Draw through DCOE won't idle

[Ohio Tom]

Hey, I have built and tuned many drawthru setups. You should be able to get a stable idle.
What is your jetting setup?
They seem to like 65 idle jets and then tweak mixture screws.
If mixture screws have no effect, then you can open up the air bypass screws, and slow down the idle rpm, then the screws will start to have effect.
I can definitely help you...

[slayer61]

Thank you for your kind offer sir. I think i have 50s or something in it now. I have some larger ones to try tomorrow. I will let you know. Thanks again Tom.
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[madmike]

Hey Paul, On my 45 doce, I had this jetting : F11 & a F7 I know ,,lol,, 55 idles, 180 mains & 155 airs,.300 Neddle & seat,, keep an eye on fuel pressure!!
I could not get the results I wanted(cruising) until i swapped to a 4 barrel QuickFuel, now 28-26 mpg !! runs 11's in the 1/4 w/15 lbs pumpgas!
hope ya get it sorted Paul!,cheers,mike
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'Black Ice'Drag Buggy 'Turbo'
Rail Buggy 1915 turbo
76 Drag/Street bug 2180cc 'Turbo' 11:85 @113 mph"If I go any faster I'll burn up the Hamster" ,gets 28 mpg. also 10/09/22 11.90 @115 mph
"If I'm ever on Life Support,UNPLUG Me, Then Plug me back In see if that Works"

[slayer61]

Thanks for your input Mike. Much appreciated
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[slayer61]

Hey Mike, what cam did you have in your 2276?
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[VdanielW]

Just to clarify: When setting your idle air fuel mixtures are you setting for what the engine wants and ignoring the afr gauge? Even one point (hell, half a point) from where the engine is happy at idle can cause it to hunt and or stall.

[slayer61]

[slayer61]

Thanks everyone for the suggestions. Seems like I just need to add more fuel to the idle. Just keep adding jets until it idles!


Link

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[Bugsy61]

Carbs have two very different systems built in that make them operate. High speed relies on velocity to evaporate fuel. The low speed and idle part of the carb relies on vacuum to evaporate fuel. You don't have any vacuum to speak of, so the fuel is not going to evaporate. You don't have much vacuum because your camshaft is way to big for a draw through set up, so there is nothing to evaporate the fuel. The power you would make under the curve would probably be greater with an Engle 100 on 112 LSA than that monster cam you have in there now. Extreme to go that small, yes, but a better choice.
Your cranking compression indicates that your dynamic compression is way, way to low. What is your static compression? With that cam, you need at least 9.5 to 1 for it to run right, and that is a compression that will limit your boost to about 6 psi boost.
You can band-aid the low compression by running 24 degrees timing at idle and adjust the rest of the curve as needed, but not the best solution.
Pull out the big cam, and put in a much smaller cam, or go blow through with dual carbs. Gravity helps dual carbs idle because the fuel will "fall into" the cylinder where combustion chamber heat will help it evaporate. Also, you will need a couple small holes in your throttle plates, but you are a long way from that.

[madmike]

FK 10 ,small motors I like fk 8..cheers,mike
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'Black Ice'Drag Buggy 'Turbo'
Rail Buggy 1915 turbo
76 Drag/Street bug 2180cc 'Turbo' 11:85 @113 mph"If I go any faster I'll burn up the Hamster" ,gets 28 mpg. also 10/09/22 11.90 @115 mph
"If I'm ever on Life Support,UNPLUG Me, Then Plug me back In see if that Works"

[slayer61]

[NJ John]

And, one of the west coast turbo builder’s told me that small cams don’t need to be 112.
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1973 standard, yellow, lowered, 3” narrowed front, 1600 blo-thru turbo w/single dell 15.4@86, so far
11.41 et buggy. Long gone
Let’s go O’s! Let’s go O’s!
https://www.youtube.com/@AirSpooledGarage

[BFB]

[slayer61]

Just to wrap this up, and put a bow on it, I have found that advancing the idle timing has allowed the motor to idle at an acceptable rate, making carburetor tuning much easier.

Thanks for your assistance, guys.

Paul
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[67rustavenger]

[slayer61]

[BFB]

[slayer61]

[BFB]

I’d say that if you dont have a stock set of rockers sitting around then theres plenty of cheap ones in the classifieds and your pushrod length is going to vary depending on combo & shims that were used to build the engine so who knows. But again, a cheap set of chromoly could be had off of the classifieds just to try out and see how it idles, dont have to go racing it down the road, doesnt have to be perfect for a trial. If it works, then get a good solid shaft rocker set and some good pushrods to cut to fit.
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"how am i supposed to torque the rear wheel nut to 250 ft lbs??? " - clonebug
An interesting thing happens in forums where everyone starts parroting the same thing and "common knowledge" takes over.
Most experts aren't.

[slayer61]

Thanks brother. That's excellent advice. Sadly I don't have any rockers laying around, but they are cheap as chips in the classifieds. Onwards!
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