4-barrel Pullover...

[Paul Thompson 'Hooligan']

I've been looking at carb tuning and came across this following couple of videos that I thought might be of interest to others too.

http://sputsengines.com/Dominator Carb View 1.wmv

http://sputsengines.com/Dominator Carb View 2.wmv

They show a 4-barrel carb on a dyno and it would appear at full throttle, the acceleration pump circuits are continually feeding fuel. I've also been pointed to the following description of the phenomenon.

Carburetor Squirting - Tech Articles - Stock Car Racing Magazine

Although this seems to indicate its a problem mainly occuring in 4-pots?
(article shows a 2-barrel unit)

Someone suggested to me that it was quite normal and safe as it made the motor rich, but I can't help but think you're gonna be up against it trying to evaluate main jet sizes IF there is a steady stream (or not) from the acceleration circuits.

So has anyone experienced this issue?

 

[Jac Mac]

Where is the problem, the pump shooter has a known size which you tailor to the requirements of the application during the acceleration phase. One would assume that the PV would also be in operation at WOT so for jetting purposes the PV, Main jet,& pumpshooter orifice areas are all added together to get the total actual main circuit size, but the main jet is the only size that can be changed without interfering in other parts of the operating envelope--acceleration--part throttle operation etc. Quite a handy feature to have when you think about it, at lower to mid range rpm when the airflow rate is not as high you have the main jet plus perhaps some PV then when your at WOT & high RPM the shooter adds a bit more. The closer the nozzles of the shooter are to the venturi, the more sensitive they will be to pullover.

On some alcohol carbs etc the metering block will have a known size drilling alongside the main jet so that normal holley jets can be used as once you start using large size single hole jets accuracy in jet flow rates is difficult to maintain.
For example the Holley #80-81-82 jets all share a bore size of 0.093", the flow rates being increased on each size by the bevel or lead into & out of the jets.
The #69-70-71-72 jets by comparison increase by0.003" for each size-0.070", 0.073",0.076",&0.079".

 

[Paul Thompson 'Hooligan']

Thanks for that, I guess providing the flow from the acceleration circuit is consistent then it can be allowed for, it just seemed to me that the videos did'nt show an overly consistent spray pattern, but I guess there are loads of cars out there running just fine... I'd just not come across it or considered it before...

Clever little buggers these 'carby things'....

Much easier typing numbers into a laptop....

 

[Randy V]

It is problematic with some engines and highly dependent upon the air-flow through the air-cleaner. If it proves to be an issue - just use the anti-pullover design squirters. These squirters to have some slight detrimental affect on air-flow overall but it's not very significant as I recall..

 

[Jac Mac]

One point I forgot to add last night, DCOE Webers have the same feature where the pump nozzle adds to the main circuit & flow fuel at WOT. The IDA does not due to internal design.
The biggest enemy any carb setup has in this day & age is the fuel industry itself, different blends play havoc with the Pump Diaphragms-Power Valves-Floats-Gaskets-O/Rings etc. Classic case is the TVR race car I play with, Demon 4bbl on Av Gas & has given zero problems for 5 years now in what could be described as a 'worst case' scenario. Races for 3 consecutive weekends then gets chucked in the corner for 10 month's. Has never developed a leak or flooded etc. Same type of carb on pump fuel in a road car that gets used perhaps every third or fourth weekend and its been in pieces several times.