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mid 80s 454 fuel flow scan installed?

D

denis_k

Member
got fuel scans installed this is the readings i am getting can anybody tell me if its close to norm 750cfm quadrajets 34' luhrs thank you
1200 rpms 2.7 gph
1360 rpms 3.4 gph
1650 rpms 4.7 gph
2000 rpms 7.2 gph
2300 rpms 9.3 gph
2420 rpms 11.6 gph
2550 rpms 15gph seems like a high jump?
2900 rmms 20 gph
3200 and 3250 at 21 gph
3300 rpms 21.2gph
 
Seems reasonable - which gear is behind the engines and how much prop are they spinning?

Hook up a vacuum gauge and see if the vacuum drops under 8" Hg around 2350; if it does, the "jump" you noted is the power piston kicking in.
 
denis_k said:
got fuel scans installed this is the readings i am getting can anybody tell me if its close to norm 750cfm quadrajets 34' luhrs thank you
1200 rpms 2.7 gph
1360 rpms 3.4 gph
1650 rpms 4.7 gph
2000 rpms 7.2 gph
2300 rpms 9.3 gph
2420 rpms 11.6 gph
2550 rpms 15gph seems like a high jump?
2900 rmms 20 gph
3200 and 3250 at 21 gph
3300 rpms 21.2gph
Click to expand...

THis looks to be on an overpropped boat. Is the 3300 WOT? I have similar floscan numbers, up to only about 2300. I get 12 gph at 3200 and
24 gph at 4500 wot.
 
It goes up to 3800 WOT i never push it pass 3200. If i go to smaller prop my numbers should improve? and what size is recommended for 34' Luhrs about 16000lbs 1.91 gears if i remember correctly.
Thank you for any info.
 
My Tiara 3100 has 18X23 props and I get to 4200rpms with the same transmissions and engines. You are quite a bit heavier than me so 18X19 may be reasonable. What are on it now? I'm impressed by those numbers. If I can interpolate those to my boat I go 26 mph at 3000rpm I get 1.24 mpg!
 
..."2420 rpms 11.6 gph
2550 rpms 15gph seems like a high jump?"

That's when those giant secondaries opened. Ideally, you want to stay away from that point for best mileage, as your Flo Scan is telling you.


You might be over-propped, but that's not necessarily a bad thing long as your boat is not underpowered. Mine will barely make 3900 at WOT, but it planes off effortlessly at minimal throttle. (It will cruise at 2,500 rpms with the same throttle setting it easily planes off at.) If yours is similar, then I'd leave the props alone.

Repropping to get max WOT rpms--as "everyone says you should"--will only reduce your gas mileage.

Jeff
 
Last edited:
fastjeff said:
..."2420 rpms 11.6 gph
2550 rpms 15gph seems like a high jump?"

That's when those giant secondaries opened. Ideally, you want to stay away from that point for best mileage, as your Flo Scan is telling you.


You might be over-propped, but that's not necessarily a bad thing long as your boat is not underpowered. Mine will barely make 3900 at WOT, but it planes off effortlessly at minimal throttle. (It will cruise at 2,500 rpms with the same throttle setting it easily planes off at.) If yours is similar, then I'd leave the props alone.

Repropping to get max WOT rpms--as "everyone says you should"--will only reduce your gas mileage.

Jeff
Click to expand...

Well Jeff my question now is can I do anything to keep secondaries closed or maybe use different model of the carb to improve my gas mileage some of my fishing trips take me 80 to 100 miles offshore. And if i keep it at 2400 i make 14.5 knt with perfect sea conditions, makes it for long ride.
Denis
 
Hummm.... You might experiment with rigging the secondaries closed, as they are when the choke isn't open, but that might not produce enough power on them primariies alone. It would take some experimenting to find out.

An Edelbrock 600 CFM marine carb has much larger primaries, so it could be run at a higher hp output on them alone. Edelbrock makes an adapted for Q-jet manifolds, and your old carbs can be sold to make up the costs. Something to consider IF tying the secondaries closed doesn't work.

Jeff
 
I doubt your secondaries are open at 2500 rpm. There isn't enough air flow to open the air valve and, until they open, the fuel flowed from the secondaries won't exceed one GPH.

There are a few adjustments you can make but before you start on them, you need to make some measurements to see where the carb is operating. Then, you need to make ONE change and repeat the measurements to ensure the desired effect occurs.

The minimal data that you need to add to what you have collected is the intake manifold vacuum values at the corresponding RPMs - RPM, GPH, Vacuum. Do both engines and don't be surprised if they are slightly different. If you wanna be thorough, add ignition timing to the list, especially if you want max economy.
 
denis_k said:
got fuel scans installed this is the readings i am getting can anybody tell me if its close to norm 750cfm quadrajets 34' luhrs thank you
1200 rpms 2.7 gph
1360 rpms 3.4 gph
1650 rpms 4.7 gph
2000 rpms 7.2 gph
2300 rpms 9.3 gph
2420 rpms 11.6 gph
2550 rpms 15gph seems like a high jump?
2900 rmms 20 gph
3200 and 3250 at 21 gph
3300 rpms 21.2gph
Click to expand...

Did yhou happen to record speed at those rpm's? Is it possible that at 2550 you are just about to come up on plane pushing a ton of water?. I have a 1985 "boating magazine" and looking at a Tiara 36 with 454's and 1.91 reduction(brand new remember) at 2500 it is burning 17.6 gph and at 3000 rpm it is burning 22.4 gph. At 3500 rpm it jumps to 34.4 gph. So I suspect somewhere between 3000 and 3500 is when the secondaries really open up.
 
I am assuming these numbers are per engine correct, not total? Suggest run it up to WOT to make sure getting proper RPM than can determine if props are right or not.

For comparison on our 1988 Tiara 31' with 18x21.5 props and 454 Crusaders with 1300hrs runs:

3000rpm 21-22kts, Port-12 & Star -14gph
3150rpm 23-24kts, Port-15 & Star -16gph
 
denis_k said:
got fuel scans installed this is the readings i am getting can anybody tell me if its close to norm 750cfm quadrajets 34' luhrs thank you
1200 rpms 2.7 gph
1360 rpms 3.4 gph
1650 rpms 4.7 gph
2000 rpms 7.2 gph
2300 rpms 9.3 gph
2420 rpms 11.6 gph
2550 rpms 15gph seems like a high jump?
2900 rmms 20 gph
3200 and 3250 at 21 gph
3300 rpms 21.2gph
Click to expand...

A total of 40 gph at 2900 rpm's for dual 454's? Seems high to me for a 34' Luhrs.
Nightstalker
 
the numbers were for my port engine right after the sensor was installed I went for a ride to test it before doing the other side. my starboard side hits 20 gph at 3600 rpms after testing the boat with both sensors in place and at 3250 starboard side burns 15.5 big difference after a while i noticed port engine before going in fuel sucking overdrive mode drops rpms alittle bit i touch the throttle and rpms pick up and fuel burn goes way up only on the port side compared to starboard side after going on 160 mile trip flow scans and gas pumps gave me same reading. Should I swap carbs before investing in the new one and see if the problem follows i check everything else by now.
THANK YOU for any info and ideas
Denis
 
The carb is probably due for an overhaul with a good kit. if carbs don't scare you, you can save several bucks doing it yourself, just get a GOOD kit that will have everything you need.
 
call Cliff - 740.397.2921 - his kits have top quality parts and are the most comprehensive I've seen. For completeness, have your 170xxxxx carb number handy before you call. He can also supply most any other "hard to get" item, too.
 
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