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Mark Ogram

Carb to Pi conversion

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Hello,

Converting my 1971 TR6 CP back to Pi. Set all the butterfies today, started the engine , all good. When I pushed in the choke cable (only connected to the MU) the revs went up to 3000RPM ish. Any ideas why this would happen, MU problem ?

Thanks in advance.

Mark.

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Hi Mark,

good first result then:) 

For 3000 rpm you need quite a bit of air.

Is air idle valve fully closed?

Are set screws on each TB fully released so the butterflies can fully close?

Are the 3 vertical linkages all 3 “free”?

If so, time for the synchrometer, assuming you have no air leaks on the manifolds.

Waldi

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Weaker mixture, compared to when you had the choke on? Engine speed rises.

How far open are the butterflies, fag paper gap, do you have the later manifold with the bypass? 

Are the butterflies centred in the bodies?

Are the right springs in the vacuum capsule of the MU?

John

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There must be too much air entering the inlet tract. Just closing the the choke enrichment lever on the MU wouldn't allow excess air in.

I would be looking for some/all throttle butterflies not closing completely, idle air valve too far open, or air leaks between throttle bodies and cylinder head. Less likely but possible would be an air leak from the servo, or a punctured diaphragm in the MU. Or possibly serious wear on the throttle spindles allowing excess air in at idle.

 Nigel

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Maybe measure the manifold vacuum at the brake booster connection. This will tell you if you have a leak.

With my PI set up and cam  the vacuum has to be around 10 inches of Mercury at idle- but this varies from engine to engine.

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Thanks for the replies. I have done some more fiddling as below:-

Loosened off the screws holding the butterflies, re-centred, and re-tightened screws.

Only have one adjusting screw on spindle lever (5+6) this is fully wound out.

Adjusted butterfly clearances (top and bottom) to 0.002", but the "bible" says adjust so that a 0.002" feeler gauge will NOT pass through - is this correct ?

All 3 vertical linkages are free and the whole linkage moves freely.

Air idle valve open one turn, but engine does not stall when closed.

Disconected the brake servo hose and blanked off - there was no difference on the RPM.

Did the tongue test on the metering vacuum hose - all good.

 

I am going to adjust the butterfly clearances further today - maybe to 0.001".

Thanks.

Mark.

 

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Hi Mark,

not sure if my engine stalled with the idle air valve closed, but I can set it to 850 rpm so am ok with that. I reconditioned my TB’s, re-using the original discs on new shafts, so hope (expect) the discs will seal a bit better over time.

I did not bother about setting the discs with feeler gages, as this is not a good indication for what is really needed, which is air flow rate (in my opinion).

So I first balanced the TB’s at idle rpm with the 3 screws on the spindles (vertical links loose) and the synchro.

Next I adjusted the 3 links “by the eye” as a course first step, engine not running, to see if they all opened synchrone.

Finally I balanced at 2000 rpm with the 3 vertical links, using the synchro. The 2000 rpm is coarsly set with the throttle cable adjuster screw.

And then re-checked at idle.

Waldi

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Search for an air leak with some inflammable spray.    Brake cleaner, or even WD40.

A light spray on any supect area will lead to a brief increase in revs if that is where the leak is, as extra 'fuel' is drawn in.

I had this and it was the spindles in the throttle bodies.    I had them rebored and oilites fitted.

John

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You really need all the butterflies completely closed at idle and let the idle air screw do the idle speed adjustment.

If you have cleaned out the throttle bodies you will get an idle speed of over 2000 rpm, I made that mistake when I fitted PI to my USA spec TR6 about 30 years ago.

There is generally enough wear in the bodies to allow sufficient air to pass down the side of the butterflies even if the edges are closed against the body.

I would suggest smearing some very thick grease around the butterfly seating area and give each set a good rattling open and closed to make sure they are properly shut before you synchronise the linkages. When you have accumulated some mileage the gunk that come from the engine breather will gum up the gaps around the butterflies, but the grease will help until then

Neil

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Thanks for the replies. I have done some more fiddling as below:-

Loosened off the screws holding the butterflies, re-centred, and re-tightened screws.

Only have one adjusting screw on spindle lever (5+6) this is fully wound out.

Adjusted butterfly clearances (top and bottom) to 0.002", but the "bible" says adjust so that a 0.002" feeler gauge will NOT pass through - is this correct ?

All 3 vertical linkages are free and the whole linkage moves freely.

Air idle valve open one turn, but engine does not stall when closed.

Disconected the brake servo hose and blanked off - there was no difference on the RPM.

Did the tongue test on the metering vacuum hose - all good.

 

I am going to adjust the butterfly clearances further today - maybe to 0.001".

Thanks.

Mark.

 

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Success - almost.

Another final adjustment on the butterflies today, started engine, good idle, even vacuum on all inlets, engine stalled when closing off air idle screw - fantastic, then the rotor arm broke in half. New one arriving tomorrow, so I am feeling very positive now. Just want to get on the road and drive it.

Thank you all.

Mark.

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Exciting progress, well done!

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Mark,

Good you solved it.

Waldi

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