Today's problem is..

Hi everyone, I have noticed on my airfuel guage that while i am cruising, it shows my engine running extremely lean, while WOT and idle seem to be alright, anything between, like cruising at a steady speed, my gauge light isnt even registering it goes so lean. I am running a performer rpm and a q-jet carb. Is there any suggestions as to what rods/jets i should run in this thing. It is running hot, I assume due to it being lean, and Ive already cracked one piston due to detonation, luckily it didnt do any cylinder wall damage, but all the valves needed to be ground down and the seats cleaned up.

Here is what is done to the engine

350ci, flat top pistons with 2.02/1.6 valves, 64cc chambers, cam is a .445/.468 lift with a 1.5 rocker, im running 1.6 full rollers, performer intake and the q-jet carb that was worked over by SMI of california. My initial timing is set @ 16 degrees, with a total of 36 coming in at 2700rpm. With the vaccuum advance hooked up, it goes to 51 degrees. Is this too much advance, or do I need to beef up the carb?

Thanks for any help whatsoever,
---Chris

i885744

Isn't that way too much spark advance??? I run mechanical distributors and i usually set them to 34-36 total advance.

68coupe

36 is perfect for a 350 without the vacuum advance.
With the vacuum advance, most people shoot for 52.

I think the timing is good
16 initial
36 total
51 with vacuum advance
that is right.

In the demon carb book (which I have looked at 1 million times), it states that if a carb is running lean, it is too big. The CFM refers to the amount of air that the carb pulls in. the bigger the carb, the more air that is pulled into the engine creating a more lean mixture. I have no idea if this true, but it is what the author wrote. I hope it will help!

page62

Have you checked to make sure your secondaries are functioning properly? Mine didn't work for 2 years due to a cheap base gasket that blocked them closed!

Also, what's your total compression ratio? (I didn't check...do you have aluminum or cast iron heads?)

Today's problem is..

According to Sean at SMI carbs, (he built the q-jet) it flows 790 CFM. My secondaries do work, that I checked, I have had the car on the dyno, and at WOT, my fuel mixture from the dyno charts was reading 12.9-13.2 I am running iron heads, and the comp. ratio was guestimated at about 10.5:1. I always run 93 octane, but thats not to say it wasnt bad gas when it detonated. But ever since the car has been up and running around, ive noticed the temp seems to slowly climb while driving at a steady speed. I am attributing this to the lean mixture indicated by my a/f guage, since the rest of my cooling system is all new and all my seals/spoilers are in place. I have been thinking of just throwing a holley on it since I have more experience with those, but I kinda like using the q-jet, especially when someone looks under the hood, "oh look, thats a stock L-48, still even has the quadrajunk"

Contrail

What rods are you running now? If WOT and idle are fine, then it seems to me you need to go a stage or two higher on the cruise circuit on your carb. Lars has a great paper on this here .

Lots of great info and charts.

lars

Just raise your power piston stop 1/2 turn. That will richen your cruise mixture, lower you temp, reduce detonation, and will not affect WOT. Your timing looks perfect - just fix the power piston height and you're set to go. No problem.

Today's problem is..

Wow, what an insight into how the carb works, thats a great read, and i found out what my carb is off of.

Lars, after reading your paper, I see where it says the power piston adjustment is externally accessible, but looking at the top of my carb, I dont see anything, do you happen to have a link to a picture of one so I know what I am looking at while adjusting it. Also, to raise the power piston, do I just back off the screw the 1/2 turn, or is it opposite?

Thanks for all the great responses everyone.

Larry B.

Most early Q-jets have the power piston adjustment between the two idle mixture screws. You have to remove the pressed in plug to access the adjusting screw.

lars

What year carb do you have? Post the number, and I can tell you how to locate and work the piston adjustment. Early and late Q-Jets are different.

Today's problem is..

Lars, the number on the side of the carb is 17057504. Going by the chart, this shows its a 77 model year carb.

Thanks for any help.

Ive got my engine all reassembled and would like to get this problem taken care of to remedy any future detonation.

lars

If you look at the top surface of your carb just forward of the air cleaner ring and just forward of the float bowl vent tube, there is a 1/4" shiney aluminum plug installed flush with the surface. If you don't want to pull the top off the carb, drill this plug about 3/16" deep and tap it for a #10 screw. Install a screw and yank the plug out. You can then use a pair of long-nosed skinny needle pliers to reach the flat-sided brass piston adjustment screw located down below this plug. Screw the adjuster 1/2 turn counter-clockwise with your needle nose to richen the carb up a tad at cruise and try it out. Re-adjust as required to obtain the A/F ratio you want. This will not affect WOT mixture.

Today's problem is..

Lars, Thanks! I looked at the carb, it has a hex plug and the hole for it had been threaded for the plug, I assume this was done when the carb was redone, but it makes getting to the power piston adjuster quite easy. It took quite a bit of trial and error to get it "out of the red" but thats what was causing it. It took about a turn and a half counter clockwise. What was happening is while accelerating moderately, it would read a little past center on my a/f guage. Center being perfect according to the guage. But under very light acceleration, like keeping the car at a steady 40-50 mph, the guage would go way lean, then completely go out. I did a little more playing with the screw and finally got it to read just somewhat past the center mark under light acceleration. If your are ever in DFW Lars, PM me and I'll be happy to buy you a round!

Thanks everyone for the great responses,
---Chris

lars

Sounds like somebody tapped the hole to gain easy access to the adjustment - and then ended up leaning it way out. You probably just corrected their Bubba-screw-up of an excessively leaned-out piston stop. If the car is running about right now, you might want to pull the plug and stick a small screwdriver down onto the power piston: depress the piston down against the spring pressure and veify that you still have some piston travel range remaining. There should be about 1/8" travel on it. If the travel is very short after running the stop up to the correct mixture range, you might consider swapping primary metering rods. If the travel range is still good, the rods are fine and you're in good shape.

Good job setting it up!