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78 GS550 running really hot

  • Thread starter Thread starter DumbGenius49
  • Start date Start date
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DumbGenius49

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Hi all,

My 1978 GS550 is running ridiculously hot in a short amount of time. For background, I have a stock airbox, a MAC 4-1 exhaust, stock jets. I've cleaned the fuel tank and the carburetors. The bike is pretty laggy when it comes to throttle and idles way too high (2.5k rpm-but that's also because I haven't adjusted the idle screw). This leads me to believe that I have a case of lean conditions. But what's odd is that the exhaust is hotter from left to right. I could hold the #4 exhaust with my bare hand, the #3 I could touch for a bit, but #2 and #1 were way too hot to even tap a few times. If the bike IS in fact running lean, what should I inspect to alleviate this issue?

Also forgot to add that it gets this hot in about two-three minutes of revving in neutral.

Thank you in advance for all the help.
 
First of all, your bike is air cooled and designed to have moving air go past the fins on the head and cylinders to cool it. Revving the bike while it's standing still will make it heat up very fast and possibly overheat.

Having said that you obviously have some issues if 3 and 4 are staying cold whilst 1 and 2 are getting hot.

I think you need to clean / rejet your carbs to suit your exhaust.
 
Aftermarket exhaust headers are all single-walled so they'll get hot a lot faster than OEM headers which are often double-walled steel. Sounds more like cylinders 3 & 4 have carb issues since they're not getting hot after a minute or two. What do the plugs look like?
 
I'll check the plugs in the morning and let you know. This bike hasn't seen the best of day, four owners and its been their first bike each time, me included. I'm trying to restore it to it's former glory, or at least something around there. If i need to rejet for the aftermarket exhaust, do I go with bigger jets or smaller and where would I even find something like that?
 
Would there be an effect if I haven't yet put clamps on the boots for the carbs?
 
Did you follow the carb clean instruction Here ? You defiantly have a problem with 3 & 4 I would not bother thinking about rejetting untill you have made sure the carbs are compleatly clean and you have verified you are getting good sparks to the plugs if you do a search you will find plenty information on how to test.
yes the clamps should be tight or you will likely have air leaks which will make it run lean
 
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You may need to bump your main jet up a size, although I have definitely heard of people running stock everything except an x into 1 and maintaining stock jetting. However...

If you're just starting it up, letting it idle and blipping the throttle a couple of times, the pipes aren't going to heat up a lot if they're not getting any fuel AT IDLE. So either the pilot jets/passages are clogged or the synch is off and two of the carbs are fully closed at idle.
The main jet doesn't affect idle much so your jet settings are independent of your cold pipes issue.

It sounds like you may also have air leaks, etc. So probably good to clean carbs + verify that your intake boots and Oring's as well as the airbox boots are all tight. If the boots aren't properly clamped down that is also going to affect things...
 
I actually have VM type carbs, would the procedure be basically the same minus vacuum diaphragm?
 
Also, make shure to check the HT leads;
when I got my bike, one of them literally crumbled into pieces, with a gentle squeeze,
being fried on the cap.
 
Clean the carbs for starters and get new O-rings, including those for the intake boots. Short cuts are long cuts when it comes to carbs so be sure to take your time and fully break down the carbs and soak the parts in carb dip to clean them. When complete be sure to set the pilot fuel screw to 3/4 to 7/8 turn open and set the pilot air screw to twice that amount. Then perform a vacuum sync. Oh, and it goes without saying that you need to be sure your points are clean and set properly, as well as you need good spark plugs. Check for visible spark on each cylinder before trying to fire up the engine next time. Be sure to check the valve clearances too. The Newbie Mistakes thread linked in my signature may help you...
 
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