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TR4/4A Stromberg CD175carb query


Jedi Warrior
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Have early type CD175's with adjustable main jets at the bottom of the float bowls.
Currently using new needle/seats and new 100 main jets with new H2 needles, revs out well, but cannot get engine to nice low idle...OK at 1200rpm...running rich when at about 700rpm....cannot get any further mixture adjustment for nice low idle. Tried altering the fixed position of the needle a bit lower..no difference.
There are no apparent air leaks.... carbs are not flooding.
Should not be the needle size as all needles will have the same idle profile within 0.0001".
The only thing that I can think of is that the carb is over fuelling at idle. The float level has been set at what the manual says ie. 0.73" or 18.5mm.
Adjust float level so that the needle/seat closes earlier?
What is the correct main jet and metering needle size?
Comments appreciated...please do not suggest changing to SU's.


Luke Skywalker
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Great carbs and have used them to replace SU's. The centering of the needle is very difficult and if it is hitting the sides of the jet, that could be a issue. The linkage on some have given me a fit. Every time you tighten up the little bolts the shaft moves slightly. Have you tried backing out the idle screw all the way on both. If it still idles up a leak of some sort is indicated.

Wish I was your way to help, getting ready for winter here.

Regards, Wayne


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The main jet should be 1/10 of an inch in diameter and perfectly round.
2H needles are .0980 at the base, then at 1/8th inch increments....0923,.0899,..0877,.0755
There's more, but those should be the measurements well within the range of idle speed.


Jedi Warrior
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Thanks guys.....idle screws, needle centering all OK.
Trying to ascertain whether altering float levels would have any effect at idle?


Luke Skywalker
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All I have left is the centering of the butterfly, if it's out even a little, it will not close off. Just another thing we overlook when putting them back together, if we have never been caught and spent two days tiring to figure out a high idle. Been there done that.



Jedi Trainee
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+1 on centering the butterflies, and also verify you have the butterfly chamfer oriented correctly....



Jedi Warrior
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Thanks again.
I do not believe it is a butterfly issue as I can achieve a low idle, but over fuelling for undetermined reason.


Jedi Warrior
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If you have a TR4A with a PCV valve, they can cause a vac. leak and high idle speed. You might try plugging the hose from the manifold and see if there is a difference.
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