hondatv

Carb help... I'm running a 540cid

Gen5 block
Callies crank
Eagle H rods
SRP pistons flat top (3cc valve relief)
Block deck to .005
.036 head gaskets
AFR 305 heads w/ cnc chambers and pockets
Merlin Marine intake for 4150 carb
Demon 850 Marine
MSD dist w/ 6m box
Cam is a RM builder custom (Thanks for the heads,cam,and lifters) Bob
@50 242/248 646/646 112

Motor is in a jet boat with a AT pump and a SS a impeller turing 5400rpms= 500hp.

Carb is a BG mighty marine 850 w/91p 99m jets and 6.5 pv. After a 20sec run the plugs are still lean,
is this carb to small? I have called Doug at BG but no return calls.

pachangalpina

I can't help but Doug has probablly been tied up with the big National event here at the drag strip. I am sure that he will be in touch when he gets back in the office.

SB

iTrader: (1)

If the carb was too small, and with those big jet #'s I would think you would be very rich. I would think the A/F ratio would go lower,lower as rpm's went up. These carbs have annular boosters and a reasonably small throttle bore size for your healthy 540.

Check for fuel supply issues.

Cool thing about the BG's is that they have glass sight bowls that will show you if you have air "froth" in the fuel supply. A fuel psi guage does not always show you these issues.

hondatv

SB if I find froth, is that coming from the fuel pump or something else and how would I stop it?

I'm also going to add a 1in HVH super sucker spacer.

SB

iTrader: (1)

I have seen pumps and/or restricted fuel lines keep psi - as shown on guage even located just before carb - but not supply enough fuel.

Another common issue is the needle and seats not allowing enough fuel into the bowl. Since the guage is before the needle and seat, this will not show the bowls going dry or getting aerated. It is showing fuel volume/psi is fine all the way to the needl and seats, but doesn't tell you about inside the bowls.

These carbs come with clear sight windows for a reason. #1 to set fuel bowl level and #2 to see if fuel remains at that level with no air/frothing while at WOT and top rpm.

Note about fuel pumps : most are rated at 0 psi and with maybe 1 foot or less of fuel line hooked to them. This is called "Free flow rating." In the real world, as psi raises the fuel volume output goes down drastically. Fuel line length/ ID " / fittings add resistance. Sharp 90 ° fittings are no-no's. Keep everything straight as possible. Bends should be sweeping.

Electric pumps 'don't like to pull', therefore they should be mounted as low as possible and as close to tank as possible. A filter should be mounted before it to keep particles from damaging these sensitive pumps, and a finer filter should be used after (before carb/fi) to keep fuel line fragments and possible fuel pump fragments (when the pumps break) from getting into the carb/fi.

An electric pump that cavitates from improper installation (as discussed just above) will not provide proper fuel and will have quickly decaying internal parts. This is a real concern!

Many so called 100gph -140gph pumps have a hard time supplying enough fuel for modern 500+hp engines, especially with the typical non-efficent fuel pick up/ fittings/ line size, and routing many people install or re-use.

Lastly - electric pumps heavily rely on voltage. Voltage is key! Every volt less/more makes a huge difference in output. Always check voltage at the pump with engine running and pump running.

SB

iTrader: (1)

How are we making out ?

hondatv

I'm still trying to get some of the handling issues resolved.

So far it's been 86mph 5600rpm.

My spark arrester was too small 8 by 3. And I'm still trying to fit a 2in spacer under my hatch.

Thanks again for the advice, the front bowl was frothing up. Turned out it was a bad needle and seat. Many THANKS

Bob280Silencer

Small spark arrestor means less air means "fat".What size fuel line? Might not have enough volumne,sucking the bowls dry,leaning it out on the big end? BOB

Bob280Silencer

My bad,SB covered that,sorry.BOB

hondatv

I'm Running #6 line.

The float level is good now mid site glass at WOT.

Motor had 4in of vac. at WOT took arrester off and picked up 200rpm's and 0 vac at WOT.