pair twin 45DCOE's: ABF race engine: unusual starting problem??

my mk1 has developed an unusual starting problem. it only wants to 'catch' when you crank if from a throttle setting of 1/2 to 2/3rd throttle and then shut off the throttle.
Its like the resulting vacuum is creating a slug of burnable mixture into the combustion chamber.
Made a lot of changes to the whole spec recently, so can't easily backtrack.
Oh, and today beaten by a 205 pug at Croft British Champ sprint, still 2nd in 1B is ok for me and the relic of a mk1 I run
Weber experts please
Jon

 

Maybe one for Jason.

 

yeah, probably. I have been reading the weber manual today and two things are possible. One, the FPR is set at too low a pressure. Or less likely I have the idle circuit set too lean. It idles and progresses onto main jet great and the dash AFR meter suggests its back on, but in the past I have always had richer idle settings.
Both are easy to prove/disprove. A twiddle or two on a screwdriver.
Will let you know
Jon

 

I would say the fuel level is too low. The vacuum created at cranking should be enough to draw fuel trough to start the engine. When you move the pedal to your starting position the accelerator pump jet adds the fuel needed to start the engine , then the running engine creates sufficient vacuum to keep it running.You must be running lean or have tuned the rest of the circuit to compensate

 

Think I agree. I have set the float heights to where I usually do, but if the FPR is set too low then I think this will create a lower than normal level at starting
Jon

 

On a side note I have always had to Prime my carburetors before starting otherwise the fuel level in the float bowls is always too low and makes the engine quite difficult to start This is why I have a button fitted across the fuel pump relay so I can manually turn on the pump for a few seconds to fill the float bowls .Why the fuel level drops in the float bowls whilst the car is standing for a few days I'm not sure but the pump has to run for about 5 seconds before I hear the pump strain a bit against the pressure.

 

makes sense, in the olden days bike carbs had a 'tickler' to raise the float fuel level for cold starting. ask your great great great granddad....
Jon

 

lots of good info in here;
http://www.apexspeed.com/forums/showthread.php?t=64306
Jon

 

oversteer, never fast. slicks in the rain, unlikely to be a good choice..
http://www.paddock42.com/wp-content/uploads/2016/03/IMG_4716.jpg

 

Have you checked the throttle pumps?

Mine (Dellorto) rattled lose after a lot of track use and the nuts came off the plungers, drove fine but was hard to start.

 

Pump jets are captive in 45's. Unloikely to be the problem, unless its something to do with the fuel supply to the pump jet.
Haven't stripped the carbs yet, or even attempted a repair.
I was wondering why it wouldn't fire after a couple of pumps of the throttle, so maybe its not squirting. Need the filters off and a 'looksee' with a mirror. If the pump jet circuit isn't working at all that may account for the symptoms. Thanks. The sprint was nearly all flat out full throttle, so I may not have missed the accelerator pumps action.
Jon

 

Just seen this...All sorted Jon?

If not, and your happy with the float heights then set the fuel pressure to 3.5psi...3 is a minimum for a comp car...4 ok tops...3.5 is ideal and enough. Also check the pump operation as I think your going to do anyway...40 or 45 pumps should work if not already fitted. Idles for a tuned 2ltr 16v should be around 55F8 as a starting point with the idle screws 2 full turns out then fiddle from there.

 

Thanks Jason.
Not fiddled yet, other projects ongoing.
For years and years I never used a pressure reg.
Rightly or wrongly, it seemed to work fine.
Probably meant I was running a higher fuel level for a given float setting, but hey hoh.
Now I'm running a reg. and I'm having start pribs.
Will report back, for the greater good, hopefully.
Still need 2 more atmospheres compression to save me getting beaten by pugs!
Jon

 

Extra 3/4 turn on the fuel reg to up the pressure hasn't sorted this. Idles out an extra turn didn't help. 38 to 39mm of fuel in centre of chamber. Two bone dry plugs and two wet?
Ongoing
Jon

 

Is it simply a couple of blocked pump jets

 

New plugs. Never have I had such a difference with just a set of new plugs. Live and learn
Jon

 

I had an issue with a lotus twin cam recently, it would not start It would cough but nothing else, put a set of plugs in and off it went, never had another problem

 

strange, how in your head you review every modification you have made in the last year looking form fundamental issues,
doubting the decisions you have made by changing stuff, to find it is something basic/obvious.
out next weekend racing on grass (surface, not the weed)
Jon

 

I think we all get to involved in what we've upgraded etc and think that's the issue rather than the basics sometimes

 

Still not fixed fully. Those new plugs only cured it temporarily. Carbs off and completely stripped.
Pump jet circuit working fine
start jets 60F5 (same ones that's been in forever, never change or even think about changing these.....)
nothing obviously wrong
200 float valve
50 spillback
thinking now its not a carb issue and maybe adding that ballast resistor into the lumenition spark system is weakening my spark.
ran without one for donkeys years, so its coming out
and
will probably take the FPR out as well, never needed it for 20+ years, so the paranoia about keeping between 3 to 4 psi to the carbs may be a myth
Jon