Carburetor mysteries

[AlanBartlett]

It has been a while since I have posted a topic up for advice, so thinking caps on as this is a complete mystery to me. Yesterday I set myself the task of getting the engine to run nicely on tick over, slow even running, as apposed to un even fast running, I got this to what i thought was a nicely running engine, ran it for a little while and then turned it off. Came back to it a few hours later, so went through the following:-

Fuel, the fuel pump is pumping nice big quirts of fuel, when the engine is turning over.
Spark, I have seen big blue sparks at both the points and the spark plug.
Carburetor, now all the jets are clear and the bowl is nice and clean. The mixture screw is turned at half a turn from fully in. which it ran fine on yesterday

Heres the thing I can get it to briefly run on easy start, it will fire up straight away. It seems that the fuel hits a dead end at the carburetor. As the plugs are bone dry when cranking over the engine, there are not wet or anything which means the fuel is not getting from the carb to the chamber.

On a last ditch attempt to get it running I changed the carb to the early bronze one, at this point it fired up straight away and ran smoothly(This carb hasnt been touched by me in 14 years, and does flood out the top of the float chamber spurting out fuel like a guizer) So on another note does anyone have any instructions for setting up this carb.

Any thoughts? Many thanks for any pointers in the right direction.

[k. rogers]

Alan, I've been giving this some thought since we last spoke and I can't help thinking there must be fuel starvation somewhere in the carb possibly caused by fuel going through and dislodging a bit lof muck after the carb has been standing all this time. This would explain why your mixture screw is only half a turn out! Is there any corrosion on the mixture valve itself, ie white crust?

[Tony Fearn]

Hi Alan.

Can't help on this one. I know nothing about carbs etc.

Tony.

[george garside]

just a long shot! My understanding from what you have said is that since stopping the engine it won't start at all on petrol but will on easy start squirted down the venturi or into the cows earhole. Also assuming that needle valve is not sticking and that float chamber is filling to correct level and that main jet, compensator and slow running jet are properly clean it could indicate that there is crap in either the very small passages in the emulsion block or in the passages from the jets to the imulsion block. Fuel has to pass through the immulsion block so that it sparays our of the 'beak' thereon into the venturi. To sort this which If my long shot is correct will be a lifetimes accululation of SHoneT unscre the emulsion block (4 screws) and carefuly poke out all passages by means fair or foul depending on degree of encrustaion etc. Similarly blow or poke out passages which TAKE FUeL fROM fLaOT cHaNMBEr VIa JeTS TO EMULSION BLOck and also make sure capacity tUbe is nOt bunged up> I presume the paper gasket between imulsion block (if fitted) is availble from zenith specialists but with care it may be re usable. As far as I caan see if any petrol was getting as far as the beak on the emulsion block & issuing forth into the venturithe engine would at least fire even if it would not run consistantly so assuming petrol is filling the float chamber the problem must be between there & the beak.

I am not recommending it but I used to completely strip float chambers & theri immulsion blocks and boil them for half an h;our in a stong detergent (DAZ OR WHATEVER - this seemed to reach the parts other things didn't .

george

[AlanBartlett]

Ken, I did have the mixture screw out, which was crusted up with black solids all around the tip, so I cleaned this off, but at the time didnt make any difference, I did also try since adjusting the screw by the one and half and two turns to see if it would fire on anything, which it didnt.

George, Ive also had the emulsion block appart, the paper gasket is still nice and intact, I was able to blow through the beak, which seemed clear nothing that i could see that could be blocking it was coming out, So the next step is to take it to work to get some proper compressed air through it, and if that doesn't work then i may resort to the boil method.

Will post again later on tonight as to whether its a go or not

[george garside]

Alan, have you tried blowing hard (or with a pump) down the 'jet holes' i.e. with the jets removed. The air may blow out of one of the other jets, Im not sure if there are any interconnecting passages in the casing, if so bung other jtes up & apply compressed air down jet holes one at a time, if all internal cas poassages in the bowl are clear air should come out of the beak. There are some very fine cast in 'grooves in the emulsion block and dthese can effectively disapear when solid with crud that is a colour match to the casting

george

[AlanBartlett]

During my lunch hour today I stripped down the carb, and got a hold of the blower, and blew through the bowl, jets, emulsion block, beak, a light mist of dust came out. Also soaked the bowl in thinners, and gave it yet another blow through. Tried it this evening on the car, and still no joy, not even wet spark plugs from turning it over so much. Tried all the postions of the mixture screw, the recommended 1.5 turn out and 2 turns, as well as the half a turn out from home postion when it was last running, but still nothing. Also swapped back to the early carb and fired up straight away, have solved the flooding problem the butterfly stops on the top of the needle were stuck, worked some wd40 into them and now nice and free. Have been tinkering with the mixture screw for slow running not quite right as of yet, but, the acceleration seems 100 percent more responsive than the other carburettor was.

I did notice the flap on the later type which controls the acceleration was very loose in its spindle on the throttle linkage side, now this has always been relatively amount of play during all the time i had it running, so just thinking it may have worn that much more its letting in air here enough to mess with things. I have it running for now, so final tinkering will pause for a later date, have a few more panels to paint and more things to put back ready for an mot in the next couple of weeks.

[Forumadmin]

The throttle butterfly should close completely; check by shining a stong light down the venturi tube.
At idle, the closed butterfly causes suction at the idle hole just past the butterfly closure point. My guess is that this hole or the passage way to the emulsion block is blocked. If you tip the carb slightly ( with correct level of fuel in bowl) you should see petrol coming out of the delivery vane just above butterfly.

Suggest boiling in vinegar for an hour.

[george garside]

? then bottle it , label it Zenith special flavour vinegar & use it on the chips!

george

[Forumadmin]

I have a 5 pint pot full of such a delicacy in the garage. Perhaps a job for our marketing manager!

[Jack]

I have a 5 pint pot full of such a delicacy in the garage. Perhaps a job for our marketing manager!

You think that someone without professional medical qualifications can help a person that keeps 5 pints of vinegar in their garage "just in case"???

I have a number of products that are particularly good for cleaning things. The machine we typically use for such things is a dishwasher. Gets things seriously clean. Then just give it a quick dry off in the oven to get any residual moisture off it. Not suitable for particularly fragile parts, but it is rather good for lumps of metal that need to be very very clean.

[Keith Andrews]

All this is common on old carbs.
Over the yrs people poke bits of wire into jets and ports..big no no, even scratches in a jet port makes a difference to flow
These , even 70yrs old are where prescision instrumemts.
Us a carb cleaner to soak and blow out with compressed gas
A quick 'clewan' side of the road fix is take rpms up, hand over pull big vac and suddenly remove the hand ..several times

A butterfly NEVER sits on a bore fully closed...this causes binding , jamming and eventuakky distortion wear in the critical part of the bore around the idle bleeds.

The biggest issue in old carbs is wear in the butterflt bushes...other than a 4 barrell holley, carbs have NO play.
Any play causes a vacuum leak...a lean condition, the most common reason for back fire or poping when lift off the throttle.
Also causes hard starting and hard cold starting....these conditions require a rich mixture.

If the idle speed and/or the idle mixture screw are outside the factory spec, something is seriously wrong with the carb, be it worn, blocked or just miss tuned engine.
Before tuning a carb the timing curve must be correct or close in the ball park.

Rebuilding a carb is not difficult...its stripped of caduim cleaned and eithed in mild citric acid...then re cad plated
U can cad plate at home...u tube....
The hardest part is the butterfly bushes.......these are inserted, and line reemed....a reemer with a starter thru the other side to get everything perfectly lineds up....
Always replace the butterfly shaft.
Then the lining up of the butterflys in the bore...when closed there is no light showing thru when put up to the lamp...it may take 4 or 5 attempts to get right...a thou here or there is not good enough.
The butterfly screws..never reuse....loch with locktite AND crimp the screw ends back on an avail...dont bend the butterfly shafts.

Rebuilding a worn carb....Properly..not patch fixed..is worth every penny, in economy, performance and smooth reliable ride.

Do not use detergents on carbs....it causes oxidation up in galleries , and if the sligestst bit is left behind, depending what gallery, may as well throw the carb away.
When reassembling smear a very light film of engine oil on gaskets...they dont stick, side of the road repairs made easy.

Alan...its quite common to run a worn carb, then one day the car is real hard to fire up or runs like its out of breath.
With toreance worn, crap often builds up, change of temp, or run over a poyhole, whatever, the crap goes and tips the prescsion intrument over the edge.

Think on the positive side...these are single throught carbs....most of mine are 4 barrel far more complex with power valves , metering blocks vac seconaries etc.

[Leo Bolter]

Jack said:

The machine we typically use for such things is a dishwasher. Gets things seriously clean.

Oh dear me . . . life can be awfully awkward sometimes . . . the half of a crankcase I just tried to put in won't fit without taking the studs out. They foul with the whirlygig . . and most of the dishes already in there need removing as well, but that's okay I guess! I'll simply put them in when I clean up the other crankcase half.

By-the-way do you use extra Dishwasher powder? . . . and/or the salt rinse?

I hear tell that the final rinse should be done with the addition of 75% Acryic enamel undercoat. But, maybe that's working on the assumption that the block being treated is made of cast iron. Jack, do you know any expert painters who could comment? (I suspect it's best to have taken the dishes out before the final rinse, in any case).

[AlanBartlett]

I'm currently using the early carburettor on my engine and am wondering what does the tool look like to get the jets out of the bottom of the carburettor, am having fueling problems or so it seems, so would like to check out all posibilities of the carb.

[Keith Andrews]

Have u been thru my post above?
This at the start of yours

The mixture screw is turned at half a turn from fully in. which it ran fine on yesterday

Std is about 1.5 turns, which means a very lean condontion
Have u checked for worn butterfly bushes?
A engine, particularly a cold engine requires a rich (choke) mixture to fire...and worn bushes or vac leak will prevent that...espec since the inlet manifold has a tiny little hole undet the carb...and if that is corroded too big will just make things even worse.
Does it fire with a squirt of engine start (ether)