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Old 01-29-2018, 12:29 PM   #1
jfnar
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Straight 6 Offenhauser Woes

I decided yesterday to start the process of putting an offenhauser intake and Holly 390 4 bbl carb on the 292 in my 68 gmc. I thought it would be fairly straight forward but I ran into a few unexpected issues. The first thing I found was that the two studs that connect the exhaust manifold to the intake were too long to allow the intake to seal where the exhaust heats the intake. I had to trim about 1/8" off of each stud. The nuts for those two studs are very difficult to get started and you have to remove the carb adapter to get to them. Next I found that some of the clamp bolts are impossible to get to with a crow’s foot to torque them down. I couldn’t even get a wrench on them until I took a cheap one and ground off the sides. I rigged up a way to connect the torque wrench to the modified 9/16” wrench to attempt to get the torque close to correct. I also found that the very back clamp would not fit onto the step on the intake manifold where it is supposed to go. I had to grind part of the clamp off to make it fit. I know this is a popular upgrade but I’ve never read where others have had the same issues. Are these common problems or did I do something wrong or miss some tricks that would have made this easier? I know I'm supposed to re-torque the manifold clamps after running it for awhile but I'll have to remove the carburetor and adapter plates to do that. Is it really necessary? When I finish getting the carb mounted and hooked up I’ll post some pictures so people with experience with these can critique my set up and make recommendations for improvements. Any constructive comments will be appreciated.
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Old 01-31-2018, 08:11 AM   #2
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Re: Straight 6 Offenhauser Woes

I remember the manifold-to-block nuts/bolts being a pain. I think I used a 12-point combination wrench and just getting a tiny bit of a turn at a time. I never torqued them, just got them as tight as possible, which probably wasn't anywhere close to over-torquing. You will have to repeatedly snug them up after running it for a while to get them to take a set.

I can't remember any other specifics (it was a while ago), but there was some monkeying to be done to get it all set up.
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Old 01-31-2018, 10:08 AM   #3
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Re: Straight 6 Offenhauser Woes

After 24 hrs I re-torqued all the nuts and most of them moved another 1/4 turn. After it goes through a few heat/cool cycles I'll check them again. I dread removing the carb to get to them though. I got it all together last night and started it up. It started pretty easy and idled good. I'll try to get some pictures tonight to show how I connected everything. It doesn't just fall together.
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Old 02-01-2018, 02:10 PM   #4
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Re: Straight 6 Offenhauser Woes

I got it all together and started it, even drove it a couple miles last night. It runs much better than it did with the worn out monojet. I still have a concern with the function of the throttle. It's a little tight when it first starts to rotate. As I continue to push on the foot pedal to get it past the tight spot it jumps too far when it breaks free. The same tight spot keeps it from idling down completely when you release the foot pedal. I broke the linkage apart and the tightness seems to be in the mechanism on the carburetor, not in my contraption that connects it to the foot feed. I'll work with it some more tonight to see if I can free it up. If not, I'll be looking for ideas, maybe even attempt to call Holley.
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Old 02-01-2018, 02:34 PM   #5
dave666693
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Re: Straight 6 Offenhauser Woes

I used a couple of socket heads (allen wrench type) bolts so I could get to them. Keep re torqueing because those manifolds love to blow a gasket
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Old 02-01-2018, 02:59 PM   #6
'68OrangeSunshine
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Re: Straight 6 Offenhauser Woes

I see you are still using the factory cast iron exhaust. Most of us who upgrade L6s also attach headers. Headers are easier[slightly] to access the mounting bolts and nuts. Like Shaky, I don't torque the manifolds on, but tighten them, and then tighten them again after running up and cooling down. [Also retighten them at tune ups.] I have a variety of 9/16'' wrenches and sockets: half-moon combos, flex sockets, stubbies, thin tappet wrenches etc. Grinding a cut in the spacers is normal. Sometimes I have double-gasketed or shot both sides of a Clifford gasket with Permatex spray copper form-a-gasket and built up the exhaust holes with a bead of red RTV. I only mount the carb after the manifold is on. I placed studs at four points where the headers hang.
Since you're using the GM exhaust manifold, keep the end guide pins there. Don't screw them out and mount studs there like headers users. The slack is necessary for the cast iron to expand and contract with heating/cooling cycles.
I ran the 8007 Holley 390 for about 25 years. After an engine rebuild, it would not settle down to a smooth idle, so I replaced it with an Edelbrock 1404, 500 CFM. I have the 5416 Offenhauser intake and long tube Clifford headers on my .030 over bored 292 L6 in my '68 Chevy C/10 Stepside.
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Old 02-02-2018, 09:13 AM   #7
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Re: Straight 6 Offenhauser Woes

I had the carb 90 degrees from what you have and used a '71 throttle cable and pedal (this was in my '66). I also had headers and may have used socket heads in some locations.
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Old 02-02-2018, 12:22 PM   #8
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Re: Straight 6 Offenhauser Woes

The problem w/ your throttle linkage is the leverage ratio you have it set at. I would move the linkage end higher up the carb lever to the large hole. This will require a bushing. Then of course check for WOT function but your linkage end is just too close to the shaft.
https://www.summitracing.com/parts/h...yABEgLppfD_BwE
But even better would be to fab a cable throttle like Shaky did & do away w/ the bellcrank system altogether.
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Old 02-02-2018, 03:04 PM   #9
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Re: Straight 6 Offenhauser Woes

More pics in case they help. I had the Clifford intake by the way:
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Old 02-02-2018, 05:47 PM   #10
'68OrangeSunshine
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Re: Straight 6 Offenhauser Woes

I have no pix of my old Holley set up. I had it side-saddle* too, only my throttle action was toward the back and I had a manual choke forward. With the supplied bellcrank and '68 push/pull rod configuration, I never got WOT [wide open throttle] until I converted to a '72 style accelerator arm and Lokar throttle cable.
[*I favor this carb arrangement, as I think it's better to have the Primaries feed the center plenum, rather than the Primaries feeding 1-2-3 and the Secondaries feeding 4-5-6. This is a matter for much debate, so therefore a personal preference until someone runs some dyno tests specifically for researching that. Out of my means and budget, anyway.]

Other problems with the push/pull bellcrank were jamming, iceing up, failure due to hairpins falling out, etc. I finally called it a ''Rube Goldberg'' and went to a cable. Much cleaner throttle response, and I do get WOT. Almost a hare trigger gas pedal now.

More recent pic with the Edselbrock.
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Last edited by '68OrangeSunshine; 02-02-2018 at 06:19 PM.
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Old 02-02-2018, 05:59 PM   #11
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Re: Straight 6 Offenhauser Woes

Quote:
Originally Posted by '68OrangeSunshine View Post

Other problems with the push/pull bellcrank were jamming, iceing up, failure due to hairpins falling out, etc. I finally called it a ''Rube Goldberg'' and went to a cable. Much cleaner throttle response, and I do get WOT. Almost a hare trigger gas pedal now.
Ha! Rube Goldberg! The "Better Mousetrap" guy! I'm sprised anybody on here remembers him!
Next you be tellin us you gotta Earl Scheib paint job!
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Old 02-02-2018, 06:17 PM   #12
'68OrangeSunshine
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Re: Straight 6 Offenhauser Woes

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Originally Posted by andyh1956 View Post
Ha! Rube Goldberg! The "Better Mousetrap" guy! I'm sprised anybody on here remembers him!
Next you be tellin us you gotta Earl Scheib paint job!
I'm an old M. F. I bought my '68 C/10 Stepside in 1973 from the 2nd owner... Now 45 years later, I'm somekinda hipster for having a ''cool'' truck. Whatever you say, Kid.

Still the original 516 Orange factory paint job. Earl Schieb's would peel.
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Old 02-08-2018, 03:18 PM   #13
jfnar
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Re: Straight 6 Offenhauser Woes

I thought I'd finish up by posting my findings and how I fixed it. I removed the carb and checked for whatever was causing the tight spot. I didn't really find anything obvious but I did find that if I loosened the small screw in the end of the throttle shaft on the opposite end from the actuation lever the tight spot went away. I'll show a pic below. It's a 10-32 screw only about 1/4" long. I replaced it with a 3/4" long screw with two nuts run all the way to the head. It didn't bottom out in the threads so I put blue lock tite on it and ran it down close to the bottom nut. I then ran the bottom nut down until it was just snug but not causing the binding. I jammed the other nut against it to hold it in place. It works much better now. It's still a little stiff to get started but not nearly as tight as before. After getting this all reconnected I adjusted the idle speed by ear; I don't have a tach; then I connected a vacuum gage. It's pulling 22" Hg so I didn't adjust anything else. Going to drive it like it is for awhile and see how it goes.
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Last edited by jfnar; 02-09-2018 at 11:32 AM.
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Old 02-15-2018, 04:08 AM   #14
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Re: Straight 6 Offenhauser Woes

Try placing a spring on both sides of the throttle plates. Old Quadrajets like this due in part to the balancing of pressure on the throttle shaft. Even works well with my Edelbrock.

A 2 dollar spring can't hurt to try!
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