Topic: '78 Gs750 running issue

[starsnbars89]

Hi guys. Haven't posted much in a while--no SOHC4 and a busy summer! Just wondered if anyone on here knew about Suzuki Gs' that they might be able to give me a little insight.

So I picked up a '78 Gs750e in good condition last week. 24,xxx miles, Lester wheels, trunk, UGLY windshield, probably OEM replacement pipes--not in bad condition by any means. Also came with a titled parts bike of the same vintage that has a (seemingly) good engine, good electrics, good plastic tail, 4-1 header, and good tires. Maybe somewhere down the line here I'll do some kind of build on the parts frame, but for now, I'm fixing the running one.

So here's the issue: the bike idles awesome, but only pulls from idle to about 4K RPM; beyond that, it stumbles, backfires, and has absolutely no power. PO tells me it "just need the valves 'aligned'". I called BS, but I bought it anyway so here it goes.

So far:
-I set the valves
-the carbs were "professionally cleaned" (by a shop for the PO and they show it)--I cleaned them again w/ spray just to be sure; jets are stock w/ 100 mains and 15 pilots
-point contacts are clean and gapped properly
-advancer works smoothly
-plugs are new
-good contact on the leads
-checked and there are no vacuum leaks on the boots (new O-rings too)
-original airbox
-it's also getting great gas flow from the petcock

All to no avail. What's my issue??? Any advice is appreciated and thanks in advance!

Pics of course:

[toytuff]

Sounds like fuel issue to me. Others will chime in.

Remember, In GOD we trust, everything else we check.

tt

[starsnbars89]

Hey TT, that's what I thought too, but choking it doesn't seem to help anything (). I did note that the outer two plugs are staying beautiful and white while the inner two seem darker, #2 being almost black. Vacuum leak I cant find?

[seanbarney41]

watch that petcock...it is vacuum operated.  I know you checked it out but it may be both leaking gas and/or vacuum...when you get it sorted, you will like that bike.  Also many known charging issues, so make sure your battery is fully charged and elctrical system is working as it should.

[grcamna2]

Hi starsnbars89,
   I think the petcock is a good place to start,as far as being sure it releases fuel when vacuum is applied,turns off fully when engine vacuum is absent;make sure it holds vacuum when manually testing it.The diaphragm may be original and too stiff..,or leaking from an internal pin hole.I'd also do a compression test after you've got her warmed up,then test the ignition coils for ohms & visually w/ a spark tester.

[starsnbars89]

Honestly guys, I didn't even think of the petcock as being a vacuum leak. I did put it on prime though (fuel does come out rapidly) and it didn't change anything. A vacuum leak is possible though. Checking the coils was my next idea. I'm thinking weak spark maybe.

[grcamna2]

stars,
   The "prime" position on your vacuum controlled petcock is only to be used to initially fill the carbs when they've been run dry.I think you should keep the valve in the "On" position at any other time except when you run low & then "Reserve"..,otherwise you'll overflow your engine w/ fuel which can really thin out the oil w/ gas & seize it.

[seanbarney41]

I think most performance guys swap to a non vacuum petcock, as they are well known to be undependable trouble on these old bike...but don't ask me where to get one or what other bikes' petcock works...I'd try Z1 Enterprises?

[Dawdlin Dog]

stars,
   The "prime" position on your vacuum controlled petcock is only to be used to initially fill the carbs when they've been run dry.I think you should keep the valve in the "On" position at any other time except when you run low & then "Reserve"..,otherwise you'll overflow your engine w/ fuel which can really thin out the oil w/ gas & seize it.

Absolutely right. I have a 78 GS1000 with the vacuum petcock on it.

Make sure there is no inline fuel filter between the tank and the carbs, if there is remove it. Most of them restrict flow to much.

Unfortunately what I think you need to do is strip the carbs down and clean them properly very few shops do it right and a spray wont do it.
Full strip down, carb dip/sonic cleaner, check and double check the jets, rebuild with new o rings and re-install with new boots. etc etc.

These bikes are very finnicky on the fuel/air mixture front.

Couple of very usefull websites :-
http://www.thegsresources.com/
http://members.dslextreme.com/users/bikecliff/

Hope this helps 

[grcamna2]

I rebuilt one for a GS1000E customer 2 years ago w/ a petcock rebuild kit from K & L Supply and it's still doing well w/ no leaks.The petcock needs to be rebuilt slowly and carefully & double check the operation before trusting it;sometimes the internal valve body casting is sharp around the area where the diaphram sits and needs to be smoothed off so that it won't puncture the sensitive and thin rubber diaphram.I'd rather rebuild a used factory O.E. Suzuki petcock than use a Cinese reproduction  ;don't go that route.