Rough Idle

[wilf]

Glynn - something just fell into place. You have a 383cu in stroker V8, but also a 600cfm carby. I have to say that your carby could well be too small for your engine - I would expect to see a 750cfm jobby on top of that lump.

Having said that, it might be, that in order to supply enough air at idle, your carb butterflies are open enough to start working on the imtermediate circuits. This is an "unusual" diagnosis, I really could not say more without seeing the thing running.

To see if that carb is actually throttling the engine, you need a vacuum guage, plug it into the "always on" port on the carb, or better still , a port on the inlet manifold itself (like the take off for teh brake servo) run it through a good length of plastic pipe into the cockpit, and have someone watch it whilst you floor the thing and run the revs up to your rev-limiter limit (5500+). Anything more than 1"wg at WOT and max rpm signifies a carb that is too small for the engine.

[neil still building it]

hi

i suppose you've checked the float height??could it be flooding slightly at tickover?? i'd be very suprised if the carb being'small' would affect anything at all even fitted to a big block the max air flow is the main problem surely??
if it was restrictive the tickover would just be slower.....only my thoughts....
NEIL

[mikey]

The other thimg is that you may be on a hiding to nothing trying to get the Edelbrock to deal with smoothing out a rough idle. They are great for mild tune but unless you have one of the later performance series I dont think the systems are easily tuneable.

Just out of interest if you have the problem that callum is mentioning due to low vacuum there is potentially another way to go without losing the benefits of initial advance. That is to look at annular discharge boosters instaed of straight leg. These compensate for lack of vacuum although they are large in size so you need to up the venturi size to compensate.

Reason I am saying this is I had similar issues which i am half way through sorting out. Unfortunately it meant the purchase of a Race demon Carb and A/F meter, the former being highly tunable including Idle feed resttrictors and air bleeds etc. Also in the AD form it comes with the Annular discahrge boosters and is sized 820 CFM which equates to around 750 if you had straight leg. The alternative is to use wires poked in the Air Bleeds and jets etc and then drill to get the best compromise. The othe thing if that your idle improved with additional air you may need to think about drilling the butterflies to let in air without screwing up transition.

If you do at any time decide to look at a carb that is more tuneable give me a bell as I will try to shortcut some of the frustrations and try to pass on the techniques of using wire etc to size jets and bleeds with minimal expense.

Mike

[Glyn_Turner]

Having spent some more time looking at this, I am not sure that the carb is the issue. The engine is kicking quite hard some of the time which makes me think it could be an electrical problem, as fuel supply problems tend to have less immediate effect. I will try removing the plug leads one at a time to see if this has any extra effect on the symptoms. What effect would you expect on the tickover if I remove a 'good' lead? The fact that the engine mounts are hitting the 'end stops' may be making the effect more noticeable.

I have to remove the engine mounts as they are not sitting right down on the chassis pads, and remove some of the metal to provide some clearance. Has anyone removed engine mounts in situ before, and if so how did you support the engine? I have a Milodon sump pan, if I spread the load with a decent piece of wood will it be strong enough to lift the engine sufficiently to remove the mounts one at a time?

Best regards

Glyn

[craigh]

are you also sure your firing order is correct,its not unknown for a plug lead to go amiss
removing a good plug lead will give you a small drop in idle speed/quality,although quite small on these brutes but still noticeable

[Glyn_Turner]

Craigh

I have checked the firing order very carefully, and the engine appears to be smooth when the revs get above 2000. Although this being my first experience of V8's I am not sure how smooth, smooth should be.

Answering other points about carb size etc. the cam is a fairly mild roller cam as I went for low end torque rather than high end BHP, and Brian at Repower assures me that the 600 cfm carb is sufficient for this engine. If it was too small I would have thought it would only be noticeable at high revs, and as I am still running it in I have not tried this yet.

When the twin cooling fans cut in, the tickover also drops by around 300 rpm is this what others have experienced?

Best regards

Glyn

[wilf]

Glyn - the drop in rpms when your fans come in is normal. All it is, is the drag from the alternator which suddenly needs to supply about 20amps more , which is about 1/3 of HP - enough to drag down the idle speed.

[mikey]

You still havent told us what the cam is. The duration etc can have a very significant effect on tickover etc. If it is extreme your initial timing may be 2 to 4 degrees below what it needs to be. Give us a chance to help and let us have the info on cam etc otherwise we are all guessing. With regards the carb - its not so much the size, but its set up to sort out a wilder cam. It is a lot about vacuum pressure at tickover. We also need to know what this is.
Mike
Mike

[Glyn_Turner]

This is the info I got back from Brian at repower on the Cam :

The roller cam upgrade on this engine is 210/215 duration at 0.050,
306/306 at 0.006 with valve lift .480 int & exh, 108 Centre Line, 90 Overlap

I will have to source a vacuum guage to check vacuum levels.

best regards

Glyn

[wilf]

You should have a smooth idle with that cam.