RE: 81 XJ550 misterious loss of power

Hi all,
Let me expand a little bit. I have a 1981 XJ550HR with 10K original miles. The bike is bone stock except for pods and a 4 into 1 Kerker (purchased the bike like this).
The carbs are properly jetted and the bike runs great... Except for a really weird thing: Once in a while after the bike is at idle (for example at a traffic light) the engine refuses to accelerate. It idles fine but when you goose the throttle nothing happens. 8O
You hear the air rushing in but there is no change in rpm. Needless to say this becomes a dangerous situation pretty fast.
After this, and as fast as the problem happens it dissappear and the bike runs perfect, and I mean perfect. The engine never stalls during this process.
It is being suggested to me that the problem could be a defective CDI box.
I've checked the fuse box and it's in good order, I've rebuild the fuel petcock and relined the gas tank.
Any ideas or suggestions?
Thanks in advance,

 

Does it have a fuel filter? It also kind of sounds like a collapsing fuel line. Make sure it doesn't look too pinched, and try replacing it because it only costs like $4 to do so.

How do the contacts look under the ignition cover?

 

Two questions (this one probably isn't related to your problem but will soon become a BIG problem if not) have you checked/adjusted the valve clearances yet?

When you say "properly jetted" what jets are you running? Stock won't cut it with pods and a pipe.

The symptom is actually that of the slides not responding to manifold vacuum as they should. Have the carbs been modified in any manner other than possibly re-jetting?

 

I've not taken the carbs apart because the bike runs so good (when it runs)
Regarding the jetting, I own 11 bikes in all state of tune and this bike was jetted properly. It doesn't hesitate of feels starving like a bike with four into one and stock carbs will do.
I like the idea of loss of vacuum to the sliders though, that or fuel starvation is what it feels like. The fuel line is a straight hose with no kinks but I will replace it just in case.

 

The part about the collapsing fuel line also applies to the vacuum line if you still have the original vacuum-operated petcock. Quite often somebody who didn't know any better replaced the vac line with a piece of plain hose which can collapse in on itself under vacuum once hot.

You can't go by "seat of the pants" when it comes to an XJ possibly running lean. They LOVE to run lean, right up until they blow up. Unless you know for sure what jets are in there vs. stock, I'd strongly recommend having a look. We had one member that got ahold of a beautiful 550 with less than 6K on the clock and TWO holed pistons from running lean.

And don't ignore the valve clearances. They were to have been checked at 3000 miles and every 5000 thereafter. If yours were ignored, you're about 7K overdue for the initial check and possibly looking at all 8 being tight. When I got my '83 it had 7100 miles on it and 7 of 8 were tight. Remember, they tighten up, not loosen.

"Starts up fine cold but loses power once warm" is a classic symptom of tight valve clearances. Don't take a chance with that nice low-mileage motor.

Also, the Mikunis on the 550s are extremely sensitive to float level adjustment; the spec is only +/- 1mm either way. Wonky float levels can also cause the symptom you're describing.

But check the valves. If ignored you're in the danger zone, mileage-wise. Ignorance is NOT bliss when it comes to valve clearances.

 

Yeah, it kinda does sound like a slide diaphragm not operating correctly. Might be worth it to pull them out and see if they slide smoothly and "clunk".

It very well could be the vacuum to the petcock. If you sit at a light, those hoses get hot.

 

does your carb rack still have the heat shield under the float bowls? this sounds like a classic symptom of vapor lock.

CN

 

Truth be told, it sounds like a bike that needs serious attention instead of just "poking" at it and making assumptions.

But what do I know?

 

What do you know? Just everything about XJs.

 

HUH?? I've never suspected the "heat shield" was in fact a heat shield. My 550 Max came without it and I wasn't aware that it should have one with the carburetors until I seen it in the service manual. To this day I have not bothered getting one installed on it and have had no problems without it (if it is in fact a 'part' suppose to be on my 550 Max). The bike pulls like a charm under throttle.

I typed in xj550 maxim carburetors in eBay to see if other racks have this heat shield. First listing I pull up and it appears to be on backwards to me. Is it?? haha... http://www.ebay.com/itm/Carburetors-car ... f3&vxp=mtr

Sorry for hijacking the thread lacucaracha; perhaps I worry too much about the little things in life sometimes. Civil engineering makes you question everything more than once especially when infrastructure is depended on by society.

 

XJ550 Maxim carbs do not use the heat shield under the bowls............only the XJ550 Seca models did.

 

yes that heat shield is definitely on backwards. it is supposed to go under the float bowls. those carbs may have come off a seca originally.

CN

 

Hey no worries moellear, not my thread. Although, I think my 650 seca wishes it was a 550 sometimes...

 

I hate to be the one to tell you this my friend; but it ain't so. The parts fiche may think they didn't come with it; but I think this is another case of "black sprockets."

The Maxim may not (can't prove it either way yet) have initially had the heat shield on the '81 models; however it was there on the Maxims for at least the '82 and '83 model years. I have a "parts rack" from a 550 Maxim (bought before I found out the carbs were different) and it has the shield.

Take a close look at this scan from the 1982 Yamaha 550 Maxim brochure. The shield is there...

 

I'm pulling the valve cover this weekend and start with the valve clearance (man the OEM valve cover is expensive) then is out to the carbs.

 

Be sure to replace the "donuts" on the valve cover hold-down bolts. Those metal/rubber composite donuts are what actually press the cover against the gasket and head. Once they get compressed and baked hard they don't do their job any more and even a new gasket will leak.