What causes heat soak, and is it a bad thing?

[wkilleffer]

Ok, you're probably familiar with the new head gasket I've put in my 74 MGB. I'm trying to drive it more than before. Today, temp got up to around 70deg with some winds. I drove the car on a couple of errands, like to the bank, where I went in a place for about 10 minutes, came back out, and started the car to drive off. Car would start, but would idle low and kind of rough, and smelled kind of like rich exhaust. After I got out of the parking lot and on the road, it seemed ok.

Now, this worries me cause it was a bad problem with the car last summer, and right or wrong, I kind of associate that with some of the problems the car was having.

Some of the things I've done lately:
-New head gasket and retorqued after several heat cycles
-Rebuilt head
-Radiator rodded out and repaired
-180deg thermostat
-New coolant and hoses. Coolant level is just below the neck of the radiator
-Oil and filter change, 20w50 Castrol GTX
-Adjusted timing, 2 or 3 deg over factory spec, no pinging on 93 octane so far
-Adjusted valve clearance while engine was warm, can still hear tappets but they're not loud
-Balanced carbs. they may be slightly rich, but they were breathing at the same rate when I put the air cleaners on

The temp gauge needle will come very close to the N, but won't get straight up and down with it nor go past it.

I've been driving it a little more enthusiastically than I used to. 3500 rpm shifts are starting to become the rule rather than the exception.

Call me paranoid, but I don't want to think that I've done all that work only to have something else go wrong or have to do it again next week. If I can think of anything else, I'll post it.

Thank you,
-Bill

 

[Dave Russell]

Heat soak is to be expected. The engine is burning fuel & making heat. Water flow & air through the radiator & over the carbs keep temperatures fairly even.

When the air & water flows are stopped, the parts get hotter because of the stored heat in the system. Carbs usually get quite a bit hotter. Hot enough to expand & even boil the fuel in them. Makes the engine run rich when restarted. If you leave it off long enough, the hot parts cool & the problem goes away.

In some regions, fuels formulated for summer often have higher boiling points than winter fuels. The problem is especially bad during seasonal temperature warm ups when the fuel has not yet been changed to summer grade.

Some engines have heat insulators between carbs & manifold which helps a bit. Some carbs have thermostatic fuel mixture adjusters to help with the problem. Electric fans that stay on after shut down until things are cooler help. Mostly, on older cars, folks just put up with it.

After restart, the air & water flows bring things back to normal in a little while.
D

 

[DrEntropy]

Dave's right.

Just diddle the timing so it starts well after a 20 minute run, I.E. starts with a "touch" of the key. You may find that to be a bit advanced of the "factory" setting.

A word in yer ear: If the dizzy is "sloppy" in any way the whole thing is moot. If you can find yer way clear to get another dizzy, send it to Jeff and have it gone thru. Then you have an "absolute" to start from.

 

[wkilleffer]

Ok, glad to know that I'm probably ok here. My car when new looks like it had an asbestos heat shield behind the carbs. All the asbestos was gone, so I jury rigged a new shield using a piece of heat shield fabric from the auto parts store. Made a difference in the way it felt on the carb side of the engine.

The car will start with just a touch of the key, but it sounds balky at first. The Flamethrower dizzy I have is tight with no "feelable" slop. Timing doesn't move all over the place while I'm trying to set it.

I'm going to take another look at the carbs as well. I'm afraid that when I adjust them, that I get them pretty close to where they need to be, then the air cleaners throw everything off a bit even though they're pretty new.

Thank you,
-Bill

 

[twigworker]

Ditto on the note above concerning seasonal gasoline formulation. If you have an excessive percentage of that stupid alcohol stuff in it it will "boil off" and cause you grief. There isn't much you can do about that other than....insert: Jack refrains to go off on an eco/political rant. LOL

Assuming that you have HIFs try screwing the mix screws back out about an eighth of a turn each and try to duplicate the heart soak situation. Repeat a couple of times until you think there is an improvement. Also make certain that the lines from the float chamber vents are clear all the way over to the canister and that the canister is not restricted. You can test that by simply blowing through the line. Make sure that the line is connected to the correct fitting on the canister too.

The idle speed will be impacted by the changes you make in the mix, probably higher, but don't mess with that until you get the heat soak thing worked out.

Carb balance is a function of throttle shaft position so you shouldn't have to go back and make any changes there, except of course as I say bringing the idle back down to where it should be.

As an aside I will add that most folks don't "begin from the beginning" when dealing with carb linkages and settings. After setting the valve clearances and ignition timing (in that order) the mechanical settings of the carb linkages and balance are NEXT and must be dealt with very precisely, more so than many realize. Final mix adjustment is the last thing that you should be doing.

Jack

 

[wkilleffer]

My car doesn't have HIFs. Since it's engine is 18GK from 1971 or '72, it has HS4s.

They've been rebushed and reshafted, and I've put new floats in the floatbowls and new jets in the last couple of years. They seem to be functioning, but next time something goes wrong with them, I'll be scraping together my nickles and dimes to buy a brand-spanking new set.

 

[twigworker]

Will, don't do that. A properly overhauled set of HS carbs can be better than a new set out of a box. Personally, I wouldn't install a new set without going through them anyway. There are several folks on this board and on MGBE that do carb rehabbing either as a full time effort, part of their commercial shop offerings or just for the heck of it on the side.

If you have HSs go back and check the float levels. If they are too high the fuel will tend to percolate into the throats more than it should. See my comment above about ethanol enriched gasoline as it causes more of this than plain gas. Liquid seeks it's own level and if the gas is too high in the float chamber then it will be nearer to the top of the main jet opening and exacerbate the percolation thing. You might want to drop the float settings and then go back and readjust everything including the mix and and linkages. If you have old needles it isn't a bad idea to replace them with new ones either. Fixed ones should be either 5s or 6s, 6s being a step richer. A good middle of the road biased needle will be an AAA and should work well at your altitude. Make sure that the bias is toward the front opening of the carb when you insert the collars.

I haven't been on this board much lately, (understatement ) but I am sure that there have been many threads explaining in detail the steps that are to be taken regarding carb set up, linkage to final tweaking. You might want to do a search and find one of those threads that meets your needs.

Remember, a properly set up set of SUs is a thing of simple beauty! Use the money saved over a new set of carbs for a night on the town with the Mrs's. Good points to be had there. LOL

Jack

 

[rick_ingram]

<snip> There isn't much you can do about that other than....insert: Jack refrains to go off on an eco/political rant. LOL

<snip>

Jack

That's OK...wouldn't fly here anyway!

 

[wkilleffer]

Will, don't do that. A properly overhauled set of HS carbs can be better than a new set out of a box. Personally, I wouldn't install a new set without going through them anyway. There are several folks on this board and on MGBE that do carb rehabbing either as a full time effort, part of their commercial shop offerings or just for the heck of it on the side.

Yeah, I know there are several people who do carb rehab. I just thought that beyond a certain point, there wasn't anything else that could be done. The problem I've always had with this car is that there is no maintenance history with it at all beyond my ownership. Judging by the shape it was in when I got it, not much had been done anyway. But I don't know what all else might have been done to the carbs in the past.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]If you have HSs go back and check the float levels. If they are too high the fuel will tend to percolate into the throats more than it should. See my comment above about ethanol enriched gasoline as it causes more of this than plain gas. Liquid seeks it's own level and if the gas is too high in the float chamber then it will be nearer to the top of the main jet opening and exacerbate the percolation thing. You might want to drop the float settings and then go back and readjust everything including the mix and and linkages. If you have old needles it isn't a bad idea to replace them with new ones either. Fixed ones should be either 5s or 6s, 6s being a step richer. A good middle of the road biased needle will be an AAA and should work well at your altitude. Make sure that the bias is toward the front opening of the carb when you insert the collars.[/QUOTE]

Mine has one-piece plastic floats that can't be adjusted, and I'm using grosse jets. Float setting is pretty low, but I don't know the measurement off the top of my head.

The needles are old, AFAIK. They've never been changed out, even when I had the carbs rehabbed. So, would you go with the 5s needle? When you start talking about biased needles, you're getting into a topic about with I know nothing. Don't know much about SU needles at all. Where's a good source for them?

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]I haven't been on this board much lately, (understatement ) but I am sure that there have been many threads explaining in detail the steps that are to be taken regarding carb set up, linkage to final tweaking. You might want to do a search and find one of those threads that meets your needs.

Remember, a properly set up set of SUs is a thing of simple beauty! Use the money saved over a new set of carbs for a night on the town with the Mrs's. Good points to be had there. LOL

Jack [/QUOTE]

Yeah, there are alot of threads out there for setting up carbs. But it's like either they don't quite describe what I'm experiencing, or I just can't get my head around it. I've already figured out that the situations explained in the SU book with the blue cover pretty much only apply to a car that's all but brand new with a little wear, and it's been self-control that's kept me from binning the book in disgust. That whole bit with pushing up on the carb's lifting pin to raise the piston 1/32" after free travel has never told me anything. Front carb causes the engine to race no matter what the setting is most of the time, and back carb stalls the engine no matter what at least some of the time. So, I get irritated and frustrated.

 

[Nunyas]

If you're really worried about the heat soaking issue, here's what you can do to minimize it.

- Get an electric radiator fan.
- Also make sure the fan kit comes with a thermal switch.
- Install the electric fan on the front of the radiator.
- Wire the fan into the electrical system so that it does not need the ignition turned on to operate.
- Set the thermal switch to activate the fan between 180F and 200F.

Wiring a fan so that it operates while the car is switched off will allow it to continue cooling the engine. With the activation set to 180F/200F it will start running when the thermostat opens. Most fan kits that come with a thermal switch will keep the fans operating until the temp in the radiator drops to 160F, and when you wire it to operate with the ignition switched off the fan will cycle on and off in accordance with how much hot fluids enter the radiator.

Installing an electric fan in the manner I described will pretty much eliminate any heat soak you had.

I installed one on my '76 MGB last year, and the engine temps post shut down were dramatically different. Prior to the fan, the engine would heat soak and the temp would spike to above 210F within 10 to 15 minutes. After adding the fan, the engine temp would be down to 160F after 10 minutes.

It's my opinion that this set up works best with a 180F thermostat, because it'll allow the engine to stay warm enough to run normally after a short-ish shut down. The engine temp will stay near 180F. With a 160F thermostat, the engine temp will drop to at least 160F post shut down, and when I had a 160F thermostat I noticed the engine ran like it was "cold" after a 10 - 20 minute stop.

 

[wkilleffer]

Just as an FYI: I dug out the sheet from the person who rebuilt my carbs. It was Jim Taylor in OK.

Sheet says the 11/2 HS-4s AUD-465 for a 1971 MGB were fitted with AAL needles. He included another sheet on setup, which I'm sure I tried at the time, but probably for nothing cause I was still learning alot about tappets and timing. I was so careful getting those tappets just right the other day that I wondered if I needed to take the MCAT and go to med. school.

What does it mean to have AAL needles?

The heat soak is not the most worrisome thing to me. I came to associate it with some of the other problems my car was having and didn't know that it's just one of those things that can happen to an older carburetor equipped car.

Thank you,
-Bill

 

[twigworker]

You shouldn't have to put up with the heat soak symptoms. Properly tuned the car should not exhibit anything that you "have to put up with".

This thread isn't the place to go into too much detail about SUs, needles, yada, yada, so I will suggest that you buy a copy of "How to Build and Power Tune SU Carburettors" by Des Hammill. It is published by Veloce Books as part of their SpeedPro Series. You can find new and used copies on Amazon for not much money. There are other publications that go further into these devices but that one will get you started.

If you have AAL needles in the carbs that means you have "biased" needles. They are spring loaded as opposed to needles that are hard set into the carb pistons. I have never figured out what the code is for needle profile designations but just understand that the Letter and Number combinations refer to specific taper profiles that have been worked out by the factory to produce specific fuel delivery characteristics for specific engines under specific conditions. That doesn't mean that you can't fiddle around with different needles, it is just that the designers wanted to use a profile for a wide range of driving expectations.

Anyway, buy the book and put your thinking cap on. These seemingly simple instruments are VERY sophisticated in their working theories and are LOTS of fun to obsess over. LOL

Jack

 

[wkilleffer]

I'll check out getting a copy of that book. Sounds interesting, and probably more what I really need as opposed to the one with a blue cover that's been such a source of irritation.

I went out and set the carbs up according to the setup sheet Jim Taylor included with the rebuilt carbs. While I haven't been able to do much of a test drive yet, the engine is idling more smoothly and not as rich as it was before. His instructions aren't that much different than anything else I've seen, but seem more intuitive to me. I'll see what happens tomorrow.

 

[wkilleffer]

Hmmm... 2 copies available new and used beginning at $105.00...

 

[twigworker]

Something is weird. I don't think that they are out of print and the cover price should be $24.95.

I know the exchange rate has gone crazy but over a hundred bucks is unreasonable. Keep searching. There is an outfit down near Charlotte, NC, Gastonia I think, called Midlifecrises Books or something like that. They are small but they might have a copy. Sorry, I can't remember their full and real name so a search must be done. LOL

Jack

 

[wkilleffer]

Ok, found one titled "The SU Carburettor High-Performance Manual" in the Speedpro series and written by Des Hammill for $22.76 on Amazon. Not sure what's going on, and they seem like the same book.

 

[PAUL161]

I had a heat soak problem on a 79 I had and found out it was the fuel line that crosses the fire wall behind the engine. The engine was converted to twin SUs. Idling in traffic on a hot day, the engine would start to run rich. To solve the problem after researching it a bit, I put a insulating sleeve on the fuel line behind the engine and it never did it again. You can get them at any speed shop.