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GS750, Ignition trouble on cylinder 1 and 4

  • Thread starter Thread starter Facius
  • Start date Start date
F

Facius

Guest
Hey Guys.

I've been fiddling with the Ignition and Carbs on my GS750.
Carbs got:
bigger jets (140)
single pod air filters
new gaskets and an overall cleanup.

The ignition got
new contact breakers
new Spark Plugs.

The bike runs OK, but not as well as i would like -shoots a bit from the exhaust.
I put my Stroboscope in action to check the ignition timing.
Cylinder 2 and 3 runs correct.
Cylinder 1 - 4 doesn't.
my stroboscope only flashes every 2nd or 3'rd round, kinda hard to tell. But in my opinion it should at least be flashing at the same speed as cylinder 2 and 3, but its doesn't.

I suspect a bad contact breaker, low voltage or ignition coil.
My money is on the bad contact breaker as its new, but i doens't really make any sense to me why it would only fire sometimes, -as far as i can see it opens ans shuts correctly.

Do you guys have any suggestions on what could be wrong?
 
Did you put in new condensers too? try switching condensers around and see if non/erratic firing moves to 2 and 3. Are these aftermarket contact points?
 
fit OEM contact breakers and condensers, check for correct plugs,plug gaps, also check the plug boots are good and a good connection onto the plug wires (snip 1/4 inch off the plug wires and refit the plug boots)
 
tom203 said:
Did you put in new condensers too? try switching condensers around and see if non/erratic firing moves to 2 and 3. Are these aftermarket contact points?
Click to expand...

The condensers is the old ones, got the breakers of ebay (Daiichi Points Set).

Switched the condensers around, now the timing is perfect on cylinder 3 and 4. But the flashes is erratic on cylinder 2 and evenmore so on cylinder 1.
Only had it running for a couple of minutes this time and didn't try a run around the block, will have to do that later and see if it still shoots.
Cylinder 1 was the only one that didn't get a new spark plug, as my local store only had 3 left :(
 
I'd get new condensers asap-and another plug. As stated, cut wires 1/4 inch back to expose caps to fresh copper. For now ,try putting new plug in #1 spot and see if new erratic firing combos develop!
 
Just took it for a quick spin around the block. It runs better then at first, but still does a bit of exhaust shooting now and then. can't tell what cylinder is comes from. idles quite good though....

But tomorrow i'll get another plug for the first cylinder, and try to refit the caps.
It's gonna take a week to get some new condensers.

Thanks for the advise everyone, have a great weekend.
 
I may be wrong, but those main jets sound pretty big. What exhaust you running?
 
Road Rash said:
I may be wrong, but those main jets sound pretty big. What exhaust you running?
Click to expand...

The exhaust is a marshall 4-1 System, and pod air filters.
I added bigger jets after having a bit of trouble with the bike "dyin" when going from no throttle to fulll quickly, and also had the cylinders honed recently as well.
On a quick note, i checked the cam timing, re-adjusted the ignition timing, plug gap and added new ignition coils and plug boots.
I no longer have any trouble with the stobo/sporatic inigion, but it does bagfire a bit at the low rpms, or when i release the throttle.

You might be right on the main jets, i'll try replacing with the smaller ones i have lying around.
 
Last edited:
What year is the bike? Your main jets are are way too big for a 750. Taken from Dynojet Stage Three conversions (no kit listed for the VM Carbs on the 750) pipes and pods for a '78-'79 GS1000 VM carbs the largest jet you would use would be the equivalent to a Mikuni 132.5 and '80-'85 GS750 (listed conversion) would be Mikuni 122.5. You are too big for the largest listed jet for either VM or CV carburetors for your pipes and pods.
 
Last edited:
Facius said:
Hey Guys.

I've been fiddling with the Ignition and Carbs on my GS750.
Carbs got:
bigger jets (140)
single pod air filters
new gaskets and an overall cleanup.

The ignition got
new contact breakers
new Spark Plugs.

The bike runs OK, but not as well as i would like -shoots a bit from the exhaust.
I put my Stroboscope in action to check the ignition timing.
Cylinder 2 and 3 runs correct.
Cylinder 1 - 4 doesn't.
my stroboscope only flashes every 2nd or 3'rd round, kinda hard to tell. But in my opinion it should at least be flashing at the same speed as cylinder 2 and 3, but its doesn't.

I suspect a bad contact breaker, low voltage or ignition coil.
My money is on the bad contact breaker as its new, but i doens't really make any sense to me why it would only fire sometimes, -as far as i can see it opens ans shuts correctly.

Do you guys have any suggestions on what could be wrong?
Click to expand...


Read the Top Ten Newbie mistakes and see how many you qualify for
 
OldVet66 said:
What year is the bike? Your main jets are are way too big for a 750. Taken from Dynojet Stage Three conversions (no kit listed for the VM Carbs on the 750) pipes and pods for a '78-'79 GS1000 VM carbs the largest jet you would use would be the equivalent to a Mikuni 132.5 and '80-'85 GS750 (listed conversion) would be Mikuni 122.5. You are too big for the largest listed jet for either VM or CV carburetors for your pipes and pods.
Click to expand...

It's a 78 GS750E. I'll replace the jets this weekend, believe the old ones are 110 or 120, cant read the numbers on em.
 
Big T said:
Read the Top Ten Newbie mistakes and see how many you qualify for
Click to expand...

Bla di blaaa.... Now i feel stupid.
no really, i believe you are referring to the pod filters, am i right?
that damn airbox is just so ****ing ugly, especially on a cafe build :)
 
I think you need to let us know what the history of this bike is. What condition was it in when you got it, what you have done to it? Jetting for pipes and pods is no cake walk especially if you have no base line for comparison of a properly running bike and possible ignition problems also. Dynojet didn't make a stage three jet kit for your bike but made one for the same year GS1000 with the same carburetor the VM26SS. I wonder why. Jetting can be a PITA for your bike. What is an overall cleanup on your carburetors? If they were not dissembled and dipped in a carburetor dip for at least 24 hrs. the fine passages in the carburetors have not been cleaned, there is also O-Ring replacement required. Have the valves been adjusted? We need detailed information and it would not hurt to put your model and year in your signature (user CP). Done with the Drill Sargent bit.:)
 
Last edited:
OldVet66 said:
I think you need to let us know what the history of this bike is. What condition was it in when you got it, what you have done to it? Jetting for pipes and pods is no cake walk especially if you have no base line for comparison of a properly running bike and possible ignition problems also. Dynojet didn't make a stage three jet kit for your bike but made one for the same year GS1000 with the same carburetor the VM26SS. I wonder why. Jetting can be a PITA for your bike. What is an overall cleanup on your carburetors? If they were not dissembled and dipped in a carburetor dip for at least 24 hrs. the fine passages in the carburetors have not been cleaned, there is also O-Ring replacement required. Have the valves been adjusted? We need detailed information and it would not hurt to put your model and year in your signature (user CP). Done with the Drill Sargent bit.:)
Click to expand...

Thank you for the hint, i'll see if i can get to do a full description this weekend.
But in short, i bought it somewhat original, but with the marshall exhaust.
Not really a candidate for an original restoration, but the engine ran fine.

I've done a complete engine rebuild, the valves was grinded and adjusted, cyliners honed, new clotch plates, gaskets and so on.. and brand new wireharness created from scratch.

That was last year, this year:

The Carbs got a full teardown, thorough cleanup, new gaskets and the bigger jets (going back to the ones the bike came with this weekend, 120 i believe)

The Pod filters and the Jetting isn't really the problem, i believe i can figure that out. The problem was the ignition wierdness, which seems to be fixed at this point :)

Thank you all for your time, its been a great help.
 
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