NHGuy

I have been working on dialing in my Quickfuel M-750. Particularly the midrange AFR at the top of my primary rpm just before my secondaries open.

Those of you who speak carburetor will know that that is the point at which the power valve opens to enrich fuel supply. I have only been working with carburetors for the last few years so I did not understand it that way until recently.

I have a Webber which is a variation on a Carter/Edelbrock, and a Quadrajet too. They all have vacuum referenced enrichment. I imagine most vehicle mounted carbs do.

I had some troubles with the way my engine was idling a couple of years ago and I am now thinking that it was due to blown power valves. It really drove me crazy. At the time I was using a marina which had a few hundred yards of idle to get in or out. And I couldn't tell if it was ignition, reversion, fuel or what?

I recently saw someone in a video using the suction cup attachment from a vacuum pump kit to test power valves so I checked my old saved PV's. Blown.
I had thought that my carburetor had a blowout protector since Holley has included them since 1992. Now, since I have some broken PVs, I doubt it.
And I remember getting backfires during some setup times. Mainly if I had the distributor badly positioned when starting my engine.

I bet that the poor idling I got was due to the power valve being leaky when I thought it was OK.

Quickfuel is now a Holley brand and I found there is a power valve protection kit available for cheap, ($13.68). It looks to be a pretty simple installation and I may do one.

Thought this would be a good post to leave in case others get a hard-to-explain rich stumbling idle as I did.

In my case I plan to install my distributor with enough advance so does not backfire, and then dial it back to it's right timing. And til the power valve is protected I will probably use a blocked valve in case I do get a pop. It's a pretty easy change.

SB

iTrader: (1)

Also Surprised the Quick Fuel has no pv protection. I would have assumed they did too.

xlint89

Forgive my ignorance. Protection from what?

I have a brandy new M850 that was perfect when installed. Now after a couple outings, I notice it stumbles when going from neutral into gear leaving the dock.

SB

iTrader: (1)

Power valve protection from backfire

NHGuy

Xlint89, I found that a 32d to 16th extra turn out on the 4 idle mix screws corrected a stumble going in to gear (once engine is warm). Just a touch of idle fuel on mine. But my idle screws are not very far off their seats. I went from around a half turn off the seats to maybe 17/32ds. And each engine is different, And you have a bigger carb too.
Mine will idle and shift at 500 or below now. Though I normally keep it around 700.

Alternatively you can tie the boat to the dock and set it in gear, or have someone steer it in gear while you set the idle screws in gear.


If it just stays sloppy and needs revving you can check the power valve this way.

Pull the carb off and/or pull the fuel supply. The supply blocks one of the bowl bolts. Remove the front fuel bowl and metering block, take out the power valve (1" wrench or socket) and apply vacuum or just suck on the flat side. If it does not hold vac you have a blown PV.


I didn't realize mine was bad and drove around with rich idle for weeks. It was embarrassing, stumbling and having to clear it out constantly.

Holley owns Quickfuel. I emailed their tech line about it, even referenced the check ball install kit that is offered by Quickfuel...they emailed back me that the carb has power valve blow out protection.

Well, How did I get 2 blown PVs? I did get some backfiring while having other teething problems on my build.

I am a first time engine builder on this high-ish compression marine 383 small block.


If you are getting backfires and don't want to drill your carb for the protector you can just get a power valve block off to put in the PV location while tuning base timing and idle. Once you are past backfires put the PV back.


HTH

xlint89

Thanks for the replies guys.

I did find vacuum gauge adjustment of carb idle screws on trailer was pretty good. Had about 16 1/2" of vacuum and quite smooth idle at 5/8 turn out. Once in the water, was a bit off, added about 1/8 turn out and that helped with the idle in water. Engine would fire right up and accelerate crisp.

Now, it fires up, sounds a bit off and slightly stumbles when put in gear. That's why I was wondering about the PV.

Never back fired, so that's prob not my issue.

I'll double check spark plugs and timing to confirm nothing else is going on.

Mine is a HP 500 carb with the stock 110* cam. Maybe I'm reverting and causing me issues?

NHGuy

Reversion would surprise me on a stock build, but folks who know these engines will tell you. You can inspect spark plugs or exhaust mani's for wetness too.

dvr4hire

Hey guys. I wont go into detail because it would be a book. I just went through about 10 hours of tuning a 750 quick fuel that would be very lean 16.0 afr just off idle and be very rich 10.0 afr at anything over 3/4 throttle. Long story short, after swapping many jets, power valves, idle restictors, air bleeds, and converting to 1:1 throttle plate opening, It was never right.The fuel curve was not linear at all. It would very lean part throttle and very rich under heavy throttle of load. If I got the top end afr good the mid range would be too lean at cruise RPM 3000ish and 15.0 afr. I think it could be fixed with different boosters and/or messing with power valve restrictior sizes, but in this case I just sold it and went back to a real holley carb and dialed that one instead. It seems they wet flow these Quick fuel carbs and set them up to be nice and lean at criuse and dead rich under full load. This is perfect for a street car, but obviously wont work so well in a boat that needs more fuel flow in the mid range CFM flow areas.

Griff

Is it hotter outside with more humidity? You may have to adjust the idle mixture slightly. Also, bump the idle up about 50-100 rpms to reduce reversion.

Carbed HP500's were notorious for reversion at idle. Merc changed the exhaust risers to help combat the issue.

xlint89

Yeah it's def been more humid than when i originally set the idle mixture.

I've got the CMI Fusion headers (one-piece header and tail pipe) installed. Not sure if they are any better than the tall risers Merc used to combat the reversion prob?