The Stovebolt Site Forums The Shop Area The HiPo Shop Yet Another Dual Rochester Offy Situation

Installed a dual Rochester Offenhauser. Bob Goodwin Racing in Murrieta California told me the vacuum signal to the carbs remains the same, meaning, I assumed, no need to re-jet the carbs. Another website says the signal is cut in half and, therefore, to obtain the proper fuel flow the jets need to be enlarged. I went ONE wire guage size larger on the air horn jet (left the main metering jets alone). Fuel mileage went down (14mpg) performance basically went nowhere. Looks like a bunch of fuel dumped into the exhaust manifold where it burned white hot and now I've got the head at the machine shop checking for exhaust port cracks, as I started running too hot and getting bubbles in the radiator. (CRAP!) Should Rochester B monojets be enlarged or decreased in jet size when dual carbs are installed, or should I scrap this setup entirely and go with Clifford's $3,000 fuel injection manifold? (Then I have to find $3,000...)

According to the instructions that came with my Offy, they say to go down on the jetting, the vacuum signal should be the same in the carb. By going up, you were over-feuling the engine. Hopefully it did not damage your head, maybe only the gasket? I'd suggest going to Carters before dumping money into the EFI system. Though it does look good.

Could be a timing problem also. Is the vacuum advance working the same as it was? Retarded timing is much more likely to cause overheating than is excess fuel.

the engine can only use so much air when you install twice the intake area the flow thru each will decrease so you will have to jet and rejet to get correct,also the power system will be over rich and activate sooner than with one carb,on timing,, vacum that each will pull will be less so some playing there..Baldy is most likly correct on your problem...doc

While I don't really like to attempt to answer questions on Rochester B's (other than replace them), there are other issues here that I would like to address.

The engine running hot, as posted by Baldy, is more likely to be a timing problem. However, if the hot condition is caused by fuel, it would be from too LITTLE fuel, not too much fuel! The change of state (liquid to gaseous) of gasoline requires external heat (the "heat of vaporization"). Many racers run A/F mixtures richer than the point of maximum power simply to increase cooling efficiency.

Vacuum, signal and airflow are separate entities!

With 2 carburetors running simultaneously, the AIRFLOW through each carburetor will be cut in half. The VACUUM will probably remain about the same at idle; but will probably decrease more at WOT than with the single carburetor.

As airflow is decreased, signal also will be decreased. Signal is dependent on air velocity, and air velocity is dependent on air flow.

Unless someone has an identical setup on an identical engine that has been professionally tuned, jetting should begin with the stock jetting for the carburetors. If one picked the correct size carburetors (THE MOST CRITICAL PART OF TUNING!!!), then the stock jetting will be close enough to do no damage and allow further tuning. Due to the decrease of signal, it is common to need to increase the size of the jets slightly; but this is not always the case!

Where vacuum really enters the calibration picture is the operation of the power system in a vacuum power system carburetor. As the vacuum is likely to be less (although never half) as throttle opening is increased; it is likely that the power circuit will open too quickly, thus creating an overrich condition at part throttle. It is generally necessary to tune the timing of the power circuit to operate with the reduced vacuum. Utilizing carburetors with a mechanical power system makes for a unit which is much easier for the hobbyist to adjust.

Attempting to use carburetors which are not close to the correct size, will normally result in either total frustration; or spending an inordinate amount of time/money trying to compensate for using the wrong size carburetors.

Remember the most expensive carburetor one will ever own, is the carburetor given to one that is incorrect for the application!

Jon.

Thanks all for expertise.

Timing: running a Pertronix 6 volt module with a new advance diaphragm. Haven't measured exact advance at RPM with my old advance timing light but before pulling the head yesterday it started at 5deg. BTDC and moved smoothly forward at throttle. I don't think timing was a problem.

Carbon fouling: the valves I pulled out were showing obviously over-rich. So if anything the engine should have run cooler, as I am aware fuel evaporation cools, too lean heats!
So the overheating (new radiator, thermo and all hoses) may have been an old brittle head whose time had come? When I had Wayne's Engine bore the block they failed to resurface the deck, and there was a gray area in communication there when I picked it up. I assumed they had checked it and decided it was OK. May be warped. But I ran this Offy setup for a couple thousand miles before the overheating began.

Offy's Instructions: to quote directly "Use two same make and style carburestors. Standard jets and metering rods are most universally used. Float level as per specification for type carburetor normally used. Set accelerator pump on shortest stroke that driving conditions will permit. Ideal Mixture--set both carburetors exactly the same."
Now how's THAT for an instruction manual?! Not a word about reducing or enlarging any jets, just shortening the mechanical power pump stroke.

Vacuum power piston/metering rod and related check valve/power jet: if I understand CarbKing correctly, maybe this small jet under the check ball in the Main Well Support will need be reduced, as the port will open sooner, due to dual carb reduced vacuum signal off throttle?
If there is a way to increase the reduced signal to the brass (?) power piston to hold it up, and thereby the check ball closed longer, as it does normally with the single carb vacuum signal I am totally in the dark as to how to accomplish that.

CarbKing's suggestion of going with mechanical power circuit carbs sounds a helluva lot easier.

So, after I've rebuilt the entire damned engine again (it's the journey, not the destination, right?) what carbs are a good replacement for an Offy designed to mount Rochester B's? I'd have to obtain some kind of adaptor...
Any suggestions on carb replacements and observations on brittle fifty-year-old cast iron are welcomed. Thanks again.

Chevr - it is not the jet portion of the power circuit that requires tuning, it is the timing which is controlled by a spring.

Generally (DO NOT ARBITRARILY MAKE THIS MODIFICATION UNTIL YOU ARE SURE IT NEEDS TO BE MADE!!!) what one needs is a weaker vacuum piston spring. Often, hobbyists will simply clip a certain number of turns from each spring thus weakening the spring. The same number must be cut from both springs. A better method is to hand wind a weaker spring of the same length with smaller wire. Obviously, if you cut the spring(s) and go too far, you are in need of new springs.

As to recommended carburetors, if you use the "search" function on this forum, you should find several threads on this subject; or give me a call during normal business hours and will be happy to attempt to answer your questions.

Jon.

I've been running dual stock Rochesters, sourced from NAPA, for six years with no problems, though I did have a mechanic with multi-carb experience set-up and balance them. Perhaps the two best things I did to the motor in addition to the dual carb set-up was the water-heating plate for the manifold and an HEI ignition, both from Langdon's. The engine starts and runs extremely well, has tons of power and with an easy foot gets 20mpg. Tom Langdon advised advancing the timing and running high-test as well as some really hot plugs. A pair of K&N air filters made a huge difference too.

I'm planning on taking my truck to a mechanic friend who has lots of experience with multi-carbs to do the tuning, and he has the tools to do it right- an exhuast gas analyzer- That is the only good way to get the mixture right as I understand it. I'm just guess, but I think he'll use 02 sensors in the ports on the Fentons to measure the front and rear exhaust gas and then adjust the carbs accordingly. There are kits in speedway and other vendors to measure exhuast gas that work that way.