Strange Misfire - Need Help

[The1970s]

Hello all, I've been a long time reader of the forum but just made an account to seek some help with a strange problem that has recently come up on my GT750. I know this forum is primarily Yamaha's but it seems the registration is no longer functional with the Suzuki 2 stroke forum, I have met Chuck (supertune) in the past and am local to his area. I figure my question is general enough that I can at least get some ideas of what to look for from the wealth of knowledge here, so here goes:

I bought my 1975 GT750 as a project about 6 months ago now. It was very tired when I bought it but still seemed to run fairly well all things considered. I took some time to check out the bottom end i.e. checked crank seals, gearbox leaks, oil injection, etc and it all checked out okay. So I decided on just a top end rebuild. I went 1.5mm overbore and had both surfaces of the block and head decked. I just got it back up and running after the top end this past week and at first everything went well. The same day I got it started I heat cycled and re-torqued, did about 50 miles of break in riding with the oil injection set to a higher setting, and kept away from the last 2000 rpm of the powerband. The next day I take it out and decide to take it to redline once to see how it feels and it starts to misfire and pop out the exhaust in the upper RPM's. From there it only got worse. I tried to take it out for a ride with my friends RD350 and after about 10 miles the right cylinder stopped firing and eventually ended up backfiring through the carburetor, blowing my uni filter apart.

Since then I've installed new points and aftermarket coils and have had trouble getting it to run right. I can test each point/coil system independently and they all produce a nice blue spark. When I put it together and go for a test ride it runs great...for about 500 yards, then starts to misfire. Even at idle it will misfire and run poorly. If I clean the points and put in a fresh set of plugs it will run great again for another short period, then back to the same situation. Here's the strange part: it's always a different cylinder. Sometimes the left will misfire, i'll clean the plugs, and then it will move over to the right. Sometimes the misfire will follow specific spark plug, other times it wont. The only thing I can think of is that the wiring to the coils may carry enough current to support one coil when I'm testing, but not enough to run all 3 coils at the same time causing the misfire. I'm at a loss now and figured I would elicit some suggestions before I go tearing the harness apart.

Sorry for the long post, but any suggestions are appreciated

[Striker1423]

Well, I don't presume to know anything about the Water Buffalo (except that I WANT ONE BADLY), but what condition is your battery in? These old bikes require a good battery that holds a solid charge to run properly. Get much below 12 volts and your bike will do exactly as you describe. This means get out a multimeter and check the battery at rest at least an hour after you charge it. Should hold charge. If the battery checks good, your off to the races checking charging output at idle and around 3000 rpm.

Idle with lights off should be close to 13 volts or just under. FYI, Lights on might cause it to dip some.  At 3k, the charging system should be around 14 volts. If it's over 15v etc. It's overcharging. That kills electrical components quick.  If it doesn't change, or dips real low at 3k, you aren't charging the battery and the bike is running off the battery itself, draining it. This causes the voltage to dip below 11 and you start losing cylinders.

Make sure your carburetor float height is correct in all three. This will cause plug fouling and generally poor performance if the floats are set too high. Too low and you risk leaning it out.

These are the last things I would check per say, but You should also make sure the spark plugs are gapped correctly and in the correct heat range. Check coil output too. There's a small tester than can be had for I think 10 bucks. It basically creates a large spark plug gap and shows a visual of the distance the spark is jumping.

[The1970s]

After posting I had some more time to work on the bike and tried a few more things. First I'll note that during the rebuild I installed a new battery. It sits at 12.58v after leaving overnight. In addition the charging system is working correctly, it puts out ~13.5 volts at 2000 rpm. I just made up a short little harness that connected the 3 coils directly to the positive post of the battery with an external switch. That let me run the bike with no other electrical load but I noticed no real change in how it runs.

I did notice that the ignition points seem to be arcing more than what is normal. I've had many bikes with points in the past and noticed that they do arc occasionally and very lightly. But these are bright blue arcs across the points. Could it be because they are new? It has all 3 new condensers so I'm unsure of what else would cause this.

[Striker1423]

After posting I had some more time to work on the bike and tried a few more things. First I'll note that during the rebuild I installed a new battery. It sits at 12.58v after leaving overnight. In addition the charging system is working correctly, it puts out ~13.5 volts at 2000 rpm. I just made up a short little harness that connected the 3 coils directly to the positive post of the battery with an external switch. That let me run the bike with no other electrical load but I noticed no real change in how it runs.

I did notice that the ignition points seem to be arcing more than what is normal. I've had many bikes with points in the past and noticed that they do arc occasionally and very lightly. But these are bright blue arcs across the points. Could it be because they are new? It has all 3 new condensers so I'm unsure of what else would cause this.

Arcing burns them, but I doubt it in 500 feet.

[Czakky]

Sounds like condensers, I've had brand new ones that were junk.

[Old Brit]

Thing out-loud 
Dropping a cylinder at random with a points/condenser set up can often be caused by a fault in the charging system but you've seemed to have checked that.
Is the fuel flow ok, does anything change if the tap is set to reserve rather than rely on vacuum in the On position.
Could the tank vent be blocked
After you rebuild I guess you've re timed her i.e. to 3.64 mm on the Left and Right cylinders and 3.42mm on the centre cylinder (allowing for the plug thread angle effect on a dial gauge).

[Barrie]

Sounds like condensers, I've had brand new ones that were junk.

I think so too , if there was a charging fault e.g low voltage the bike would run till the battery was almost dead.

[1976RD400C]

Me too on the condensers. Make sure the wires on them are connected properly to the terminals.

[motodreams]

Skip worrying about the battery, They aren't even needed to run these systems.  Sounds like you have a good starting point.  Fuel would usually target a specific cylinder.  It certainly sounds like timing or ignition related given symptoms.  Heat often makes electrical issues worse too.

[sav0r]

Blown head gaskets will do funny things.

[m in sc]

the bike doesn't have a cdi, its a n old school battery and point setup, so battery condition is absolutely important, but doesn't seem to be the issue here.

Keep in mind, these motors are rubber mounted. is the ground strap in good shape?

It does sound like condensers, for sure. however its easy to chase your tail with this problem. you say they were new. were they -new- or new -nos-.  they DO break down over time sitting. also, verify the spark plug caps and wires ohm out ok.
Also, verify the kill switch and key contacts are in good order.

BTW, we are RD heavy here, but not exclusive at all. And, welcome.

[The1970s]

Well it certainly seems like the condensers where at fault. I pulled the new ones and found that they where all essentially acting like a direct short to ground. I put in the old ones, re-cleaned the points and reset the timing. So far its been running great. I still notice some light arcing at the points, maybe I need to get another new set of condensers, or maybe that's just the freshly cleaned points "wearing in". I guess time will tell

I always had it in the back of my mind that the condensers could be the only thing causing the arcing, but for some reason couldn't believe that all three of the new (not NOS) ones I received didn't work.

[m in sc]

glad you found it. 

for what its worth, you can use automotive condensers, but they are bigger so need to be located elsewhere, up at the coils is a good place if you cant find any - new - new ones. 

[RDFL]

Very inexpensive too. -- https://shop.advanceautoparts.com/p/bwd-ignition-condenser-capacitance-rating-022-g635/3031810-P?searchTerm=G635

[The1970s]

I would really like to buy one of the Accent electronic ignitions but I have been told that they are produced in batches and are currently out of stock. They expect to have them available again in 6 weeks. So until then the points will have to do. If I have any more problems I'll likely just wire in an automotive condenser since they are readily available.

Now I'll have some time to start planning my porting changes for then next rebuild  ;D I put it back to stock specs while I get all the frame and suspension dialed in but I know I will be looking for more power in the not so distant future