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MG MGB Technical - New Engine Help Please

I'm attempting to get me 74 MGB started after a 4 year restoration. The engine will start right up, but only on 3 cylinders. I've checked the plugs for spark and each one is sparking. To check the spark I pulled the plug and held it against a ground while my wife turned the engine over. I checked the compression and all cylinders are at 130 to 140. I've got HIF carbs on that are also rebuilt. I've check my fuel flow and it is good.

Any suggestions of where to go next would be greatly appreciated. Our club show is next Saturday and I'd really like to drive the car.

Thanks,

Ken
Ken Waringa

With spark at 3 of 4 plugs, I would say almost defintely a bad wire or cap or plug itself, as everything else is common to all cylinders.

Paul K

Paul, thanks but I have good spark on all 4 plugs when I tried holding them against the block. I first thought was a bad wire or bad plug, but I've tried different plugs and wires with no difference.
Ken Waringa

If it isn't spark...then it's fuel.

First, check for manifold and vacuum hose leaks.

Since your carbs are freshly rebuilt - then I suspect
that they haven't yet had their mixture jets adjusted and idle settings synch'd.

This is not unusual as carb rebuilders cannot
fine-tune carbs without actually bolting them up
to a running engine in order to tweek them.

In your case - it sounds like the carbs has:

(1) one or both of it's needle jet(s) set severly lean;

(2) the throttle plates on the carbs are not synch'd
to each other;

(3) .one of the fuel chambers is dry (sticking
float valve).

My money is on # 1 or #2 - or both.

I'm not too familiar with HIF style carbs and so
I feel that I am not qualified to give detailed steps
on how to tune and correct them, exactly.

Either someone who is more knowledgeble here on the BBS can do this, or I suggest that you
get a hold of an SU manual that will guide you through the steps on how to get your carbs tuned and synch'd after a total teardown and rebuild.
Daniel Wong

Plug leads in the right order?
Steve Postins

With one carb serving up two cylinders I can't see how fuel could be the cause unless a valve isn't openning properly or maybe a critter crawled up in the port.
Bob Ekstrand

My guess is the same as Steve, check the plug leads are in the right order, I had similar and only found my error after many hours of scratching head not believing I had mixed the leads up. Engine started and ran but rough with the offending lead in the wrong place.

Colin
C J Bryan

Ken. I tend to agree with Daniel, it is probably a carb tuning problem. Did you set the jets about .060" below the bridge for initial start up. If not, you might try that as, with the HS seris, this is very close to ideal.

Checking for a good spark by holding the plug leads about 3/8" from the block and looking for spark is a good idea, as is checking the spark with each plug. But, both the wires and spark plugs can work in open air and have problems firing under compression. Paul Hunt has suggested that the use of a timing light, on each wire in turn, can show us if the lead and plug are firing properly under compression. An engine analyzer is also a good idea if you cannot find the problem by adjusting the carbs properly and doing the basic ignition checks.

Bob. Each carb services two cylinders, but there is also the balance tube which has some effect. So, for some combination of reasons, the outer two cylinders, 1 and 4, seem to be running slightly rich when the inner two cylinders, 2 and 3, have the correct air/fuel misture. Back in the old days of leaded gasoline, it would be common, with the carbs properly adjusted, to see the two inner spark plugs a golden/tan color and the two outer plugs a little black and sooty. Thus, a mixture which is weak, but capable of firing number 4 cylinder, might cause number 3 to be too weak to fire because of a lean miss. Les
Les Bengtson

Front Carb. Thanks for all the inputs. I finally tried lifting the pistons in the carbs. The rear carb made a big difference, but lifting the front carb piston made no difference at all. I pulled the bottom off the carb, which is the float bowl on the HIF and it was dry. I rebuilt the carbs about a year ago and the valve must have hung up in the opening. I got it freed up and now it's running on all four. It's still a bit rough, but not to bad. This is the first time since I bought it over 4 years ago that I can move this car without pushing it.

Thanks for all the help.

Ken

Ken Waringa

Ken, you might also want to considering putting a synthetic oil in the carbs, and oiling slightly around the pistons to help them respond quicker. I just finished the rebuild on my engine, and carbs and had almost the same problem as you did. I found that while filling the piston with synth oil was good, a little dab on the outer pistons kept them moving along right quick. I also greased (high temp grease works best) most of the moving parts in the carbs after running the engine, and found that the carbs will respond right damn quick afterwards, and be easier to synch up.

Quick little tip for anyone with HIF-4, or SU's, wire brushing the carbs with a dremel (Cup, flat, and long wirebrush set) while making them look nice and shiney also takes off the decades of grime, and helps them keep just abit cooler *used a IR therm to test that out, and trust me it works*. Takes the better part of a day to do them both but it'll look and work excellently afterwards.

Have fun! CJD
CJD Dark

CJD,

Surely just the reverse! By cleaning them up you have reduced their efficiency in radiating heat (hence the apparent drop in temp on the IR guage). Perhaps painting them matt black will actually drop their operating temp as they will be better at radiating the heat away?
Chris at Octarine Services

How hot do carbs get during opperation? If it is less than the surrounding engine bay then shiney would be good (assuming you want them as cool as possible). If it is higher than than the immediate engine bay then I'd agree with Chris. However, I have no idea how hot they actually are.

Iain

I D Cameron

By the MkI hand my carbs run fairly cool when pulling air and vapourising fuel. They get much hotter after switch-off, which is why HIFs with their temperature compensating jets can be better for hot starts than HSs. Sometimes on the V8 it sounds like the fuel in the float chambers is boiling! Certainly turning the ignition on afterwards causes the pump to chatter for several seconds as if the carbs were empty, unlike in winter.
Paul Hunt

I have got to find some way of getting heat into my carbs on the V8 - this damp cold weather sees them icing up. On the motorway at constant speed the throttle plates ice up so they don't close and the engine "idles" at 4000 rpm + and when you ask for more power there isn't any! Stop the engine for 2 or 3 minutes and the ice melts and all is back to normal for the next 10 miles.

Presumably this is why the factory V8 has the "frying pan" air filter extensions to take hot air off the exhaust manifolds.
Chris at Octarine Services

Actually Chris, by removing the decades of grime build up I've decreased the chances for pre detonation to occur by removing the build up of oils, dirt, scale that held in the heat on the carbs. The car starts and runs easily, and the carbs stay cool to the touch more so when the heat shields in place. Now while the reverse my be true for northern climes, down here its pretty much essential that we keep the fuel cool, because of the ungodly heat during the summer, and the rather warm winters we experiance. And think of this, the cooler the fuel is, the denser it becomes, and the more likely it'll burn hoter and produce more power. The same princible applies to air, the colder it is the denser it becomes and the more power it'll make in the engine.
CJD Dark

Chris why not have carb spacer added to your V8 with take offs for the hot water supply from the heater? Or buy a carb heater system from a chevy hotrod dealer? It's really nothing more then an a electric blanket for the carb that's powered by the electrical system, not sure what kind of carb your running on your setup but that could be a good option there to add, you'd need zip ties to hold it in place, as its made to be removed before the cars driven. Or you could fabricate a heating shroud around the carb, and re route your heater hose to a steel line inside it, wrapping from the side to the back of the carb and off back into the heating system. You may lose a few degree's in terms of your interior heater but it should in theory take care of your v8's carb icing problem.
CJD Dark

This thread was discussed between 06/03/2004 and 13/03/2004

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