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650 or 750 carb for my 351W?
I think I may have touched on this a while ago, when I first put the motor in, but I can't recall. Here's the details of my engine.
351W bored .040 over World Products Windsor Jr. heads, 1.94/1.60 oversized valves 9.3/1 compression ratio Crane Cams Roller cam, 216/224 duration at 0.050", and .520/.542 lift Edelbrock Performer RPM dual plane intake Holley 750 double pumper carb I just brought it in for state inspection. My first long drive since the fall. What I'm noticing is a sputter and hesitation sometimes when I first hit the throttle. It'll cough a little, then go. Last year, I know my mechanic turned my secondarys down, and it helped a good bit, but it has surfaced again. We're debating between using my 750, or the 650 carb I had on my old 302. My use for this car is a mix of street, autox and open track. I like the low end torque, but want to be able to run open track too. What do you suggest? Steve |
Sounds to me like maybe one or both of your accelorator pumps are not functioning correctly.
John |
If it hesitates on the initial throttle drop I would agree to check your accel pump adjustment. It is just in front of your thottle linkage.
Hold the locknut ( 3/8?) while lowering the adjusting nut until it contacts your accel pump lever on the carb. The Holleys usually like slight contact or zero lash. My old BG required a small gap. If there is play between the lever and the nut that may be the cause of your hesitation. You need that squirt on time for the throttle response until the vacuum pulls fuel from the jets. |
Not addressing your hesitations, sounds like that motor is better suited to a 650...
Motor is not stroked, not huge compression, not a real big cam, sounds like a 750DP would be a bit much.... Just my $0.02 |
Steve,
I agree with MidOHasp, even with 100% VE at 6K RPM, 625 is maximum draw. So a 650 would give you plenty of headroom. If turning back the secondaries helped the bog, you may be over carbed at 750 CFM. When the secondaries open too soon/fast there is not enough air flow over the venturis so it leans out - the accelerator pump offsets that but won't result in as even a mix as the venturis. It may also be, as suggested, the accelerator pumps, particularly if the change occured suddenly. Remember too that cold air (this time of year) will make for a leaner mixture, aggravating things if it is marginal to start with. Another possibility is to use vacuum secondaries with the quick change diaphram spring kit from Holley. You'll get great drivability for street using a stiffer spring and can change it to a lighter spring for track (acting much like a mechanical secondary) by just removing a cover held on by two screws. |
"Sputter" then go, means too rich and it is loading up... Just a dead bog then go means it is too lean. I am talking in terms of the accelerator pump, not the jetting. Try different nylon pump cams before anything else. Cheap and very very easy to change out.
Ed |
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