Carburetor Woes

[Finnius]

Hey folks,
I think am having a carburetor woe on one of engines at the moment.
I am running a 38/38 Weber carb on a 4cyl and had it tuned and running fine for many thousands of kilometers.

Then the other day whilst driving, it sounded like there was a popping coming from the intake. I pulled over and the popping noise stopped (it actually sounded like a belt had come off. Now the engine wont idle smoothly and is lacking in power and runs rough.

At idle the engine revs up then down - almost stalls then powers up again and repeats.

Anyways, checked spark, which is good, checked compression which is normal, checking timing with a light and pulled the rocker cover to manually check that the correct valves were open at TDC - also all good.

So i am thinking that the carby is either running too rich or too lean. Maybe a blocked jet (even though it would have had to make it past the newish fuel filter).

I thought once a carby was setup it couldn't really change. The mixture screws aren't going to just undo or do up... Anyone have any thoughts?

I guess the clue is that the engine revs up past 1000 then down to stall then up past 1000 repeating at idle. If i increase the throttle the engine smooths out - but still runs rough. For instance you can drive it around if you keep the throttle up.

I also swapped the fuel pump with a known working one (thinking it might be a fuel supply problem) but the problem persists.

Thanks for any direction on what to check next. I don't really want to just change the mixture screws to get it running right again (if that even works) if it will just mask a problem i have overlooked.

[bee utey]

Been a while since I've worked on a Weber but from memory the main and idle jets on many of them were screwed in to the main body from the outside. Just look for a slotted brass screw in the side, unscrew it and look for the jet in the end of the little brass holder. Blow it out and reinstall. It's not exactly hard for dirt to block them as the idle jet size is only around 0.5mm from memory.

[FisherX]

Poping noise could be backfiring caused by a lean condition. And the engine reving up at idle with poping points to a vacuum leak.


Check your carbie is still bolted down and hasn't come loose, intake manfold is secure and gasket still in place and all the vacuum hoses are in good nic and not leaking especially the one for the brake booster.


Good luck.

[Tank]

Does this carb have a Power Valve (power enrichment) that works off vacuum with a rubber diaphragm, backfiring thru carb can rupture these valves, Regards frank.

[Finnius]

It has an accelerator pump on the side with a rubber diaphragm.
Not sure what it's purpose is actually! The old strombergs i am used to didn't have one...

It is number 41 in this pic:
https://www.aulro.com/afvb/images/imported/2017/01/328.jpg


I will be double checking vacuum lines and also a little fuel filter that is built into the carby tonight when i get home.

[bee utey]

Your idle jets are #34 behind screws #36. The accelerator pump supplies fuel during the lag between the throttle being wanged fully open and the fuel starting to issue from the main jets. Down draft Strombergs have their accelerator pumps internally. Side draft Strombergs don't have one because all fuel issues from the main jet, there is no idle circuit. Your Weber doesn't have a power valve as metering is controlled by calibrated emulsion tubes #89 and air corrector jets #94.

[Finnius]

So i found the problem: :o

https://www.aulro.com/afvb/images/imported/2017/01/308.jpg

The whole end of the PCV valve is missing, obviously causing a pretty significant vacuum leak. I obviously blocked up the hole an retested the engine to double check that this was the cause.

The question now is: do these valves just fail over time, or was there a pressure wave (backfire) in the inlet manifold that popped the little ball and end of the valve out?
Presumably pressure from the crank case wouldn't pop the end of the pcv valve off - only pressure from the inlet manifold would do that...

If there was a backfire, why? The engine purrs like a kitten now i fixed the vacuum leak and it also purred like a kitten before hand too... And i have put many thousands of kms on the carby setup without a hiccup.

[JDNSW]

I have had what looked and sounded like carburetter issues turn out to be distributor - in particular, if you draw a blank on the carburetter, have a look at the internal earth wire in the distributor - if it is broken, earthing will be haphazard, and while the spark may be good with the engine stopped, with the vacuum advance operating, the broken wire may no longer work, so the spark may be weak or intermittent.

This is just an additional thing to think about - I agree that a likely issue is a vacuum leak.

John

[Finnius]

Bit of a side topic, but is anyone familiar with the F10 fuel treatment they make in Perth? Apparently it will clean your entire fuel system, kills growth in diesel, breaks up water, makes you live longer etc.

I have a bottle and wondering if i should put it through my system as the only change to the engine when the pcv valve popped was in the last few weeks i had put a lot of fuel through it, and was just returning from an over 10,000kms trip.

Right when my auxiliary tank was finishing emptying into the main tank (gravity fed) the popping started. Maybe a bad batch of fuel made the intake pop, blow the pcv valve and then run rough as there was now a vacuum leak?