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MG MGB Technical - where are the rpms

My 1800 was rebuilt by an experienced shop. It's a GA with std bore, std cam, diy head w/3angle grind, bronze guides, H4 carbs and only GS needles. Currently have a DM2 dist with pertronix, good wires, and I've tried the timing all over the place. We get 92 and sometimes 93 octane premium here.

Engine now has 1500 miles on it. Broke it in properly and at 1,000 miles I began to take it to 4500 - 5000rpms. At first it didn't want to go over 4500 but now it's getting up to 5000 with some power.

1. shouldn't I be getting peaks up to 6,000 rpms?

2. I can't get rid of pinging even after retarding to 7*btdc@600rpm and disconnecting the vac adv.(carb vac port). Could it be rocker or lifter clatter when under 1/2 open throttle?

Thanks, Fred


Fred Horstmeyer

Fred-
1) No, 6,000 RPM is the readline, not the power peak.

2) The lash in the valvetrain is in no way connected to throttle opening. I suspect that you've either got an inappropriate distributor on the engine or the advance springs have gone weak with age, allowing too much at too early an RPM. All MGB engines used either the Lucas 25D4 or the Lucas 45D4 distributors, never the DM2 variant. If you have a high compression 18GA engine then you should have a Lucas 25D4 model 40897 distributor. If you have a low compression 18GA engine then you should have a Lucas 25D4 model 40916 distributor.

Centrifugal advance specifications for the 40897 distributor are: 4 @ 600 rpm, 6 @ 700 rpm, 9 @ 900 rpm, 15 @ 1600 rpm, 20 @ 2200 rpm.

Centrifugal advance specifications for the 40916 distributor are: 6 @ 600 rpm, 8 @ 800 rpm, 9 @ 1000 rpm, 18 @ 3000 rpm, 24 @ 4400 rpm.

Static timing for both high and low compression GA engines should be 10 BTDC, with Dynamic timing being 14 BTDC and 12 BTDC for the high and low compression engines, respectively.
Steve S.

Thank you Steve! I've been waiting to hear from one of you experienced fellows.

1. Sorry, wrong term I guess. I meant to say peak rpm. I really can JUST reach 5K then it feels like it's going to explode.

2. I've got a 24D in the boot with new points but haven't used it because the adv springs are weak. I'll put together a curve chart, but I did have it at the following curve for quite a while with similar problems: dynamic....
600 - 14
900 - 17
1600 - 25
2200 - 28

3. 9 or 9.5/1 is considered high compression right?

I'm heading to an MGMGMG sponsored show today. Thanks for the suggestions, I'll get on it.

Fred Horstmeyer

Fred-
With a spark curve like that, I can well believe that you're having problems. After consulting my sources, I can say that no MGB distributor ever had an advance curve like that! 9:1 to 9.5:1 is indeed a high compression engine. You should be feeding it high octane fuel only.
Steve S.

Steve,
1. You wrote, "With a spark curve like that, I can well believe that you're having problems". But your 40897 #s are pretty close aren't they, if you add the 10 degrees static.

600 - 4+10=14 mine was 14
1600 - 15+10=25 mine was 25
2200 - 20+10=30 mine was 28

2. It should be able to run on 92-92 octane shouldn't it?

3. How high do you rev your engine? How high can you rev your engine?

4. I recieved some advise at the show yesterday to retard and then retard some more....but I just lose too much power. So I played around last night. I had it at:
600 - 5
1000 - 7
2500 - 20
3000 - 24 and it still pinged with the add benefit that it ran bad bad bad.

Reset to:
1000 - 14
2500 - 27
3000 - 30 and it still pings.....BUT runs really well! Guess I'll stay with that for now.

Tonight I'll probably put in the 25D with points just for kicks.

Fred Horstmeyer

Fred
I would say that your compression ratio is a little high for 92/93 octane fuel, I am currently running a fairly standard spec with 8.8 to 1 CR on 95 octane. No pinging at all even on full throttle up hill. Timing is 14 @ 1000 dynamic (vac removed).

Think you may have problems stopping the pinging with that setup and fuel, but bear in mind that pinging can eat away at the pistons and in extreme cases blow holes in them, not something that should be allowed to carry on for long.
Hope you can get it sorted soon, good luck.
Graham
Graham

European Research Octane Numbers (RON) are 95 and 97/98 for super unleaded. US octane numbers are usually slightly different and are quoted as a lower number, and 92/93 is equivalent to super, I think.
Martin

This thread was discussed between 15/06/2003 and 20/06/2003

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