[JBanna]

Hey all, I think it's time to replace my 455's Rochestor Quadrajet - I'm not sure how complicated the throttle linkage will be to adapt...any suggestions?

Thanks!

[grivera]

Are you replacing the intake as well? If so, Lokar throttle cables and brackets make it easy.

[steve25]

Why is it time to replace it ?

[67drake]

If it’s a ‘78 Q-jet, I’ll buy it off of ya. Seriously

[chiphead]

What is carb number? Put me next in line

[steve25]

Q-jet issues?

I took the time to look back through some of your previous post.

How long ago did Cliff go thru that Carb?

Was it when you were still running the Olds 403 motor?

If that carb was set up for the 403 then that’s part of the reason why you blew out the passenger side head gasket when it was used on your Roller everything 455.

[chiphead]

If you are looking to use a different type carb, go with a 750-850 vacuum secondary Holley 4150. Or send your Q-jet to the guy that now runs Cliff's Q-jet biz.

[Tim Corcoran]

Don't get rid of that Q-Jet, send it to one of the Q-Jet specialists here on the board and have them fix/rebuild it. The Q-Jet is a much better carb than you can buy today. Keep it, use it.

[JBanna]

Thanks for all the responses!

My carb number is 17058253, which I believe was the one I had on the 403 according to Cliff, but my memory isn't good enough to remember for sure (I also apparently had a 17057262 from a '75 Pontiac 350 in June of 2009, but I can't remember what happened to that carb); anyway, Cliff rebuilt both in the time period of 2005-2009). Since Cliff isn't personally rebuilding carbs anymore, I asked him to send me a rebuild kit for the 17058253, using my engine specs. Because I was going through my engine to diagnose and fix the blown head gasket, I decided to go through the carb too, which was probably overkill since it looked fine upon disassembly; in any event, the motor is malfunctioning in that I can't get a smooth idle and acceleration, no matter the mixture or APT screw settings, and I have to advance the static timing to about 30 degrees to even get it to stay idling or running, especially under load and decelerating (my manifold vacuum is less than 10 inches at idle and only about 20 inches at 2000 RPM). I still have to do a compression test, but the leak down test showed problems with #5 and #6 cylinders, and my machine shop says that's probably because the hydraulic lifters in those cylinders were just hanging up a bit and will rectify itself, something common in a rebuild start-up?

So, the bottom line is I originally thought I screwed up something during the carb rebuild, but I was very careful and specifically followed the guidance from Cliff, his book, and other online videos, so I'm thinking it may be something else...oh, I also checked for vacuum leaks using both carb cleaner and propane, which was negative. The PCV valve is also new and functioning properly.

A few other things - my initial setting of the rockers was too tight, and of course I had no compression in any cylinders; however, I went back through them and noticed the leak down issue with #5 and #6, but the motor started and initially idled ok (but not great), so I went with the advice from the machine shop who went through my heads and buttoned it up, only to have the problems described above. I had also pulled the motor and removed the oil pan and timing cover to inspect the cylinders, pistons, timing chain/gears, etc. - it looked pretty pristine and there was no metal to be found anywhere. Finally, I'm pretty sure a slipped harmonic balancer led to the head gasket failure as I was chasing the timing for a few months before the failure - the new balancer is marked very well and cannot realistically be the timing issue now.

I think that covered all questions and comments above, but in case I want to send the carb out to Cliff's successor, can someone give me the contact info please? That may be my course of action if I can rule out a mechanical issue; I don't dislike Rochester Quadrajets, but in this day and age of declining expertise in older vehicles, I'm reaching out for that expertise...

Thanks again to all responses!

[JBanna]

More: I swapped my 403 for the 455 back in 2005, with the 455 being rebuilt by a local and reputable performance engine shop, and it has about 25k miles on it now. I have no need to replace the intake manifold as it appears to be "like new", especially after I cleaned it up! Hopefully that now answers all the queries...

[JBanna]

...and one more thing seems worth mentioning - my carb used to take a few days for the bowl to leak out, but now it only takes a few hours...I checked the bottom plugs for leaks when I had the carb apart, and they were fine...are there other potential routes for that gas to take? Cliff wasn't concerned about the few days as he thinks it's the nature of the fuel these days, but I don't think a few hours would be ok with him...

[steve25]

First on your check out list is to see what you actually have for hot cranking compression with

1) all the plugs out.

2) the carb held wide open on the secondary side at least.
3) power to the HEI killed so it can’t harm its spark coil.
Then report back.

Your poor idle / low vacuum condition could be that the Cam is not timed properly even though the shop might have assembled the motor, which leads then to the question of did they degree the Cam?

If not then the marks they were going by on the timing gears might be off, this has happened to folks before.

Your be fuel now totally evaporating or going too low to start the motor sounds strange.

Yes, with today’s fuel the 10% Alky of its make up will evaporate off fast what’s left of the gas should not.

If your sure that your balancer is correct then make a mark 2 1/8” around and that will be 34 to 36 degrees by the time the motors at 4000 rpm with the vacuum advance unhooked and plugged off.

If you do have timing related issues the motor should then run much better and especially at low rpm and idle conditions.


Your having the valves adjusted too tight on cylinders 5 and 6 could have harmed the lifters and this could also now be the source of what is seemingly your carb issue .

Low vacuum levels created in this way can lead to a mistaken carb issue.

To do a quick check for this kill the power to the HEI and crank the motor over a good number of times.

Any speed up of rpm indicates cylinders with lower compression then others which then need to be determined as why.

I get the feeling from what you have posted that you seem to be confused with the carbs you have.
Once again if the your mistakenly runnng the Q-jet that an expert like Cliff did up for your 403, then you will never get it running well on your 455.

Please also post up your Cam specs and what ratio rocker arms your running.

[JBanna]

Update! I finally figured out the real issue with the poor running - I've taken apart and reassembled nearly everything on my car over the years, to include pulling the engine three times, but I never got into the guts of my engine past the manifolds, so had never set the valves/lifters. I went online and saw many videos on how to set my rocker arms, but as it turns out, they didn't like the extra "pre-load" - I went through them again only tightening them as much as my hands could, on the TDC of each cylinder, and now my compression and vacuum are great, and it runs great with no valve noise - I still have some carb tuning to do, but I'm happy!

[JBanna]

Yes, I followed the proper steps to do the compression and leak down checks, and I know my balancer's timing marks are correct, and every 90 degrees took each cylinder to TDC in the firing order, so once I realized the lifters/rockers were still too tight, I went through them again with less "pre-load" and now things are how they should be, and there is no perceptible damage to any of the components.. My engine specs are below, which Cliff had when he tuned my 403 carb to my 455 specs:

Pontiac 455 bored 30 over, with 6x-4 heads ported with my Edelbrock Performer intake, as well as a set of Hooker headers and custom dual exhaust (turbo mufflers)...

- The cam is one from Comp, the XR64HR: 212/218, .487/.495, 110 degrees lobe separation.  
- Forged pistons, balanced crank, etc., estimated compression ratio of 9.5-1.
- I'd have to dig up the roller lifter and rocker specs for sure, but I think they are also from Comp, selected to be compatible with their cam.
- The distributer is curved to 14 degrees static timing with a total about 45 degrees with vacuum/mechanical advance.

I just need to do the final tuning of the idle and APT mixture screws, but they are really close...

Thank you for helping!

[steve25]

If your still running the stock bottle neck studs and using the factory nuts they only need to be tightened down to 20 ft lbs,

hydraulic lifters have about a .100” to .120” travel range to there plunger for adjustment.

The factory RA4 lifters where called limited travel types and only had .060” of travel range.

The factory setting of the rocker nut to 20 ft lbs should place the plunger to around the middle of its travel range.

[JBanna]

Although my motor and engine bay appear to me stock, the guts are not - I have roller rockers (along with the lifters, cam, timing chain, etc.); so, I have poly locks with the allen lock nuts - they are now set with zero lash and the only preload is what I could do with my hand...the motor seems to really like it!

[steve25]

I don’t understand what it is you mean by “ what you could do by hand”?

With poly locks and straight 7/16” rocker studs you should be able to tighten them down to the point that the plunger has run out of travel and your then holding the valves open.


Also if your lifters only have a round clip holding the plunger in you need to know that if you set them at zero lash and you happen to over rev the motor the round type clips can pop out and then you have big problems.

Do you have stock rocker arm studs that are are 3/8” at there top and 7/16” on the head side.

The side of the roller rocker trunnion that has a round recess in it should face the poly lock.
Are you roller rocker arms themselves installed right with the flat side of the trunnion facing the head ?