88305tpiT/A

iTrader: (2)

So I had my non-cc q-jet tuned pretty well I thought. Changed jets and rods till I arrived at a pretty good 14:1 at cruise and 12.5:1 at WOT for the 383 its on top of. Took .48 main rods with 76 jets (pretty fat I know).

So then I went on a long trip (CT to NC and back) and had a major problem on the way back. Under power the Motor began to run very lean and cut out as if the fuel bowl was running dry. I had seen this happen to me before so I stopped at an autozone and changed the fuel pump and filter there on the road. That got me going again and the motor no longer cut out. But the carb was still lean and went from 14:1 to 15:1 and has been there ever since.

I tried looking for a vacuum leak but couldent find one. The fuel filter got changed too when I changed the fuel pump in that parking lot. when I cut it open at home it was pretty dirty. But I thought as long as the fuel bowl has fuel in it it shouldent affect the mixture of the main circuits... I guess maybe I got a bad tank of gas or something and that might have gummed up the carb pretty good?

Anyone seen anything like this before?

Thanks for any advice.

Sonix

iTrader: (1)

see if the primary power piston got jammed down. Shut the car off, and see if you can push a screwdriver down the float bowl vent and feel the power piston.
Are you monitoring your vacuum level at different AFR ratios as well?

88305tpiT/A

iTrader: (2)

No, I dont have an in car vacuum gauge. Probably could get one but $ is tight these days...

I will check the power piston when I get home but I doubt its to blame as the AFR does respond and drop when I go to wide open primaries, its just all leaner than it used to be.

I am taking the carb off tonight if I can to clean it. I am basically just going to take it off and drain it. Then air horn off and basic rebuild without taking the throttle body off the main body. I will replace everything I can (needle, seat, accel pump, check ball, power piston spring). Then clean with some aresol carb cleaner through the primary jets and in and out every hole in the main body. Then blow out with compressed air. If that doesnt do it I think Im in the market for another qjet cause this one requires some huge jets and to get the right AFR would require .046 main rods and likely larger than .077 jets.

Sonix

iTrader: (1)

Well yea, but i'm thinking the acc pumpshot and secondaries could be covering it up. If it's lean all over the place, it's one thing to check...

48 rods and 76 jets sounds ok to me... It is on a 383 after all... You could always plug up and redrill your air bleeds...

Rayzor32

iTrader: (1)

yeah i've had this happen before on my buddies 400 we ****ed with that carb forever, he wanted the qjet but we ended up putting a holley on it and then he switched it to an eldebrock and I took the holley.

88305tpiT/A

iTrader: (2)

Ok.... Any idea what was wrong with the carb or is your post to endorse me buying another brand of carb?

Rayzor32

iTrader: (1)

basically we never figured it out so we swapped carbs i dont care what you do i dont work for holley or eldebrock do whatever you want... just saying what we did. we ****ed around with that carb for a while, I think it might have been something with the air bleeds or a porosity problem maybe could have just swapped a different qjet but we just threw the holley on and was done with it

88305tpiT/A

iTrader: (2)

Power piston looks fine, not sticky at all. I blasted all the passages with berrymans cleaner. Am waiting for a bunch of air horn gaskets from carbs unlimited. I got more than one in case I have to go back in a bunch of times and make jet changes...

88305tpiT/A

iTrader: (2)

Ok so I put the thing back together after cleaning it and its STILL LEAN!!!

I am getting super frustrated here.

This all happened on a long trip from North Carolina to Connecticut. About halfway home the fuel pump started going bad and the carb got lean. I replaced the fuel pump and just cleaned the carb but its still lean. Only things I could recheck are that the power brake booster is leaking, intake manifold bolts could be loose?

I suppose the float could be going bad but would that really change the A/F ratio by more than a point???

When I did clean the carb out I sprayed cleaner IN all the holes in the boosters and not in the jets to try to force stuff back out of the main passages. I also didnt take the throttle body off cause I dont have another throttle body to main body gasket.

I am at my wits end here, I already have a 76 jet in it now and I have a feeling going to a 77 wont make enough of a difference to bring it back under 14:1. And besides, arent you supposed to tune WOT with the jet and cruise with the rod? I backed the APT screw out another 1/2 turn but it didnt seem to make a difference this time.

Sonix

iTrader: (1)

yes, if you're lean at cruise you want to play with rods only.
That is, if you are ok at WOT (lock out secondaries).

48 rods are fat, so you can easily step down a bit.

The fact that it changed on it's own is a bit of a worry. Idle is unchanged? timing curve is good?

You used a propane bottle to check for vac leaks?
How do all the plugs looks? No misfires anywhere?

88305tpiT/A

iTrader: (2)

Idle is lean too. Seems to get rich again closer to WOT you are.

I never could find any thinner rods than .048's If I could find some .046's I would put them in in a heartbeat but edelbrock's smallest is .048's for 75 and later qjets.

I will check -
Intake manifold bolts,
Plugs,
try to get some starting fluid or something to check for leaks

I dont have a propane bottle for a grill or anything - could I just get a little mapp gas torch or similar? Maybe that is what you meant and I am just dense thinking to use a white propane cylinder.

Thanks so much for your help Sonix, I really appriciate it.

Sonix

iTrader: (1)

yes, i'm talking about the 1lb propane bottle (like the MAPP ones you find...). Like for soldering plumbing in your basement.

I've never even seen rods as fat as 48. I have a stack of 39-45 size. I think I just bought some 46 (might be 48 though...) from summit racing...

idle will be lean due to a slight amount of excess air coming in. WOT sucks up so much air a slight vac leak is no longer a big deal. I'd run the propane over all possible areas while idling. Look for the area that bumps up your idle speed...

88305tpiT/A

iTrader: (2)

Ok.

Supposedly the 75 and later qjet primary rods are different than the 74 and earlier and they dont interchange.

Will try the gas next.

Sonix

iTrader: (1)

I know. the '74 and earlier are shorter I think.
I only have post 1975. yous is as well right?
http://store.summitracing.com/partde...5&autoview=sku

88305tpiT/A

iTrader: (2)

Yes I do but the part you are looking at on summit is for pre 75's

Check Edelbrocks website:
http://www.edelbrock.com/automotive_...ss_q-jet.shtml

Sonix

iTrader: (1)

right! that's where that is...
and that's weird, because i've only ever tinkered with post 1975 q-jets, and i've never seen anything fatter then 45 in my days. In fact usually 43 is as fat as I can find... yet edelbrock only makes 48 and thicker.... I don't get it....

anyway, it doesn't re-rod itself, so i'm guessing if it was set up ok before, then something else has changed, and re-rodding it will only be chasing your tail...
let us know what you find when you get a chance..

88305tpiT/A

iTrader: (2)

Exactly my thoughts!

Will do.

88305tpiT/A

iTrader: (2)

Well, I did the propane trick and no luck. Couldent get idle speed to change or A/F to move at all. I will be replacing the intake manifold gaskets next. Hoping that they are leaking into the valley.

Incidentally I did notice that vacuum was around 23 inches at idle in park but in drive it dropped to around 18. Is that significant? Before tearing off the intake I will perform a compression test and check all the valvesprings for breakage.

Damon

Edelbrock QJet carbs are calibrated differently than your older Rochester in almost every case. They use later "M" style metering rods that have a larger diameter .036" tip. You probably have earlier "K" style metering rods that use a smaller .026" tip. That's why they only have the very larger diameter rods- becuase the WHOLE rod is fatter than most original Rochesters- the metering taper AND and the tip.

Long story short, the difference means a 48 Edelbrock (M) rod would be about equivalent to a 42K Rochester rod, but the jet size you need to work with them to get the overall calibration right will be different.

Example of equivalent combos using the different style of rods:

76 jet + 44K Rochester rod

80 jet + 51M (Edlebrock rod)

Both of those combos produce the same flow area at both part throttle and WOT.

A 76/44K combo is my usual starting point for most 75-up style QJets. It's usually about right for a modest street driven performance motor.

I'd be looking for WHY everything went lean on you all of a sudden. You sure you've got good fuel pressure? No kinked lines or clogged filters? Qjet's don't just fall out of tune like that, short of getting dirt in them or having a vacuum leak pop up.

And is it really lean everywhere? What happens if you smack it wide open?

88305tpiT/A

iTrader: (2)

Yes I agree with your assesment.

I am only lean at idle to part throttle. I think its a vacuum leak from the intake gaskets into the crankcase.

I didnt think that a fuel restriction would cause that kind of leanness at idle and part throttle but be ok at WOT but I will certainly check the hardline from the pump to the carb. I suppose I could also blow out the line from the tank to the pump with compressed air.

Thanks for the suggestions and explination of edlebrock parts vs. rochester parts.

88305tpiT/A

iTrader: (2)

Well I dont get it...

I got the intake off last night and all the gaskets looked fine. Good squish around all the ports and the gaskets didnt look damaged at all.

I think this eliminates a vacuum leak as the culprit because now I have ruled out leaks from everything: Intake manifold gaskets, Distributor vacuum advance hose, A/T vacuum modulator hose, Brake booster hose, PCV system, and carb base gasket have all checked out fine.

So when I took the carb apart last time I didnt take the main body off the throttle body. Could there be some passages between those two that got clogged or something?

I also blew out some of the fuel lines last night with compressed air. The pump to carb hard line was totally clean. The tank to pump feed line seemed to flow fine. Is there any way I can test the pump? I bought it new three months ago when this started but who knows maybe its bad.

Incidentally I am traveling to OH next week and If I cant get this fixed by sunday night I will go buy this:
http://store.summitracing.com/partde...D36001&FROM=MG
Or this:
http://store.summitracing.com/partde...D90670&FROM=MG
From summit.

five7kid

iTrader: (14)

Just because this is in a truck is no reason to buy a "Truck" Avenger carb.

Stick with the q-jet or get a double pumper Holley.

88305tpiT/A

iTrader: (2)

Who said this was in a truck?

I have the motor all back together and will let the RTV dry till tommorow morning.

I put 77 jets in and went to 5 turns out on the APT screw.

Also cleaned it some more and took the throttle body off this time.

If it isnt pig rich then I guess Ill just get the JET carb I listed this week when Im in Tallmadge.

88305tpiT/A

iTrader: (2)

Ok so after all that its about where it used to be. A little over 14:1 at cruise. So something changed that I havent found yet.

I did find one interesting thing though. When I looked closely at the distrubutor gear I noticed that the drive side of the teeth were showing massive wear. Like not smooth - like melted a little. I can only assume the cam gear is the same way. So now I have to replace the gear and the cam at least.

I was thinking that this could be due to the .010 clearance I shimmed the shaft to. Its an aluminum body HEI.

88305tpiT/A

iTrader: (2)

Ok now I am doing more reading and it seems that with my aftermarket comp cams retrofit roller I might have needed a bronze tip pump pushrod. I guess I could have ground my fuel pump eccentric down?? Also the distributor drive gear wearing badly suggests the cam material is not compatible with my old flat tappet style drive gear and pump pushrod. Low fuel volume due to a reduced pump eccentric certainly could cause a lean condition couldent it?

88305tpiT/A

iTrader: (2)

ok so after all it was a combination of a bunch of things.

Firstly I took the dissy drive gear to my engine builder and he laughed and said "yeah, they make these summit brand distributor gears out of stale peanut butter". SO that was throwing my timing off - changing my F/A ratio.

Then my fuel pump failed for some reason which had nothing to do with the F/A but did almost strand me.

Then I noticed that the header that the O2 sensor is in had a hole - right in front of the sensor. Caused excessively lean reading.

New fuel pump, good melonized distributor gear and new headers and problems gone.

Damon

We should make this thread a sticky with the heading of "why 90% of all carburetor problems are found in the distributor." Old mechanic's saying.

If you keep dirt out of them, QJets will run nearly forever without needing adjustment. They don't often fall out of tune for no explicable reason.

I'm glad to hear you got to the true root cause of the problem. Always feels good when you finally nail it dead-bang. Now go have fun with it.