Power loss GS750

[PeterBoomhut]

I've tried to get an answer on the Dutch MF but not succeeded. Normally he top speed is 160 but now 120 km/h
Someone tells me to execute an rpm drop down test bij disconnecting each spark plug and see which cylinder does the less attribute.
Cil 1: 1300
Cil 2: 1430
Cil 3: 1740
Cil 4: 1290

So cyl 3 does do a little.
On the MF they told me it's an ignition problem, so I Replaced coil 2-3 (same covers) , did a coil swap, changed out spark plugs against new ones.
Did an switch witch another TCI , an some other pick-ups.
So the entire ignitions was changed but nothing helps.

Now i'm thinking It's a fuel problem .
Should I first adjust the pilot screws ( With colortune) or do I have to synchronise the carbs first?

I did clean the carbs 2 & 3 with carb cleaner (removed the jets ) an dried every thing with compressed air.

Please help

[tom203]

It's not clear which year 750 this is....what carbs? standard ignition?
If you suspect power loss, have you done a compression test on each cylinder? Or is that whatyou mean by disconnecting each spark plug in turn

[PeterBoomhut]

This is a single disc from 1977. The ignition is replaced by a GS850 TCI and Pickup coils.
The original VM carbs with original airbox.
Th cycle has ride a whole year before last month the failure appears.
I did a compression test :
19-4-2018 : 12,5 | 11,5| 12| 12 | Bar
In PSI: 181| 167 | 174 | 174

[Steve]

Thanks for the meticulous report.

Your compression numbers are not bad, so I would not suspect valve adjustment. After all your changes, there is not much chance of ignition-related problems. One question that I do have, though, is: when you changed the coils, did you also change the caps the snap on to the spark plugs? You might try your RPM drop test after swapping the #2 and #3 spark plug leads. Just remove the leads from those two plugs, swap them and put them back on, then do your test again.

To answer your other question: I would recommend checking the carb sync before making any adjustments. That is to ensure that the carb is open enough to be contributing to the power mix.

.

[posplayr]

I would still check charging system with a Quick test even though it is unlikely to affect only #3. Op did not mention plug wires and whether it was swapped between 2 and 3. The only other thing I can imagine is plugged jets or perhaps bad Petcock. Doesn’t that effect #3?