Crossflow carb/plug problems

[Duckpit]

In my Lotus I have a 1600 xflow with a 32 DFM Weber. I seem to have a problem with plugs fouling up, ie it starts failing to run on one cylinder.

The problem can be overcome by lifting the plug lead off the plug slightly and she then runs sweet.

It was initially with No1 cylinder, but yesterday I stripped carb down and raised float slightly and now it is No2 cylinder and No 1 is great.

I took some advice off Weber a while back and they told me to look for dripping petrol in the carb which is solved by lifting float. This has helped but I am confused by liffting the plug lead off slightly.

Yes the plug is wet when I take it out.

Thanks

[Jonathan Kay]

The problem can be overcome by lifting the plug lead off the plug slightly and she then runs sweet.

Is that so far that you're sure it's disconnected or might you be lifting it into better contact?

What happens if you withdraw that lead completely from the plug?

Jonathan

[elie boone]

Is the distributor cap clean inside ? it could be that the current is finding a less restricted way to earth.

 

[Duckpit]

Interesting.

Withdrawing plug lead completely means shes running on 3. Its when the plug lead is just off the plug top that she runs sweet ie .

if its the dizzie then why has it swapped  from no1 to 2.

Will lift dizze tomoz

Thanks

 

[Duckpit]

Interesting.

Withdrawing plug lead completely means shes running on 3. Its when the plug lead is just off the plug top that she runs sweet ie .

if its the dizzie then why has it swapped  from no1 to 2.

Will lift dizze tomoz

Thanks

 

[Jonathan Kay]

Withdrawing plug lead completely means shes running on 3. Its when the plug lead is just off the plug top that she runs sweet ie .

Not sure what you mean:

1. Rough when "fully fitted"?
2. Smooth when slightly lifted?
3. Typically lumpy when fully removed? Sounds the same as 1?

That implies bad contact from lead to plug. if that's the cause it might have swapped as you played with both 1 and 2 and the contact varied.

Jonathan

PS: Experts please: can you ever improve spark on other plugs by having a small gap on one contact? As with a trembler coil?

[Jonathan Kay]

Is the distributor cap clean inside ? it could be that the current is finding a less restricted way to earth.

Definitely worth checking.

if its the dizzie then why has it swapped  from no1 to 2.

If it's tracking in the distributor that could wander a bit, or it could be that + marginal spark + my idea.

Jonathan

[OliverSedlacek]

Lifting an HT lead slightly can in some cases give a 'better' spark. It's a little trick used by some professional garages that needs to be used with caution as a shock from a modern ignition system is seriously hazardous. You need to be clear whether you have an ignition problem which leaves the plugs wet when a plug fails to spark or whether you have a fueling problem where the fuel quenches the spark.

[Duckpit]

Yes you are perfectly correct, thats why the title is "carb/plug" but how can i determine which it is ?

[Colin Cooper]

Duckpit,

It may be worth checking the fuel pump. I had a similar problem some time ago. All seemed well, then a misfire & loss of power. On stopping to check leads, it gave the pump time to work up to pressure.

This was eventually found by James Whiting putting a pressure guage on the pump. Bloody annoying at the time.

Good luck,

Col

[Jonathan Kay]

... how can i determine which it is ?

Start with the cap, and could you do the three stage test in #6?

I have a feeling from previous discussions that in the absence of HT testing equipment it's quite common to replace the cap and leads and plugs quite early in the diagnostic process.

...

Thanks, Oliver.

Jonathan

[OliverSedlacek]

Replacing HT components if they are a few years old is rarely a waste of money, and even professionals use "diagnosis by substitution".

[Duckpit]

!!

IT has new dizzie cap, rotor, leads, plugs and coil also refurbed pump, the obvious things

Yesterday I increased fuel level in carb, because it had become difficult to start, and problem moved back to cylinder 1

[Piers300]

Have you recently fitted a new fuel tank?  Sealant can block the feed line. Using an air line from the engine end and blowing down the line can unblock it ,but you may still have crud in the tank. I had this problem in 1986.

Piers

[Duckpit]

Thanks Piers

I dont think it is an undersuppply fuel problem as the plug is very wet when I takeit out. I can also see the fuel pumping in on the carb when I pull the throttle. More likely is an oversupply problem ??

[7 wonders of the world]

Valve clearences ??

Compression test too

[Colin Cooper]

I would still suggest checking the fuel pump. There was no obvious sign of fuel shortage, and misfires as you describe. Of course once the engine was off load, all was well. It took James Whiting quite a time to find, and then only by putting a pressure gauge in the line.

Its also a cheep part to replace.

Col  

[Duckpit]

Good shout, will check compression & valve clearance. Recently I had  valve spring go and took off the head etc, but the problem was there before the spring went. Engine was rebuilt in about 96 but never used until after restoration last yer.

Definitely not a problem with lack of fuel, once plug is cleaned and dried she runs a treat and I can floor the car and keep he revs going for sometime with no change.

The problem occurs after start up when cold say after 3 starts, then clean and dry plug and she is fine.

The fact that it shifts between No1 and 2 depending on the level of fuel o=in the float chamber really is confusing though.

[Dave Rayner]

I think I would be tempted to throw on a couple of twin 40s and have done!!  

[Jonathan Kay]

if its the dizzie then why has it swapped  from no1 to 2.

Withdrawing plug lead completely means shes running on 3. Its when the plug lead is just off the plug top that she runs sweet ie .

Is this still reproducible?

Jonathan

[Duckpit]

No chance of 40's even though I have a set. Its a wholly original Lotus 7 and the value is in its originality, if I put 40's on it I would have to cut holes for the intake on the bonnet as well.

Yes the problem can be reproduced

[Jonathan Kay]

No-one has yet offered HT test gear in the register... any generous volunteers?

You've told us that that you have all new HT components. So with that fault I'd now be looking for a bad connection somewhere or a faulty new bit and I'd start with the contacts of the leads on the plugs (can you swap the leads?) for the former and a marginal coil for the latter.

Jonathan

[Duckpit]

OK I may have found the problem.

I have just tiled my garage and there is now visible spatters of sooty material by the tail pipe which suggests to me that it is running rich.

On looking through the records of the car I see that the previous owner rebuilt the engine, left it out of the car and never used it 1997.

I have the invoice for the rebuild of the engine and a BCF2 cam had been put in and I believe this to be different from the  standard 1600GT. And of course the carb jetting has not been changed.

 

Could it be that the inlet valves iare opening for longer and taking in too much fuel and the mixture is all wrong ?

 

Nick

[Duckpit]

Aren't Webcon a brilliant company, the technical advice is fantastic.

We agreed that it is running too rich. We think the problem is the main jets and I need to reduce size, also I have the wrong plugs for the car. I have been using B8ECS as per my 1700xflow in the supersprint when  the ones for a 1600GT are BP7ES.