our project

[Reklaw]

Hi Peter,
I would think that the sooty plugs could have stemmed from a myriad of issues. Like ….. Choke partially on, too much fuel pressure, float levels not right, needle and seat stuck open, jets too big, engine burning oil etc. Might be best to go through your existing carboy, fix any issues with it and call that your base point and decide after you have run it for a while whether or not to put a bigger (or smaller) carby on.
Bob W

[screwit]

Peter , just to confuse you thinking now with early motors on unleaded you need to run hotter plugs [ 1 or 2 heat ranges ]as the chemicals now in fuel burn differently it will change the plug colour, in a old engine that was deigned to run on super and now run on unleaded with no other changes from the factory the plugs burn black giving most people the impression that it is running rich only to down jet size or down size carby to get a better colour, then only to find a burnt piston later on . We have had unleaded petrol for a long time now , was your boat running it when last in the water ?
I was told many years ago that a direct swap for a 327 chev in a road car was a 465 holly 4bl.
Americans tend to over carburettor their hotties but this stuffs up the fuel air mix if throats are to big for for the air speed created buy the vacuum.
Stock carb should be right size . 500 Holley runs 253 with no problems , 350 Holley may be to small as a boat is like driving up hill all the time.
Ross

[piquet95971]

This is a simple question. Which way is best to put the holley onto the manifold?

I know all the smarty pants answers gasket, nuts and washers etc.

Fuel bowl to the bow or to the stern?

[piquet95971]

The oil drain kit was going to cost $61 from a place in Frankston. They were difficult to deal with so I went to Enzed and had one made up by them. Cost from Enzed $33.00.

Tomorrow it will be fitted.

[Reklaw]

Don't know about any fitment issues but, in principle, bowl to the front.

[piquet95971]

Bugga, have to re route the fuel line. Another job done that will have to be redone. I had it round the other way and it looked so good too!!

[Alan]

Pete, you can leave it that way, but you will need to get a set of jet tubes to stop the fuel running away from the jets when accelerating.

[The Spook]

Folks

There is a formula for working out the size of a carburetor for your engine, surprisingly it was formulated by Holley. Who would have thought that they would have a good idea of how to work out the size of a carburetor for an engine. One of the things that surprises me is the number of people that tell me that Holley don't know what they are talking about when it comes to carby size and automatically put some huge carb on and wonder why it doesn't work so good.

So here it is:

(engine size in cubic inches X maximum use-able rpm) divided by 3456

Here is an example using my engine:

357 X 7250 divided by 3456

becomes 748.9 CFM

So from that I use a Holley 750 CFM vacuum secondary carb.

Holley vac sec carbs go up in 100cfm increments and are a more forgiving carb. The next one up is a 850 CFM.

Mechanical sec carbs go up in 50 CFM increments.

Spook

[WoodRay]

Don't forget (engine size in cubic inches X maximum use-able rpm) divided by 3456
assumes 100% volumetric efficiency. Most average engines would loose around 20-30% from frictional loses and flow restriction with regards to filling the cylinder. You could knock the carby size down slightly if your not going to be running at full noise and this would help with better low down torque. Spread bore Holley's are a good alternative if you wish to keep the large WOT of a 750 and still retain small primary throttles.

[The Spook]

Woodray

You are correct in saying that there are some losses of volumetric efficiency in the process and in the early days Holley did actually refer to this. The last time I looked up Holley's website I could find no mention of such efficiency losses and it just quoted the formula.

There have been a lot of gains in port design, camshaft design and overall engine efficiency during the last few decades.

For those of us who are concerned about how much efficiency they may lose from the cylinder fill then Woodray is correct in saying that you are able to drop the 100% figure by about 20%. This will improve engine torque in your lower rev ranges.

I have found that for me using the 100% figure gives me a good size carb without being too big. However you choose to use it, the formula will give you a a carburetor size which is commensurate with the cubic inch capacity and RPM capabilities of your engine.

Lets use the example of a stock 253 with standard camshafts and it has the ability to rev to about 5500rpm.

We get (253 x 5500) / 3456 which equals 402 cfm at 100% efficiency.

If you feel that that will be too much then 402 x .8 equals 322 cfm of air. Once you have made the calculation then you can go and find a carburetor which fits that size range.

If you change the camshaft and valve spring combination of the engine and it now has the capability of revving to 7000rpm it obviously changes the calculation.

You now have: (253 x 7000) / 3456 which equals 512 cfm of air. If you assume 80% efficiency of the engine then 512 x .8 = 409 cfm. Once again you would then go and search for a carby which fits these calculations.

The beauty of this formula is that it works for all engines whether they be 4cyl, 6cyl, 8cyl or 12 cyl, its universal and your carby doesn't have to be a Holley as long as its possible to work out, or know, or find out, how much air the carby will flow. Holley just make it easy because they actually rate all of their carbs in CFM of airflow.

Spook