82 XJ750 Seca - Performance degrading

I picked up this bike beginning of the winter... ran like a champ, 9900 miles on it. Rode it a couple times a month over the winter... no problems, crisp clean running.

Getting ready for the season I have so far:

• Changed air filter
• changed gear oil
• new plugs, gapped

Changing main oil / filter and fuel filter next. When the shop has time I DO plan on eventually getting the carbs cleaned and synched but she was running so good initially I thought I might make it into the season when the rush dies down...

Over the last month and a half I've noticed:

• When the engine gets nice and hot and as the outside air temp has gotten above 60, above 4K rpm a slight knock or popping... tightened all exhaust and it seemed to help a little for a while
• This last week or two with outside air temps above 70, just crappy performance... sluggish and what feels / sounds like missed combustion strokes below 2500 rpm. When up at 55 mph and 4200 rpm, pegging the throttle does accelerate, but with hesitations and skips along the way...
• It does not look like I have any stuck choke mechanical / plungers, etc.
• It seems to continue getting worse each time I ride and am afraid to go further in case something mechanical is going on.

One big thing: At the onset of these issues and with advice from a long time older bike owner, I started putting in high octane fuel... I'm hoping it may all be just that and will drain it this weekend. I did also treat one tank ago with Seafoam to get the carb cleaning started...

Any advice MOST appreciated, thanks

Tim

 

check the valve clearances
clean those carbs
check out ignition system ohm it out
trim plug wires back 3/8 inch wire could be corroded

I only use Hi test non ethanol fuel at the end of the season. Run tank low fill with HT last 2 to 3 tanks and store bike with it.
High test does nothing for your bike as it is a low compression motor big waste of money

 

I never use high octane in the tank. It doesn't do anything for a bike or car that doesn't call for it. I would use fuel that doesn't have ethanol if there was a station near me that carried it.

 

Carbs. Higher octane can't mask vacuum leaks or compensate for plugged fuel passages. It also offers no benefit for an engine that was designed to operate on low test fuel.

 

Air filter? Small rodents can mess up an filter in days

 

Do NOT let a shop clean your carbs unless it is Hogfiddle's shop.

 

Like others have said, there are basic things to check first. Carb cleaning/tuning is all for naught if the valves aren't in spec. Start there.

It does sound vacuum leak-ish to me, but switching to higher octane gas is just masking a problem to make yourself feel better... it's like Seinfeld changing the 34 on his jeans to a 33 - makes him feel better, but doesn't actually solve anything.

Most common areas on the carbs are the intake boots themselves (that rubber breaks down and cracks over time), and the butterfly valve seals (which also breakdown over time, and our bikes have seen much, much time). IF the carbs are gunna be cleaned and haven't been to church (full teardown, clean, rebuild), that is probably a smart move, since they will be down for maintenance anyway. Saves you from pulling the rack twice. Expensiver, yes. betterer than wasting your time with flacid solutions.

 

when the valves get hot the clearance gets smaller, so if your valves are tight to start with they get tighter.

 

I'm betting that 9900 miles =zero maintenance from new.
Consensus is tight valves and dirty carbs.

 

Everyone wants a quick fix, but those are like all the diet secrets on the internet - Invented by quacks who tried something and got lucky once.

 

a quick fix is air in the tires, valves and the church of clean is not.

 

Well my update is fuel in the oil so hope I didn't drive it a little too long

Tearing it apart now...

 

I'm hoping that you're just breaking the carbs for a refresh, and not breaking down the whole bike.

 

I'm with k-moe. Probably just needs a petcock rebuild and perhaps new needles and seats to solve the gas in oil issue. Of course all the other stuff mentioned probably needs attention also.

 

Your valves won't be able to get that tight yet. That's not the issue.

My bet is you're overheating due to thinned oil. Smell your crank case to see if there's a gassy smell in there.

 

Today: drained all oil, replaced filter, removed all exhaust ( have another one to put on minus the mufflers)

Tomorrow: new exhaust, drain gas, inspect petcock, new inline fuel filter, inspect and clean floats-needles, then replace oil if pretty good.

I left it on prime not running once by accident when I got the bike... Hoping it was just that and a stuck float.

Valve cover is leaking just a little anyway so figure why not check the valves and fix that next.

 

If you're leaking at the valve cover, you'll need a new valve cover gasket and donuts for the valve cover bolts.

 

I checked my local Yamaha shop this morning and surprisingly they have one

 

The dougnut rubbers are as important as the gasket is. They are what puts pressure on the gasket so that it seals. Once hey harden, even a new gasket will leak.

 

Out with the old

In with the new(er)