'78 Gs750 running issue

[starsnbars89]

Hi guys. I've lurked around a bit but haven't really posted much on here (speaks to my '80 550's reliability).

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 here to fix 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:

There's the 550L in the background for anyone who likes I-spy.

 

[eil]

Some random thoughts:

All the regulars here are going to tell you to do a proper carb cleaning yousrself, by breaking it down and and giving all the parts a good soak in Barryman's or equivalent before doing anything else. Also replace the o-rings with a kit from cycleorings.com.

Aftermarket pipes usually require larger jets.

Has the airbox been properly sealed up?

 

[JAG]

Hi guys. I've lurked around a bit but haven't really posted much on here (speaks to my '80 550's reliability).

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 here to fix 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:

There's the 550L in the background for anyone who likes I-spy.

How is the cam chain tensioner / cam chain? Just an idea, but put under load this may be stretching and upsetting cam timing.

 

[starsnbars89]

Hey guys, forgot to mention, I checked and the cams are in time. I did reset the tensioner, which seems to be working fine. The tensioner was completely loosened when I got it though so I did think timing was the issue. The manual said that there should be 20 rivets between timing marks on the sprockets (weird timing process or what?), which there are. Both keyways line up with the head too (which I figure usually means something).

I didn't soak them, but the carbs are literally spotless and were definitely done in a shop (paint pen to mark torqued screws, etc.). I sprayed them out good as well and the o-rings seem "plump"; i.e. new/replaced.

The mufflers may be OEM replacement, but they're still OEM so I would assume stock jets, no?

The airbox is sealed and good. Air filter is seated tightly against the box too.

 

[starsnbars89]

Also, I found that the outer two plugs are staying clean while the inner two are darker, #2 being almost black. Vacuum leak I can't find?

 

[Road Rash]

Check your points.

 

[tkent02]

When my 1000 had a clogged exhaust, it ran about like you are describing.