Stumped - Fire on #1 & #3 cylinders only???

[1978GS750E]

Trying to sort the wiring on my GS1000 my came across an issue that has me stumped! I get fire on the #1 and #3 plugs and nothing on #2 and #4. I've got a brand new, fully-charged battery and cannot get any fire on the even number cylinders??? Doesn't one coil fire #1 and #4 cylinders and the other fire #2 and #3?? How can I only get fire on one cylinder from each coil?? As you can guess, I loathe wiring issues and any help is greatly appreciated.

Any ideas?

The only electrical change that was made to the bike was the addition of a SH775 R/R. But that doesn't have anything to do with the firing, does it?

Do the coils have to be grounded to the frame? I had the frame powder-coated and where the coils mount it is really thick. Just grasping at straws here!

Thanks,
Ron

 

[chuck hahn]

Things to try:
trim 1/4 inch off wires and screw the caps onto fresh connections
try new plugs if they are old ones
remove the connector in the caps that grip the plugs. dump out the resistors and replace them with copper or steel slugs about the same diameter and length of resistors...resistors are not needed. and this mod permanently removes resistor failures forever. look in caps and youll see they are slotted for a screwdriver. NOTE.. 1 and 4 do not use a spring. 2 and 3 use a spring. put spring in and then the replacement

 

[1978GS750E]

Thanks Chuck! On a whim, I took my dremel and removed some of the powder-coat where the coils mount to the frame and guess what.....all 4 cylinders are firing now! I guess they DO have to be grounded?? Now to check the timing one more time. I replaced a head and base gasket and did everything per the manual, #1 arrow on cam pointed level with the top of the head, 20 pins between arrows #2 and #3 (counting the ones that the arrows point at) so I am hopeful that is correct. Who knows, this thing may be running this weekend!

 

[chuck hahn]

I made the jump they had ground based on both were firing respective cylinder...I still suggest removal of the resistors, First thing i do when building a bike.

 

[1978GS750E]

The powder coat was really thick, about 1/16" on the mounting tabs, including the holes. I suppose they were grounded just enough to fire 2 cylinders??? Now I'm getting a massive backfire when I try to start it! Must have done something wrong with the cams?

 

[chuck hahn]

Flooded cylinders and wet plugs in the two that werent firing would be my first suspects. Dry the plugs and dry the jugs a while with the air hose. Air movement will help evaporate excess fuel.

 

[GregT]

The powder coat was really thick, about 1/16" on the mounting tabs, including the holes. I suppose they were grounded just enough to fire 2 cylinders??? Now I'm getting a massive backfire when I try to start it! Must have done something wrong with the cams?

Not neccessarily. Your description of cam timing was correct. Check you've got the right coils to the right cylinders.
Plugs out,laying on the head, spanner on the crank end, you'll hear and see which coil fires. Look at the timing marks to check which pair of cylinders are coming up to TDC.

 

[1978GS750E]

What do you mean when you say "spanner on the crank end"??? I've verified that the #1 and #4 cylinders are at TDC when the "t" mark is aligned with the ignition. Going to look at the single point ground later today. I'm thinking that the powder coat may be an issue there as well??

 

[Brendan W]

To help identify the coil plugs, have a look here http://members.dslextreme.com/users/storagecliff/images/750e-s-z_wiring.jpg
Both coils have an O/W supply. 1&4 has the W trigger wire. 2&3 have the B/Y trigger.
Check that the SPG has connections to the r/r, the harness returns, the frame and the battery negative.
By spanner on the crank Greg means turn it over with a spanner on the crank at the ignition pickup and watch for the sparks as the 1/4 and 2/3 firing marks go by.

 

[1978GS750E]

Thanks Chuck, Greg T and BrendanW! Used the wrench (aka: a spanner in some places!) on the crank and found that my timing was off 180 degrees. My coil that should fire #1 and #4 was firing on the #2-3 F mark. All the color coded wiring is correct per the colors and the plug wires are marked 1 thru 4. So....did a quick switch of some plug wires and hit the starter button, and she fired right up! Still lots to do as far as cleaning up this wiring, but good to hear it run for the first time since December 26th. Makes getting into the garage easier tomorrow!

 

[chuck hahn]

Should have had to just swap the coil leads at the harness