Ford Muscle Cars Tech Forum banner

1 - 16 of 16 Posts

·
Registered
Joined
·
2,617 Posts
Discussion Starter · #1 ·
I've got a high-rpm stumble that I am trying to fix on my 289. First the combo:

289, Holley 750dp (milled choke horn, secondary jet extensions), Holley red label electric fuel pump, Weiand Xcelerator intake, World Windsor Jr. heads, Comp 282S cam (236 @ .050), 1.5" headers, 2.5" dual exhaust.

It did not have any problems with my old carburetor (659DP), and haven't had time to tune on the 750 very much, but I'm running 72/78 jets, and normal driving on the primaries feel really good. Basically, with a rolling 3000rpm start, if I floor it in 1st, it pulls fine to 7000rpm. Same rolling start in 2nd, it pulls to about 6000rpm and then totally starts blubbering and falling on it's face.

Two ideas in my head... first of course would be jetting. I figure, it only takes a couple seconds to rev out in 1st gear, so if it was jetted lean, the accelerator pump could be compensating for it over that short period of time, whereas it takes longer to run through 2nd and the accel pump gets burned off and all that's left is lean jetting. If this is the case, I figure I'll go up 2-4 sizes and all should be well.

The other possible cause would be plugged fuel filter or fuel pump going bad, causing the fuel bowls to run dry. I'm not so sure this would be the cause, though, because the instant I back out of the secondaries, everything smooths out... so it would have to be ONLY the secondary bowl running dry. Is it possible for the 289 to drain the secondary fuel bowls in less than 4 seconds?

I'm leaning towards jetting on this one... but wanted to see what you guys think... it just seems weird that if it's jetting, why is it like a light switch, instantly going from pulling strong to stumbling?
 

·
Registered
Joined
·
87 Posts
sounds kinda like too small of a power valve and it is closing at wot, this will cause you to go 9 jet sizes lean.
 

·
Registered
Joined
·
2,617 Posts
Discussion Starter · #4 ·
KenKidd: I only have a power valve in the primaries (stock 6.5), and I'm pretty sure a 750cfm carb on a 289 has zero vacuum at WOT so I can't see it being the power valve closing, especially since I didn't have the problem with the 650main body.

king: I've got rubber fuel line right from the tank to the fuel pump (mounted in the frame rail in front of the tank) and up to the carb... I think 3/8" if I remember right. Like I said it didn't have the problem with the 650, so that's why I'm leaning towards it being jets, but I suppose the filter could have clogged or the fuel pump could have worn some on the past 6 months... hasn't been driven maybe 500 miles in that time though.
 

·
Registered
Joined
·
440 Posts
Motorhead.... Does it pull clean in 3rd or 4th gear up high? You could be having an ignition problem that only shows up under heavier loads. 2nd gear is not a heavy load but it's heavier then 1st gear. Bruce
 

·
Registered
Joined
·
1,627 Posts
Sounds like a lean condition.

78 rear jets sound a little light for your engine combo. Things change a little bit with the warmer weather too.

80 - 82 is a good next step try. Also is your fuel inlet restrictors... I would assume they are .110 size already? If not, you maybe running low on gas in your bowl, and slowly leaning out your mixture.
 

·
Registered
Joined
·
2,617 Posts
Discussion Starter · #7 ·
bprigge: I haven't tried in 3rd or 4th, mainly because 3rd gear 6000rpm is about 90mph and I haven't had the chance to take it out on the highway. I guess I should probably check that out also though, so that when I try a fix I will have more to compare it to.

Mike: I agree on the jets... stock jets for a 750dp are 70/80... but when I put this main body in my 650, the biggest jets I had were 78 so I put those in the rear and 72 up front. I have a pair of 80s now, but as bad as the stumble is, I think if it IS jetting it will take more than just 2 sizes to clean it up and so I wanted to wait 'till I can get ahold of some 82s and put them in. Also I'm not sure what size needle/seat I've got, they are stock Holley rebuild kit size so might be a little on the small side... they were never a problem with the 650 carb, but maybe the 750 is pulling just a little more fuel, just enough for a problem to show up.
 

·
Registered
Joined
·
2,617 Posts
Discussion Starter · #9 ·
Well Friday after work I took it out on the highway and flogged it through 2nd and 3rd a couple times, and guess what... it ran fine! Damn I hate intermitten problems. If anything I'd say the weather was a few degrees cooler, but I wouldn't think it was different enough to make THAT big of a difference. It does seem to maybe do it more often when it's hotter outside, but I don't drive it every day so could just be a coincidence... normally for hotter weather you lean out the jetting right? But if it were too rich, I'd expect it to stumble as soon as the secondary accel pump squirts in, not 2-3 seconds later after the pump shot is all gone! I dunno, maybe there is some junk floating around in the secondary bowl... I think since it ran fin Friday, maybe I don't want to change the jets TOO much, so I'm gonna throw the 80s in there and keep driving it.

iskwezm: that does sound just like my problem... thinking it almost has to be jetting, or like I said junk in the carb, but we'll see


_________________
'86 Bronco fullsize, 460, E4OD
'85 Ranger, 340hp 289, T5


<font size=-1>[ This Message was edited by: Motorhead on 7/11/06 12:53am ]</font>
 

·
Registered
Joined
·
2,617 Posts
Discussion Starter · #11 ·
That brings up another thing on my mind Mike... when I swapped in the 750 main body, I also added the jet extentions and nitrophil float in the rear bowl... and when I went to adjust the floats, I found that I had to have the adjustment pretty much all the way down in order to keep the fuel at the right level. Since then I have bumped the adjustment up about a half turn, because it just didn't seem right that it was adjusted so low, but still if I remove the sight hole screw in the side of the bowl the level is higher than it should be. Is the nitrofil float less bouyant, and thus needs to be adjusted further down for it to close the valve completely?
 

·
Registered
Joined
·
1,627 Posts
Intermittant....

I'll stick with the statement about the larger needle seat size. .110

Depending on how long your at WOT, should be easy to verify.

Keep it in second ( or third) and keep it at 6000 RPM's and see if it starts to degrade as you leave it there. That will be the proof.

10 seconds or so should be enough time. A second set of eyes to look at your instruments and speedo would help a lot.
 

·
Registered
Joined
·
2,617 Posts
Discussion Starter · #13 ·
When I took it out on Friday I ran it all the way through 2nd and then shifted to 3rd and ran it up to 7000rpm... probably had it to the floor for over 10 seconds, and no stumble or blubber or even slight miss, it was fine. Larger needle and seat would probably be a good thing to have, and is really easy to swap out, so maybe I'll do that at the same time that I change the secondary jets. That way even if the fuel pump is a little bit weak, the larger opening will make it easier on the fuel pump.
 

·
Registered
Joined
·
2,617 Posts
Discussion Starter · #14 ·
Well, it seems like the jetting fixed it. I went from 78s to 82s in the secondaries... figured four sizes would either make it a lot better if it was jetting, or a helluva lot worse if it was anything fuel delivery related. Did a full 1/4 mile pass out on the highway, from 1st through 3rd, and the tach just zing'd past 7000rpm in each gear, so it's DEFINITELY up on power now, and no stumble... well, except when I tried to floor it at 1800rpm, doesn't work well with a 750dp on a 289 lol!
 

·
Registered
Joined
·
1,627 Posts
Great to hear, I was waiting to know how that turned out for you. It seems that the 750's on our displacement size really likes the 82 - 84's in the rear. The fronts should be 72 - 74, plus the power valve brings it up to 82-84 for the front.

This is with the HP Holley Body or the Proform?

Still, everyone I talk to, when I say I have 84's in a 750 on a 302....they tell me I am nuts. While that may not be far from the truth, I stand my ground and then turn the engine over, and paint thier street with two black stripes.

I am sure you can get the low RPM bog undercontrol too. Mine only very slightly bogs off of idle now, and I am still working on that issue.

I am confident I can get this carberated motor to run like a fuelie. I have yet to install my 4 hole 1" spacer. Maybe soon.... just hesident because everything is running so good now.

I think the 4 hole 1" spacer will help that a lot. It should create better low airspeed velocity and a better pulse, by making the runners a little longer. You are ruinning an open plain manifold? Maybe a 4 hole spacer may help you for your low RPM bog keeping the exit velocity a little higher coming out of your carb, and giving you a better venturi signal???
 

·
Registered
Joined
·
2,617 Posts
Discussion Starter · #16 ·
Actually this is just a plain Holley 750 dp main body, 4779 carb number, that I bought used off eBay, ground the choke horn off, smoothed the sharp corners off, and swapped into my 650. Since stock jetting on a 750DP are 70/80, I figured I was probably a little lean with the 72/78s, but those were the closest jets I had at the time.

The low RPM bog doesn't really bother me, it only happens when I get into the secondaries too soon, and you almost have to expect that with such a big carb on a small engine with a manual transmission, there just isn't enough airflow to pull a vacuum signal on all four venturis below 2500rpm... I mean, that's the whole reason for vacuum secondary carbs, so that you can't accidentally open the secondaries too soon. I think no amount of tuning can ever fix that on a mechanical secondary carb, you just gotta know when the engine is ready for WOT.

Yes I'm running a Weiand Xcelerator low-rise single-plane intake. Also when I did the 750 main body swap, I also added on a 1" 4-hole aluminum spacer that I made into a tapered spacer with a little grinding and filing. I think it's probably the reason why I didn't loose any low-rpm driveability when swapping from the 650 to the 750.
 
1 - 16 of 16 Posts
Top