I'm helping a friend rebuild a Q'Jet and am running into some things I could use some help on. He claimed he was having a miss when the Secondary were fully open. He felt it was running out of fuel, but I never got to see it run on the car.

The car is a 1975 Corvette, mild 350, 4spd, ??gears. The carb was 7045203 (Is that correct for '75 Vette?)

1. The jetting seemed overly rich on Primary Side to me, but Secondary seemed fair to maybe a little lean. Any comments?
Primary Jet 72
Primary Rod 46K
Secondary Rod CH
Hanger I

2. I think I have found his problem too, but am not positive ... and wish I would have seen this before it was apart. The Hot Idle Compensator is actually missing and just open in the back. I thought I understood how this worked, and would provide a huge vacuum leak to the engine ... but I can't find any ports where this actually goes into the intake. Did they seal these off for some reason?

3. If I can find the vacuum leak at the HIC, then I suspect that would explain the Idle screws being out 5 TURNS!! Not for sure, but isn't something 2-3 turns more normal.

I made the mistake of telling my buddy I would help to throw a rebuild kit in without driving the car first ... Now I'm finding enough oddities, that I could use some help answering. Any help would be appreciated.

Thanks, Neal

I just finishe dgoing through Doug Roes book on ROchester carbs and would suggest you check at the local library for a copy. The numbers correspond to a 75 Vette and are correct so you have a good starting point. If you have not disassembed the carb I would put it back on the car and run the tests prescribed in his book. Also I found a site with good info on a Corvette site but can't remember it. the author of the article states you can contact him for technical assistance at lars.grinsrud@lmco.com.
Both sources stated the primary rods contribute more to WOT response and the secondaries just compliment them.
Let us know how all works out.

Originally posted by Neal Wright:
He felt it was running out of fuel, but I never got to see it run on the car.

1. The jetting seemed overly rich on Primary Side to me, but Secondary seemed fair to maybe a little lean. Any comments?
Primary Jet 72
Primary Rod 46K
Secondary Rod CH
Hanger I

2. I think I have found his problem too, but am not positive ... and wish I would have seen this before it was apart. The Hot Idle Compensator is actually missing and just open in the back. I thought I understood how this worked, and would provide a huge vacuum leak to the engine ... but I can't find any ports where this actually goes into the intake. Did they seal these off for some reason?

3. If I can find the vacuum leak at the HIC, then I suspect that would explain the Idle screws being out 5 TURNS!! Not for sure, but isn't something 2-3 turns more normal.


Are you sure the float level is high enough? That can cause some of those symptoms.

1. Go to the Corvetteforum.com and do a search for Lars papers. He has a paper that should list the stock jetting and rods for that car. Download it.

2.I recently rebuilt a Qjet, and had an open hole for the HIC. If the screw holes arent tapped, it isnt needed. Look for any sort of outlet from the port to a vacuum or other source. IF there is not an obvious one, the problem is elsewhere. I have seen a few Qjets- including mine- that is open in the back. I am not sure, but I think it hooks to the throttle somehow, as I have read that when a car has one something must be done to it keep it closed (or open)

3. 5 turns does seem excessive, but my Haynes books says to start at 4 turns out? I'd double check the float level before messing with anything else.

4. Good LUck !

Wow, BuzzBomb hit it right on the money!! Here is the link to Lars Q'Jet papers. The site is actually CorvetteFAQ.com, but who's counting:)

http://www.corvettefaq.com/c3/QJet_Tuning_Paper.doc

I do already own Doug Roe's book, and understand it quite well ... but Lars put down on paper what I have been looking for years. The factory Rod/Jet numbers for just about any Chevy/Pont around!! On top of that, his papers are quite insiteful.

The other part of my question was about the part number. Carbs Unlimited now has a very good part number reference. Check this out if you ever want to know the EXACT application for your Q'Jet number ...

http://www.carbsunlimited.com/Number/oemnumber.asp

P.S. As it turns out the carb I have is actually from a '75 C/K 350. Interesting enough it does have all the correct Rods/Jets for our '75 MT Vette.

P.S. Yeah, I know about the comments on the float. That's what started the rebuild. It's obviously been apart by someone, and I had told him we'd probably find the float misadjusted. It was late last night and I actually haven't had a chance to check the float level yet.

Thanks for the help ... and definitely check out the links I found. They're going in my Q'Jet reference file!!

Neal,

I'm glad you can use the Lars stuff! That guy is the Qjet king on the 'vette forum and has a HUGE following for redoing and tweaking Qjets. If you didnt get that Excel file for the ENDLESS rod/metering jet combos, get that too. IT may be in the Roe, book- not sure. I almost bought that book, but the Lars stuff is SOOO good, its almost overkill graemlins/beers.gif

I'll bet the float is waay off. That is usually the first and best place to start.

BTW- if you like tuning Qjets for best performance, go to the SUper Stock forum.com, and do a search in teh ENgine category for Qjet. Look for posts by a guy named Danny(?). TONS of GREAT info for flogging the Qjet to be had there too.

Alright, I got a chance to look at the float tonight ... but am having troubles figuring out where it should be. My carb kit SUCKS!!

I can't hardly tell if it is supposed to 5/16" or 15/32"? Huge difference, and the carb kit is very unclear. The old one came out about 9/32".

Can somebody help me out that has a '75 book to look up what my float level is supposed to be. Carb #7045203.

Thanks, Neal