erratic fire on center cylinder, gt550

durgam · Post by **durgam** » Tue Oct 27, 2009 12:23 pm

Im running a 74 GT550 with stock points. I noticed the problem when I had the bike at Idle. I put my hand over the tail pipes of all cylinders. The left and right had good rapid 'POP's while the center would pop about half the time. Going by sound and pressure on my hand. I then checked the temp at the headpipes and found the center to be running about 150 degrees cooler than the other. The bike is real boggy until about 3200 revs. I know its a two stroke, but i have next nothing until than. Ive got good plugs. canI swap condensors to see if the problem follows the condensor. The points are old and the timing is set correctly. Anyhelp would be greatful, thanks. Kris Heres a pic.

tz375 · Post by **tz375** » Tue Oct 27, 2009 12:53 pm

Kris, Nice bike. Did you build it yourself? That oil tank and shallow tray for the electrics are neat.

Miss at the bottom end is probably related to the carbs rather than ignition.

I'd start by checking the adjustment of the slow screws and then I'd pop teh filters off and see if al 3 slides raise at exactly the same time.

If those are spot on, the carb will probably have to be removed for cleaning.

durgam · Post by **durgam** » Tue Oct 27, 2009 1:24 pm

Thats the first thing i though of too. Sorry i forgot to mention that. Ive cleaned these numerous times and synced the carbs when I set the timing. Im running the same jets in all three carbs as well.

thanks though. I did do it all myself. cept the paint. I made the oil tank from 4 in aluminum angle plate with the sides welded on.

tz375 · Post by **tz375** » Tue Oct 27, 2009 4:36 pm

It's possible that the center pot has low compression from bad crank seals or maybe a leak at the base gasket.

If not, my money is still on carbs and specifically the pilot jet system. Do you have a copy of the Suzuki carb manual?

Are you sure there isn't a leak in that silly vacuum hose arrangement from the center carb to the petcock?

Coyote Chris is the resident GT550 expert and has had a number of curious problems to fix. He also has a really nice looking custom GT550.

Suzukidave · Post by **Suzukidave** » Tue Oct 27, 2009 5:03 pm

If you have a timing light that clamps to the HT wires try them while the engines running and watch the flash .

durgam · Post by **durgam** » Tue Oct 27, 2009 5:49 pm

HT wires???

I dont have a copy of the carb manual. I just passed up on one on Fleabay too.

I checked my compression and its at 120psi in al three cylinders. I just put a new top end in it last month. Would a crank seal leak show up in a standard plug type compression check?

These thjings are why I though it mught be electrical. Ill check that vaccumm line tonight maybe.

If the problem lies in the pilot jetting, would it show its ugly head by playing with the screw?

thanks for all the help and advice.

TLRam1 · Post by **TLRam1** » Tue Oct 27, 2009 6:12 pm

I didn't read the entire post but mine was diagnosed to a bad condenser on my Center Cylinder.

durgam · Post by **durgam** » Tue Oct 27, 2009 6:35 pm

I read your post. I didnt have the ARC you did, but a similar problem non the less. Did you try to switch condensers from one cylinder to another to see if the issue went with the condenser?

Suzukidave · Post by **Suzukidave** » Tue Oct 27, 2009 8:42 pm

durgam wrote:HT wires???

Oh sorry .. spark plug wires

when you trigger the timing light you should see a nice steady flash . If you see the timing light flash and then no flash its a good bet that the problems somewhere in the points or condenser

TLRam1 · Post by **TLRam1** » Wed Oct 28, 2009 2:30 am

durgam wrote:I read your post. I didnt have the ARC you did, but a similar problem non the less. Did you try to switch condensers from one cylinder to another to see if the issue went with the condenser?

I didn't swap condensers, replaced it with another. I think I was lucky with the condenser issue, never had a bad one before. I did not crack the condenser open, must have been something loose inside a few times it would run fine.

Post by **Suzsmokeyallan** » Wed Oct 28, 2009 7:02 am

The condensers are nothing more than a roll of foil sandwiched in paper rolled up tightly as an insulator between the layers.
All sorts of things can go wrong with that design so if you suspect a condenser just swap it with one from another cylinder to check its operation and to see if the problem also swaps over to that cylinder as well.
Id suspect an electrical related issue on this one as well from the info given, more than a mechanical one.
By the way whats the battery voltage on the bike when its running at idle after about five minutes, is the headlamp circuit on all the time?
If it drops below 12 volts at idle the spark becomes weak and erratic on one or more cylinders, usually it seems to affect one cylinder first, but not always the same cylinder on the same model of bike.

durgam · Post by **durgam** » Wed Oct 28, 2009 7:45 am

I didnt get a chance to touch it last night, I had to go to my powder coater's. Ill swap the condensers today and check the voltage as well. Ive had a couple asks for a build thread, so im gonna start one today. Thanks for all the advice.

Suzukidave · Post by **Suzukidave** » Tue Nov 03, 2009 6:24 pm

Any luck getting it firing on all cylinders ?

jaybob · Post by **jaybob** » Wed Nov 04, 2009 9:38 pm

I had one cylinder running cool and firing erratically on my 550. Found out that the carb float on that cylinder had gas in it and it was flooding.

durgam · Post by **durgam** » Sat Nov 28, 2009 9:23 am

I rode for about 2 hrs on last sunday with no problems at all. I tried a ford 390 condenser and got no spark at all. i might have had grounded it out i was told. But It ran really good. I never checked the temp though, but it felt pretty strong.
I did remember one thing, as I was doing my initial tuning after adding the chambers and pods, I moved the needles, I know that wont affect any at idle conditions, but it will affect all mid range. With that said, Its winter here now and she's gettin put in storage soon. Ill get it out in late january, to start doin my next round of mods. Thanks for the help on this. Hopefully it wont haunt me again in spring.