John et all...
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From: John M. Trindle[SMTP:jtrindle@tsquare.com]
Sent: Saturday, March 02, 1996 12:29 AM
To: MG Mailing List
Subject: Re: Freeway Running with Choke On
>On Fri, 1 Mar 1996, David M. Hall wrote:
>
>> I believe that I have my carbs correctly adjusted for the fasted lean
>> idle. I have a fairly free flow tuned exhaust system, Dual SU Carbs,
>> some type of hot (PO) cam, Crane Electronic Ignition, Vaccume advance
>working
>> correctly, static timing about 20 BTDC.
>
>STATIC timing 20 BTDC? Zowie! That's a little much, isn't it?
>Over-advance has given me a lean-mixture indication in the past.
>
- I came up with the by setting to 10BTDC, and then slowly advancing until
backfire went
away when shifting, and also while no pinging while going up a hill. I kept
advancing, and
kept getting more power, and no pinging. I stopped at 20 because, yes it
seemed to be
getting a bit excessive! Perhaps I shoulde back it off a bit! Also, I think
that I noticed that
the idle went up a bit. I can't seem to get it below 1000 RPM now.
>> b. Are my needles wrong?
>Almost certainly. A cam change and an exhaust change will certainly
>change the optimum needle profile.
>
>> c. Is it OK to run with the choke on?
>
>Yes and no. Yes, if you remember to take it off when you don't need it.
>Too much gas in the mixture causes excessive ring wear, pollution, plug
>fouling.
>
>> d. What's the best way to get the performance increase I noticed while
>running
>> with the choke on?
>>
>
>The absolute, positive, best way is to get new needles profiled on a
>rolling road dyno.
Sounds like the thing to do. Can any shop which has a dyno tell me what kinda
needles to get? My guess is that I need to find a shop which is familiar with
working
on MGs or other SU Equipped cars.
>
>The problem is that each combination of bolt-on modifications and exact
>engine is going to result in a slightly different optimum. In the case
>of a header system, K&N filters, and a long duration cam the flow pattern
>is going to be SIGNIFICANTLY different than stock.
>
>You can play the game of swap needle (consulting the profile charts or
>the Haystack program to move in the right direction) but it is not cheap
>and quite frustrating on HIFs. Only a few profiles are available in
>biased needles due to emissions considerations.
>
>Right now I have some fixed needle in an adapter in the rear carb, and an
>AAE biased needle in the front. Yes, it is confused, but I ran out of
>stock and patience at this point.
>
>I'd check that advance with a light against the book value before going too
>far. I played the "over-advance it, increase richness to compensate"
>game for a while when my rings were in the process of segmenting
>themselves. It seemed to help acceleration around town but didn't affect
>top-end (and it sounded AWFUL). When I rebuilt the engine the temptation
>went away as I had enough power to get out of my own way.
>
I think that I'll back the advance off a bit, and then check around town
(Portland OR,
Vancouver, WA) to see if anyone can do the rolling dyno check. (Any one know
of a good shop in this area?)
Thanks for the input! :)
Dave Hall
dhall@vcd.hp.com
72 MGB/GT
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