Category Archives: Technical

Technical Section – MGB Engine Replacement

Please note: Questions and answers and Upkeep and Performance Hints are provided for information and advice purposes. No liability either express or implied is assumed by reliance on the information presented either by the writers or the AMGBA.

Some or all of the below is from our message board at http://board.amgba.com, Facebook group at www.facebook.com/groups/americanmgbassociation. This is just a part of what appears there and in the member magazine.

Also be sure to see our message board on our website and Facebook group for immediate help from fellow members.

Q:     I’m hoping you or another member can help. I recently bought an 18GH engine to replace my worn out 18V in my ’78 MGB. The problem is we can’t seem to get the drivers side engine mount to fit correctly. We have both the old style and new style mounts, but neither lines up to both the block and the frame. Is there any modification or kit we can get to accomplish this. Please let me know as the summer is here and I’d like to get back on the road.

Brian Turner, Beachwood, New Jersey

A:     The 18V and 18GH engines are certainly interchangeably replaceable as compete units, but there are differences in the engines and bodies.

While it is said the B was unchanged for it’s entire run, the rubber bumper body looks the same, but was heavily altered to meet later crash standards and there are differences that make the swap a bit more than removing one and sliding the other into place.

In your case, the major difference you see is in those minor differences in the body sheet metal and require that you swap the front engine plate with your old engine’s to assure the mounts line-up and to use the later motor mounts.

In addition to changing the front engine plate and mounts, you may also encounter some interference between the left front motor mount and the bulge of the oil gallery that runs just inside the engine block in that area. So you may have to use a grinder or some judicial use of a BFM (mallet as opposed to a hammer) with a block of wood to move (or remove) the sheet metal flanges and panel enough give you the clearance for it to fit.  You should be able to do this without doing damage to the car.

I hope that helps you.  Take photos to share and let me know what you find when you get into it.

Safety Fast!
Art Isaacs

Q (part 2): We got the “new” engine in and running, but it heats up and over very quickly and the headers became cherry red also. We blocked the hose that would have gone to the choke outlet as described earlier, but it doesn’t seem the coolant is circulating. Is there something we missed about this outlet that is preventing circulation?

Brian Turner, Beachwood, New Jersey

A (part 2): There are any number of reasons the coolant wouldn’t flow or the engine would overheat.  Blocking the choke water outlets is not one of them.  Fast questions:

The cheap stuff to check first:
– Did you check or change the thermostat?  A stuck stat will stop or restrict coolant flow.  Just remove it to see if anything improves.  You can replace it afterward.
– Did you flush the engine before installing it?  You can still do that as you remove the stat.  Take the lower hose off  as well and flush both the engine and the radiator from the top to see what comes out.  Run it with just water now (no fear of freezing) and replace the proper coolant when you are satisfied with its operation.
– Have you removed and inspected, cleaned or changed the water pump? Hoses? Have you checked the heater control valve to see if it’s blocked?  If it is, that often means gunk or corrosion in the engine.
– Have you checked the timing?  That should be set per the engine spec, not as the car would be. Too far off and they do overheat.
– Is the carb running too lean?  Tends to run hot if it is, but not to the extent you seem to describe.
– Have you tried running the heater with the fan on to see if it runs cooler? If it does run cooler, that indicates other possible problems than flow.

The big stuff:
– Do you know the history of this engine?  Why was it available?
– Have you done a pressure check on the cooling system?  Head gaskets leaking internally are a fairly common issue on these engines, as are cracked heads.  A pressure test generally shows this up, even if there is no external leakage.  Often the only external leakage seen might be a moist line of coolant at the head gasket below the spark plugs.
– Having run it hot and had it overheat, is there coolant in the oil? Or white smoke (water vapor) exiting the exhaust?  Or coolant backing out of the overflow pipe? If there is, see above.
– Was the radiator checked for blockages or the cap replaced or tested?

Check these out and then let’s talk further.  Talk to you later.  Good luck.
Art Isaacs

Q (part 3): Here are some pics of a question we have.

What do we do about this?

Brian Turner, Beachwood, New Jersey

 

 

 

 

 

 

A(part 3): Not a major issue.  Of course the difference is the outlet fitting for your water heated choke from the Stromberg 175CD carb on your old engine.

Older engines with twin SU carbs never had them because they all used a manual choke.

If you are continuing to use the single Stromberg, then you would either have to change the heads between engines or covert your carb to a manual or electric choke and blank or plug the other end of the choke heater hose.  The manual choke conversion kit is available from Moss (part number 386-325, about $125), which is more than I can say for the replacement choke heater hoses for the Stromberg.  I have not see the electric kit of late.  These conversions are easier to do than changing the heads.

It is also much cheaper than swapping to an HF44 carb (Victoria British part number 3-740, about $750), which includes a new air cleaner and the manual choke kit, but is otherwise a bolt-in conversion.  But the performance difference may be worth it.

There is also the Weber downdraft conversion, which requires new air cleaner and intake manifold (both included) as well as an exhaust manifold and engine pipe (both not included) and linkage modifications, but you could get the electric choke version of that carb (Victoria British part number 3-450, about $700), which does not require the manual or water choke connections.  Again the performance will be much better than the original 175CD carb.

If you have the twin SU carbs, manifolds and linkages from the donor car, and your local emissions laws allow the swap, that might be the best way to go.  The head on the engine you are swapping-in does not have the outlet, so it’s just the front that needs to be blanked and you need the choke cable, which can be the actual MG part or one available from the universal parts rack at any Advance, NAPA or Pep Boys shop.

I hope this helps you, but write or call any time if you have any other questions.  Good luck.

Safety Fast!
Art Isaacs

MGB Timing Adjustments

Q:     I am a new AMGBA member and I have a tech question. I have a 1977 MGB with twin HIF carbs and pertronix ignition. My question deals with timing. When I got the car (my 4th MGB) the vacuum was not hooked-up . I was advised to hook it up. When checking the timing with a light I found it to be 10 degrees BTDC @ idle with approx 30 degrees @ 1500 RPM. I reset it to 13 degrees idle.

My issue is when I hook the vacuum advance, the timing will advance to about 45 degrees idle and even more @ 1500 RPM. Back in the day when I had cars with a vacuum advance, I set the timing and hooked up the vacuum and did not look to see what happened. I looked at the B to insure the advance worked and it did. Is this normal or should I not use the vacuum. The car runs fine without it but if it works better with it I would use it. What do you think? Thanks.

Chuck Gonyeau, Island Pond, Vermont

A:     Your car has been modified, so the manual does not exactly apply. I am not sure, but there may be some things that you are doing that can effect the timing.

First, when you disconnect the vacuum tube, it should be from the distributor side and you should plug it before attempting to set the timing. The car should be warmed-up and at idle (800-950rpm).

Being that your car has a twin HIF4 and you are saying it has a Pertronix conversion, you should have either a 25D or 45D4 distributor, not the stock electronic unit. I can make some suggestions based on that.

To begin with, the vacuum advance would be necessary with this set-up. And failures of the advance systems, mechanical and vacuum, are fairly common on the 25D4 and 45D4 distributor.

From what you are saying, the timing advances too far as soon as you connect the tube, even at idle. It sounds like the vacuum advance is working, but the rapid advance at high idle may show something in the mechanical system. Weak or broken springs will cause the mechanical system to advance too quickly, but don’t jump to trying to find replacements for those before looking at the more method related possibilities.

For one thing, the mark for this set-up should be about 8 degrees BTDC at idle. This is more typical of the older set-up. I have a 73 with twin HIFs and a 25D4 that this works well with it.

I also connect the vacuum line to the intake manifold, not the carbs. The photo below sourced from the web shows the stock position and the alternative pick-up. I’ve always had it hooked up to the manifold since removing the AIR smog system the car years ago. May not make too much difference, but I know it works.

These are no cost changes, so I’d suggest you start there and see what happens. Let me know how you make out and we can take it from there.

Safety Fast!,

Art Isaacs

Top Installation

Q:     I am installing a new Robbins top on my ’77 MGB. There are no instructions included. How is the loose material hanging at the rear window attached to the moveable frame bar? Any other tips will be much appreciated. Safety Fast!

Don Boudwin
Clayton, Delaware

A:     A trick taught me on changing my ’73 top was to leave that rear bar loose and not attach it to the top. It leaves some slack in putting the top up, making it easier (after it’s up, you then move the bar into place and it tensions the top nicely). And gives the same in taking it down.

It also reduces creases and allows you to fold the top own that reduces damage to the plastic back windows and top itself. It involves pulling the top back flat on the boot and using 3 beach towels in stowing it. Takes a few minutes extra, but my AMCO vinyl top (considerably cheaper than a Robbins) is almost 20 years old and the windows are clear and unscratched and it is only now starting to show some wear at the attachment points at the rear.

Using Velcro also gives you the option on how to handle that bar. The way I do it is to put a towel on the boot, lay-out the top flat onto the boot, lay a second towel on top of the center window, fold the side-panel windows at the cloth divider (between them and the center pane) and one more for good measure on top of the folded windows. I fold the ends of the towels such to keep them from falling off in handling and further protect the plastic windows.

This covers all the clear plastic, ensures none are creased and protects them from the frame. The last part (partially done simultaneous with all the above) is to bring the frame back and down, draping the opaque part of the top partially into the well below the frame and just the window portion, now cushioned in towels, draped over the front top bar.

The window section now hangs mainly behind the seats and is mostly hidden by the front of the boot cover or the front of the tonneau cover when put behind the seats once opened.

The photos will make it clearer. Safety Fast!

Art Isaacs

QA_Side_window_Folded_on_Divider,_2_Towels QA_Top_Up,_Towel_on_Boot QA_Top_Up QA_Towel_on_Boot,_Rear_Windows_Flat

Setting the Vacuum Advance

by Art Isaacs

The easiest way to check if the advance is working properly is to set-up a timing light and watch the timing mark move under various conditions.

If you have not used a timing light on your car or don’t own one, this may help. Otherwise, set up the light as if setting the timing, which is not a bad idea to check first anyway.

If you are buying or borrowing a timing light, try to get one with a clip-on (induction) coil for the spark plug wires.

Older types required an in line adapter that meant removing the wire from the distributor cap or spark plug and the connections were often not as good and sometimes fell off during use. Clip the light to the #1 spark plug wire (though for this purpose, any wire will do as long as you can see the timing mark with the light on). With the battery behind the sets and under the floor, it is easier to connect your timing light to the cigarette lighter on the console.

The adapter is cheap and available at any auto parts store or Radio Shack. Make sure the adapter has fairly long wire leads. Then just connect the 2 leads from the light to those on the adapter. With the engine off, clean and paint the timing mark on the engine damper pulley (crankshaft belt pulley) with light paint or even “white-out”. The mark should be a slot in the pulley outer ring. Also paint the mark on the engine, usually on the timing cover just above the pulley. It looks like a saw-tooth metal tab sticking out from the cover at about 10 o’clock facing the engine or a bit closer to the passenger side of the car. Finding the correct tooth to mark is often difficult, but you should paint the right one to avoid any confusion later.

Start the engine as normal and warm it up. Remove the vacuum advance hose from the distributor and plug it. If the idle drops, the diaphragm of the advance unit is not blown, though at low idle there is often no difference.

Aim the timing light at the marks watching if the mark moves at idle. Too much motion may be indication of mechanical wear in the distributor, but hold that thought for a moment. Slowly raise the idle and then race the engine a bit. Gradually raising the idle allows the mechanical advance to work. Quickly racing the engine is like acceleration without the advance unit and you can see how the engine reacts. If there is no motion of the timing mark on the pulley (relative to the casing), there is a mechanical problem in the distributor, usually a stuck advance plate. I don’t think this likely as you say when you open the choke, the car runs better. Raising the rpm will mechanically advance the timing, if jammed, even if the vacuum system works, nothing would happen. Next, reconnect the vacuum line to the distributor and repeat the above. If the engine stumbles and the timing mark does not move any differently than with the line removed, the advance pot is probably leaking and the vacuum advance unit needs to be replaced (about $70 from Moss).

he original 1980 system is a Lucas electronic unit on a unique distributor. If you are OK with some modification to your car, I would highly recommend going to an earlier model 45D points-type distributor (still available new) and equipping that with an aftermarket electronic conversion, such as Pertronix. The installation of both the distributor to the engine and the conversion kit to the distributor are bolt-in with no modifications to either the car or distributor, save possibly some wiring connections. The 45D is easier to maintain and the electronic conversions are more reliable than the Lucas CEI or Opus units. Just a suggestion to consider, especially if you find both mechanical wear and the advance unit faulty.