Morgan +8 Forum

Doesn't surprise me - as I mentioned above, I think it's the airflow/updraft effect - the exhaust only has to be a smidge too long or too short and the exhaust fumes flow right back into the cockpit - think about what your hair does when driving - blows forward rather than back. If you drive with sidescreens it is apparently worse.

You could spend a fortune messing about with your engine, or have a look at the exhaust. A simple £5 bolt on end tip may do the job.

If the plugs are a light "latte" colour then it is probably running a touch lean, they should be a darker tan colour on modern fuels.

I went through this with my +8 shortly after I purchased it. Too much exhaust, and hydrocarbon smell, in the cockpit. Checking the plugs found relatively good color, changing the exhaust pipe length, and the pipe turn down didn't make any difference at all (and this over rich condition was a complaint that the previous owner had after the Edelbrock was installed), even when the turn down tail pipe fell off the car.

A tune up helped only a little, until I re adjusted the carburetor, which resulted in a far leaner setting. The jets and needles (yes, Edelbrock carburetors have jets and control needles) remained unchanged. After the carburetor idle screw adjustment, the problem was gone. Plug color remained unchanged, as the running mixture, with the carburetor's jets and needles being the controlling factor, remained unchanged. However, idle mixture was substantially leaner with the adjustment.

At idle, the engine was way too rich, due to the adjustment of the idle mix screws being set using a rich mixture favored method (a hot rodder's favorite the two methods that Edelbrock cites to set up their carburetors). I re adjusted the idle mixture using the lean mixture favored method, by closing down the screws, then re opening them until I had smooth running. This was opposite to the method that was used to install the carburetor, where the screws were opened until roughness was encountered, then closed until smooth running was attained. Edelbrock's documentation states that this is a "performance setting". The difference in the mixture screw setting was nearly a full turn richer than using the original lean mixture adjustment method. Using the lean "economy" adjustment method eliminated the smell of hydrocarbon after a short drive.

Note: I have not driven the car with the top up since the day I purchased it. I have since installed a dual, side exit, tuned header exhaust system. There is no hycrocarbon smell. I was able to lean out the idle mix a bit more as well.

The SUs have needles as a primary jet setup. The plug color would probably be in the ball park, but the idle would still be too rich. With the carburetors having over 100,000 miles of use, it is possible that a fresh set of standard needles and jets be installed, if not just packing and general tear down and re assembly.

If you want to check the idle (slow speed) adjustment, you should look at GoMog, as there is a proper adjustment procedure here: https://www.gomog.com/mi/GoMoG/allmorgan19.html. If you can get a good adjustment, then your problem is solved. If not, you may need a carburetor rebuild, then try the adjustment procedure again.
That said a quick method to see if you have an over rich mixture is to simply raise one of the SU pistons slightly with a screw driver and see if the idle increases. This indicates too much fuel. That will give your quest for an exhaust free cockpit some direction.

It doesn't make any sense that Morgan would sell a car that pumped CO into the cockpit, as that is extremely dangerous. Something is wrong, but if you can't find an exhaust leak, the problem is a too rich idle mixture.

I have driven a few +8s, carbureted with PCV systems, and fuel injection. None of hem had any problem with exhaust in the cockpit. All had rear exhaust systems.

Drove the +8 again yesterday and same thing. I'm wondering how a too rich idle mixture can create the fumes when driving the car? It does indeed smell at idle as well.

With the SUs, the problem of adjustment is throughout the range. Rich at idle can result in rich throughout the range. Are the damper reservoirs properly filled? Have the mixtures been adjusted? Have you checked for a rich mixture by lifting the carburetor pistons to see if you get an initial increase in idle speed?
Nothing beats a good carburetor adjustment or rebuild to go with your tune up.

I had a 1976 +8 with the original twin SU setup and I got consistently 23.5 mpg, mixed driving. I now have a 1985 +8 with the Bosch injection system and get a similar consumption. (There are limits to what modern electronics can do to change the basic laws of physics). So I think you have a carb problem that makes the car try to burn far more fuel than it should.

The SUs are remarkably simple and easy to tune. Twin SUs are a bit better to keep a man usefully occupied on weekends, but not hugely so. I have lived with twin SUs on my Morgan +8, a Volvo P1800 and a 1936 Derby Bentley 3.5 liters. Essentially the same carbs.

So assuming that the carbs are fine, gaskets are tights, there is oil in the damper cup, the carb pistons move freely, there is no false air etc.

The cards should be syncronized so that the two carb pistons lift equally with throttle movement. There are some VERY fancy and expensive tools with air flow measurement etc. The simplest consists of two rods with a piece of coat hanger wire sticking out from the top. You unscrew the damper piston at the top and replace them with the rods, bend the wires so that they point towards each other and are at the same height. When you push the throttle, the two should lift simultaneously and by equal amounts. If they don't, you need to have a closer look at the throttle linkages and make sure that the carb pistons move freely without sticky points. But no need for hi tech. It is a bit like the Haynes manual for my P1800: "Remove the gear wheel with Volvo tool SVO 991178 (or a screwdriver)"

Assuming that the two carbs are synced, you check/adjust the mixture:

There is a small "pin" sitting under the carburettor barrel that you can push upwards a mm or so. That lifts the carb piston. If the revs drop off when you push it, the mixture is too lean. If the revs increase, it is too rich. When it is just right, the revs will increase a little bit and then drop back to the original level. I personally tested that with the air filters on. They do give a bit of air resistance, so testing without them will throw the result a bit. The advantage of this procedure is that you can tune the two cards independently with undoing throttle linkages etc.

If a carb is too lean or too rich, adjust the jet position by one flat at a time on the adjustment nut until you get it right.

You may find these links useful:

https://mgaguru.com/mgtech/carbs/cb130.htm



Changing to other carbs is iffy business in my experience. The resistances to air flow in the intake manifold, the carbs, the air filters etc. are all matched to each other. And an overhaul kit for twin SUs will run you 100 - 150 $. So it might be a good idea to make sure that what you have works as intended before forking out for other carbs

I did two things today: 1) A bit embarrassing but the carb pots needed damper oil, one of them was almost empty. I filled both up to approx 1-1.5 cm below the top with SU carb damper oil. I should have attended to this as first thing, as I have owned many sports cars with SU carbs during the years. 2) The sealing around the air intake for the heater is missing, which means air is sucked into the cabin from the engine bay. I temporary blocked the intake with a clean rag. 3) On both carbs, at idle I pushed up the Lifting pin, which rise the piston and there was no change to the engine rpm, which I take as the mixture is ok. It's also in line with the colour of the spark plugs - coffee latte. Tomorrow I will make sure that the choker is activated equally on both carbs and importantly, fully deactivated on both carbs when not using the choker function. The result is that there are no excessive fumes in the cabin as far as I can judge, except the usual "Old British car" slightly oily smell. So, hopefully it has been rectified, the ultimative test will be wifes judgement ...

Usually, the oil in the dash pots is filled so that when the cap is put back on, resistance is felt at about 15mm before the threads of the cap touch the top of the dashpot. Filling the dashpot is not necessary, and it slows the throttle response. The level is set by feel of the damper resistance, rather than by the actual level of fluid.

Hi Martin, nice to hear from a countryman driving Morgan! And thanks a lot for the comprehensive advise and tips and the link to SU carb adjustments. For Austin Healey 3000 - funny enough we are currently restoring a AH 3000 BN7 form 1959 and I will certainly need your advise there too!! I tried to push up the pins and there was no reaction when idling, so I anticipate that the mixture is ok. This is also whjat vthe colour of the spark plugs indicates. I saw your mail to me and I will write you back there too. Cheers /Tak Finn

We finally got rid of the exhaust smell in the cabin. The rear pipes are rather big being 63 mm Ø and it may be that the velocity of the exhaust gases are lower than when using the original tail pipe size. Anyway, we took off the tail pipe and installed a 45 degree exhaust tip just in front of each rear wheel and... no more exhaust smell! The exhaust note has changed slightly, more mellow and slightly louder at low speed, but not when driving on highways. Problem solved, but we may install original tail pipes later on...let's see if we can live with the side pipe look. Thganks to all for your valuable inputs. Cheers. /Finn

Glad you got it sorted.