800 cfm too big?
 

I am running the QFT 800 cfm dbl pumpers on 500 hp engines. They had 77's and 86's for main jets and the thing was really rich. It ran well but really rich. Soot on the transom and all of the plugs were completely covered in soot. I decided to order and install stock jets and go from there. I reached out to Holley and got the info and ordered them. The mains are now 74's and unfortunately I threw the other box out and don't have the info for the secondary's. The problem I have is that the thing is still rich. Low 11's everywhere and actually in the upper 10's when idling around 800 rpm which brings up the other "problem" I think I have is that the idle screws are only out 1/2 turn. I did turn them in till the AFR was in the 12's but that only made it lean pop(?) when I hit the gas so I went back to the 1/2 turn. The vacuum is low too. The QFT's have 2 small ports in the front and one on the side. None of them showed any vacuum at all. I hooked the gage up to the large PVC(?) connection in the rear and got the gage to move however never in the "green". FYI the cam is 230/[email protected] and only .547 lift. I had to buy new wide band 02 sensors for this season and I did go through the process to calibrate them however the port side only worked for the first few starts now just stays on "HTR" so all of the info here pertaining to the AFR readings is from the starboard engine. I am wondering if I should drop back to 750 cfm? Will 50cfm make any difference?? Just for reference, the last 1/2" or so of throttle movement (full throttle) makes no apparent difference. I did not pull a plug to see if they are covered in soot. I am also wondering if the AFR unit I have is working correctly. Thanks for any help.

David Vizard has a great Holley Tuning book. Worth the read and reference for sure.

The 750/850s won't flow exactly the their model cfm, but the 750 is obviously smaller and will produce a better signal to the down leg boosters. Since you have the 850s, they should work great for your 500hp level.

Getting a bunch of jets in the ballpark of pri/sec is a good idea. The Holley Jet Kit has a lot of them that you'll likely never use.

Are you using the pri/sec power valves? What are they?

Aftermarket metering blocks enable more tuning.

What size are your PVCR holes vs primary/ secondary jetting vs PV ? You need to get working 02 sensors otherwise you have no idea where its actually rich. IF you assume its rich at wot but its actually rich at cruising part throttle, then you pull secondary jetting you might end your boating season EARLY.

You bought these carbs used ?

Are they 800cfm marine (model # m-800) or something else ?

vac guage in green you said. What is the “hg number the needle is at ?

Why would you not calibrate new 02 sensors ?

What is your initial timing set at ?

Are these 500hp motors 454’s or 502’s or ?????

What exhaust manifold/tailpipe system on these ?

Float levels are where ?

Fuel psi is what ?

SB,
1) I bought them new from Holley.
2) They are the Marine carbs with mechanical secondary's.
3) The vacuum gage did NOT go into the green. I'm going by memory but I think it was around 15. Maybe this info is incorrect because I left the drives in neutral and maybe it was supposed to be in gear? I can retest in gear if needed for a correct reading. Just let me know.
4) I did go through the process (successfully) to calibrate the new 02 sensors. I think they are full of soot already and the one has quit working.
5) I have a run ready marine distributor from MSD. You guys will know these parts but it has the "black" advance part installed along with a light spring and a medium spring. 33 degrees total advance all in around 2200 rpm. This info is with the drives in neutral.
6) Stock bore and stroke 454's
7) Thunder exhaust from Eddie Marine. I have the long stainless risers. The tips are angle cut straight through. No mufflers. I had reversion trouble last season so for this year I added silicone hoses with a 90 degree bend.
8) The float levels are correct. Maybe a little high but marginal.
9) I don't know the fuel pressure. Still running the stock belt driven pump that is combined with the sea pump.
Thanks for your help.

I wish I had know to look at the power valve(s) when I swapped out the main jets. Everyone here knows that I have much to learn and it's going slowly. I have an email to Holley to see if they can give me the info. If they send the info I'll post it here. If they don't I'll take them back apart and find out. FYI, every time I looked at the AFR for the one engine still giving info, the numbers were always in the 11's.

All good.

It's a good idea to get bowl gaskets that are designed to be re-usable, changing jets can be fast and painless.

If you got the carbs new from Holley they should have known PVs, as part of the standard offering.

If you're up to it and it's likely and easy access, you can check your O2 sensors and swap them to confirm the non-working afr system is the O2.

From:
https://www.holley.com/products/disc...ct/parts/M-800
Click on ‘Specs’

SPECS Barrels 4 Billet Color Diecast-No
Billet Booster Down Leg
Brand Quick Fuel Carburetor
Use Marine
CFM 800
Choke Electric
Circuit 2 Clearance
Category Carburetors and Components Condition New
Discount Percentage 26-50%
Emission Code 3
Finish Black
Fuel Gasoline
Fuel System Carbureted
High Speed Air Bleed 28
Idle Air Bleed Size 70;67
Marine Engine Size Universal
Marine Use Yes
Material Aluminum Model 4150
Primary Main Jet 78
Primary Power Valve 65
Primary Pump Nozzle Size 32
Secondaries Mechanical
Secondary Main Jet 86

SB,
This is interesting. I asked Holley directly "what do the carbs come with for jets from the factory" and received the wrong info. This probably explains why the boat is down on power. I bought a new pair of 28 pitch B-1's and wasn't able to get past 4500 rpm. I'm going to order David Vizards book and try to educate myself on this topic. I may have the components confused but the power valve shown is 65. Isn't that supposed to be 6.5? That opens quickly correct compared to a 3.5? The funny thing is I was told that my engines had a ton of vacuum but when I checked it was below the green range. Should vacuum be measured at idle in gear? Also with that in mind I have the idle set at 800 and when in gear it drops to 650 ish. I bring it up to 800 to 1000 rpm when actually underway. My thought was it's easier on the drives while shifting with the lower rpm. If I check vacuum in gear what rpm would you suggest I use to check vacuum? Thanks again for the help.

Different topic but cleaning the 02 sensors is possible? I looked online and there were several different idea's on how to clean them. Mine are new with less than 5 hrs on them. I was going to use carb cleaner but a little hesitant for fear of wrecking the sensor. Thanks for the help;