MGB What's the best way to tune an MGB?

[wkilleffer]

Hey everyone, been awhile since I stopped by.

Anyhow, I'm having some frustration with tuning my MGB. It's a 74 with an 18GK engine and HS carbs and a Flamethrower dizzy and coil. Kind of a bit mongrelized, which could be part of the problem, but it's run better before, and it's not like these engines are rocket science or anything like that.

It's been pretty cool down here in southeast TN, and it just seems like my car runs pretty crappily at outdoor temperatures of 50deg or lower. It's hard to get it tuned where it's not running too rich or too lean, and the cold air seems to keep the engine from reaching full Normal operating temperature.

I've wrapped the oil cooler up, but haven't done anything to the radiator. There's no insulation under the bonnet.

Truth be told, I find the whole tuning process to be an exercise in frustration. I don't know anyone around here right now who could help me out.

I know some of the basics of tuning and balancing the carbs, but no matter what I do, the exhaust seems to have that poppy, irregular sound to it that I've been told isn't correct. Last time I checked the compression back near the end of last year, everything was fine, and I've not driven the car much since then. I check the timing before doing anything to the carbs, and it's where it should be.

So, what am I doing wrong? I'd drive the car a whole lot more even in cold weather if I didn't think I was slowly damaging it or something due to an improper tuning.

Any suggestions would be appreciated.

Thank you,
-Bill

 

[bob67bgt]

First thing I would do is get at least a 180 thermostat in there. Get it up to operating temps the way it should be done. Then be sure the mechanical weights in the distributor are advancing. With the newer distributor is probably ok but check it anyway. Be sure the valves are set to spec and then get the carbs sync'ed and adjusted properly. Its going to be something easy, just finding it may be more difficult. Bob

 

[DrEntropy]

Hi Bill!!!

If you've followed Bob's "order of events" the carbs ~should~ fall in place. Is it rich as the engine gets to temp (15 minutes run-time, at least)? Jet tubes return "home" with choke cable pushed all the way in?

How old is the fuel you're trying to burn?

I know, iknow... too many questions. :wink:

Just tryin' to get a feel for the overall picture.

 

[wkilleffer]

Ok, following up...

I *think* it already has that 180 thermostat in it. It's a pretty new 'stat, hasn't been subjected to overheating or anything like that. Could go back to 2007 as that seems like when I had the head rebuilt. Kinda having to fly by the seat of my pants on a few things as the maintenance file isn't handy right this sec.

It gets really hot and humid here in the summertime, so 180 might have been as high as I was willing to go. But even with a hotter stat, which I used to have, maybe 190? Even with that, it seemed really reluctant to get up to the Normal mark on the gauge in the cold of winter. In summer or just warmer times, it's not a problem.

Did the valves before it got too cold last year, and was trying to be as precise as a surgeon. Can check again, of course, but it's getting cold outside... Now, what's this about being able to set the valves when they're not precisely open or closed? Seems like John Twist talked about that once upon a time, bumping the starter while adjusting, but didn't make sense to me. Haven't driven much since then.

Fuel could be over a month old, but it's almost gone.

Ok, balancing and adjusting the carbs... *That's the hard part for me!!!*
I know how to do it according to the books and stuff, and I can usually get them balanced using a Gunson Carbalancer. But it's the mixture adjustment that gets me everytime.

Ok, the book seems to apply to engines and carbs that are brand new. Not a car like mine. All that stuff about "push the lift pin up 2/100" and listen to the idle" doesn't seem to work for me. I try, but it doesn't seem to explain anything. Usually, by the time I get thru trying all that s%$#, I'm about ready to wring someone's effn neck, possibly even my own.

Want to see me frustrated and losing my temper? Tell me to adjust and balance your carbs...

Ok, anyhow, there's the sticky wicket. Can't get to normal operating temps and can't adjust the carbs. Any ideas to cure it?

Thank you,
-Bill

 

[DrEntropy]

Sounds like all else is fine, my attack plan would be to set the jets to "zero" (wound right up into the carb body 'til the top of the jet was the same level as the stage), and drop each one the "twelve flats". Start the beast, blip the throttle periodically to keep it "clear" (put a piece of cardboard in front of the rad and WATCH th' gauge) until up to temp, get the idle to just under 1k, 750RPM is a good enuff speed... use the balancer to adjust draw to even on each (one side linkage free'd up so they're independent) and try it out after securing the pinch clamp again. If it snaps and bucks a bit, lower each jet one flat. Keep the idle speed to around 750 max. if possible.

If it gives you grief after that, there's something else goin' on.

 

[wkilleffer]

Weird... the jets don't seem to screw into the bodies at the same rate. I bought them new just a couple of summers ago, so they should be matched. Front carb is labeled either AUC or AUG 1300 and the rear is something like AUG 1346. Problem? Not sure if I've noticed that before, and I've had the car about 10 years.

 

[DrEntropy]

The jet threads shouldn't vary from one unit to the next. Two turns is two turns, should be the same on both. If you remove pots, springs and pistons then use the "depth" end of a vernier caliper are they really different? That'd be a good test.

IIRC you did all the vacuum leak checking a while back with a squirt bottle or such.

I know it can be frustrating, but to paraphrase Sherlock Holmes: process of elimination. I think you're 9/10ths there.

 

[wkilleffer]

I think I got it...
One of the jets has a green band on it, and the other has a red band. Not sure if that's a big deal or not.

Seems like two flats on the back carb were the equivalent to one flat on the front. Wish I knew why, but may not matter.

Got the car started, warmed up. Balanced the carbs and left the jets at "12 flats" down. Tach is roughly midway between 500 and 1000. Exhaust note sounds much more regular. Exhaust smells a little gassy. I don't have any gas-testing equipment.

Haven't been able to do a test drive yet, and still wonder about the whole temperature thing. But feel pretty optimistic so far. Makes me wonder just how long I've had it set up wrong.

Thank you for your help.

-Bill

 

[DrEntropy]

Go conservative and dial the front one only, back up. One flat at a time... drive to test.

If the front is fat (rich) the idle will mellow out. If it gets worse, lower that one and lower the other.

A VERY useful tool instead of EGA (exhaust gas analyzer) is the ColourTune thingie. Two are better'n one. Not cheap but highly effective for dialing dual carbs in on our low-tech engines.

Glad you're making progress!!

 

[bob67bgt]

Bill,
The green and red bands are normal on the jets. Happy to hear you are making progress..Bob

 

[beachbumbarry]

wkilleffer, Have you checked your dash-pots for oil. If the car has been sitting for a long time, you could pull off the vacuum chambers and clean everything and add new oil to the dash-pots. When you lift up on the piston with you finger do both carbs have the same resistance? Just my 2cents, I'm not an expert but that gave me problems once.
BarryE

 

[davester]

I'm surprised that nobody has asked when your carbs were last rebushed. In my experience, the number one cause of difficult to tune carbs is air leaking past the throttle shafts. If the carbs haven't been rebushed and throttle shafts replaced in the fairly recent past I can guarantee that you'll never get it tuned properly. One quick way to check whether there's an air leak is to spray some carb cleaner at each end of the throttle shaft on each carb. If that has a big effect on RPM then you have a leak that is causing you problems.

 

[wkilleffer]

I'll try the spray leak test soon. Seems like the carbs were rebushed and shafted back in 2003. Haven't put that many miles on the car since then, though I can probably post an exact figure tomorrow after a look at the glovebox log book.

We had some balmy days last week. Car seemed to run well, though at idle it smells a little gassy. I tried the front carb-up one flat trick, and the idle seemed to get a little rougher. So, I left it as-is.

Now, a fault seems to be once the car reaches NOT according to the gauge, there's the chance that the fuel will start to boil. The carbs are insulated, which has made a difference. But the fuel line in the back of the engine compartment is not. Any ideas for that?

Thank you,
-Bill

 

[wkilleffer]

Ok, I haven't driven the car today, but according to the log book, the carbs were rebushed and shafted about 13k miles ago. I had that job done because the car had become almost undriveable. The shafts were leaking so bad that the carbs wouldn't stop backfiring.

 

[DrEntropy]

Then the carbs themselves aren't the issue. Gaskets, joints, etc... Do the "spray test" around the fittings.

 

[wkilleffer]

I didn't spray test, but I did check the tightness of the nuts that hold the carbs on, and they were a couple of flats loose.

The problem that came up today happened I think due to fuel boiling in the lines due to the heat of the engine.

The carbs are insulated, so I'm inclined to think it's the fuel line at the back of the engine compartment. The engine wasn't running hot. It was at or slightly below N most of today's driving. Sometimes the idle would drop really low when stopped, and it was often difficult to start after sitting for short periods of time. Or, I'd get it started, only to have it stall at the nearest stop sign. I had a coworker with me, and that was a little embarrassing.

 

[MGjoc]

Fuel boiling in the lines is "vapor lock", which is not very common at all. I've only had it happen to me one time and that was in Aug in California in the mountains in my TF -1500 while driving really hard. If there is fuel flowing thru those lines, you don't have vapor lock. I wish I could offer more than that.

Cheers - Dennis

 

[wkilleffer]

Well, it just sounds like there are moments when the carbs are getting too warm.

 

[DrEntropy]

I'm for checking both fuel delivery pressure and VOLUME.

 

[JPSmit]

Then the carbs themselves aren't the issue.

Bill, I wasn't sure whether to post this on this thread or your vapour lock thread, but, everything you are describing points to too much air in your system. Here is some of the issues I had with mine. (Midget)

1. started, ran fine, sounded rougher and rougher but OK at speed, then wouldn't restart. That turned out to be that the manifold had worked a little loose. It worked fine cold, but, as soon as the car was hot the gaps expanded open and I was done. (I even installed clear fuel line - which had lots of bubbles which I thought was boiling - wasn't)

2. once I corrected that, it ran much better, but, still not great. when I got mine roadworthy (before the manifold loosened) it would start but not accelerate - it ran out of gas. My mechanic thought it was an issue with the carb being too big - solved it by putting a plate over the air intake. problem solved. Then I was driving and smoke started coming from the engine - turns out I had a tiny leak in the fuel line from the float to the carb and it was dripping onto the catalytic converter. fuel out means air in when vacuum applied, it couldn't get the mix right. replaced hose, sealed leak and didn't need the plate anymore.

3. Then I went out New Years Day - 27 deg or so, lots of cold air. ran rock solid at 800 rpm when I started, drove 1/2 an hour and it idled rock solid at 1200 rpm. got home and checked the carb shaft and sure enough, I could do morse code.

So, I have just had the carb rebuilt and weather permitting, later today, I will see how it all pans out.

I know that you had the carbs rebushed but, don't want you to assume that they are all right - don't want to question the rebuilder but, I've had more than one part that I "knew" was right and wasn't.

Bottom line, your car shouldn't be running the way you describe but I don't think it is a fuel issue. One way you could test is to partially block off the carb intakes. This will restrict the air intake and I think the car will run stronger.

The way you describe it may be the kind of issue I had (Manifold) where heat is opening a gap as you seem to have the issue restarting (not initial starting) At any rate, you shouldn't have to put up with erratic behaviour with these cars.

good luck and keep us posted