Lotus Elan

Fuel dripping from airbox

PostPost by: lotusroads » Sun Jan 09, 2011 4:24 am

Hello,

My '69 +2's standard (not big valve) twin cam engine was recently rebuilt but running rich; black spark plugs, smoke at start up and when shifting gears, high fuel consumption, lots of surging in second gear.

The carbs are Weber 40DCOE 151's. I checked the jets and the mains were 115 and the idles 50F9. I changed to 110 mains and 45F8 idles based on the Lotus manual. All the problems listed above were solved. However, I now have fuel in the airbox (damned engineering conundrum).

While I was synch'ing and adjusting the mix on the carbs after installing the smaller jets, no excess fuel was observed at the trumpets with the airbox cover off.

After test driving at highway speeds (60 to 80mph), and noticing much smoother driveability, I inspected the engine bay and noticed the fuel below the airbox. I removed the airbox cover, and while revving the engine noticed fuel spitting from the engine breather tube next to #4 carb throat (nearest to firewall). There was some emulsified oil/fuel at the breather hole and on the airbox cover below the hole. #4 also appeared to have the wettest throat (also suggesting the excess fuel coming from the breather).

I'm grabbing at straws for what could be causing the fuel coming from the breather tube. My only thought at the moment is float height which wasn't checked following the re jetting.

Any other thoughts out there? Your input would be greatly appreciated because I don't want to be starting any fires in there.

Cheers,
Greg
lotusroads
First Gear
First Gear
 
Posts: 17
Joined: 16 Aug 2010

PostPost by: sparkey » Sun Jan 09, 2011 11:29 am

Although not much help to you, mines been like that for years and no amount of adjusting float heights etc seems to solve it. I've even swapped to different webers and it was the same. I use a remote cone foam air filter in the nose feeding into the original airbox and I thought it may just be that with an empy airbox the excess fuel is more noticeable. I console myself in the fact that in normal driving it seems to be sucked back into the carbs, and it's only at idle that it can build up - that and the fact that there's nothing that's likely to ignite it under the rear of the airbox !?!

Hopefully someone will come and give us both some inspiration for a fix.

Steve.
User avatar
sparkey
Second Gear
Second Gear
 
Posts: 68
Joined: 07 Jan 2006
Location: High Peak

PostPost by: sparkey » Sun Jan 09, 2011 12:33 pm

Well since my last post I decided as I'm doing a few jobs on the car today that I'd have another go at stopping the fuel in the airbox problem and surprisingly there's none there either at Idle or after a drive :? In fact the airbox is very clean and dry ! So my advice would be to ignore it for a long time and it will fix itself....... possibly.

Steve..
User avatar
sparkey
Second Gear
Second Gear
 
Posts: 68
Joined: 07 Jan 2006
Location: High Peak

PostPost by: terryp » Sun Jan 09, 2011 1:11 pm

Adjust the floats to the revised setting in the workshop manual and try to get hold of viton tipped needle valves.

Terry
terryp
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 1667
Joined: 29 Nov 2007

PostPost by: alaric » Sun Jan 09, 2011 2:28 pm

Hi. Also make sure that your thackary washers are not tightened up too much - should be about 1mm between the coils.

Sean.
alaric
Fourth Gear
Fourth Gear
 
Posts: 900
Joined: 07 Apr 2005
Location: South Dorset, UK

PostPost by: lotusroads » Sun Jan 09, 2011 6:49 pm

Thanks for the input. Every little bit of insight helps and is much appreciated.

What I can't get my arms around is how reducing the size of the jets is causing raw fuel in the breather tube to the airbox. I thought by using smaller jets less fuel would be going into the throat so less would be available elswhere. Hence, the plugs are now cleaner following the re-jetting (indicating complete fuel burn).

I checked the manual and there is no schematic showing the path of the breather tube through the head and into the airbox.

Does anyone have a description of this breather tube or know how fuel might get in there?
Greg
lotusroads
First Gear
First Gear
 
Posts: 17
Joined: 16 Aug 2010

PostPost by: elansprint » Sun Jan 09, 2011 7:57 pm

Greg i had this problem there was a pinhole in the brass float where they are soldered together so the float fill with petrol & held the needle valve permanently open. Solder it up everything has been fine since.
Ian
elansprint
Fourth Gear
Fourth Gear
 
Posts: 502
Joined: 12 Sep 2003

PostPost by: UAB807F » Mon Jan 10, 2011 7:55 am

Hi,
I had a similar problem on my elan, fuel leaking from the airbox, spasmodic uneven running and spitting back. Sadly I must admit that the problem was most likely caused by my own incompetence although on the plus side I have hopefully sorted it now.

I suspected flooding which I first noticed after fitting an electric fuel pump & new lines. The car didn't run as smoothly as I hoped and that started the downward spiral of tinkering, float heights, pressure regulator, etc. In the end I stripped the carbs again to check for a leaky float or sticking inlet valve and found that the rear carb had the valve seat slightly loose in the cover and thus not sealing properly. I can only think I've not tightened it up enough previously as I can't see it coming loose on it's own, so my fault.

On the plus side, new valves/gaskets, float heights set back to standard and it's a different engine. Hence anything to do with excess petrol & I'd do a basic carb strip/rebuild and ensure everything is set to the manual as a start point. If only I'd followed my own advice.......

Brian
User avatar
UAB807F
Fourth Gear
Fourth Gear
 
Posts: 645
Joined: 20 Dec 2010
Location: Lincolnshire

PostPost by: gjz30075 » Mon Jan 10, 2011 8:59 pm

The 151s have a pointed grub screw and locking nut to hold the chokes. Make sure the grub screw is snug against the choke and the locking jam nut is holding it tight.

Greg Z
Greg Z
45/0243K Sprint
36/5727 pre airflow coupe
User avatar
gjz30075
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 2975
Joined: 12 Sep 2003
Location: Roswell, Georgia, USA

PostPost by: GP Surf » Thu Jan 13, 2011 5:43 pm

I had this problem and it was due to the Thackery washers breaking up. They can be of very dubious quality. I switched to the rubber compression types and ditched the double coil thackery washers.

In the 4 years I've had them they've been brilliant and I'll never go back. Use new nylocs, don't overtighten and you're away.

http://www.rdent.com/pages/carbmount.html

Graham
GP Surf
New-tral
 
Posts: 4
Joined: 16 Dec 2010

PostPost by: rgh0 » Sat Jan 15, 2011 2:44 am

Excessive fuel in the air box is normally from leakage between the body of the carb and the venturi / trumpet assembly. If this assembly is not tight then the fuel flowing to the auxillary venturi from the main jet leaks back to the airbox. Depending on the model of Weber there is either a spring clip on the venturi body or a screw on the outside of the carb that holds the venturi in place to seal the passage from the main jet to the aux venturi outlet.

However you appear to describe fuel coming from the engine breather. This would only normally occur if excess fuel had got into the oil and was being boiled off. if the engine is running rich then fuel can wash past the cylinders and into the oil. If you only do short runs then the fuel can build up in the oil as the oil never gets hot enough to boil it all out through the breather. Given that your problem appears to have gone away a little time after fixing your jetting it sounds like it took some time for all the fuel to boil out of the oil and now that its gone the air box is dry again.

cheers
Rohan
User avatar
rgh0
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 7514
Joined: 22 Sep 2003
Location: Melbourne, Australia

PostPost by: lotusroads » Sun Jan 16, 2011 6:15 am

Rohan,
Good suggestions. :lol: I don't suspect the venturi's are loose, but will check them out. The fuel in the oil is a good observation that a quick oil change will eliminate (fresh oil and filter never hurts either).

I was adjusting the carbs today which has the motor running smoothly, unfortunately there is still fuel finding its way into the airbox, not much but its there.
My Regards,
Greg
lotusroads
First Gear
First Gear
 
Posts: 17
Joined: 16 Aug 2010

Total Online:

Users browsing this forum: No registered users and 6 guests