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MG MGB Technical - URGENT, carb enq

Hi. Please can someone help. I have just purchased a 1978 MG roadster and while doing routine checking, the car started miss/backfiring a lot. Having checked plugs, leads, points ect ect, we turned our attention to the carbs. According to my data, the car should be fitted with twin su's, model no AUD434 HIF4. What is fitted is twin su's, model no AUD445 HS4. These are apparently marina 1.8 carbs. Could this/Would this be the problem. One of the needles in the carbs is very rounded at the tip, the other is quite pointed.
D J P Hewett

Clearly the needles are wrong, but engines move from car to car. Mine is a 76, but the engine is from a 75, and I like many others, have fitted HS4s in place of HIF4s because they have less problems with fuel overflow. Check the engine number, this site is useful.
http://www.zeke.ne.jp/~glitter/main/html/mgb/mgbdata/MGBdata2.html
c cummins

If it was OK to start with, then clearly something has gone wrong, which won't be one needle pointed and one rounded even though that is incorrect. Pretty-well any SU carbs, like any distributor, should allow the engine to run relatively smoothly, just maybe not at its' most efficient and effective. Backfiring in the exhaust is usually an ignition problem, missing sparks allowing unburnt fuel into the exhaust, then the next spark ignites it. How did you check the ignition? A good starting point is to clip a timing light onto each plug lead in turn and watch the flashes. If they remain regular and consistent on all leads while the missing and backfiring occurs, the only ignition part left is the plugs. Whilst on 1 and 4 point it at the crank pulley and see if major timing jitter occurs with a misfire or backfire. If some or all plug leads exhibit erratic flashing then check the coil lead as well. This will flash four times faster than any one plug lead of course, but if *that* is erratic then it is down to the points, coil or connections. If the coil lead is OK though, then it will be cap or rotor.
PaulH Solihull

Make quite certain that you have the plug wires properly oriented in the distributor cap. Remember, the rotor turns counterclockwise, unlike most other cars. RAY
rjm RAY

Sudden misfiring and backfiring when the engine has been running well beforehand could be symptoms of a bad condenser. This happened to me just last week about two weeks after fitting a new one during a major service. I refitted the old one and the problem was solved.
Miles Banister

The Roadster needs a three point seat belt. The inertia reel bolts onto the anchor at the rear wheel arch using a bracket. I found that with the folding hood down, it fouled the belt. I cured it by buying a stowaway frame off an early B.
c cummins

Not that this is anything to do with a carb problem, but where is your mounting point on the arch? I have inertia reels on the rear arches and the folding hood drops right into the space behind the seats, see below.

PaulH Solihull

That is how mine is with a similer bracket. I notice your reel cover is missing, my hood would dislodge it as I folded it, although my frame was not in the best condition. And being 6'2" my seats are back to the rear shelf.
c cummins

Hi all. Thanks to everyone for their wisdom, the good news is that having gone through all suggestions, the engine now runs very well. It turns out that number 3 plug was only sporadically firing and each time we attatched the strobe light, it worked, disconnect strobe, it didn't. As for the seatbelts, well I think it's meant to have inertia but the previous custodian had the hood fouling problem and went down the static changeover route.
With luck the car should be mot'd by wed, so i hope to attend my local MGCC natter in Needham Mkt in it.
D J P Hewett

How big is the reel cover? When my hood is down and in the straps the belts still retract.
PaulH Solihull

I am embarassed to admit I have made the mistake Ray mentions. After unplugging all the leads I once put them back on in the right 1-3-4-2 order and starting from the right 1 o'clock staring position but going clockwise around the distributor. Engine doesn't run so well then!

My seatbelts are inertia reel ones and they do work with the hood folded although I found I needed to put a small piece of plastic hose in the housing to make them work. The body has slots for the belt to run in. Kind of like fork tines. But the opposite ends of the centre forks don't meet in the middle leaving a gap. The belt would end up half in one slot and half in another making them stick. A small piece of clear vinyl tube over the forks closed the gap so the belts don't catch now.

Simon Jansen

The reel cover pivots in the top. My straps are missing so that does not help I guess. But the only time I use the hood is when I take it for the MOT, seems to always rain on that day.
Failed this year, on the petrol filler cap! Some slug pinched the chrome cap, and I had fitted a tempory push in one.
At least there was no retest fee. Surprising what will fail because it is deemed emergency use only. A few years back my Astra failed because I had the spare wheel on, it was not a spacesaver, but a steel wheel instead of an alloy, same size wheel and tyre.
c cummins

This thread was discussed between 08/08/2010 and 13/08/2010

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