Boggs at high RPM - Help

[GrillMaster]

Getting a stumble/bog from my 351w when I floor it and rpms go way up. running a 650 4 barrel holley with vac secondaries. Pulls hard for the first 75 feet then I het a pop pop stubble. Where do I start to troubleshoot? Timing, Jets?? Thanks

 

[KyleQ]

Sounds like you are running out of fuel -

 

[GrillMaster]

think I should just go up 5 jet sizes or so in on the secondaries?

 

[Explorer]

"Could" be as simple as low fuel pressure or secondaries coming in too soon or too late. I haven't used your specific carb, but the quick change spring kit made tuning a lot easier on the other Holley's I have owned. I know nothing about your MSD setup, but timing could be an issue.

 

[KyleQ]

Running out of fuel as in plugged fuel filter, pickup, or something -

Although jetting at hi-rpm could be the case as well. Its fuel related-

 

[Rustytruck]

Let the spark plugs determine if you need jet change. Since this is in transition from different systems in the carb I would be looking at the plastic accelerator cams and worst case squirters. Maks sure your accelerator pump spring is properly adjusted each time you change something.
Was this carb mounted and running properly on this Bronco before these issues or a new install?

 

[bax]

High RPM stumble and pop or backfire is a reaction to a lean situation. With a Holly it could be as simple as too low of float level. fuel pressure is also a cause for a high RPM lean condition. it's not squirter size they are already used up after WOT. Jetting could be a cause. Any fuel obstruction areas- filters, kinks. line ID. stuff like that need to be looked at. I would first check for obstruction. Then see if I could raise the floats in the bowls as much as I could get away with. Try it and see. Then move on to installing bigger float bowls on the carb. yes you can do that. jets I would do last. keep in mind bigger jets will richen up your mixture at idle. You should not tune to WOT unless thats where most of your engine operation is. Fuel pressure is hard to check unless you run a gauge into the cab where you can see it during your WOT pulls. If you can keep +4 pounds at full pull then your good with your delivery system up to where the gauge is installed

 

[broncnaz]

Im going to bet that its the secondaries opening to quick. Get you a set of secondary tuning springs and throw in the heaviest one.
I will disagree with the statement about bigger jets making your idle richer. Holley carbs have idle feed restrictions changes in main jets have no bearing on idle mixture. unless you install jets that are way smaller than the idle feed restrictions. Which is oprobably not even possible.

Id pretty much rule out fuel pressure at this time as with a holley you have 2 fuel bowls just stomping the throttle and within 75 feet you bog really doesnt point to fuel pressure as you can run a good little distance with full fuel bowls. So as long as the float level is correct. You should be able to go a lot farther than 75 feet before running out of fuel.

It also could be accelerator pump related either pump squirter needs to be a little bigger or they are to big or the pump itself is runing out of fuel and you need a larger pump. But I would check to see what size squirter you have. Ifs its smaller than 31 then you probably need to go bigger.

 

[jckkys]

The power valve may not be opening or the power valve restrictions could be too small. Some two stage power valves are too restrictive. Remember fuel from the power valve is added to that passing through to the main jets to produce the total WOT mixture. A blockage in either circuit may explain your problem.

 

[methcat]

Running out of fuel as in plugged fuel filter,...

this....

 

[GrillMaster]

New fuel pump and filter so don't think that is an issue. I have a heavier spring I can try. I used that power valve that came in the rebuild kit what ever that is. What size should I be running? I'll get some more info for you guys.

 

[broncnaz]

I think the power valve is probably fine unless you just have really low engine vacuum. Usually the kits come with about a 6.5 power valve.. Even if it is opening to soon you really shouldnt be bogging. it may run richer but shouldnt bog.
May be a timing issue as well. how much are you running? also hows the engine built stock? modified? what cam?

 

[jckkys]

If the engine is running without surging in high vacuum or cruising conditions the jetting is adequate. If the power valve opens at 6.5" of Hg it will be open at WOT (or when you floor it). The amount of fuel contributed by the power valve is limited by the PVCR or power valve channel restriction, if the power valve is functioning correctly. A book like Holley Rebuilding & Modifying will explain how to tune this. Simply installing bigger jets will cause an over rich condition at cruise and destroy fuel economy. If your Holley has the correct metering block, the restrictions should be close. That leaves a bad power valve or a blockage.

 

[GrillMaster]

It's just a bone stock '95 351w with performer rpm intake, MSD6, holley 650 carb and headers. Looks like I have some things I can check now. Thanks! ...and let me know if you think of anything else.