Another SU carb thread

[adam]

Hi all,

 

So I have a little issue with my SU carbs. First off, a bit of background information...

 

I'm running an L28 with 240z round top SUs, only mods are 'Trust' headers and a twin 2" exhaust system, I'm still running the standard 240z airbox.

 

I have adjusted my valve clearances, adjusting timing to approximately 7 degrees BTDC at an idle of about 750-800rpms.

 

I have checked my float levels in each carb and they're spot on. So I go and balance the carbs, adjust their mixtures until they're running awesome.

 

It drives around town (from idle to 3krpms) perfectly and when I give it some stick, it'll rev out freely and pull hard..

 

My issue is, I go to start her up the next day, warm up and whatnot, driving around town, she's sweet. But when I go to give her some stick (above 3krpms), she hesitates pretty badly and loses the pull she had 'yesterday' when I tuned it.

 

So my question is, is this normal for SU carbs? Are they really this temperamental?

 

She's a weekend car so I would like to give it a squirt here and there but this is starting to frustrate me, any help would be appreciated.

 

 

Cheers,

Adam

[d3c0y]

That is some low ignition advance to be running if you are running high octane fuel in it.

 

 

I've never had any problems with SUs behaving like that. Maybe your fuel pump is on the way out?

[adam]

I'm not entirely sure what the go is with the timing, my original dizzy struggled to run below a certain timing, and ever since I've upgraded to the electronic item, I can't get it to run any higher.

 

I'm still running the standard mechanical fuel pump so that's crossed my mine but it doesn't explain why it would run perfectly fine after I've adjusted the mixtures the day I drive it..

 

It's driving the car the following day or week that's causing the issue. Like the slight change in air temps affect the tune or something.

 

 

[d3c0y]

The distributor base plate can be adjusted itself. If you pull it out and flip it over you will see you make big adjustments that way and then fine tune with the top adjustment you are familiar with.

 

 

Actually now i think about it when did you last change your fuel filter? What colour are you plugs when you pull them out after a drive? It sounds like a fuel issue to me, probably starving. If you take it for a long drive does the problem go away. Is it the fact the car sits around for ages while you tune it, but when you drive it the next day its going out for a quick lap?

[adam]

Interesting.. I'll take a look into that (distributor base). Thanks.

 

The fuel filter was changed about a year or so ago.

 

Carbs are tuned so that plugs are grey at the firing end.

 

Going for a long drive doesn't solve the issue either unfortunately. I know getting stuck in traffic for a while can foul my plugs and cause a lot of hesitation when I get out of traffic, and it takes a while for it to drive nicely again.

 

I tune the carbs roughly while the car's sitting but fine tune it by cutting the motor out while accelerating up a hill then pulling out and reading the plugs.

 

I'm really stuck on this one..

[WA240Z]

If you are finding the issue to be underload I would look at the spark side of things. Replace all 6 plugs and the coil, very cheap way of getting a base to start with, and replace the fuel filter as well, also cheap.

 

As for the 240z hitachis getting out of tune, I havent touched mine in 8 years. I replace the damper oil every year and run some carb cleaner through them yearly. Once adjusted corrected they do not need to be played with at all. My car ownly getts used about once or twice a month, apart from the 6 to 7 secs of cranking to get her started initiallly with plentty of choke, once warm it idles perfectly and will rev out to 6k without issue under load, year in and year out.

 

Agree though the idle ignition advance is very low. Check the manual, but as yours is modified, L28 etc, you are unsure what static compression you have you may need to start low and then see how you go. Run 98 octane.

 

Good luck, Billy.

[d3c0y]

Just keep winding the timing into it until it goes ping!... Then back it off a bit.

[adam]

The issue is under load but it is fine when I give it a touch up tune, so I am lead to believe the ignition and fuel delivery stuff is fine. The plugs, leads and ignitor are all fairly new though. Rotor and cap on the other hand I haven't replaced but I don't believe they're the cause since it does pull hard, just not consistantly day after day.

 

I've run carb cleaner through it a few times and experimented with the damper oil also. What's currently in there works great.. When it's recently tuned!

 

Maybe the carbs need a rebuild?

 

I'll have a play with the ignition hopefully tonight and might get onto replacing the dizzy and rotor to eliminate it.

 

Oh and I don't run anything other than 98 through the car.

[oldmates260z]

I had a similar problem with my old Datsun 1600 but it had an aftermarket air filter....at revs and load when it needed the most air it would suck the filter in...is there anything in the air box that could do that? A flap or something?

[1600dave]

What do you define a "touch up tune" as ?

 

Mixture ? Float level ? Idle speed ? If its mixture, are you making it richer or leaner ?

 

Checked your floats for leaking / sinking ?

 

The only thing that really "wears out" on an SU is where the throttle shaft runs in the body, and that will usually only cause problems with getting a consistent idle.

 

And your ignition advance is very low...

[adam]

I don't think there's anything loose near the intake that could possibly get caught.

 

'Touch up tune' is simply the adjustment of the mixture knob to get the right colour showing on the plugs (grey instead of black/white).

 

I have already checked the float levels when I did the valve clearances and they appear to be spot on.

 

I can understand if I struggled to get the carbs to run nicely at all there might be other issues such as spark or float levels, but they do and can run nicely, just not for the following day or week. My confusion is with their sensitivity to changes in air temp or something..

 

I haven't had the chance to adjust the timing, looks like I need to get onto that asap.

 

[1600dave]

When you adjust the mixture each time, are you adjusting it the same way every time, or is it random ?

 

SU's aren't sensitive at all, they are basically one tapered needle going up and down in a fixed jet.

 

 

 

There are really only two things that control the mixture :

 

Fuel level in the jet (controlled by height of the jet, and / or the level of fuel in the float chamber)

 

Where the tapered needle is in relation to jet (controlled by piston going up / down in the dashpot on top of the carb)

 

 

Things I can think of that could cause variation in mixture from day to day :

 

Sticking piston

Sticking choke

Leaking float

Leaking needle & seat in float chamber

 

 

There are other things that will cause the mixture to change, but not on a daily basis.

 

Are you sure the choke lets the jet return to its correct position when its off ? The choke pulls the jets downwards, make sure they return fully to the "up" position (push them up with your finger while you're down there near the mixture adjusting nut).

 

Does the piston fall freely after you lift it up with the lifting pin and let it drop ? Should hear a nice metallic "clunk".

 

Keep at it - they're good, simple carbs and ideal for a dual-purpose engine. And don't write off other factors, make sure your ignition and other compnonents are in good order.

[adam]

The mixture adjustments have been quite random.

 

I have completely disconnected the choke because it doesn't return the jet to its original position (not sure whether this is the issue?). The car would run really rich when it was off and I noticed that the jet never returns to its original, most upper position. I've disconnected it completely and push the jets up as far as they go and checked them every now and then when I touch the mixture nut.

 

I've checked the piston and it rises and falls smoothly, leaving a clunk.

 

I'm not sure whether the choke not returning the jets to their original positions could be a sign of what's wrong, but I thought having removed it altogether.. it would eliminate that as the cause?

 

And after reading many threads on the topic of triple carbs and SU carbs, for my application, there's no reason to upgrade hence why I have made this thread to try and rectify this minor, annoying issue.

 

In the mean time, unless there's any other ideas out there, I might sort out the timing and go over the ignition and other obvious fuel related stuff first.

[d3c0y]

Have you ever pulled them apart and given them a good clean? That would be a good place to start if you haven't.

 

 

How far are your mixtures from the base setting? Two and half turns out or whatever it is? (You will need to look that up because its ages since i had SUs on my car.) If you are miles off that and both carbs are very different in setting, it will at least confirm something weird is going on in the carbs

[adam]

I've only ever sprayed carby cleaner into it removing any visible carbon or whatever it is built up.

 

And the mixtures are around there, 2.5 to 3 turns or so from their most lean setting.

 

I did have issues with them running too rich but the choke not being returned to their highest position was the cause.

[d3c0y]

Unscrew the tops, pull the pistons out and give them a really good scrub if they are all black inside. Spraying in a bit of carb cleaner wont do shit.

 

 

You may need to pull them off and give them a bit of a soak and a pull apart.

 

 

Your mixtures sound like they are in the normal range, maybe the chokes getting stuck is a lead, even though you dont use them.