cause of a misfire???

[bradders89]

I run a 1.9 dg and it has a bad misfire when pulling away or changing down gear goin up hills. Its like it clogs up instantly but clears again if i rev it hard and it puffs out a lot of black smoke then seems fine again. Its been serviced recently with new plugs and leads and the timing has been checked. Any ideas what it could be would be gladly recived! Thanks.

[Mickyfin]

It could be a number of things, so here's a few suggestions on what to check...

How old are your HT leads, and plugs?

Is your timing set up correctly.?

How is your tickover?

[bradders89]

Plugs and leads were done in september and timing was done spot on. Ive taken plus out and checked them and there all sooted up.

[kevtherev]

So you get like a hesitation as you try to accelerate... and then after revving it harder it clears up and away you go?

does it do the same if you slowly and gently depress the accelerator?

[bradders89]

No doesnt seem to. only when i put my foot to the floor then it just coughs and splutters untill it clears with a puff of black smoke!

[kevtherev]

Ok three reasons why that might happen.

Air leak on the induction side...brake servo hose vacuum hoses, gaskets, choke pull down unit diaphragm, distributor diaphragm leaking.

or

the auto choke is not opening fully.. take the pancake duct off and check it's operation from cold (search my posts I have answered this a lot)

or

the transfer jet is blocked.. again take off the duct look into the carb and you'll see a tap like thing pointing into the venturi, operate the throttle and you should see a healthy squirt of fuel go down the venturi

[ghost123uk]

I'll go for the auto choke not going off when hot.
Try wiring it fully open as a test, just to see if that cures it.

[California Dreamin]

Yup..sounds like over fueling (rich mixture) I remember choke unit problems associated with snapping bi-metalic spring (snail spring breaking off at the end) resulting in the choke flap...'flapping' about and not fully opening.

Seriously though..this could be down to lots of things: but if all four plugs are sooting up on a hot engine (remember they will look sooty when cold and on choke) then a issue on the fuel side of things looks more likely.

Martin

[sodea]

Bradders, it sounds like you have exactly the same problem as me. I have a 1984 T25 which has started to pop and backfire under load and when it loses power, I have to ride the clutch to keep the revs high so I can move away.

Same thing with sooty sparks too.

I'd be interested to find out which of the above solutions worked.

Keep us updated

Steve

[kevtherev]

Same thing with sooty sparks too.

after a run at motorway speeds?
or just tootling around

[sodea]

Truth is, I've only owned it for two months and the longest run its had is from Llandundo where I bought it to Galway where I live (150 miles)

It's done very little of anything since it got here as I'm still waiting for all the paperwork to be finalised, so I expect it's from the M-way run.

That said, the problem didn't materialise until after that particular run (couldn't say when the sparks got sooty)

Having read a few of the other threads and associated symptoms of a sticky choke, I'm gonna have a poke around under the carb lid tomorrow.

Will let you know how it goes.

[ghost123uk]

To both of you, remember you could temporarily fasten the choke flap fully open with a bit of copper wire or similar on the choke lever, just to see if it improves things (once the engine has warmed up).

Or you could just monitor the choke flap as the engine warms up, making sure it is fully open when up to temp.
The only "issues" that might come a long doing it this way is, that in this weather, a ticking over WBX might take some time to get to "choke fully open" temperature and if say the thermostat it not working correctly it may never come up to "choke fully open" temperature". Mind you, that is a test in itself

[sodea]

Further to me hijacking somebody else's post, I had a look under the Carb Cap today, and gave the choke cap a good ol' spray with WD40. It didn't seem stiff and all my attentions didn't really make any difference to the problem.

I even tried Ghost's suggestion, holding open the choke open with some galvanised wire.

When I did this and put my foot to the floor on the accelerator, I got the same result as before: a delay, engine spluttered and popped (there seemed to be explosions down the carb when this happened).

The explosion either caused the engine to stall, or occasionally the revs would catch up, either way, it was fairly significant explosion. This would definately explain the balckened sparks.

I also tested the seals with the carb cleaner spray, and I couldn't determine any significant difference in engine noise, so reckon these are sound.

So if it's not the choke flap, my next step is to look at the fuel delivery into the carb. Is there a test for this?

the transfer jet is blocked.. again take off the duct look into the carb and you'll see a tap like thing pointing into the venturi, operate the throttle and you should see a healthy squirt of fuel go down the venturi

Kev, you mention looking down the venturi. What is this exactly? Also, when you say operate the throttle, presumably you mean with the engine off? Either way I'll need to get a mate to help out.

[sodea]

I've just found this link to cleaning out the second filter. If it is a Jetting problem, this is going to be my first port of call.

https://club8090.co.uk/wiki/Fu ... ting_carbs

I have to say, having been a T25 owner for only two months, I'm extremely impressed by the resources and level of support available.

Yours gushingly

Steve

[kevtherev]

OK
take a look on the side of your carb there is a little pump that works when you depress the acc.
it has four screws holding a plate, behind this plate is a diaphragm pump.

no 6 here



when the casing is removed you should see this..



This pump squirts neat fuel into the throat (venturi) of the carb via internal drillings, when you press the accelerator.

I had mine block up with rust after I had water ingress into the petrol tank, the water rusted a spring in the fuel cutoff solenoid in my LPG system... which is post filter.

This extra fuel compensates for the initial gulp of air that your throttle butterfly allows into the inlet manifold... and the lag of the main air and fuel jets.
If you accelerate very slowly it won't hesitate.... or shouldn't.
It is one reason why a heavy foot on acceleration pushes the MPG down alarmingly....especially when you see the amount squirted into the throat.



so you can see here the little protrusion in the throat with the choke flap open (no 12 on the diagram)... engine off operate the throttle with you hand or get a friend to press the accelerator