Technical Rochester 2 Barrel Tuning

Anyone have experience tuning these carburetors? I have a Jet Rochester 2 barrel on my race truck. When I come around the corner at about 3000 rpm and floor it as I exit corner my AFR shoots up to about 17:1 and then slowly drops down to about 12.5 (main jet taking over). My thinking is that maybe the accelerator pump is lean which is why my air ratio is so high until it settles down to the main jet. If that is the case, do I just put more bend in the acc pump rod to give it a bigger shot? Thanks for any advice, maybe this is a normal condition I do not know.

 

Does the engine stumble or hesitate when this happens? If not, you may be looking for a problem where none exists.

Years ago I noticed that on hard braking entering the corner at one certain track, my oil pressure would drop down to zero and come back up when I got off the brakes. I had it in my mind that the oil was being thrown to the front of the oil pan and starving the pickup. I should note that I had been racing for 4 or 5 years with this oil pan setup and had no bearing problems. I dropped the pan and added a set of baffles around the pickup with no change in the perceived problem. One night when there was a lull in the action, another racer asked me if I had noticed that on hard braking you could actually stop the engine for a second (sprint cars are direct drive). However, your forward momentum would instantly restart the engine when you got off the brakes. One conversation and the problem was solved.

 

Put electric tape over the gauge.

 

I don’t dirt track race but if your turning left and the accelerator pump well is on the left side of the carb which requires a water fall flow of fuel to fill it. Could it be centrifugal force at play?

 

Thanks for the replies! Im kinda thinking its normal behavior as it doesnt stumble/ bog or anything. Really just want to make sure Im not leaving any power on the table.

 

These carbs are so undersized as it is for these engines need every little bit

 

If I recall correctly Carbking mentioned that there are different power valves used through out all the models.

 

Yes that is correct, this is a Jet Performance 500cfm, it has the big power valve

 

Call Jet

 

Have tried, can not talk to anyone and leave message for a call back. Has been terrible customer service for years

 

Time to find some Central Texas dirt trackers who run at Heart-O-Texas speedway. Or ran there years ago when they had to run two barrel carbs and I think they ran the little carb in stock class and the bigger two barrel in modified class.
In the 70's they sold wedge carb spacers that kept the carbs more upright in the turn. The theory being that the fuel would stay more level in the float bowl. They still do for 4 barrel. Offenhauser 5045-B Offenhauser Carburetor Wedge Plates | Summit Racing

 

Been a long time since I built 2-G Rochesters for the circle track folks; but maybe I can make some suggestions.

(1) The large 2 barrel with the 1 3/8 venturi was rated 435 CFM. There was a larger 1 7/16 venturi on a late governed truck carb that flowed about 450 CFM. Never cared for that as a starting point. The 1 3/8 venturi was good for about 6800 RPM on a .030 over 350 CID on a 1/3 mile dirt track.

(2) Scratching my head on being lean exiting the corner. Does this carb have the 2 ball check system (inlet in bottom of pump well, outlet under venturi), or the less efficient slot? If the slot is being used, perhaps (as mentioned above) centrifugal force could be preventing fuel from entering the pump cylinder. We always used carbs with the 2 ball checks for circle tracks.

(3) The operating rod can PROBABLY be bent to increase the volume of fuel under the pump; BUT a better method is to install a shorter pump. Rochester made those pumps in a lot of lengths.

(4) Tuning the power valve is an art, not a science. Rochester made at least 19 different of these power valves. They differ in orifice size, but also in the height of the plunger (the short plunger will activate so many milliseconds slower than the long plunger). Also need to look at the vacuum going through the corner to see if the power valve actuating valve is keeping the power valve open during the corner. If so, then a lighter spring on the power valve actuating valve (shameless plug - we sell a set of 3 different) would allow the power valve to remain closed (desired) through the corner, requiring larger main jets. The larger main jets, plus the opening of the power valve exiting the corner would provide more fuel quicker. The largest orifice may or may not be the best. Need to tune the power valve and the main jets so the combination is correct for W.O.T.; then tune for the jets ALONE being correct for the corners. It may be necessary to use a smaller orifice power valve and larger jets. Just need to test.

(5) A "closed" air horn gasket will act as a fuel baffle keeping the fuel level in the bowl more level than an "open" air horn gasket

(6) The above 5 items are ideas which you prove/deny by testing. Don't just arbitrarily start making changes (other than maybe item 5).

One other comment, while we are on the subject. We found the acceleration exiting the corner was improved by leaving the choke valve in the carburetor. We used to remove the valve and shaft, and narrow the shaft until it was so thin the screws for the plate wouldn't hold. We used pop rivets to fasten the valve to the shaft. We then fixed the choke valve to always be wide open. When discussing this with my mentor at Carter, my mentor, an engineer, suggested the presence of the valve allowed "dirty air" (eddy currents) to be minimized, thus the air flow was significantly improved at lower RPM's. This should get some discussion!

Jon

 

Excellent suggestions thank you very much, I will open this carb up and see whats in there for power valve, slot, etc. Thank you very much for the suggestions!