'78 XS500 bogging @ 3/4+ throttle - how many cans of Seafoam before tearing into the carbs?

[ehud42]

Disclaimer: I'm _very_ new to bikes, much less vintage ones. Previous owner babied this one. Has 16,000kms on the odometer.

I'm building riding confidence and am now noticing that at or attempting highway speeds (100km/h), at ~6,000rpm or about 3/4 throttle it bogs down and loses power. If I ease off on the throttle I can accelerate through to 7,500+ rpm and it wants to run! What I've noticed is it seems to be more related to throttle position - if I can shift down, I can hit higher RPMs at a lower throttle setting. But full out is bog-ville.

I just started running Seafoam at the recommended does (15ml / 1L), but a brother-in-law who has a lot more riding experience says to dump it in. So that will be next, increasing the dose.

I did check the points, and there was a little bit of crud that I tried cleaning off with a bit of fine grit sandpaper.

The spark plugs were gapped tight. Manual says 0.027 - 0.031" , but the feeler gauge would not even go in at 0.0200. So I've increased that to just below 0.0300 as pictured.

Speaking of plugs - any tea leaf readers? Do they look ok? I have no idea how many km's are on them - I've put ~600km on them since taking ownership.

After reinstalling the plugs, the right side seems to pop a little when releasing the throttle.

I'm gonna keep tinkering, but any guidance is appreciated!

 

[5twins]

I'd replace those plugs. Their center electrodes look to be worn pretty short. That's how plugs wear, the center electrode gets shorter and shorter. It's hard to see unless you have a new plug to compare it to ......

...... and yours definitely look worn short. These old points ignitions are tough on plugs. I change mine every 3 to 4K miles because by then, as the above pic shows, they're visibly worn.

 

[Highpower]

Sometimes it’s best to just jump into the deep end of the pool. In over 60 years of working on bikes the only sure fire solution to gummed up carbs that I have found is to take them apart and clean them properly. But before you do that, do a complete service on the ignition system. You didn’t say what year your bike is, but if it’s ‘80 or later, or has an aftermarket electronic ignition, about the only thing you can do is new plugs and check the timing.

If it’s an earlier model with points you need to go through the setting the points drill.

However, I do have a question, does it still have the original air box and filters? Engines run much better with the stock system. Over the years I have found that pod style air filters are hard to tune so that an engine runs good throughout the rpm range. It can be done but, IMO, the juice isn’t worth the squeeze.

Personally I don’t think they look all that cool either.

 

[ehud42]

@Highpower - so I am heading in the right direction thinking the primary source of the high throttle bogging is gummed up carbs?

'78 XS500, original airbox and filter (I have removed the nylon stocking that someone sewed into an additional "pre-filter"!)

The points seem "tight". A 0.3mm feeler trips on a bit of grit on the contacts. 0.35mm doesn't go. I've tried cleaning them with some medium/fine grit sand paper. Do condensers typically need replacing? or are they pretty robust?

@5twins - local store has the D8EA plugs in stock, will will try and get those installed by the weekend.

 

[mrtwowheel]

I dump in the Seafoam. 8 ounces to a tank of gas. Worked for me many times.

 

[ehud42]

I dump in the Seafoam. 8 ounces to a tank of gas. Worked for me many times.

Alrighty then - here I'm fretting like nervous nellie that 4-5oz to a tank is too much. LOL!

 

[Highpower]

After sanding the points, run a business card through them to clean the surface. One thing to check, is that you aren’t using resistor spark plug caps. It has been my experience that point ignition systems don’t need any additional resistance between the coil and plugs.

If everything else is good, it sounds to me like your main jets may have some crud in them. I would remove the float bowls and take a gander in there. On my bike the carbs were chock full of fine rust powder when I first got it. There could be all sorts of different types of debris in yours. Honestly, I don’t think the Seafoam is going to fix your issue, but it won’t hurt to try.

 

[ehud42]

Put new plugs in. Sanded the points a bit better to get the crud off. And then it wouldn't start -

Noticed that one set of points I could slide some paper in while in the closed position. Clearly took a bit more off than anticipated.

So, I did my first re-gapping of the points. 0.35mm.
And then it started up! Phew!

Took it out for a run - and I think it's running better in the city. So much fun!
But still bogging at 3/4+ throttle/5,500rpm/90+kmh.

Dreading the carb job as I don't want to open pandora's box and have nothing to ride. Will keep running foam until I have some time to tackle the carbs
I may also take it a shop in town that specializes in older bikes (KRM Mastertech Motorwerx has been recommended to me).

 

[ehud42]

I get some courage Sunday to remove and open the carbs. Long story short - nothing obvious to my naive eyes, and no change in performance.

Anecdotally, I like the hard pass/fail test of pulling wrenches. No vague, AI involved subjectiveness. Do it right, the bike starts. Make a mistake it doesn't.

I put the carbs back together and reinstalled them. And the bike started up right away. Nice! Shut it down, and got ready for a quick test ride. Started it back up, warmed it up, started turning the choke off and gave it a bit on the throttle - and it died. Instantly. . Rechoked, started, throttle. Dead. Panic. What did I mess up in the carbs?????

Looked down at them. Oh. I had turned the petcocks off.
Fixed that and off for another fun little rip around the neighbourhood.

Jets are 132.5 - probably originals. No varnish, etc on the pin from the vacuum plunger. Bike has 16,000kms.

Brother-in-law came over for a quick look. I started it for him, and the right exhaust got hot much faster than the left. Also, he thought the exhaust smelled too gassy. So points issue? (cylinders not warming up equally) Running rich? (gassy exhaust)

 

[Jan_P]

Is the battery OK and charged ?
Most likely the points ..but can be other
first step take off cover over points start machine observe if there are small sparks at both points opening closing
then look at the spark plugs take them out report how they look here
Ground them against the engine and se if there is spark at the plugs .

 

[AUTOMAN]

Excess sea foam will destroy things.
Rubber
Plastic
Fuel

 

[6fifty]

How about one of those cleansing fluid, like the one with P.E.A. (polyether-amine) ?
Maybe GumOut still has it in one of their products for Carbon Cleanser or Fuel System Cleaner.

 

[Highpower]

I'm going to go out on a limb and say that your jetting is off. At the very least the main is a few steps too small. I looked at my notes just now and this is what my bike is currently running:

Note: I live at sea level, so if your altitude is much above 1000 meters (maybe less, I don't know that metrical stuff real well,) you may need to jet down.

30 Pilot
142.5 main. (Your 132.5 is, IMO way too lean.)
Stock air box with green foam filters, probably made by Uni, but I see no manufacturer mark on them.
Aftermarket mufflers.
13K miles on bike

Mine starts and runs perfect. It is a little cold blooded, but usually by the time I reach the end of my 150 foot driveway it settles down. I went up one tooth on the countershaft sprocket and it pulls hard from just off idle all the way up.

If one cylinder is running hot, the timing may indeed be off on that side, or you might have an air leak. I set my points at .014" and then set the timing.

 

[ehud42]

I'm going to go out on a limb and say that your jetting is off. At the very least the main is a few steps too small. I looked at my notes just now and this is what my bike is currently running:

Note: I live at sea level, so if your altitude is much above 1000 meters (maybe less, I don't know that metrical stuff real well,) you may need to jet down.

30 Pilot
142.5 main. (Your 132.5 is, IMO way too lean.)
Stock air box with green foam filters, probably made by Uni, but I see no manufacturer mark on them.
Aftermarket mufflers.
13K miles on bike

Mine starts and runs perfect. It is a little cold blooded, but usually by the time I reach the end of my 150 foot driveway it settles down. I went up one tooth on the countershaft sprocket and it pulls hard from just off idle all the way up.

If one cylinder is running hot, the timing may indeed be off on that side, or you might have an air leak. I set my points at .014" and then set the timing.

Interesting. I'm at ~239m (785'). I have to drive almost 2 hours to get above 1,000'. Welcome to the Red River flood plain...
Also my exhaust is not stock - not sure exactly what I have.