Off idle stumble with new 331 rebuild...update...problem solved

I just got a custom tuned 750 DP for my 331. The engine was just newly rebuilt and one addition was a single plane ported and knife edged Hurricane. I will be running this engine to nearly 7000 rpm so I needed a 750 for the job. I had my old 770 street avenger on the engine just prior and I had no off idle stumble with it.

The engine stumbles quite a bit off idle. This happens even before the secondaries are opened. Timing was double checked and it's good. Idle mixture screws are set 1 turn out on all 4 corners. I don't have any vacuum leaks. I can see the accel pump squirters squirt when I open the primaries. My idle vacuum is about 8 inches at 800 rpm. Once the stumble recovers, the engine is running great.

The carb has pink accel pump cams. I am not sure what the squirter diameter is. The PV is a 4.5. Jets are 72 primary and 81 secondary.

I plan to call the carb builder, but I was curious about your recommendations. I read that I should raise the float level and/or enrichen the idle one turn or so. I seem to remember having this issue before with a different carb and a dual plane manifold and I fixed it by putting a higher number PV in.

_________________
Tracy Blackford: Corona, Ca
'65 FB Mustang 331, 282S cam, ported 351W heads. T5z, 3.50 9" posi.
346 RWHP@6400 on a warm spring day (335 RWHP SAE corr.)


<font size=-1>[ This Message was edited by: blkfrd on 4/7/07 4:23am ]</font>

Tracy, go to 74 and possibly 76 primary jets. You should see a big improvement with the 74s, if so and you still got some stumble go to a 76. The single plane intake is doing that to you. Bruce

Increasing your base timing will help it some too but as you know you gotta keep the full advance in the sweet spot.

<font size=-1>[ This Message was edited by: bprigge on 4/4/07 2:29am ]</font>

<TABLE BORDER=0 ALIGN=CENTER WIDTH=85%><TR><TD><font size=-1>Quote:</font><HR></TD></TR><TR><TD><FONT SIZE=-1><BLOCKQUOTE>
On 2007-04-03 11:07, bprigge wrote:
Tracy, go to 74 and possibly 76 primary jets. You should see a big improvement with the 74s, if so and you still got some stumble go to a 76. The single plane intake is doing that to you. Bruce

Increasing your base timing will help it some too but as you know you gotta keep the full advance in the sweet spot.

<font size=-1>[ This Message was edited by: bprigge on 4/4/07 2:29am ]</font>
</BLOCKQUOTE></FONT></TD></TR><TR><TD><HR></TD></TR></TABLE>

I read that increasing the main jets was another possible solution, but not if the main circuit is already tuned. The carb was custom tuned based on my engine and car specs so if I change the jets it may help the off idle stumble but the main circuit may be too rich. Is that a fair assessment? Any other suggestions?

_________________
Tracy Blackford: Corona, Ca
'65 FB Mustang 331, 282S cam, ported 351W heads. T5z, 3.50 9" posi.
346 RWHP@6400 on a warm spring day (335 RWHP SAE corr.)


<font size=-1>[ This Message was edited by: blkfrd on 4/4/07 8:51am ]</font>

Was the carb tuned on your engine?

Tracy, what is your initial and total advance and at what RPM are you at total? This may have some bearing on you problem.

I bought a custom built 750 for my 393 and found that, in my case, I needed to drop 3 jet sizes on the primary with my original build in order to be steetable. Now after freshening the motor again, raising the compression a half point, and doing some basic head/intake porting mods, I find that the original larger jet size (actually the whole original carb combo) is nearly spot on.

_________________
Dennis

65' STANG, 3160 lbs

393W NA, 2.78 Toploader 4 Spd, 9" 3:70 Posi
Pro Comp Heads, Vic Jr. Intake, 750dp, CI Custom SFT, 10.2 CR, 1 3/4" Headers, Lakewood, Subframes, Caltracs



<font size=-1>[ This Message was edited by: dennis111 on 4/4/07 7:23pm ]</font>

Tracy,

Yes, that's a possibility. A single plane intake is going to allow mixture to be pulled from all 4 barrels instead of just two like a dual plane intake so the signal at each barrel is going to be smaller, requiring bigger jets to get the same flow. I know you know all this stuff but maybe hearing it again will clarify what's going on in your set up. Assuming you were running a dual plane before you are now going to need bigger jets with the single plane to get the same mixture, even at cruise throttle settings. Because you bought the carb as 'tuned' doesn't mean, in my opinion anyway, that it's going to be spot on for your set up. If the builder/tuner got it within 2 jet sizes I would say they did pretty good. You could also try switching to a 50cc excelerator bump to cover up the lean bog but you might still be running lean at part throttle with that single plane intake. It would be sweet if you could put a wide band F/A meter on it, that would take away the guess work. Bruce

<font size=-1>[ This Message was edited by: bprigge on 4/4/07 7:43pm ]</font>

Distributor was curved for my car when it had the dual plane intake. 16 BTDC initial and 36 BTDC all in by 2400 or something like that...going by memory right now.

The same builder tuned my other carb off the engine based on all of my engine and car specs. He got the jet sizes right on. That was a 770 street avenger and a weiand stealth intake. Jets installed were 72s and 80s.

New 750 DP is jetted 72s and 81s...almost the same as the old one, but the innards are different. Both carbs had the better drop down or dog leg boosters installed. Both carbs had 4.5 PVs installed.

Given that he got my last carb spot on and he has gotten other peoples carbs spot on that i've spoken with, I am assuming the jetting is OK. The engine runs like the jetting is OK or very close.
[addsig]

Besides the jetting, have you played around with the pump cams and squirters yet? My 66 had a terrible stumble just as you floored it from idle. My problem was too much fuel and duration from the squirter and pump cam. This occurred when I switch out my C4 and 3.00 rear with a 4-speed Toploader and 3.50 rearend. Went with the smallest squiter and cam...just about fixed the problem.

$15.00 and 15 minutes, is all it takes to up-jet to 74's in the front and see if it gets better or worse.

Having the throttle plates too far open for idle, or the secondary plates mis-adjusted for idle will also cause your issues. The carberator is very sensitive to the position of the front and rear throttle plates.

Tracy, If 72s worked in the primaries with a dual plane I don't see how they could work with a single plane, unless the boosters are much more sensitive on the new carb. Like Mikes66 says, it's cheap and quick to try it out. Bruce