SU issue : MG Motorsports Forum : The MG Experience

SU issue #1
Tincture77 WILLIAM RANKER REDDING,CA, USA Sign in to contact 1970 MG MGB GT 1972 MG MGB "Abingdon Ava" 1973 IHC Travelall "The Tuna Boat" 2008 Maserati Quattroporte	Topic Creator (OP) Oct 24, 2014 07:20 PM Joined 14 years ago 130 Posts

I just set up my SU HIF 6 (44s) with spring loaded needles on a new race engine (FAB-TEK). It started up fine, and I adjusted balance, idle and AF mixture as usual. After it reaches full temp. and is shut off, it has trouble restarting. I have to push on the throttle a bit to get it started, and after it does start, it struggles to stay running (have to hold the throttle). Fully actuating the throttle at this point does nothing. I am running 113 octane leaded mixed with 91 unleaded 1:1. Static timing was set at 10 deg BTDC. Timing at 5000 rpm was 30 deg. Is it the needle profile, timing, vapor lock, or am I missing something obvious.

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Re: SU issue #2
fast-MG.com Dave Headley Cortez, 4 corners, Colorado, USA Sign in to contact	Oct 24, 2014 10:01 PM Top Contributor Joined 15 years ago 9,605 Posts

Bill ,do you have dashpot dampers and oil in them? You also may find it helpful to shorten the advance curve so you have at least 15* initial advance while maintaining 28-30 all in.

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Re: SU issue #3
dcharnet Donald Charnetski NA, NA, USA Sign in to contact	Oct 24, 2014 10:50 PM Joined 13 years ago 1,788 Posts

Bill:

These are classic "my Fab-Tek wants a Weber" symptoms. There is only one known cure.

Don

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Re: SU issue #4
dickmoritz Dick Moritz Philly 'burbs, PA, USA Sign in to contact	Oct 25, 2014 07:03 AM Top Contributor Joined 14 years ago 26,263 Posts

Bill,

You may find your float levels are set a bit too high. If that's the case, heat soak can cause the fuel to expand up through the jet, and dribble out into the carb float, causing a flooding and hard start condition. You might want to look carefully into the throats of the carbs on hot soak to see if you can see fuel being forced up and out of the jet. If so, you might try dropping the float level just a bit.

You probably already know this, or Dave may have suggested it, but try starting with jets down 0.060 - 0.070 down from the bridge (if your carbs still have bridges...). With fuel pump on and bells and pistons removed, you should be able to "puff" down into the jet and see fuel burble up and over the top of the jet.

I'm sure Dave can offer more detailed suggestions...

Dick

Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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Re: SU issue #5
Tincture77 WILLIAM RANKER REDDING,CA, USA Sign in to contact 1970 MG MGB GT 1972 MG MGB "Abingdon Ava" 1973 IHC Travelall "The Tuna Boat" 2008 Maserati Quattroporte	Topic Creator (OP) Oct 26, 2014 12:12 PM Joined 14 years ago 130 Posts

Thanks guys. I reset my float levels, advanced the timing to 15 degrees at idle and revamped the insulation on my heat shield. It now restarts after reaching full temp. I then took it out to break in the engine by going full throttle in 2nd or 3rd from 2000 to 6000 RPMs several times and coasting back down to 2000 again. However, it pulls great from 2000-4500 and then sputters for a second or two, then pulls great again up to 6000. I have my fuel pressure regulator set to 3 lbs and it remains steady. What do you think, too lean? Perhaps a thinner profile needle in the middle?

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Re: SU issue #6
fast-MG.com Dave Headley Cortez, 4 corners, Colorado, USA Sign in to contact	Oct 26, 2014 12:36 PM Top Contributor Joined 15 years ago 9,605 Posts

Bill, unless you recurved the dizzy you may be over advanced. Did you get all of the E85 out of the cell? Do you still have Santa Fe needles?

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Re: SU issue #7
Tincture77 WILLIAM RANKER REDDING,CA, USA Sign in to contact 1970 MG MGB GT 1972 MG MGB "Abingdon Ava" 1973 IHC Travelall "The Tuna Boat" 2008 Maserati Quattroporte	Topic Creator (OP) Oct 26, 2014 03:29 PM Joined 14 years ago 130 Posts

No ethanol in system, same needle though.

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Re: SU issue #8
Duncan M Duncan M (Disabled) Disabled Account, Antarctica Sign in to contact 1949 MG TC 1968 MG MGB GT	Oct 29, 2014 03:13 AM Joined 9 years ago 449 Posts

I doubt oil is run in dampers, (first reply) as this is a racing car, and racing cars always run rich. Any particular reason you run at 3 psi fuel pressure? Have you tried a bit higher? I have found in similar racers that your special racing spark plugs should pretty much always show some signs of fowling, or you are too lean; unless you really know that engine. Just leave 'em alone unless they are getting fowled over time. Right, a narrower needle would make richer, but so would shoving more fuel through with higher fuel pressure available. Richer needles would present a whole new set of problems to sort out. A sort of mid-range thing like you mention may not be as easy as just richer needles. Some people will tell you that you cannot run SU's at much higher than 3 PSI, and you need webers. Well, you can run SU's a good clip higher than 3. Or you can try richer needles at 3 psi. Just throwing some things at you. Let us know what works.

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Re: SU issue #9
fast-MG.com Dave Headley Cortez, 4 corners, Colorado, USA Sign in to contact	Oct 29, 2014 08:40 AM Top Contributor Joined 15 years ago 9,605 Posts

Duncan, FYI, bill's carbs were tuned to race at 5-7000' elevation so are likely too lean for his new west coast location. Dashpot dampers with oil can help low speed response especially if not enough advance.

Bill, mid range sputtering like you describe can be carb vibration related although I think less likely with HIF as opposed to HS. I don't remember if your carbs are rubber mounted. Did you by any chance go to solid engine mounts during the rebuild?

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Re: SU issue #10
Tincture77 WILLIAM RANKER REDDING,CA, USA Sign in to contact 1970 MG MGB GT 1972 MG MGB "Abingdon Ava" 1973 IHC Travelall "The Tuna Boat" 2008 Maserati Quattroporte	Topic Creator (OP) Oct 29, 2014 01:39 PM Joined 14 years ago 130 Posts

UPDATE: I took it to the dyno at ThunderHill in Willows, CA (500ft elevation). We found that it was too rich initially, and after several sessions we continued lean out the carbs. Also I took the suggestion to advance the timing to 15 deg BTDC at idle. After we exhausted carb rich/lean adjustments, I retarded the timing back to 10 deg BTDC @ idle. This adjustment added 2.5 HP and 1.5 ft lb of torque with no issues restarting. End result 120 hp at the wheels 107 ft lb torque. Only 1 hp less than my best corrected hp in Albuquerque. I guess letting a race engine sit unused for 2 years reduces the potency a little.

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Re: SU issue #11
Duncan M Duncan M (Disabled) Disabled Account, Antarctica Sign in to contact 1949 MG TC 1968 MG MGB GT	Oct 30, 2014 02:43 AM Joined 9 years ago 449 Posts

7,000 feet to 500 feet would be a relevant detail, and something that should result in over lean condition if not adjusted. I guess that's how they got so rich and had to be leaned out. No wonder he was running 3 psi fuel pressure. Guess I missed the altitude part that was never mentioned. Makes you dizzy, eh?

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Re: SU issue #12
ghnl Eric Russell Mebane, NC, USA Sign in to contact 1961 MG MGA "Calvin"	Oct 30, 2014 07:31 AM Top Contributor Joined 12 years ago 12,733 Posts

In reply to # 2837108 by Duncan M Some people will tell you that you cannot run SU's at much higher than 3 PSI, and you need webers. Well, you can run SU's a good clip higher than 3.

I admit to confusion. My understanding is that SU's have float bowls that provide the fuel the carb uses. The float bowls are vented to the atmosphere. It'd seem to me that running higher fuel pressures would have no effect on the fuel in the float bowls (because they are not under pressure from the fuel pump). But - it might overwhelm the needle valve that stops the flow of fuel into the float bowl.

Thus, inadequate fuel volume could be a problem, but increasing the fuel pressure wouldn't solve that issue.

Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100

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Re: SU issue #13
fast-MG.com Dave Headley Cortez, 4 corners, Colorado, USA Sign in to contact	Oct 30, 2014 09:03 AM Top Contributor Joined 15 years ago 9,605 Posts

Eric, an increase in fuel pressure can result in very slightly higher fuel level in the bowls. But(IMO) not enough to measurably richen mixture,

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Re: SU issue #14
Tincture77 WILLIAM RANKER REDDING,CA, USA Sign in to contact 1970 MG MGB GT 1972 MG MGB "Abingdon Ava" 1973 IHC Travelall "The Tuna Boat" 2008 Maserati Quattroporte	Topic Creator (OP) Oct 30, 2014 11:35 AM Joined 14 years ago 130 Posts

Just as an aside: I did have to lower the fuel pressure from 3lbs to a little over 2 lbs due to over flow in the front carb. This goes against theoretical air pressure differences. I had not touched the fuel pressure regulator since I raced at 5000 plus feet (with no carb overflow), where floats vented to atmosphere should have had less resistance (lower air pressure) than in Willows (higher air pressure).

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Re: SU issue #15
fast-MG.com Dave Headley Cortez, 4 corners, Colorado, USA Sign in to contact	Oct 30, 2014 01:09 PM Top Contributor Joined 15 years ago 9,605 Posts

Bill, that suggests it's time to replace needles and seats.

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