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45 dcoe carburetor jetting

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Joined: 08 Oct 2019
Posts: 32
Location: Hudson ohio

PostPosted: Fri Dec 27, 2019 4:10 pm    Post subject: 45 dcoe carburetor jetting Reply with quote

Hey, I was told to come here because you guys seem to know everything. I recently put 45 dcoe carbs on my 1978 924 and it runs rough and was wondering if anyone had done this and what jetting they used. I don't know carburetors extremely well and jetting goes a little about my head. Thank you

Side note - I've seen the jetting and sizes for the 40 dcoe but I don't know if that will translate to the 45's
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Joined: 08 Jun 2010
Posts: 971
Location: Maysville, Colorado

PostPosted: Sat Dec 28, 2019 1:16 am    Post subject: Reply with quote

I have 45's on my race car and they have not been set up yet( I started out with some old used ones and rebuilt them). The only real way to do it is find the local race car engine builder/tuners and there will be one or two who are experts at this. Their method will be to put it on a chassis dyno. The local(Denver) fellow is a Datsun engine builder, it takes him about an hour to get them dialed in, typical cost $200-300. I'll take my car there this spring. You can spend a fortune on jets, tinkering around and not getting anywhere!!!

A Denver fellow who races(quite fast) a TR4 with twin 45's and is street legal said they dyno tune was the best $200 he spent on the car

The first thing to do is make sure your carbs have NO vacuum leaks, all those rubber O rings are goofy. Nobody has ever devised a good vibration mount/sealing system. New O-rings available cheap from McMaster-Carr. Run a vacuum line only to the brake booster, no vacuum line to the distributor. Advance the distributor about 10 degrees. Mine is current set at about 40 degrees advance at 2000 rpm for the race car and 15degree advance at idle at 8000ft for a street car.

The local tuner's first suggestion was to check the venturis and if necessary replace them with 34mm. The rather poor breathing NA head is an impediment to getting good air velocity thru the carbs. The old 933 race cars ran 36's, but that was at 6000-8000 rpm with the somewhat better flowing 931 head and long duration cams.

My setup is for an altitude of 5000' approximately so completely different than your requirements. But my guess's are:

The 40 DCOE jets should be a good starting point at low altitude, the 45F9 idle jets the same as the 40COE. Do the standard Weber check method. If leaner is needed, I believe there is a 40F9. If leaner still the next leaner jets are the F8's-I have a bunch. The Main and Corrector jets from the 40's is good starting point also. There is a traditional flat spot about 3000 rpm, the transition from idle jet to main jet. You can change the pump bypass jet to 0, but that is a fuel eater and just covers up the other jet problems. Many times a flat spot at transition indicates the main jet too rich, but the only way to really get rid of that is by dyno tune. A flat spot on flooring it indicates that the pump is putting in too much fuel, so put in a higher number pump jet to bypass more fuel. Don't worry about the emulsion tubes, they seemed to be tailored to individual applications, I think the F11 I think were the most common.

My race application is a little more simple as we aren't worried about fuel consumption currently about 1 gallon every 6 minutes, or the low speed transition, and idling is not very important, even with my slow driving it is never under 3000 rpm.

PM me if you want more information.
Poco a Poco, #033 '78 Vintage racer, SCCA Dp-81
'77 924
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