inline 250 ??'s

[BAGNDRAGIT]

ok guys and gals, stumped here. i bought a 71 short step about 3 weeks ago (technically paid 1800 bucks for it) so i cant complain, mostly complete and already lowered.

anyway, i have been trying to get the truck to idle correctly since day one, i have rebuilt the carb (rochester monojet) 3 times, i have timed it twice i have replaced the plugs, intake and exhaust manifold gaskets all vac lines, wires are newer the distributor was upgraded to electronic..not by me, i have replaced the batt cables and coil as well along with a new pcv and fuel filter and all rubber line, it also thee vac moidulator on the trans. still misses at idle, i am stumped, although when i pulled the carb apart the main metering rod didnt seem to wanna seat in the seat. is that normal?? seemed too short... i am going to change it to HEI cause i dont trust the electronic setup since i didnt install it. is there anything i might be missing?? she runs great off idle, real smooth, but as soon as i stop at a light all it wants to do is start missing and die while in gear, but in park my idle is really high. i also have my mix screw on the carb turned all the way in so im leaning towards the main rod being the problem but i wanted some opinions and experiences from here.
i hope im not missing any info. just to be sure its a 71 250 i6 with a turbo 350. non ac ps anyway, its late im tired, tell me if i missed something...

also does anyone know what years and models had the 6 cyl HEI in them??

thanks in advance!

[68lwb292]

Don't know what to tell you but keep chasing those gremlins. Look in 76 and older Chevy camaros, malibus, novas, and pickups for HEI Straight 6s. I've seen brand new HEIs on Ebay for $60 and up.

[68lwb292]

Ebay Link for HEIs
http://cgi.ebay.com/ebaymotors/NEW-H...item45f6de001f

[sixtyeightchevykid]

i have had the same kind of issues, the best thing is to get a "new" carb, more than likely the carb body where the throttle butterfly rod goes is worn out and is sucking in air. the way i checked mine was take some carb cleaner with the straw on it when the truck was running and spray it around the throttle rod where it goes thru the carb and it changed the rpms and got another carb put on, that made it idle like a champ.

[Old Bluey]

When my 250 missed at idle, it was spark jumping to from the cap to a hard fuel line, and between the wires (Taylor 8.5mm). I found this out by getting shocked when I was doublechecking the spark wires were pressed into the cap. Cap was 2 years old but had cracked as well.

My suggestions:
Make sure nothing conductive is close to the cap
Separate your wires with a loom
Install a new cap (Unless you've done that already)

I thought mine was carb related too, which it may be, but checking that you're getting consistent spark would be a good start.

PS - I've got the pertronix swap for my distributor too.

[BAGNDRAGIT]

ok i will go ahead and do both the carb check and the distributor check, im leaning more towards the carb cause the cap and rotor look pretty new. but just to be safe i will do it.

thanks for all the input. i am going to start looking at carbs, any suggestions? or should i try and find another monojet?

[Old Bluey]

Those monojets are hard to beat in terms of reliability. I wouldn't look for an upgrade until you can get a 4 barrel.

If you do get conductive materials close to a new cap, and it jumps (and the engine misses) it can cause it to crack. Both my cap and wires looked pretty new too. when the engine is running, try to separate the wires with a loom and see if it makes a difference. You may even hear snapping coming from a jumping spark. This problem sounds way too familiar to mine. I learned the hard & expensive way. Good Luck!

[Wrenchbender Ret]

There are 2 different gaskets used between the throttle body & fuel bowl. Make sure you have the right one & it is installed correctly. The metering rod does not "seat" in the jet. It has steps on it & rides up & down to regulate flow. Make sure it is installed on the bracket correctly & the adjustment is correct acording to the direction sheet. Make sure the float level is correct.
Some of the early 6 cyl. HEI 75-77 had the coil mounted externally. The coil in cap is better. 78 up till they went to the computer controls.
The Monojet is a good carb. There are some improvments on the later models like electric choke.

[swamp rat]

>>>i also have my mix screw on the carb turned all the way in so im leaning towards the main rod being the problem<<<

I would say this is probably one of your problems right here, the mix screws should be around 2 1/2 turns out from the lighty seated position, by turning them all the way in you are not allowing any fuel to pass thry the idle circut, if you do this your probably compensating by turning in the idle screw on the linkage and this opens the butterflys and pulls fuel from the main jets and or accelerator pump, then when you push on the pedal the accelerator pump cannot travel thru its full stroke...

>>> Main rod<<<
I think maybe you are refering to Metering rods? they move freely up and down inside the main jets.

Yoy have not mentioned if you have done any compression testing, a cylendar that is low on compression can also act like a miss as well.

I like to fire up the engine and use a pair of ignition wire plyers and pull the plug wire off of each spark plug one at a time to try and isolate the miss, if you pull the wire and nothing changes in the way the engine runs you have found a plug not getting spark. If you pull a wire and you instantly hear a miss the cylindar is firing.

I have also went out at night and fired up the engine and raised the hood and looked for any electrical arching in the wires, wear a pair of gloves and move the wires around a little looking for sparks.

Also not sure of your distributer but pull the roter and a cap and look for any dirty looking film inside and clean it well, look for any signs of arching there as well.

[BAGNDRAGIT]

Originally Posted by swamp rat

>>> Main rod<<<
I think maybe you are refering to Metering rods? they move freely up and down inside the main jets.

Yoy have not mentioned if you have done any compression testing, a cylinder
that is low on compression can also act like a miss as well.

you are correct, metering rod is what i meant. and no i have not done any compression testing. but i will. and i will give your other suggestions a try.

Originally Posted by Wrenchbender Ret

There are 2 different gaskets used between the throttle body & fuel bowl. Make sure you have the right one & it is installed correctly.

is there a way to determine the correct gasket? or is it pretty straight forward?

[Wrenchbender Ret]

If you match up the holes its pretty straight forward.