MN80Vette

I am trying to tune my '80 with a GM 350 HO crate that has a 600 CFM Holley 4160 carb. After I put on full length headers and true dual exhaust a couple of years ago, I started having an engine shudder problem when I stomped on it. The concensus here on the C3 CF was that my engine was leaning out when I opened the throttle quickly.

I bought bigger jets but did not put them in because the car ran with a steady throttle at any RPM. Instead, our son gave me an accelerator pump cam he took off his V8 S10. That took care of the the lean shudder but was squirting too much too soon. My neighbor was following me and said that my car smelled "really rich" when I accelerated. The right cam is somewhere between the original (whatever that was) and the fast and big cam I have now.

I bought this set of cams:


Which cam are you using?
Which CFM/model Holley carb do you have?
What engine is under it?
Does that combo work well IYO?

I am going to try one or two out this weekend, but I don't want to try any more than I need to.

I'll post the results of my cam test.

Thanks guys!

MN80Vette

The silence is deafening.

sundevil74

Sorry! No can help. But I'd like to see the responses myself. Going to rebuild the same carb soon for a 327/350.

olescarb

most often for the gasoline we have here in California we use the pink pump cam (#330) with a 0.031 squirter, we often must also use 2 or 3 3/32" flat washers on top of the duration spring on the pump arm so the pump becomes more active. most newer Holleys come with a pump duration spring that is not as strong as the spring that Holley used back in the 1960s and 1970s.

i hope this helps Henry @ olescarb

toddalin

I also run the pink cam on my 600 dp that has been substantially modified for better flow.



In addition to playing with the cam, you can also increase the pump shot from 35 to 50 ccs, but that shouldn't really be necessary.

My combination and dyno sheet:

331 CI, pump-friendly 9.5:1 CR
K&N 14"x4" air filter, Corvette drop base and lid
Holley 600 dp, choke horn milled, polished
1/2" Aluminum open spacer port matched to manifold, exterior polished
Edelbrock Performer RPM, port matched, exterior polished with all extraneous castings and lettering removed
Homemade lifter valley splash shield to keep hot oil off manifold bottom
Camel hump 1.94/1.50 heads hogged out to 2.02/1.60, pocket ported, port matched, pump-friendly hardened seats, 3-angle valve job
Cylinder bores clearanced to unshroud the valves
Comp Cams 1.52:1 roller-tip rockers
Crane Cam Vintage Muscle 327/350 hp cam, 222 degrees @ 0.05, 0.447" lift (with 1.50 rockers)
Doug Thorley headers, dechromed and ceramic-coated
2.5" mandral-bent exhaust (including tips), 2" cross-over just before rear axle
DynoMax stainless Ultra Flow mufflers
Mallory Hyfire IV CD ignition box triggered off Accel points
Mallory high voltage chrome coil
Mallory spiral-wound coil wire
Mallory solid copper plug wires, ends soldered to wires
Champion plugs
37 degrees total ignition advance (now 34 degrees per Lars tune-up)
Carter high volume fuel pump
Melling high volume oil pump
Open breathers
Polished aluminum high flow water pump
Flex fan with polished aluminum spacer
Polished aluminum one-wire 100 amp alternator
Muncie M-20 CR 4-speed (now Keisler TKO600 5-Speed)
Hurst shifter (Now Keisler With Hurst Tower)
3.70:1 positraction
225/60/15 Firestone Firehawk SZ50s on 7”-wide Western 30-spoke Turbine Wheels

At the time of the dyno pull the engine had never been professionally tuned up. The car did 293 ft-lb and 293 hp @ 5,500 rpm on the chassis dyno. The engine pulls to 6,200 rpm and does over 200 ft-lb from 1,900 rpm. This was prior to some clean-up work around the junction of the carburetor and base of the air cleaner that smooths the flow through this area and a Lars tune-up and should be good for a few ponies. The car ran 107 mph in the high 13's (old, bad tires) at the quarter prior to the carb-area clean-up and Lars tuning. The car is extremely streetable and could probably even run 87 octane if it had to. (I can't get the engine to ping no matter how far I advance the ignition.)

The secret to power is in the attention to details!

MN80Vette

OK, it sounds like pink is the de facto standard. I assume you have them pegged in hole #1 (default for GM - immediate squirt) vs. #2 (Ford - delayed squirt).

HOWEVER, I am still going to start with a less agressive cam with a long duration squirt cam because pink is the next-to-heaviest. I don't know for sure, but I think the cam I have is already squirting too much for too long.

Well, the weather is going to be decent here tomorrow. I am going to line up the 8 AED cams and compare the one on my Holley now and pick a cam a notch or two less agressive and go from there.

MN80Vette

That is an impressive carb! My stock 600 CFM Holley seems kinda' tame compared to yours. Sweet.

toddalin

Just a matter of milling off the choke horn and blending it into the bowls. I actually got this one on the 'bay and replaced a stock 650 dp with electric choke that now occupies the garage shelf. I thought that the 650 had a crack, but upon close inspection (after replacing it), it was just a casting line in the metal. Oh well, this one seems to work as well (probably better), as the modificationss should certainly be good for more than 50 cfm, the flow is more laminer, and it has more WOW factor than the stock unit.

Recognize that the 600 dp and the 650 dp use the same size venturies. The difference is that the 650 uses the same base plate as the larger models. But even the 600's base plate is larger than the venturies so its not really the critical restriction in flow.

MN80Vette

Well, I put a different cam on the accelerator pump today. I have some good news and some bad news. First, the good news.

I figured the cam I had on was shooting too much fuel too soon but not long enough and decided I want a more gradual but longer shot. I put each cam on a finishing nail through the #1 hole and put a 3/8" bolt in the shaft opening so I could visually compare the rate and duration of all the cams like this:


Some cams had a definite "corner" to them and others had a curve. The one I used to have on this carb is the one next to the blue cam on the far end (green). The green cam that cam in the AED assortment is the third from this end. As you can see in the picture in the initial post, the orange cam has a rounded curve like the red cam but a longer shot. It worked pretty well!

Now the bad news. I couldn't test other cams because fuel was running down from the the accelarator pump every time blipped the throttle. There were some pretth significant puddles of fuel on the intake after driveing it aways.

I need to take the carb off and fix the accelerator pumo befure I do any more tests.

MN80Vette

I installed the biggest, longest shot cam in the kit and a new diaphram (the old one was leaking) and I still had a lean-shudder problem when I pressed the throttle "assertively" and even a little backfire when I stomped on it. The cam and diaphram were not the problem. I checked ignition timing 2 times and experimented with it a few times too. Still the same problem. On the positive side, when engine finally got above 2500 rpm, it starts to run smoother and pull harder. Vacuum secondaries kicking in?

At this point, I'm getting nervous because our son and I have a dyno appointment Saturday. (He's bringing his custom 355 CID '93 S10 pickup.) My '80 with a GM 350/330 HO crate felt like about 110 RWHP today at best.

Our son and I were talking about what to do. He thought maybe the squirter was too small and that I should just try a bigger one tomorrow. When I took the squirter out, I learned that 1) it was a #31 and 2) it had a large black fleck (rubber?) on the inside of the squirter housing and over the driver's side squirter hole. I remove that and back-blew a little compressed air through each nozzle. The passenger side just sounded like moving air, but the driver's side made a "thup" sound before the moving air sound. It had been blocked!

I put the squirter back in and fired it up. It was late so I didn't drive it, but I'm looking forward to trying it tomorrow!!!
I'll post the results.

R96IMP

Watch the angle of the primary accelerator pump arm.The arm can sometimes be too flat from the factory and it will delay your pump response.Gently bend it as needed to accomodate your pump cam geometry.It should provide a continuous shot of fuel while on the ramp of the 330 cam in hole "1".
Also pay attention to the free play in the adjuster screw/spring.You want "0" play and if you just barely bump the top of the spring/screw there should be a squirt of fuel(trickle).
I think you are on the right track,the jets don't do squat until after the transition circuit.The accelerator pump/squirter covers the transition between the idle circuit and the main jetted circuit.
When you get it where you want it,put in a fresh set of plugs and take it for a very spirited ride.Then check your plugs for coloring.Thats how you will tell how its running(too rich/lean).
Keep us posted on your progress and have fun!!

MN80Vette

Thanks for the tips.

I do have a question about the arm on the cam. I never looked at it before I had the carb off to replace the diaphram. Is the cam arm supposed to be resting on the flat top of the cam in the neutral position? Here is a picture of the cam I had on when I started this little project.



Originally the arm was touching the cam ON the hump. I remounted the cam so that the arm is close to the hump, but I couldn't get it to rest right on the flat spot. It seems I'd be getting less shot if the arm starts on the hump already.

MN80Vette

I took my car out for a test drive yesterday with the cleaned-out squiter. It was much more responsive but it still studders for quite a while when I open the throttle a lot at cruzing rpms until 2500 where it wakes up. That tells me it's still starving for fuel when accelerating below 2500 rpm. (Do you agree?)

I am going to get a bigger squirter today. If I have a #31 in there now, what size squirter do you think I should try?

I am going to replace the fuel hose between the fuel filter and carb too. The piece of rubber that was blocking the nozzle hole had to originate there.

R96IMP

Try a 37 with a hollow screw.
I didn't notice before that you had a -3 holley.....Those plastic linkage arms were done away with by Holley after many complaints about durability.
Get a METAL arm for the pump lever and adjust it.They will fit in exactly like the plastic arm.Appx 8.00 from Jegs,Summit,etc.
I'm starting to think your jets are too lean.....or you have a plugged jet-like the squirter was.Take it apart and check for crap in the jets,maybe go up about 3 sizes,the squirter is doing its job now,its downstream of that(in the carb).Which powervalve do you have?I think stock is an 8.5

enkeivette

I've gone up from 28s to 40s and I still have a slight hesitation. It's almost gone, but I'm going to try a bigger pump cam next. If that doesn't work, I'll try the 50cc pumps.

MN80Vette, the blue cam appears to be the biggest, but does it shoot the soonest? It looks like the big black one might have a more aggressive rate.

MN80Vette

Those are good ideas, R96. I got a metal arm with the 50cc pump kit I bought. I haven't had time to put it on yet. When you say "adjust it", do you mean just the bolt through the end that pushes on the pump lever?

Previously, you mentioned bending the arm. How so? What is the right bend?

Olescarb,
Are you talking about adding "2 or 3 3/32" flat washers on top of the duration spring on the pump arm", are you talking about the red spring in this picture or a different spring?


I appreciate your help.

olescarb

Olescarb,
Are you talking about adding "2 or 3 3/32" flat washers on top of the duration spring on the pump arm", are you talking about the red spring in this picture or a different spring?


Yes, you can put 2 or 3 washers on top of the red spring to make the pump more active (this is a trick Barry Grant did back when he was modifying Holley carbs) or we use a stronger duration spring on most of the holleys we tune.

Henry @ olescarb

enkeivette

MN80Vette

I didn't try the blue cam because I thought it would give a BIG shot and then nothing. Instead, I put on a cam I thought would give a longer shot.

I have the distributor out now so I can experiment anymore yet. I need to stabilize the timing first.

olescarb

I my mind the pink cam is the best one to try first since it is the most progressive one unless you are only accelerating from closed throttle, if the pump cam comes in to quickly there may be no pump squirt left if you hit the throttle from a part throttle cruise. henry @ olescarb