Warts and all GT40 Australia #48

Peter Delaney

GT40s Supporter
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

Lifetime Supporter
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
 
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 McCluskey.

Lifetime Supporter
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 Delaney

GT40s Supporter
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 McCluskey.

Lifetime Supporter
Peter, in answer to your question "Dunno":eek: 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.
 

Attachments

Peter Delaney

GT40s Supporter
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.
 

Doug S.

The protoplasm may be 70, but the spirit is 32!
Lifetime Supporter
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
 
Last edited:

hoppy

Lifetime Supporter
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 McCluskey.

Lifetime Supporter
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.:embarassed:.
Good thread here on heavy steering. http://www.gt40s.com/forum/gt40-tech-chassis-brakes-tires-wheels/22076-what-causes-heavy-steering-racing-speeds.html

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.
 

Pete McCluskey.

Lifetime Supporter
Peter D, thanks to Mick I discovered the throttle body adjusters in the picture below. They make balancing the throttle bodies a piece of cake.
You can get them from here in Sydney Hi-Performance Parts Online - Tweakit
The ones pictured are for Weber TB but you get the idea. I used a dremel to cut out the existing joiners and fitted the adjustable ones . A bit fiddly but not difficult.
 

Attachments

Peter D best mod I have made to the car. You need to piss the origonal DC&O linkage off they are flawed in there design. Once they are tightened the grub screw marks the shaft from then on you cannot change the setting they will always want to track back to the same position and rotating them doesnt make any difference. The new linkage Pete has shown allows for very small changes. Since the last dyno tune after fitting the new links not one pop backfire or so much as fart. Off the throttle at low RPM its faultless. I would never have thought that I would get 8THB to work as well a 4BRL in terms of drivability. Of cousre the Dyno tuner was a bit of a genius as well.

Mick
 

Peter Delaney

GT40s Supporter
Pete - thanks for the heads-up PM on these recent posts.

Mick & Pete - this looks like a similar setup to the one I have in between each of the TB pairs on each side - see 1st pic. (The only DC&O standard moving parts I have are the spindles & butterflies). I am pretty sure that I have never had trouble with the inside set of 4 TB's/butterflies - it seems only to have been the ones at the ends which went out of sync.

The interesting thing is that my setup seems to have now "settled down" & hasn't need a sync tweak for about 12 months !! Maybe there was a little bit of binding on the outer butterflies, & that with lots of use, things have "worn in" to the point where it all runs freely & stays put ?

In the olden days, RF refused to use any part of the DC&O linkages / spindles / butterflies - Robert had told me that he had them all "custom made", and even had similar adjusters in between the inner & outer TB's - see 2nd pic. These outer adjusters allowed you to remove any butterfly binding due to axial misalignment on a TB pair (my original problem, I think), as well as to independently adjust sync between the pair.

Personally, I think it was an over-complicated "band-aid" for a problem that should not have been there in the first place. It looks like the new DC&O TB setup with siamesed TB pairs solves this problem.

Thanks again guys - and MERRY CHRISTMAS !!

Kind Regards,

Peter D.
 

Attachments

Pete,
Great car! Well done (bit late but still deserving).Its an Inspiration. Lots of good ideas.
I want one, just gotta convince the cheese and kisses.
Kind regards, Gus.
 
Last edited:
Top