Petrol Mist from Carbs

[Gordon Craigs]

After fitting a fast road cam when I blip the throttle I get a fine petrol mist coming back out of the carburetor. Under hard acceleration there is a strong smell of petrol inside the car which disappears as soon as I back off on the throttle. Any thoughts?

[Peter Cresswell]

This might be because of the overlap on the cam, which is the amount of time both the inlet and exhaust valves are open. The hotter the cam the greater this is. Which cam have you fitted?

What you are getting (I think) is called the 'stand off effect', and the engine isn't sucking all the mixture in. Basically the revs are too low! This is why it clears as soon as you go full throttle. It's not too much of a problem unless something sets fire to the vapour. This can be the hot exhaust, or if the engine spits back. Don't fit sponge or paper type filters as these can become soaked in fuel and cause a serious fire.

Barrie (Whizzo) Williams almost lost his modified Datsun 240Z whilst driving it on the A43 with this very problem, and I used to occasionally drive my racing MG TC on the road to run it in. This had an MG Factory racing cam and also had a huge stand off effect when driving on the road. At night you could see a huge ball of flame occasionally when it ignited!

[Peter Cresswell]

Hi Gordon,
I forgot to add that it will be well worth while getting the car set up on a rolling road. This may well help minimise the problem you are experiencing, but it will also make sure you are getting the best performance from your new cam and engine - including mpg.

[Gordon Craigs]

Hi Peter. Thanks for your reply. It's a Kent 717 cam. Good idea about the rolling road.

[John Watson]

Hi Gordon, you have not stated where you are located. I can recommend Pitstop at Brize Norton in Oxfordshire, Ralph Saunder the owner really understand standard and modified B series engines, this is where we have our MGB Register Rolling road day. His number is 01993 850 654

[Gordon Craigs]

I'm in Northumberland so bit of a hike to Oxfordshire but thanks anyway. Might go down to Peter Burgess

[Peter Cresswell]

Hi Gordon,
You might find this useful to help locating a rolling road.
http://www.rollingroadregister.com

Put your postcode in the box on the right plus a few other details and see what comes up.
I have heard that Peter Burgess is very good though - and if you buy one of his engines you get free rolling road set-ups for the life of the engine.

[Ian Dixon]

I can recommend Motorscope Nortallerton, great guys and a lot of experience.

[Jim Haines]

I am absolutely amazed to learn about this 'problem'.
22 years ago when my car was rebuilt II ordered a complete rebuild of my engine, and specified a Piper high-lift cam. Coupled with the fact that the cylinder head was also skimmed for - probably - the 2nd time I effectively got a Stage 2 engine and thought I was going to sail off into the sunset with a seriously sporty MG.
Little did I know............ What I actually got was an engine that just wouldn't settle down at tickover, had serious running on problems at shutdown, generally ran hot and was nothing like what I had hoped to achieve.
I even corresponded with Don Hayter at the time, who told me that they had run-on and heat problems with their development models as soon as they fitted twin carburettors (aparently it did not happen on single carburettor installations of that engine at that time) and recommended that I ensured there were no sharp edges in the combustion areas following the head skimming as these could easily cause pre-ignition (run on).
In fact, I sold the head as a Stage 2 unit and bought an 'original' replacement which was carefully measured and 'dressed' before refitting. I also got shot of the camshaft and replaced it with a bog-standard unit, all of which got me closer to a 'normal' engine. Sadly, I now know that the new radiator I fitted at the time was actually partially blocked so I still had overheating issues until I finally bit the bullet and changed it, and now have something I can rely on (until something else goes wrong).
Whatever, being fitted with a Borg Warner automatic box the car is still seriously fast off the mark if I am feeling 'sporty'.
I can leave a lot of modern cars behind from traffic lights without even thinking about it, although they haul me back at around 40 mph without problems..... The torque at low speeds is very good - however, the trade off is that I am only doing 50 mph at 3,000 rpm..
Hope you sort out your problems - the high-lift cam was not a good experience for me.....