rattle rattle rattle

[Freakshow]

so here's the thing.

i have 2 x 250cc bikes now both making the same rattling - deto noise......

1 has a PVL - set at 2.3 BTDC
1 has a standard yamaha ign set at factory BTDC

one has a 36mm carby
1 has a 34mm carby

1 rebuilt motor
1 original motor

NOw here's the thing, they were jetted seperately and by differant people and with a plug chop there looks to be tan and plenty of gas on the main jet on BOTH.

And they both sound clean and crisp off idle on the stand.

NOw this is where it goes pear shaped,  after the first FT lap or 30 sec, they both rattle from what sounds like in the head/ barrell like a can of spray paint.  and i mean rattle ugley although it seems to get noisey as they warm up and is only under riding throttle.

I have stuffed about with the timing and main jets and results stay prity much the same.  There must be a common denominator as the bikes are unrelated so there is no duplication of a mechanical error.

Although they were ridden seperately, and 6 months apart , the fuel came from the same bowser outlet , as i happened to have a empty can on me at the time.  It was shell V power or what ever they call there 98 at both times.   LAter yesterday after we had a tune up day bike 2 displayed the same characteristics as its brother had 6 months earlier at a race day.  Later in the day i drained the tank and used a mix of BP 98 and TTS the pinging/ rattle had moved from across the range under load to just the bottom to mid under load and went away at full revs, throttle.  so i suspect its in the Fuels burn or the timing of the burn. 

The motors are stock so the amount or style of fuel is my problem i suspect.  What does my head in is, both the bikes are displaying  the same issue.  "Rattle can in the motor'  albeit now i have got it down to the low end - loaded.  Late in the day After about 6 laps i think it nipped up So i pulled in , (will pull it down this week to see what was going on there) as it warmed up, and maybe up the pilot incase its leaning off the gas.  But these bikes are to far removed to be displaying the same fault based only on set up.  iT has to be a fuel related issue in quality or quantiy characterised by the Shell or BP 98 that has changed the previous baselines that worked.

Question anyone else seen this change and if so what correction did you make to move or solve the issue ?
What concrete ( not guesses please ) adjustments should be made to chase out the problem.

I plan to pull them both down in the next week or so and record them separately to see what is mechanically the same and what if anythign is differant in the barrel and head, squishes and comp etc.  Incase there is a need to refer to this data and make changes.

[shorelinemc]

definately do not use the shell fuel it will cause problems

[asasin]

 had the same thng but both my motors failed . it was a blocked feul tap they would run good for about 2-3 laps then sh88 them selves.It seems that those motors will run very lean and detonate befor seizing.

[Slakewell]

You can try BP 100 then at least you will know if it's the fuel or not.

[Freakshow]

But why on a plug chop was the plug tanned out ? at some point it was getting Juice in.
Only one has a Fuel filter, but in saying that i actually checked it was flowing on at least two occasions.
cant beleive both have had blocked fuel taps.  both were on 25:1

Its strange its worse on the shell and only 25% better on the BP.  IM yet to try i higher octane again, but i would have thought the timing and flow would have made a differance there but not that i could notice.  Standard heads so its not over compted.  

if i had the chance i was going to strat messing with needles.   ITs just weird that both are dipsplaying the same issues so there is a common issue.

[steve234]

Float level setting, it could be that the fuel level is to low resulting in not enough fuel at FT ?

[shelpi]

Hi whats spark plugs are you useing and what heat range? can you get your hands on some AVGAS as it still has lead in it ANTI PING AND KNOCK (just to test for fuel sake) , have you tried other fuel additives and other 2 stoke oils ?  recommend belray MC-1 or PENRITE TS40C ,what happens when you richen the fuel ratio? start at 30:1 then 25:1, 20:1 even 16:1, are they running cdi or points? did you use a battery powered timeing light to see what the timeing curve is doing in real time? have the heads been modified or are they stock? try lowering compression ratio (2 head gaskets) are both the port timeings the same, have they both been modified from std (ported) WHAT AIR filters are you useing? HOW much back pressure? have you done a engine pressure test to see that they are not sucking air from a seal, gasket or crankcase join, all the best and hope they come good and soon.

[Freakshow]

NOT float as both Carbs do it.
As i said HEads barrels are standard.
2 differant timings/ bikes shelpi they cant both be out.
one motor has new seal other ran fine a few years back so the seals were fine.

Its got to be something in the way the fuel is or isnt going bang its the only common denominator and the relations ship it is having on the spark and the delivery of fuel.  In 2 differant,  but otherwise identical models and cc bikes.

[oldfart]

Freaky ... I normaly run caltex fuel, this weekend I ran shell in My Rm 125 and it was running like shit at high RPM changed 2 plugs and still the same so o the trailer it went .  In my case it's either the High speed coil ( stator ) or fuel.   

[bazza]

Start with fuel change. Check float and fuel tap. Let us know when you find answer.

[Freakshow]

i changed fuel like i said went from the shell to the BP ( drained tank and refilled) as it was the only thing i could think to try off the top of my head.

went better on only rattle its face off up to half throttle but the nise is still inthere at low to mid revs and under load.  increasing with heat...............

[Freakshow]

I just did a search on Shell 98 pining its guts out and it had more hits than * insert politically incorrect stament here*

I suspect more an more its a definate fuel issue.  I just have to figure out how we can tune around it.

If a harley complains of pingin there must be issue with te juice itself
http://www.hdforums.com.au/Default.aspx?tabid=71&view=topic&postid=312592&forumid=27&tpage=1

THEre was a write up that all feul comes out of BP qld and then each state blends it out ? so if that the case Shell SA may be way diffeant to another state at the pump.  iSee that have added inhibitors to increase economy ? what ever you need that for ? 

I had had a lot of success with a batch of that ethonal stuff in the TM and it went like a cut snake, but i never had the can with me in the car when i was going that way.  Might be worth me making a special trip to get a can filled to suss if it makes any differance in these bikes.

and as such if its true as they say that ethanol based fuels have "a natural tendency to resist compressive precombustion in engine combustion chambers."  Its probally worth me testing an outing to see if the inherant 'effect of lifting octane and allowing exposure to greater heat and compression in engines without 'pinging' (precombustion)." actually does anything to move this problem away.

[JohnnyO]

It sounds like they are detonating through too much compression or the timing too advanced and you may need a higher octane fuel or add an extra head gasket or base gasket and try that.

[Lozza]

Lift the head and look for signs of deto on the head and piston. If you can see signs of sandblasting the fuel per se will not be the issue more the O2 content of the fuel.

[Nathan S]

If the noise is detonation, then the problem is that your compression ratio is too high relative to the fuel you're running.
Or, the compression ratio is too high relative to how crappy the combustion chamber shape is...

I went down this path with my AT1 and had exactly the same symptoms as you. It was great for a couple of laps from stone cold, but would then ping like mad when it was fully warmed up, no matter what you did with jetting or timing or riding style.

Its a particlar problem for the early xT1 Yammies - a reed valve head on a piston port barrel (or visa versa, I can't recall) will do this every time.