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Cold weather engine tuning

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wkilleffer

wkilleffer

Jedi Knight
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My apologies if this is an FAQ that could be found elsewhere.

The weather here in TN is getting more nippy, with highs in the low 60s, and morning temps sometimes in the low 40s or even 30s. I don't yet want to stop driving my MGB, but the cold seems to be causing it some problems.

I use the choke to start the car, and it idles around 1700, and seems to be run ok. It might stumble a bit, but not always. Once the temp gauge gets around to the N, I release the choke. That's when all the trouble starts. It stumbles on accelleration, and carb backfires quite a bit. It will stumble and backfire after shifting gears. Also, it looks like getting to the N on the gauge is a struggle. During the summer it goes a bit higher, but not in the cold.

My commute's kind of short, which might aggravate the situation, but I've noticed it to some degree as long as I've owned the car.

Here are a few things I did after yesterday's drive:

Enrichened both carbs by two flats.
Slow idle speed is around 800-850.
Filled dashpots with Marvel Mystery Oil to bring about a richer accelleration.
Wrapped the oil cooler in cardboard and duct tape.
The car has a 190deg thermostat.
The timing's at around 17 or 18deg BTDC, and might could be advanced further.

Any ideas and suggestions would be welcome. I know there are people out there who have little or no trouble in the winter.

Thank you,
 
If you have problems in the cold when engine temp is up to normal you will have the same problems in the middle of the summer. Same rules apply.
 
[ QUOTE ]
If you have problems in the cold when engine temp is up to normal you will have the same problems in the middle of the summer. Same rules apply.

[/ QUOTE ]

Well, yeah, but it ran better in the summer. Also ran hotter. It sometimes seemed to bog a bit under quick accelleration, but not like this cold weather trouble. I'm stymied.
 
Can't help much with B's. They strange little critters but Doc and Tony will be along soonish and will have a few ideas for sure.
 
If it's chuffin' back thru the carb it sounds like a lean-run condition. The two flats should have changed the stumble characteristics. Did it?

Are you static timing or running with a light?
 
[ QUOTE ]
If it's chuffin' back thru the carb it sounds like a lean-run condition. The two flats should have changed the stumble characteristics. Did it?

[/ QUOTE ]

Not much. Maybe even none at all. Maybe less outright backfiring.

[ QUOTE ]
Are you static timing or running with a light?

[/ QUOTE ]

I'm using a light, testing at 1500 rpms.
 
Hm. IIRC you've already gone thru the dizzy and are sure the advance mechanism is working. Weird. I'd suggest static timing it at 8 degrees BTDC anyway and try it, just as a test.
 
[ QUOTE ]
Hm. IIRC you've already gone thru the dizzy and are sure the advance mechanism is working. Weird. I'd suggest static timing it at 8 degrees BTDC anyway and try it, just as a test.

[/ QUOTE ]

The dizzy's a new flamethrower. The advance works, and doesn't leak.

How do you do static timing? I've never done it, and may not have the tools to do it.

It's conceivable that the manifold could be loose enough to allow a leak, but doesn't seem likely. The carbs are fastened down tight. The throttle shafts and bushings have been replaced and are still tight. The vac hose on the manifold to the dizzy is new. The other vac port on the manifold is sealed with a plug which could be removed if needed. The other port had been used for emissions and has been removed, and the hole sealed with a bolt.

Thank you,
 
Think just for the heck of it I would change the condencer before I did anything else.
 
[ QUOTE ]
Think just for the heck of it I would change the condencer before I did anything else.

[/ QUOTE ]

Don't think a flamethrower has one. They're supposed to be a Pertronix in a new dizzy unit. The old dizzy I took out was pretty well trashed. I installed the new one in September.
 
I vote for a leaking manifold gasket, carb spacer gasket or some other situation causing a lean condition. Get the car running and spray carb cleaner on supected problem areas. When the idle speeds up, you've found your problem.

Dave
 
And it ran great after you installed the new unit??? OR??
 
try incrementally turning the choke down as the car warms up. You may have full choke for too long and car is then loaded up. Just a shot in the dark, I know I run better when i do that on the 30f morning drives to work.
YMMV
 
[ QUOTE ]
And it ran great after you installed the new unit??? OR??

[/ QUOTE ]

It ran much better. It was more likely to try and overheat with the old unit. It never did, but the engine temps seemed less regulated and it used more gas. Getting the carbs anywhere near to properly tuned was an infuriating exercise. Now, they're somewhere around close, but not quite.

It seems to run ok when it's hot outside. Makes me think I've run it too lean all along, and as it gets better overall, more hidden defects get exposed.
 
I also vote for leaking manifold gasket or carb spacers...Also try a new coil, thats an easy thing to do and can solve an number of these problems.
As far as running too cool goes cover up half the radiator with cardboard, if thats too much cut some away(or add a bigger peice) until it runs at normal temp. My wife drives her MGA all year round and right now I think she's got about half of the rad covered, just remember on those warm days to uncover it or it'll run too hot.
 
Is the choke cable too tight & not allowing the choke to operate fully (all it takes is a hairline opening)?...are the 'discs' centered?
 
Good point Tony, what ever it is I bet it has been a continuning problem all summer but was masked by the heat.
 
Could a crack in the heat shield cause this? The spray test seemed inconclusive, but I just noticed a crack that runs from the top of the heat shield to the edge of the right side of the rear carb. It runs about halfway down that edge.
 
If you spray it while running it will tell you if it's leaking.

BTW: Forget trying to static time an electronic iggy. Not practical.
 
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