Howdy all,

I'm rebuilding the carb on my 239. The carb is from a late 50's early 60's 292.
(Holly COTE-9510-x)

My 239 was running a little rich so I also got a set of #45 jets. (it's got #52's now))

Does anyone have any experience swapping out jets? Does anyone think #45 will be too lean?

Thanks,

Rick

 

Holley does not rate their jets by diameter, they rate them by flow, which is a truer method of indicating what you can expect when they are placed in your carb.

Each jet size differs from the next number size by 4.5%.

Consider: Air is 10% thinner at a mile high than it is at sea level, so the jets must supply 10% less fuel (flow 10% less) at a mile high than they do at sea level.

The old rule has always been one jet size for every 2000 ft. in elevation. You can see from this that 4000 ft would be 2 jet sizes, or 9% less fuel, and 6000 ft would be 3 jet sizes, or 13.5% less fuel, and so the old rule brackets the 5000ft - 10% science pretty well.

But this assumes that the original jetting is spot on. Sometimes, jetting is rich to cover all sorts of situations.

The real test is done by driving. When you put your new jets in, and the engine is fully warmed up (very important!), make a light acceleration. The idea here is to load the engine, say from 15 mph or 20 mph in a high gear, but you do not want to open the power valve -- this will defeat your efforts! This is why having a vacuum gage in the cab and knowing what power valve you are running is vital.

When you accelerate under a load, but without the power valve opening, if you feel any hesitation or flat spot or sag, then you need to go richer by a step until that sag is taken out.

Another test, if you are familiar with the engine and live in an area where there is colder weather, is to study how much choke is used to get drivability.

If the 52s were close to being right to begin with, the 45s will be too small -- but that is only a guess. You can only tell by trying it out.

If you are working your truck by pulling or hauling loads, you must not err too much on the lean side of things, or you could start burning valves.

A second test is to look at your spark plugs. Generally, if the center electrode is a tan color, you are close to being right. If it is sooty black, you're too rich, and if you are bone white, you are too lean. If you are shiny and crusty black, you are burning oil.

The third test is the best, and that's to use an exhaust gas analyzer. The meter will tell you whether you are getting the 14.7:1 air/fuel for cruise and 12.5:1 for power that you need, and will dial you in very closely right away.

 

Outstanding!


Well I do know a bit about rich vs lean I guess having flown little airplanes for many years and using an exhaust gas temp guage to set mixture.

Aircraft engines are air(and fuel) cooled. They run much richer than liquid cooled engines.

I have always wanted to know how carry this over to an auto engine. The problem I guess is that mixture is adjustable on an aircraft engine(that runs at a mostly fixed rpm and throttle most of the time)

With fixed jets in an auto engine I'll have to use trial and error as you suggested. (I don't have access to an analyzer!)

It seemed to be very rich using the #52's but I'll bet the gaskets were leaking internally and maybe I should try the #52's before I try the lower ones.

From what you're saying it appears that the #45 jets might be more appropriate for high altitude applications? The replacement jets were only about $6 so it'll be fairly cheap to experiment.


I don't know about the power valve. The kit has a replacement. I don't think there was a choice and I haven't found a source for them.
(When you say "Power Valve" that's not the accerator pump is it?)

This will be my first attempt at changing jets in a carb..

Thanks for the info!

Regards,
Rick

 

Rick: head down to the library and grab a Holley carb book, or buy one. HP books (Mike Urich) and SA Design (Dave Emmanuel) both are OK. They will give you an idea of how the systems work.

The power valve operates by several methods, but on most modern carbs, is actuated by vacuum. the idea is that as the engine sees more load, the manifold vacuum drops, and a spring overcomes a piston to open a bypass around the main jet, from the bowl into the main well. This permits about a 20% to 30% enrichment. You want a 14:1 air/fule mix or leaner for cruise and economy, and about a 12.5:1 air/fuel ratio for power, like when you mash the pedal to pass or go up a hill. The main jet is calibrated for cruise, and the power valve opens to enrich the mixture by opening another orifice that parallels the main jet.

Usually, a jet step or two change at a time is sufficient, but if you are using a carb that was originally from another engine, then you may have to up or downsize by more steps.

Fooling with carbs and ignition is a great way to make your engine run sharp, both for power and economy, and it isn't very expensive. A little thinking and lots of patience is all that you need.

 

Thanks!

I think that'll be fairly easy to do. I believe the power valve that is screwed in operates by vacuum. it should be fairly easy to swap the jets around and come up with a fairly good running combo!

Thanks again,

Rick