View Full Version : forgot to add main and air specs bmw dcoe jetting

10-31-2002, 05:31 PM
mains are 155 airs are 175

11-01-2002, 10:04 AM
A very crude analysis would suggest that you are running too rich at the top-end.???

Your spread between the mains and the air correctors may be too narrow.

You may also be over-carbed (40 DCOE(s) may work better)and the compression is a tad on the low side for such set-up.

Try increasing the air correctors to 190.


11-01-2002, 02:36 PM
Too rich, try 150s or even 145s for the main jets, 180 to 190s for your air correctors. Rule of thumb, your main jet size should be roughly 4x that of your venturi size 36x4=145 and your air corrector is main jet+50.

11-02-2002, 03:43 AM
Have any of you guys tried tuning with an EGT gauge? I 'm still trying to figure out exactly what temp i sgetting a bit too lean. I've heard of FI cars running up to 1400F or slightl more. But i never really run my carbs hotter than 1350F or so. But it has helped me to see how the mixture is and what way I'm going to go for next summer.

11-02-2002, 12:29 PM
Yes, I tried it with a Lotus 7 and a Ford with a dual DCOE set-up.

Couldn't solve the problem that car had. I figured it out with an idle jet change.

For rich/lean, I still trust a plug and pipe read. Afterall, we are talking carbs.

11-02-2002, 09:37 PM
Yeah I still check the plugs as well, and my fuel gauge is a good indication of mixture too lol! I like the egt for extended full throttle runs like on the back straight or whatever. I found that I was a bit lean up around 5500-6000 so I made some adjustments and it worked a bit better. My idle circuit is another story though, Its pig rich but I'm putting a 2L 16V in next spring so I'm going to wait before I buy jets. They are pretty fun to play with though.

87 ITB Scirocco
HighSpeed Motorsports (http://www.geocities.com/highspeedconnectionracing)

11-02-2002, 11:45 PM
tried 40 dcoes clearly to small especially with the amount of flow the head delivers at .500 sucks [email protected] 28" swirl is minimal which is what you want. What everybody says on top end cars from honda to chevy to dodge to vw high swirl is for low rpm street action. I'll try the 190 air thanks. What nobody mentioned it the bowl jet. One set of specs for a factory 1800 cc bmw 4 is as follows and the thing snaps your head back hard with a factory 1 7/8 exhaust pipe to a regular muffler. 38mm venturies , 5.0 aux. vent., f9 emulsions, 40pump jets, bowl jet is 70 idle is 45 f9 mains 125 airs 175. 300 deg cam 460 lift 110 lobe separation 42x 36 valves no porting. Would exhaust if to small for a particular motor that is modified prevent top end pull or just give more torque. One individual added that the compression is to low. I was wondering that myself. But remember I had a 65vw bug I didnt build 2180 48ida carbs a true low 7.5:1 cr 44x 37.5 valves fk87 cam essentially 320 degrees 600" lift and used a wopping 42mm venturies had a 12 lb fly 1 3/4 header and the thing reved like a jap race bike. Out of the hole a little slow but once it was on the rpm band damn the thing ran the way everybody said it wouldn't run.Little hard to drive but 1/4 mile 2 slips 12.99/ 12.98 the other three when tested ran respectively 13.01 13.05and 13.02. when I built this car bmw .I myself was wondering if the venturi are to small. On the vw end regardless of compression you run venturies 2mm smaller than the size of the intake valve which is the industry std on hipo aircooled action I'm no pro just quoting. I'm confused would runner size and mani length have anything to do with venturi selection remember I have 46mm intake valves Most weber charts claim motors turning 7000rpms need at least 40mm vent. to provide sufficient air to the motor. sorry lots of ?? but like they say inquiring minds want to know

11-04-2002, 12:43 PM
Plug cuts and EGT work well together. Nothing beats a good plug cut though. I found that EGT was a good indication of what was happening when I had a rich 'misfire' problem. My racecar wouldn't climb above 6000 rpm earlier this year. I was suspecting an ignition problem not firing the cylinder, but it would dump fuel so much as the rpms climbed. The EGTs would be in the mid 1300s and then fall to the low 1200s as the rpms reached near 6000 rpm and the car wouldn't pull through it. Then we rejeted to 145s (from 155 mains), and voila - reved like a banshee, and got temps up in the low 1400s and it pulled to 8000. Anything over 1450 is probably getting in to the risky zone of motor damage. Anything over 1500 is asking for trouble IMO.