carb jetting

bullseyeprec · #1 (**permalink**) 12-11-2003, 10:03 PM

I just got my BDR 392 stroker and it makes great power!!!!!! My question is that the builder adjusted the carb and re jetted it for his altitude (2000 ft above sea level) The car runs really rich for about the first minute or so, I think adjusting the electric choke will take car of that but it pops a little under de-acceleration but I think most cobras do that. My main concern is that under wide open throttle (cant open it up in 1st or second gear due to tire spin) but in 3rd then I pin the throttle it seems to miss a little under wide open throttle. Performs very well but It does need some card tuning I think. Its a 750cfm vac sec. and I am about 400 feet above sea level. It ran really good at his altitude. Is the missing under WOT due to lean jetting?? He installed 69 jetts primary and 78 on the secondries. Also my carb has dual metering blocks. Is my problem jetting?????? The problem does not occur under normal driving at high speed, Just under wide open throotle. Thanks!!!

blykins · #2 (**permalink**) 12-12-2003, 05:40 AM

Hmmm....it seems like there would be less air at 2000', so you would jet down....so it's logical that if you have more dense air at lower altitudes, you would need more fuel...i.e. bigger jets. But the symptoms you're talking about sounds like running rich. I'm really confused. Does it idle rich/lean?

bullseyeprec · #3 (**permalink**) 12-12-2003, 06:00 AM

Idles a little rich..no smoke just the strong odor. ...But I can expect that with a .576" roller cam I think

blykins · #4 (**permalink**) 12-12-2003, 07:04 AM

Well you can always tune a carb...no matter what size cam you have. I have a .544" hydraulic roller in mine. It idles and runs fine. The weather plays tricks with it...but I guess that's part of running a carb.

I'll be the first one to tell you I'm a newbie when it comes to carbs...I just started fooling with one when I built my small block. It's a Mighty Demon 750...it's got 4 corner idling, and you can interchange all the air bleeds and jets...so I've tried to learn a lot from my dad...

You should be able to adjust the mixture to get it to idle a little more lean. If you run out of adjustment of course, you can change the idle air bleeds.

I suppose the easiest way to tell would be to just pull a plug and see what it's doing. I would think that if it's rich/lean enough to cause it to miss at WOT, then you should be able to tell by reading the plugs.

Some Cobras pop due to the baffles in the sidepipes.....however, according to what popping you're getting, it could be that you have an header leak...or it's running rich.

blykins · #5 (**permalink**) 12-12-2003, 07:05 AM

Oh, one more question....does it just miss when the secondaries open, or does it miss after they have been open a while and you're running wide open?

bullseyeprec · #6 (**permalink**) 12-12-2003, 04:58 PM

Well I cant get it too hook up in 1st or second gear but in 3rd gear it seems to miss a littlelike 1 time per second until its time to go to 4th gear. I am going to go back to the factory jetting (they were changed to 69 on the primaries and 78 on the secondaries and the local speed shop said at our altitude (400 feet) it should have bigger primaries. It was tuned at high elevation so I thougth I would need to swap jets when I got it here since it was tuned at 2000 feet ASL I believe its off. I got an appt Tue to swap jetts and adjust the choke and idle mixture screws. Will post what I come up with. Should be Wed before I know anything.

Mr.Fixit · #7 (**permalink**) 12-12-2003, 05:16 PM

69's are kinda small for a hot stroker small block, but before you get too far, put in fresh plugs, make a 1/4 mile pass and kill the engine, coast off the road, pull a plug, see if it looks rich or lean. Popping is more often header gasket leaks in these cars than anything else, re-tighten your header bolts, if one is pretty loose, replace that gasket, it is probubly burned through around the port.

bullseyeprec · #8 (**permalink**) 12-23-2003, 09:57 PM

carb update...I rejetted the carb (770 street avenger double pumper) to facotry jets and factory power valve...runs better! I have a very slight off idle hesitation and I picked up 2 pump squirters..I am replacing the 28's with 31's and I think this will cure my hesitation. I noticed by revving it up under the hood if I manually stroke the accelerator pump there is no hesitation so I just need more fuel. I guess thats common with big cam and single plane intake. Will update you when its done. Thanks for the input..

Perf. Eng. Eric · #9 (**permalink**) 12-24-2003, 12:21 AM

Bullseyeprec, you may want to try the pump squirter cams on position #2. Also make sure the pump lever arm screws are adjusted to leave only a slight gap (.003/.005) between the plunger rod and the discharge arm when the car is at idle. Sometimes we have found not enough base ignition timming can cause an off idle hesitation also . Most of our stroker Widsors like between 16/18 deg. initial timming at around 950 to 1000 RPMs Your total advance will probably be at 34/35 deg. You may have to re-curve your distributor to get these results. Check with your engine builder to be sure. Hope this helps.

bullseyeprec · #10 (**permalink**) 12-24-2003, 07:17 PM

Perf eng eric I saw some specs that call for .015 clearance at wide open throttle between pump lever and arm and I have at least that but at idle my pulunger rod and discharge arm contact eack other (I dont have the .003 -.005 gap you mentioned. I had always thought I wanted a squirt as soon as I crack the throttle, Are you saying I need a gap at idle? That was delay the squirt intil I cracked the throttle enough to overcome the .003 -.005 gap, is that correct??

bullseyeprec · #11 (**permalink**) 12-25-2003, 12:10 PM

Ok, Got it done. squirters were 25 on the primary and 28 on secondary and I changed them to 31 on both sides. Things got better, no off idle hesitation but I did get a stumble at about 2K if being gentle on the throttle. Backed the cam into poosition 2 and all is well. No stumble at any rpm and it runs better wipe open (not sure why the squirters would have effected that) but maybe I was just dumping in too much gas to early? At any rate life is good. Gotta go do Christmas stuff....Best wishes to everyone!

Perf. Eng. Eric · #12 (**permalink**) 12-25-2003, 02:44 PM

Bullseyeprec, Glad you got it fixed. The gap I was talking about just keeps the lever from vibrating fuel in when the engine shakes at idle,even more important when using solid rod carb. linkage. Merry Christmas

bullseyeprec · #13 (**permalink**) 12-25-2003, 03:09 PM

Eric, I have a throttle cable and if I have any prob will back off on the adjustment but I think I am good to go. Thanks for your input. I am glad I got things lined out, I am sure the neighbors are too!!! Merry Christmas. BTW, If you are in the forum in Christmas, YOU HAVE BEEN BITTEN!!!!