1977 GS750 Running on Two Cylinders

[Guest]

Hi All,

Joined the forums since I just bought a GS750 last week. The previous owner did some work to the bike to get it running, but it is not running well. It idles high and when I rode it, it had absolutely no power and it would lose more power if I went past 1/4 throttle. I thought it was because he put pod filters on it and some out of carbs in need of cleaning. However after digging deeper, I found that cylinders 2&3 were not producing power (oily and wet plugs, exhaust pipes were warm while the others were hot to the touch) which made me suspect electrical since it smells like it’s getting gas and air is being pushed out when the plug is out? from what I’ve gathered the left coil is supposed to run cylinders 1&4 and the right coil fires 2&3. On my bike this is reversed. The coils were replaced by the previous owner so I suspect that this might have been done incorrectly. I tried pulling plugs and grounding to the engine but even on the cylinders that i knew were firing I couldn’t see any spark, not sure why, probably me not grounding it well enough. The old wiring is spliced into the new coils, 1 orange wire feeding two yellow wires, black wires are connected to two separate old wires. I tried to swap the wires from the 1&4 coil onto the 2&3 coil and all I got was explosions coming out of my carb. Swapped it back to how he had it and started right up and ran like it did before. Anyone have any suggestions? Are the spark plug wires just all on the wrong plugs? Links to wiring diagrams? I know the bike has multiple issues but I would like to sort this one and move on to the other ones after I know my electrical system is working correctly.

Thank you

[LAB3]

Welcome to the board! Can't be of much help with the wiring on a four but it seems you're on the right track to get started. Follow this link to find the wiring diagram and a trove of other stuff you'll be needing. http://gsarchive.bwringer.com/

[gustovh]

Check your points gap for both sets of points. That,s the place to start. Check gap and timing, another thing that could be an issue is a bad capictor for those cylinders. Have you checked for power to the coils?

V

[Andrewb.95]

Yes unfortunately I have a lack of tools at the moment which is why I’ve been testing temp with my hands and looking to see spark lol. I need to re invest. how do I check timing on these bikes? And where are the capacitors for this circuit?

[canuckxxx]

You need to know, for sure, if the coils are causing sparks. If you take a known good plug, connect it to a plug lead and lay it on the cooling fins so the metal part of the plug is touching the fin. Then turn the engine over and watch for sparks. Do it in a dark or dim garage to make it easy to see.

Make sure the switch on the right handlebar is in the ON position.

[tom203]

The capacitors (i.e. condensers) for the ignition are next to contact points....right front lower crankcase behind round cover. As far as wiring diagram ...this colored one from the 79 gs850 is similar as to ignition layout - it's easier to read than black and white ones

http://members.dslextreme.com/users/bikecliff/images/1979_GS850GN_wiring.gif

ignition coils are very durable, points ,condensers and bad wiring are better culprits

[Steve]

ignition coils are very durable, points ,condensers and bad wiring are better culprits

Agreed. That is the primary reason why Suzuki and most other manufacturers had switched to electronic ignition systems by 1980. Those have problems, too, but not as many.

Since you need to invest in tools, your first two electrical tools should be a multimeter and a test light. The light simply tells you that you have power, the meter can tell you how much. Unless you actually need the numbers, the light is usually quicker and easier.

In this case, you already know that you have power to at least one of the coils. Use your test light to verify that you have power to the other coil, too. That would be on the orange/white wire. You should have power whenever the key is ON and the kill switch is set to RUN. Next to the orange/white wires will be a black wire on one coil and a white wire on the other. Check for power there. You may have to rotate the engine to verify. If the points are closed, your test light will not be on. When the points open, the light will be on. As you crank the engine, the light should blink. The blinking light will tell you that the points work, but won't tell you about their condition or timing.

As far as which wire goes on which plug, the coil with the white wire should be connected to plugs 1&4, the coil with the black wire should be connected to plugs 2&3. Yes, the 1&4 coil is usually on the left (as it was from the factory), but I had one that was reversed, too. Easy enough to change.

.

[Andrewb.95]

Thank You all for the help, I'm going to take a look at all that after I get home from work today. I'll let you know how it goes.

[Himble]

And may I suggest that you get hold of the GS750 service manual here (if you haven't already done so):

http://gsarchive.bwringer.com/mtsac/~cliff/storage/gs/GS750_Service_Manual_early8valve.pdf