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MG TD TF 1500 - CARBURETTOR HEAT SHIELD
| Can anybody supply a diagram for a heat shield .I have 52 TD that is showing percolation problems. It is hard to start after it is shut down and left for 15-20 minutes. When it starts it runs very rough until surplus fuel is cleaned out and then runs perfectly till the next shut down. It is definitely a heat sink problem and not ignition.|
|Here's how mine looks. Very simple and works like a charm. NEVER any over heating!|
Check the archives,
K seem to remember at least one reference to a diagram and also to someone who was makes a heat shield for the TD.
Sorry I can' be more specific, the age showssometimes.
I would suggest advancing the static ignition by around 4mm on the circumference of the crankshaft pulley - it works well for my TF, engine runs much cooler. Also check out distributor bob-weight springs, and ensure advance matches revs. Heat shield helps as well but why MG suggested TDC for static timing goodness only knows! Even contemporary Morris Minor saloons were 4-5 degrees BTDC!
|I recently had the distributor for our TD rebuilt and am running with at least 5° of initial advance on it and while it has made the engine run considerably cooler, today on a run of over 100 miles in 90°F+ temperature, over some rather hilly terrain, I was running the temperature right up to 100°C on the temperature gauge and even with a heat shield in place, was experiencing vapor lock each time we stopped right along with the 5 other T series cars on the run. The situation with vapor lock has gotten progressively worse in recent years with the reformulation of the fuels we use. Since the problem seems to be the vaporization of the fuel in the passage between the float bowl and the jet in the carburetor. This bubble of vapor is trapped in that passage, pushing the float hard against the needle valve and stopping any additional fuel from getting into the float bowl and the engine, upon being started, is starved for fuel until the bubble of vapor can be cleared. Pushing the tickler pin will often upset the equilibrium and clear the bubble. If that doesn't work or if you just want to deal with it from the cockpit, just pull the choke out. You may have to run with it out for a bit to get everything settled down, but I am finding that it works quite well.|
Sandy - I got your e-mail and will send you the info on the heat shield. Cheers - Dave
I agree with your comments about modern fuels making overheating worse. Even with older leaded fuel, in the 70s my then TD was prone to vapourising during short stops for fuel etc and in the UK we don't get the very high temps. you experience.
Sandy, if you tackle the problem in different ways, as suggested in the Archives, I'm sure you'll improve matters considerably, except in the most extreme conditions.
Good luck, John.
years ago I fitted a fuel pump isolating switch as part of an anti-theft system. Since I changed to unleaded petrol I switch it off before I stop the engine. Since doing this I have had no problems with hot start up.
I got over my senior moment and found Dave DuBois heat shield article.
It can be found at: http://homepages.donobi.net/sufuelpumps/ and select other Tech Articles.
In the meantime I see above that Dave has e-mailed you about his article.
Way to go Dave
|Ray - Good idea to defeat the vapor lock problem - no fuel in the float bowl/passage, no vapor lock. I too, installed a fuel cut off switch (actually used the unused fog lamp switch), but quit using it after our son was taking the TD to his senior prom and I had the switch off. The car died on him after getting out of the drive way and the look on his face as he walked back up the walk to the house was enough to make me never use the switch again. Besides, the flyoff hand brake is a better anti theft device anyway, but I may have to go back to using the switch as an anti vapor lock device. Cheers - Dave|
|I am convinced, together with SU specialist Don Jackson, that a carburettor heat shield makes the problem worse. What you need instead is a shield that deviates the heat from the fan which reaches the carburettors. I have temperature sensor on both carburettors and the front one is always about 4C warmer than the rear one, which indicated heat does not come from the outlet manifold but from the fan.|
I may have described this before since praising the heat shield is a common mistake.
Denis, 1950 MG
|Denis L. Baggi|
|Denis.. My problem is caused by heat sink after the engine is shut down and left to rest for 20 minutes. Continuos running never gives a problem.At 89 degrees F today it never missed a beat till it was turned off and left to rest for 25 minutes. This leads me to believe that the heat is coming up from the exhaust area and not the radiator.|
|I agree with Sandy. Yesterday was 94 degrees in Seattle (everyone up here thought we were going to die!) and on our drive, where I ran the temperature up beyond 100°C when going up some long climbs on I5 while maintaining 60 MPH and the car never missed a beat, but 5 minutes with the engine off or even 10 minutes of sitting still and idling and the car would vapor lock. If, on the other hand, I opened the right side of the bonnet while the car was sitting, there was no problem. Whether the heat is coming from the exhaust or the engine or both, it is heat soak that is causing the problem in my estimation. Find a way to remove the heat from the engine compartment or a way to prevent the fuel in the float bowl or the passage from the float bowl and the jets in the carburetors from vaporizing in a high heat environment and the problem will be solved.|
Denis, I have to agree with you regarding the apparent uselessness of the heat shield, since I have a heat shield on our car and the other five cars with us had no heat shield and there was no difference in the occurrence of vapor lock in any of the six cars. Neither did any of the cars experience vapor lock while driving, only when the cars were stopped.
I have seen reports of cars not being bothered by vapor lock in which the exhaust manifold has had a Jet Hot coating applied inside and out. The reasoning is that the Jet Hot coating moves the exhaust gases through the manifold fast enough that there is not as much heat transfer as in an untreated manifold. I'm not sure that I buy into that explanation and having not seen a car that has had the manifold treated so I have no first hand knowledge that this, in fact, works. Cheers - Dave
|After fitting a five speed box to my TD with the resulting repositioning of the engine about 10mm forward I installed a 'suck' electric fan to replace the blades connected to the water pump.|
However, the new engine position meant that I was unable to install the new fan in the middle of the rad since to do so meant that I couldn't get the fan belt between the water pump and the fan.
So I offset the fan to the right. This has the effect of blowing 'colder' air across the front carb in particular, and the exhaust manifold.
Sitting in the car in traffic and putting my hand by the running board, the amount of hot air blown through the side louvres because of the position of the fan is considerable. The result is that in stop-go traffic or just idling, I have no problems with vapour lock.
What works for me to avoid the lock when parked is to pull out the choke and leave it out when parked, and then to turn on the ignition just prior to closing the choke and starting the engine so that the pump primes.
|Mike - "What works for me to avoid the lock when parked is to pull out the choke and leave it out when parked, and then to turn on the ignition just prior to closing the choke and starting the engine so that the pump primes. "|
It's interesting that clearing or avoiding vapor lock when the car is stopped always seems to involve the choke. I will give your method a try when we get some more hot weather here. We have gone from 90+°F on Saturday to mid 60°F on Monday - spring time in the Pacific Northwest. Cheers - Dave
|Now I thank everybody for the comments and I have to admit that I am not going to be too scientific about it. Temperatures in Lugano in the Summer run above 90F and I never had vapor lock. My belief is that this occurs if you try to crank up the car you have just driven when it's hot, and then the front carburettor gets blown with all the heat from the radiator. If that is the case, install a shield between the radiator and the front carburettor. As stated above, I can prove the front one is warmer than the rear, because I have a digital thermometer connected to both.|
I trusted Don Jackson on that, please do read his booklet "My SU Companion" published by the Octagon Car Club, he worked decades for SU.
|Denis L. Baggi|
|Denis - Is the booklet by Don Jackson still available? I would be interested in reading it. Also, what is the altitude where you live? I remember as a child back in the 50s, living with my grandparents who had a summer cabin in the mountains at 3000ft altitude. Their main home was down at sea level and it seemed that there was always more of a problem with the cars vapor locking at the higher altitude, probably due to the depressed boiling point of liquids at higher altitudes. Cheers - Dave|
|Hi Dave. That booklet was sold by the Octagon Car Club and I am not sure it is still available. It is a source of info about SU carbs and fuel pumps, however the info on the shield was given to me by Don in a letter, he sent me kindly various parts to rebuild my carbs. He was very kind and had a TF on which he mounted 1 1/4" carbs. |
If you are truly interested I'd be glad to copy it for you, and send it by snail mail - my university will pay!
I live at about 270 m above sea level, so that this is not an option, I do not see any difference when I go down to sea level.
|Denis L. Baggi|
|Denis - If you would e-mail me at the address above, I'll send you my postal address, I would very much like to have a copy of the booklet.|
270m above sea level shouldn't make much of a difference although it would be around 3.5°F - 4.0°F in the boiling point of water. At 3000 feet and above, there would be a significant difference. Cheers - Dave
I had my manifold jet-hot coated over the winter in hopes of avoiding the vapor lock...last week it was 95 degrees in Boise and I was doing very well up the hills...kept @ 90C while last year without the jet-hot coating it would be @ 100C...unfortunatly...the vapor lock after shutting down has not stopped...still runs ragged after a 10 minute stop...I'll try the choke...also think I'll put in a cut-off...that seems the best...no fuel in the bowl...no vapor lock...
|Bob - Thank you for the information. I rather suspected that your experience would be the result of the jet hot coating. I found on our outing last weekend, that propping the hood open on the carburetor side of the engine eliminated the vapor lock when stopped. Cheers - Dave|
This thread was discussed between 17/05/2008 and 23/05/2008
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