GS750 not faster than 80km/h, with lots of struggling

[Guest]

Short summary:

Bike was standing still for 5 months. Cleaned the carbs, airbox, got new gaskets and boots on carbs, head, clutch cover, ignition cover, breather. New fuel line. Valves have been adjusted, carbs are synced, ignition timing checked and corrected. Brand new 4-1 motad exhaust mounted.

I took it for a spin, driving 130km/h on the highway easily. Then I took some countryroads, not going faster than 60km/h. After that, it wouldnt run any faster than 80km/h anymore. Second time I tried, it worked for 5 minutes and then started having the same problems. As soon as they start there is lots of popping and hissing from the exhaust.

Running to rich? Or something else? I put it away for now with disengaged clutch and killswitch and let it roll into the garage. Will do a plug chop this afternoon. Other than that, I'm at a loss for info...

Any ideas?

Drummert.

[Brendan W]

Popping is sometimes a sign of ignition trouble. Are any of the exhaust headers cooler than the others ?
Are you still on contact beaker points ignition ?

[mmattockx]

Bike was standing still for 5 months. Cleaned the carbs, airbox, got new gaskets and boots on carbs, head, clutch cover, ignition cover, breather. New fuel line. Valves have been adjusted, carbs are synced, ignition timing checked and corrected. Brand new 4-1 motad exhaust mounted.

Standing still for 5 months isn't an issue, my bikes do that every winter. Did you do any jetting to match the new 4-1 exhaust? Just the exhaust isn't a huge change but it will still need tweaking to suit. Did the bike run well before sitting?

I took it for a spin, driving 130km/h on the highway easily. Then I took some countryroads, not going faster than 60km/h. After that, it wouldnt run any faster than 80km/h anymore. Second time I tried, it worked for 5 minutes and then started having the same problems. As soon as they start there is lots of popping and hissing from the exhaust.

If it is intermittent then it isn't jetting, it is either ignition or you possibly have junk in the fuel that is clogging things up while floating around. Depending how long you were running at highway speeds it could be a tank venting issue as well. Electrics often work OK cold, then act up once they are hot so that is one place to check. Does it idle and run well at slower speeds when it begins acting up? Or does it run like garbage everywhere when it happens? Are you running a fuel filter?


Mark

[Drummert]

I'm still running on points. I brought it to a shop a few days ago and they checked the timing, which should be ok now.
I didn't do rejetting yet, because a guy from motad told me it wasn't necessary. That did surprise me and I still wonder if this is true. For what I've seen around here, I should rejet.
I don't run a fuel filter at the moment, but one should be coming in quite soon.

As soon as the problems start, they also appear at slower speeds, although not so obvious. I did a plug chop today and the plugs on 2 and 3 where quite black. The others were a nice brown. I did some adjustments to the pilot fuel screw and side air screw and got it running a bit better. Tried driving around and opening the throttle. As soon as i'm above 80km\h and I open the throttle, the bike starts popping and hissing.

The bike ran before sitting for 5 months, not smootly, but ran. Until the exhaust pipe on 4 broke and blew off at the head.

[Brendan W]

2 & 3 share an ignition coil. Coils are fairly bulletproof so I would start at the coil supply plug followed by a condensor. Is one point set arcing more than the other ?

[chef1366]

Points are medieval

[geol]

Since the problem appears to be combustion related on 2&3, it may be the coil or the points/condensor. Easy enough to check the coil. Swap the leads from the points from 1&4 to 2&3 and swap your spark plug wires. If the problem moves. coil is suspect. If not, point and or condensor suspect. The exhaust would not cause this dramatic of a power loss. More likely the bike is firing on two cylinders at times. You can also take a spray bottle of water with you when you ride and spray the pipes. If the water vaporizes, pipe is hot and cylinder is firing. If it pops and sizzles and bounces off the pipe when sprayed, the pipe is hot from being next to the firing cylinder pipe but the pipe in question is likely on a cylinder not firing.

[Drummert]

You can also take a spray bottle of water with you when you ride and spray the pipes. If the water vaporizes, pipe is hot and cylinder is firing. If it pops and sizzles and bounces off the pipe when sprayed, the pipe is hot from being next to the firing cylinder pipe but the pipe in question is likely on a cylinder not firing.

Tried this one. Drove around a little, then sprayed the pipes. 1,3 and 4 evaporates. 2 bounces off. So I figured I could very quickly check if the points arced as well. Points arc on 2&3, not on 1&4, which is confusing me now, because I thought it was working properly if it did arc.
Tried readjusting the point gap between 1&4, because it seemed off. Broke the screw which keeps it in place with the last part of it stuck in the plate...


I'm quite a patient guy, but this is testing me.

[Brendan W]

The condensor controls the rate at which the points switches off a coil and reduces the burning or arcing. All points arc. A correctly working points and condensor will have a gentle spark that may not be visible in bright sunlight. A heavy yellow arc is most often a sign of a bad condenser.

[Drummert]

Ok. I checked them outside in the sunlight. The arc between 2&3 wasn't yellow, but just a little blue/purple spark. Again, nothing between 1&4.

Next step: get myself some new screws and redo the point gaps? Then check the coils after that one?

[Drummert]

So I finished my exams, which means again some time to try and figure the problem out. What I found today:

- If I measure the ohms between the leads of the coils, I get some 6 ohms resistance on both coils.
- If I measure from one of the leads in front of a coil to one of the plug wires, I get either no reading or too much resistance to show... I didn't switch the coils yet, so it ran with these coils. How is thhis possible?

I took of the points plate to get the broken screw out, easy enough with some pliers. Now these moments always get me wondering whether I should buy some replacement screws from the hardware store or really look for the right screws online. I'd guess hardware store will do?

Reading my last post saying I had a spark between the points for 2&3, might me problem be a broken condensor? I do remember switching them around and still finding the problem...

[Brendan W]

6 Ohms sounds a bit high but more important the coils read the same. Generally I would expect 3-5 but I'd say the low side of the coils are ok.
To measure the high side remove the two spark plug caps and inspect the core of the HT leads. Corrosion in there will cause problems and can be eliminated by cutting back 1/4" of the lead to get clean core. Test the resistance between the two and you should get say 15000 Ohm plus or minus a few k. Both coils should be more or less the same.
If you have a defective condensor switching it only moves the problem to the other set of points. The only way to know is replacement unless you have specialised gear - a condensorometer. Marked difference in spark at the points is a clue.
I don't fully understand where you took the 'front side' measurement.

[mmattockx]

- If I measure from one of the leads in front of a coil to one of the plug wires, I get either no reading or too much resistance to show... I didn't switch the coils yet, so it ran with these coils. How is thhis possible?

Assuming you are talking about measuring from the primary side wires to the plug wires then you should get a resistance of infinity. There is no electrical connection between those two sides, current moves only through inductance between the two sides of the coil.


Mark

[Drummert]

I took the measure for the plug leads from the leads into the coil to the end of the lead, so plug end. I already thought this would give infinite, but I saw a guy do this on a youtube video somewhere, so I figured I might as well try. So from one plug end to another plug end should work and give about 15k ohm?

I just measured the condensors by charging them with a resistance test and then switching back to dc voltage. Both seem to work... Hmm....

[mmattockx]

So from one plug end to another plug end should work and give about 15k ohm?

There should be continuity and the resistance should be whatever your service manual specifies. My 1100E spec for the secondary side (plug cap to plug cap) is 31,000 - 33,000 Ohms, but it has the electronic ignition and your points system has different specs. If you don't have a service manual you should download one from Basscliff's site: http://members.dslextreme.com/users/bikecliff/


Mark