several reasons then.

I just wanted to check with you guys before I pull the carbs off again.

Scudder

[rustybronco]

The bike will run fine (maybe not perfectly) at lower speeds and then when I hit higher speeds 60-70mph and rpms, then it starts missing. Then I will limp home missing all the way, even at low speeds. I assume this is from the fouled plug which are black and gassy when I pull them.

Fouled plug or plugs? what do you mean by gassy?

if all four are fouled, I would suspect the fuel levels being too high or the clip placement on the needles in the wrong positions.

[Scudder]

The last time out it was all 4 plugs. They all have been pretty black on a number of episodes. The last time out the plugs were black and wet with gas. If I don't take it too fast the plugs will become black, but if I go expressway speed/high rpms then they are black and wet with gas. I'll check the needle to make sure clip is where it's supposed to be.

Thanks,

Scudder

[rustybronco]

I cannot highly stress the importance of setting the fuel levels correctly. if in doubt, double check the 'wet' fuel level on each carb. also, make sure the sealing surfaces of the 'choke' plungers are not damaged and are fully seated when the choke lever is off.

are you sure 100's are the correct jets required? not 97.5's? http://thegsresources.com/_forum/sho...&postcount=149

[Scudder]

My engine number indicates 100 mains, but I haven't checked the wet fuel level. I guess I'll have to learn how to do that. Thanks.

Scudder

[Coady]

Those main jets won't cause the fouling.

Could be the other carb stuff as suggested. Check that first.

But if not, I'd suspect that you're either not getting good spark at higher rpm: check voltage at the coil and resistance of the coils. And check the battery voltage at 5000 rpm to make sure charging system is OK.

And/or you're actually oil fouling the plugs.

[Scudder]

Took'er out for another good ride. I checked the electrical and that doesn't seem to be a problem anymore. I think bad sparks was contributing to the plug fouling (possibily?).

Pulled the plugs after the last ride and all the plugs showed her running lean. I suspect it could be the airbox not being sealed up enough. Going to weather strip it soon. THe reason I think this is I went from 1 1/2 turns on the air screw, down to 1 turn, and that seems to remedy the missing a bit.

Does this sound right, or am I heading in the wrong direction?

Scudder

[BassCliff]

My engine number indicates 100 mains, but I haven't checked the wet fuel level. I guess I'll have to learn how to do that. Thanks.

Scudder

Hi,

Some have made their own "wet" level tester by drilling out a drain bolt and affixing a clear tube.



If we are talking about VM carbs, the general rule is to start with the fuel screw about 3/4 to 7/8 of a turn out from lightly seated. Then set the air screw approximately double that. Fine tune the air screw using the "highest idle method".

This might be a handy reference, the Mikuni VM Tuning Guide: http://www.mikuni.com/pdf/vmmanual.pdf

Thank you for your indulgence,

BassCliff

[Coady]

Took'er out for another good ride. I checked the electrical and that doesn't seem to be a problem anymore. I think bad sparks was contributing to the plug fouling (possibily?).

Pulled the plugs after the last ride and all the plugs showed her running lean. I suspect it could be the airbox not being sealed up enough. Going to weather strip it soon. THe reason I think this is I went from 1 1/2 turns on the air screw, down to 1 turn, and that seems to remedy the missing a bit.

Does this sound right, or am I heading in the wrong direction?

Scudder

It's a bit confusing cause you posted about possibly running rich and fouling plugs and now you're suddenly running lean?

[Big T]

Took'er out for another good ride. I checked the electrical and that doesn't seem to be a problem anymore. I think bad sparks was contributing to the plug fouling (possibily?).

Pulled the plugs after the last ride and all the plugs showed her running lean. I suspect it could be the airbox not being sealed up enough. Going to weather strip it soon. THe reason I think this is I went from 1 1/2 turns on the air screw, down to 1 turn, and that seems to remedy the missing a bit.

Does this sound right, or am I heading in the wrong direction?

Scudder

The VMs are not as sensitive to airbox sealing as are the BS (CV) carbs

What are your fuel screws set at?

I assume you've been referring to the air screws (on the side, near the back)

Do all your screws have tips?

Did you replace the condensors with the points?

Timing advance working properly?

[Scudder]

Agreed. It is confusing. It was fouling plugs before I timed up the points, now it is running lean. I did not change out the condensers when I did the points.

Fuel screw are one turn out. I haven't check the tips on the air screws.

Last night I took it out again.

Plug 1: White lean
Plug 2: Black but not wet, rich I assume
Plug 3: White lean
Pulg 4: Brownish Black (Pretty good I think)

I haven't replaced the O rings, so I plan to do that as soon as I get them.


Thanks

Scudder

[Big T]

Agreed. It is confusing. It was fouling plugs before I timed up the points, now it is running lean. I did not change out the condensers when I did the points.

Fuel screw are one turn out. I haven't check the tips on the air screws.

Last night I took it out again.

Plug 1: White lean
Plug 2: Black but not wet, rich I assume
Plug 3: White lean
Pulg 4: Brownish Black (Pretty good I think)

I haven't replaced the O rings, so I plan to do that as soon as I get them.


Thanks

Scudder

Go out and pull the ignition cover. Start the bike. See if there's sparking at the points (best done in the dark) If there is - there's your "miss" and replace the condensors.

Were the "old" points all burned up when you replaced them?

I think you need to sort out any ignition issues before you can move on to the carbs. This will be:

1. No condensor failure
2. Proper timing
3. Proper advance

Did you do the timing statically, or do you have a light?

Your original problem was due to the timing being incorrect, so get that completely sorted and any carb problems will be easier to diagnose

[Scudder]

Yes. There is spark at both points. So, the miss is because the condensers are bad? Thanks. I will follow your line of solving the problem. I feel like I've been bouncing from electrical to carbs for a while now. I'll order some condensers.

Scudder

[Big T]

Yes. There is spark at both points. So, the miss is because the condensers are bad? Thanks. I will follow your line of solving the problem. I feel like I've been bouncing from electrical to carbs for a while now. I'll order some condensers.

Scudder

Bad condersors indeed

Bad condensors = misfire= worse at high speeds = can continue due to heat build up = improper firing of spark plugs = wet, black plugs

Sound familiar?

Once you get the ignition up to snuff, then dial in the carbs

Use your downtime to strip, clean and install new O rings in the carbs

[BassCliff]

Hi,

Good call there, Mr. Big T. Being the spoiled rookie that I am, with electronic ignition, I never think about points and condensers.

Mr. Scudder, please keep us informed. Once you get good, consistent spark the carbs should be easy.


Thank you for your indulgence,

BassCliff