Alright... one more EASY tuning question...

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Got the coupe back on the road after rebuilding the 3x97's, and it's running great except for one MINOR problem. It idles fine at 800 rpm and has great response off idle - however, when driving it'll die when I left off the pedal approaching a stop sign, etc., sorta like the rpms drop too low-too fast and it can't recover. If I feather the throttle a little to keep it from dying, it'll recover and idle all day long at 800 rpm.

Not looking for too much 'how-to' info here - I guess I'm hoping that someone with more experience tuning multi-97 setups will recognize this symptom and suggest an area for me to focus on. Tighten idle screws? Loosen throttle screws?
Other?

It's a '53 8BA with 3x97's that I just rebuilt and re-jetted to .042's. The linkage is not progressive. Idle screws are set at one full turn out from seated. Timing is set at 5 deg advance at 800 rpm idle.

Again, runs/idles great except the problem above. Any suggestions?

[Unkl Ian]

Points ignition? Check the dwell angle.If the points gap is just a little too tight,it will want to die when you back off.

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Forgot to mention - it's a Mallory Uni-lite.

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Anyone else..? This is gotta be an easy one for the experts.

[hudson_hawk]

thats funny you mention this, i have the exact same problem w/ my 2gs so BTTT

[BLAKE]

C'mon... you gotta be kiddin' me - 119 reads and only two suggestions, one of which has since been deleted...?! Someone out there has to have an idea on this one. Two 'off-HAMB' suggestions were 'float level too high' and 'more fuel pressure' - will likely adjust my regulator tonight.

Someone else have ideas since there are two of us looking for info...?

[Elrod]

I had a car that would do this and I couldn't figure it out. I added a small breather to the carburetor one day and the problem went away. I can't explain why, but do you have breather elements on the tops of your carbs, or are they open? Try that. I know. Very un-scientific!

[Unkl Ian]

Are you running vacuum advance?

[buzzard]

I looked twice and still don't know. Actually, this is a problem I have been fighting for a while. I think it has to do with the old floats. I've always just bumped the idle up a bit and it doesn't die out with a quick stop.

[Smokin Joe]

I've never run 3 carbs non-progressive. I'm not an expert.
Most of the guys I knew who did were either racing or went back to 2 carbs on the street. Or they blocked the center and ran on the end carbs or blocked the ends and ran on the center. Or they went progressive.
3 carbs on a stock bore and stroke flatty probably doesn't leave much flow in each carb to play around with at idle. I can see how it would be tricky coming off the throttle with too much gas as you let off and all the vacume going away to mix it into vapor in the carb and manifold. That would seem to me to cause the puddled gas to make the engine act flooded and sputter a bit till you clean it out. Had the same problem with dual quads on a high rise SBC if you used the 500 carbs instead of the 390's. Anyway, that's my guess. I'd run 'em progressive. Better gas mileage that way too, by a big margin.

[Elrod]

Well, here's a thought.

Those bowls are facing toward the front of the car. As you stop, the gas in the bowl is going to slosh forward and raise the float, in essense closing down the needle. Do you know how much fuel pressure you are getting to those three carbs? Perhaps not enough to keep the needle open.

Electric fuel pump, or the stock mechanical?

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That's more like it - thanks for responding.

I'm running K-N filters. The Mallory Uni-Lite is mechanical advance, not vacuum. I'm hearing that the three carbs may be why I can't idle worthwhile below 750-800, but 800 rpm idle doesn't bug me, anyway. Coming off idle, there's no bog or anything - it has great response. I ran a progressive linkage before this rebuild - still had this problem, just not so severe but it was much slower to respond to off-idle acceleration. I re-tried the progressive setup yesterday and didn't like the slower response to off-idle romps, so returned it to non-progressive.

I'll increase the fuel pressure a little tonight to see if anything changes. Anything else while I'm in there...?

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ELROD - the fuel pump is stock mechanical with Holley pressure regulator. In your scenario, if I had enough gas in the bowl to slosh forward and close the needle temporarily, I should also have enought gas in the bowl to keep it running while the needle was closed... correct?

[Elrod]

Aren't the jets at the back of the bowl.

I started to think that perhaps the float would just rock forward and close the needle due to inertia, but the float is in the bowl sideways, so only if you turned a hard right would that happen! http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

Hmm... If you up the idle and it still does it, I would think the carbs are starving for fuel. I would look for the solution there. Need more fuel!

[buzzard]

I do know the solution to the slow response off idle with the progressive linkage......

SMALL BLOCK CHEVY, BABY!!!....SMALL BLOCK CHEVY!!!

[BLAKE]

Gosh - I wish! If only good rebuildable cores weren't so hard to find...

Italics=sarcasm http://www.jalopyjournal.com/ubbthreads/images/graemlins/tongue.gif

[buzzard]

Or, you could put mechanical brakes back in for something REALLY traditional. Then you won't have to worry about it stalling out because there will be know chance for quick stops.

I set my idle at around 900 or so. It solved the problem for me.

[BLAKE]

Gotcha. I'm gonna try a few things to see if I can lucky but it runs so good now I don't wanna go in too deep and screw something else up. If I can't figger it out, 900 rpm idle it is!

Was just hoping someone on the board would jump out and say, "Needs richer idle", or something similar - sounds like it's not that easy... dammit. http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

[buzzard]

Actually, I should say it bandaided the problem. Not solved it.

[Revhead]

Buford,
Here's a couple of paragraphs from a carb book I have. It sounds like the same exact problem, but how to fix it without a dashpot I dunno. May give some insight into the problem though.

blah blah blah...you have no choice but to lift your foot off the gas and stomp on the brakes. And you engine, of course, dies - unless you have a anti-stall dashpot. This kills the engine because a glob of air/fule mixture is still on the way to the cylinders. In effect, the combustion chambers are flooded by this mixture. the anti-stall dashpot solves this problem by letting the engine return to it's normal idles slowly, giving the combustion chambers time to burn off the extra rich mixture.

Dashpots on Holley carburetors are mounted on the carburetor. A dashpot consists of a chamber, diaphram and spring. A plunger protrudes and just touches the throttle linkage. When the throttle pushes against the plunger, the diaphragm tries to force air out of the chamber, but the only exit is through a small hole which lets teh plunger in slowly. The spring returns the plunger for the next stroke. Dashpots are adjustable by adjusting the length of the plunger wit ha screw.

and more on the subject...

uring decelleration, particularly when you snap the throttle shut very quickly, a high vacuum is created in the intake manifold. This high vacuum is caused by the engineturning over at higher-than-idle speeds with the throttle plates shut, which pulls extra fuel out of the idle circuit, creating a very rich mixture. One way to reduce the manifold vacuum is to have the throttle held open slightly. another solution is to provide a special passageto let air bypass the throttle valve. air allowed past the throttle valve reduces the manifold vacuum. then the mixture will elan out. the device that holds the throttle partly open on a carburetor is known as the dashpot or throttle controller.

Well that's all I got on the subject. I looked through all the "troubleshooting" areas and they are worthless. Hope this at least gives you some ideas of what to check.

[BLAKE]

Thanks REVHEAD - that's good stuff and I owe you big Lone Stars for typing it here. It also jibes with DRJ's earlier response. If nothing else, it's somewhat comforting to know it's a relatively common problem and not some boner I pulled during the rebuild.

I actually tried one quickie fix this afternoon that's sorta along the same lines - my throttle return spring can mount to different positions on the linkage attached to the throttle shaft of one carb. Mounting it farther 'up' the linkage piece provides more leverage/tension, snapping the throttles closed more quickly - I moved it 'down' a little to see if it would close more slowly - too slow. I'll try somewhere between tomorrow afternoon.

Thanks for the thoughts on this guys - you renewed my faith in this forum for quality info. http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

[badpat]

this could be so many things.... timing, advance, vaccum leak in the carbs... and the list goes on. I would really look into going with a progressive linkage on a flatty- yer droppin a lot of fuel in there. i assume you have yer timing set properly, after that, go around yer carbs with a propane torch and check fer leaks.

[hudson_hawk]

[ QUOTE ]

uring decelleration, particularly when you snap the throttle shut very quickly, a high vacuum is created in the intake manifold. This high vacuum is caused by the engineturning over at higher-than-idle speeds with the throttle plates shut, which pulls extra fuel out of the idle circuit, creating a very rich mixture. One way to reduce the manifold vacuum is to have the throttle held open slightly. another solution is to provide a special passageto let air bypass the throttle valve. air allowed past the throttle valve reduces the manifold vacuum. then the mixture will elan out.



[/ QUOTE ]

so as i understand this, adjust the linkage on the middle carb so that the plates don't close all the way and allow some air to enter the motor

am i correct?

[Revhead]

Hudson,
I don't know exactly what they meant to do to get air to bypass the throttle. If you left the middle one open a bit then the idle would be higher all the time. but maybe it would work with the idle lowered and taking some of the load off the idle circuit. I don't know really I am not a carb guy, i'm just throwing out ideas.

Buford - Holley sells a dashpot PN 11-4 with several brackets PNs 20-72 20-58 20-17 These are all for holleys. The dashpot is universal and could be adapted if you wanted to go that route.

oh and in the holley description the dashpot is to prevent stalling on sudden decelleration on manual tranny cars. so another source with the same answer.

oops forgot one: Holley PN 11-61 dashpot replaces ford PN C8OZ-9B549C

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REVHEAD - thanks again for the research. I'll poke around to see what I can do - if I figure anything out, I'll post an update.

[porknbeaner]

Buford,
I had the same problem with 3 dueces on a Y block. Granted the Y blobk probably took a deeper breath than your motor but...
An old guy(older than me) said I should look at two things:
First he said I need at least 6.5 psi fuel pressure to feed the carbs. 7 would be better and 7.5 max.
He said after boosting the fuel pressure if it still had the same problem to start looking for a vacume leak.
Boosting the presure cured it. But I did look for a leak anyway. Just to cover all the bases.
BTW I ran progressive and didn't like it either. So I went to a locked linkage. Screw the gas milage unless you have to have it. http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

[BLAKE]

Good input - thanks. I did tweak the fuel pressure last night, but I have a little more room to go - I don't wanna get above 2.5 or 3.0 psi with the 97's.

[Unkl Ian]

Just thinking out loud here.What would happen if you set it up so that you only ran off the middle carb at idle? Close the throttle plates and mixture screws completely on both end carbs,and set your idle speed and mixture with the middle carb only.Worth a try?

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UNKL IAN - tried it this evening... one of about 50 things I tried. In the end I went back to the non-progressive setup that I started with since each change affected some aspect of drive-ability but never fixed the original problem... when you let off the gas it'll stumble and try to die before it levels off and idles.

[Unkl Ian]

How hard would it be to hook up a vacuum advance mechanism?It would help with drivability.

[choprods]

could it be simply flooding at that low an RPM?