Bobs69BB

Background:

Full restore of a 1969 L36 427/390 coupe that I bought partially finished from PO. Engine was finished when I got it, but sat for 6+ years before I got it fired up for the first time.

Engine is the original 512 block, with numbers matching carb, intake, and side exhaust. So looks stock from the outside, but looks are deceiving...

Engine has been bored and stroked to 496 cu in. with an Eagle forged rotating assembly. Hydraulic flat tappet with roller rockers. Custom cam from Dema Elgin, 240/244 @ 0.50, 112 LSA. Stock distributor with Pertronix Ignitor, stock coil, wires and plugs. Quadrajet carb. The carb was supposedly flow tested after a discussion on the cam with Dema. However, recently discovered leaking primary well ports with fuel puddling in intake, ports were epoxyed only, so I am not assuming the carb was rebuilt properly. Currently fixing the leaks with set screws, the epoxy is drying as I write this. As I had the carb torn down: Primary jets: 72, primary rods: 45B, secondary rods: AX, secondary rod hanger: K.

Had the timing curved, 12 deg idle, 36 deg all in at 2800. Reset the valves twice with 1/4 turn to be sure they were not the issue.

Runs great at idle, rumbly given the cam, pulls like an SOB, sweet as hell from 2500 - 3500 but peters out around 4000 rpm with a noticeable stumble. Power drops off and acts like a high speed miss. Have not really pounded it as the engine has less than 200 mi on it, but I'd like to at least taste the extra HP from 4000 to 5000 rpm.

Looked initially at the ignition. Changed the plugs to non-resistor, threw on a spare cap. No difference.

So now the questions. Ignition or carb, or both? What should I try next? Is a Qjet man enough for 496 cu in? Any comments on carb tuning ie carb jetting/rods? What should I expect as max RPM for this combo?

Oh and to answer the what I want to do with it. Flog it like a race horse, drive it like God meant big blocks to be driven. And make Prius owners turn up their noses in disgust!

vetteman1976

Id take a serious though to rebuilding/ cleaning your carb. if its been sitting that long...Or least go through it and clean it out..There a few videos on the tube to help with it.. If your going through it looks to be to much,
Lars, I believe on the forum has quite a reputation for being the king of the quadrajet.. See if he's able to help you..

may not hurt to run a can of seafoam through it, spray out your carb with cleaner, change in-line fuel filter, Check your plug wires for burns, cracks, positioning etc...
always start with the easiest stuff (usually cheapest) relevant to your problem and move your way up the checklist...It also gets you in tune with your car....Make a spreadsheet and write down dates/ times when and what you do...For me, the older I get the faster the years fly by and the more forgetful I get!!! now, what was I saying I forgot...
my nickels worth....
J

vetteman1976

here's a video to get you close on adjustment as well..Good luck..I just did mine couple weeks ago...I think my mixture screws are not set at 2-1/2 turns out both sides..idle at 700..
Be careful when you screw the mixture screws in, do it just so it touch don't crank the screw tight!!!



J

Bobs69BB

Carb was rebuilt, looks like new, done by a supposedly quality firm that I won't name. Other than the well ports, looks OK, but what do I know. Carb filter clean as a whistle. Fuel filter is new, fuel pump is new, fuel lines are new, gas tank redone, wires are repro date code correct, etc etc etc. PO spared no expense. Although new does mean sitting around for 6+ years collecting dust ...

What puzzles me is that up to 4000 rpm it runs great.

And I do get you on writing everything down ...

But what is next on the checklist?

Bobs69BB

Idle screws wouldn't affect how it runs at high rpm? It idles fine, somewhat rich until the choke warms up.

Shark Racer

I'd check fuel pressure, float level, and then go on to things like air-valve tension etc.

Are the throttle blades opening to 90 exactly? (not before or beyond)

7t9l82

i have seen air valve tension do that, easy fix. hope that works for you

BigBlockk

I believe the primary jetting is WAY lean. That 72 main jet is getting you a WOT metering area of 407. The stock '70 LS5 had a 78 main jet that gave an area of 477. The '70 LS5 tune was 78/49 primary and the AX secondary.

This is what I think is happening. The primary side is so lean at WOT that the engine begins to breakup before it can get up on the secondaries.

Your jet/rod combo gives a 248 cruise/ 407 WOT. Both of these are very lean. The '70 LS5 had a 289 cruise/ 477 WOT tuneup. I would say you should start there.

If you make these changes and it runs worse it is a fuel delivery problem (fuel pump).

BigBlockk

Later.....

Faster Rat

I fought a similar problem for several years after rebuild. Finally got my distributor on a Sun machine and found that the lower bushing in my housing was worn. Found another housing with a good bushing. Probably explains the timing mark moving around and the intermittent nature of the higher rpm misfire. As Lars says...90% of all carburetor problems are really ignition problems.

jimboscarbs

This sounds like an ignition problem however if your engine is adecent piece you are running out of fuel even a healthy small block wil cause this I will post a more detailed explanation later, mobile right now

BEst regards , Jim

Shark Racer

At 4000 RPM he should be well into the secondaries with an engine like that. If his primary size is really lean, the easiest verification would be to do a half throttle run up a grade and see if the car stumbles and stutters (lean miss).

Faster Rat

Could be both. Mine more than likely is...I have been told by Cliff Ruggles that my '84 vintage service replacement Q-Jet air bleeds are way too big. I also need to change my jets and rods. When we had the engine on the dyno the builder told me it was acting like it was blowing the flame out...and we were using the test stand HEI distributor.

BigBlockk

I have no doubt he is into the secondaries at 4000. I'm thinking that the engine begins to breakup before the richest portion of those AX rods is actually metering the fuel to the secondaries. With this kind of engine it is probably a good thing it won't go any higher. He has to have some kinda compression to be running that cam. High engine speed, high load and a lean mixture is a recipe for disaster.

The engine may have ignition problems but they may be a gift from Heaven. If the ignition were healthy he may have melted this thing already. Unless they have done something really strange with the main air bleeds I simply don't think a 72 jet can provide enough fuel for an engine like this.

BigBlockk

Later.....

Bobs69BB

Thanks everyone, good suggestions! Now I have a few things I can try. I keep re-reading Lars papers and the Doug Roe book, but there is no substitute for the level of experience on this forum.

The secondary air valve spring was way tight, unfortunately did not get a turn count before I loosened the set screw. Backed it off to 3/4 turn per Lars papers. Float level was OK at .375 . Slight adjustment in secondary hanger height, reduced from 44/64 to 41/64. Fixed the primary well leak.

Also, the throttle blades open fully on the bench. I am tweaking the throttle linkage to get the same on the car, they were at about 80%. Hope to get it back together later this week to see how it runs. Chip away a little at a time ...

Found the needle valve clipped through one of the holes on the float, another item that does not give me confidence in the rebuilder ...

BigBlockk, your suggestion to start with the Qjet 454 LS5 tune makes a lot of sense to me. This thing has to need a lot more fuel. That will be my next step after this round of changes. 454 is closer to 496 than 427, so it is a logical place to start. FYI, the compression ratio should be around 10:1 according to the PO, so close to the stock LS5 ratio at 10.25:1.

If I am still not there, I'll check the disti bushing. FYI, when I had the disti recurved there was a lot of play in the gear so had it reshimmed and the timing cleaned up quite a bit.

How can I tell if the air bleeds have been messed with?

Shark Racer

Size them with a numbered drill bit set and then compare notes with someone who knows what they should be.

Sadly, I'm not sure what the bleed size would be for that one.

Actually, now that I'm looking at the list, it looks like your carb would have had 71/45 rods from the factory, along with a BC secondary. Those AX rods are QUITE a bit richer than your factory rods were, including earlier parts in the "rod curve". I'm not convinced that jetting is the issue here. When you say the carb is #s matching I'm assuming it's the correct carb for a 427/390 - 7029201 right?

You really need to do a hill-pass test or lock out the secondaries to check for lean stumbles under power in order to test that theory out. The hill-pass test is basically find a long uphill and give the car about half throttle (The mech secondaries engage around 2/3 of the throttle travel). This will put the car under enough load to get the rods to unseat so you can test the "power tip" part of the rods.

73ss

Try a test run without the gas cap. Are the return lines plumbed & working? Is there a "Whoosh" when you take the cap off? Also, measure the needle & seat size. The large .149 edelbrock n&s do not work well in the pre '75 carbs. at least from my experience. Will cause the symptoms you are describing.

BigBlockk

I went back through this thread and there was something I missed. I believe it was late in the '69 model year the air bleed arrangement changed from small bleeds to large bleeds. There was a strike at GM about that time and the change over was not done at the same time in all divisions and models. If your carburetor is a small bleed it would be a lot closer to the proper calibration. The Roe book shows the difference between the two. The smaller the bleed the more fuel will be drawn into the air stream.

Also I don't see anything about the color of the plugs. What do the plugs look like.

I hope the previous owner didn't find a numbers matching carb on ebay for this car. If that is the case the only thing correct on the carb may be the number. If this carburetor has been remanufactured there's no telling what is inside. Part of this process is to remove all air bleed bushings. Then they mix and match all of the carb bodies, tops and base plates just to keep you guessing. Over half of the parts carburetors I get off ebay are reman jobs.

BigBlockk

Later.....

jimboscarbs

Personally I think you are running it out of gas...

With that said for good measure you should also do a once over on your ignition a stock HEI module (if so equipped) is not designed to run in the upper RPM range >4500 (not enough dwell time to saturate the coil) it will cause you much pain if you are not aware of this... (a hard pull and a look at the plugs would let you know) gives the same symptom as running out of gas experience is something you get right after you need it; I learned this the hard way...


If your rat labors above 3800-4000 rpms the engine runs;makes noise but just wont pull and your ignition is ok it could be a fuel supply problem not necessarily a carburetor problem.

Running a healthy small block with a mechanical fuel pump it takes a -8 AN line from the tank to the carburetor with modification to the carburetors fuel inlet to keep the float bowl on a quadrajet full.

Dyno results will tell you different ( a dyno sits still though...).

The above is from real world experience

The fuel demand of a big block only makes this worse...

If you are forced to run a mechanical pump you need to run a six valve pump (Edelbrock or such) with the lines sized to -8 AN from the tank to the carburetor to accommodate your set up.A positive displacement pump pushes better than it pulls, add the G forces of a hard launch and you get the picture...

Any less of a fuel system on a hard launch for those not faint of heart will equal a busted nose on the steering wheel with a RAT the will run...

I would run an electrical pump with your set up mounted at the tank, you can get away with smaller lines maybe stock depending on the pump and line routing and how long you are on the wood... woohoo don't be skeeered





Needle and seats
personally I prefer the flat disc type needle and seat (Daytona Carb parts carries them) this is old technology but they work and will flow better than the tapered ones if you do the math on it.

There is way more to this than a casual response can cover but you get the jest of it.


Also if you have your engine dyoned make sure you go to a facility that has a wide band O2 sensor, this will find your problem...

if the dyno guy does not know carburetor tuning find some one that does there are ponies in a correct fuel curve...

Horsepower per dollar, save a blower installation or nitrous, a good carburetor is the best performance dollar you will ever spend!

I hope you get it figured out, Jimbo

Bobs69BB

FasterRat, carb is 7029215 which according to the Prince book is correct for a 69 427/390. Stock jetting was 71 45 AX from the Lars list.

73SS, gas and return lines and hoses are new, along with the fuel pump. Am getting "woosh" when cap removed. Will check needle and seat dimensions.

In fact, I plan on measuring all the orifices before I start swapping out jets or rods to see if I am dealing with a "FrankenJet". I ordered the Ruggles book as I saw that it includes the bleed sizes.

BigBlockk, plugs are black and wet. I am burning fair amount of oil, assuming this is due to the rebuild as I have read this is fairly common the first few hundred miles? If I was running significantly lean wouldn't the plugs look differently?

The ignition is not HEI, has repro 284 coil with Pertronix Ignitor.

Still need to get it on the road with the tweaks so far and report back ...

Bobs69BB

BigBlockk, thanks for the pointer to the Roe book on the air bleeds. I found the diagram you referenced (p47). The primary air bleeds are the small bushed type, look to be slightly smaller than 0.05". If an 1970 LS5 has large bleeds, does this now not make it good starting point for jets/rods?