Bob Kelly III
Ranch Kid from way back,.... that got Old !
But Pete.... I liked the sermon ! 'twas a good explination !
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Bob.......
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Bob.......
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Downhill = backfire and fouled plugs
- Thread starter XS650-Spark
- Start date
Care to explain or expand this?
Geeezzzz - that's not very nice MTW. I guess they forgot to take their medication that morning - or perhaps they were trained by those nice folks at United Airlines...
While the engine may run with only 85 psi, it will not produce the torque or HP that it could if the engine internals were in better condition. After a top end re-build, a few years ago, my dry compression readings were in the 140 to 145 psi area.
Completely agree RG - the bike would produce a lot more power and get far better fuel consumption if everything was sealing properly, but it is still quite rideable and starts reliably even with much lower compression values.
DanielBlack
XS650 Junkie
It's true, Stella is performing very smoothly and reliably. I'm thoroughly enjoying this machine and the knowledge of how far I've come with her.
No doubt, when the time and budget comes around, an engine overhaul will be very satisfying. Until then, her even albeit low compression keeps her quite serviceable. I think that's the key. Over maybe 50K miles she had been kept in fine enough trim that she shows even wear. I'm optimistic about the findings when it comes time to measure the lower and more expensive internals for spec.
No doubt, when the time and budget comes around, an engine overhaul will be very satisfying. Until then, her even albeit low compression keeps her quite serviceable. I think that's the key. Over maybe 50K miles she had been kept in fine enough trim that she shows even wear. I'm optimistic about the findings when it comes time to measure the lower and more expensive internals for spec.
Daniel: from what I have read, it is very unlikely that anything in the bottom end (crank, big ends etc.) will need any work at all. These things seem to have been built like a battleship down there with everything running on balls and rollers.
P
P
Bob Kelly III
Ranch Kid from way back,.... that got Old !
Weekendrider: OK.... with the throttle off and going down hill the engine is being turned by the rear wheel not the engines Normal mode of running...via the cylinders fireing.... the throttle off stops fuel and air from entering the cylinders, the Air pump that is your Engine Now has nothing
to suck in... so vaccuum is created in the cylinders because your engine RPM is higher than at idle and your going down hill the push of the rear wheel keeps the engine turning and the maximum vaccuum you can achieve is reached inside the cylinders... something like 29 to 30 lbs of vaccuum
due to the elastisity of air you can't get any more vaccuum in air it simply pulls apart.....
so down hill with the throttle off is the maximum vaccuum that you can get.... this vaccuum isn't only in the cylinders it seaps past the rings and lowers the pressure in the bottom end as well ....perhaps sucking up oil from the bottom end as the vaccuum tries to equilize itself
.... the engine normially has vaccuum in it, that is what makes it run ...it can't run without it..... going down hill with the throttle off simply amplifies that vaccuum ..... the only relief to the vaccuum that the vaccuum sees is the throttle position, opening the air way into the cylinder past the intake valve
if that's closed the vaccuum is maximized.
an engine truly is just an air pump with a few pecularities....the timing isn't perfect tor pumping air the best,.... but it's great for getting power from an air fuel mix....
hope that answered your question...
Bob.........
edit.... I have a 5hp B&S engine I have made into an air compresser.... all i did was take out the cam and put 2 check valves in the head, one in the spark plug hole.... one next to it, when the engineis turned the air is sucked in through the one check valve, then compressed out the other check valve.... simple !
to suck in... so vaccuum is created in the cylinders because your engine RPM is higher than at idle and your going down hill the push of the rear wheel keeps the engine turning and the maximum vaccuum you can achieve is reached inside the cylinders... something like 29 to 30 lbs of vaccuum
due to the elastisity of air you can't get any more vaccuum in air it simply pulls apart.....
so down hill with the throttle off is the maximum vaccuum that you can get.... this vaccuum isn't only in the cylinders it seaps past the rings and lowers the pressure in the bottom end as well ....perhaps sucking up oil from the bottom end as the vaccuum tries to equilize itself
.... the engine normially has vaccuum in it, that is what makes it run ...it can't run without it..... going down hill with the throttle off simply amplifies that vaccuum ..... the only relief to the vaccuum that the vaccuum sees is the throttle position, opening the air way into the cylinder past the intake valve
if that's closed the vaccuum is maximized.
an engine truly is just an air pump with a few pecularities....the timing isn't perfect tor pumping air the best,.... but it's great for getting power from an air fuel mix....
hope that answered your question...
Bob.........
edit.... I have a 5hp B&S engine I have made into an air compresser.... all i did was take out the cam and put 2 check valves in the head, one in the spark plug hole.... one next to it, when the engineis turned the air is sucked in through the one check valve, then compressed out the other check valve.... simple !
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DanielBlack
XS650 Junkie
Exactly, Pete. I have full expectation of most everything being in spec. It wouldn't run so well now if it wasn't nice and tight in there. The timing wanders a bit, but that's probably just a worn chain.
Bob Kelly III
Ranch Kid from way back,.... that got Old !
ok back to the subject at hand.... 5Twins may well have said it ! float level ! there may well be a 1mm diference in float level to keep the gas from spilling out into the throat of the carb when going down hills
..... probably very steep hills....
How steep is this hill that keeps giving you problems ? it must be a doozy !
I like my idea of the chokes being activated due to extream vaccuum..... has anyone ever ran into this before ?
I must admit this is the first i've ever hurd of a bike foweling plugs only going down hill !
...but there has to be a reason for it !
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Personally I think 5twins has it ! dumping raw fuel into the engine going down hill would certainly do it.....
the soultion would be reduce the float level a small amount.... so the level in the carbs can't over flow when going down hill.
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Bob..........
..... probably very steep hills....
How steep is this hill that keeps giving you problems ? it must be a doozy !
I like my idea of the chokes being activated due to extream vaccuum..... has anyone ever ran into this before ?
I must admit this is the first i've ever hurd of a bike foweling plugs only going down hill !
...but there has to be a reason for it !
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Personally I think 5twins has it ! dumping raw fuel into the engine going down hill would certainly do it.....
the soultion would be reduce the float level a small amount.... so the level in the carbs can't over flow when going down hill.
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Bob..........
TimeMachine
SeventyEighte
Oil fouled vs gas fouled. Ok Im chiming in. Firstly, I ride hills. Yesterday & today I run "laps" on a route that does include some steep downhill (10mph lol) twisty no throttle riding. My own BS38 equipped mostly stock tuned engine does stumble & complain slightly at the bottom and wants to clean out with some throttle. And then its all over and fine. I accept this as normal ! It does not die, foul plugs, or backfire because of the descent. I am fortunate to have a 12,000 mile stock engine which passes Bobs thumb compression test well ! Lol. After following this thread I am inconclusive as whether DB's plugs are gas fouling or oil fouling ? I am leaning towards oil fouling due to engine vacuum sucking oil either by rings, head gasket, valve train or all of the above. I recommend further diagnosis. Is there any sign of gasoline in the oil ? Can you simulate a hill with the engine running ? (Point it down something) hell, i've done laps around the high school parking lot helmet off, tuning ! There must be a clue ? And if you really cannot determine the cause, no joke, avoid long downhills and enjoy the ride anyway thats how simple I am and yet by further test riding you will put the puzzle pieces together. So, is it oil fouling or gas fouling ? I am curious as well.. -RT
thats how simple I am and yet by further test riding you will put the puzzle pieces together. So, is it oil fouling or gas fouling ? I am curious as well.. -RT
Bob Kelly III
Ranch Kid from way back,.... that got Old !
OK RT.... lets beat this one out.... there has to be an answer to it !
if Oil foweled.... what could be the cause ? too much oil in the crankcase that Hits the crank on a steep down hill ? unlikely.
Never seen a head leak that didn't leak at least MOST of the time and never seen one on vaccuum only...( but there is alwayse a first time !)
I have seen perodic head leaks from head gaskets causing very strange sysmtioms ! and can't rule a head leak out because of it...but usually a head leak is a constant leak and very appairent. just not always.
surely a head leak would show up in a compression test they usually do....
Rings that would allow that much oil through to fowel the plugs would definately show up on the compression test as being in the 50's or lower ! and probably not even run at all.... so the rings are ok......
valve guides and seals only let in a small amount of oil usually.... and is easily noticed by the smoke from the exhaust...
so they must be good.
their 's not much else that can cause oil consumption !
I have seen head gasket leaks into the oil line supplys before and those are appairent on one side usually , not both, and show up in the compression test...... so I think we can exclude Oil as the foweling agent...it's gott'a be gas !
...so if it's gas what would be the cause, when only going down hill ? not many things change when going down hill.... the vaccuum does and the angle of the machine is about all...... that leaves only 2 possable causes vaccuum or angle..... I think we can eliminate My sujestion of increased vaccuum because you get the same amount ( or darn near it when just throttling down on the flat land) and he hasn't mentioned that that is a problem.
so it leaves us with only one.... the angle..... if the floats are so high that they are right at the brink of over flowing sitting on the flat.... ( which in my mind they certainly are with stock settings !) then it would only take a slight angle to get them to over flow into the throat body and be sucked in
..... that or perhaps the choke/enricher is being misteriously activated.....( very unlikely).
so taking the only thing it can be is float level !
if it were mine , I would take the carbs off pull the float bowls and increase the float height by 1/16" to 1/8" on each carb equally... put it back together and try it.... but ONLY do that to isolate the cause.
can you find any objections to this theory ?
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Bob........
if Oil foweled.... what could be the cause ? too much oil in the crankcase that Hits the crank on a steep down hill ? unlikely.
Never seen a head leak that didn't leak at least MOST of the time and never seen one on vaccuum only...( but there is alwayse a first time !)
I have seen perodic head leaks from head gaskets causing very strange sysmtioms ! and can't rule a head leak out because of it...but usually a head leak is a constant leak and very appairent. just not always.
surely a head leak would show up in a compression test they usually do....
Rings that would allow that much oil through to fowel the plugs would definately show up on the compression test as being in the 50's or lower ! and probably not even run at all.... so the rings are ok......
valve guides and seals only let in a small amount of oil usually.... and is easily noticed by the smoke from the exhaust...
so they must be good.
their 's not much else that can cause oil consumption !
I have seen head gasket leaks into the oil line supplys before and those are appairent on one side usually , not both, and show up in the compression test...... so I think we can exclude Oil as the foweling agent...it's gott'a be gas !
...so if it's gas what would be the cause, when only going down hill ? not many things change when going down hill.... the vaccuum does and the angle of the machine is about all...... that leaves only 2 possable causes vaccuum or angle..... I think we can eliminate My sujestion of increased vaccuum because you get the same amount ( or darn near it when just throttling down on the flat land) and he hasn't mentioned that that is a problem.
so it leaves us with only one.... the angle..... if the floats are so high that they are right at the brink of over flowing sitting on the flat.... ( which in my mind they certainly are with stock settings !) then it would only take a slight angle to get them to over flow into the throat body and be sucked in
..... that or perhaps the choke/enricher is being misteriously activated.....( very unlikely).
so taking the only thing it can be is float level !
if it were mine , I would take the carbs off pull the float bowls and increase the float height by 1/16" to 1/8" on each carb equally... put it back together and try it.... but ONLY do that to isolate the cause.
can you find any objections to this theory ?
.....
Bob........
TimeMachine
SeventyEighte
Bob, no objections to your theory. Also no conclusions. I believe, if it is in fact gas fouling the engine oil will show gas contamination. (hoping) I also have no conclusive info that DB's rings are ok.. actually there is stated history of high mileage wear and low compression.. I suppose exhaust smoke would be a great indicater and would like more info on that ? Anyway, I just hope my own example of carb behavior can help DB determine cause.. doesn't matter to me to be right or wrong.. Thanks for your help as well ! -RT
Bob Kelly III
Ranch Kid from way back,.... that got Old !
its hard to say actually we have very little info to go on.... but he at least did the pree tests for is and answered a bunch of questions to start....
that's far more than most ! he's a sharp guy ...he'll get it figured out !
...thank YOU RT.... your a big help here ! the more minds on a problem the faster it's solved !
Bob......
that's far more than most ! he's a sharp guy ...he'll get it figured out !
...thank YOU RT.... your a big help here ! the more minds on a problem the faster it's solved !
Bob......
DanielBlack
XS650 Junkie
The discussion about Stella was a tangent from the OP. Apologies for the confusion.
Bob Kelly III
Ranch Kid from way back,.... that got Old !
No problem Daniel.... that happens all the time ! no biggy !
I'm glad Stella is still running good ! I wish I had liscnses on mine so I could ride her ! LOL
oh well maybe next month ! HAHAHHA!
.....edit.....
RT : I was talking about this original thred.... not Stella , just incase ya missed that !
LOL..... typing is a weird way of comunicating !
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Bob.......
I'm glad Stella is still running good ! I wish I had liscnses on mine so I could ride her ! LOL
oh well maybe next month ! HAHAHHA!
.....edit.....
RT : I was talking about this original thred.... not Stella , just incase ya missed that !
LOL..... typing is a weird way of comunicating !
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Bob.......
Next time you go down that hill, pull in the clutch, let the engine idle, use brakes to control speed. If it still fouls, think fuel slosh.
Also, have the bike bend over and spread it's cheeks. Run a clean finger around the inside of the tailpipes. Feel for dry/sooty or slick/oily. Smell your finger...
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....oh my.....
Bob Kelly III
Ranch Kid from way back,.... that got Old !
LOL ! 2m your too much ! HAHAHAHHA !
very good idea too if it still loads up going down hill then it has to be fuel slosh ! good thinking !
Bob.....
very good idea too if it still loads up going down hill then it has to be fuel slosh ! good thinking !
Bob.....
