Motorsports Village

How to fix a rich A/F cruise issue
Page 1 of 1

Author:  nxtruck [ Thu Apr 26, 2012 3:36 pm ]
Post subject:  How to fix a rich A/F cruise issue


I recently added an AEM uego wideband to my truck to help fine tune the carb. The combo is as follows: 434 sbc, older casting AFR 227s, 10.6:1 compression (93 oct. pump gas), Cam Motion .641/.630, 267/273 @ .050, 112ls solid roller, SV 4150 intake, 1 7/8" x 3 1/2" headers into a short 3 1/2" exhaust, and a Holley HP 1000.
What I'm seeing on the wideband confirms my darker plugs. At idle, the w.b. is showing 13-14.5 or so and at WOT I'm seeing 12.4-13.0 or so, which I'm being told is in the ballpark. However, at part throttle, I'm seeing 11.6-12.4 or so, which I'm thinking is on the rich side. I'm running an NGK FR4 plug, which is as hot of a plug as I feel comfortable using and have the timing set at 38* using a crank trigger. At the track, after making a few passes, it looks like I should be using a step colder plug, but when street-driving it, those FR5 plugs darken quite a bit.
Since I'm new to the whole wideband thing, am I overlooking anything obvious here? Is this part throttle rich condition something I'm going to have to live with?
Thanks in advance!

Author:  shrinker [ Thu Apr 26, 2012 3:55 pm ]
Post subject:  Re: How to fix a rich A/F cruise issue

A lot depends on how much power the engine is making at your cruise. If its just into the booster flow then you could test smaller HSAB or blocking the lower emulsion holes and or reducing main jet size and increasing PVCR size to compensate. The solution depends upon how much CFM is flowing at the problem area. Ignition timing etc is very important too.

Author:  nxtruck [ Thu Apr 26, 2012 4:20 pm ]
Post subject:  Re: How to fix a rich A/F cruise issue

One thing I did try was increasing the IAB from .75 to .77. I also reduced the primary jets from 82 to 80. I did consider reducing the mains even more and increasing the PVCR to compensate, but am not sure how much to increase by.

Author:  shrinker [ Thu Apr 26, 2012 4:46 pm ]
Post subject:  Re: How to fix a rich A/F cruise issue

I havent worked on a holley 1000hp here yet but from reports I've been told the calibration in them is closer to the mark than their other previous stupid HP efforts. But if you need to lean the top of the tslot area then reduce the IFR, leave the air bleeds at .069" to .072 range and control the mixture with the jet. I would guess that the HSAB should be left as stock because this carby would have to have a large venturi and I'd bet its got the wrong booster in it for the venturi size so it will need lots of HSAB air to make the booster start in time to match the tslot runout.
Your engine has a big cam for that low a compression so I'm not going to get into helping you tune it, My advise is to be careful about the tuning because if you go the wrong way you will end up making NOx at light loads and damage the engine when you stomp on it. If you run it rich it will be safe and the reading on a WB cannot always be trusted. If you used a gas bench and a dyno you will find that engine has plenty of unused Oxygen in the exhaust and thats a bad thing, it means there is unused hot Oxygen's left in the cylinder and thats dangerous,thats why you need to be rich and no where close to stoich. Pick the wrong ign timing at cruise and you will have hot carbons glowing waiting for you to open the throttle and give you some preignition woes. So id put up with a rich cruise, maybe not as rich as you describe but more in the 12.5 12.9 range. I wouldnt advise to go leaner than that unless gas bench and dyno confirmation showed otherwise.

Author:  nxtruck [ Thu Apr 26, 2012 7:02 pm ]
Post subject:  Re: How to fix a rich A/F cruise issue

Thank you, sir, for your input and guidance!
The compression in this engine is low, as you noted, as this engine started out as an effort to run in Hot Rod Magazine's Pump Gas Drags. This meant that it was to be ran on 93 octane fuel while spraying it with a 150-200 hp shot. I know that the cam isn't optimal for what I'm doing with it now, but it's what I have. I do grudge race the truck at the local 1/8 mi. track and street-drive it quite a bit.
I'll put the .75 IABs back in it and go from there. I try to keep an eye on the plugs when at the track, and was wanting to use the w.b. as an additional tuning tool. I don't have access to a dyno or a gas bench, so I guess I'll error to the rich side and keep it in one piece.
Thanks again, shrinker, for all of your help!!

Page 1 of 1 All times are UTC - 5 hours
Powered by phpBB® Forum Software © phpBB Group