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MG MGB Technical - Rough starting & Misfiring while Cruising

My 72B has developed some new starting and driving problems, and I'm wondering where to look next.

First: the 1/2 tank of gas left over from last Fall has carb cleaner in it, and gas stabilizer. It is 10% ethanol blend. So gas is roughly 6-7 months old.

Second: car ran essential just fine last Fall when parked for winter. Car needed full choke to start when cold, but it just barely cranked and it fired right up and idle just fine. It was a "tap" the key, and it was running, barely needed the starter it was so quick. As engine warmed, choke was slowly opened and it was smooth and took throttle without problem from start to fully warmed up.

This Spring it requires full choke to start, requires 5-10 seconds of cranking (< 1 second last year) to start, misfires if given throttle from cold start until fully warmed up and choke is open. The test to see if choke can be partially opened is to see if it takes throttle, if it does I open choke slightly and repeat.

So the first point to make is that the rough starting was there before I did any alterations this Spring.

Now to the Spring alterations.

I re-installed the gulp valve, as I've detailed in an earlier posting. Prior to this - port for the gulp valve in the intake manifold was plugged, the charcoal canister was still connected and hose went to valve cover, but the hose coming from the cover plate on side of engine vented to open air, so there was no vacuum drawing air from canister to valve cover.

With gulp valve in place, and hose from engine side cover going to top tube on gulp valve, and the bottom tube on gulp valve plugged, the car still had the cold start problem described above, but as I explained in the earlier posting, the car ran just fine when warm and smelled cleaner and appeared to run fine with the gulp valve installed. I haven't had time to determine if there are no longer gas fumes in trunk, but it appears to be gone.

So the second point to make is that when the gulp valve was intially re-installed, car appeared to be running fine, but still had the cold start problem.

NOW - the car continues to have the cold start problem. Yes, it starts, it is this recently new misfiring when trying to give throttle while warming up the engine and slowly opening choke that has me confused as to cause. Oh, and the warming up process is done while setting in garage, not driving to warm it up.

But now it won't run properly on the road. It accelerates just fine, when warmed up it is glass smooth and sounds just fine - at idle.

But when crusing along at a steady speed the engine is popping-misfiring-jerking the car. The misfire is enough to jolt the car. It appears to more of a popping/misfire sound from rear of car, but I sense it is also coming from engine, although more to the rear. It is a single jolt/jerk motion and it clears for a few seconds and does it again.

Acclerating at any highway speed is fine, quick response and smooth. It is when using a flat throttle speed that the hiccup - popping happens. One can go for awhile and it is fine, then it starts in and may last a mile or more, then smooth again.

My first thought was a vacuum leak. Last night I sprayed carb cleaner on every vacuum line, fuel connection, canister, carbs, floats, gulp valve, etc., with no noticeable change in engine speed.

I've checked inside distributor cap to see if there was carbon tracking or mositure, nothing.

I've checked in the dark for any sparks coming from plug wires, nothing.

I've ruled out fuel filter plugging as it is only a couple thousand miles old, same for fuel pump, besides it takes hills and rapid acceleration just fine.

I'm thinking this is more than just old gas, but I may draw the remaining gas out, and replace with fresh. I'm also thinking of disconnecting hose from side of engine and plugging gulp valve. But my recollection is that it ran fine initially after gulp valve re-install.

Your thoughts? Its been a frustrating day so far. Only made it half way to work before turnig around and going home, to switch cars.

Thanks for patience and any assistance.

BobA
R.W Anderson

You may have fouled the spark plugs. Under light pedal they fire okay but with increased demand they could have a fouling coating that leads to misfire,This coating could have been caused by extended idling during your warm-up.
Chgampon spark plugs had a warning leaflet one time explaining how the coating on the plug's ceramic becomes glazed when facing higher power demands. sometimes it can be self cleaning by feathering the pedal up to speed.
Sure cure is change the plugs.
Sandy
SANDY SANDERS

Could be bad fuel I suppose, although I've had (UK) fuel older than that over winter and it hasn't caused any problems next spring.

Could be fouling of plugs, but that should clean itself once the car has started, glazing from higher power demands doesn't seem likely in this instance although again fitting new plugs may help.

Looking for cracks or tracking in a cap isn't reliable in my experience, but clipping a timing light onto the coil lead and each plug lead in turn is - if you can reproduce the misfire or iffy starting at a standstill. Look for any erratic flashing on one or more leads.

After that it is a matter of trying to isolate any of the emissions stuff, then (as long as you have removed any possibility of ignition issues, which are the cause of 99% of carb problems) I'd be setting up the carbs from scratch and seeing if there were any oddities raised during that.
Paul Hunt 2010

Spark plugs have <5,000 miles on them, but were installed in '07, so I'll replace them anyway. I think I'll take car out for a road test where it will most certainly act up, then I'll immediately pull plugs to see if there a sign of any problems.

I don't suspect plugs, as under steep incline and all modes of acceleration car performs without a single hiccup or misfire. I don't suspect plug wires for same reason. Car performs fine under load, it just doesn't like crusing along at a steedy speed. I'll do plug wire test anway.

Car never misfires at idle once engine is warmed and choke is open. Car also does not misfire during revs without load, parked.

I'd like to do an acutal volage at plugs test, but don't have equipment for high voltage. Just to see what is getting to plugs. I suppose I could do the ole' plug wire end with metal rod to engine block test, and watch for length and color or spark, but I usually do something wrong and I'm the test instead.

I will add though that prior to any of this I did lean the carbs 1 flat, to try to improve gas mileage; but car drove just fine after that and that was months ago.

I may re-balance carbs after the rest of this is done, just as a check - but I need to adj valves first. I was hoping to clear up the problem before starting other things.

Thanks so far.
R.W Anderson

Another thought would be that the the diaphragm in the vacuum capsule has developed a leak. If the the vacuum advance capsule has ruptured, it can cause the timing to advance and retard, resulting in the cruising on flat ground problems that you are experiencing. RAY
rjm RAY

Try disconnecting the vacuum advance and plugging the manifold (I assume) port. If that stops it it could well be the spark trigger point out of phase with the rotor being by the cap contact under high levels of vacuum i.e. light throttle cruising, and idle if the source is the manifold. I.e. it could be that the rotor hasn't reached to cap contact yet when the spark occurs and is being asked to jump too big a gap.
Paul Hunt 2010

I've always been suspicious of the vacuum advance diaphram on the 72B, and I've never thought of going to someone else's and doing what I find suspicious on mine.

And that is... I remove the vacuum hose at the intake manifold when engine is running and I get the immediate change in rpm. I plug port on intake manifold and rpm changes, thus confirming I have vacuum and its affecting the distributor.

But I'll turn the engine off, hose still connected to distributor but disconnected from intake manifold. I blow in and I'd swear I don't have resistence. I suck air back and I'd swear I don't have resistence.

To me this implies a leak, but vacuum appears to be getting the distributor to respond. So I'm stuck with being suspicious.

And being a gentleman and in consideration of manners and etiquete and all, it just wouldn't seem right to come up to a strangers B and start blowing and sucking on hoses. They may get suspicious of my intentions!

Oh, and I did buy a new diaphram for my PCV valve (which I've been incorrectly referring to as the gulp valve - I've always been referring to the valve that is bolted to the intake manifold and ported directly down into manifold).

So as can be seen by all my recent posts, Spring has arrived and its warm enough for car repairs again, and my B isn't lacking in opportunities to scrape knuckles, spend money, and get frustrated.
R.W Anderson

Maybe you're not sealing your mouth round the pipe properly. It should be possible to suck on the pipe coming from the distributor, *hard*, with the cap off and see the points plate twist. You should then be able to seal the end of the pipe with the tip of your tongue and the points plate will stay twisted round, until you pull your tongue off whereupon it should snap back. You could have a small leak in the diaphragm, enough to cause a slight change in rpms when you remove the pipe from the inlet manifold, enough to affect the mixture and disable vacuum advance. Note that the rpms will only change if you remove the vacuum pipe from the inlet manifold at idle, it won't change if the pipe is on the rear carb and you remove it from there. However sucking on the pipe in those case with the engine running should cause the revs to rise. If you really suck...
Paul Hunt 2010


Hiccuping, popping at cruise speeds usually indicates a lean condition. Maybe that 1 turn on the jets to lean it out a little pushed it over the edge.
JM Morris

It could be a partial blockage caused by the old fuel and additives. Sometimes thst stuff turns into a jelly like substance. You may need to partly dismantle the carbs and float chambers to check for particulate blockages.

It could also be a fuel pump problem in that the pump is running but not delivering enough fuel. Easy enough to check by disconnecting the fuel line after the filter, switching the ignition on and seeing if the flow from the pump into a container is OK.

J Tait

I may have missed this somewhere in the story, but have you tried merely changing the ignition coil and/or ht leads?

Nigel
Nigel Pratt

Simple test. When the engine is hiccupping, pull the choke to richen the mixture. If it goes away, it proves the mixture is to lean under those driving conditions. Richening the mixture by one or two flats on the jet may be the answer. Alternatively a slight air leak has developed causing the lean mixture to show up at the rev. range and throttle opening you are experiencing with you car.
I have an O2 meter in my car and pick up your cars symptoms on my car when I have it set to lean. The O2 meter drops to very lean at the same point as the hiccupping and is correct in most of the other conditions (some are a bit rich). The lean hiccupping it magnified on cold mornings until the engine reaches correct operating temperature. I sometimes use the choke to correct the mixture at that point until the engine is warm and the hiccupping has cleared.

Richard.
RH Davidson

Pulling the choke can actually 'compensate' for some ignition problems.

Sticky fuel in the carb shouldn't need a strip-down, squirting carb-cleaner into the jet and fuel inlet a couple of times should be enough.

Fuel pump should deliver at least one Imperial pint per minute, and in practice closer to two, in a steady stream of pulses with negligible bubbling.

The easiest way to check the ignition, bar plugs, is to clip a timing light onto the coil lead and each plug lead in turn and watch the flashes while cranking or running. Any erratic flashing on one or more plug leads while the coil lead is regular points to cap and/or rotor problems. Irregular flashing on the coil lead points to points or other trigger and coil, or connections.
Paul Hunt 2010

I think some of this will need to be proved out following a valve adjustment and rebalancing of carbs in the weeks ahead.

I have a hard time considering this to be a lean problem, as the car historically has had such an unburned fuel rich smell to it; that and terrible gas mileage.

After adjusting valves and balancing carbs a year ago, the rich smell diminished noticably.

Recently hooking the pcv valve to intake manifold also reduced fuel fumes.

The hiccups have decreased, but still happen several times during a 30 minute drive. They decreased once I really plugged the lower tube on pcv valve at intake manifold.

Here's one: the hiccuping increases when going into overdrive, slipping it back out of overdrive and hiccuping decreases.

Yes, it is more pronouced during cold drives early morning at 50F and decreases as engine is at full operating temperature, than in afternoon at 70F.

I should just drive my Honda...
R.W Anderson

I think it would be wise to check the complete ignition system for correct operation prior to doing anything to the carbs.
J Tait

In fact the first test of the ignition is simply to look at the tach when the misfiring/jerking occurs. If it is jumping up and down it is an ignition LT problem. If it's steady it's HT or fuel, or possibly condenser, which is a known issue with rubbish quality and premature failure at the moment, as are rotors.
Paul Hunt 2010

Car had a loose condenser once, caused by a partly stripped/worn condenser screw. Would nip up the screw, drive the car and the condenser would work loose causing the car to hiccup intermittingly. Took time and patience to find the cause.
Richard.
RH Davidson

Watched tach this morning on drive in to work, when engine hiccup ocurred, no movement in Tach.

Pulling choke out minimizes hiccup.

Going into overdrive at highway speed, increases occurence of hiccup. So if already in OD, and cars hiccups, maintaining speed but going out of OD, decreases hiccup occurence.

And on the "loose condenser", a couple years ago I found the small wire coming into side of the distributor was only surviving on 2-3 strands of bare wire. Replacing this wire eliminated the hiccup back then that was almost constantly popping up. The hiccup now feels different.

I'm still looking, still learning, still driving.

Thanks to all.

R.W Anderson

If going into OD increases it that makes it sound load-induced, which could well be HT. When you put more load on a cylinder it makes it harder for the spark to jump the plug gap, so the HT voltage rises, so if there are any weak points in the HT the spark might choose to jump those instead.
Paul Hunt 2010

If the hiccupping feels like a sharp ether on off action, then it is an electrical problem. If it feels like a soft power loss hiccup then it is a fuel supply or compression problem. Changing engine speeds with the OD changes the engine load which magnifies the feel of the hiccup. You’ll have to decide which one and follow your instincts.
Richard.
RH Davidson

This thread was discussed between 14/05/2010 and 20/05/2010

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