Rochester Rod / jet questions.
October 15, 2014


I am running an original Rochester’s 4BBL (7041250 MC 0491) on a stock 350 71 cutlass.
Recently my 45 year old single exhaust system fell off so I replaced it.
I have upgraded from a 2.25” single to 2.25” duals (all no name brand parts).
As expected the engine is running much leaner.
Everything still works, just slower.
Starts easy hot/cold, No off idle bogging, accelerates smoothly at all speeds.
However, my top end is about 10 mph lower and hard acceleration seems slower.

I tried a Rich/Lean test by setting the RPM to 2,000 and the put my hand over the primaries.
The RPM increased to about 2,500 RPM. Less air, more RPM, guess my car is lean.

According to 442.com, the stuff in my carburetor should be:

Primary Jets, 7031970, Sizes 0.070
Primary Rods, 7034849, "49B", size tip 0.026, size rod 0.049

Questions:
Can I keep my original Primary jets and just change the Rods?
And if so, what size ranges of rods would work the best for a daily drive?

Thanks, Miles

 

I'm surprised by this. My carb has 70 jets, 49B rods and I haven't had any issues with my non-stock engine configuration.
How much vacuum does you engine pull?

 

Replacing only the rods with smaller rods will richen up the part throttle but have no effect on wide open. That's because all the rods (except one for motorhomes, I think) have .026 tips. Bigger jets alone will richen up both midrange and wide open. You have identified that you are lean at midrange, so smaller rods would be in order. If you are also lean at wide open, you will need larger jets.


Doug Roe's book has a good section on that. If you need to richen both, maybe 2 jets larger, and one rod larger. You don't want midrange too rich or it will kill response.

 

Stange how a hopped up 355 manages just fine on stock jetting, but a stock 350 needs more.

I wish there was more of an exact science for carb tuning, my engine is the same way. I've had to tune it much more rich than I should to get it to run okay, and it still falls on it's face in 3 gear.

 

Carbs were enginner to fit many applications so there close not perfect.heavy to light cars,high altitude hot and cold they had to run OK everywhere.45 years of wear and tear ,dirt ,vanish buildup can cause havoc. Try a fresh rebuild first. Then if not to your liking. Try up jetting one or two sizes is most likely all you need.they were sometime set lean for emissions not performance.

 

Short carburetor engine history:
I bought the car with 19k all original 3 years ago (71 CS, 180 hp, 8.5:1 stock, gears 2.73).
I had Hobbs in NH rebuild the carburetor in 2011.
The carburetor and engine have been running great on a single exhaust and I now have 29k.

Last year I checked my engine settings: Vacuum was 17.0 to 17.5 at 800 RPM neutral.
Gas millage city/highway has been 12-14 MPG (87 octane Gulf) for the past 3 years.
I am still a beginner at carburetors; however, I figured out the balancing of the Mixture Screws and the Idle screws and have eliminated bogging issues.
The car starts great, hot and cold and always accelerate smoothly (stock points).
The car never had any real power to it. Now it seems to have less power.
On October 5th, the top end was about 110+ MPH (single exhaust).
On October 12 with similar weather, top end was 100 mph, with dual exhaust.

Now it’s time to learn about Jetting.
This month will be researching on what parts I need to buy before I crack open the carburetor.
This weekend ill re-check all of my engine and carburetor settings to see what may have changed.

Question:
Is the hand over the primary still a good Lean-Fuel test?
Should I test the RPM changes at other RPM settings?
I have only tested the RPM / Hand test at Idle, 1000 and 2000 RPM.
Did anyone ever make a 5” Can cover for the Rochester with a variable Air control to get better Air mixture test settings?
A controllable ”Can” on top of the carburetor would be are much more precise measurement tool.

Tentative future parts list:
Primary Jets: 0.071 and 0.072
Metering Rods: 0.048/0.026 and 0.047/0.026
Rebuild kit
Fuel filter

 

 

What I wrote above, needs a little clarification. It should not say wide open, because that would open the secondaries too. At low engine load, you have high vacuum which pulls the rods down into the jets, which puts the fat part of the rod in the jet and making the mixture leaner. When you press the accelerator, the vacuum drops and the rods will lift up, putting the thinner part of the rod into the jet and richening the mixture. When I said wide open, I was thinking when the vacuum dropped to the point that the thinnest part of the rod (.026) was in the jet and the primary side was richened to the maximum. Most street driving is done in the area between rods fully down or rods fully up, and having the best mixture will help throttle response, drivability and fuel mileage.


I always used the hand over the primaries for checking at idle speed. At idle, most of the air doesn't come down the venturies, but through the idle passages. At some engine speed, you will kill the engine put blocking the air, because the idles passages are small. I am not sure what idle speed is the limit for that test.


If you are going to open the carb up, I would buy a good kit from Cliff www.cliffshighperformance.com and rebuild the carb. I would not buy a kit from the local parts store, as many of them do not do well with the ethanol in the gasoline, and will fail in a short time. My 72, same engine, runs very well on stock jets and rods, so I'm not clear why yours would be different. Also make sure when you take advice, mine or anyone else, it is pertaining to how you plan to use the car. Mine is street driven only, not raced. I don't adjust it for reducing ETs, only for the smoothest running and driving on the street.

 

This site has a lot of Olds specific tuning for Quadrajets :
http://www.route66hotrodhigh.com/PowerTuneQjet.html
I run a 73 Quad with 69 jets /44B rods on a stock 350 ,works fine no issues.

 

I thought that a difference between jet and rod of about 0.030" or "30" in the numbers was a good starting point- that's what the 403's carb worked with. I had to scavenge needles from a Chevy parts carb to get this. The info was from one of the QJet books.

But, nothing is better than what works so if a 69/44 = 25 thou diff works for your engine, all the better.

 

Yeah it works for my engine , it pulls good too when i step on it from part throttle and cruises on the highway fine too.I pull 19-20 inches of vacuum also.The Quadrajet is kinda of a bugger to take apart so making changes every week is not gonna happen so finding a recipe that works is perfect . It's not a race car and I don't expect a gas miser out of it so if it works good ,I'll leave it alone. Just drive it and have fun !

 

Interesting issue. I'm not convinced your jetting is the problem yet. Have you checked things like the fuel pressure and float level yet?

 

I read that magic "30" difference also, and that is what prompted me to start a similar thread a couple weeks back. My question was why are Pontiac jet/rod combos so much different that Olds? From the factory, Pontiacs come close, or right at the 30 difference, but Olds are more like 20, which means strictly from a jet/rod combo, Olds are MUCH leaner. But I was told by someone more knowledgeable than me, that the internal passages ion the carb body also plays a role in the rich/lean debate.

 

Oldsmobiles of pre 1975 most have small Air bleeds .there are on average three types of air bleeds small medium and large. Small use smaller jets and bigger rods,medium use little bigger jets and slightly smaller rod,large use bigger jets and the larger power tip rods. Some just basters in between.
What ratio rod to jet is on the caburator try to stay in that range.quadrajet with apt one rod can cover about 5 richer in the cruise range.a 50 rod can ajust down to a 45 that why there a little easier to tune.

 

Reading over this again your vacume seems low for a stock cam engine.it should have a steady 20-21 inches.@750.how many miles are on the engine. How the compression balance.has it ever had a new timeing chain installed.fuel pressure/volume might play into things.

 

^this^

The air bleeds are sized in conjunction with the rod/jet combo to make the correct calibration for an application.

Take Chevy carbs for example. They have huge air bleeds, and use huge jets, on the order of 77 for the BBCs.

The Qjet calibrations are set up for the intended application. One make will be different from the other in "what the engine likes" because of design differences. Intake and head design and flow, emissions equipment, you name it. These all have an effect on how the carb should be set up.

But in general terms, Quadrajets across the board are set up a bit lean, especially the later ones, and can benefit from a bit more metering area in the rod/jet combo. a .030 rod/jet difference is a good starting point.

 

Get yourself a vacuum gauge and hook it up to a manifold vacuum source and adjust your idle mixtures to the highest vacuum reading at idle. If you have low vacuum it"s a sign something else is amiss in the engine. Check your carb base gasket and all your vacuum hoses . Check all your ignition pieces, it has to be properly working cuz if it's not you'll never get the carb adjusted correctly.Even a small vacuum leak is not good for tuning a vaccum dependent carb.JMO

 

Why would having a new timing chain installed play a part in this? As long as it was installed correctly, it should be fine, right?

Is there a better test, because a fuel pressure gauge to test a fuel pump? Most of us have the stock style mechanical pumps,and they put out about 5psi.

 

Running the dog snot out of 45 year old time chain could stretch it.some stock chains had hard plastic teeth that can chip off and make them really sloppy . I am not to sure if Oldsmobile use them or when .I know I have change a lot of GM that had them.(how sloppy can they get well you push on the gas on let off it can pop through exhaust really bad some people drove them that way till the Chain failed)

A sloppy timeing chain can retard the camshaft where it effect idle vacume and power.low inital timeing can also effect idle vacume and power.most people here run at least 12 up to 20 inital advance.it just matters how much mechanical advance you have .most Oldsmobile's like 34 to 36 total advance (initial plus mechanical).fuel pump are cheap if it old I change it.while you got it out put your finger on the chain and see how loose it is.if it a new chain and the problem started way after the install its not a problem.

 

Hello all, hope you had a good weekend.
I’ll try to answer some of you questions.

The goal for this car is to be a weekend driver and showing in Original unmodified class. Plus some fun driving. It’s never going to be a street racer, but it is fun to open her up once and a while.

The CS and Engine are original with 29,000 miles. I am the second owner.
I started rebuilding each section as the $$ and time permits three years ago.

I replaced most of the fuel system in 2011, including Fuel pump, Sender unit, All Fuel and vent hoses with Ethanol approved hoses. The only original fuel parts are the Metal fuel lines and the Tank/cap.
The carburetor was professionally rebuilt by a local reputable shop and I am pleased with the results.
I have enjoyed tweaking the carburetor and I think I have mastered the art of Bog elimination.

I have NOT checked the fuel pressure yet. It’s on my list of things to do.

I am still running the 45 year old nylon timing gear.
The engine has never bee opened.
Compress ion are all between 149- 156 (around 2012).
Oil pressure (hot), Idle (700 rpm) 22 psi, (1,000+ rpm ) 50 psi.
Car has AC and Power Drum.

I am a little concerned about the low vacuum of 17 (at 700 rpm hot).
Normally when I test for vacuum, I use the Air breather feed from the center rear of the Intake Manifold.
I usually disconnect the feed to the distributor vacuum advance and plug the carburetor side of the port.
I never leave any vacuum line open or unplugged.

I usually leave all other normal vacuum sources connected, since that’s the way the car is usually driven (Transmission, Heater manifold, Brakes, Charcoal canister and PVC).

I have searched the vacuum system for leaks on a few occasions and replaced most of the vacuum hoses.
The biggest leak offenders are the Breather temperature switch and the Heating manifold.
The Breather temp switch does not hold a vacuum greater than “5”. I have tried two switches with the same results.
The Heater manifold controls under the dash also have a slight leak. I can pump up the reservoir tank and in a minute the pressure will drop from 17 to near zero.
I think both of these leaks are normal for most cars.

Rebalance Carburetor:
This weekend I warmed up the car and re-adjusted the Mixture screws for peak RPM and Vacuum.
This week’s numbers for similar to last year’s numbers. I did end up, 1 half turn more RICH than last year.
Hot engine, 700 RPM, Vacuum 17.
Today’s Road test was excellent.
Car cold engine started first try at 40F. Dead stop acceleration was smooth and there was no Accelerator pump surging at all.
Morning highway run, 15 miles, good overall performance (at speed limits).

Other Tests:
After readjusting the carburetor, I checked for LEAN by manually closing the Primary choke plate.
I used a Digital tachometer to monitor the RPM. I figured closing the choke plate is more precise than putting your hand over the primaries.

At idle, 700 RPM, I did not notice any Increases in RPM as I gradually closed the choke plate. I did see a reduction in RPM.
At 1,200 RPM, I closed the choke plate slightly and got a stable 1,500 RPM (25% increase in RPM with less air).
At 2,000 RPM, I closed the choke plate slightly and got a stable 2,500 RPM (25% increase in RPM with less air).

I read somewhere, that at 2,000 RPM, I should only see a slight Increase in RPM (50-100 RPM, 5% increase) for an ideal mid range mixture. Is that True?
What do your stock engines do with a similar test?

I have about 2 more weeks of driving and shows.
I plan on pulling the carburetor off and rebuilding in early November. Thanks for your suggestions.

To Do list:
Test Carburetor fuel pressure.
Check engine timing.
Check for new vacuum leaks.
Order Carburetor parts.

Tentative future parts list:
Primary Jets: 0.071 and 0.072
Metering Rods: 0.048/0.026 and 0.047/0.026
Rebuild kit
Fuel filter

And have fun.

 

Honestly, don't be in a hurry to change primary jetting. What secondary rods and hangers are in there? I would put in a G hanger and something like a CV or CK rod, both are .527. I know the later 350 carb I had ran great with 71 jet and 44K primary rods. I went up two sizes on jet and the plugs read rich. If you insist on changing it, go up one jet size.

 

To say you're playing on borrowed time would be an understatement.

IMHO this needs to be remedied like YESTERDAY.

 

I may be in luck, sort of.
My 2 year old Milodon water pump is leaking!

The shaft is squeaking and the Inside of the Pump pulley has a puddle of antifreeze. The only way for antifreeze to get into the pulley bell, is if its
squirting out the squeaking shaft.

So After i get the carburetor sorted out, Ill tackle the following:
Water pump (closed impeller), timing gears and chain, Clutch fan, and maybe redesign the timing cover to replace the 4 self tapping screws with real Nuts/bolts.

 

I completed the Carburetor rebuild.
I removed the factory Jets 0.70 and Rods 0.49 and replaced them with Jets 0.71 and Rods 0.48.
This was my first rebuild. I read the Ruggles book a few times.
The only issue I had was installing the gasket over the rods. After about 15 tires I was successful.
The road test was great. The car started right up and had a good idle. Acceleration from a stop was very good, no bogging or surging. On the highway, the acceleration was noticeably better, zero to 110 in 1 mile (it's only a 200 hp low compression engine).
I have a little fine tuning to do this weekend to get the idle mixture perfect and ill collect more data on Vacuum and lean fuel tests.

The only issues I had were not related to the carburetor. The green liquid is leaking faster from the pump shaft. I think I’ll replace the pump and timing chain/gears next spring. That will be new thread.

 

This months carburetor update:

I checked my fuel pump pressure and it looks ok.
Idle 650 rpm, 7 PSI.
The same at 2,000 rpm 7 PSI.

Tested the following Rod and Jet combinations:

Jet .070/ Rod .049 (Stock)
This combination would show a 25% RPM increase when I tipped-in the choke plate at 2,000 RPM. This indicates the engine is very lean and the car was noticeably slower after the conversion from single to dual exhaust system.

Jet .071/ Rod .048
This combination would show a 22% RPM increase when I tipped-in the choke plate at 2,000 RPM. My fuel volume calculations indicated I should have about 18% more fuel for rod /jet pair compared to the stock setup.
I measured only a slight improvement with the RPM lean test.
However, I did notice some acceleration improvements.

Jet .072/ Rod .048
This combination had only a 5% RPM increase when I tipped-in the choke plate at 2,000 RPM. Or about 100 RPM increase as I carefully closed the choke plate on the primaries.
I think I will stick with this combination for a year or so and see how it works.
The fuel volume calculations were about 30% more fuel than the stock 70/49.

The engine runs great and starts easily. Acceleration is smooth.
Since this is only a low compression engine, I do not expect to be able to spin the wheels.

I did run in to another unrelated issue. After installing the carburetor I took the car for a test run. The ignition coil wire slipped off coil while cruising down a steep hill. I was in heavy traffic and there was no place pull over. There were a few small backfires and then a kaboom.
Both of the mufflers were destroyed.
Oh well, I did not like those mufflers any way, they droned too much.
Now I have an excuse to try a different pair.

 

1 year later.
I replaced the timing chain and gears successfully.
The engine still runs great and there no leaks in the areas that I have worked in.

The Carburetor was still running the old combination Jet .072/ Rod .048.
The car is slow and the mixture is still very lean since adding the Dual 2.25 exhaust system.
The Rod selection was limited so my next rebuild includes Jet .073 / Rod .046.
I also installed new fuel float and Power-pump.
After the installation I did have an extra part which I believe is the washer for the Power-pump?

If my Excel volume calculations are correct, my original 70/49 combination had a volume of .00196 sqin
The new setup with 73/46 has a volume of .00252 sqin.
That comes out to 49% more fuel than stock.

After the install, I rechecked engine timing, Dwell and re-balanced the two Idle Mixture screws.
Then I checked my RPM Rich/Lean by closing the choke plate by hand on the Primaries at 2,000 RPM. I had a digital tachometer mounted under the hood to monitor the RPM changes.
The RPM held steady for most of the choke plate closing process, then finally began to drop.
I also tested at 1,500 RPM with similar results.
I did not measure any “Increase” in RPM’s as I closed the choke plate.
It looks like I solved the Lean issue.

Next the road test.
I now hear something that I have not heard before. The sound of squealing tires (Rear Tires: P275/60 R15 107S). I have never been able to spin my tires before. Now, without too much effort, they spin.

The car does feel like it is quicker at low speeds. I think my car is back to its original horse power range of 180-200. Now I need to figure out the Secondary’s. I am not sure if they ever open?

Top speed of the car still seems to be around 110 (Rear gears 273).
The car accelerates smoothly to over 4,000 RPM. I do not notice any surging below 4000 RPM. I was having issues before the rebuild with surging after 3,500 RPM.

I am still running on damaged mufflers. Somehow the noise of the mufflers covered up the sound of gasses leaking out the split seams, during my last state vehicle inspection.