Holley marin carb tech advice needed

[carburetorguy]

Hello all!
I am working on a Indmar 350/320 hp engine. Uses a List Holley 75021-4. Marine carb AD boosters, 600 cfm size universal unit. Have done 4 of these as off lately and the quality is nothing much to brag about. More so, the customer complains about performance lacking, max rpm is appr 4000. I my world that is about 500 rpms off.

I have a OEM Volvo Penta AQ 271 hp carburetor in stock. These are rated at some 715 cfm from the factory.

I am thinking of replacing the smaller carb with the Volvo Penta carb.
This to gain some additional airflow and hopefully performance.

What is your thinking about this sugegstion of mine?


It seems that the Indmar 320 hp engine realisticly was in the area of 290 hp not the advertised 320 hp rating.

I live in Sweden so not feasible to check it on the lakes due to the ice present.

Thanks
Peter

[rgalajda]

"Have done 4 of these as off lately and the quality is nothing much to brag about. "

Motors or Carbs? What kind of quality issues?

One would think 600cfm would be well sufficient for a 350 spinning 4000-4500 rpm , which I assume is where it produces its highest torque for a boat. Since it is a vacuum secondary carb , have you confirmed secondary opening.
Would be interesting How the 715cfm carb would work.
Have you confirmed good tune of engine timing , proper prop etc?

[carburetorguy]

Hi!
Thanks for your interest.

Carburetor issues)
Loose throttle shaft linkage brackets, debris i metering blocks, poor maching of transfer slots, loose boosters. Butterflies loose or maladjusted. All this naturally not unique to any modern production Holleys in my experience.

This Indmar 350 engine was bought brand a decade ago. The owner when interviewed says it never has reached the same speed/rpm’s as the old engine. The AQ 271 was a pretty fine engine. 9,5:1 compression, decent OEM cylinder heads, 2,02 in valve. Naturally not a world beater engine but a performance jump over the base 350 engine rated at 260 hp w.Qjet carb.

Pete

[gntkllr]

The newest "Numerical Listing" I have only has a -1 version in it... and the listed specs seem off for a 600 vacuum secondary... #68 primary main jets and #66 secondary main jets, 0.031" shooter, and white secondary spring (the base, no dash 75021 lists #70 and #76 main jets, and a red (?) secondary spring)...

First order of business is to get rid of any ridiculous power valve that is rated at 2.5"Hg, and install at least a 6.5"Hg. Can you tell us about the transfer slot widths and lengths (those might suggest installing transfer slot restrictions), the metering block PVCR sizes, the idle feed restriction sizes and locations, the main and idle air bleed sizes, and the "emulsion" bleed location and size situation?... because while I also believe a 600 is on the small side for a healthy 350, there's no reason it shouldn't be able to be made to work well, easily above and beyond a 4000 rpm limit...

[GTO Geoff]

Might sound stupid, but have the basics been checked:
- fuel pressure & adequate supply
- is the carb getting full throttle? You would be surprised how many do not.....

A muuuuuuuch better carb would be the Edel 600 AFB or AVS. Use the Holley for a boat anchor.....

[rgalajda]

[GTO Geoff]

..which doesn't sound like the carb currently being used...

[carburetorguy]

Hi again!
Sorry for the slow follow up.
In this -4 I don’t have any complaint about the transfers slots per say. It was a description on the several I have worked on in total. At my present location I have not means of measuring the blocks.
Thanks for your interest, I will make test later with the AQ Holley when the lakes are icefree.

Peter/www.carbguy.com