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Stumble over 3500 rpm

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C

CanyonCarver

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Hey everyone, it's been awhile. Usually that means one is enjoying lovely, trouble-free motoring, and I guess that's been true for me, but only in my daily driver.
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The problem appeared when I swapped over to a set of dual HS2's from my old single Stromberg. The car is an early '70 mkIII, and was Honda reliable for the year I drove it daily. Well, long story short, I never could get it to run right with those carbs, as it would stumble at anything over about 3500 rpm. Pulling out the choke would improve the problem, but not cure it. I fiddled and ****ed with those carbs for quite a while, started a thread on this board about it (can't find it now) finally gave up and threw on the trusty ol' Stromberg.

No luck. Same exact symptoms. Every once in a while it will rev up just fine even in 3rd or 4th gear, but 90% of the time, it just goes something like poppity-poppity-poppity under full throttle and over 3500 rpm. Pulling out the choke helps, but it runs way too rich at idle.

Any suggestions welcome, and thanks in advance!

Josh
 
Hello Josh,

when I have that sort of problem, I go over all the basic settings. I check the valve clearances, then run a compression check, points, plugs and timing. Last of all I check the fuel system and that it is set as it should be.
If that uncovers nothing obviously set wrongly, I'd run the car at the troublesome point and then pull the plugs to see what colour they are.
This should show how the mixture is at the miss fire and whether all the cylinders are the same.

Try that and let us know how you get on.

Alec
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Check the fuel filter. I once had a truck that ran perfectly when not under load, but pulling up hills would stumble and quit. The filter was partly plugged and would flow enough for low fuel demands, but when you needed lots of fuel (i.e. higher revs or higher loads) it would run out of fuel.
 
Alec, how long should I run the car at that point?

Just to clear things up, the car will run at full throttle, and it will run over 3500 rpm, but it won't do both. As far as I can tell, it'll run at full throttle all day long, as long as I keep the RPM down. I also have a new fuel pump.

Eric, any pics of that turbo Miata?
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Hello CC,

a couple of minutes should be enough.
I don't understand how you can run full throttle "all day" and keep below 3,500 rpm.

Alec
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<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by CanyonCarver:
Eric, any pics of that turbo Miata?
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<hr></blockquote>

No pics yet - waiting for better weather to haul it out. I haven't driven it much (just got it in January), but with 219 hp and 206 ft/lb at the rear wheels on the dyno it should be amusing...
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"I don't understand how you can run full throttle "all day" and keep below 3,500 rpm."

Okay, ya got me there.
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I guess I'd have to find a good hill.

So just to make sure I've got this straight, I should make it stumble for a few minutes and then look at the plugs? If it's too lean, don't I risk damaging the engine if I do it for that long?
 
Well, maybe if you're making almost no horsepower or going up steep hills ... That'd keep it at low rpm
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Well I went out and drove a little today, and systematically tried some different things. Sometimes the car will pull right up to 5k or so with no problem, but if you do it too much, sure enough, that power fade comes back. Definately seems like it's just running out of gas when there's a higher demand for it. Still no problems at low RPM, though. I floored it and rode the brakes for a bit to keep it at about 3k and no problems. I filled it up and put in some STP fuel system cleaner...maybe it's just got some buildup in a line somewhere?

Fuel pump and filter are spankin' new.
 
Hello CC,
running lean for a couple of minutes won't do any harm.
You say that pulling the choke helps, so surely there can't be a fuel supply problem. Is the mechanical ignition advance system working? (Not the vacuum side).
If you remove the distributor cap you should be able to rotate the rotor arm a few degrees and it should then return under it's own spring tension.

Alec
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