Carb Stumble or Bogging?

[KRH1326]

Hi Everyone,

Now that I have my Major problems worked out for now, I can work on some of the Minor ones.

I idle out of harbor at or under 1000 rpm, because of no wake zone. Once I hit the bay, I can throttle up. As the RPM's come up, mostly at about 1500 to 2000 rpm under load, trying to get speed up for plane, the engine kind of bogs. At that point it will either stall, or if I goose or bump it a little, it will overcome the bogging and continue on, climbing rpm's.

All of the ignition parts are new as, this is the first season on the water with this project. It has a new fuel pump and a new fuel filter and seperator with a gauge hard piped in the discharge, and am getting steady fuel delivery. The engine is tuned real well. Plugs, point gap & dwell, timing setting, even checked with advance curve. Beautiful idle @ 500 rpm. All seems well.

The carb is a Holley 4 bbl. List 7163. It was rebuilt by Lyle Stahle , at Stahle Performance Carburetors, Inc. a reputable carb shop. It basically is :

600 CFM Holley Vacuum Secondary Carb List #7163. Original equipment on a Ford Marine 351 engine, this carb would be well suited for 350-400 ci applications. This carb has been rebuilt with a genuine Holley Kit and is ready to bolt on and go.
Standard Features:
? #65 Primary Jets
? #25 Power Valve
? #25 Primary Pump Shooter
? 30 cc Primary Pump
? New .097? Viton Needle & Seat
? New Vacuum Secondary Diaphragm
? Power Valve Blowout Protection
? Electric Choke Standard
? "J" Style Vent Tubes
? Universal Marine Throttle Lever


I am usually alone in the boat so it is difficult to see exactly what is happening, but recently, I took the old man out, and tried to snoop around while he was driving.

I am not a carb guy. I never ?got it? and never will. Not that I know what I am looking for, but the carb seemed to be doing its thing. If he increased throttle, it got a little shot of gas, but I did notice that when it was trying to bog down, if I assisted the secondary bttfly, it immediately took off like nothing was wrong.

I have the OMC Select-Trim, which actually raises and lowers the front of the engine. I have been researching and it may be possible that if the engine is tilted aft, and natural g-forces could affect the bowls? I have not noted what position the engine was in as this happens, that will be my next mission.

Could you please give me a direction to look in, that may help?

 

[Maclin]

Re: Carb Stumble or Bogging?

Sounds like the accelerator pump shot is not quite dialed in yet. Several items affect the duration of the shot and the modulation, meaning more at first then wanes or even shot all the way thru. It is also important to be sure the pump lever is adjusted corrrectly so the pump shot starts right away as soon as the throttle is moved.

There is a cam that moves the pump lever. There are several different ones that can be used. There is also 2 positions for the cam installation, for this engine the #1 screw hole would be correct. The white cam provides an even shot all the way thru. The green cam moves the lever more at first providing more gas. Big blocks usually like this type of shot. The squirters determine how long the shot lasts. Sometimes it is hard to tell the difference between a lean bog which wants more pump shot and a stumble type bog which needs less. If the pump shot lasts too long then you can actually have both, lean at first then rich. Your engine may be wanting more gas faster, and as the shot runs on it bogs down. When you open the secondaries the remaining pump shot is used immediately and the engine wakes up.

You could try #28 squirters, that way the pump shot would not last as long. My best tune for 383+ cube MoPars was a green cam with #31 squirters, that is what they wanted. A lot of fuel at first to cover up the lean, then less as the mains started flowing and the secondaries began to open and flow. Along with this is a quick advance curve which I believe your distributor is setup for.

Coupled with all of this is the secondary opening rate. Thsi is determined by the spring used in the vacuum diaphragm can. Your engine may want more carb sooner and a lighter spring may be in order. The purple spring is probably close to what your engine needs. If it has the plain spring then I think it needs to be changed out.

It is imperative that the pump lever be adjusted correctly. There should be no gap between the lever and cam, the lever should move anytime the throttle is moved. The correct adjustment is made with throttle all the way open, the lever should not bottom out but have just slightly more travel left at that point, Holley has a spec for it and I am sorry I do not have it memorized as I just make sure the lever is not bottomed out. Some misread the adjustment and think the clearance is at the first part of the throttle movement, but not so. The adjustment is to make sure the lever does not bottom out and bend itself or rupture the pump diaphragm.

I would try #28 squirters and the purple spring and see if it gets past the bog and wakes up some. I am also thinking the primary jets may be too small, that they should be 69-70's but the best way to determine that is reading the spark plugs once the bog is tuned out. And they will not cause as severe of a bog as you have described so worry about that later.

There are some informative books out on Holleys, and if you want to tune it yourself they can really help you decide what to do next.

 

[KRH1326]

Re: Carb Stumble or Bogging?

WOW ! ! ! GREAT ANSWER ! ! !

Thank you for the ammo, I'm goin' huntin'.

 

[ringmaster72]

Re: Carb Stumble or Bogging?

I believe he said that when he manually opens the secondaries it took off fine. If that is the case, wouldn't the problem be in the secondary area? Vacum Secondary Holleys do not have an accel. pump for the secondaries, only the primaries. I am by no means a carb expert, but I am learning now as I am running into similar problems.

When I manually kick open the sec. the boat runs great, pulls hard to over 60 mph. When I left it up to do it itself, the motor runs to 3K rpm, then fell on its face. I went back and watched the carb and notice that the sec. would not start opening until 3Krpm and then the gas would hesitate flowing out. I tried all of the springs to no avail, no change. I then discovered that I had a fuel pressure problem which afffected the float level, which affected the sec. gas coming in.

So, the fuel pressure prob was the check valve in the holley electric fuel pump, which I have repaired now. So, I am going to adjust the float levels today and give it another shot tomorrow. However, I bought a Edelbrock Carb in case the Holley is still giving me problems.