• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

TR2/3/3A SU H6 Carbs all out of whack!!

Share this page

HighAltitudeTR3

HighAltitudeTR3

Jedi Trainee
Country flag
Offline
I am having a string of issues with my SU's. I set my carbs to what the book recommends for a baseline, then I lean it out. The engine warms up, idle increases from about 650 to 1500 RPM. I reduce idle rematch the carbs (the suction and fuel mixture (with colortune). I discover I'm running SUPER rich so I tune back my carb to run lean, I end up closing the mixture screw all the way. Now the car is running (somehow) like a champion!! Test Drive = Perfect. Fast forward a few hours, car won't start, not even close, I press on the accelerator and I barely get it to run. It backfires through the carb and stalls easily. I try to open the mixture screws about a full turn and then I just stop because even if it works it's not telling me anything. The fuel is some how finding its way in to the carb via some other channel than the jet on both carbs.

I have ordered the leanest needles from Moss (I'm at ~9000 ft) and another rebuild kit. My plan is to rebuild the carbs again reseat the leanest needles and start from scratch to set the carbs. Is there anything else I should do? My jet doesn't look worn so I don't think it needs to be replaced. I think I need to soak the seals in oil? What oil should I use? Any other tips?

TIA
 
I'm not using the choke... haven't really needed it. But I try to make sure it's completely off.
 
You can't see the wear in the jets; I'd replace them before going to non-standard components. I would also replace the spring inside, unless I knew no one else had touched it from new. People sometimes try to fix a gland leak by stretching the spring, which causes problems with the jets not returning after release the choke.

Also sounds like you might be having float level problems, maybe a leaky float valve or a bit of crud caught in one.

But when you tried to start it "after a few hours", it was really lean because the engine had cooled off. You needed the choke at that point. Don't touch mixture nuts unless it is thoroughly warmed up.

Also pay careful attention to jet centering. The piston must fall smoothly and land with a distinct click.

Obviously, check the gland washers for damage and proper installation. There should be a little cup washer for each of them, with the concave side towards the washer. Personally, I much prefer to use O-rings instead of the cork. They last a lot longer. Also make sure you have the copper washer on top of the upper bearing.

What does the "lift the piston" test tell you? Also, have you double-checked valve lash, ignition timing, and (if there is any possibility it's wrong) valve timing?

BTW, the "lean" needles don't change idle mixture. Having to run the nut all the way up is a sure sign that something is wrong other than needle profile.
 
Thanks for the reply Randal. I never really use the choke. At my altitude and especially this time of year the choke doesn't do a lot of good. A lot of people with carb'd cars here disconnect the chokes. The lift the piston test was perfect before the test drive, everything was... the colortune was just right, the suction of the carbs were synced and the car ran amazingly. This was the most work I had put in to tuning the carb ever which is why I'm so confused how far backwards everything went. When the car cooled down I'm thinking the alternate path for the fuel to come through had been blocked. I'm thinking the corks I just installed are not seating quite right.

rhe Pistons move freely.

When you say the needles wont change the mixture, do you mean that they are all the same size? Not sure what you meant... at this altitude I've almost always had to tune my carbs down, but usually it's jets not needles on the carbs I've worked with in the past.
 
The "lean" needles change the mixture profile, meaning they are thicker lower down than the stock needles to give a leaner mixture under cruise conditions. (I forget offhand about full throttle, and don't have my charts handy). But the idle station at the top of the needle is the same size.

Yes, you'll have to set the nuts leaner than usual, due to the altitude. But only by a few flats, not all the way. I've set mine at 11,000 ft (Hoosier Pass) and it was only a flat or two; maybe 5 flats total since leaving sea level. And I still used the choke to start it.

Don't have a photo of me, but here's one of my late friend Fred Thomas.

CNa9HVk.jpg
 
Cool shot! i love the white walls.

I can only only guess what is going on with my carbs, but I know gas is getting passed the jets. I think the only place the gas can get by is the two corks around the jet. Is there any where else?
 
If you set it up to run with the perfect mixture at 9k, you will need to choke it to start. Those not using the choke are running a rich mixture to begin with.
 
I second replacing the inner spring like Randall suggested. You could turn the nut up but if the spring in the gland washers is too long and holding the jet out then nothing happens-- plus you would not need a choke because if the spring is wrong/ too long, the choke is out. When doing repairs like this, I keep about 3 sets of new spark plugs on hand. Sometimes you can buy the Champions cheap on sale and the Bosh work good also.
steve
 
Thanks guys. I'll add springs as well. Makes sense about the choke and running rich. My plugs are black!
 
Fuel can also leak into the throat past the wear inside the jet; and the copper washer on top of the upper bearing. Plus if the fuel level is too high, you'll get extra flow through the jet.
 
I'll check the float level. You have any tricks to getting the float level right? I know they offer rod kit for spacing, but I have enough of those tools I only use once every other blue moon.
 
What can be done with the play in the jet lever. The lever the choke is connected to. I have a 1/16th inch of play there that is effecting the idle. (I have a feeling new springs is going to fix this, but just in case....)
 
HighAltitudeTR3 said:
What can be done with the play in the jet lever. The lever the choke is connected to. I have a 1/16th inch of play there that is effecting the idle. (I have a feeling new springs is going to fix this, but just in case....)
Click to expand...

The springs should be strong enough to push the jets fully up. If they stick, it is either the choke cable binding, or the jet outer surface is corroded or rough in the seal. If the jets are returning full up, but there is still some play, you can take that out by adjusting the choke cable on the choke lever.
 
The first step to get the fuel level right is to see where it is.This is easy if you pull domes and look down the jet ,fuel should be just below the top of jet.
If this checks out and stays the same after some off ignition cranks the fuel level is ok and no need to touch the float valve assemblies.
If it doesn't check out you will know which floats need attention and which way you are going.
Too many variables like repro needle assemblies,float wieghts etc to use the factory setting for final adjustment of the float without checking the final level of the fuel.
Tom
Tom
 
Back
Top