Someone wrote with problems with the 34 PICT/3 carburetor on his 1600cc twin-port engine (new manifold boots and gaskets, new plugs and leads, 009 centrifugal-advance distributor). I have read lots on the common “flat spot” problem (stumbling at low-rpm acceleration) and have tried the adjustments, but I still have the flat spot. I have opened up carburetor and checked the float, needle valve and seat — all okay. I also cleaned and blew out all visible rubbish. The motor seems to idle okay, but it spits and coughs when accelerating. If I up the size of the main jet and idle jet by one size or two will this help?
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Rob responded – Carburetor troubles can be related to all sorts of things — fouled plugs, bad plugs, bad or weak coils, blocked fuel filters, damaged accelerator pump diaphragm, etc., etc. But a significant source of “flat spot” trouble stems from the combination of the 34 PICT/3 carburetor and the 009 centrifugal-advance distributor.
Someone wrote with a suggestion that many people try, but we don’t recommend. Here’s what he says to do to get rid of the “flat spot – The answer to your problem is simple. If you don’t care about fuel consumption too much, run a 1/16th drill bit through the main jet. Try it as it works, and easy to get another jet if you don’t like the fuel consumption. And as for your 009 distributor, if you see a 050 for sale BUY it.
Rob responded to the jet drilling suggestion – I hope that “1/16th drill” wasn’t serious??? That’s X158.75 in Solex jet size equivalent!!! The normal jet size for a 34PICT/3 with vacuum distributor is X127.5 and with a 009 distributor is X130 or X132.5 at most. With an X158.75 main jet you’d have about 24% increase in fuel consumption!
The guy responded (paraphrased) – Yes, my suggestion to drill the main jet with a 1/16″ drill was serious. As I said, if you don’t care for the fuel consumption you will find that it works. Try it — its only a jet.
Rob – I wonder if you realize just how BIG 1/16th is (hence the Solex size equivalent I noted.) If drilled to 1/16th” the jet will supply so much excess fuel that you’ll have…
About 24% increase in fuel consumption — worse than the average Holden V8.
Serious plug fouling.
Serious wash-down of the oil film on the cylinder walls, resulting in massively increased wear.
Oil dilution and contamination (more wear).
Smokey exhaust (the cops may be interested).
Need I go on?
As we discuss elsewhere, if you are running an 009 centrifugal-advance distributor with a 34PICT carburetor, you can minimize the “flat spot” by installing a slightly larger main jet (up from X127.5 to X130 or X132.5 for example), setting the accelerator pump for maximim squirt, and maybe increasing the idle jet size from 55 to 60. Then set the maximum distributor advance to as much of the 28-32 degrees BTDC as it can take without the engine detonating/pinging.
Some have recommended filling in the throttle valve bleed hole. This should only be tried if the above modifications don’t eliminate the flat spot. Doing so results in the carburetor running extra rich at low and middle speeds (butterfly closed or slightly open), as it allows more air to “leak” through the throat without sucking in fuel. So filling the hole mean less air for the same fuel pulled through the jets. It won’t harm the engine any more than other run-rich methods needed to work with the 009 distributor, but if you expect the engine to last a long time then be aware that running rich (which you NEED to do with the 009), results in oil wash-down in the cylinders which will speed piston ring wear somewhat.