Formula78

I am in the process of building the motor for the Formula again and have some questions about the carb. Here is some back ground:

Motor: 385"SBC, Sportsman 2 heads, Performer RPM intake, XM-272-HR Cam, full roller valve train. Timing at 8 deg BTDC @ idle.

Carb: rebuilt marine Holley 650 Vac secondaries off the shelf. Probably intended for use on a Merc 260.

Last year when I had the boat out I ran it for two hours before it came apart due to improper clearances in the rod journals. So this year we have a fresh shortblock (again) 4340 forged parts and all the goodies. We never got the carb tuned right the one day we had it out and I am hoping to do something before we get to the water again.

It was a DOG once throttle was applied it acted as if it were lean (poping through the carb) and would frequently die trying to accelerate. Hole shotting it was totally out of the question as it would instantly die. It would then require a few pumps on the throttle to start it again. It idled like a champ, but had no acceleration off idle. Once planed it ran like a champ when given healthy throttle from 3,000 RPM to 4,500-5,000. Really felt very good but did not want to push it hard in the break-in process.

I am guessing it has the standard 30cc accelerator pump and was wondering if I should slap in the 50cc pump and hope for the best or is there something I can do to get it closer to tune given my combo above? I know that whatever I do will require 'tweaking' and tuning on the water but I am hoping to get it closer so that I can at least plane it out with relative ease on my first outing.

I have only worked with EFI in the past and am new to the carb thing so any help at all is welcome!!

Attached are some pics of the motor in the pre-blown-up state, as well as some nice carnage shots.

articfriends

iTrader: (1)

Sounds like you got fuel delivery issues or mismatched gskts on primary side of your carb. The thing i know for sure is a 50 cc accelerator pump isn't the answer. If your carb is only a 650, and i assume its a square bore,not spreadbore,then you really only need # 25 squirters,maybe # 31's but doubtful. Ive used # 37's with std accelerator pump on bigger motors and it was never a issue. Things that come to mind off the top of my head is 1.Check to make sure you still have needle under squirter in the primary,you can do this by carefully reaching down primary side in the center with phillips screw driver and removing squirter,at this time note the number on the squirter,most likely # 25. 2. Check accelerator pump linkage,verify that someone hasn't preloaded the cam arm against cam so much that it isn't getting a full stroke. Also,make sure your seeing some fuel squirt out of primary squirters if you rotate throttle linkage. If your not seeing anything then disasemble carb,maybe whoever rebuilt it used the wrong gskt or put it on upside down and it blocked off passage between jet plate and the bowl 3.Check primary float bowl level,you can do this by just unscrewing sight plug on side of carb and idleing motor(do this actually before you do step 2). Whatever your problem is it leans towards something on the primary side of your carb (as it runs ok w/o). If in doubt,buy a holley guide book for 20$,it will detail all the things i mentioned and alot more,Smitty

BAD-HABIT

I had the same symptoms on my 358SBC. Turned out to be a leaking throttle body/plate. I tried everything to fix the symptoms was to install a 3.5 powervalve. Even this was only a band-aid. I ended up getting a BG Sea Demon carb and it was PERFECT right out of the box! With my old Holley I would get a 200rpm loss from Out of gear to in-gear. With my BG I get a 50rpm Loss!!! I spent ALL summer tracking my carb problems. Buying the BG was the BEST thing I did. I read a tech article from Teague and he reccomended the going with a 2.5-4.5Powervalve with a similar problem. Main thing is check your plugs!

BH

BAD-HABIT

With the BG I got the best results with the out-of-the-box 6.5 Powervalve.......

blkhorse

iTrader: (1)

debri in the carb can also cause that problem

RumRunner

Formula 78,

There shouldn't be any need for a 50cc accelerator pump if your carburetor is setup properly.
There are a few things that can cause this type of situation:
1) Squiter nozzle size too small for engine
2) Butterflies out of adjustment
3) Float levels too low
4) Dirt, or debris in carb
5) Too much ignition timing at idle (not very common)
6) Carburetor incorrectly tuned

Your best option would be to get a book on carburetors, a kit, and go through it before going back to the water.

formula31

Ummm< I had to reduce the size of my squirters and increase the size of the cam to maintain a longer squirt to provide a pump stream until the engine rpm was high enough to take over the mains. Bigger squirters hurt that effort. I use 21's on my 780's. No one had talked about the vacuum secondarys. If they are opening early,especially under acceleration loads, they would cause the bog. I am running a 50 cc pump to maintain a stream until the engine rpm creates enough pull on the carb to draw from the mains. each engine and combination is different. Have someone (with no hair)park their head above the carb as you go through this and watch what is happening. If the secondarys open when it bogs, you have a weak secondary spring installed. You dont want the secondary to open until you have a maintained load, 4000 rpm and up, roughly. A bigger cam will reduce the vaccum and change the "pull" characteristics on the carb. It took me 3 years to dial in my 3310 780's I use on my bbc's. Twas not easy and I had to go against "common knowledge" I did find an old engineer at Holley who told me what to try. He was right. If you can afford new carbs, do that. If not, be prepared to experiment.

RumRunner

Formula31,

Unless there is something completely wrong the secondary spring should not have any affect on his off idle acceleration. Having to make the changes you've made is not very common, it may be an indication that something in the intake tract is too large where there is not enough air velocity, or there is a problem with the primary circuitry in the carburetor where its not starting quick enough. If it's working for you that is great, but it may there be other causes.

Doug

Badtoon

Call Nickerson!!!!!!!!!!!!!!!!!

Attack

Call Dean Nickerson 215-781-1370. He is doing the carbs for my 750's. They will be dialed in specifically for my engine set up.