81 Special fuel level drops with rpms

[ckahleer]

I can only ride the bike slowly. if I try to accelerate it starts missing. My attempts to correct have included: replaced coil, wires, plug caps and plugs. Disassembled and cleaned carburetors, Installed gn250 igniter.
Today I checked fuel level in carb bowls with clear tubes. When I rev the engine the fuel level in the tubes drops very low.
See video. should it do that?
I hate computers! When I click "attach files" it does't see the video. It is an MP4.

 

[jetmechmarty]

I hate computers! When I click "attach files" it does't see the video. It is an MP4.

You have to upload the video to YouTube, Facebook, etc. Then you paste the link using the film icon between GIF and " above.

I'm going to take a WAG at your problem. Try it with the fuel cap open. It might not be venting.

 

[ckahleer]

Just tried it with the gas cap opened. No change. Its like the fuel in the tubes is being sucked back into the carbs when revving.
Are the float chambers on the carburers supposed to be vented? I don't see any vents.

 

[jetmechmarty]

Are the float chambers on the carburers supposed to be vented?

Yes, but by 1981, not to atmosphere. I'll let someone with more carburetor experience take it from here.

 

[ckahleer]

Here's the video

https://www.facebook.com/video.php?v=566658881604075

 

[jpdevol]

Stock BS34's?

I think you'll find the sight tube's volume is getting sucked up by venturi vacuum while running - normal

No video yet

 

[jpdevol]

I'd verify ignition with timing light (if possible) before more carb investigation.

Test diaphragms, both drop test and "blow" test?

 

[Jim]

Just tried it with the gas cap opened. No change. Its like the fuel in the tubes is being sucked back into the carbs when revving.
Are the float chambers on the carburers supposed to be vented? I don't see any vents.

Yes, they're vented to atmosphere.... the two holes in the bellmouth.
You running K&N type filters by any chance? The lip on 'em has been known to mess with the bellmouth holes. Try taking the filters off?

 

[jpdevol]

Video viewed. Seems to rev up ok there. Does this issue only arise while riding under load?

 

[ckahleer]

It has stock air box and filters.
If you listen to the end of the video you can hear the missing and backfiring while I was holding it at 6K rpm.
While riding it starts at 4k rpm.

 

[Jim]

Whenever you rev the engine, low pressure in the venturi sucks fuel up out of the bowl. That creates a slight low pressure in the bowls. That in turn pulls fuel back out of the sight tube.
That doesn't mean the level in the bowls is dropping. As fast as it recovers when you close the throttle.... indicates the bowl levels are staying up.... else there'd be a lag while they refill.
I've never ran one when checking fuel level. I suspect however, what you're seeing is normal.

 

[jpdevol]

Can you verify ignition timing? Can you check compression?

The carbs can be re-verified, but best if the above questions resolved first.

 

[Jim]

For any suspected carb problems, the normal order of troubleshooting is... everything else first.

 

[jetmechmarty]

For any suspected carb problems, the normal order of troubleshooting is... everything else first.

I agree. Carburetors very often have electrical problems as well. I've ridden my bike with the carburetors all jacked up and didn't even know it! I thought it was running fine!

 

[ckahleer]

I think we are getting somewhere. Pickup coils ohm out within spec, but timing is jumping from full advance to between the U when revs are held over 3.5k RPM.
Any ideas what may cause this?

https://www.facebook.com/video.php?v=409245237853306

 

[Jim]

I think we are getting somewhere. Pickup coils ohm out within spec, but timing is jumping from full advance to between the U when revs are held over 3.5k RPM.
Any ideas what may cause this?

https://www.facebook.com/video.php?v=409245237853306

Try swapping the pickup leads from the crank pickup to the GN box.

 

[ckahleer]

The problem may have been cheap Chinese junk. Swapping pickup leads did not help, did not hurt either. Since these igniter boxes were only $15.00 each, I had ordered two. I put the 2nd box in and advanced much better. The first box (in video) would jump far past full advanced and jump back to forth to the U mark. The second box gradualy moves to full advance, (about 6:30 position) while engine speed increases,and stays there while above 3.5k rpm.
Raining today. I'll take it for a test ride tomorrow.

 

[Jim]

The problem may have been cheap Chinese junk. Swapping pickup leads did not help, did not hurt either. Since these igniter boxes were only $15.00 each, I had ordered two. I put the 2nd box in and advanced much better. The first box (in video) would jump far past full advanced and jump back to forth to the U mark. The second box gradualy moves to full advance, (about 6:30 position) while engine speed increases,and stays there while above 3.5k rpm.
Raining today. I'll take it for a test ride tomorrow.

Yeah... one of my GN boxes acts screwy like that. Ya gets what ya pays for....

 

[jetmechmarty]

I have not yet tried the GN box, but I also bought two. Great minds think alike!

 

[ckahleer]

Today I went for a ride with the 2nd gn250 box. A little, but not much better. Reinstalled origional stock igniter box, still no better.
Low rpms runs fine, but breaks up on acceleration and above 4K rpm.
I read somewhere on this forum that the pickup coil magnet on the rotor can become weak. Could this cause my problem? Can pickup coils be bad even though they ohm within spec, (750 ohms). BTW compression is 150psi cold.