What is a good idle speed for my RV8

irony · Post by **irony** » Mon Apr 13, 2015 3:58 pm

I am currently building a kit car with a 3.9L RV8 with a Weber 500 and Megajolt, standard cam. The Carb came of a 3.5L which was known to be good so I am hoping the jets etc will be okay. The idle is currently quite high at 1200rpm. I have been unable to get it much lower without the engine really beginning to struggle.

What sort of idle speed should I be aiming for?

I am making a bit of a list of stuff I want to check.

1. Vac leaks with some easy start
2. PCV working correctly
3. Accelerator mechanical stop now allowing the idle to go lower
4. Anything else anyone wants to suggest.

Sorry for the newbie questions but this is my first engine build and the search function didn't turn up much.

Cheers

kiwicar · Post by **kiwicar** » Mon Apr 13, 2015 4:48 pm

Hi
I like an engine to idle between 500 and 700 revs, any more and it is difficult to drive in traffic.
Are you sure you are idling on the idle circuit and not on the transition circuit just if it is insensitive to the idle mixture screws and does not want to idle below 1200 revs then it sounds to me like you are on the transition circuit. If the carb has been sat on a shelf of just not used for more than about 6 months then I would get a rebuild kit, strip it, clean it and blow out all the little passages channels and jets in the thing (tee pee dental brush pick things can be a big help here). These carbs are made of mega-cheap zinc/ally die-casting metal and it will corrode given the slightest excuse. This is not helped by all the little drillings and passages and the random use of dissimilar metals, i.e. brass jets and steel needles/adjuster screws.
There is a very good book on 4 barrel carbs by David Vizard http://www.amazon.co.uk/David-Vizards-Holley-Carburetors-How-/dp/1934709654/ref=sr_1_1?s=books&ie=UTF8&qid=1428943630&sr=1-1&keywords=david+vizard+carburetors well worth the money.
Best regards
Mike

irony · Post by **irony** » Tue Apr 14, 2015 9:20 am

I foolishly left the car unstarted for three months over winter and it wouldn't start. I sent the carb away for cleaning and it has now come back so it should be okay.

I shall look into buying that book.

irony · Post by **irony** » Fri Apr 17, 2015 8:45 am

I have recently replaced the accelerator return spring on my carb to a dual spring. I noticed that one of the vacuum ports on the carb (the manifold vacuum port, the one on the right) has lost its little plug. Also there is a open port on my manifold as well. I am going to block these off.

Could the open ports be causing my high idle? I'll start the engine tomorrow and see if it makes a difference. Can't start the engine easily in the week as the nipper is asleep by the time I get in from work.

kiwicar · Post by **kiwicar** » Fri Apr 17, 2015 9:32 am

Hi
Yes open ports will make it idle high and prevent you getting a stable mixture from the idle circuit. I would also defiantly check that you are on the idle circuit at idle.
best regards
Mike

irony · Post by **irony** » Tue Apr 21, 2015 8:41 am

Over the weekend I capped off Manifold Vacuum inlet/outlet on the carb. The idle went down immediately and the idle screws are doing their job now. Problem sorted.