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After close to an 8 week hiatus, I finally drove the MGA off the trailer and into the garage.
My internet investigations suggest that my SU H6 high speed bog is most likely caused by a lean condition rather than a rich condition. At the last event, Eric Russell re-tuned the carbs and at idle, they sound sweet and the engine is smooth. The jets are now about 0.05" below the bridge (I normally run with the jets about 0.07" to 0.11" below the bridge). Eric felt he could not lean out further. I normally run KW needles and we also swapped in SM needles (at least we think they are SM based on limited eyesight, but they were my stubbiest needles, so clearly would run leaner than the KWs.)
I'm going to try to make a methodical assessment of the carbs to determine if they are not damaged in some way (I recently installed a new jet on the front carb), to try to identify my piston springs and perhaps get stronger springs. I should probably also get new piston dampers. Then if all looks good, I might try to make an appointment to run the car on the dyno.
...
Well, that's the plan that I started to implement last night:
Carburetor analysis, Part 1:
Carbs are SU H6 1 ¾” throat size with 4 bolt flange. Twin Cam type with square intake. Carbs have H type jet stamped “1”. Intake Manifold marked AEH 198 VB.
Lifting pins deleted and lifting pin holes blocked off, front with steel epoxy, rear with what appears to be a copper wire.
At one time, tags on carb float bowls appeared to read: Front AUC 884 or 894, Rear AUC 868. The only number that makes any sense is AUC 894 for the front carb which was listed for a 1959 Jaguar MK1 with engine size of 3-4 liter, red spring, needles MO3(std) and SJ (lean). (The 1958 Twin Cam, 1588cc motor came with twin H6 carburetors (AUC877), red spring, needles RH (rich), OA6 (std) and OA7 (lean)).
I set up a scale and a spare HS4 piston and spring to try to measure – or at lease compare – the piston springs. I set the piston (no needle) on the scale and placed the spring over the piston oil damper tube. Then I zeroed out the scale. Then I pressed down on the spring until my fingers just touched the top of the oil damper tube. I also measured the length of the spring with a ruler.
The HS4 test spring measured 5 1/2” long and required 2.7 oz of pressure to compress so that my fingers touched.
The first anomaly:
The spring on the front carb measured 6 1/4” and required 3.7 oz of finger pressure. The spring on the rear carb measured 5 3/4” and required 3.0 oz of finger pressure.
I then proceeded to do the piston drop test. I removed the dashpots and pistons and labeled each of the four parts for the front (F) or the rear (R). I blocked off the piston vent holes with tape.
The second anomaly:
The F/F combination was about 2 seconds slower than the R/R combination. On average, I felt the F/F combination dropped in about 8 seconds whereas the R/R combination dropped in about 6 seconds. This is consistent with the actual feel of the pistons in the carburetor body when installed on the car. When checking the piston drop clunk, the rear piston seems to drop faster and clunk louder than the front piston which has a more muffled sound when it hits.
I tried swapping the pistons and running the piston drop test with a F/R and R/F combination. I felt that this combination resulted in a more even drop, but after many tries my results were inconsistent at best. My feeling is that I should stay with the original dashpot to piston combinations.
I clearly don’t know the history of the carburetors. I’ve always tried to be careful, however, not to separate the pistons from the dashpots. But I never noticed or paid much attention to the springs. My current intuition is that I’ve managed to reverse the springs and that the 6 1/4”, 3.7 oz. spring should be moved to the rear carburetor and the 5 3/4”, 3.0 oz spring moved to the front. In theory, that should slow the rise of the rear piston as compared to the front under acceleration. To be continued ...
You are awesome Jeff. I’m not sure I would have the patience and perseverance that you possess.
NJMP Time Trials was fun. Didn’t win, but had a good battle all event long with a BRZ and while he beat me in two out of three sessions, I won overall as I was more consistent.
I did the piston drop test again and concluded that the F/R and R/F combination dropped more equally. Ultimately, I should change to this combination.
I, however, reassembled the carbs in their original configuration and with the unequal springs also in their original location and with the SM needles to try to recreate the high speed hesitation. Gunning the engine in the garage did not produce any noticeable misfire or hesitation. I was using the SU carb tuning kit with the wires in the pistons to compare rise. I did not notice any discrepancy.
But with the Uni-Sys, I did notice that the rear carb was not drawing as much air at idle and I attempted to adjust to equalize the balance.
Then, in performing the piston lift test, I did notice that the rear carb did not seem to be accelerating and then stumbling like the front. I attempted to lean out the mixture (perhaps I was going the wrong way?) with the jet adjusting nut. The third anomaly: In doing so, I determined that the jet was not moving freely up and down within the jet bearing assembly and that I also seemed to have a lot of side to side slop in the choke lever. I’m thinking that the clevis fitting on the jet is probably loose and that some of the other parts are worn.
I will try adjusting the nut on the rear carb to richen the mixture just in case I was going the wrong way. But then my plan is to remove the carbs to analyze why I have so much slop in the choke lever. And once the H6 carbs are off, I’ll try to install the set of H4s that I have and see if I can get them running. Will need to swap manifolds and modify linkage.
Jeff, you certainly like a challenge. Some Triumph racers that had carb problems gave up on the old ones and bought new carbs. Problem solved, but at over $2000 for a set now, I can understand your approach. The prices have gone crazy. The last I looked they were around $500.
What's life without a challenge? I've never dealt so carefully with the carburetors, so I'm learning a few things as I go.
H6 Carburetors Part 3.
I tried to fine tune the carburetors and felt that I was successful in getting the rear carb sorted. But when I went to the front carb, I was unsuccessful. Ultimately I decided that the piston was sticking. At first I swapped pistons based on my piston drop down test. No success, so I pulled the carbs.
I first addressed the slop in the rear carb linkage. Part of the problem was that the clevis on the bottom of the jet was quite loose. I was able to find a substitute jet in my spares. Some of the other parts were worn, and I substituted a few parts in better condition from a set of old carbs. I also fill soldered some of the pivot holes in the lever arm and drilled new holes. Tightened things up - but maybe too tight. Afterwards, it took me quite a while to center the jet so that the piston moved freely. Finally successful and quit for the night.
Will move to the front carburetor tonight. Since the H6s are off, thinking I should test fit the set of H4 carbs and see if I can get them running. Requires a manifold swap and some modification of the throttle linkage.
I am shocked that we have not received a report on Pitt Race.
According to a Facebook post by Mr. Aashish, Pitt Race delivered just as much fun as it was last year and was made even more memorable in the company of many friends and also fellow local region members from the Mohawk Hudson Region. Here's a few pictures of the action from my perspective.
Haha, just too tired and dealing with work again. What can I say. I had a lot of fun. Would have been nice to dip below 2:10 on the full course, but at least my co-driver did. I was happy to knock 2 seconds off on the South Course and another 2 seconds off on the Track Sprint on the north course.
08-09-2022, 04:25 AM
