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PO put a 25D distributor in a 1979

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PO put a 25D distributor in a 1979 #1
senior citizen Dale G Cumming Georgia, USA	Posted: July 15, 2012 08:58PM Registered: 11 months ago Posts: 20

Hi guys,
I recently purchased my first MGB, a 1979. Ever since I got it, it has had a problem falling on it's face at full throttle, 4000 rpm. Just noses over like it is running out of fuel, and if you lift, it runs just fine until the next time.

I've been through the entire fuel system from tank to carb (replaced the diapragm--no help) and checked fuel system pressure and volume. I also have gone through the entire emissions system, repaired some bad vent lines and rebuilt cannisters, no help. I also have done a complete tune-up without improvement.

I finally bought a dial-back timing light (The fan ate my old one!!), and lo-and-behold, absolutely no mechanical advance!! Vacuum canister dead too. The timing was set at 10 degrees initial. The original distributor had been replaced by PO with a 25D #41491. I pulled it and went through it. Now it is fine (except the cannister), and I plan to run it as mechanical until I decide what to do. The problem is that I loosened the clamp to get the distributor out and now it won't go back in. Engine is at TDC, No.1 firing position and I noted the rotor position prior to removal. It will go in easily 180 degrees out, but I cannot seem to get it to go in where it belongs. Is there some trick to this or do I just need to keep trying???

I now know that you are supposed to leave the clamp on the distributor and remove the two 7/16 bolts from the block. I would have if I could have figured out how to get at those two bolts. I have to know, how do you guys get them out???

Thanks in advance for your input..

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Re: PO put a 25D distributor in a 1979 #2
rundjk David Kinsey Virginia Beach, USA 1980 MG MGB "Runaway"	Posted: July 15, 2012 09:18PM Registered: 3 years ago Posts: 1,476

Similar thing happened to me, Dale. The clamp has likely twisted and won't let the dizzy shaft in oriented the right way. You'll have to either get the clamp bolt out and then really pry open the clamp, or maybe go from under the car and get the two bolts out. Have you moved the car at all or turned the engine (crankshaft) at all? If so, you may have moved the notch inside. Keep trying. One of the experts on here will probably chime in soon.

David J. Kinsey
Virginia Beach
Tidewater MG Classics
1980 MGB "Runaway"

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Re: PO put a 25D distributor in a 1979 #3
j-blueridge Jamie Bourland C'ville VA, USA 1968 MG MGB GT "Antichrist"	Posted: July 15, 2012 09:41PM Registered: 8 years ago Posts: 664

I always leave the clamping plate in position on the block. Only had the 180 out of phase thing once. I think I loosened the clamp more and spread it with a screwdiver as a lever.

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Re: PO put a 25D distributor in a 1979 #4
senior citizen Dale G Cumming Georgia, USA	Posted: July 16, 2012 12:45AM Registered: 11 months ago Posts: 20

Thanks for the response David. It is nice to know that someone else has had a similar problem and I am not being picked on because of my old age!!!

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Re: PO put a 25D distributor in a 1979 #5
senior citizen Dale G Cumming Georgia, USA	Posted: July 16, 2012 12:49AM Registered: 11 months ago Posts: 20

Thanks for the info. No, the car hasn't been moveed or turned over. I will work on the clamp, sounds like a likely suspect.

Thanks

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Re: PO put a 25D distributor in a 1979 #6
ErnieY Ernie Y Nr Cahors, France 1970 MG MGB 1976 MG MGB GT 1978 MG MGB	Posted: July 16, 2012 02:07AM Registered: 1 year ago Posts: 2,896

Although the 25D will be a complete mismatch for your engine a lack of vacuum advance will not be the cause of your problem at 4000rpm. Since there is little or no vacuum under full or near full throttle conditions the lack of an advance function would not affect high speed running in any way.

There are many articles describing the purpose of vacuum advance, here for instance:

http://www.460ford.com/forum/showthread.php?t=117504

Despite your checks on the fuel system I'm willing to bet that you still have a supply problem.

Is the float level correct ?

Have you tried running with the gas tank cap not fitted ?

Edited 1 time(s). Last edit at 07/16/2012 02:08AM by ErnieY.

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Re: PO put a 25D distributor in a 1979 #7
senior citizen Dale G Cumming Georgia, USA	Posted: July 16, 2012 12:49PM Registered: 11 months ago Posts: 20

Thanks for the response

I realise that the lack of vacuum advance will not make a difference, but it also had the components of the mechanical advance rusted solid, so no advance. I thought that this might be the root cause of the problem.

I have not checked the float level, but I will now that you have suggested.

Thanks

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Re: PO put a 25D distributor in a 1979 #8
Dave Braun Minnesota, USA 1952 MG TD "Tommy" 1970 MG MGB "Maggie" 1974 Triumph Spitfire 1500 "Sammy"	Posted: July 16, 2012 02:39PM Registered: 1 year ago Posts: 2,005

Fixing a frozen advance may not solve the problem if the springs are weak and giving the wron curve, or the advance stop is hammered or wrong.

Typical installation is vacuum advance straight up and number one at 2 o'clock.

If the drive dog has been left alone in the engine, the distributor will still engage in the correct position.

Hope this helps,
Dave

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Re: PO put a 25D distributor in a 1979 #9
B-racer Jeff Schlemmer Minnesota, USA 1958 MG magnette 1971 MG MGB "POS" 1973 MG Midget MkII 2006 Dodge Charger "Daytona" 2008 Ford Super Duty	Posted: July 17, 2012 07:06AM Registered: 6 years ago Posts: 12,514

In reply to # 2121289 by ErnieY Although the 25D will be a complete mismatch for your engine a lack of vacuum advance will not be the cause of your problem at 4000rpm. Since there is little or no vacuum under full or near full throttle conditions the lack of an advance function would not affect high speed running in any way.

Wow - what a bunch of wrong advice!!!

The 41491 can be made to work properly. While it is a great distirbutor to replace the 45DE4 that came witht he car, you will need to get the advance working. Once it does work, you'l go from too little to too much advance. That's where a recurve helps performance drastically!

As for the vacuum advance, your engine can make vacuum at ANY rpms, if the engine is not under full load. Vacuum is developed any time the throttle is held constant or closing. When it opens drastically, vacuum is reduced drastically by decreasing the overall veloicty of incoming air. More air in a smaller space = lower velocity, lower vacuum.

ANY street driven engine needs vacuum advance for performance AND economy. The vacuum unit on the 41491 distributor is ideal for manifold vacuum and also works very well on ported vacuum if you have the option. That's why I really like this distributor for your application. Get it rebuilt and you'll be loving the perormance!

jeff@advanceddistributors.com

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Re: PO put a 25D distributor in a 1979 #10
rundjk David Kinsey Virginia Beach, USA 1980 MG MGB "Runaway"	Posted: July 17, 2012 01:00PM Registered: 3 years ago Posts: 1,476

David J. Kinsey
Virginia Beach
Tidewater MG Classics
1980 MGB "Runaway"

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Re: PO put a 25D distributor in a 1979 #11
ErnieY Ernie Y Nr Cahors, France 1970 MG MGB 1976 MG MGB GT 1978 MG MGB	Posted: July 17, 2012 01:22PM Registered: 1 year ago Posts: 2,896

In reply to # 2122246 by B-racer The 41491 can be made to work properly. While it is a great distirbutor to replace the 45DE4 that came witht he car, you will need to get the advance working. Once it does work, you'l go from too little to too much advance. That's where a recurve helps performance drastically!

So you are confirming that a 25d is a mismatch unless you have it recurved, wot I said I think !

I don't disagree with what you say about vacuum across the range but referring back to the OP 4000rpm equals nearly 72mph without O/D or getting on for 90mph with, both pretty well full throttle scenarios which will be producing little or no vacuum.

As I read it the car gets to 4000 rpm but then falters, classic symptoms of fuel starvation, is it your opinion or contention that vacuum is a possible culprit for this reported fault ?

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Re: PO put a 25D distributor in a 1979 #12
72bwhite Larry McKinney Thousand Oaks, USA 1972 MG MGB	Posted: July 17, 2012 04:09PM Registered: 2 years ago Posts: 1,338

Distributor will only fit in the drive dog one way, you will notice it's wedge shaped so where the rotor is pointing is fixed in relation to the cam. It will either appear right with relation to the crank timing mark or 180 degrees out, remember crank dose 2 tuns for every turn of the cam.

I always pull the dizzy with the clamp attached and then rotate it in the clamp to get the timing right once its back in the block, clamp is lose so it can rotate.

Now as to your original problem a distributor in the condition you described could defiantly be the problem, actually kind of surprised it would run at all. The B engine should just be getting happy at around 3K and rev up to 5k maybe more assuming you are in 2nd or 3rd gear.
With a 79 you would also need to make sure you don't have any obstruction in the exhaust system.

I would defiantly consult Jeff about your distributor and strongly consider sending it to him to be rebuilt properly.

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Re: PO put a 25D distributor in a 1979 #13
gerry22 Gerry McNulty Nevada, USA 1971 MG MGB GT "Sick" 1974 MG MGB GT "Rusty" 1979 MG MGB "BB"	Posted: July 17, 2012 04:30PM Registered: 1 year ago Posts: 321

I'm an old dude too and I prefer to take the dist and locking plate out all at once How? You need a short 7/16 wrench and you can get them both out from the under side. The reason is you can bet the holding plate is bent. I just lay it on a block of wood and hit it back straight. Not a big job and will you have the dist out you can see and set the points

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Re: PO put a 25D distributor in a 1979 #14
senior citizen Dale G Cumming Georgia, USA	Posted: July 17, 2012 04:42PM Registered: 11 months ago Posts: 20

I agree with what everyoone is saying and appreciate the input, but perhaps I did a lousy job of explaining the problem: It falls on it's face at 4000, no matter what gear it is in or even at no load. If you lift the throttle, it comes back. Also sounds like fuel starvation to me. I have checked the entire fuel system to the carb and have good volume (1 pint in less than 20 sec.) and pressure (3 psi at deadhead). I also cleared all of the vent lines and tried it with the cap removed. The Diaphragm was suspect since the piston would drop at 4000rpm, steady throttle, with aircleaner off and no load. Changed it and no help. Any Ideas???

This situation is what brought me to the distributor. I felt that it was an unlikely cause, but thought I would give it a shot. The 25D was in the car when I bought it (1973 41491). Dwell was 55 degrees, and it was set at 10 degrees initial. When I checked it with a dial-back timing light I found the no mechanical advance situation. The vacuum advance is also dead and will not hold vacuum. I have torn apart the distributor and gotten the advance working and installed the medium weight Mr Gasket advance springs. My general experience, (although not on a MGB),tells me that these springs, and the weight of the advance weights should bring in total mechanical advance at around 2500 to 3000 rpm. This would bring me to about the 36 degrees total that everyone is talking about. Is this a logical conclusion? and, if so, is this a decent curve for the MGB???

I would love to replace the vacuum advance but cannot right now. My Toy fund is running low!

Thanks in advance for your input.

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Re: PO put a 25D distributor in a 1979 #15
senior citizen Dale G Cumming Georgia, USA	Posted: July 17, 2012 05:15PM Registered: 11 months ago Posts: 20

You are the man!! I had the entire dizzy disassembled and paid no attention to the positon of the drive dog when I put it back together. I didn't notice the taper. I'll bet I have it 180 degrees off. I will check it now. Thanks again!!

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Re: PO put a 25D distributor in a 1979 #16
rdgreen Robert Green Adelaide, Australia 1975 MG MGB MkIII	Posted: July 17, 2012 05:27PM Registered: 1 year ago Posts: 755

Hate to put a dampener on your enthusiasm but having the distributor 180 degrees out will make no difference to how it performs, assuming that you have moved the plug leads accordingly. If the leads are as per the manual it's not wrong. Ran mine 180 out for 16 years before recognising the error when replacing with one of Jeffs excellent 25D distributors.

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Re: PO put a 25D distributor in a 1979 #17
senior citizen Dale G Cumming Georgia, USA	Posted: July 17, 2012 05:33PM Registered: 11 months ago Posts: 20

RATS!!

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Re: PO put a 25D distributor in a 1979 #18
rdgreen Robert Green Adelaide, Australia 1975 MG MGB MkIII	Posted: July 17, 2012 05:37PM Registered: 1 year ago Posts: 755

Checked your float heights?

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Re: PO put a 25D distributor in a 1979 #19
pinkyponk Adrian Page Nova Scotia Canada, Canada 1971 MG MGB GT 1973 MG MGB MkIII	Posted: July 17, 2012 06:47PM Registered: 2 years ago Posts: 2,745

Is it possible you are losing spark when the point plate rotates and stretches the little cloth covered wire inside the distributor?

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Re: PO put a 25D distributor in a 1979 #20
mac townsend Fairfield, CA, USA	Posted: July 17, 2012 08:45PM Registered: 7 years ago Posts: 18,136

if you have the distributor in 180° out it will most certainly affect how the car runs.

That would mean that the drive is not properly engaged and is being driven only by the tension of it's being cinched down in the wrong place. The pin is offset and cannot be properly engaged @ 180° out. It might be forced in (still wrong) or just crammed up against the face of the drive and free to shift as it may. In either case stuff isn't going to work as it should. And bearings will suffer.

Now...if the gear itself has been removed (requires removing an additional piece most folks are unaware of, so it can't happen "accidentally" winking smiley

and replaced 180° "out" then all you've done is moved the #1 plug port on the distributor cap to the port formerly used by #4. If the wires are then redistributed accordingly, all should run fine. The distributor will appear 180° out to someone not aware of the transfer because the plug ports are not where they are expected, but the engine doesn't care and you can continue timing on #1.

Or don't move the wires and time it on #4. And totally mess with any observer's head!

Now it is possible to field strip a distributor and put it back together wrong so that it is 180° (or even 90° IIRC) out without regard to the drive dog. This is accomplished by hooking up the advance springs to the wrong posts thus shifting the distributor cam with respect to the locating pin in the drive shaft. (The pins are connected to the lower drive shaft, the cam connects to a short upper shaft which is connected to the lower shaft by the springs). I've done this more than once without realizing what I was putting myself in for<G>

1973 Roadster. A nice 50-footer!
SUs, Datsun 5-speed
MGB Tips and Tricks: www.mgrescue.com

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