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914World.com _ 914World Garage _ question on backfire

Posted by: crash914 Nov 3 2004, 06:26 AM

OK, I know that I don't have my fuel ratio down yet...laptop hard drive failed....so I hooked up my MS that was set up for the stock 2l on my 2.7....

I have changed out the stock FI system for twin dual throttle bodies...

I havent got it running well enough yet to get a low rpm to set timing...but I can blip the throttle to maintain 2 3K rpm.

I know that my cold start is lean, and when cranking it sounds like running on 2 cylinders...when it warms up, I can get it to rev...

again, my fuel maps are off.....when cranking, it sounds like cylinders 3 and 4 aren't kicking...when I get sustained running, it spits back in cylinder 4, cylinder 3 seems ok and the firing order is correct. I have swaped injector wires to see if it was a signal problem....no....

What would cause backfire/spitting back through the throttle body?

1. lean condition in cylinder----yes, most likely...although I do read 14.5 on my wide band...
2. Loose exhaust valve clearance....leave pressure in cylinder that spits back through the intake valve
3. Tight intake valve....see above
4. ??????

I am running a 163/86b cam on a 103 by 80 motor......

I should have a laptop to hook up to it by this weekend....
thanks, herb

Posted by: crash914 Nov 3 2004, 09:46 AM

Anyone?

or was everyone up late.....

Posted by: mike_the_man Nov 3 2004, 11:12 AM

Not sure if this will be any help, but my car was doing the same sort of thing. It turned out to be a few pretty major exhaust leaks right at the heads. It wouldn't hurt to make sure the exhaust is all sealed up.

If that's not it, then it's probably running lean.

Like I said, probably not much help.

Posted by: crash914 Nov 3 2004, 12:07 PM

you could be right, I did think that I heard an exhaust leak....but it was doing the same thing with no exhaust on at all....I will look at it..thanks!

Posted by: rhodyguy Nov 3 2004, 12:23 PM

like dual carbs, do the butterflies open and is the flow at the same rate? the linkage must still be dialed in.

kevin

Posted by: crash914 Nov 3 2004, 12:49 PM

I did look at the linkage...both seem closed the same amount and open to the same position....

I have not yet set the air bypass for idle air yet...this is something I want to do after I can get somewhat of an idle....

THanks for the insite...herb

Posted by: Mueller Nov 3 2004, 01:26 PM

http://www.innovatemotorsports.com/index.html

you really should invest in one of these

to help ease the pain, get a few local people you know/trust to go in on the purchase....

sounds like it's really lean right now....

Posted by: crash914 Nov 3 2004, 02:09 PM

Muller....it is lean at idle....I do have a WBO2 set up..AFR is arould 14.5 at 2-3000 rpm...my laptop is broken so I can't adjust the pulse width until I can get the laptop back in operation....

IF it is just an AFR problem...that is an easy fix....it could be as it is very hard to start cold...thanks, herb

Posted by: john rogers Nov 3 2004, 06:48 PM

I would say lean is the problem. A lot of the vintage race cars I race with are lean at about 1200 RPM or so and they (as does mine) spit and pop back through the carbs a lot. Once the RPMs come up the mixture goes richer and the poping goes away. If it were me I would not run it until you can richen the mixture a bunch as the 14.5 is too lean as I like it 13 to 1 or so. I would also pull the injectors and put each into a jar or container and see if they are flowing equally and have good spray pattern. Good luck.