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John McCormack

Poor idling

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I went to Bayview (about 40km away from where I live across the centre of Sydney) in the TR2 today and got caught in traffic on the way home, about an hour of stop start stuff.
The engine was heat soaked but the electric fan kept the gauge in the middle.
When the engine gets hot after about half an hour the car doesn't idle well. The first half an hour or so it idles at 800rpm but when it gets really heat soaked the idle will be at 800rpm for maybe 10 seconds after stopping in the traffic and then it drops to 500-650rpm and very rough. It doesn't stall or even feel like it will but it isn't right. I usually juggle the brake/throttle to keep the revs at 800-1000.
If I bump the idle up it then idles too fast for the half an hour before the engine gets really heat soaked.
It feels like the engine is rich cutting, but the mixture is right for normal driving.
Any ideas?

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I have the same problem John. I put it down to the build up of heat under the bonnet and the engine breathing that very warm air.

 I have a duct that feeds cooler air to the engine when I’m driving but obviously that’s not going to work when I’m stationary in traffic. 

Rgds Ian

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Hi John

I had exactly the same issue some time ago......H6 carbs for ref.

Firstly i suspect the choke mechanism was not returning properly and therefore influenced the idle ( when it shouldn't have been), it was.

Having sorted that i had it on a Rolling road. The SM needles could be adjusted to give a reasonable idle on modern fuel, but leaned out higher up the rev range (actually quite dangerously.)

 Changed to RH needles set on RR. ( there is no other way to do it in my opinion as you can not replicate load). Problem gone, and 3000 miles round the Alps showed absolutely no issues.

Iain

Edited by iain

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+1 for Ian's explanation above, I have adopted the fix suggested by Ben Freer, which is to insert an in-line axial fan into the fresh air tube. Mine switches on with the main cooling fan, so providing cool (outside) air to the carbs while stationary.  Roger H has the same arrangement.

Bob.

Edited by Lebro

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My solution wasn't my idea but I got an inline fan from Merlin Motorsport. It's original purpose is to ventilate boat bilges. Hence it is known as the "bilge pump". It's made of plastic and isn't particularly pretty but could be painted to help it blend in. When I was at Merlin I was intending to purchase a far more expensive, metal model made by a well known electric radiator fan manufacturer.  However the "huff" - technical term - was far better from the cheap plastic one. It's mounted behind the grill with a piece of ducting up to the carbs.  I only switch it on in traffic queues and it makes the car far less grumpy. 

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+1 for the in-line fan.

haven't got a clue if it works but I do have a rather pretty green LED to indicate it is on and and I can also watch the ammeter needle move slightly

when turning it on to keep me amused.

Seriously, if the engine is in otherwise good order then supplying slightly cooler fresh air direct at the carbs must be helpful.

 

Roger

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9 hours ago, Lebro said:

+1 for Ian's explanation above, I have adopted the fix suggested by Ben Freer, which is to insert an in-line axial fan into the fresh air tube. Mine switches on with the main cooling fan, so providing cool (outside) air to the carbs while stationary.  Roger H has the same arrangement.

Bob.

Hi Bob,

Can you tell me which fan you used.

I have fitted a 120mm 12 Volt computer cabinet cooling fan under the front panel just above the radiator with a duct leading to the carburettors and with a switch under the dash.

The problem is that when I switch it on the air flow-rate is very poor and doesn't  seem to have any effect. The spec. for the fan states 84 cubic feet per sec.  It is possible that the computer motherboard which the fan would normally be plugged into would have better control. 

Regards, Colin

 

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9 hours ago, Ben Freer said:

My solution wasn't my idea but I got an inline fan from Merlin Motorsport. It's original purpose is to ventilate boat bilges. Hence it is known as the "bilge pump". It's made of plastic and isn't particularly pretty but could be painted to help it blend in. When I was at Merlin I was intending to purchase a far more expensive, metal model made by a well known electric radiator fan manufacturer.  However the "huff" - technical term - was far better from the cheap plastic one. It's mounted behind the grill with a piece of ducting up to the carbs.  I only switch it on in traffic queues and it makes the car far less grumpy. 

Hi Ben,

Can you tell me what size and make of fan you used.

Regards, Colin

 

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4 hours ago, Colin Fairhurst said:

Hi Ben,

Can you tell me what size and make of fan you used.

Regards, Colin

 

Bob is correct (and has saved me the trouble of a Google!) Thanks Bob:)

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A quick question gents . . . and without wishing to highjack the thread (apologies John). On the subject of cold air and carbs, would anyone happen to know if there has ever been a 'cold air intake box' made specifically for the sidescreen H6?

Cheers, Deggers

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The idea of using a bilge blower to enhance air flow under the bonnet to cool the carbs is explored in some detail in a number articles like this one at the MGA Guru site: https://mgaguru.com/mgtech/carbs/cb208.htm

and here: https://mgaguru.com/mgtech/carbs/cb208b.htm

The MGA has air duct fittings under the bonnet that lend themselves to this type of set up.

 

bilge_blower9.jpg

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Thanks Bob and Ben. That fan looks interesting. The air flow-rate and the amps rate are impressive. I will check measurements and will probably order one. I may even consider the larger size one.

Regards, Colin

 

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I'm using this fan, http://www.attwoodmarine.com/store/product/Turbo-3000   it is a bit noisy but delivers a lot of air, quite impressed. I'm using it hot days in slow traffic via a manual switch. It can be found on Ebay and Amazon for about GBP 25-30 and it looks, and seems to have same data as the one sold at Merlin motorsport

Edited by ijonsson

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Thank you all.

These posts reinforce the view down under, the carbies just don't like very hot air. The temperature on Friday was low 30 degC meaning the temp under the bonnet must have been very warm indeed.

I would appreciate a photo or two as to how you mounted the fan and ducting to the carbies if anybody could help.

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Will take some photos later, & post them.

Bob.

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Bob's pictures will be a lot better than mine partly as I'm currently about 200 miles away from the car/photos and also you really wouldn't like to see the state of my engine bay when I did take some pictures. (The view is greatly improved now). Whilst you're awaiting the demonstration of Bob's photographic skills suffice to say that I fixed the fan to the edge of the ducting on the inside of the apron with a right-angle bracket. The flexible ducting was secured using cable ties attached to the radiator stays.

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20191208_093259.thumb.jpg.bcadff513a1b9d2314817f50d91b1d7f.jpg 20191208_093112.thumb.jpg.a95a1971ae67a7f985eb4797b32ed027.jpg

20191208_093317.jpg 20191208_093345.thumb.jpg.1f84f3fbcfd7b22a8443079867a8fe99.jpg

Pipe attached to front grill with 1 cable tie, fan clamped to radiator tie bar with an aluminium strap see photo No.3

I did originaly have the fan on the other side, but it got in the way of doing any work on the steering box.

The pipe had to be slightly flattened where it passes over the rocker cover.

There would be a lot more room for the fan if the original LH horn was not there !

Bob.

Edited by Lebro

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Mine is on the other side and the ducting points at the front carb only. My bonnet hinges are on raised blocks and when it's running you can feel the draft by the windscreen so it is clearly shifting the air. 

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4 hours ago, Ben Freer said:

Mine is on the other side and the ducting points at the front carb only. My bonnet hinges are on raised blocks and when it's running you can feel the draft by the windscreen so it is clearly shifting the air. 

Until the car is moving at any speed.

Car manufacturers aren't stupid, examine the area in front of the windscreens in many different new models from many different manufacturers. The inlet for the in car heater comes out in a grill immediately in front of the windscreen, why do you think that is ? When the car is moving at any appreciable speed (30 mph onwards) the air from the inclined windscreen creates a high pressure area at the base of the windscreen, it seeks an escape avenue which is happily supplied by the manufacturers into the heater vents. The air you feel being displaced out of the gap given by the lifted hinges on blocks WILL stall when the increased pressure of the windscreen starts asserting itself and stuffs high pressure air in from the windscreen. Happily when your car is stood the raised rear of the bonnet will allow the escape of hot air because of course the heated air layers the underneath of the bonnet and the raised rear edge improves the layer dispersal rearwards.

We tested this out on my race car TR7 V8 which of course has a similar windscreen angle to many modern cars with a considerable rake against the much more vertical angles of the TR4 and of course the earlier sidescreen cars. To improve the air flow down the rear of engine immediately in front of the windscreen we removed (by hacksaw and snips) the existing rear of bonnet rain channel on the firewall and had an open gap of about 2" behind the bonnet trailing edge between the bonnet and rear of engine firewall. We then carried out traditional "wool tuft" testing along the bonnet, wings and windscreen and the photos (polaroid...this was the early 1990s) show the wool tufts on the lower area of the windscreen turning around and diving into the rear of bonnet gap and the high pressure area from the windscreen stretching forward about 18" in front of the windscreen. We had an underfloor alloy engine cover complete with extracting vent at the rear of engine bay firewall area to remove this hot air.

If you seek to lower the carb area air temperature you need to build a sealed area that the carbs breath from, ideally from a sealed carb airbox (if the feed is via a corrugated pipe the turbulence from the corrugations slows down the airflow sufficient not to cause pressurisation of the carbs) or at least from carbs which breath from an area the inlet pipe with cold air vents into which is sealed as closely as possible against wing, bonnet and against manifolds heat intrusion. It doesn't have to be perfect the throughflow of air from the front should help reduce the temps in that area.

Mick Richards 

Edited by Motorsport Mickey

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This is my effort ( see pics). I have found that the computer cabinet cooling fan to have very little effect.

I have ordered an Attwood fan.

Regards, Colin

 

IMG_20191208_122755650Forum.jpg

IMG_20191208_122921197Forum.jpg

IMG_20191208_123044007Forum.jpg

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