odd carb. problem...
Thread Starter
Instructor
Joined: Apr 2009
Posts: 186
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From: Pensacola Florida
odd carb. problem...
I had to replace the needle and needle spring on my 74 q-jet. I also put in a different metering valve because my old one wouldn't stay down with the original washer. Now I have an idle issue--seems to do fine at higher speeds but not well at low rpm's like at a stop. Also has pretty bad throttle response. Is it possible my metering valve and rods could have been wrong?
Any help or thoughts would be appreciated
Any help or thoughts would be appreciated
Racer
Joined: Nov 2009
Posts: 289
Likes: 0
From: Franksville Wisconsin
http://www.corvette-restoration.com/...arb_Tuning.pdf
Try that link, it helped me set up my Q-jet. I found it while several searches and saved it.
Try that link, it helped me set up my Q-jet. I found it while several searches and saved it.
Thread Starter
Instructor
Joined: Apr 2009
Posts: 186
Likes: 0
From: Pensacola Florida
I guess it's not a metering valve if you say there isn't one. It's the mechanism with a long thin rod on either side. I replaced the need and retaining spring because after having someone rebuild my carb awhile back I began having problems with flow into carb. I pulled the top off and found no spring that goes from float to needle so I bought a kit to get the spring.
Tech Contributor
Joined: Aug 1999
Posts: 14,379
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
Possible causes for your problems (but read my post on why Q-Jet problems cannot be diagnosed via Internet posts):
1. You replaced a perfectly good power piston with one that has been destroyed by having the lower end cut off. There is never a reason to replace a good power piston just because the temporary retaining collar is loose in the casting. Fix: Remove your "new" power piston (there are no new pistons available - any pistons out there are used ones of questionable quality) and install your original piston. Shrink down your casting bore top lip to secure the temporary retaining collar.
2. You did not get the primary metering rods engaged in the main metering jets when you installed the piston. Fix: Inspect the primary rods when you re-install your original piston and verify that they are not bent at the tips. Install the old piston with the rods, making sure they are positively inserted into the jets.
3. You installed the needle clip backwards (it does not need to be installed - it serves little, if any, purpose). Fix: Make sure the clip is installed over the rear lip of the float arm and not through one of the little holes on the arm. Or, remove the clip.
Lars
1. You replaced a perfectly good power piston with one that has been destroyed by having the lower end cut off. There is never a reason to replace a good power piston just because the temporary retaining collar is loose in the casting. Fix: Remove your "new" power piston (there are no new pistons available - any pistons out there are used ones of questionable quality) and install your original piston. Shrink down your casting bore top lip to secure the temporary retaining collar.
2. You did not get the primary metering rods engaged in the main metering jets when you installed the piston. Fix: Inspect the primary rods when you re-install your original piston and verify that they are not bent at the tips. Install the old piston with the rods, making sure they are positively inserted into the jets.
3. You installed the needle clip backwards (it does not need to be installed - it serves little, if any, purpose). Fix: Make sure the clip is installed over the rear lip of the float arm and not through one of the little holes on the arm. Or, remove the clip.
Lars
Tech Contributor
Joined: Aug 1999
Posts: 14,379
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
http://www.corvette-restoration.com/...arb_Tuning.pdf
Try that link, it helped me set up my Q-jet. I found it while several searches and saved it.
Try that link, it helped me set up my Q-jet. I found it while several searches and saved it.
Do not download any of my papers from the Internet. I do not post any of my papers: My papers are only available from me by e-mail request. Any of my papers posted by others on the 'net are outdated and obsolete. The paper you referenced in your post is 23 revisions out of date and completely obsolete - it should not be used.
Lars
V8FastCars@msn.com
Thread Starter
Instructor
Joined: Apr 2009
Posts: 186
Likes: 0
From: Pensacola Florida
1. You replaced a perfectly good power piston with one that has been destroyed by having the lower end cut off. There is never a reason to replace a good power piston just because the temporary retaining collar is loose in the casting. Fix: Remove your "new" power piston (there are no new pistons available - any pistons out there are used ones of questionable quality) and install your original piston. Shrink down your casting bore top lip to secure the temporary retaining collar.
You are correct---I removed the "new" piston and the lower end is broken off. The problem is that the old one is no longer in my possesion. I trashed it which apparently was a really stupid move. I didn't realize at the time that I couldn't get one anywhere.
Do you have any advice for getting one. I tore down two older q-jets and both of them had broken pistons.
By the way, thanks for the advice Lars
You are correct---I removed the "new" piston and the lower end is broken off. The problem is that the old one is no longer in my possesion. I trashed it which apparently was a really stupid move. I didn't realize at the time that I couldn't get one anywhere.
Do you have any advice for getting one. I tore down two older q-jets and both of them had broken pistons.
By the way, thanks for the advice Lars
