Best weber DCOE18 jets for a big valve long stroker

Dear all

I wonder if anyone can assist. I have recently had my engine rebuilt on a Ford 1600 bottom end (which I think gives a capacity of around 1650cc). At the same time I had big inlet valves fitted. Camshafts remain standard.

The carburettor also remains standard, ie with:

Main jet 115
Air corrector 200
Idle jet 45/F8
Accelerator pump jet 40
Accelerator bleed valve 40

There are no obvious problems but the car seems a little restrained under full throttle and pings under load.

Has anyone built to a similar spec and fiddled with the jets to compensate for extra capacity and valve size? If so I’d love to know your jet sizes

Regards

Robert

Robert, - a lot would depend on your chokes, among other things. My best advise is a rolling road test, by someone who knows old cars, webers etc. ! The other option is just guesswork, as all engines are individuals…!

Dag

Robert,

Yours is a case where you need to take a look at the whole picture.

Did compression ratio change?

Is the gas up to snuff?

Are temperatures acceptable?

Is the oil clean?

Are the air filters clean?

What do the plugs look like? Which ones are you using? Buy some sets of new plugs and change them as required.

Are the plugs of the proper design and heat range?

Ensure cam timing and ignition timing are where you want them. This is best accomplished by ensuring you know for sure where TDC is and not just assuming the crank pulley and cam timing marks are correct. These marks are at best only approximate. You made major changes to the engine. Check cam timing with a dial indicator and degree wheel and ignition timing with a strobe over the rev range. Also ensure float level is correct and all carb jets and air correctors are clear and clean. These items are a prerequisite for successful carburation changes. One doesn’t want to cover up or compensate for timing deficiencies with carburation changes.

If timing events are satisfactory, then determine where pinging occurs. I like to break down the operating ranges and address them systematically, from lower rpm’s to higher rpm’s, under load. I like to work form steady state response to transient response.

Does pinging start early when on the idle circuit, or later, when the mains are on, at say 3500 rpm, or does it occur even later at higher rpms, like 5000 rpm?

The additional assumption I’m operating under is if pinging is occuring, make the mixture a little richer. If this doesn’t improve things, you might try the opposite, making the mixture a little leaner.

If pinging only occurs early on, when you are on the idle circuit, then you might try to jump up the idle jet size. This will make the idle and transition range richer. You will likely have to redial in the idle mixture screws for this adjustment. Carb balance is important at low rpms. Use an SK synchrometer or manometer based system that measures manifold vaccuum. Get the balance correct. Using a hose to judge the relative hiss between the carb barrels or a Unisynch are not acceptable IMHO.

If pinging occurs after the transition to the mains, try an increase in the main jet size.

If it happens only at high rpm, then you might try a smaller main (200–>190 or smaller) air corrector, which would give a richer mixture, mainly at high rpms.

The suggestion of using a rolling road should speed tuning, especially if air/fuel (A/F) ratios can acquired during the run. Measurable A/F ratios take the guess work out of the process.

Changing jets and air correctors is reasonabley easy on Webers. You are using Webers, aren’t you?

Lacking a rolling road, satisfactory results may be obtained on a long straight away in an upper gear, say 3rd or 4th. The higher gear allows some time to judge the results of your tunning changes.

Change one thing at a time and evaluate the results. Keep a log as to what was changed and what the results were.

I’m sure you will arrive at you want, but it may take some time and dedication.

Remember, carburation is one of the last things you want to change, so be sure all else is correct before you start down this road. You’ve already taken the first step, by recording your carb settings. Do the same with the timing events, even include the ambient conditions of the day.

Post your progress. This board has some pretty savy contributors who are willing to help.

Work the issues in a systematic way and good luck.
Bill

When I was trying to sort out the jetting on an inherited 1558 Twink I found the site below which contains some very interesting articles and also a small DOS Jetting program.

hometown.aol.com/dvandrews/webers.htm

None of this gives exact answers and I would certainly endorse the use of a rolling road, however, in the absence of such an expensive item the information on the above site is very useful, combined with the usual Weber info contained in books (Passini, Weber manual, Wilkins, Vizard etc).

It is productive to think about what you are trying to get out of the engine. Going for peak power is not always realistic - driveability can pay huge dividends!