# 68 GTO carburetor problem and jet size



## 68GTOJim (Oct 21, 2020)

I need a little help on getting my carb to work right on my 400HO engine. The original carb had the input threads strip out even with the larger fitting. So I found a #7043251 R3 carb in my junk box. I modified the accelerator cable bracket to fit and the choke to fit the engine. The engine ran great sitting still and revving it up but terrible on the road under load except when the carb was wide open. I now have the carb on my workbench to see if i can see a problem. I can not read any marking on the jets. does anyone have a recommending jet size on this engine. the only modification is an HEI. Also does anyone have theory on why the engine doesn't run right. 

Any help will be greatly appreciated. I'll wait before putting it back together. many thanks...


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## BearGFR (Aug 25, 2008)

Well, first off, we need a much better and more detailed description of what "doesn't run right" means. What exactly is it doing, or not doing, how is it behaving .etc. that causes you to say it's not running right? Also, be aware that the majority of the problems that get blamed on carburetion are actually caused by ignition problems, so before you start finagling the carb it would be a good idea to find out exactly where you are,. ignition-wise: what's your initial timing set at? How much total advance (vacuum diaphragm hose disconnected and plugged) do you have at 3000-3200 rpm? And at rpm is the mark 'steady' or is it jumping around a lot?

Bear


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## 68GTOJim (Oct 21, 2020)

Thanks Bear for the quick reply. I just typed a whole page and lost it. I'll try again...


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## 68GTOJim (Oct 21, 2020)

Ok, I'll try again. The initial timing is 12 degrees which I believe you recommended in some post. 10 degrees vacuum. I didn't check total at higher rpm but it sounds real good sitting still. On the road it accelerates but "jumps around a lot". at wide open throttle it runs great. so I have the carb off and apart. the jets have no mark so I don't know what size they are and so the recommendation request.


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## 68GTOJim (Oct 21, 2020)

I need to add that this is not the original carb. I stripped the threads on the original carb even with the larger fitting. I found a #7043251 R3 in my spare parts bin. I think it's an Olds carb so I modified the choke and accelerator cable bracket to fit my engine. that's why I asking for recommending jet size.


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## 68GTOJim (Oct 21, 2020)

the HEI has been recurved to the same curve as the original distributer so I don't think that's the problem. Before I restored the car it ran great...


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## PontiacJim (Dec 29, 2012)

68GTOJim said:


> I need to add that this is not the original carb. I stripped the threads on the original carb even with the larger fitting. I found a #7043251 R3 in my spare parts bin. I think it's an Olds carb so I modified the choke and accelerator cable bracket to fit my engine. that's why I asking for recommending jet size.


You might be able to save the original carb using a heli-coil. Maybe contact them for more info.









Fuel Inlet 7/8 x 20 insert coils for thread repair (FIVE)


Quadrajet service, parts, kits, restored originals




quadrajetpower.com





Your replacement carb is the 1973 Olds 455. being 1973, it would be leaned out. I found a listing for the jets:
The 251 jets are.070, and sec rods are .057.

1968 400HO/Automatic Q-jet # 7028268- .073 primary jets from what I can find. The 7028267 used .075 jets.


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## BearGFR (Aug 25, 2008)

68GTOJim said:


> Ok, I'll try again. The initial timing is 12 degrees which I believe you recommended in some post. 10 degrees vacuum. I didn't check total at higher rpm but it sounds real good sitting still. On the road it accelerates but "jumps around a lot". at wide open throttle it runs great. so I have the carb off and apart. the jets have no mark so I don't know what size they are and so the recommendation request.
> 
> the HEI has been recurved to the same curve as the original distributer so I don't think that's the problem. Before I restored the car it ran great...


Perhaps it's fine, but the only way to know for sure is to --- be sure. The amount of vacuum advance you have in the can is irrelevant for performance. At WOT, there is nearly no vacuum so the can is inactive anyway whenever you have your right big toe pushed through the floorboard and hanging over into the carburetor --- which is also why you always set timing with the can disabled/hose plugged. The ONLY things the can does are to help with fuel economy at light throttle/steady cruise and also help with engine cooling under those conditions (mixtures tend to be lean in those conditions, and lean mixtures burn more slowly and hotter which tends to heat up the exhaust valves and heads). Initial timing also matters very little. What you're really after is what the maximum total (no vacuum) advance is at RPM, and at what RPM is it "all in". The total figure will be the SUM of your initial setting plus how much 'mechanical' movement happens in the distributor. Let's say, just for the sake of an example, your distributor has 25 degrees of "movement" available, and you want a total mechanical advance of 35 degrees (which is fairly common for what Pontiac engines with cast iron heads with open chambers tend to "like"). To get that 35 with that distributor (which will add a max of 25 degrees), then you'll need an initial timing of 35 - 25 = 10 degrees initial. When we set timing on an engine during a 'tune up' --- we're REALLY setting the total number. It's just easier and "'less stressful" to make the adjustment with the engine at a low idle than it is to do it with it sitting there spinning at 2500+ RPM while your leaning over the fender and trying to read the light while turning the dizzy. But to do it 'right' you have to know how much movement is in your distributor. Say you had a dizzy that only had 20 degrees "in it" - then to get that same 35 you'd have to set initial to: 35-20 = 15 degrees initial. Anyway, the point is to not get "hung up" on a specific initial timing setting - you're really after the total. Choose whatever initial setting gets you the total you want with your distributor.

How good it sounds "sitting in the driveway" means nothing. The only time that matters, ... is when the car is sitting in the driveway.

You said "jumps around a lot" --- did you mean that the car surges while driving? Or are you talking about the timing marks moving around a lot at RPM while you have the light on it?

If it's a surge while driving -- when do you notice it the most? At a steady speed (and what speed/RPM) or at WOT/heavy acceleration? If at a steady speed, then yes it could be a lean surge indicating that you need larger jets (or primary rods with smaller tips/more taper) in the primary side - or possibly a power valve system that isn't working (like if someone has had the carb apart and left out the spring under the piston). If it's mostly/only at WOT acceleration, then that points towards needing more fuel on the secondary side. You get that with secondary rods that have smaller tips and/or more taper, or a different rod hanger (holds the rods higher). It can also be caused by a previous "trick of the month club tuner" who has the secondary air valve spring set WAY too tight and didn't know the correct way to tune the air valve opening rate.

If it's the timing marks that are jumping around a lot while you're watching them with the light, then that's an indication of a distributor/ignition problem such as a wonky mechanical advance mechanism, loose/floppy "breaker plate", defective vacuum can (that's loose enough to let things move when when it's disconnected), distributor with way too much shaft end play, worn bushings, etc. "Some" mark movement "occasionally" isn't awful, but "a lot" that happens "nearly all the time" (such that you're never really sure what the actual reading is with the light) is a problem. 

You car will tell you what it wants. You just have to learn how to listen and understand what it's saying.

Bear


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## 68GTOJim (Oct 21, 2020)

Thanks so much to Pontiac Jim and Bear for all the good info. It's a lot to digest and I'm studying it. The Olds 455 carb I'm going through again is on my bench. I drilled out the .050 to .078 which I realize is a little more than the .075 of factory carb on the 400HO std trans. When I finally solve my problem I'll see what the plugs look like after I drive it for awhile. The jumping around I mentioned previously is when under mild acceleration the car acts like it misses but different than an ignition miss. I'm hoping it's in the carburetor. I'll also check the timing at 3000 and see how steady it is. won't be able do this until tomorrow and I'll report it then. 

Thanks again guys, sure nice we have Pontiac encyclopedia folks to help us...


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## BearGFR (Aug 25, 2008)

> The jumping around I mentioned previously is when under mild acceleration the car acts like it misses but different than an ignition miss.


That sounds like it could be a part throttle lean surge. If that condition goes away under full throttle, then your secondaries are adding enough fuel to balance things out.

One thing you need to overcome is the idea that every mixture problem on a QJet is properly addressed by jet size. Sometimes, that's the answer - but sometimes it's not. As vacuum starts to drop off as throttle is applied, the spring under the power piston will push it up, raising the metering rods in the primary jets so that the 'thinner' part of the rod is now in the jet, and thus adding fuel. You can make much finer adjustments to fuel mixture by tuning the power piston / metering rods than you can my working with jet sizes alone.

Some things you should definitely check while you have it apart is to 1) make sure the spring under the power piston is in place and 2) that there's no binding/rough spots that potentially prevent the spring from pushing it up. One way to test this: remove the baffle, push the piston all the way down with your finger, let it go. The spring should pop the piston up and hit the plastic retainer with enough force to knock it loose so you can remove the whole assembly. If the piston is binding or not working properly, then you'll have a surge like the one your describing under light acceleration. Yes, you can solve that with larger jets but if you go that way, then you're probably going to be too rich at steady state cruise. 

This whole mechanism is the equivalent of the 'power valve' on a Holley, but because you have the ability to use different metering rods with different rates of taper and diameters of those tapers as well as using different springs to change how it reacts to vacuum, this system is much more tunable than on the Holley where all you have is the ability to change out the whole valve for one that opens at different vacuum levels. 

Highly recommend a couple things: 
Invest in a good air-fuel meter, either one that's temporary/portable (like the Innovate LM-2) or one that's in the car full time (like the AEM X-series UEGO) so you can see what your fuel mixtures are in all driving regimes. These take most of the guess work out of carb tuning. 
Grab a copy of the Cliff Ruggles book on tuning QJets. 

Bear


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## 68GTOJim (Oct 21, 2020)

Thanks again Bear for the quick reply, just got home so I'm late in reading the suggestions... i can see I've got a whole lot more to consider other than simple assembly. Also I haven't worked on a Qjet for 30 plus years. one of your comments has me thinking that you may have hit it. i might not have been careful enough when I put the carb together that the power piston may have been in a bind, thus my problem as you described. i just checked the carb after I assembled it. with a small screwdriver I checked through the horn that it is moving freely as you recommended. I'll put it on the car tomorrow. Another thing I didn't know is that there are different metering rod sizes. I have 8 or 10 old Qjets in the junk box so I'll check them when I get a chance. Is ther a size you recommend for the 78 jet size I have. At this point I'll try the carb as it is now just to see if it runs. 
I'll get a copy of the book you recommended on tuning. any suggestions on where to get a good air-fuel meter?

thanks again for taking the time to help. greatly appreciated...

Jim


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## 68GTOJim (Oct 21, 2020)

68GTOJim said:


> Thanks again Bear for the quick reply, just got home so I'm late in reading the suggestions... i can see I've got a whole lot more to consider other than simple assembly. Also I haven't worked on a Qjet for 30 plus years. one of your comments has me thinking that you may have hit it. i might not have been careful enough when I put the carb together that the power piston may have been in a bind, thus my problem as you described. i just checked the carb after I assembled it. with a small screwdriver I checked through the horn that it is moving freely as you recommended. I'll put it on the car tomorrow. Another thing I didn't know is that there are different metering rod sizes. I have 8 or 10 old Qjets in the junk box so I'll check them when I get a chance. Is ther a size you recommend for the 78 jet size I have. At this point I'll try the carb as it is now just to see if it runs.
> I'll get a copy of the book you recommended on tuning. any suggestions on where to get a good air-fuel meter?
> 
> thanks again for taking the time to help. greatly appreciated...
> ...


Got the carburetor back on and road tested it for a mile or so. extremely better. car revs very smooth through the gears, no more jumping around. I really believe it was the power piston. it still doesn't feel exactly right. I broke my timing light so I couldn't check the timing advance curve. Maybe tomorrow.


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## 68GTOJim (Oct 21, 2020)

BearGFR said:


> That sounds like it could be a part throttle lean surge. If that condition goes away under full throttle, then your secondaries are adding enough fuel to balance things out.
> 
> One thing you need to overcome is the idea that every mixture problem on a QJet is properly addressed by jet size. Sometimes, that's the answer - but sometimes it's not. As vacuum starts to drop off as throttle is applied, the spring under the power piston will push it up, raising the metering rods in the primary jets so that the 'thinner' part of the rod is now in the jet, and thus adding fuel. You can make much finer adjustments to fuel mixture by tuning the power piston / metering rods than you can my working with jet sizes alone.
> 
> ...


Bear, I sent an update on the problem but i don't think it went to you. I'm slill trying to learn how to use this...
Jim


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