Holley tuning?

[JayBee]

Well, I finally replaced the antique Holley spreadbore 580 double pumper with a Holley mechanical secondary marine 850 dp. The 850 is overkill, but hopefully a little better match than the old 580. But, now I have some tuning to do. Here is what I was seeing:
When you initially hit the throttle the engine jumps to about 4600 rpms for a moment, then changes tone a bit and settles down to about 4200-4300 rpms. If you let off then hit it again it does the same thing. It is NOT a pump issue - the pump is tight, and it did not do this with the 580 - which consistently pulled 4400 rpms.
So, am I seeing a lean condition at WOT that is momentarily compensated for by the accelerator pump? I didn't have any trouble with pinging or running on, which I did experience with the 580 and was part of the reason for the upgrade. I didn't have tools with me to do any ignition cut plug checks.
Any suggestions?
Jason
[ July 07, 2003, 05:44 PM: Message edited by: JayBee ]

[JayBee]

Forgot to add:
Oval port 396 (bored to 402)
9.6-1 c.r.
Comp Cams Xtreme marine XM278H
Edelbrock Torker II (old twisted carb model)
A/B impeller

[FastRat1]

i had the very similar set-up..except mine was a 410CI..different hyd cam..Edelbrock Tranchula manif (carb sits diag.)..mag ign..850 cfm carb (mech sec)..Berk jet (stock)..OT headers..in an older (70's) 18' Southwind..ran real strong..only thing i did to the carb was put 50cc reo pumps on both ends and re-jet to 80's all the way around..advanced the timeing 4 deg..thats it..no stumble.. anywhere...the 850 u have..should run great..rite out of the box..if its used..u need to ck the actuater levers that operate the pumps..they should just "kiss" the pump levers..also ck the pump cams 4 the rite 1's...good luck...let us know
Ron
Ron

[Moneypitt]

850 CFM on a 402, or a 410, or for that matter, even a 500??in a jet!! come on guys, get a grip!! And a calculator, do some math, and learn something about those internal combustion air pumps that we call engines.....Moneypitt

[JayBee]

As I mentioned above - I already know the 850 is overkill on the 402. It was a deal I couldn't turn down - and if it doesn't work out well I'll sell it and get a 750 which would be much closer to the right size.
So, do you have any helpful information?
Thanks,
Jason

[Oldsquirt]

JayBee, I think what you are experiencing is the difference between a carb that can supply MORE than enough air/fuel and one that was marginal at best. Having your rpm flash and then come down is a sign you are now able to overpower your pump. Your old, smaller carb just couldn't supply the air/fuel fast enough to do so.

[Moneypitt]

The "HELPFUL INFORMATION" was contained in the sentence, "Do the Math". Even 750 cfm is too much carb for a 5000 rpm 402. All the tuning in the world won't show you anything at all with the 850, except for polluted oil from over fueling. To sell the 850, and buy the 750, try that, and still not know what size the engine really needs, certainly defies any logical HELPFUL INFORMATION requests.... Just my opinion.....Moneypitt

[Omega21]

According to the math:
402ci = 0.232638 cubic feet
.232638 * 5000RPM=1163.194
1163.194/2(intake once every other rev)=581.9 cfm
581.9*. 85(Volumetric efficiency)= 521 CFM.
521 cfm @ 85% VE (a little better than a stock motor)
581 cfm @ 100% VE (complete perfection, better than a very well tuned race engine)
You wanna rethink that 580?

[oldphart]

come on guys lighten up on him, if you do the calcs we would all be running tiny 2 barrells on our engines. Jet boats run at 3500 & up all the time and that throws the calculations out the window.
I race circle track and have a lot of back to back testing experience. The main difference between an 850,750,650 or 390 is in throttle response for the rpm run. we had carbs that screamed at Altamont (1/2 mile) that wouldnt come off a corner at Roseville (Tight 1/4 mile). the same for porting, small ports mean crisp in traffic, while big ports mean flat in traffic but runs like hell by itself. its all about velocity. installing a larger carb doesn't overfuel, depending on jetting its still so many cubic feet of air to so many gallons of gas. at idle we are all running the same carb.
To answer your original question, you do now have the power to overcome your pump out the hole. Thats a good thing!