GS750 compression

[Jeff Adams]

Hey guys, I am finishing up a 78 GS750, but I'm having trouble getting it running right. Been thru the carbs, even had 3 different sets of carbs on it with the same issue. 3 cylinders have compression around 180lbs, but no. 3 is 130 lbs. I know that's low, but I didn't think that it would keep the motor from running better. No 3 cylinder doesn't seem to be firing at all. It has fuel and spark, but the temp is much lower than the other 3 cylinders. Can that difference in compression cause the issue? Thanks
Jeff

[Nessism]

130 psi is more than enough to fire the cylinder.

It could be a spark problem. 2-3 are on the same coil, so maybe it's the 3 plug wire or cap problem. The wires and caps can be changed. All this said, have you adjusted the valves yet? Synced the carbs? Properly rebuilt the carbs with new O-rings, and checked fuel level using the "clear tube" method?

[Jeff Adams]

It's getting good spark and I have been thru all 3 sets of carbs that I have tried with the same results, so it's not the carbs or spark. Also has a new electronic ignition and yes, I have adjusted the valves, too. That's why I'm at a loss as I have tried everything I can think of.

[Nessism]

Well, it's got to be something. Fuel, spark, compression. You have compression.

[Rijko]

How do you determine "good" spark ?
I have seen plugs spark nice and strong, but turned out to be broken.

What else have you tried ?
Did the bike ever run right for you ?
pssst .. may want to rethink dismissing advice from an experienced guy like Ed

[Jeff Adams]

New plugs, but I don't know the bike's history. I bought it out of a bike shop whose owner died with no title and swapped everything onto a titled frame. This issue may have been why the bike was in the shop in the first place. Seems to me the next step would be pulling the head, though I'm reluctant to do this as it was only a project to sell. The odd thing is a also have a 78 GS1000 with number 3 cylinder acting the same way.

[Rijko]

Is one of the 3 carb sets you tested from that GS1000 perhaps ?

Number 3 has the vacuum hose, coincidence ?
Is that connected properly and not leaking ?

What happens if you squirt a tiny bit of gas or brake cleaner into that hose while running the engine ?

[Nessism]

Did you rebuild the carbs properly; full teardown, soaking in carb dip, reassembly with new o-rings and gaskets?

[Jeff Adams]

2 sets of the carbs are from GS1000's, but they look like the same carbs. The the petcock hose comes from cylinder 2. 3 and 4 are just vent hoses to my understanding, unless I understand that wrong. That's the way the 78GS750 I had last year was set up and it ran fine. School me here guys if I'm missing something. To me it's obviously not a carb issue when the motor runs the same on 3 different sets.

[Nessism]

2 sets of the carbs are from GS1000's, but they look like the same carbs. The the petcock hose comes from cylinder 2. 3 and 4 are just vent hoses to my understanding, unless I understand that wrong. That's the way the 78GS750 I had last year was set up and it ran fine. School me here guys if I'm missing something. To me it's obviously not a carb issue when the motor runs the same on 3 different sets.

Then the problem is ignition.

[chuck hahn]

petcock vacuum is off 3 not 2. 2 and 4 are vents and they do not get capped off. A length of vacuum hose off each vent ran down between swing arm ant rear of engine will suffice. Number 3 isnt firing right because it has a MASSIVE vacuum leak because the venting and petcock vacuum nipples are all screwed up. Arrange as i said and youll see a huge difference.

EDIT... IF youre running it off a gravity bottle you need to plug the hose off number 3 to eliminate the vacuum leak same as if it were hooked to the petcock

[Rijko]

... School me here guys ..

ok. Most important lesson when troubleshooting, is not jumping to conclusions and dismissing possible causes.
You are doing both.

First, you dismissed Ed's advice.
You also dismissed my advice about the number 3 hose because you were convinced the carbs are not the problem.
Should have tried it instead of dismissing.

... the petcock hose comes from cylinder 2. 3 and 4 are just vent hoses to my understanding, unless I understand that wrong.

Wrong. Number 3 is a vacuum hose.
Start the engine and put your finger on the top of that hose, you will feel it suck air.

Did you not wonder where to find the hose for the vacuum port on your fuel tap ?

... That's the way the 78GS750 I had last year was set up and it ran fine.

Believe me, that bike did not run fine. With the vacuum hose open, that engine will probably run so lean that number 3 may be damaged.
Sold it ? Warn the buyer.

.. To me it's obviously not a carb issue when the motor runs the same on 3 different sets.

If you hook up the carbs wrong, it's actually quite logical you have the same issue with 3 different sets of carbs.

.. The odd thing is a also have a 78 GS1000 with number 3 cylinder acting the same way.

Did you think that through ?
So you have 2 engines, both having an issue with cylinder 3 not running.
What could be the common factor in that ? You used the same carbs on both engines !
Hooked up wrong in this case, but the issue could be anything in them carbs.

Just well meant advice, hope you learned something.

[zed1015]

As others have advised.
Number three ( 3 ) "Vent hose" is NOT a vent hose and if not connected to a vacuum tap MUST be CAPPED OFF otherwise number three ( 3 ) cylinder will run weak.
This excessivley weak mixture can cause issues which may lead to the compression loss you have found.