Warts and all GT40 Australia #48

[Peter D]

Pete - sorry for the thread drift, but it seems relevant to your interest in the vacuum gauges & tuning.

Trevor, I see where you are coming from - perfect 90deg setting at WOT should translate to a perfect idle setting. However, I doubt that I have a perfect set of TB's / spindles / butterflies / linkages !

As mentioned previously, the only problems I have had are related to throttle positions near idle - specifically, fast openings off idle (most often when blipping the pedal on down-changes - the odd cough & spit, & sometimes some backfire). When Steve does his magic with the vacuum gauge or rubber tube (getting the butterflies all synched at idle & just of idle), the problems disappear - for a while. In good or bad idle tune, the engine has always run clean & strong at part or full throttle.

This all lead me to think that the culprit was the fact that the butterflies get slightly out of synch at idle over time (non-perfect linkages, heat cycling, vibration, etc).

So I reckon that tiny variations between butterfly positions equate to significant % differences in air-flow between TB's at closed/near-closed throttle, but at WOT, the % differences are much less noticable - hence the problems at near-idle & the lack of problems at WOT.

Does any of that make sense ?

Kind Regards,

Peter D.

[Trevor Booth]

Peter D,
I would suggest that there is a basic problem somewhere, you should not need to constantly adjust. As I recall you have 4 sets of 2. hopefully your linkage is connected between each set of two and your idle stops are on the same end of the spindle as the linkage is connected.

your linkage should only be connected to one spindle per engine bank with the other spindle being "driven" off the spindles to which the linkage is connected. What is critical is that no torsion be intoduced into the spindles, they will twist at the butterflies

[Gary Kadrmas]

Peter,

I had a set of 48 IDAs on my Cobra for awhile. I experienced "problems most often when blipping the pedal on down-changes - the odd cough & spit, & sometimes some backfire"...........and the problem source was an intake manifold vacuum leak.

Not sure that this is part of your issues you are trying to solve, but it is worth a check over if you have not already done so.

In addition.....are both throttle shafts on both banks of carbs fitted with a fastener that locks both carb shafts "together?" If you have this lock on one side and not the other, you will experience the same type of symtoms.

Good luck!

[Pete.]

Peter, another thought is, how big is your cam? Dependant on the valve overlap it may be a cause of some of the problem at idle and low throttle settings.

[Peter D]

Thanks for your input guys.

Trevor - you are right - I have the old DC&O setup with separate TB's, but each pair joined only by the spindles - ie, no adjustment between the butterflies on each pair. I think that DC&O now have a newer version which has each pair cast as a single unit (2 TB's joined by a bracing strut thru which the spindle passes). With my older setup, you are relying on perfect machining of the bases of the TB's & the tops of the manifold, and accurate seating/bolting, to get each pair of butterflies to freely operate over their whole range - it took a bit of fiddling to achieve this.

From the centre capstan, I have one pushrod going to the front pair's spindle on one side, & the other pushrod going to the rear pair on the other side. Each spindle pair on each side is connected to its mate via an adjustable gizmo, BUT, there is only an idle stop on the pair that is driven by the pushrods. I think that this could be contributing to the problems of going out of synch over time with heat cycling & vibration.

Vibration is certainly an issue in this area - believe it or not, the old DC&O setup had circlips on the spindles to hold the bearings into the TB's, BUT, no grooves in the spindles for the circlips to fit into !! I noticed this when I found a couple of circlips had worked their way 1/4" along the spindles, allowing the bearings to nearly come right out of the housings ! This was fixed by adding collets to the free ends of the shafts, & by fitting lengths of slit fuel hose between the circlips of adjacent pairs of TB's.

All in all, I think that the old DC&O design had some serious flaws - I recall that the old RF refused to use the DC&O linkages/spindles/butterflies & had their own made up (complete with separate spindles for every TB & adjusters between all).

When I next have things apart, I'll add idle stops to the 2 spindles which currently have none.

Gary - as my problems completely go away after a resynch of the butterflies, I suspect that they relate to the linkages. If it was a manifold leak, the problem should always be there ??

Pete - For similar reasons as above, if it was a cam issue, the problems should be persistant ?? Whilst my cam is "mildly aggressive", when all is in synch, the engine idles at 600rpm like a sewing machine (no loping or lumpiness) & jumps off idle perfectly.

BTW Pete, what setup do you have on your car - the old DC&O like mine, the modified RF linkage setup, or the new DC&O design ? (and, have you had any problems with the butterflies going out of synch) ?

So all up, I reckon the problems relate to poor design & the lack of idle stops on all 4 pairs of spindles.

Kind Regards,

Peter D.

[Pete.]

Peter, in answer to your question "Dunno" You may be able to tell me by looking at the pic below.

The butterfly shafts are joined where the arrows point with a shim, (for want of a better word) fixed with multiple Allen screws. A & B are the throttle adjusters. The ends of the shafts are fixed with a circlip, but I can't tell if there is a groove machined in them without taking them off.
They have not moved and there is no end play. So "if it ain't broke"
I agree they could be better designed.

[Peter D]

Pete, it looks like you have the newer DC&O TB castings - each pair appear to be joined together with a cast "web" - thus eliminating mounting/adjustment between the TB's on a pair (lucky you) !

If your circlips haven't moved, I would guess that they are located in grooves on the spindles, so its all good. (Mine must have been made on a Monday - some sleepyhead just forgot the grooves) ?!!

I notice that you have idle stops on only the 2 driven shafts like mine - so if you have not had any synch problems over time, maybe that's not an issue.

So in conclusion, I reckon that my problems come from the variability introduced by the mounting of totally separate TB's in each pair. Its not such a big deal - I can live with a professional tune-up every 3-6 months (but I still want to eventually get a synchrometer so that I can do it myself).

Trevor - I am partially deaf, so I would not trust myself with the rubber tube approach to synchronisation !

Kind Regards,

Peter D.

[Pete.]

DRB #48 is another year old this Month. Happy Birthday.

[Doug Sainlar]

Originally Posted by Pete

DRB #48 is another year old this Month. Happy Birthday.

Did you ever get her fitted with power steering, Pete???

I've said it before, I'll say it again--man, I really did those space-age door mirrors!!!

Doug

[hoppy]

Congrats mate,

In some ways it doesn`t seem that long ago that I was at GT40 Australia to discuss having one built when yours came back from being registered,I sat in it and was impressed with the build quality,then said to PR "build me one".

Living the dream of young boys.

[Pete.]

Originally Posted by YerDugliness

Did you ever get her fitted with power steering, Pete???

I've said it before, I'll say it again--man, I really did those space-age door mirrors!!!

Doug

No Doug, I didn't at the end of the day it was impractical and would have make the car illegal for road use.(outside the ADR"S)
We fiddled a little with camber, tire pressures and greasing the steering rack helped enormously..
Good thread here on heavy steering. What causes heavy steering at racing speeds

Hi Hoppy, I wasn't aware it was my car that motivated you to go ahead with yours. I'm really pleased about that. PR must owe me a bottle of bubbly.