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MG MGB Technical - HS6 carbs

Today did not go nearly as well as yesterday.

Worked on the 68 MGC. No fuel is getting past the float bowls.

Traced the supply of fuel from the tank all the way to the carbs.

Fuel will deliver all the way to the bowls.

After the bowls fill, there appears to be a lock of some sort happen...where fuel is no longer supplied to the carbs...I have a clear filter in the engine compartment and can watch the delivery trickle down to nothing. This happened with one SU fuel pump and two electronic pumps, each delivering #2 - #3.5 psi.

When I pull the feed lines to the bowls, fuel delivery is normal....into a pan.

The car would start when I tried spraying some starting fluid not only into the neck of the stock air filter, but directly into the carbs themselves.

I opened each bowl...neither was overly dirty.

Can the needle of each carb be varnished or blocked such that this will cause the delivery of fuel to completely stop....

No fuel is forced from the vent tubes of either carb.

Thanks in advance...

rick
rick ingram

Are you sure both the float bowls are filled? The only way to know for sure is to pull the lids on both after the problem has occured. I've had dirt get into the top of one float bowl lids in the metal pipe before the needle valve, causing similar problems. Otherwise, is a vent pipe blocked? I'm guessing that might stop one of the carbs being able to siphon from the bowl.
Steve Postins

I pulled the tops to both float bowls, emptied them, and cleaned them. I also blew carb cleaner through the vent pipes.

Next step is to pull the dash pots and take a look at the jet/needle assemblies, I guess.

Thanks for the input!

rick
rick ingram

What you describe is normal, in that when you turn on the ignition there will be fuel flow until the float chambers are full, then the float valves will shut off the flow. The fact this is happening shows the vent pipes are clear, if they weren't fuel would continue to flow and be pumped up the jets flooding the engine. I'm not sure what the problem is here, is it that the engine isn't starting? If the float chambers are full but it won't start and after cranking for a bit with the choke out the plugs still have no fuel smell and are dry, then maybe the jet pipe from the float bowl is blocked, although I wouldn't expect this to happen to both carbs at the same time unless they have been incorrectly assembled. With full float chambers, by blowing gently *into* the vent pipe you should be able to see fuel bubbling up the jets (pistons raised). Have you got the needles correctly set into the pistons? For fixed needles this is the shoulder flush with the base of the piston. Are the jets correctly set to their starting positions? This should be two full turns of the jet adjusting nuts down from being *flush* with the bridge (NOT down from being screwed up as far as they will go which I have seen stated), for HS4s at least.
Paul Hunt 2

Paul..Thanks for the comments.

No...the car will NOT start....it WILL start on starting fluid sprayed in to the air filter housing...and when that is taken off, when sprayed directly into the throat of the carb. Hence there is a block somewhere between the bowl and the intake?

On BOTH carbs.

Additionally, when everything is connected up, the pump will NOT pump any fuel through the system...the filter becomes empty. When I disconnect the feed tubes, a good amount of pressure is released, and the pump will flow gas as anticipated into a catch container....This occured with one SU pump and two different makes of electronic fuel pumps...bot with a delivery rate of 2 to 3-1/2 puounds.

rick
rick ingram

Have the carbs ever worked on your engine (ie are they new to you)?

If so, maybe someone has fitted small bore jets which are being completely blocked by the needles?

The choke lever should pull the jets down quite a bit so would sort this problem out. Are the choke levers set right?

The jets come with a piece of pipe which fits directly to the float bowl. Either could be blocked (easy to check by blowing).

Are the dashpots fitted correctly such that that the engine is actually pulling a vacuum through the carbs? There's supposed to be a little plastic thing which spaces the dashpots and hence the needles up from the bridge - are they missing?

Is the filter fitted the right way around?

Another stupid question, but worth checking I suppose. Are the fuel pipes connected to the fuel inlet connections (ie not the vent connections). Have no idea if this is possible! Just wondering why the pumps can manage to fill the filter once but give up once the bowls are full. Fuel would fill the bowls and then the float valve would (in this scenario) block off the vent pipes which might cause the airlock/pressurisation? This might be poppycock.

Neil
Neil

Neil....There are NO stupid questions.

Yes...This car was running about 3 months ago. Quite well.

No, the carbs are not new...original to the car. Unknown if there's been any work done to them, but after this many years, I'm willing to bet yes.

I tried it with the choke out.

Dashpots fitted correctly as are the fuel supply tubes and vent pipes.

This is an MGC with stock air filter assembly...there is only one way thatit can fit.

I'm pulling the dashpots tonight if I have time and energy.

rick

rick ingram

Rick,
I'm probably wrong but I'm a little doubtful that the fuel filter is a good indicator of what's going on - I've had ones that vary from constantly full to always empty, with the car running fine.
Steve Postins

I was thinking about the fuel filter rather than the air filter. Otherwise I guess that I am stumped then!

I'd be filthy rich if I had a pound for every time my GT was working perfectly when I put it into the garage and then was not (at all) the next time I went to take it out of the garage again... Have you checked for the garage fairies???

Neil

Rick

Could a vacuum leak cause fuel starvation to both carburetors?

Chuck
Chuck Hassler

Other than a blocked jet tube (both carbs?) the only other thing that comes to mind if memeory is correct, on my 69 CGT there were gaskets between the carbs and the airfilter adaptors these have a hole which only lines lines up one way. If blocked this may keep the dashpots from lifting under vacuum.

HTH

Ron
Ron Smith

I don't know if this applies to the MGC, but the MGB has some plugs in the ends of the intake manifold. If one of the plugs comes out the engine will not run due to the huge air leak.

Clifton
Clifton Gordon

Would be interested on how this one works out. It the "both carbs" issue that is begining to make me rethink this. Both carbs fail/clog at the same time? Possible, I suppose. Obviously the removal of the piston & chamber is a quick and easy investigation to the clog theory. Hmmm...massive vacumm leak...you guys could be on to something. Let us know, Rick.

Paul
Paul Hanley



I don't know much about these car, but I am trying to learn, and I know absolutely nothing about an HS6 Carb. I have a '74 barn-find MGBGT, with HIF4's, that I am trying to get running, and it had almost the exact symptoms. The car has been de-smogged but the charcoal canister and related plumbing is still in tact. I read somewhere, possibly on this BBS, that a clogged vent might be the problem. I disconnected the hose to the charcoal canister and the car now runs fine.

Rick, I don't have a clue how an HS6 on a '68 MGC vents. But opening up the vent on my HIF4's cured my problem. It might not work in your case but what have you got to lose? Try it and see.

Mack
A Sneed

Ditto the comment on seeing my fuel filter anything from full to empty with no running problems. If a pipe removed from a carb pumps fuel then you can forget the filter and everything up to that point. If the float chambers are 'full' with the lids moved try blowing into each overflow pipe and see if fuel bubbles up the jets, air filters removed of course and pistons manually raised. If that shows fuel all it can be is incorrect needles/jets, way out of adjustment, or the massive air leak mentioned, although I would have thought that would prevent it from firing with starter as well, unless you used copious amounts to offset the massive weakening. Is the choke functioning and visibly moving both jets down?
Paul Hunt 2

Thanks again for the comments....will give a progress report when I get out to the barn.

rick
rick ingram

Rick, and something else. What type of float jets do you have? The original type or the type with the two ball bearings. The Gross Jets stick on me anymore and I don't use them. Also, you might want to check for critters that have crawled up into your vent pipes. Usually spiders. Had this happen on a 67 B and it took me a while to find those things plugged. Motor had a difficult time starting with those plugged.
Mwhitt

The float jets are indeed Grosse Jets...but are free and clear.

I also sprayed carb cleaner through the vent pies and got a steady return stream.

rick
rick ingram

I'm thinking air leak here.....
P J KELLY

This thread was discussed between 08/05/2006 and 11/05/2006

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