Downhill = backfire and fouled plugs

XS650-Spark

Spark
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Hi, I'm Looking for some ideas on why this could be happening...

Problem:
It runs great until I go down a long steep hill and then the bike begins to hesitate, then backfires and eventually dies. It doesn't matter if I'm on or off the throttle. Every time I find both spark plugs are fouled. I believe it is oil, but I'm not 100% on that. If I pop in two new plugs everything is dandy until I go down a long steep hill.

My BIke:
I've got a 1977 XS650 stock bike, except pamco which I installed and works like a charm.
Plugs: BP7ES NGK with gap to spec
Carbs: I've cleaned inside and out. Pilot screw 1.5 turns out.
Fuel: is flowing fine, filters clean and clear
Timing: checked when pamco installed
Valves: to spec
Battery: tests perfect and recharges fine
Compression: Left 144lbs / 170 with oil. Right 149 / 172 with oil

Any ideas or help would be much appreciated.
 
It sure sounds like a fuelling problem related to the floats....?

Is the fuel tank pretty full when this happens? I wonder if it could be as simple as your tank is low and it starves as you nose downhill.

Can you fill it up and see if it happens - or put it on reserve and try it?

Interesting problem!

.....could also try moving to Windsor or even Holland. There are NO steep hills here or in the Netherlands. ;)
 
Simple, Do not go down hill.......!

On a more serious note I recently read a thread somewhere and they pointed out that going downhill with the throttle closed increases the vacuum in the the intake port. If there is a leaking valve seal then the oil can be drawn through more quickly by this vacuum.

A petrol fouled black plug nornally dries quickly because petrol is volitile, but an oil fouled plug stays wet looking. Perhaps you could eliminate either the petrol or the oil from the equation?

Best of luck with the detective work.
 
Excellent ideas. Here's what I know...

The fuel is good, and runs through the system well. This issue happens with a full tank, or like today I was fairly low but not into my reserve.

I like the "float" idea - although I've double checked the position of the float according to the specs.

The fouled plugs look dry to me, and that's pulling them out right after it happens. It's also interesting that it's always both plugs. So we are thinking fuel fouled, which makes the "float" / fuel the culprit? Sorry for the dumb question but does that mean too much fuel in the mix? I'm trying to picture how the float position would change with the bike facing downhill - allowing more or less fuel through? If anyone has ideas on adjusting the float one way or another please let me know.

Good thought with the leaking valve seal idea - I suppose if the fuel or float solution doesn't solve it I'll need to crack the engine and overhaul. Never did that before, but there is always a first time.
 
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;)....so, you're not looking into moving to a flatter part of the planet?
 
That's Quebec north of VT. The Canadian border becomes Ontario over by Massena, NY.

I went to school in Plattsburgh, on the NY side of Lake Champlain across from Burlington. 20min directly north put us in Quebec. An hour got us to Montreal. With a drinking age of 18 in Quebec (19 Ontario, 21 US) it made for a popular trip for freshmen.

Pete mentioned Rickards Red in another thread. The only time and place I've ever had it was on those trips to the Peel Pub. Big ol' pitchers!
 
That's Quebec north of VT. The Canadian border becomes Ontario over by Massena, NY.

I went to school in Plattsburgh, on the NY side of Lake Champlain across from Burlington. 20min directly north put us in Quebec. An hour got us to Montreal. With a drinking age of 18 in Quebec (19 Ontario, 21 US) it made for a popular trip for freshmen.

Pete mentioned Rickards Red in another thread. The only time and place I've ever had it was on those trips to the Peel Pub. Big ol' pitchers!

Oh, I should know better, I drove through Canada once to go to Boston, just because I wanted to throw Niagara Falls into the trip, long time ago.

Scott
 
C'mon over MTW - we're fun, the beer is good and we drive on the same side of the road!

Pete
 
C'mon over MTW - we're fun, the beer is good and we drive on the same side of the road!

Pete

I used to drive expedited freight to the Honda plant, north of Toronto. My last trip to Canada was a nightmare. I was driving a leased pickup with a topper on it, no company name on the truck. I appeared to be a "Traveler" (modern day gypsy) to the border agents. I was sent to Immigration for interrogation. I was called a liar, over and over by the Interrogator, this was not a short ordeal. It was not a good experience, this was pre-911. I refused every trip in to Canada after that, even with a passport.

Scott
 
OK, so we think at this stage the issue is not oil. If it were then what is the probability of it happening on both sides at the same time?? With both plugs fouling it suggests the problem is too much fuel. Perhaps it is a carburetor or ignition problem? But I would expect the ignition problem to also show up on the flat unless there is an electrical short that only shows up going downhill.

Regarding the carburetor, it is possible for me to imagine the fuel level in the carbs being too high and at that critical point where going downhill allows excess fuel to spill forwards. But this must be happening for both carbs so both must be set too high. If you take the air filter off and turn on the fuel can you feel fuel slashing around the main fuel jet with your finger? If so then the level is too high. If you do this test on the centre stand then this will simulate a hill.

Do your carburetors allow you to make a fuel height measurement using a plastic tube?? I know the later model bikes allow this but there may have been a change around 1977 going by the pictures in my manual. If this is possible then it is an easy test to make without pulling anything apart.

Best of Luck.
 
XS650-Spark :
if the plugs are dull black that's usually oil foweled, if their shinnie black thats usually Fuel foweled.... just from my experience....mind you.
when going down hill and the throttle off a trimendious amount of vaccuum is created in the cylinders more vaccuum that at any other time.
this is a very good indication of the engine sucking OIL from somewhere... because a gasoline fouled plug may clear up by itself an oil fouled plug won't
.... so with that I am guessing a head gasket leak.... the compression readings rule out the rings...... their fine
sense there is no practical way of putting the cylinders under a large vaccuum short of a vaccuum pump which few of us have
I sujest first re torqueing the head to specks and see if that cures or lessons the problem....
if that does nothing then I think a New head gasket will be in order be sure to coat both sides of the gasket with Copper-coat gasket sealer, if you can
it comes in sprey cans now and is perfect for a light coat on each side of the head gasket !
.... if on the other hand you can smell gas on those plugs i could be way off base ! but I doubt it !
if it runs good at all times untill you do down hill and increase the vaccuum the only other thing I can think of is the choke /enrichers being opened by the extream vaccuum in the carbs.... and new springs on their plungers should cure that . they may not open up all the way but you should be able to put your finger on the outside one and feel if it's opening up a bit on you.....
good luck ! that is indeed a puzzeling one !
please keep us posted !
Bob...........
 
Just a side to the main topic, on the XS what increase in compression do we expect to see after adding oil? Is 25 psi increase typical? I have ordered a compression tester and it is due this week so need a bit of educating (I know the engine needs to be pre-warmed and throttle wide open, but that is about it).

Thank you.
 
Bob, Lucky you have 2 thumbs then ..... I am checking our various threads, forums and Youtube so will see what I get.
 
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You really need to look at the "dry" and "wet" numbers together Paul.

It's a bit of a simplification, but atmospheric pressure is 14.7 PSI absolute and so a typical engine fueled on gasoline will have a dry number that is about 15-20 times its compression ratio. So, an XS650 with a CR of 8.4 would have a dry number of about 135-165 psi, I think. The "wet" number is usually a bit higher - because the oil helps to seal the piston rings if they leak at all.

Ideally, the two numbers will be high and nearly the same, indicating that the rings and valves are sealing well.

On the other hand, if the dry number is low and the wet number goes up a lot (like 30-50 psi say), then your piston rings are not likely in good condition. If the dry number is low and the wet number is about the same, then the problem is with your valves - and likely not with the rings. This is because adding oil will not help the vales to seal any better if they are already leaking.

Here endeth the sermon.

Happy Easter everyone! It's my birthday today, so I got Frosted Flakes for breakfast and they're ggggrrrrrrrrreat!
 
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....I like Bob's method better. He always has his compression tester with him, but I sometimes I forget where I put mine. :D
 
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Cool!

People may recall that Daniel Black got readings of about 85 psi dry on Stella - and she runs great and he rides her all the time, so it doesn't seem like these things are all that critical. As we all know, these old XSs are pretty tough.

Pete
 
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