draw thru 4age update and question!

jakes_hachiroku
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draw thru 4age update and question!

Postby jakes_hachiroku » Thu May 24, 2018 5:02 am

so ive finally got this beast to start up and the turbo sounds sooo good! BUT I cant get this thing to idle. I have to keep blipping thr throttle. so I found a huge vacuum leak and fixed that. still the same result. advanced timing, same. retarted timing, same! but something strange happened when I plugged the vacuum leak I said I found! so whith me hitting the throttle quickly to keep it running, each time I hit it, the idle runs away from me a few thousand rpms. so if I quickly hit it to 2000 it climbs on its on the 3500 or 4000!!! and it seams like im off a tooth or something in my distributor but please correct me. also for your info im on the lowest setting for my idle mixture screws on my weber dcoe carb. any help would be great thanks! :D :D :D :?:

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jondee86
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Re: draw thru 4age update and question!

Postby jondee86 » Fri May 25, 2018 4:01 pm

I like it when someone tries to be different :) Got a pic of your setup? At a guess
it could be that there is enough air/fuel mix in the tubing between the turbo and the
combustion chambers to cause the engine to flare even after you close the throttle.

Probably depends on the size of your turbo. If it is small and spools early it could be
making a few psi of boost at 2000rpm. Bearing in mind that I don't know anything
about turbo systems... is it possible that you need a vacuum operated bypass valve
to route that compressed charge backto the intake side of the compressor when the
throttle closes?

Cheers... jondee86
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sirdeuce
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Re: draw thru 4age update and question!

Postby sirdeuce » Sat May 26, 2018 8:38 pm

Cam timing?? Does adjusting the idle screw make ANY difference? Dirt in idle circuit or jetting could be off. If the cam timing is off idle will be really difficult to maintain and can cause the that kind of surge in RPM. Been a long time.
No bypass would be needed with a draw through as the throttle is before the turbo. Wouldn't be anything to recirculate.
WEBER is a good carb, no doubt, but if the jetting is off it can be a real bitch, especially for a newbie. Draw through is easier than blow through, but turbo + carbs tends to need a different approach than N/A. Watch your plugs for clues.
One shot, one kill.

sirdeuce
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Re: draw thru 4age update and question!

Postby sirdeuce » Sat May 26, 2018 8:57 pm

For idle screw setting, 1 1/2 - 2 1/2 turns out. If you can't get response from a full turn in either direction then your jetting needs attention.
And we go back to cam timing here too. Carbs need a good vacuum to operate properly in the idle and bypass circuits(fuel draw, atomization, etc..). Cam timing being off a tooth will kill manifold vacuum.
Do you have a vacuum test gage? In your set-up anything less than 12inHg will give you hideous problems. Should be over 15inHg at idle.
Haven't had a draw through system in close to 30 years.
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jondee86
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Re: draw thru 4age update and question!

Postby jondee86 » Sun May 27, 2018 12:44 am

Interesting. I was thinking that with the throttle in front of the turbo, closing
the throttle would unload the spinning turbine and allow the turbo to speed up
momentarily as the engine consumed the last bit of charge left in the intake
ducting. Hence my expecting the engine to flare up before settling.

However, if an engine with a draw thru system does not flare when changing
gear, then obviously the situation I was thinking of does not apply.

Cheers... jondee86
Give a person a fish, they eat for a day. Suggest they search before posting, and they learn a skill for a lifetime.

sirdeuce
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Re: draw thru 4age update and question!

Postby sirdeuce » Sun May 27, 2018 9:20 pm

Not that I've experienced. Intake systems for a draw through system are small, no intercooler and piping to increase the volume of the intake. Reason for such a design is to reduce the possibility of driving around with a bomb under your bonnet. An inline engine may have a bit more piping, a VW would have the turbo in approximately the same location as the stock carb, than a boxer, giving it a bit more volume but not enough to cause a surge in the RPM.
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