QuestionQUESTION: Hi Howard,
I’m trying to get my 74 MGB to pass Boise’s emission testing… need to be < 6% CO and < 900 PPM Hydro Carbons… for tests at idle <1200 rpm and at 2500 rpm
I’ve just rebuilt the SU’s, all new internal seals, gaskets and can’t detect any vacuum leaks (spraying carb cleaner around shafts, ports… ) I did a valve job about 500 miles ago, have great compression tests, and have replaced points, condenser, cap, rotor, wires and plugs, and fine tuned everything. Dwell 60 degrees, timing at 12 degrees (set at 1500 rpm with vacuum line disconnected and plugged) carbs balanced and adjusted to “lean to best idle” Running 89 Octane 10% ethanol (highest I can get….) But I fail 3 of 4 test… at 2500 rpm - CO 7.15% HC 298 and at 1000 rpm - CO 8.32% HC 1272 PPM.
I’ve had it tested 4 more times playing with the mixture +/- half a turn around “lean best idle” but couldn’t get it to pass. So I tried advancing the timing to 17 degrees and retested: at 2500 rpm - CO 4.00% HC 263 and at 1000 rpm - CO 3.26% HC 2018 PPM. Tried to get the idle HC down by increasing the mixture a quarter turn, but bumped both CO test above the limit to 7+ but it did lower the idle HC down to 1470 at idle.
Boise’s at 2700 ft elevation and it hot, 98+. Do I need to adjust the timing more for the elevation? (no ping detected, and increasing the advance still raises the rpm) Or test early in the day (cooler) for better result?
Other ideas???
Any insight would be appreciated…
Jim
ANSWER: Hi Jim,
Keep in mind that CO is mainly fuel mixture and HC is either mechanical or electrical but can raise if too rich or too lean.
Most engines run about 5% CO at full throttle and full load (peak torque) on a dyno. The 7 & 8% readings you got indicates the jets were set too rich. You should not have to alter timing to try to get correct readings. You should use what ever the sticker under the hood says. If it is different than what your book says, the sticker always over rides what the book says.
The setting of 3 & 4 % CO is too lean and could cause a lean misfire thus the high HC when you had the low CO readings. High HC is an indication of a misfire of some kind. But you need to have the CO correct and the timing correct before trying to pin down a cause of high HC. It seems like a catch 22 because a very high CO can cause high HC and a very lean CO can cause a high HC too. So first get the CO into the 5% area and then if HC is still high look for electrical problems as good compression indicates the mechanical is ok.
Are you familiar with the side lift pins to set mixture? Also try this. Be sure the two carbs are in sync first then with the engine up to operating temp set the idle as low as you can make it run. So it is on the edge. Then adjust both jets at the same time in either direction (up and down) which ever makes the idle either speed up or smooth out. Then set the idle stop screws on both carbs down again to get the lowest possible idle. Then adjust the jets which ever way makes the engine either smooth out or speed up. Repeat this process until you get it idling very slow and if you move the jets in either direction the engine will die. Then reset the idle stop screws up to get an 750 to 800 RPM idle. Then lift the side pin and see what it does. It should drop off and run like an old John Deer tractor. It should do that by lifting either pin.
Let me know,
Howard
---------- FOLLOW-UP ----------
QUESTION: Hello again Howard,
I reset the timing, to 11° BTDC per the emission plate recommendation, and adjusted the carbs per your process, and managed to pass 3 of 4 tests.
At 2500 rpm - CO 4.32% HC 721 and at 900 rpm - CO 7.23% HC 660 PPM
Took it back for the second test of the day, after leaning the rear carb ¼ trun. Better but not passing…
At 2500 rpm - CO 4.8% HC 784 and at 900 rpm - CO 7.03% HC 562 PPM
What next? Richen the front and lean the rear carb?
Observations: When tuning, I could keep it running at slightly below 600 rpm where the adjusting screws bottomed out. At that point it was running pretty rough, so I was using a tach and vacuum gauge hooked to the manifold, as well as sound to estimate best mixture, and a synchronizer tool to balance air flow. The vacuum needle bounces rapidly but smoothly between 12 and 14 in. Hg below 750 rpm centered at around 13 in.(It smoothes out completely at 15 in.Hg at around 800 rpm.) The vacuum gets really erratic as I adjust away from the best idle mixture at 600 rpm… I also tried using the synchronizer to max out the air flow to each carb (one at a time) while I fine tuned the mixture, then rebalance them, this seemed to work pretty well…
After adjusting back up to 800 rpm, when I lift the rear lift pin, the engine slows slightly and runs like the old John Deere, but doesn’t stall… When I lift the front lift pin, it slows, then stalls completely in a second or two. Figuring this means the front is too lean, I tried it again after richening the mixture by 1/2, 1, and 1 ½ turns, but it still stalled within a second or two of lifting the pin. (I adjusted the front carb back to the indicated best idle for the test results above…)
When I rebuilt the carbs, I was very careful to center the throttle plate and make sure they closed completely. I did notice that when I replaced one of the cold start valve O rings, that one seemed a little less tight a fit to the housing when I re-assembled it could this be a problem?
At the first test above settings, the engine has a very verrrrrry slight miss at 3-4 second intervals, not a stall type miss, just an irregular tone… At the leaner setting, I’m getting more irregularity in the smoothness of the idle sounds.
Thanks again for any suggestions
Jim
ANSWER: Jim, you are adjusting it backwards. When you lift the front carb pin, you are adjusting the rear carb not the front. You said when you lift the front carb pin you were adjusting the front carb.
Start from scratch, Loosen the linkage between both carbs and back off the fast idle stop screw and the throttle stop screws. This allows both throttle plates to seat closed. Now adjust the linkage so that there is free play when you pull the throttle cable before it opens the throttles and that it opens both at the exact same time.
Now, using a feeler gauge, run the throttle stop screws (idle set screws) down so that there is exactly the same clearance between the stop screws and the throttle. I use a .005" feeler gauge while some mechanics use a piece of cellophane. Now you have the two linkages so that if you pull on the throttle cable both throttle plates lift exactly at the same time and there is exactly the same Clarence between the idle stop screws and the throttles on both throttles.
Now carefully turn both throttle stop screws in the same amount (the screws are hex head so count the number of flats. 6 flats = one turn) (I usually turn mine in 9 flats (1 and 1/2 turns))
With the top pots off I run the jet adjustment nut all the way up on both carbs and then turn one of them down by counting the number of flats (6 flats = one turn) until I see the jet is recessed about 1/16th of an inch below the surface of the bridge of the carb. Then turn the other jet down the exact same number of flats that you counted on the first carb.
Now you have both butterflies in sync and if you pull the cable there is some free play until it opens both throttles at the same time. At this point you don't know what the idle speed is nor do you know if the jets are set correctly but the throttle plates are set together and if the cable is pulled both will open at the same time and the jets are at the same level but maybe not correct.
This is where you start from and you need not change the relation between the throttle plates unless you have a Unison gauge.
The engine should start at this settings. Now, ANY TIME YOU ADJUST THE THROTTLE STOP SCREWS, YOU MUST TURN BOTH THE EXACT SAME NUMBER OF FLATS.
Set the idle at about 800 RPM by turning both stop screws the same number of flats. Lift the FRONT CARB pin and listen for the results.
If it drops off and runs like a old John Deer tractor then the REAR CARB is correct. If it speeds up than the REAR CARB is too rich and you must turn the 5/8 nut up but never adjust more than 3 flats and recheck. You should rev the engine between adjustments so the plugs will not get fouled while adjusting. If the engine drops off and dies, You either didn't adjust the linkage careful enough or the REAR CARB is too lean. Always adjust no more than 3 flats and recheck. Once the engine responds correctly you can then lift the REAR CARB pin and do the adjustments on the FRONT CARB.
When finished you should have it idling at about 800 RPM and if you lift either lift pin the engine should drop off and sound like an old John Deer tractor.
The importance of setting the linkage so that it lifts both throttles at the exact same time is that when you are being tested at anything above idle you want to know that both throttle plates are open the same thus all four cylinders are pulling the same not two pulling harder than the other two. Even though there is a balance tube in the intake manifold the front carb mainly only runs the front two cylinders. etc.
Use a thin oil or just engine oil in the damper. This is important because it dampens the movement of the piston between pulses at idle and it takes the place of an accelerator pump on a domestic carb.
Let me know,
Howard
---------- FOLLOW-UP ----------
QUESTION: Hi Howard,
It finally passed!!
At 2500 rpm - CO 2.71% HC 667 and at 900 rpm - CO 4.49% HC 702 PPM
The throttle linkage was a few hundredths off, which I corrected. I also followed your procedure to set the throttle screws, it duplicated the results I was getting using a UNI SYNC to balance the air flows.
Thanks for telling me about how to use the opposite lift pins to test the mixture… my SU manual covered them for single carbs but didn’t indicate how they indicate the opposite carbs mixture. I used them to tune the carbs for the starting point today’s tests…
The carbs seem overly sensitive, so I wonder if I have something else wrong… To get it to pass, I had the emissions tested 3 times in around 10 minutes, leaning each carb by ½ turn then richening by ¼ turn (HIF4 has an X slot screw) and saw the CO reduce by 2.5% at 2500rpm and 3.5% at 900rpm, and the HC go from 1550 to 650. From your previous comment, It looks like I’m now running too lean and should probably increase the mixture by ¼ turn to get the CO up a bit at 2500 rpm, till next years test.
I’ve been recording the relative height of the jets after each emission test for the past 4 years. (taking the top pots off the carbs and counting the number of turns above or below the level of the jet guide). The front jet is always noticeably higher, about 1 ¼ turns higher (leaner) than the rear jet. Is this within the normal range? Or do I have worn or mismatched jets and needles… Also when I put the UNI SYNC on the front carb the rpm drops by around 10-20 rpm but drops by 75-100 rpm when placed on the rear carb (same diaphragm setting, and the air flow through each carb as measured by the UNI SYNC is identical). This is also after I’ve used the lift pins to adjust the mixture so they both cause the engine to run identically, like the same old John Deere.
Any additional insight would be appreciated.
Thanks again for all the help,
Jim
AnswerHi Jim,
Sounds like you have a good handle on this. Sometimes there is a difference between the carbs due to ware on the jets or needle and some difference in vacuum between the front two cylinders and the rear two cylinders. Even though there is a balance tube on the intake manifold the front carb is effected by the front two cylinders and the rear by the rear two cylinders. The position of the metering needle in each piston is also a factor.
The gauge measures the air going in and raises the ball in the tube but it also restricts the air flow slightly so you may see a slight drop in RPM and you have picked up that it changes the front a little different than the rear. There are many things that can cause that like valve lash adjustment, difference in compression in the front two than the rear two, a slight difference in clearance in the front butterfly shaft than the rear.
If it runs good and passed emission tests and passes the lift pin check, it may not be worth tracking down the reason for the difference. In the dealerships I worked in back when those cars were new, we felt good if we got the lift pin test ok as we would do several cars a day including some cars with 3 SUs like some of big Healeys and E-Type Jags.
I liked the SU carb as it was the step between a conventional carb and fuel injection.
Howard
