Learn me troubleshooting SUs

Keith,

Short answer for me is .016" usually means .016."  I hadn't thought too much about if you changed rocker ratios, so here's where the long answer may change.  Most of my advice comes from dyno work I've done with Dave Anton at APT on A-series and B-series engines.  Dave taught me to try opening them up a bit more than the cam manufacturer recommended if it's a higher ratio rocker than stock.  So he may have added .02-.04" clearance if the ratio is higher.  What we'd do (and I still do) is try it on the dyno and get the best numbers we could.  Generally, the additional clearance got us some low-end without sacrificing top-end and improved idle quality as I mentioned before.   

I haven't thought about it anymore than that until you posed this question, but maybe the long answer is that .016" means .016x(% increase of new rocker ratio)?  That sounds smarter than my guess-then-dyno (aka trial and error) system anyway.

Carl

The reason I asked about the rockers was due to this article on Mini Mania - a better than average source of info. I set my lash to 0.015" (ish, this is the sort of task where there is skill involved in judging the perfect amount of drag) at the valve so we'll see how that works.

The rebuild kit arrived yesterday, so I swapped out a few parts (new jet, new float needle, some gaskets) but left the shaft seals alone because I preferred not to mess with those return springs if I don't have to. I also checked the float level per the manual and it was right on, so I left that alone. The car fired up and seems to idling happier. It's trapped in the garage behind a disassembled CRX (more on that in another thread) so I wasn't able to test drive. Hopefully tomorrow or tonight!

Adrian_Thompson said:

I have zero useful to add.  I was going to mention the shaft bushing, but that's been covered.  I'm just here to say, that when Keith comes asking for help it's either the end of days.  Either that or the GRM forum is evolving into it's own  AI fountain of all knowledge and Google will have to buy out the whole empire or GRM will surpass Google as the go to place for any and all questions in the known universe.

Urrrgggg....

I kinda get your drift, but I get a similar reaction when I post a question on a certain marque-specific site I frequent, and the tendency is to not want to ask questions at all because of it.

Aside, everything I know about SUs comes from fighting those damned Mikuni smog-era copies on the '73-74 Z cars, so this is a case of "if you have nothing nice to say, don't say anything at all."  One might retort that these are not SUs and are only vaguely related in that they are constant-depression carbs, and I would reply that that is my point entirely, and "constant depression" is an apt term for not only how the Mikunis worked, but also the operator's state of mind...

I have no problem asking questions. 

So, the little booger is up and running. It idles okay but pops occasionally. Driving, it’s cranky until it gets a chance to clear its throat and then it’ll run pretty well. It does not like anything between a carefully-approached idle and wheee! Plugs are brown at idle. I’ve screwed with the mixture and it’s happier, but I need to set the timing. According to the online guides, this is done by basically turning up the dizzy until the engine speed stabilizes, then backing off a bit...

Seems very unscientific. I have a wideband I can stuff in the tailpipe, but do I look for EFI numbers of 15ish at idle and, what, 12ish at WOT? I’m a chassis guy :) I have a timing light if the little thing has marks, but I don’t know where to set it because dizzys are all different. It’s set in the same place it’s been forever.

Do I seem a little lost? I am a little lost.

It’s getting a bit late at night to have a lumpy little car idling in the garage with the doors open, so I’ll give it another shot tomorrow evening. I may say “screw it” and just drive it to work tomorrow to give it an Italian tuneup. 

If the timing is right and you don-t have a vacuum leak, I would guess that the fuel level in the jet is too high.

Started over with the mixture and stumbled into the happy zone! The little nutcase is out on the street. Idle is a bit high at 1500 and the throttle seems slightly sticky (probably cable routing) but it’s happier than it’s been in years. Thanks a bunch. 

Now it it has a flat tire and is sitting at the side of the road with a bunch of new oil leaks, probably from aged out seals. Can’t blame the mini for the first...or the second, really. If they’re not leaking, they’re empty, right?

Always timing first, then mixture.  Set the timing with a light (32 all in, hopefully around 20 at idle) in before you do anything with the mixture.  If your light isn't a dial back unit, put a mark at 32 degrees on your pulley. (geometry involved:  put a tape on it, measure the circumference, work out the degrees/unit of measure, etc.)   Make sure you've got the vac advance reconnected before you set the mixture.

Baseline the jet 2 turns down from the bridge.  Set the idle as low as possible.  Then, the traditional method is to lift the piston slightly (1/8" or so).  Ideally, the idle will rise about 50 RPM, then fall off.  If it rises more than that and doesn't fall off, it's too rich (you gave it more air by raising the piston and leaned it out).  If it falls, it's too lean.  Adjust about 1/2 turn and repeat the process.  Between each change in jet position, make sure to rev the engine a little bit to clear things out. 

Regarding the 50 RPM rule of thumb published everywhere, I don't worry too much about that.  I mainly want the idle to not change when I raise the piston.

Regarding wideband, I rarely see idle above 14 with SUs and often they idle best in the low 13s.  They've got to be a lot fatter than FI, and fatter still if you're stuck with E10 gas (sometimes in the 12s).  You should see 12.5ish at WOT just like a FI car, but you'll see much bigger variances.  You'll also see a pretty big lean spike at tip-in.

Again, this works in Michigan at our 700 ft altitude.  The timing should still be 32 all in at any altitude, but the mixture may vary.

I don't want to sound like a jerk or a broken record, but I see so many tuning issues (in the shop, on the phone/e-mail) where the timing isn't right, so it's really important to start there.

Glad to talk off-line if I can be helpful.  You've been really helpful to all of us!

I’ll check the timing tonight, thanks. I don’t think this HIF44 has the ability to lift the piston, I couldn’t find the lifting pin that’s on the diagrams. Is that possible or did I just miss it?

The reason I’ve been concentrating on mixture is because of the way it gradually got worse and the fact that I had that gummed up carb. 

This Aldon dizzy doesn’t have vacuum advance. Set at 32 at idle then?

The dizzy has a centrifugal advance that should be fully advanced by around 3000-3500RPM.  Rev the engine until it's not advancing any more, then set it at 32.  So 32 @ 3500RPM is probably a safe place to set it.  Ideally, it will be around 20 at idle.  I really like to plot the advance curve, so if you want to be compulsive like me, take a reading at 3500, 3000, 2500, 2000, 1500, down to idle and see where it is.  If it's 10 at idle or 15 at idle, that's probably okay, just not an ideal curve.  Aldon's usually have a good curve, so that's probably not going to be an issue. 

Do you have points?  If you do, that could cause a gradual change as they wear.  But even if you're electronic, your timing may have floated a bit just like your mixture.  Things move and wear.  

The other issue you could have is that the centrifugal advance gummed/locked up and your timing isn't advancing.  If you plot the curve, you'll see that.  Given how gummed your carb was, that could be an issue. 

I usually use a little screwdriver to lift the piston if I can't find or get to the lifting pin.  Just put the blade on the edge of the piston and gently move it.

I find that a lot of driveabilty problems these days are more than one thing.  In the old days, it was one thing and fairly easy to find the smoking gun.  Since these cars are so old, sit a lot, and have had a lot of hands on them over the years, it's often a few small things adding up to a bigger issue.  

Hope that helps--otherwise keep asking!

That helps a bunch. I'll work my way through the list. The car's running pretty happily right now so it's just a matter of fine-tuning instead of the gross problems I had at the start of this conversation!