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;

The green is for typical bone stock automotive engine in a car/truck.

Performance motors will have less idle vacuum. How much less depends on camshaft vs cids vs initial timing vs idle rpm….etc

I’ve had non marine performance engines idle with 3-4” at 1000rpm. Braap ! :)

You’re 15” is more typical of a mild marine performance engine. Again, forget the color, just look at the #.

A 65 powervalve is one rated at 6.5”hg
A 45 pv is one rated at 4.5”hg

I ran a similar spec'd 800cfm on my 425hp/454. It was a little rich, but worked fine after I started using Rapid Fire spark plugs. They wouldn't foul out like stock MR43's

6.5 Pri power valves are pretty standard.

When your intake vacuum drops to 6.5" hg, it opens, and flows extra fuel. It's tunable on the metering block for the jetting and air bleeds.

As an example, different props will load the engine differently. Lower pitch will be easier on the engine at the same rpm, and have higher intake vacuum. The engine is not working as hard, and less throttle is used etc.

The DV book is well laid out and easy to read. He wanted to put more info in there than the publisher wanted. I think he has some good YT vids on carbs too.

Any effort you put into research will exponentially help you out. It will also enable the folks here to get you dialed in even better.

If you want to feel better about your situation, go check out my thread on dealing with carb tuning. Has been a journey!

I'm not impressed at all with anything Holley says on the phone - call with the same question three different times and you'll get three different answers. Putting the carbs back to stock with what SB listed and trying again is the right direction.

Keep us posted.

Hey Tom, I have read through some of your carburetor woes. I'm hoping to skip to the end where it runs perfect after one more change :D
I have a few questions for you guys while I wait for Vizzards book to show up.
1) Do I check vacuum in gear?
2) Thinking about the idle mixture screws, they are only out 1/2 turn. I saw a guy talking about this very thing and he said you DON'T want them just cracked open. IF he is correct, do air bleeds affect the idle circuit? If I installed larger air bleeds it would require more fuel at idle? I could turn the screws out to make the entire "circuit" run more efficiently??
3) I posted that my vacuum is around 15 out of gear which would indicate that I need a larger power valve than the 6.5 that comes with the carb? If the power valve is improperly sized, is it possible that it is dribbling fuel at idle which is why my adjustment screws are only out 1/2 turn? The power valves are adding fuel when they shouldn't be?? Or would it be more likely to dribble fuel if I installed a larger (7.5) power valve??
Again, I ordered the carb book and I'm looking forward to being less dangerous and more educated on this subject.
Thanks for your help.

This guys has a pretty good video on tuning the holleys...helped me out earlier this year when i was dialing in a carb for a guy here at work that has a 71 mustang

Power Valves are spring loaded in the open position.

When the engine fires and pulls vacuum, they close up. A 6.5" PV only opens when the engine is throttled up and cruising at speed. When the vacuum reaches 6 5" of course.
​​​​
​​​ Power Valves can leak and have ruptured diaphragms. But if the carbs are new, they should be gtg.

It's nice to have a vacuum guage in the instrument cluster. It can also be used as an engine health meter.

Just an update for this thread. I have been working with a guy at Holley and he sent over a different spec sheet for the M-800 which I believe is the correct info. I had asked my engine guy to richen up the carbs and when I first pulled the carbs apart there were 77's in the primaries and I think 86's in the secondary's. The other sheet shows 78's in the primary so this can't be correct based on my scenario. Anyway I put the carbs back to completely stock spec's including the main jets back to 74's and 84's.
TomZ has been helping me and he ran across a thread (car site) where a guy says that there is a hole in the baseplate that is connected to the transfer slot in the secondary's that shouldn't be there. It allows unmetered fuel to flow rendering your idle screws almost useless which was what I was experiencing. He used JB Weld and filled them in and then everything worked fine. I was thinking about replacing these carbs with actual Holley's so if this didn't work I was prepared to replace them. I took a look and my carbs did have this hole he was referencing. I filled them in and my afr readings are way better from idle which is now mid to upper 12's to wot which is in the mid to upper 11's. Idle used to be in the low 10's and even some 9's. Soot on the transom etc. Idle screws are now out 3/4 of a turn instead of only a 1/2 and it is pulling almost an inch more vacuum at idle. Thanks again to TomZ for his efforts.
Fix that Rich Running Quick Fuel - Club Cobra

i have a 4360 cubic inch radial 28 cylinder you should see the size of the carb on that!

crazy ! And thanks for telling us.
Thanks to tomz , of course too.
:thumbs

And a shame on and FU to quick fuel and holley. Wtf people ? Twice the $$$ over last 1/2 dozen years and make it junk ? Not cool. :(

I meant to attach this sheet I received from Holley. Just FYI
https://cimg1.ibsrv.net/gimg/www.off...2e95937d41.png

What a cheap out move on Holley/QFT’s part. They know this is a problem but have not addressed.

And their tech guys are clueless to boot.

Rob, glad I could help you get her figured out!

Like I mentioned in our conversation a while back, I have yet to see a QF holley that wasnt a hot mess, the last one I tuned for a boat engine needed IFRS drilled larger, jet feeds reamed, transfer slots doubled in length, other passages pin gauged and adjusted, work on their stupid emulsion garbage, extra emulsion holes added where holley usually has them when they don't have bleeds, base plate needed circuit work, PVCR enlarged, different IAB, had .028 hs bleeds from holley because they needed to bandaided the main jet signal because fuel passages weren't conducive to flow and signal sucked, got rid of the J hooks because they put vacuum on bowl and disrupt fuel to main jets, added the offroad truck crossover vent tube, the sets screws in rear plate that fill the passage holes for when they drill it for fuel were loose and falling out, the throttle stop had bigger threads then the set screw so it was falling out and needed fixed then epoxied, the arm that opens the secondarys needed tweaked because once stop was actually correct, blades werent square. These are just the things off top of my head, there were others. ITS main issue was it was blow up lean at 15 to 17-1 at part throttle and too rich at wot, the circuits needed serious work because just adding MORE jet to get part throttle killed the afrs at wot and it transitioned terrible. Ran perfect once all that was done and i dialed it in on dyno then boat in no time fwiw, Smitty
OH yeah, this was brand new out of box carb too. Once all this was done, I was able to go down 8 to 10 jets sizes and afrs were on target at part throttle and wot, transition was good, idle was good.

I can concur with this mess via the research I did to fix my Demons (not really Demons at this point). I’m not sure how QFT/Holley is getting away with this garbage, but they sure are and they sell a lot of them! Similar stories are coming from the other specialty carb shops, too.

One thing that stands out… the guys that really know how to fix these contraptions just about always suggest going back to the tried and true Holley 3310’s metering. This will get you much closer on a marine carburetor (for cars it’s a miraculous fix).

Looking back at my dyno notes, this carb I was describing did NOT have J hooks, the holley "truck" tube vent kit though richened it at least equivilant to 3-4 jet sizes from regular vents. Removing the j hooks and going to regular tubes also richens them when you get rid of whatever evil thing the vacuum on bowl assembly from them does.

I chucked the J-tubes.

Sorry to side track, what's a good carb to buy in the 800-850 CFM range? Thanks

Believe it or not, i’ve never done this. How do you guys pull them out ?

You can stick a large screw driver thru the J and twist back and forth or use good vise grips and a hammer.. By time there out, they are NOT reusable BTW.

so, with carb bolted down it seems .

Thanks !

I think I'm all set. It turns out that these are not too big for my application. I put the timing back to 33 degrees. I messed with the idle screws again and now have them at one full turn out at the 4 corners. AFR numbers IMHO look good from idle to WOT. I put up another video at the end of the Top speed run with B-1 26's using the 28's and the speed and rpm are up from my first disappointing run. Still going to order a set of 26's because the overall performance with them was way more fun but it's running pretty strong now. Thanks to everyone for your help.

Keep reading Vizard's Holley book. You'll need it again eventually.

Glad you got it sorted out.