engine stumbles
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Fleet Owner
Joined: Jan 2004
Posts: 26,007
Likes: 575
From: Brentwood,CA
engine stumbles
i just started up my 390 today after in stalling a streetmaster intake and holley 600 cfm carb.the engine has a lack of power and just stumbles if you lay on it and it idles a little rough
Elder User
Joined: Jan 2004
Posts: 867
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From: Eastern Pa
Check for a vacuum leak, however usually you would get a high idle.
Check where you have your distributor vacuum hooked. Should be manifold vacuum. Make sure the choke is open fully once warmed up. What does it sound like out the tailpipe? Do you hear missing? is it belching black? Always go for the obvious and easily discounted things first.
Check where you have your distributor vacuum hooked. Should be manifold vacuum. Make sure the choke is open fully once warmed up. What does it sound like out the tailpipe? Do you hear missing? is it belching black? Always go for the obvious and easily discounted things first.
Elder User
Joined: Jan 2004
Posts: 867
Likes: 0
From: Eastern Pa
Which port on the carb? It should be on the baseplate. As far as the pump goes, I believe you push down so the spring is compressed all the way and set the gap between the pump arm and the linkage. I'll have to check what it is but I know it's only a few thousandths.
Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
Ford390; You hooked it to the right place. Never ever connect vacuum advance to manifold vacuum for normal running. Always to the ported vacuum on the metering block. The base plate ports are manifold vacuum and give you a false idle speed by advancing the timing at idle, a no-no. You adjust the accel pump by moving the throttle to wide open throttle(WOT) position and adjust the screw and nut so you have .015" of movement left in the accel pump arm that the screw pushes down. This way the pump doesn't bottom out and act as a throttle stop and possibly rupture the pump and still gives you quick accel pump response. Did you adjust the mixture screw on the sides of the metering block? If not, you need to adjust the carb to match your engine combination.
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Fleet Owner
Joined: Jan 2004
Posts: 26,007
Likes: 575
From: Brentwood,CA
yeah i need to adjust them i tried yesterday but no luck what size main jets should i be runnning i have 66 size in there now
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Post Fiend
Joined: Mar 2004
Posts: 6,056
Likes: 8
From: Union, Washington
The 66's should be close. But you realize that the main jets only control the fuel flow at at or near wide open throttle(WOT). The metering at intermidiate speeds is controlled by air bleeds(and engine vacuum) that are not adjustable and at idle by your mixture screws. If the engine has backfired and the carb is an older model you need to check the power valve as all it takes is a single backfire to blow one without a protection valve.
Elder User
Joined: Jan 2004
Posts: 867
Likes: 0
From: Eastern Pa
Sorry, my bad on the vacuum connection. I was thinking of mine with an Edelbrock carb and a cam that lessens the vacuum considerably, when the straight vacuum port is recommended. The Holley must be below 1000 rpm to adjust the mixture screws properly. Above that you start to get the intermediate circuit involved and you'll never get a proper adjustment then.
