Hi,
For about 10 seasons Ive been using a holley 750 double pumper on a 350 HO crate engine in my 20 ft boat.The engine has been smooth and strong all this time. This year I'm having a lot of problems with idle and/or hesitation during take off. Ive played with the air-fuel mixture screws every weekend since May with no luck. Any suggestions? I was thinking of changing to a less sensitive quadrajet. Also, timing seems to be dialed in. I would really like to set it and forget it again for awhile. I'm also reading conflicting articles for the 350HO. Some say 750cfm others say 600 cfm.
Which is it?

Thanks
Tom

600,,,the 750 is to big for a 350 engine !!!

In 2000, my friend and I took a 2000 260hp 5.7L Vortec with a 2 bbl carb and switched it over to an Edelbrock RPM with a 600CFM holley square bore (4150??, don't remember anymore) with a 1" spacer matched to the carb and intake and a K&N marine air filter. Also matched the intake gaskets to the intake and heads. Boat went from 50 mph to 56 mph! We were impressed! We did back to back run on the same day for speed comparision (we had everything pre-assembled so we could do the switch rather quickly). Anyways, the 600CFM was perfect right out of the box and only required minimal air screw adjustments. Jim V helped help guide us getting all of the "right pieces". I think a 750 would be too big, giving poor fuel/air signal around idle, IMO.

750 is way to big for a 350 in stockish form, get you a 600-650 edy on there, fix all the bs...

John jr

Thanks for all your advice gentleman.
Any suggestions for a good 600-650 cfm and where to buy? Eldebrock model#?

Thanks again

Tom

http://www.summitracing.com/parts/ED...?part=EDL-1409

Sorry to add this to the mix, but I just dug up the documentation that came with the engine. Believe it or not GM recommends a Holley 750. Thats probably why I put it on back then. Anyway, I'll definetly look for the 600 or 650. BIGJIM, thanks for the link. Nobody has mentioned a Rochester Quadrajet. Stay clear?

Tom

before I would run a Q-jet I would run a edelbrock carb, I run them on my 454's in my gibson and have used then several times on boats...work well and easy to tune if needed, also merc used a variation of them OEM for the last several years before EFI...I run 600's on my big blocks, but they rarly see over 3500, if looking for economy run a 600, more performance a 750...Rob

A Q-jet is a 750-CFM. They did make an 800 CFM version that mainly came on the 455 Buick including the 1970 455 Buick Grand Sport. When GM put the Q-jet on the 305 they put a little limit tab on the secondary air valve which would cut the CFM back to around 600. However all you have to do is to cut the little tab and then you have the full 750 CFM. A Q-jet is a little more complicated to work on than the Holley is. Due to the small primary butterfly’s, it will deliver great fuel economy and throttle response providing that you stay out of the secondarys.

Not true (except for sq bore dbl pumpers), most are vac 2ndarys and they are on demand carbs that work on anything from an i6 to big block.. Qjet is the best carb ever.. small primaries = best torque / economy.

"Although the basic four-barrel design proved quite successful, carburetor engineers discovered that throttle response and fuel economy could be improved by decreasing primary size, and by increasing the secondary size, the engine’s maximum flow requirement could be fulfilled to maintain strong full-throttle performance. A highly efficient, well-balanced unit that was practical for virtually any driving condition was the end result."

http://www.hotrod.com/articles/hppp-...et-carburetor/


On most qjets the primary cfm was from 180 to about 230 cfm..!