Cobra carb help please

[Iancider]

We have an upcoming new member - Duncan and the proud owner of a brand new shiny AK Cobra.

He will be joining he club in the next few days but until he is registered I would like to help him out.  Being brand new it has a few teething problems.  It seems to me that that the engine has only been crudely set-up and it is running rough.  It is I believe, a 5.7 L Chevvy small block with alloy heads, Edelbrock 4 port carb. 

I found that there was an air-leak on the servo vacuum pipe and that caused the revs to rise every time he braked - now cured with a jubilee clip.  Tickover is rough and the engine is hunting.  Taking the air-filter off and it is very apparent that fuel is dripping into one of the primary chokes.  It looks like the acceleration jet is dribbling all of the time and I suspect the float needle is not shutting off properly. The engine pinks on a modest throttle opening on lower revs.  I have little specific knowledge on these - can anybody help and is there a specialist that could set it up properly nearby?

Regards


Ian

[Quarrycars]

You mentioned the poor idle, take a look at the idle air screw it is likely that the throttle plates are so far open that the carb is trying to run on the main circuits.  Check the basics, compression, timing, intake air leaks etc. before messing with the carb.  What distributor does it have?

[Iancider]

What distributor does it have?

I dont know the make but it is very large cap and has the vacuum advance underneath rather like Lucas dizzies.  The exhausts are also looking pretty black but I guess that is not abnormal for such a big engine.  I think I have cured all of the air-leaks - there was a jubilee clip missing in the servo pipe and on the vacuum advance.  the leak was causing the revs to rise every time he braked.

Regards
Ian

[Cob68]

Hi, If you go on u-tube there is a set of videos that shows you how to strip the Elderbrock carb, very easy to follow. Worth checking carb is clean and floats working properly even if just to eliminate this, take a look at the video's.

[Iancider]

Thanks

[Camber Dave]

Sounds like a float level problem.
Too high and the Vacuum will draw excess fuel.
Works just like a cistern on the Bog. Water to high and it dribbles into the bowl or overflow.

When we fitted Holley's to Rovers they had a window in the body to view fuel level. And with some carbs the needle valve could be adjusted from outside the body.

[damouk]

Random question but does he have appropriate fuel pressure for a carb setup?

[Quarrycars]

I dont know the make but it is very large cap and has the vacuum advance underneath rather like Lucas dizzies.

Sounds like the regular GM HEI distributor, is the coil in the cap?  Try disconnecting the vac advance, it may be causing the light throttle pinging.  Also check the mechanical advance, if a spring is off that may be over advancing at low RPMs.

['Mendip Wurzel']

Changing the subject a bit, but could I just ask a question about the Edelbrock carb

I have an RV8 with Edelbrock.... if I brake hard into a bend and then accelerate hard there is a engine lull, and then the engine jurks forward almost as if to much fuel is getting chucked into the engine. The carb is fairly new so everything should be good internally with hopefully floats at the correct level etc.

I have tended to live with the problem as I don't usually drive fast enough to notice the problem.

Thanks     

[Iancider]

Wow a veritable torrent of good ideas - thank you all.

I took a look at the tube last night and it does seem to be an incredibly easy carb to work on.  Currently my guess is that because it is all brand new that there is a bit of crud under the float needle but clumsy assembly is sometimes responsible for bending the float up and setting the fuel level too high.  It is only on one choke so that look like a possible. 

The distributor cap is enormous so I think the coil is in there - good thought Quarry cars about the advance spring being off that too is a possible.

Damouk - Fuel pressure - yep that is possible too but I have not looked into that yet.  I suspect that is not the problem because it is only happening on one choke.

Mendip Wurzel.  I think I know this one.  Most likely is a vacuum leak on the servo - presuming you have one.  I have just fixed that on the cobra.  When the brake was applied the revs would rise then it would cough and splutter on gentle acceleration  - solution fit the missing jubilee clips.  Another possible is that the manifold vacuum spiggot is not capped-off.  It is the lower one of the two that is normally used for auto kick-down and if you are driving a manual then it should be capped off.  As you look at the front the carb you should see the advance take-off pipe on the left and the lower and right hand spiggot should be air-tight capped off.  Another possible is an air-leak at the base of the carb because the bottom gasket is leaking.  A good and quick test is to get some Bradex easy start which is highly volatile and spray that at the base of the carb while it is running and if the revs increase than the gasket is leaking where you just sprayed. 

Hopefully we will all know what it is in a few day - watch this space.

Regards
Ian

[Dixie]

Changing the subject a bit, but could I just ask a question about the Edelbrock carb

I have an RV8 with Edelbrock.... if I brake hard into a bend and then accelerate hard there is a engine lull, and then the engine jurks forward almost as if to much fuel is getting chucked into the engine. The carb is fairly new so everything should be good internally with hopefully floats at the correct level etc.

I have tended to live with the problem as I don't usually drive fast enough to notice the problem.

Thanks   

Kev check the accelerator pump.  STD set up from the factory is quite rich, you may also want to reduce a rod size.  Eddlebrock have a good web site, it shows the graph to use to work out what rod/jet combo to use.  Accelerator pump would normally be on the 1st hole.

Mark

[Iancider]

Update,

Worked on it for most of the day to day but failed.  Took the Carb top off - one of the floats was a little high, the float needles seems okay.  There was a little debris in the float chambers but all taken out.  All reassembled and initially looked promising but then the troublesome dribble in one of the primary ports came back.   It looked like it was coming out of the accelerator jet but without the throttle being pumped.  We did manage to prove that increasing the fuel pressure made it worse and that if we lowered pressure below 1.5 bar is would not run at all well.  It would seem that there is a path into the accelerator jet that bypasses the pump. The pump diaphragm seemed fine.

Has any body got a clue as to how that might bypass?  It is not overflowing out of the top.

On oddity:  I tried to swap the float needles from left to right and they seemed identical - just to prove if the dribble switched sides as a result BUT the float then sat one high and one low - like they were different lengths.  I could not see how that happened but it did.  Is that a clue?

I am beginning to think of something obscure like a porous casting or a blanking plug left out or similar.  any ideas?

Regards
Ian

['Mendip Wurzel']

Changing the subject a bit, but could I just ask a question about the Edelbrock carb

I have an RV8 with Edelbrock.... if I brake hard into a bend and then accelerate hard there is a engine lull, and then the engine jurks forward almost as if to much fuel is getting chucked into the engine. The carb is fairly new so everything should be good internally with hopefully floats at the correct level etc.

I have tended to live with the problem as I don't usually drive fast enough to notice the problem.

Thanks   

Kev check the accelerator pump.  STD set up from the factory is quite rich, you may also want to reduce a rod size.  Eddlebrock have a good web site, it shows the graph to use to work out what rod/jet combo to use.  Accelerator pump would normally be on the 1st hole.
Mark

Thanks Mark, excellent information
Kevin

[Quarrycars]

Update,

  We did manage to prove that increasing the fuel pressure made it worse and that if we lowered pressure below 1.5 bar is would not run at all well. 

1.5 bar?  Should be no more than 0.4 bar (3-6 PSI).  Check the throttle plates, they should be nearly closed, just the idle ports showing, if the plates are up past the transition ports it may be trying to idle on the main circuits.  If the engine has low manifold vacuum due to ignition/cam timing someone may have set the idle screw to far in.

[Sumov8]

Gents  If it is of use I have the Eldebroke DVD and booklet that explains how to set the carb up

[Hairy Santa]

fuel pressure way to high, looks like your running the wrong pump

[Iancider]

Gents,

Thanks again.  I am guessing the regulator is calibrated in Bat but it would not run at all below "1".  I couldn't check the timing because I do not have a timing light nor do I know where the timing plate is on that engine (80's Chevvy 5.6L V8 with replacement alloy heads).  Nothing wrong with the distributor as far as I could see and the spring is operative.  The firing order seems to be different to a UK car:

1, 5, 4, 8, 3, 7, 2, 6.  Is that correct?

Sumo 8 - I would appreciate the DVD.  Will you be going to the Riverside?  Or can I pick it up from you?

Regards
Ian

[Dixie]

Tony Searle will be able help.

But my SBC book states 18436572, which strangely is the same as a RV8?

Even all the other capacity's use the same order.

Mark

[Dixie]

Look at this link.
http://www.mre-books.com/chevy/engines/firing_order.html

At the bottom of the page.

[Dixie]

fuel pressure way to high, looks like your running the wrong pump

Pete is correct.

Edlebrock state 5.5 psi as Max pressure.  I use a Carter pump, with cheap and simple reg.

Eddie web site has all you need to know.

http://www.edelbrock.com/automotive/mc/carburetors/thunder.shtml


Mark

[Hairy Santa]

[Iancider]

Wow how different it all is.  I was numbering the pistons the wrong way so I will recheck the wiring now I understand the nomemclature. 

Regards
Ian

[Quarrycars]

If the engine history is unknown it's a good idea to get back to basics, find TDC on #1 cyl - or #6 if it is easier to get to.  Even if there are timing marks it is useful to check, different dampers, aftermarket timing covers, slipped dampers all can throw the setting off.
Check the cam timing, pull a valve cover and watch the valves on cylinder 1 or 6, the exhaust valve will be closing as the piston approaches TDC.  The intake valve should start to open just before TDC and the exhaust close just after TDC, if the intake opens too soon the cam timing is advanced, if it opens late the chain may have slipped or stretched, if there is a large overlap between the exhaust and intake valve opening it has a 'wild' cam, any of those conditions will affect the idle.
You can set the basic ignition timing by lining up the reluctor teeth in the distributor.
The carb always gets the blame, it may be the problem - but check everything else first.

[Iancider]

Thanks QuarryCars,

This is all very useful stuff.  I had suspected a false air-path due to a faulty casting and that seems it might have been right.  We swapped the carb to try to remove the problem and it went.  Tat is about 80% of the troubles now resolved.  It is now going to be booked into Intrpro for a full set-up on the rolling road.  I will let you know how we get on.

What a great club this is - so much good advice and so quickly - thanks all.

Regards
Ian