Fine tuning Q-jet

[bob69]

I need help fine tuning my Q-jet. I'm working on a 07029207 1074 SR crab. I have Cliff's book and have been following recipe number two for my application:

1) 1969 Camaro, 350 sbc, 200r4 trans, 1800 stall, 3.55 axle, 9 btdc.

2) Cam. 214 int / 224 exh at .050, .444 int / .446 exh, 112 lsa.

3) summit 152123 heads, 165 cc int runner, 72 ccv, 2.02 int / 1.60 exh. Flat top pistons.

The carbs idle was very rough. After several adjustments, with no success,  I hooked up a A/F ratio tester. The idle A/F came in at high 15s to low 16s. Too lean. However, the A/F at steady state running (2000 rpm) was a very nice 14.6 to 15.0.

I increased the jets two sizes. The idle improvement was significant.  The car now idles very smoothly at 650 rpm. The throttle screw was turned down by over a full turn. Idle screws now have adjustability when they did not before. Pretty sure closing the transfer slots is why.

The idle A/F changed to around 14.1. However, steady state running (2000 rpm) changed to low 13's. Way too rich for normal driving.

The cards spec's are:

Jet.                   71       (was 69)
Rod.                 37       (was 36)
Idle tube         .039
Idle dcr.          .053
Upper IAB      .081.   *
Lower IAB      .063
Upper MAB    .070.   (was .050)
Lower MAB    .070.   (was .050)
IAB.                 .098.   (was .093)
Idle discrage  .095
Secondary.       AN

I think the UIAB is too large. I would like to decrease it to something closer to what Cliff suggest. Any suggestions how much. The jets should also be dressed. How much? I would like to get idle around 14.1 A/F and steady state around 14.7 to15.0  A/F. I prefer to keep the UIAB in the body and not use the air horn air bleeds. 

[old cars]

 I believe 7029207 Original jets were 66. Increasing jet size would not have fixed your idle. You have missed something. The original setup of this carb ( before modifications ) would have been closer. If the original setup was still stock ( not modified )

[Cliff Ruggles]

We don't change main jets to tune idle fuel.  You may see a superficial improvement there simply because all the fuel to the idle tubes has to go past the jets, but this is not how idle tuning is accomplished.  Trying to add fuel by changing the jet/rod relationship throws a monkey wrench into the rest of the tune as you are finding out.

Several things to mention.  You are working with a factory "high performance" carburetor.  Most likely it is a Service Replacement not an original 1969 unit. 

The originals and SR's are not set up the same internally.

Either way both as a relatively "generous" idle system and support engines with big cams in them without needing much done to them.  The originals have more idle fuel to the mixture screws than the SR's if anyone is taking notes.

Since you opened up the Main Airbleeds then the stock jets and metering rods are out the window.  You will need larger metering rods to lean it back down since you moved up to 71 main jets.

I would recommend removing the APT screw in the baseplate and getting one of my external/adjustable ones.  This will save having to take the carb back apart for fine tuning part throttle A/F.

Second I would quit chasing the A/F meter and tune for best results instead.  You shouldn't have had to touch the idle system in the first place assuming your 350 engine build has over 10 to 1 compression with that camshaft.  That is a very mild cam and makes plenty of vacuum in a 350 engine build provided the pistons aren't way below the deck and TDC and you put a thick head gasket on it.  The compression ratio should be over 10 to 1 with tight quench and with that cam you should be seeing plenty of vacuum at idle speed and full control of idle A/F with the mixture screws with the stock carb set-up.

Since the 207 carb is a factory "hot-rod" I don't open up the MAB's when building them here.  Instead I'll employ the APT system and go up one or two jets sizes from stock and larger .042" primary rods, then use the APT system to fine tune part throttle A/F.

Just curious what rebuild kit you used?  Mine would have contained the correct high-flow N/S assembly.  Using a smaller one will effect the fuel level at any setting/pressure and lean it up a bit everywhere.  The float setting is a big deal with tuning so make sure you aren't setting it too low or using the wrong N/S assembly.........Cliff

[bob69]

Thanks for the quick reply. I've visited you site for years looking for suggestions. This is the first time I've submitted a post.

I've played with q-jets for years (since the 80's) and this one has not responded the way I was expecting. It is a service replacement carb. I believe the date 1074 indicated a 1984 carb. The only reason I went to the A/F ratio tester is because nothing else I tried seemed to work.

I've checked/adjusted almost everything on the setup over the years. The distributor works best at 9 btdc. Float settings from 3/16 to 1/4 (at 3/16 now). From stock 66/36 jet/rod to 69/39 jet/rod, to the latest at 71/67. Double checked  all the settings and adj those that needed adj..

The only modification to the idle circuit was to open up the bypass from .093 to .098, which did not make much of a difference. All other idle specs are as they came on the carb. None appear to have been modified.

It had 186 heads with a factory 10.5  comp ratio, with a 350 hp 350 cam. But the engine would "run on" with 91 pump gas. I had to leave the trans in gear to get the engine to stop. I changed the heads to a larger chamber volumn to stop this. I calculated compression to around 9.10. This is a factory 1969 350 4 bolt main block with the original chevy std pistons. When I took the engine to be cleaned the machine shop offered $1000 to buy to. They said the block casting was used on Z/28 and desired by restorers.

The engine has good vacuum.  With the 350 hp cam, the vacuum was around 15. After the head/cam change the vacuum is around 16.5.

I did not expect the jet change to do what it did. It was the last thing I could try without modifying the idle circuit. I can go back to stock jetting/rod or anything in between. I have a large selection of quality used early q-jet parts that I've collected over the last 40 (yikes) years. Weekly trips to the yards for late 1960 chevy parts can do this.
 
So, besides changing to an adjustable apt, what other changes would be advisable? I have the tools to modify anything on a carb.

Ps. I've purchased dozens of rebuild kit over the years from different manufacturers. A few have had the larger N/S. Most with widows, some without. I always check gaskets to make sure no required passages are blocked. I don't recall what's in there now.

Pss. The car is a camaro ss with 350, auto, P/B, P/S, and A/C.

[Cliff Ruggles]

ALL of the rebuild kits you are buying in the past 15 years or so come from the same source.  They will NOT have the correct high flow N/S assembly in them and it should NOT have windows in it.  They blue pumps seals are "soft" as well, and why I have good USA ones made with a lifetime warranty on them.

I would recommend one of my kits and set the float to 1/4".  I also have the "orange" PP spring reproduced and would put one of those in place as well.

Go back to the small MAB's and close to the stock jetting.  With this new fuel it helps to increase delivery by apprx 5 to 7 percent across the board.  Install one of my adjustable APT screws in the baseplate so you can nail down part throttle A/F without pulling the top off the carb and changing parts.

Pull the idle tubes to make sure they are not varnished up and size them at .038".  Nothing else in the idle system would need any help for what you are doing.

Lowering the compression ratio NEVER helps for power production (torque).  The factory 186 heads are excellent, right up there with the excellent 041 castings used the same year.  The combustion shape is excellent and they will do fine on pump gas at high compression ratios with the right cam choice.

Those 327/350 and 350/350 engines will run fine on currently available 92-93 octane without running hot, overheating, pinging or "running on" with careful tuning.  The factory really did know what they were doing when they designed cams for them.  They also used .020" thick steel shim head gaskets and pistons near the top of the block at TDC to keep quench really tight.  Most folks miss the mark considerably building those engines putting the pistons deeper in the block and TDC and using a .040-.060" "builder" head gasket on them.  That NEVER works out well. 

Neither does lowering compression and going to a smaller cam (short seat timing, tighter LSA and earlier intake closing point) with a point or so less compression just kills off power (torque), pulls power down in the RPM range, and considerably LESS upper mid-range and top end power.

I prefer to use the 327/350hp cam in 350 builds with 64cc heads, squish around .035" or so, and 10.5-10.6 to 1 compression.  You will have a hard time finding a cam from anyone (doesn't matter what they tell you) that will idle better, and make as much power at any RPM over nearly as broad of an RPM range.  Another excellent cam for the later blocks is the GM LT4 "hot" roller cam with high ratio rockers on it.  Comp Cams wishes they could grind one that works as good but they insist on going tighter with the LSA so the end user has "menacing" idle quality, not the most power at every RPM with a smooth idle and more vacuum for power brakes, etc.......

The "run-on" deal comes from not quite enough initial timing combined with not quite enough idle fuel and bypass air, or a little of all three.

The current engine in my car is 11.3 to 1 compression, 200 psi cranking pressure, 13.5 vacuum at 700rpms and manages pump fuel now since 2009 with zero issues anyplace.  IF I drop back to 87 octane fuel it runs fine on it, but will also "run-on" once in a while but only on really hot summer days with the engine fully heat soaked.

[bob69]

Thanks  for the suggestions. I'll order the rebuild kit, adj apt kit and return jetting close to original. The carb was very clean when I bought it. I used long shaft numbered drills to measure the idle tube size, .039 fit - .040 did not. I also ran a slightly larger drill in the tubes to make sure they were clean.  They were. The carb has the original short pp spring. Is the orange pp spring different? Can I get by with changing the upper mab and leave the lower mab alone.

I live in Calif and 91 octane is highest available in my area. Also, a gallon of 91 octane is over $5. Gets real expensive when you go for a drive. And the state keeps changing it's formulation over the years to make it "cleaner". My cars run hotter and get less gas mileage. One one the reasons I try to build an engine to run on 87.

Which brings me to another observation.  When I pulled the 186 heads, all of the exhaust valves were bright white and you could see where the heat discolored the head about 2" around the exhaust valves. I had 67 jets and 36 rods in the carb at the time. I changed to 69 jets in the hope the engine would not run so lean. Any suggestions on how to tune the carb for Calif reformualted gas.

I agree completely with your comments about using the 350hp 327 cam. My machine shop also recommended the same. They rebuilt my 1968 camaro vert 327 / 4 speed using the 350 cam. They decked the block to get zero deck height with the 30 over pistons and shaved the heads to get a clean flat surface. The engine started as a 275 hp 327 with 291 heads. After a little tuning the car is a blast to drive. Runs smooth at 750 idle, does not "run on" with 89 octane, revs easily to 5500, and never went hotter than 190 degrees on the drive up the sierras to Hot August Nights.  Which is why I could not figure out why the 1969 ran so badly.

I use the 69 camaro for cruising. So max power is not a priority.  Just clean running. When I have a need for speed  I  jump into my 1969 Nova SS 396, which has a stong 454 in it. 😁  And all have q-jets.

[quadrajam]

bob69, I tried to send a PM but your message box is full...??..

[bob69]

Quadrajam, sent you a pm. Did you get get?

[quadrajam]

Yes, thanks. but was unable to reply. Check your e-mail and see if that worked.

[bob69]

Finally rebuilt the carb using one of Cliffs rebuild kits. The MAB and idle tubes  were changed back to their original sizes.  The jets were changed to 68 (stock 66), rods changed to 39 (stock 36). Float set to 1/4. The engine started right up and ran good. Had to turn down the throttle screw as the idle was high. About 1/2 turn. Idles good at 820 in park - 600 in drive.

Idle vacume is relatively stable at 15 inch. The steadiest reading has the idle screws at 3 turns give or take 1/16 turn.

Light acceleration is about the same. Really jumps off the line. Most likely due to timing. Initial set at 11°. Timing goes to 17° at 1000 rpm. Distributor vacuum advance pluged for the test.

Moderate acceleration is not as good as before. Seems down on power. Just don't know if the jet/rod combination is lean or rich. Heavy acceleration is also down some from before. But need to work out the moderate acceleration first.

Cliff, the engine still has a tendency to run on. Just not as bad. I think the timing is ok. Could the carb be lean?

Can you explain a little more on the APT. What impact it has on idle, cruise and acceleration. I measured the original apt height before replacing it with your adjustable APT set to the same height.

[Cliff Ruggles]

APT changes the metering area at cruise, or light engine load.  You MUST use the vacuum advance to tune with the APT, otherwise your engine is missing a lot of timing and can not effectively burn a lean mixture.

Hook up the VA and start over.  I would also replace the primary rods with 42B-44B's.  Since you have APT you can raise them up and richen things up if/as needed, but will have more control leaning it up.

Running on typically means the throttle is too far open at idle.  This can be caused by not enough timing, not enough idle fuel, bypass air or just the idle speed is too high.......

[bob69]

Update. WOT was too lean with 68 jet on E10 gas. Changed jets to 69 with 41 rods. Better wot but lean in part throttle cruise. Turned apt 1/2 turn richer and pt cruise now running in mid to high 14s. Turned idle screws down to 2 1/2 turns to get best idle (was 3). Set drive Idle speed to 600 rpm and park idle dropped 100 rpm to 700 (was 800). Idle af in 13s. No longer have run on.

Still have one issue to figure out. The af goes very lean momentarily when throttle is opened. This happens at all engine speeds, from idle to steady cruise, when throttle is opened. This reflected in the af meter readings/recordings.

The accelerator pump was adjusted to stock measurements at 5/16 from air horn to top of plunger (measurement was 6/16 before adj). The af does NOT go rich. The check ball from the kit was used. I see fuel coming from the shooters with the engine off.

The choke pull off provided no delay at all (parts store replacement). This was causing a major stubble on fast acceleration.  A brass restrictor from an old choke pull off was inserted into the hose to the pull off. The delay is now about 1 1/2 to 2 second.

The secondary air valve rod was adjusted to keep air valve closed when not needed. Air valve spring set to 1/2 turn.

What else should I look at to fix this monetary lean condition.

I think this is part of a larger issue as acceleration is not what I expected. I have another car with the same cam profile in a 396. Even with the displacement difference the acceleration (in a 350) is still too low.

[old cars]

"Update. WOT was too lean with 68 jet on E10 gas. Changed jets to 69 with 41 rods. Better wot but lean in part throttle cruise"
Wot should be adjusted with secondary rods after the primary side is figured out.

"Still have one issue to figure out. The af goes very lean momentarily when throttle is opened. This happens at all engine speeds, from idle to steady cruise, when throttle is opened. This reflected in the af meter readings/recordings."
How much throttle opening ? What is very lean?

Cliff said
"Second I would quit chasing the A/F meter and tune for best results instead."
I believe Cliff is making this statement because some people do not understand what A/F ratio's should be at..... idle/cruise/light acceleration/wot...

[bob69]

old cars, the afr I'm getting  is in the mid 16 range. This is lean especially since the afr should be rich due to the extra fuel provided by the accelerator pump. I see this at all throttle opening rates, from idle to steady cruise.

I learned some time ago that trying to tune by afr does not give the best performance.  This is based on first hand experience tuning my four classics.  Three 1969s and a 1968, all chevys with different v8s (327, 350, 396 and 400). All have/had q-jets.

Lars has an excellent write up on how to tune a q-jet that I follow.  He is a well respected Corvette tuner and has written several articles on tuning corvettes.


Back to my current situation. I use afr to help identify possible problems. I'm not seeing a momentary rich spike as expected when opening the throttle. I see the monetary rich spike on the other three q-jets. I don't see why this one does the opposite.

The car's acceleration does not come close to what I see for the other cars. I want to fix this before moving on to other tuning adjustments.

[old cars]

AFR ratio going in to 16 range on light acceleration with a quadrajet is quite normal. A jab of the throttle may show  the accelerator pump (richness ) momentarily. I tune with AFR all the time. I don't usually need AFR to identify possible problems.