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Guys

 

I'm tearing my hair out here. Car has been running great and been upon Dartmoor 2-3 times 200-300 miles no problem since fitting the twin 40's. I go to China on a 10 days job and on return start the car - it ticks over fine. BUT as soon as I put load on the engine it miss-fires badly. Easy off the throttle and it runs fine. Ignition comes to mind so I change plugs/capacitor/points/rotor/cap/leads - EVERYTHING apart from the coil and the wire from it to the distributor. Problem still there. Colour tune says the Webers are OK on tick over - slightly lean when going from idler jets to mains - I had noticed an occasional spit back when it was running fine on light throttle at 1500 RPM - i.e. going from idlers to mains. A temperature sensor on the exhaust ports hinted that No 4 was running cold (miss-firing?) I blanked off the servo vacuum - still the miss-fire. So where do I go from here? I do have an electronic distributer - never fitted - I could go back the SU's - don't want to!

 

Save my hair!

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Before you get into the Webers I suggest taking your electrical work one step further - don't rule out a bad coil. Swap it out if you have a good spare - misfiring under load is one of the potential symptoms of a bad coil.

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Is your car +ve or -ve earth?

 

Is the coil LT wired correctly for the vehicle earth set up?

 

This is assuming you have a standard points set up not any electronic gadgetry.

 

On +ve earth cars the + terminal should be connected to the distributor.

On -ve earth cars the - terminal should be connected to the distributor.

 

This is a similar problem to one I encountered years ago on an E type, that, in the words of the owner, ''has had everything electrical renewed and it still will not rev, and fluffs under load''

 

Cheers

Peter W

 

PS if your coil has CB and SW next to the LT terminals you may find the + or - sign stamped on the coil somewhere. If not, you have precious little way of knowing which earth system it was designed for.

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Thanks Guys - I'll change the coil next.

 

The car is wired negative earth and since the car rn fine before the China trip I'm assuming the coil is wired correctly but I'll check it anyway.

Edited by bullstreetboy
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Fuel-wise: did you check the pump's capacity in combination with the amount of fuel there's in the tank? A full tank can mask the pump's incapcity to feed the Webers properly.

 

Furthermore: do you always use the same fuel station? I had some bad experiences with that: my car didn't run properly after a fuel-up at a 'different' station.

 

Menno

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Just a thought, but as I had a similar problem with my 45's try checking the little fuel filters just beyond the fuel pipe inlets. Webers really don't like any sort of fuel restriction, my filters were partially blocked with some waxy goo picked up from a continental supermarket petrol shop, no name no pack drill on the pump, but I was desperate and running on fumes at the time.

cheers Rob

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I'm with Randall and Peter W.

 

This sort of misfire is normally electrical, so as you have

replaced everything else but the coil, replace that first,

then see where you are with other possibilities.

 

My newish coil gave up the ghost in the middle of parking

the car after a gentle run. It happens with modern coils.

 

AlanR

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Thanks for all the feedback Guys

 

I think a coil (Accuspark - I wasn't impressed with their leads) change is the next step. Then tappets. The fuel is the same its always had - even when on SU's. Premium unleaded from Tesco's ( at 20p a litre off where else would I go?) with the lead replacement sack in the tank.

 

I did remove a Venturi some time ago to check it's size. I'm pretty sure it's in place correctly and definitely all screws are there. As I said a colour tune indicated all cylinders firing light blue. A second heat check on the exhaust ports with a laser heat sensor gave me cylinders 1 and 4 running colder which hints to me that is not weber related.

 

When I do eventually find the cause I'll post what I found

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I had a similar problem for a little while and eventually traced it to the vacuum advance pipe dropping out of the bottom of the carb. Ok, this was SU's and I'm guessing that on Webers you don't have one (?), but a similar air leak could cause misfire when moving under load when all looks normal at rest. (the Venturi securing screw would be a good one, but if you've checked all of those, try anything similar).

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Ashley is right - I had similar problems and couldn't find an answer. Took it to Peter Baldwin for rolling road diagnostics/setup and quickly discovered a plastic fuel filter collapsing under load. You would never find that in stationary running. The rest of the system was fine but he made a few tweaks and transformed the car. A couple of hours and a rolling road in the hands of a man that really knows his job can achieve more than an enthusiastic owner can in weeks! [And probably cheaper than ad hoc replacement of items one by one].

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Guys

 

Thanks for all the suggestions and support.

 

latest situation is:-

 

The coil is wired correctly for negative earth battery. I.E. + goes to ignition switch and - goes to earth via the points. I did change the coil - same miss-fire. So apart from changing to the electronic distributor I've changes everything that may be the cause ignition wise. So back to the beginning - just about to re-do the tappet - wind stopped the process today(not mine but the weather!). Compressions are not the best but OK and in the green when cold. If tappets don't solve it then in my opinion it points to the Webers/airleak. So off with those again if necessary to apply vac grease or silicon to the "O" rings. I did have a look at the Weber internal fuel filters and although dis-coloured they look OK. I really don't want to go back to the SU's as I never did solve the Kangaroo effect on light load with those.

 

BTW - I don't have any timing marks on the crank pulley. Webers , I believe , need a little more that standards and no vac advance.

 

Ratt Racing is just down the road with a rolling road so perhaps Mike will agree to have a look at it again if I fail to solve this frustrating problem.

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Indeed, mine is lousy under any load when not warmed up, which I guess is normal; but after a decent hot run it gets wheeeeezy (also, the gearbox get rough changing down at the end of a run, is that thin oil?)

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I just came across a similar thead on the Dutch TR forum (yes we have that here too!).

 

Making a long story short: same symptoms, etc. The owner tested the output of the pump (liters/hr) and found out that the pump didn't have enough capacity to fill the carbs properly.

 

Worth checking: jerry can & stopwatch will do the trick, I think.

 

Menno

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Having been through everything else, there's a good chance its the Webers and fuel delivery. Could be **** somewhere which limits flow under load only.

 

Had a similar problem with my 3A which has 45s together with an electric fuel pump and fuel pressure regulator to ensure appropriate fuel delivery. Agree you will need to get it warmed up on a rolling road but with someone who undestands Webers (many claim to, fewer actually do). My problem was solved by immediately changing to different idle jets and spark plugs - instant transformation which was fine tuned after 40 minutes on the rolling road.

 

Hope you are as fortunate.

 

Miles

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Guys

 

Solved at last!! Tappet adjusted yet again and it made no difference. Brand new set of plugs - no difference. So it had to be the Webers/fuel delivery.

 

It was a lose Venturi on cylinder No 2 - the one I had taken out to get the size before my trip to China. I must have replaced it and just started the engine without taking the car out for a run. Stupid mistake! It's running OK now if a little rich on the mains - the plugs are black. So if Mike at Ratt Racing can have it in his rolling road to get the jetting right I can then leave things well lone for a while.

 

Thanks for all the support Guys

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