I have been seriously trying to get the 140-E back in better driving shape (Driven 4 times in the last 4 weeks! Probably a record!). I’ve had some strange running characteristics that I’m starting to nail down… found out I was missing 1 of 3 throttle return springs so the middle carb wasn’t coming back down to full closed. Thankfully I identified that before chasing mixture.
But one thing I noticed while “tinkering” yesterday is that the mixture adjusting screw does not seem to move regardless of choke position.
I pulled this image off the interwebs, and my understanding is the feature A should rotate up and contact feature B and lift B slightly. My A never touches my B. The feature A does move, just not enough to move B.
I have the Bentley’s service manual and have searched online and can’t seem to find any information on setting the choke. I’ve been dealing with hard starting and think I may have found the culprit if my understanding is correct… but how would I adjust this interaction?
Brandon,
IIRC there is a spec for the distance between the screw on B and item A. Look at your other carbs and see if the distance is the same, if not adjust.
pauls
Well it’s not the cable binding… actually looks like the main jet adjustment screws are just adjusted so rich that the choke doesn’t engage. Looking like time for a general carb tune up.
I see this fairly regularly. I think it is in part due to the fact that it has been so long since SU HDs were in common usage that there aren’t many people left who know what the individual adjustments actually do.
Fairly often when a car comes to me I end up removing all the additional throttle return springs (or adding the missing ones), following the book to set the carbs back to the initial tune, checking for leaks, then fine tuning. About 80% of the time this is enough to save a car over which the dreaded “It needs new carbs” curse has been pronounced. (SUs are pretty rare birds here, Webers and Dellos are more in the common knowledge domain.)
Hmmmm. in my experience the only sure way to resolve these issues is to strip the carb(s). normally to find the neoprene section of the main jet has gone rock hard and the main jet spring is partially disintegrated. Additionally the choke operating rod is at best stiff and worst stuck in the body of the carb.
The SU’s are superb simple pieces of engineering, once set up they work brilliantly. We should all realise that every so often a strip and clean is necessary, especially with infrequent use and modern days fuel cocktails.
Just my opinion of course.
Best regards
Phil D.
It’s strange because I had it running quite well about 10 years ago, and I don’t know that dad ever fiddled with it after that.
I did replace some sinking floats and readjust the float levers to 7/16" recently so just going back through them.
Also as mentioned the center carb is for some reason missing the return spring. Had been chasing a 2k-2500rpm idle and noticed play in the center carb followed by noticing no spring…
I’m thinking the carbs were “just rebuilt” but it has now been about 14-15 years, so needing an internal refresh isn’t out of the question I suppose.
But I will start by readjusting everything per the manual and go from there…
Really I think the best thing you can do for any car is to drive it… I let it sit too long last year. I’ve been out 4 times so far this year, and it seems to run a little better each time.
Phil’s correct. There’s plenty of spots for little brass shafts to bind from corrosion in the carb body. It’s productive to pull it all apart, clean & lube. Once each carb all moves smoothly, the threaded rod that actuates the choke on the three carbs needs to be “timed”. Or more correctly the jet drop in each carb should be nearly identical at full enrichment. One can pull off the suction chamber and measure jet drop with Vernier calipers. IIRC jet to bridge drop somewhere around .2-.250" works. Best done on the bench, because access from under the car is…challenging.
My car would start on a pair of cylinders then gradually fire on all. Once adjusted, it lights off like a champ.
Dave
sozfingers
(Keith. P. series two roadster,,,UK)
12
Just helped a friend with a Mk nine he bought recently, the auxiliary choke was on a switch, but when the weather got colder the car would not start,checked to find no fuel at the base of the choke, just a half rusted out banjo bolt, both float chambers full of residue and both jet springs collapsed and rusted, the previous owner has screwed the mains down to compensate for the lack of choke leading to extreme fuel consumption, water had obviously laid for a long period at the bottom of the float bowls, surprised it ran at all,
Do you have some diagram of this jet to bridge drop you’re talking about? The service manual (I have Bentley’s) says nothing about adjusting/setting choke.
I don’t really notice any binding… The little “lever” (Feature A in the picture I posted in OP) moves freely up and down… it just doesn’t touch feature B. I do think I could see slight movement on the rear carb.
I have this same issue of starting on not all cylinders I think…
And remember I’m dealing with a 140, so basically anything with the carbs in the car is “challenging”
The front carb especially is about half under the wing and really only accessible through the wheel-well.
Brandon, can you easily pull the suction chambers off? 4 screws each (if you can reach them).
Take off the polished chamber, piston & spring. When you operate the choke lever the center of the jet is pulled down. This increases the fuel coming around the needle which richens the mixture.
Its the same way the fuel mix screw works, except the “choke” is a gross adjustment where the screw is a fine adj.
You’ll see the jet height difference as they drop. Use the depth measuring “stick” of the Vernier calipers to measure each jet’s drop from the fixed brass outer part. You’ll likely find a large variation between the 3 carbs.
Your space challenge is that the adjustment (threaded rod & couplers) is under the carb. Unless you can access from underneath…I think you’re SOL. I’ll look for pictures…
Dave
I have the air box and battery box removed at the moment so I actually CAN get to the bottom of the carbs through the wheel-well… Just not the easiest thing in the world.
But first I need to get it to where the choke actually actuates the mixture adjustment, no?
I think I can remove all the suction chambers… That needs to be done to change the main mixture needle right? I know I swapped to the UO needles with the carbs in the car 10 years ago so I presume I can get the chambers off one way or another.
In this attachment you can see 3 arrows. Notice on the opposite end of the “arrow point” there is a small round bar running transversely from the center carb to the F&R carbs. SU choke adj.pdf (1.4 MB)
If your choke cable & carb linkage is adjusted and operating properly, the center carb is the “master”. The rods leading to F&R carbs can be adjusted for length (cotter pin/clevis/yoke on threaded rod). The length of the rod determines how much the jet drops at full actuation.
I checked my notes, and I have my full choke actuation set at .200" jet drop from the fixed outer tube.
Its somewhat “relative” to your needle height in the piston and where your mixture screw is set. Make sure you have the carb tuning done & correct clearance on the fast idle screw before you start.
Dave
