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MG MGB Technical - SU Carbs Lean at Speed

Good Afternoon All,

I have HIF-4's on my 74 1/2 "B". I'm getting ready for a several hundred mile run, and decided to check things out. Set the timing (18 btdc @1000 rpm, max 38 btdc @ 3200, no vacuum advance) per my engine builder, and decided to check the plug color after running about 65 mph. Got up to speed for about 5 miles, turned off key & coasted to a stop. All 4 plugs are almost white, just a slight grey tinge, looking very lean. At idle they are a nice light chocolate.

I'm runnung AAA needles, switched from ABD after installing K&N pancakes. The car runs great. No overheating or pinging. Oil in dashpots is fine, I checked that the spring loaded needles are installed with proper bias. Also checked for vacuum leaks and could fine none. Checking with the lifting pins I get a slight speedup for just a second then back down, just as the book says.

So, why could it be running lean at speed? BTW, 65 mph is just over 3000 rpm. Any chance that the fuel is causing it? I'm running Conoco 93 Octane...no ethanol.
Doug Keene

What?...did I stump everyone?
Doug Keene

The mixture curve is rich at the bottom,it kinda leans out from off idle thru mid range & then starts fattening up to the top.
Lean to me is an engine that spits & sputters back thru the carbs during normal driving & during HARD driving leaves aluminum on plugs.
It sounds like your mixture is in the ballpark to me.

I don't like to run as much advance as your builder recommends(but he built it, so what do I know!!!) 28 deg. total advance is safe for all driving conditions year around. 38 deg. total advance in moderate temps is OK, but in traffic on a 110 degree day your gonna get you some Pinging.

I drive my 67 GT everyday on regular gas 87 RON & it doesn't seem to mind, as a precaution during the heat of the summer I'll run 93 RON. I don't notice any gain in performance, but it doesn't PING.

Are those K&Ns, or those Bob Filters that your using????
Dauntless

Dauntless,

The filters are actual K&N pancakes. Not sure why you ask, but my 74 1/2 does not have a brake servo, so there is no interference problem. Re; the advance,I know it sounds like a lot,but even running 91 octane I don't get any ping.
However, since I posted the message I have found a couple of web sites that discuss plug color , and one of them did list too much advance as a reason for a plug burning white!
I recurved the "Eurospec" distributor I got from Brit-Tek because it's advance curve was so slow, so I may have to do a little more work on it.

Cheers, Doug
Doug Keene

Regarding the "Eurospec" distibutor from Brit-Tek. I sent mine back to them because it was not the right spec and did not have the right advance and weights. This, from info from the fellow who ported my head and modified my carbs. The dizzy they are selling might be right for certain applications, but not mine. I guess, know your application and know what you are buying. Hope that helped someone?
Brian Johnson

If your car pulls when accelerating from 65 and doesn't flat-spot I would say it is just fine.
Paul Hunt

Brian,

I was told that the "Eurospec" (45D 41610)is/was the only new Lucas 45D sold these days. I say was because Bob Ford at Brit-tek had told me that it would no longer be manufactured. The stock advance curve was for the European 18V846/847 HC engines per Paul Hunt's web site, and as I said it's very conservative. I modified mine using springs for a GM HEI distributor, and it's running great. I was just a little concerned about the plug color.

Paul,

Thanks for your input. It does pull well at all speeds, so with your reassurance I feel more comfortable that all is OK.

Cheers, Doug
Doug Keene

Doug, I was told to purchase a 41427 with #10 weights to match the performance of my head and carbs. I beleive Bob had to rummage through to find one. I am by far no expert at the special tuning of these engines, so I rely on others advice. -Brian.
Brian Johnson

There was a very good thread on 28 March, 2003 titled "HIF4 Needles." I won't reproduce the whole thing here, but will copy in some highlights:

"To anyone out there running HIF4 carbs, what needles are you running and what is the state of tune of your engine? I have a relatively mild 18V with a 270 degree cam, 1:55 roller rockers, bored 30 over with 9.75:1 CR. I am currantly running ABD needles in a set of Burgess modified HIF4s... it seems lean on the top end..."

chris, heswall england
You probably won't do much better than either AAA or AAM. The sequence of needles that most people use, starting with the weakest, is ABD AAU AAA AAM AAB Rolling road testing I did with a spec fairly similar to yours showed AAM to give the best results, with AAA not far behind , but AAB being far too rich and being appreciably less powerful from about 3500 rpm onwards Hope this helps Christopher Storey"

As to the Eurospec distributor, I plan to start a new thread.


Matt Kulka

Hi,

A few years back I had the same symptoms with my 73 B not much power at speed, but idled well. I have 2 su's and a cam that is a bit hotter - not much. As it turned out the actual problem was that the banjo bolts ( the fittings at the fuel pump) were not lined up with the interior of the fuel line hose. In other words the outlet from the fuel pump was a bit(about half)off center relative to the fuel line hose. What was happening was that with the restricted fuel flow, the pump could not fill up the fuel bowls fast enough to maintain fuel at the correct level in the bowls at higher engine speeds, causing the thing to leanout as the level in the fuel bowls dropped. At idle - no problem. So I loosened the banjo bolts. Made an index mark across the top of the bolt to indicate the proper orientation of the holes inside the bolt, and then lined up the fuel line hose so that the mark across the top of the banjo bolt pointed to the center of the fuel hose. Problem solved. Full fuel volume and no more flat spots or leaning out. Soon after I replaced the banjo fittings with modern brass 90 degree elbow fittings, rubber fuel line and hose clamps. FWIW Alan
Alan

I experienced lean running symptons on my 73B after installing the K&N "Bob" filters - i.e. the engine was not pulling well. Changed from ABD to AAA needles in the HIF carbs after looking at the WinSu program and the problem was solved. These two needle sizes have the same initial diameter and hence result in essentially the same jet position when adjusting the carbs at idle per the classic process.

Barry
73B
B.J. Quartermaine

The fuel mixture is set weak at part load (when you drive on a level) for the sake of fuel economy. When you need maximum power (foot down), it is set rich to give you the boost.
With leaded gasoline, it was easy to read the mixture strength from the color of the spark plugs with the help of the lead salts taking various colors. With unleaded gasoline, the spark plug color cannot give you any reliable information. The Colortune tune plugs can help you for few $. For $$$$$$, an exhaust gaz analyser is what you need.
Regards,
Jean-Marc
JM Thély

This thread was discussed between 19/10/2003 and 21/10/2003

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