[jaamzg]

been working on the RC and everything seems to be where it's supposed to be (90 318 TBI). The problem I'm having is a rich condition at idle only. I've plugged up the scan tool to it and the map sensor reads about 2.5 volts and 7.5 Hg" at idle. The cam is a hughes rv cam (9204al http://www.hughesengines.com/cams/la_hyd_roller.asp)

As I raise the rpms, it starts to run normal at about 1000 rpm, the map volts come down to about 1.8 or 1.7 and the preassure starts to read about 14 to 16 Hg". I suspect the condition is being caused by the cam. My question is if I put in a small "T" in the vaccum line between the TB and the MAP sensor and attached a small valve to "leak" in some air, would that help solve the rich condition at idle and would it cause any new problems above idle?

Thanks,

James G

 

[Evildriver-3]

I wouldn't do that, i don't know how you would adjust that outta there without getting into the computer and maybe reprograming it

 

[Runin' With The Devil]

It would be worth the experiment, (Costing no more than the parts would), to Tee in an adjustable air bleed. Everything you need is at the aquarium / fish store. I know a lot of guys who do this to increase boost pressure, temporarily in the turbo Daytonas...

 

[Evildriver-3]

You have a cam in a motor with tbi, the tbi's computer not understanding the motors low rpm characteristics is most likely why it won't get the mixture right along with the cam and it's profile and how little air the motor is pulling in at that rpm.

So now lets introduce a vacuum leak to help at idle, this is what you would be doing, this isn't going to help, about the only thing that would or might help would be redoing the computer to the motor, now im no computer man so i can't help with what would have to be done how to get into it how to reprogram it or anything similar to that

 

[Smokey]

Personally, I would just set the idle up mechanically to about 1000, and avoid the issue.

 

[jaamzg]

Unfortunately, the computer is the old kind that has to be sent of to be reprogrammed, to someone like JET. I tried about 4 or 5 times, but they would never respond to my requests for a precise quote (just 400-$500) is what they said. I thought about setting the idle higher, but as it is, it's hard to stop going down an incline (ie mountain road) in 4LO with a 600 rpm idle, it would be very scary a 1000 rpm idle.

So, if I understand what you're saying marty, the cam i have is inducing a low vacuum and causing the map sensor to send out a higher voltage? So inducing a small controlled leak would only cause even less vaccum? Some explain this to me. Is a higher map voltage output indicative of more vaccume or less vaccum? If a higher voltage ouput means less vaccum, then a introducing a leak won't help. I might as well save my $2.00 in supplies.

james

 

[big10]

Except for +2 degrees LSA and + .029 exhaust lift, the Hughes cam specs are very similar to the computer controlled Comp 254HR-12 specs which requires no computer modification.

 

[jaamzg]

When I ordered it from Hughes, they told me it did not require any computer mods. They had another cam that's a step up from the one I got that required computer mods, so I just decided to go with the tamer one.

 

[89on35s]

I have an 89 pickup with the same problem. If you adjust the idle it will get better, but not go away. I was told that the Jet chip that installs inline with the computer harness will make this go away.

 

[Chump]

First thing I would do is to check the base condition of the MAP sensor and then work your problem from there.

Specifications

All models with TBI.

Apply 5" (125mm) Hg to unit and record output voltage. Apply 20" (500mm) Hg to unit, the difference in voltage should be 2.3 - 2.9.