Spitting & Back-Firing

[PhilW]

I bought a 2b Hood about 1 1/2 years ago, one of the faults it had was spitting back through the carbs, mainly on starting but less when running, also back-firing through the exhaust, most noticable when on over-run downhill.

I have made changes but the problem is still there.

I had the carbs, 2 off twin choke 40 DCOE Weber, serviced & refitted them with new seals and gaskets. The ignition was at 8 degrees this is now reset to 14 degrees. ( as per Dave Andrews spec sheet, also carb settings in line with his spec sheet)

Engine is 2 litre Pinto about 1987, SOHC fitted with Kent fast road cam KCFR32, 'red' cylinder head, new exhaust manifold gaskets fitted when engine came out for other work. 4 into 1 exhaust pipe.

Any suggestions to a solution would be greatly appreciated.

Phil W.

[Guest 2b cruising]

Have you checked that the valve timing is correct to the type of cam you are using.

If this is ok, because you cannot shut the fuel completely off on over-run when using carbs, fuel will still get through to your exhaust.

If you have a hotspot in the manifold, exhaust ports, or exhaust the fuel keeps ignighting either in the same places when on over-run.

There is not to much you can do to stop this apart from a complete strip off with your head and check for any carbon spots, or metal sharp edges or casting lumps. Make sure you have a clear gas flow through all ports and no weld spots through the inside of your exhaust.

The most common cause is without doubt valve timing.

Best done for performance reasons on a rolling road with vernier type cam drive gear.

Other than that, you are causing no power loss or internal damage and most people like such a set up on their kit cars. It's a very common occurrence on track days with carb type fuel systems.

[Bob Tucker]

Be careful with 14 degrees advance unless you have also limited the upper advance limit

[theduck]

Have you checked your dizzy is advancing properly?

[Guest janis]

Mine does it all the time i brake with gears just to hear it..sounds racey..love it..

[Guest janis]

This post while ago has cover everything..read throu and u might find some tips..

http://www.rhocar.org/index.php?showtopic=42787

[Peter Bell]

Spitting back through the carbs can be a sign that the idle mixture is a bit lean.

[Guest Pauljordan]

I had a small crack in my manifold on my pinto, that gave the same issues. Only found it when it finally broke. It was worse when cold as expansion caused the crack to close up.

Just a thought may be same issue........

 

PJ

[Guest 2b cruising]

Spitting back through the carbs can be a sign that the idle mixture is a bit lean.

I wrote this in a post around 12 months or more ago.

Open exhausts of the style we use make it hard to avoid such symptoms, but you are correct about lean mixture.

 

The response I got was bullying by two other members that thought they knew all about it.

Edited January 12, 2016 by 2b cruising

[Bob Tucker]

Guys you are dead right....spitting back thru'the carbs is often a weak idle/transition mixture.

 

So....Yah boo sucks to the bullies.........unless they are bigger than me.....

[Guest Jasony939]

Hi, I had the same spitting back through the carbs and it was the valve clearances. Just when starting or blipping the throttle from idle.

[Guest mower man]

Weak mixture and or air leaks on exhaust system [ oops I've dug that old chestnut up again] are the main cause progession jets /tubes swaps can help mick

[Snapperpaul]

The long 4 branch manifold and straight through exhaust will bang a bit especially when in my situation mixture was rich.

Carb spitting back can be a whole host of things so check all of them.

Cam timing, valve/rocker gaps, ignition timing (doesn't help if dizzy vac is disconected), fuel lean/rich.

No one is wrong, no one is right.

The answer is in there somewhere but with all engine diagnostics on a forum without all the info it's just a guess, educated or not.

I always looked at the advice and though "haven't tried that, didn't think of that, can't measure that.

By the end of this process I had eliminated most of the doubt leaving only a few things to check.

[PhilW]

Thanks for the advice, I will go through them and check out each one, will start with the mixture and inlet manifold.

I believe the cam timing was checked.

Phil W