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MG MGB Technical - Rough Idle and some questions

Hi Everyone,

I have a 1979 MGB with HS4's fitted with K&N filters and a mega jolt ignition system as well. I have the standard MGB ignition timing map loaded up.

My problem is I have a rough idle to where it seems like a cylinder isnt firing. I checked all wires and plugs and everything is sparking though. I have video as well if you would like to see.

I can get it to idle down to 600-900 but its always rough. Its not as smooth as it should be at 1200 as well.

The spark plugs are showing that it is running rich. But...

I did John Twists method of lifting the piston and getting a raise of 50rpm then drop back down on the HS4's, they are balanced as well.

The car revs great and drives fine but I have that rough idle problem which of course makes me assume that it should be running smoother.

I sprayed carb cleaner over everything and no signs of a vacuum leak...

I take it I should lean out the carbs a lot ?

I had the mixture nuts set 13 turns out to start as my base but I had to keep enriching the carbs to attain the small rise and fall in rpms. Otherwise the car wanted to stall itself out.

Is there something Im missing here? What needles should I be running?

All choke linkage is not interfering anywhere and the fast idle screws are not engaged at all... Crank case ventalation system is plugged up on the carbs and open on the engine.


I also started to get the little bubbles between spark plugs 2 and 3 indicating a head gasket leak. The oil is fine and the coolant system purged itself once due to too much pressure but everything was fine after that...

I had originally replaced the head gasket before the car sat for a while and had the head milled etc so there is no reason that I can see that there would be a crack in the head etc... Just a case of the dreaded crappy gasket failure that Ive heard so much about.
JRB Mr

I have also replaced a bad engine mount a while back.
JRB Mr

The increase in revs when using the lifting pin should be *momentary*, dropping back even while you are holding the pin up. If the revs stay up it is rich.

Did both carbs have to be enriched further to get to what seemed right? If one carb has to be turned more than the other that indicates a problem with one or other carb. If both carbs needed it then there is something else wrong, possibly technique.

It needs practice and experience to use the lifting pins correctly, the change in revs is very slight on HSs. Be thankful you don't have HIFs, it's almost subliminal on those. To train yourself keep adjusting each way until it is detectably weak, and detectably rich, reducing the number of flats to get from one to the other as you tune your ear in. With HSs you need to run the engine at a fast idle for several seconds every couple of minutes to 'clear its throat' otherwise you will get false results. If you are still having trouble identifying the sweet spot, then count the number of flats between detectably rich to detectably weak, and set it mid-way between them.
PaulH Solihull

Hi Paul,

Both carbs had to be enriched further. They are pretty identical but I will double check how many flats im out.

I guess a follow up question is does having the carbs running rich really cause an engine to shake that much at idle?

I also just saw on the archives someone had a problem with idle when their fans kicked on. I have mine hardwired on for the time being. Since I have the megajolt unit I can test by using another source of good 12+v that I was originally running the car with before I wired it into the car. I do remember it idling rock solid when I had that power unit attached... Hopefully thats the case as I hard wired it right in afterwards so I could drive her around the block.

Regards,

James
JRB Mr

Has this rough idle occurred since you installed the Megajolt. If so I would check the ignition map at low revs. I'd also check the Megajolt site for any info on using it with the 'B. They used to have a forum there. I fitted a Megajolt to my midget when I had it and seemed to remember just how solid the idle was. Have you a timing light to check the advance a low revs?
Steve Church

Hi,

The megajolt is pretty rock solid. The timing light matches whats being shown on the computer. Im using the standard mgb map that someone loaded up, I played with the timing as well to see if there would be a change but no change at all. Loaded up the map that comes with it as well...

The rough idle was there before the megajolt but after fitment of the SU's as they werent setup. I bought them as a freshly rebuilt kit.

I have fitted HS6's on my 80 TR7 as well and did not come across these symptoms other than running a little rich from my tuning.

-James
JRB Mr

Unless I too am missing something I can't see any reference to balancing the carbs! This needs to be done "on throttle" and at idle. Only with matched (balanced) air flow is adjusting mixture a worthwhile exercise.
Allan Reeling

The fact that the engine ran fine with an outside source of electrical power is something to look into. It may be that you are not getting enough voltage to the distributor, especially since the later cars are wired for 6 volts at the coil instead of 12. RAY
rjm RAY

Ray that is my concern as well... Should finally be able to check tonight.

Alan, thanks for your concern... Very well balanced... Its always the first thing I do since its sooo easy to do.

JRB Mr

once I check the power supply I can go down the megajolt path as well as sometimes its a symptom of a bad coil as well although I checked all spark plug wires and spark plugs....
JRB Mr

My 73 rocks if the idle is turned down low enough, and it got worse with the switch to unleaded. With that and the tendency of HS carbs to gradually bog down if left idling a long time I have mine at about 850-900 instead of 750. Similarly the V8 with its twin electric fans slows with them running, although stays smooth right down to stalling, so I have that set for the recommended idle speed with the fans on, i.e. slightly higher with them off.
PaulH Solihull

Ok so its not the power source... I leaned out the carbs and ended up using the baseline setting of 13 flats out and it seems to like it there.

Still not idling the way it should. You can feel it in the car when your in the drivers seat. It shakes.... Although not as bad visibly as before.

Im going to have to buy another coil and swap it out to check.

The way I checked the spark plug wires was to just pull the plug real quick while the car is running and I can hear it firing away.

Spark plugs I checked all on one plug but just swapping the plugs out and hitting the starter and having the plug arch to the engine block.

Im going to have to check all wires again but really dont think its the case.

Anything else I could be missing here?

I turned the idle up and its still not acceptable.

I have a video I can upload if you want to see.

-James
JRB Mr

Any misfire in the exhaust - regular or irregular?
PaulH Solihull

http://www.youtube.com/watch?v=JjYIc0I7rxY&feature=plcp

Here is a video of it at a low idle
JRB Mr

The previous post was when I had it running rich and idling at a low rpm.

I have a new video from last night after I set it to 13 flats out and turned the idle up but its quite dark.

http://www.youtube.com/watch?v=Vleq7CN3WiA&feature=plcp

JRB Mr

Burnt valve possibly or HG failure, have you done a compression test?

Bob
R.A Davis

I have bubbles inbetween cylinders #2 & #3

Ill have to pick up a gasket this weekend...

JRB Mr

No coolant in the oil...
JRB Mr

Head is coming off... Compression is horrendous... Not getting a reading on #4 Cyl... Kind of scared... lol. No idea why #4 has no reading.
JRB Mr

It's only firing on three, so not surprised at your subsequent posts.
PaulH Solihull

Yup... Ugh... Going to pull the head tonight... I hope I dont see anything ugly. Im hoping that the studs were streched and I cant get a good seal. Going to pick up a cometic gasket if disaster is averted after pulling the head. I dont want to get ahead of myself though.

JRB Mr

Go with a Payen or Fel Pro head gasket. These are the best head gaskets on the market today and are reasonably priced. If you can budget it, replace your head studs with ARP units. Your present studs are 33 years old and have expanded and contracted countless times. They will not hold proper torque and the new head gasket will fail in short order. The ARP studs are well worth their price. I write from personal experience. I'm running a supercharger on my '67B with no problems. RAY
rjm RAY

Common cause of no compression is no valve clearance, due to recession and poor maintenance.

Contrary to popular opinion, I've never used anything other than OE head studs, and I do not replace them, and I do not have gasket failures. And they do hold torque. Obvious damage is a different fettle of kish.
Payen/FelPro gasket will be fine.

FRM
FR Millmore

Hi Ray,

Look up cometic... They are far superior. I might go felpro though if they have it locally this weekend if everything looks ok.

FR, we will find out when I finally pull the head. I had adjusted the valves and fitted a new gasket when i had the head off the car. Had it milled, so their might be a difference in flatness from block to head which is why the cometic gasket is worth the money. Poor maintenance isnt suspected but hey you never know.

Ill worry about studs after I actually see whats going on. Price of arps are just crazy. Might be able to source similar studs with the same build quality and tensile strenght.
JRB Mr

I've seen too many head studs snap off when a head is being re-installed and this is at less than 50lbs. of torque. The stock studs, on the market today, have a bad reputation. I invested in the ARP units so that I wouldn't have to do the job a second time. Reusing ancient hardware is a recipe for disaster. It's your money, spend it as you see fit. In any event, good luck in your endeavors. RAY
rjm RAY

The Payen head gasket has proven to be the best for the MGB, and the price difference would go towards ARP studs and it would be bullet proof.
I have found that some Felpro engine gasket sets have the Payen head gasket but not all, some just a plain composite. (both made by Fed Mo) It may just be the ones here in Australia but I would stick with Payen to be sure. Denis
Denis4

Going to check compression again today after putting some oil in the spark plug holes to seal up the rings and see if thats the problem and the compression readings go up. I hope its not...

Im just saying I might be able to get similar studs made for cheaper than the arp bolts with the same tolerances through the company my father works for.

Keep cometic in mind...
JRB Mr

OK so it was the valves way out of adjustment on cyl 4 !!! Combined with a compression gauge not reading correctly.

I took off the valve cover gasket and adjusted the valves all the way out on cyl 4 and compression jumped right up to where it should be. Thank god it wasnt worse.

I used another gauge that is not screwed into the spark plug thread but rather has basically a rubber stopper you hold up to the spark plug hole.

There are the readings I got

Cly 1 145
Cly 2 125
Cly 3 165
Cly 4 10 - before adjusting valves

After putting oil in the spark plug holes.

Cly 1 220
Cly 2 200
Cly 3 230
Cly 4 220 Originally was 30 before adjusting valves.

Since i do have a known leak in regards to bubbles where the head and block meet in the corner of Cly 1, then in between cly 2 and 3... That can account for some of the loss of compression, have to adjust all the valves as well.

Does that sound about right? Also how are my numbers with the oil in the head? Id average in a 5 psi variable with the gauge...

Ends up it was in fact user error!!!

Thanks for your help,

James

JRB Mr

User error in those horrible valve adjustments... No clue how that happened.
JRB Mr

Comp readings way high for late engine, Sounds like you put way too much oil in for the wet test. I posted this on MGA board lately:
"What is the safest type of oil to use for the "wet" compression test? Is standard 5/30 motor oil OK?"
>> Any motor oil. On most MGA and other engines with dished pistons, a common error is to squirt the oil in such that it just sits in the dish, which accomplishes nothing as to ring sealing, but does give erroneous (high) compression readings. leading people to conclude that it has improved and therefore you need rings - NO!. You must squirt the oil against the cylinder walls so the rings seal, and get as little as possible in the dish.
In fact, I rarely do the "wet" test, because of this problem, and because worn rings are pretty apparent from low comp + oil use + smoke."

You replaced the head gasket and adjusted valves. Once it got hot, the gasket settled and the valve adjustment got tighter. The nuts also got looser, so you got bubbles. You may be able to save the gasket at this point by retorquing hot, AND ALWAYS check valve clearance after retorque! If all appears well after retorque and running for few days, retorque and readjust again. You have somewhat compromised the gasket's sealing by having coolant and maybe oil leak across surfaces which should be dry, but I've saved many.

FRM
FR Millmore

Hi FR,

Yup, way too much oil in there....

Thanks for the info!

I'll let you know how the gasket turns out and will definitely post some video of what should surely be a nice idling B.

I have no problems putting on another gasket but I will see how everything turns out with the one fitted at the moment and follow what you said.

Thanks,

James
JRB Mr

James,
OK, gettin' there!
Another point:
"The way I checked the spark plug wires was to just pull the plug real quick while the car is running and I can hear it firing away. "

Checking for spark this way (how far will it jump?) is the prime cause of HT failures - cap, rotor, coil. Once a spark has been created it WILL go someplace. The harder it is to get there, the higher the coil output goes. A system that will fire at 10KV normally (usual), and 20KV under full load, may reach 50 or 60 KV, and trash the whole system. If there is not a reasonable path to ground through a plug gap, it will punch a hole through the weakest link in the HT insulation. If you are lucky, it will bite you and you'll drop it so it shorts to ground. Otherwise, it punches holes through the plastic or other insulation. This is the common cause of failed HT, and often is caused by bad wires which are effective disconnects. More confusing still, such damage is cumulative and progressive; you can abuse the system today, but it may not fail for 6 months or two years.

FRM
FR Millmore

Hi FRM,

Point taken. Ive shocked myself pretty good when using a colortune plug in the past.

I am now running a megajolt system with a coil pack. The brand new wires (I know they can be bad out of the box) are for a ford escort and have a long rubber piece to fit down into the cylinder head of the escort engine. Ill have to attach a link as I havent had enough coffee yet to fire the neurons off in my brain to describe it accurately. Its heavily insulated. I took it off for at the most 3 seconds. I will keep in mind what you said.

The funny thing is I kept thinking the rough running was due to the distributor but in fact im pretty sure it did absolutely nothing wrong! Took the head off, had it milled, put on new SU's and kept thinking it was timing or the carb. Then I got wrapped up in the megajolt project thinking it would give me a solid baseline and was as cheap as a new distributor. Too many things going on without doing a thorough inspection of everything!!!

It was a lot of fun setting up the mega jolt system though. But I could have had the car back on the road at least 2 summers ago but hey thats how hobbies go. Cant believe Im That guy right now but hey, at least we are making progress.

Thanks for the help,

James

JRB Mr

James-
Three seconds is a lot of sparks!
The better the wires are, the more likely it is to damage something else. I don't know anything about the Megajolt, but sooner or later there is a device that makes sparks, aka a coil. That's the fail point, if there is no cap & rotor. Some coils have voltage limiting spark gaps to protect them, and the Megajolt may have that. These normally have limiting voltages somewhere above rated spark output, so if it says it is a 50Kv system, the gap voltage might be 60 or 75Kv - more than enough to kill wires - or see below. In that case, any failure would be after the coil.


A note for guys with their own electronic modifications: it is said that HT voltages are less than good for pacemakers and the like!

FRM
FR Millmore

adjusted the valves and its running a lot better.

http://www.youtube.com/watch?v=EkmLLecp5uo&feature=plcp

http://www.youtube.com/watch?v=6ahKlJ2Z5AE&feature=plcp


Tappets still loud.

I currently have the fans hardwired in so they are running as well...

JRB Mr

This thread was discussed between 29/05/2012 and 18/06/2012

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