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Q-Jet uphill tuning question ***updated with new ? in post #6*

boz42

boz42

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I am running a new Jet performance q-jet. It runs & drives awesome. I have plenty of power & throttle response is quick. There are no drivability issues on the street.

The problem I am having is when I go up a 30 degree or steeper hill it stumbles terribley & will eventually die. I can hold it wide open & it will not stumble, but I need to quit launching the front into the air.

I do have the vent tubes plumbed together with a copper line.

Any suggestions would be appreciated.
 
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Raise the float a SCH ( just a little bit ) and make sure the timing is set right with the dizzys vacuum plugged at idle . The adjust the air fuel for best idle . When done adjust the idle speed up a hair , but keep it low enough not to pull through the brakes or diesel .
 
x2. Float too low.
 
Thanks guys, I hoped it was just a float problem. This thing runs perfect until you try to drive up a steep hill.

It also runs perfect going down hill. If that tells you anything.
 
In my experience, setting the float higher for climbing will starve the engine on descents. But only on nose-in-the-ground descents.
 
Okay, so I finally got around to changing my float setting. It created another question. I was looking at Doug Roe's Q-jet book in the Offroad section & it shows some pics of floats prefered for offroad use. My float is shaped oppisite of what the floats he says are good to use. I can see how this shape would make my carb starve on uphill climbs.

Do yall think I should try to find a flaot that has the larger side to the front.

This pic shows my float & a couple from Roe's book. These are 2 of the floats he claims to be ideal for offroad use.

p1010002.jpg
 
boz42 said:
...

Do y'all think I should try to find a float that has the larger side to the front.
Click to expand...

I'd say no. It'll allow the float to control the needle better on climbs, but shut down fuel earlier on descents.
 
But theoretically fuel demand should be much decreased on descents, correct? As long as the float bowl is full you get quite a bit of runtime, and with little/no demand, as long as it didn't STAY shut on descent, lower than normal float bowl level shouldn't hurt.

I hear what you are saying though. Trading one problem for potentially another.
 
It's a compromise. I thought about float shapes, and decided I didn't want to get into Q-Jet details that badly...

All I can say is that I really don't like it when the truck's nose is pointed to the ground, said ground is still ways down the darn hill, and the motor runs out of fuel.
Then again, there are usually more severe climbs than descents where I'm 'wheeling, so...
 
Interesting thoughts,,,,,I did consider the downhill concern, but the float is further away from the front of the bowl than the back so I felt like the float shape would have less effect on the fuel shifting to front of the bowl. Looking at the design of my current float, I fugured if I raised it high enough to feed going uphgill it would never close the fuel inlet.

I bought a new float shaped like the ones Roe pictured. It would not fit in my Q-jet because the Jet carb uses a late model carb body. Jet has a part throttle enrichment adjustment that is in the center of the float arms. I modified the new float to work & then sprayed it with shellac according to Roe's method. I am going to try it this afternoon.

I'll report the outcome.
 
Okay so yesterday I put the new float in. It worked perfect. Downhill, uphill, & flatground. I also did not have to set the float higher than stock setting to get it to act right. Hopefully the shellac will keep gas from soaking into it. If it does I am all set. Thanks for the help guys.
 
Is that a new float boz? You could also try a brass one.
 
cybrfire said:
Is that a new float boz? You could also try a brass one.
Click to expand...


Yes it was a new float. The problem is the Jet perf. Q-jet uses an 80 something carb body. There is only one style float I could find for that carb. It is the one pictured above. I bought a new float for an older carb(it was for a late 60's truck) like the ones Roe has it the picture & had to mod it to fit. I could not find a brass one same day, so I went with what they had in stock.

79chevyk10 said:
That is some fantastic tech!!! :saweet:
Click to expand...

Thanks, but I can not take credit. All the info I gave came straight from Doug Roe's Q-jet book.
 
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