Re: Trying to fire on all 4s
From your post, two things are obvious. The problem is in the ignition system or wiring, and something is gnatzelfratzed.
Since we dont know precisely where the problem is, then the most efficient way of fixing it is to check the complete system.
A stock 78 750 would have a points and condensor ignition system.
1. gap the points and set the static timing for the 1-4 cylinders to the |F1 setting and the 2-3 cylinders to the |F2 setting. Point gap should be 14 thousandths. Iif you do not know how to precisesly set this up, say so and I will post a step by step set of intructions for gapping and timing)
2. Remove the gas tank and with a multimeter, check the voltage on the coil + leads (orange/white wire) on both coils. They should read almost the same as the voltage at the battery terminals. Ignition is switched on for this check. (multimeter scale set to 20 V DC) voltage measured should be greather than 12V.
3. Remove the positive (orange/white) wire and the white or black wire from both coils and check the resistance between the + and - terminals on the coils. Resistance should be 3 ohms. It will commonly be between 3 and 4 ohms, and anything between 2 and 5 ohms is within servicable limits. (multimeter scale set to 200 ohms)
4. Remove the plug caps from the 1 and 4 cylinders. Check resistance between the metal connectors inside those two caps. Resistance should be between 30k and 50k ohms. Do the same check between cylinder plug caps 2 and 3. (multimeter scale set to 200K ohms)
5. On the orange/ white lead wire (positive) to your coils, check the voltage of the wire by switching ignition to on, placing + multimeter lead on orange/white wire and black- meter lead on engine cooling fin or frame to ground it. Voltage should be about the same as voltage at battery terminals.
6. Check frame to coil mounting bolts for corrosion and clean if need be.
If the plug cap reading are not within 30k-50k, you can try removing the plug caps (they unscrew from the wires) and trimming 1/4" of insulation from the end of the wire, bending the core wire over the end of the spark plug wire end and then screwing the plug caps back into place. (you may not now have a good connection on the ends)
The voltage of the battery at the terminals with nothing on the bike on (ignition switch off) should read 12.7-12.8 if full charged and about 12.4-12.5 at 50% charge. Anything less that 12.4 is not what I consider within servicable range and I would replace the battery if it will not hold a higher charge than that. 12.0 -12.2 is pretty much a dead battery and useless.
Let me know what you find and depending on test values, we will know where to go.
Earl
seadrive1 said:
My GS750 (1978) had been in storage for 10 yrs. Last summer (August) I cleaned out the carbs, changed the oil, replaced the plugs, and got it running, but sometimes only on 3 cylinders, sometimes on 4. A month ago, I tried the bike again, and although it starts, never on 4 cylinders. When i removed a spark plug , and held it close to the engine ( 1/4 inch) the spark was strong. When I grounded the plug to the engine..... no spark.... so .... I then changed the spark plug caps.... and still.... same problem ! By the way... same thing happens on all 4 spark plugs.
Shouldn't I get a good spark when I ground it to the engine ??
So... Im thinking... do I have a grounding problem ?
Oh, well.... whats what im asking.... im out of ideas. Any advice ?
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