still confused: carbs or electrical

[dtkid]

My bike runs great in low gears and up to a speed of about 45. When I shift into 5th gear and hit fifty...it falls on its face (80' GS750L).....Now, it is as if the throttle has a "dead-spot" because when the bike refuses to accelerate I can back off the throttle and the bike sorta "straightens out" and behaves normally. Also, it seems to get worse the longer I ride (as the bike heats up). My thinking which rules out carb/main-jet problems is that all four cylinders suffer at once. If it were carbs all four cylinders wouldnt' fall flat at once, would they?? (I wish it were carbs). My battery is charged, my plug wires are old but I've got good rubber insulating tape around the boots (is there any way to use the old coils and replace the plug-wires on these?) I can't find any bare-spots on wires. The chargin' system seems to be working. I'm not blowing any fuses (anymore). I can try the coils from my 81' but I want to hear what the experts say first.

Please provide knowledge!!!!

Much Thanks from the Mountains: David

[ddaum]

Not an expert here at all, but I just got my bike out of the shop after waking her up from a two year nap. Just before I left the shop the mechanic gave the whole bike one last look-over and found a fuel line that was not very ridgid and actually kinked a bit. the bike was running and idling fine, but he mentioned that at higher speeds with more fuel demand that the kink might starve the engine a bit on the highway. He replaced the whole line with a more ridgid tube and made sure there were no kinks. the bike runs great at all speeds.

Try checking/replacing your fuel filter and have a good look at the fuel lines and the way they are routed to eliminate any small kinks. Might not be a bad idea to remove and wash your air filter too. That's my two cents, Good luck.

[duaneage]

Easy and cheap things FIRST

Check fuel flow. Drive with the petcock onPRI and see if that cures it. Could be the petcock is not working properly .

Next, check the advance mechanism to make sure it is working properly, You could use a timing light to see if the timing advances. If your not sure how this is done, ask and I can elaborate. The advance weights are on the right side behind the Suzuki cover. There is a spring that allows them to fly open at 3400 rpm to advance the engine. IF they are too loose or do not move apart then there you go.
Onto carbs, if you have vacuum carbs (CV type) make sure the choke is not always on. Check that the linkage moves fully in when you have the choke off.

Check plugs at 50 mph by killing the engine. White and we have e fule dilvery problem. Black and your having carb cleanliness issues and probably need to replace some old O-rings.

Good Luck and keep us posted.

[xifer]

Re: still confused: carbs or electrical

My battery is charged, my plug wires are old but I've got good rubber insulating tape around the boots (is there any way to use the old coils and replace the plug-wires on these?)
Much Thanks from the Mountains: David

You can remove the old plug leads and replace with new, there is
a pin on the sec. connector that sticks into the copper inside the HT lead,

then seal it up using silicone sealant

[dtkid]

she just died

On my way to work this morning it refused to run even at low speeds, my darlin' is sittin' outside in the parkin' lot and won't start. Turns over good, no blown fuses, no gas line kinks.....I'm baffled. Guess now I'll have to put the other coils on....this is two strange. It started up fine this morning and I rode slow for the 5miles into work but when I got-on-it a little it fell on its face and refused to come back??!!!

I'll keep yall' informed. Thanks for the input...David

[duaneage]

Fuel starvation. Two coils would not fade away at the same time like that. If you open one of the drain screws on the carbs does any gas run out?

[dtkid]

fuel tank sending unit?

Strange: the bike starts right up after is has sat for a while. Took it home, changed plugs, coils, fuel filter, gas lines and bottom and top of CV carbs yet it still falls-on-its-face at around 5K rpms. I'm gonna check the magnets in the timing advance today. However, I was wondering.....what does the "fuel-tank sending unit do? Could this possibly be the culprit. Also, the line that comes off the sending unit is a vacuum line I presume. I mean the distal end don't "hook-up" to nuthin does it?


Again..Much Thanks: David

[FTGRET]

The hose on the sending unit is just a drain line in case the gasket leaks. Still sounds as if you have a fuel flow problem. You may want to remove the tank and pull the petcock assembly to check that the filter screen isn't clogged.

[duaneage]

Re: fuel tank sending unit?

I mean the distal end don't "hook-up" to nuthin does it?

What are you referring to?

There is a vacuum line from the #2 carb to the fuel valve(petcock) that turns on fuel when the engine is running. It must be hooked up and not have a leak.

The diaphram can be checkedmanually by taking a suction on the hose and seeing of gas runs from the outlet. It should stop as soon as the suction is taken away. How you provide the suction is up to you.

[GS700Jim]

sounds like the same problem I'm having... does it start to die out around2500-3000 rpms'? I'm starting to really listen to everyone saying fuel related.... dirty carb.. filter clogged.. line kinked... more digging for me tonight!!

[dtkid]

perhaps petcock

I really have been hesitant about the problem being fuel-related, until just about 5 minutes ago. Mine shots down at 4500 rpm (almost exactly and consistently) however, I have a good flow of fuel when I'm just looking at the line. Gas gushes out of the tank. But, a few years ago I had to place an in-line stop-cock in my main fuel line because the pet-cock knob got screwed up. I thought that since I keep my pet-cock on the "on" position and because I have good fuel flow (when I'm not running) that it couldn't be fuel. But I just checked my new plugs and they are indeed a little white. And since I've eliminated all other possiblities (I hope) it must be the pet-cock. QUESTION: Is there anyway the pet-cock could cause a decrease in fuel delivery at higher rpms? And that this decrease would be mighty consistent (always at 4500 rpms)??

I have another tank and I'll put it on later tonight to see if that solves my mystery. I let you know what happens in the morning.

Thanks much for the assistance: David

[redliner1973]

Re: perhaps petcock

QUESTION: Is there anyway the pet-cock could cause a decrease in fuel delivery at higher rpms? And that this decrease would be mighty consistent (always at 4500 rpms)??

Most definately YES!!! You sound like you have rust/trash in your tank clogging something up. Or defective petcock..

[dtkid]

Shazam!

IT WAS THE PETCOCK!!!!!! Put my other tank on last night and drove 80 mph all the way to work without a hitch. (Well, not actually all the way).
Once again this site has rescued my spirits.

I just put a check for $25.00 in the mail to support this wonderful place.

Much Thanks: David