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Collective wisdom needed...Misfire at hi revs ...fuel starvation?

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SpitnSawdust Avatar
SpitnSawdust Richard Simpson
Lewes, east sussex, UK   GBR
1969 Triumph Spitfire MkIII "Little Blue"
Hi,
Now I've got the correct float bowl on the front HS6 so it doesn't pi$$ out fuel and soot the plugs...
Am I right in thinking my previous attempt to sort it by lowering float valves with washers might be causing fuel starvation at high revs (>4k on a tc2500 engine)?
TIA
Rich

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Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
It can also be caused by the ignition skipping or by flooding.

What do your plugs look like?

SpitnSawdust Avatar
SpitnSawdust Richard Simpson
Lewes, east sussex, UK   GBR
1969 Triumph Spitfire MkIII "Little Blue"
Not exactly a plug chop but...they look ugly. Oil issues mostly but much much less sooty with the correct front float bowl. I suppose the fouling could easily cause misfire at high revs.
I'm going to take out the float valve seat spacers and also bring in hotter plugs and try it in the week.
Cheers
Rich

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clshore Carter Shore
Beverly Hills, FL, USA   USA
In the 'old days' a plug cut was about all we had.
Nowdays there are Wideband O2 sensors.

Plug cuts are only as accurate as the most recent combustion cycles before shutoff.
The plugs only reflect what deposits were made onto the insulator.
Unless you start with a fresh (or freshly cleaned) set of plugs, on a motor that is otherwise in very good
working order (no oil or other contamination, valves in perfect shape, ignition close to optimum)
your chances of accurate readings are seriously degraded.

WBO2 also have their own set of pitfalls, owing to the fact that they only indicate the proportion of O2 levels
remaining from combustion events, not the fuel content of the measured gases.

If you are misfiring, then by definition there is some combustion malfunction that will result in unburnt mixture.
But misfires may be due to excess fuel (rich), or excess air (lean), or some combination, or electrical issues
The WB02 aggregates the results, rather than reporting the O2 remaining from individual combustion events.
If you are misfiring 20% of the time, the indicator may report peak, may report peak low, or may average the values.
Even if it faithfully follows the values, which change about 100 times per second, how can your human eye follow them?
Absent an oscilloscope that can actually resolve the O2 signal from each individual ignition event, the WBO2 instrument
will lie to you.

I recommend that you 'sneak up' on the misfire.
Start each test run with a clean set of plugs (a good plug cleaner device will rapidly pay for itself, given the high costs of plugs).
Get the motor up to full operating temperature, ensure that the cold enrichment is off (jet position on an SU).
Do runs at WOT at a specific RPM (use the brakes, or test going up a steep hill) for at least a few seconds, and cut ignition.
Accelerate gently up to the desired RPM, then apply WOT and modulate the brakes to keep RPM constant.
2,000 RPM intervals should work.
Once you have identified what RPM range the miss issues occur, go to 500 or 1000 RPM intervals in that range.

Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
I'd clean the inside of the dizzy cap check the plug wires as well.

SpitnSawdust Avatar
SpitnSawdust Richard Simpson
Lewes, east sussex, UK   GBR
1969 Triumph Spitfire MkIII "Little Blue"
Thanks for the info chaps.
Lucky I have a private airfield for those WOT tests or my license might misfire ;-)

clshore Carter Shore
Beverly Hills, FL, USA   USA
Depending on what you discover, a high speed fuel mixture related misfire may be correctable by suitable needle taper.

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SpitnSawdust Avatar
SpitnSawdust Richard Simpson
Lewes, east sussex, UK   GBR
1969 Triumph Spitfire MkIII "Little Blue"
Thought I was running BDQ needles. So imagine my disappointment when I went to fit the BAG ones only to find they were already n there.

It will keep pulling past 4krpm as long as I don't plant the accelerator, in which case it back fires out the exhaust.

I wonder if it's just over carbed for a clapped out 2500tc lump...oil fouling is a problem

I might try lighter carb piston springs. I know it's not under load but even at high revs stationary the pistons only lift maybe a half inch. And theyre not sticky, worn etc

Still....keeps me out of trouble. Appart from buying stuff;-)

Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
I you are backfiring at speed that's timing.

Is it consistent?

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Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
It may be a weak spring on the points. At high rpm they could be bouncing.

SpitnSawdust Avatar
SpitnSawdust Richard Simpson
Lewes, east sussex, UK   GBR
1969 Triumph Spitfire MkIII "Little Blue"
Hi,I

Its only at full tilt...4th OD on WOT.
it's got electronic ignition ...acusparc retro fit I think.

Re timing ...too much advance...or not enough? The dizzys in quite good nick. New advance springs and everything moves freely.. vacuum advance is not connected.

Doug in Vegas Avatar
Doug in Vegas Douglas D
Las Vegas, NV, USA   USA
Try pulling and blocking the vacuum advance line and see if it stops.

clshore Carter Shore
Beverly Hills, FL, USA   USA
Posted to wrong thread



Edited 1 time(s). Last edit at 2018-05-16 07:40 AM by clshore.

carChips Avatar
carChips Victor Harnish
Kelowna, BC, Canada   CAN
1933 MG Magnette
1973 Triumph Spitfire 1500 "Chip"
1989 GMC Sierra 1500 "Bush Truck"
Seeing you don't have an advance hooked up, I guess not enough advance. Try setting it at 12 Degrees BTDC.



'S all for now
Vic

Wolfcreek Steve Steve P
Central, WI, USA   USA
Coil? Condenser?

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