Gadgetman Posted February 27, 2021 Report Share Posted February 27, 2021 Where do people normally start investigating a miss fire under load? The engine runs beautifully with the car stood but out driving it has a miss fire when under moderate load, I.e accelerating. It’s a rebuilt engine with new electrics, ie plugs, leads, coil, condenser, re-build distributor. apart from replacing components one at a time it’s going to be a pain to troubleshoot. I need an old fashioned Crypton analyser! Link to comment Share on other sites More sharing options...
Pete Lewis Posted February 27, 2021 Report Share Posted February 27, 2021 start with spark plugs here some clues Link to comment Share on other sites More sharing options...
Paul H Posted February 27, 2021 Report Share Posted February 27, 2021 I found my misfiring was caused by the rotor arm was loose . The rivet hold the arm was becoming loose Paul Link to comment Share on other sites More sharing options...
poppyman Posted February 27, 2021 Report Share Posted February 27, 2021 If on Strombergs check the dash pot oil, my 1850 Dolly is a sod for that. Tony. Link to comment Share on other sites More sharing options...
Pete Lewis Posted February 28, 2021 Report Share Posted February 28, 2021 engine oil for the dash pots thats the flat spot pump are you on std air filters ??? or something silly ?? Pete Link to comment Share on other sites More sharing options...
Gadgetman Posted February 28, 2021 Author Report Share Posted February 28, 2021 Std filters with the flexible tubes running to the front grill. what’s the best grade of oil to use in the dash pots? Back in the 80’s I used to use 3 in 1 oil but I don’t think you can get that now? Link to comment Share on other sites More sharing options...
JohnD Posted February 28, 2021 Report Share Posted February 28, 2021 There is no "best oil", although as Pete has said, engien oil is the factory recommendation. The dashpot is there to slow the rise of the piston on acceleration, speeding the air through it, lowering the pressure at the bridge and drawing more fuel up, richening the mixture to aid acceleration. Use a thinner oil, and that richening will be less, and your consumption less. Thicker, if you have a heavy right foot. Everything from sewing machine oil to diff oil has been used, try it yourself! JOhn Link to comment Share on other sites More sharing options...
Pete Lewis Posted February 28, 2021 Report Share Posted February 28, 2021 think the orig spec was SU a straight 20 and for strombergs engine oil i have no idea how thin 3 in 1 goes when hot seems to resemble cats pee its not an automotive oil and unlikely to be temperature stable in a engine situation makes me wonder why triumph and others do all the development work to then feel a door hinge/ bicycle oil is better doesnt add up the dashpot id quite clever in cold oil gives stiffer damping and as the engine warms up gives somewhat less but damping piston rise is essential unless you like flat as a pancake performance and as you then need wider throttles you get poor MPG combined with poor flat acceleration......and no SMILE Pete Link to comment Share on other sites More sharing options...
Ian Foster Posted February 28, 2021 Report Share Posted February 28, 2021 You describe the fault as a missfire rather than a flat spot, but it depends what you consider to be a missfire. If it just an occasional miss it could be carb related...lean hitching. If it a continuous stutter above a certain revs then it is more likely to be ignition related. I would start by rechecking the points gap and closely inspecting the rotor arm, but my best guess would be that you have got a dodgy condensor. Some aftermarket offerings can be pretty rubbish. Ian Link to comment Share on other sites More sharing options...
Gadgetman Posted February 28, 2021 Author Report Share Posted February 28, 2021 45 minutes ago, Pete Lewis said: think the orig spec was SU a straight 20 and for strombergs engine oil i have no idea how thin 3 in 1 goes when hot seems to resemble cats pee its not an automotive oil and unlikely to be temperature stable in a engine situation makes me wonder why triumph and others do all the development work to then feel a door hinge/ bicycle oil is better doesnt add up the dashpot id quite clever in cold oil gives stiffer damping and as the engine warms up gives somewhat less but damping piston rise is essential unless you like flat as a pancake performance and as you then need wider throttles you get poor MPG combined with poor flat acceleration......and no SMILE Pete Thanks Pete ive some sae30 running In oil so I’ll try that and go over the electrics again and start swapping bits if that doesn’t cure It. Link to comment Share on other sites More sharing options...
Clive Posted February 28, 2021 Report Share Posted February 28, 2021 47 minutes ago, Pete Lewis said: think the orig spec was SU a straight 20 and for strombergs engine oil i have no idea how thin 3 in 1 goes when hot seems to resemble cats pee its not an automotive oil and unlikely to be temperature stable in a engine situation makes me wonder why triumph and others do all the development work to then feel a door hinge/ bicycle oil is better doesnt add up the dashpot id quite clever in cold oil gives stiffer damping and as the engine warms up gives somewhat less but damping piston rise is essential unless you like flat as a pancake performance and as you then need wider throttles you get poor MPG combined with poor flat acceleration......and no SMILE Pete I reckon there is a serious misconception that performance improves with a thinner oil as the piston can move faster. People do not understand that a lower piston with greater air speed over the jet is a richer mixture, they think piston higher, so needle thinner so more fuel. Wrong, that is the "cruise" position. But still, it keeps us busy. I have always used 20/50 oil (or nearest I have available) as the viscosity changes less with temps. The SU straight 30 is probably from when that was the triumph recommended oil, before multigrades were the norm. Link to comment Share on other sites More sharing options...
Pete Lewis Posted February 28, 2021 Report Share Posted February 28, 2021 12 minutes ago, Gadgetman said: ive some sae30 running In oil it might be a sae30 but has it got any guts ? just normal engine oil 20/50 etc why confuse it if you do what it says in the book it will do what it says on the tin Ha Pete 1 Link to comment Share on other sites More sharing options...
GrahamB Posted February 28, 2021 Report Share Posted February 28, 2021 I have experienced misfire when accelerating but runs fine when cruising. As you open the throttle, more air enters the cylinder and the pressure at TDC is greater. A higher pressure requires a higher voltage to spark across the plug gap and the higher voltage can cause the insulation else where in the circuit to break down. My most recent occurrence was on my wife's Toyota due to a faulty coil pack. The previous time was on a Hillman Imp due to over large plug gaps and contamination on the distributor cap. That was nearly 50 years ago but I knew exactly where to look on the Toyota last year when I experienced the same symptoms. Link to comment Share on other sites More sharing options...
Gadgetman Posted February 28, 2021 Author Report Share Posted February 28, 2021 So.... plugs all look good, nice light biscuit colour, 25 thou gap, Oil changed in dashpots to 20-50, points gap re-set to 15 thou. Compressions checked all 160-169 psi hot. Took it for a run and all was ok... for a short while...then when under load it hesitates when the throttle is slowly pushed down, maybe not a full misfire? What about a weak mixture, would that do it, remember I said the plugs were a nice light biscuit colour.....maybe a Vitesse needs builders tea colour!! Link to comment Share on other sites More sharing options...
daverclasper Posted February 28, 2021 Report Share Posted February 28, 2021 13 minutes ago, Gadgetman said: What about a weak mixture, would that do it I have had those symptoms when running weak. I think too much advance timing can also cause it?, though you should also get pinking as well I think?. I'm no expert though! Dave Link to comment Share on other sites More sharing options...
Paul H Posted February 28, 2021 Report Share Posted February 28, 2021 19 minutes ago, Gadgetman said: So.... plugs all look good, nice light biscuit colour, 25 thou gap, Oil changed in dashpots to 20-50, points gap re-set to 15 thou. Compressions checked all 160-169 psi hot. Took it for a run and all was ok... for a short while...then when under load it hesitates when the throttle is slowly pushed down, maybe not a full misfire? What about a weak mixture, would that do it, remember I said the plugs were a nice light biscuit colour.....maybe a Vitesse needs builders tea colour!! Have you tried retarding the timing ? Paul Link to comment Share on other sites More sharing options...
Gadgetman Posted February 28, 2021 Author Report Share Posted February 28, 2021 It doesn’t pink so doubtful if it’s timing Link to comment Share on other sites More sharing options...
Pete Lewis Posted February 28, 2021 Report Share Posted February 28, 2021 easy richen up 1/4 turn see what happens Pete Link to comment Share on other sites More sharing options...
NonMember Posted February 28, 2021 Report Share Posted February 28, 2021 Yep, sounds likely enough to be just weak mixture and so easy to test a quarter turn richer. Link to comment Share on other sites More sharing options...
daverclasper Posted February 28, 2021 Report Share Posted February 28, 2021 Always wondered if. When setting up an engine with some wear (first do the book/base settings for carbs and timing), then as I understand, timing affects fuel burn/mixture and sort of vice versa (chicken or egg situ?). Is this correct?. If so, just a case of some twiddling to get it running ok/well, rather than spot on?. For most of us DIYers, then the only way to analyze the state of tune is a rolling road? More interested than I'm wanting total perfection. Link to comment Share on other sites More sharing options...
Iain T Posted February 28, 2021 Report Share Posted February 28, 2021 Gadgetman, if you've done work on the head (valves, porting etc) then the better breathing will require a richer mixture. I had to change my needles as under acceleration mine went very weak. I installed an air/fuel gauge and all was revealed! Iain Link to comment Share on other sites More sharing options...
Gadgetman Posted February 28, 2021 Author Report Share Posted February 28, 2021 Thanks, yup ported and compression raised ( hence the 169 psi compression reading!) I”ll richen it up a 1/4 of a turn and see what happens then if all else fails it’s rolling road time. I have a local old school guy with a rolling road who understands strombergs and has been know to fettle needles to get the mixture right across the range! Link to comment Share on other sites More sharing options...
Steve P Posted February 28, 2021 Report Share Posted February 28, 2021 If you wouldn,t mind,who and where is the rolling road chap?. I have been looking for one for a racing Metro with Webers. Steve Link to comment Share on other sites More sharing options...
Gadgetman Posted February 28, 2021 Author Report Share Posted February 28, 2021 Here you go http://www.aireytuning.com Link to comment Share on other sites More sharing options...
Pete Lewis Posted March 1, 2021 Report Share Posted March 1, 2021 a tip its a lot easier to help if we have clues about engine mods in the first post sorry we dont have a crystal ball were all guessing raising the CR will need a revised needle profile or added weight to the damper top ring or spring Pete 1 Link to comment Share on other sites More sharing options...
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