Power/Carb issues - '77 GS750

[Anonymous]

I encountered some problems on the road yesterday with my 1977 GS750. Since my field observations were affected by traffic, what I'm giving below is what I've seen in my shop. I hope it makes sense enough to get a good dialog started. I am at a loss about what is going on.

First, what happened on the road: loss of power, engine bogged down when I'd give it some throttle. Couldn't get above 40 mph. Engine ran as if it had something stuck in its throat. If I gave it a lot of throttle, it sounded as if it were drowning. (Previous rides were without any problems. Bike was rebuilt this year, about 300 miles on the rebuild. Rebuild included carbs. Previous owner had installed larger jets and air pods. I kept those.)

Back in the shop I checked the plugs. All 4 were black as coal. Cleaned them up so I could do some testing.

Bike starts with sputters and pops. Backfiring out of all 4 cylinders. Idles at about 1200-1500.

If I turn the throttle slowly, the bike will sputter and protest, but still accelerate RPMs. If I hit the throttle suddenly, the bike dies.

At times I can ease the throttle up so the bike is at 2500 RPMs. I let off the throttle, the bike stays at 2500. If I hit the throttle hard while it's at 2500, the engine will respond properly--it revs right up. When I drop the throttle, it'll return to 2500. After about a minute, it drops on its own to 1200-1500.

And take a guess on the condition of the plugs after about 20 minutes of running. Yep, black as coal again.

All of this comes after carb rebuild, synch, idle and mixture adjustments, etc etc. I've done so many things that I've kinda lost the starting point I think.

Any advice?

Thanks in advance!

[SqDancerLynn1]

Do you have rust in the tank??? It could be the carbs are dirty again even tho you just cleaned them. Do you have the original coils?? if so I would consider replacing then even with some used later model GSX/Bandit coils

[Anonymous]

Re: Power/Carb issues - '77 GS750

I did clean out the tank, so there's no rust--well, as far as I can tell--feeding into the carbs.

The coil is the original, as far as I know. Which brings up a good question: I've tested each plug for spark, but how do I know if it's a strong or weak spark? I have nothing else to compare it to. Any tips?

[Big T]

Pull the #1 and #4 plugs, hook them up to the plug wires and lay them on the head so you can see the tip

Crank the engine. Look at the spark:

Red is dead
Blue is OK
White is even better

Best done in the dark, so the spark is easier to see

[KEITH KRAUSE]

What happened on the road sounds like you lost a coil and were running on two cylinders. Or, a rich mixture fouled one or two plugs.
With a new plug, the spark should be fat and blue. What plugs did you install? Timing must be correct too. If you still have the points, check their gap and be sure they're clean and not pitted.
Air filter clean/replaced?
Where did you set the side air screws at?
Where did you set the pilot fuel screws (underneath) at?
The slow to return to idle sounds like an intake leak. Did you replace the manifold o-rings and check the manifolds too?
Did you change the jet needle e-clip position?

[Anonymous]

It sounds to me that the problem, whatever it is, is common to all four cylinders equally. I suggest that bad ignition would not lead to black sooty plugs. I would concentrate on carburation myself. It sounds like it is super rich. Last time my bike did that was when my air box filled with gas because of a bad fuel tap and a plugged airbox drain hose. Fuel enrichment circuit O.K.? Try turning off the gas and riding it to see if it runs better just before it dies as it leans out.

[Anonymous]

Do you have a mechanical advance on the end of your crank under the points cover? If so, it sounds like it may not be working. If you do have a mechanical advance, it needs to be removed, cleaned and lubricated as a standard maintenance chore now and again.

[KEITH KRAUSE]

It sounds to me that the problem, whatever it is, is common to all four cylinders equally. I suggest that bad ignition would not lead to black sooty plugs. I would concentrate on carburation myself. It sounds like it is super rich. Last time my bike did that was when my air box filled with gas because of a bad fuel tap and a plugged airbox drain hose. Fuel enrichment circuit O.K.? Try turning off the gas and riding it to see if it runs better just before it dies as it leans out.

A bad igition can't lead to sooty plugs?? Try telling that to countless owners who have had electrical problems. A weak spark won't ignite the mixture completely, so combustion is poor and the plugs are black/sooty.
Turning the fuel off (not possible with vacuum petcocks) will just give you a bike that's running out of gas, along with the other problem.
Removing his airbox lid is a better test if he wants to try it. But that still doesn't prove electrical or carburetion related.
He needs to verify the spark quality and timing/point dwell/point condition first. Then make some basic adjustments to his VM carbs and check other basic maintenance points. Then re-test.

[Anonymous]

Power/Carb issues - '77 GS750

Keith, I'm going through the timing/points/etc checks. What I found first was that the points for 2 & 3 had shifted. That could be part of the original problem.

However, now I've encountered a condition that confuses me. The two sets of points are sparking at greatly different intensities. The points for 1 & 4 appear to be in ecstasy. The points for 2 & 3 hardly spark at all.

Thinking it might be a faulty coil, I swapped the two coils around. Same condition: points on 2 & 3 hardly spark at all, while 1 & 4 are going crazy.

In addition, it's impossible to get the timing marks to line up, especially for 1 & 4.

I'm really getting confused now. Any assistance would be appreciated.

(By the way, regarding carb settings: mixture screws are at 1/2 turn; the pilot fuel screws are at 1 3/4; filters are new (pods); did not change the e-clip position, and just realized that I forgot to note the position.)

[SqDancerLynn1]

Uneven spark from points I would Replace the condensers

[KEITH KRAUSE]

I too, would replace the points and condensers. They're cheap.
If you want to try though, make sure the points are clean, not pitted, and each side of the point is parallel with the other. Set the gap. Also check the plug caps for good clean connections. As for the timing, if the engine is missing at all, the timing will jump around and be impossible to set. You could do a static check, but that's not as accurate as using a timing light. I'm guessing the manual should show how to do a static check. Never done it to a points type ignition myself. If any doubt, especially the possibility of failing condensers and you don't have a tester, just replace the points and condensers or get a Dyna "S" ignition or similar, if you have the $. After any of this you should have good spark.
As for the basic carb adjustments, I'm not sure what screws you're talking about when you tell their adjustments. So we're on the same page, the side air screws, mounted on the side, for allowing more AIR into the mixture...and the pilot fuel screws, mounted underneath and engine side of the bowl, for allowing more MIXTURE into the carb throat.
The SIDE AIR screws should be adjusted using the "highest rpm method". They GENERALLY end up between 1 1/2 and 2 turns out. Do you know how?
The PILOT FUEL screws adjustment varies from bike to bike. To allow for your pods, a 1/2 additional turn out (richer) from their factory setting is a good starting point. Fine tuning is necessary.
Another basic maintenance item is a good carb vacuum synch. If your carbs vacuum is unbalanced, you'll have poor performance and can foul plugs. Float level has to be correct too. Also, with pods, you must remove the two float bowl vent tubes to avoid fuel starvation. Leave the nipples open. This is a requirement mentioned in jet kits.
If you didn't replace the manifold o-rings and check the manifolds too for cracking, that's most likely the cause of your slow to return idle, unless you have incorrectly routed throttle cables. Sounds like an intake leak to me though.
It's still hard to say what's causing your problem. You've only had it running about 300 miles since the rebuild. It could be poor carburetion from the beginning that's creating a rich mixture that's taken a while to accumulate. It could be the points/timing/spark quality and/or a loose connection. It could even be low compression/poor combustion if the rings haven't seated. It could be a little of everything. That's why I suggest first checking the points/spark/connections, then the basic carb settings and the synch. If it still has the problem, these things needed checking and adjusting anyway and we're narrowing it down.

[Anonymous]

Power/Carb issues - '77 GS750

Ok, gents, here's the latest...

First, to answer a few questions from Keith:
Agreed on nomeclature for the adjustment screws. We are accidentally on the same page there. I do indeed know how to adjust the side air screws. And I have been using a carb synch tool, the mercury type from MotionPro. It was a challenge to balance them out, of course...

Thanks for the information on removing the float bowl vent tubes. I didn't know about this recommendation, for the previous owner had installed the larger jets and didn't pass along any of the kit's documentation.

I have trimmed the spark plug leads and have fresh connections. The points and condensers are new.

So, the current situation:

Decided to test the condensers by swapping them to see where the sparks end up. In the process, I encountered some very worn wiring on the leads that go into the points (the ones that come in from behind the timing plate). Patched those up and now the 1&4 point set has settled down a bit, the 2&3 set remained the same. My assumption is that they are both sparking in an appropriately subdued manner. Or so I thought...

For when I hooked up the timing light to test 1&4, I got no strobe. I put the lead onto the #1 spark plug wire, then tried it on #4. No strobe. Put it over on 2, got some strobe, put it on 3, got some strobe.

So I did a very simple test: with the engine running, I pulled the wire off of spark plug #1: engine kept running. Ditto when I pulled it off of spark plug #4. If I do either #2 or #3, well, the engine dies. Seems my 4-cylinder has now become a 2-cylinder...now that's efficiency...

So am I correct at this point in assuming faulty coils? Or are there still too many variables?

[SqDancerLynn1]

It does sound like a bad coil?? What do the plugs look like??? IF it was mine I would get a set of late model 3 ohm bandit coils & replace both of them. I have three ohm coils on my 79GS850 I am still using the points. I modified the ign by installing a 2 ohm balist resistor into the 12V line to get the required 5 ohms for the points & can remove the resistor when installing a dyna

[KEITH KRAUSE]

That test would certainly point to a coil or the wiring to the coil. Double check the wire connections.
Because of the sudden problem you experienced on the road, that was mentioned at the beginning of my first reply.
OK, OK, Lynn and Big T mentioned it too.
After you take care of it, let us know how it goes. I still think your slow to return idle may be an intake leak. Most likely the manifold o-rings.
Hopefully the jet needles have been raised to allow for the pod filters. A lot of people think you just pop in larger mains and call it done.
Let us know if you think the jetting may be off.