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Why can I not go any faster?


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Firstly, Hello to all on this forum! Dunno why but I have been posting a lot on the 80s forum but since my 1600 dl is in fact 1979 I think I should hang around here more often.:banana:

 

Anyway, I'm having trouble getting the old girl to do much over 120k (75mph) on a flat straight. Not long ago I would have been able to cruize at that speed and if I put my foot down get up to 135k (83mph)

 

Iv'e done a lot of work to the engine lately and probably farted around with the carb to much but generaly she is in fine shape. She starts first time on even the frostiest mornings which suggests to me that I can't be far away but I can't help feeling she has a lot more to give.

 

Coments please on where to direct my attention and why. Or am I just asking too much from her

 

Some details to help:

 

Recently rebuilt engine.

New ignition system (bar the dizzy)

Timing at 8 degrees

Hitachi Carb (cleaned and rebuilt to the best of my abilities)

Recent compression test perfect

 

Cheers

 

Dirk

 

PS I did get her up to 155k (96mph) yesterday but it was down hill and I got the feeling that the engine was actualy stopping the car from running away from it any further.

 

PPS Naught to 100k (62mph) is fine at somewhere between 13.5 and 14 seconds so the power is there, I just can't find it at the top.

Edited by Dirk
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Cheers Rob.

 

No cat but it is the classic pea shooter exhaust. Maybe its being a little restrictive but I guess subaru knew what they were doing when they designed it.

 

I've been playing with the timing a lot lately but have found it quite difficult to tune by ear because of other noises confusing me. Like I said, it feels like its dragging its heels at high speed. I wonder if then the timing is either too advanced and slowing the pistons down or too retarded and not putting enough bang behind the pistons.

 

I'm also curious about fuel/air ratio. My spark plugs are very clean at the moment. Perhaps too clean.

Edited by Dirk
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Thanks fellas.

 

Valve clearances are as per manual specs.

 

I haven't purchased a rebuild kit for the carb. Iv'e been rebuilding it with donor parts from other secondhand carbs. I have been playing around with all the jets and valves to work out the best combo. Currently the main jets are one and three numbers larger than the orgionals repectively. Plus a slightly larger power valve (I think its clean) and idle jet.

 

I think I know whats coming here. Buy a re build kit! That or a Weber.

 

Primary main jet. origional: #97 new: #98

Secondary main jet origional: #140 new: #143

 

Despite the larger jets, the sparkplugs are still squeeky clean and my exhaust gasses are clear.

 

Does the power valve operate at high speed without acceleration?

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Hey,

 

The power valve opens when engine vacume drops to say 6 and dumps an additional load of fuel to richen thing up. That is, when you have your foot in it.

 

Doug

 

Cheers. but i'm still confused. If I have my foot on the floor and im doing highway speed in 5th gear what is happening then? Surely under these conditions there would be plenty of vacum? Or not?

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The power valve is activated by the Vacume piston in the top of the carb. Air rushing through the primary side draws a vacume on the piston against spring pressure and suspends it above the pin sticking out of the power valve.

 

When you put your foot in it and vacume drops to say about 6 in the primary throttle bore, the spring in the vacume piston is no longer overpowered by the vacume, and drops onto the power valve pin. That opens the power valve.

 

Doug

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Sweet, Thanks Doug. I'm looking at the donor carb at the moment and can now see what is going on. I always wondered how that valve worked and never really took much notice of the spring mechanism above it.:rolleyes:

 

So this could indeed be and issue. Also the age and condition of the carb means it is leaking air from most places so I guess the vacum is a little less than it should be.

 

edit: What am I saying?! Less vacum equals more power valve.!

 

Again I am drawing to the conclusion that I should at the very least, buy a new rebuild kit.

 

Many thanks for the help.

 

Dirk

 

P.s. Still fiddling with the timing but have not found a deffinate sweet spot yet.

Edited by Dirk
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picture.php?albumid=162&pictureid=1886

This is a PV from a 28 32. I filed the bottom where the plug is then drilled the plug out. That spring goes on the other end, not where it is. What was left in this after cleaning could possibly break loose and plug the tiny discharge hole. I soldered a cap back on it and made a couple more holes for discharge.

picture.php?albumid=162&pictureid=1885

This was a test fit of the plug from an old marine carb, tilison? Anyway, it's also pre mod on the extra holes. Some carbs have multiple discharge "ports", some have large windows.

 

Yea, new carb kit.

 

Doug

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Awsome! Thanks Doug.

 

Just pulled out the power valve and gave it a good hit with the air line. It would appear to be clean but as you say it is a tiny hole.

 

Will do the kit thing and replace the fuel filter as well.

 

Cheers.

 

Dirk.

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"If I have my foot on the floor and im doing highway speed in 5th gear what is happening then? Surely under these conditions there would be plenty of vacum? Or not?"

 

Oh, you're welcome. What happens should be a low vacume reading. High vac at idle.

 

If you have a lot of vacume at full throttle, your carb would be too small.

 

Doug

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What happens should be a low vacume reading. High vac at idle.

 

If you have a lot of vacume at full throttle, your carb would be too small.

 

Doug

 

This make sense to me since the power valve does not opperate at idle.

 

However, how is it then that my vacum advance works harder at higher rpm?

 

Is there more than one vacum system working in the carb?

 

By the way, it seems that we differ on opinion about how to spell vacum(e)

 

Oddly enough I just tried to spell check it on MS Word and it corrected me with VACUUM! I had a look on Google and all three versions are being employed in about equal quantities.

 

P.s. I am toying with the idea of increasing the size of the secondary main jet by 4 numbers (just happen to have that size in the garage). It doesn't take much effort to do so I may as well have a little experiment. Any ideas on what might happen?

Edited by Dirk
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P.s. I am toying with the idea of increasing the size of the secondary main jet by 4 numbers (just happen to have that size in the garage). It doesn't take much effort to do so I may as well have a little experiment. Any ideas on what might happen?

 

Yea, I should use spell check.:) You'll get a richer secondary mixture which is what you may need. You could probably go larger than that if you had the jets. No worries there.

 

In general: That power valve opening is like increasing the primary jet 8 to 10 sizes. Say, from 14.5 to 12.5 air/fuel ratio, for economy, then power.

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Been tinkering in the garage and have made myself a nifty little tool that allows me to replace main jets without having to remove the whole carb!:)

 

I replced the secondary main jet #143 with #147.

 

So far the results are good. I deffinately have more high end power. The acceleration seems to be smoother and more progressive too although I feel I may have lost a little low end power but not much. This I can live with. Previously I have had this jet installed but also with a larger primary main. This setup was not successful since I had a drop in power across the board and could see tell tale signs of unburned fuel in my rear view mirror. I am much happier now with just the larger secondary.

 

Can't wait to test top speed. Unfortunately its blowing a hooly out there and is not pleasant driving. 50-60 knot cross winds on my favourite test road.:mad:

 

The car origionally had a four speed transmission but now has five speed. I wonder if this has been a factor in choosing jet sizes.

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Hey,

 

Good move on the jet-driver and yes, the 5 speed changes things. In fourth you may allready be in overdrive...5th even lower rpm.

 

Remember, the primary gets a shot from the accelerator pump to get things moving. Under load or driving fast near fulll throttle, the power valve bumps the mixture up. So the primary jet supplies fuel for cruising... with the addition of the other two when needed.

 

Edit: Here's my newest Hitachi.

 

picture.php?albumid=162&pictureid=1890

Edited by Quidam
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Four barrels! What engine is that going on?

 

Iv'e been reading up some more and fiddling some more too.

 

I upped the primary slow jet quite a few sizes which has made a good difference too. For the first time since I've had the car the idle mixture screw actually works the way it is supposed to.

 

With the changes now made I can feel a deffinate power curve that was not so obvious before. In forth gear the power comes on between 55 and 70mph. Actually in fouth gear I can happy get up to 80mph but I have no idea what rpm im doing so I don't want to push it too much.

 

From what I understand the primary idle jet is most effective between 1/8 and 1/4 throttle but I guess it is always active.

 

What purpose does the secondary idle jet have?

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Four barrels! What engine is that going on?

 

I upped the primary slow jet quite a few sizes which has made a good difference too. For the first time since I've had the car the idle mixture screw actually works the way it is supposed to.

 

From what I understand the primary idle jet is most effective between 1/8 and 1/4 throttle but I guess it is always active.

 

What purpose does the secondary idle jet have?

 

That's for an EA-82T. Idle jet? Just at idle. Secondary idle jet? I'm going to have to think about that. You have two corner idle?

 

I would drill the power valve out. These carbs have small ones and it makes a difference.

 

If this carb is set up right, It willl hit hard. Small primary on a stock engine is better than a Webber, midrange about the same, and the Webber will give you more horsepower on the top end provided both are tuned the same. And if your engine can make that horsepower. But you'd have to use that adaptor, and unless you change the intake manifold...you're not going to get all that's possible with the Webber. The intake is too small.

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