Lack of Free Reving beyond 2500RPM 1972 V8 3500S Manual.

Lovel

Lovel

Member
Having recently just completed a full rebuild of my V8 engine and after replacing the following I still feel my engine is not as
free reving as it could be. Relevant information from the rebuild:

Composite head gasket fitted C/R? less than 10.5:1?
EFI camshaft fitted
P6 cylinder heads as per original
Standard air filters
Stainless exhaust
S type exhasut manifolds.
6 deg timing at idle BTDC set with strobe
Vacuum checked with good healthy readings
Cylinder compression pressures range from 175-180psi engine hot
Electronic ignition with new std plugs at .025" gap, new ignition leads
Fuel pump pressure 5psi, new filter, Octane booster castrol valvemaster
Full advance on distributor is being achieved ,checked with strobe


I need to confirm the needle coding for the SU's as I cant recall what is fitted atm, they will be STD I'm sure

The vehicle runs sweet enough at lower revs, but when you want to push it a bit more, it nearly gets to the red line but only
reluctantly/slowly. My old SD1 used to fly all the way round to redline without a groan, so it should be posoible with the P6?

Can anyone advise if a needle change will help? either I should go for smaller or larger needles?

I cant think of anything else that I should be checking to try and eliminate this issue ?


:?:
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

Changing the throttle linkage bushes completely sorted mine, cheap starting point if you haven't replaced yours :wink:
Jim
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

corazon said:
Changing the throttle linkage bushes completely sorted mine, cheap starting point if you haven't replaced yours :wink:
Jim
Click to expand...

Changed them too.

Needles fitted are BBG's
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

BBG is a bit weak, but dont think that'd cause what you're describing. My first thing to check would be the electrics i think?

Is this the first time you've tried the electronic ignition?

Rich.
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

Richen the mix a bit and try that!
What oil is in the dashpots?,as 20/50 wil be too thick,it needs to be thin or will restrict slow down dashpot movement,drain them out and try it!
Looking down the carbs,do both throttle butterflies open fully at full foot down on throttle pedal?
May need operating rods adjusting or one may be bent!
Carpet too thick under acc pedal can restrict that!
With eng ticking over turn/adjust dizzy everso slightly left or right to get the fastest tickover point!
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

rockdemon said:
BBG is a bit weak, but dont think that'd cause what you're describing. My first thing to check would be the electrics i think?

Is this the first time you've tried the electronic ignition?

Rich.
Click to expand...

Yes have just changed to elec ign, but at the same time I discovered that my original ballast resistor points setup had the coil wired back to front :shock: (previous owner, honest) . I had ran her for about 400 miles like this and it was running ok up a 2 or 3 thousand RPM but above that was poor, you could see the rev counter dropping out at the same time with the cutting out, hence I thought go for pointless ignition. Now she runs really sweetly with a 3 ohm coil wired correctly, but seems lethargic at higher revs.
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

Pilkie said:
Richen the mix a bit and try that!
What oil is in the dashpots?,as 20/50 wil be too thick,it needs to be thin or will restrict slow down dashpot movement,drain them out and try it!
Looking down the carbs,do both throttle butterflies open fully at full foot down on throttle pedal?
May need operating rods adjusting or one may be bent!
Carpet too thick under acc pedal can restrict that!
With eng ticking over turn/adjust dizzy everso slightly left or right to get the fastest tickover point!
Click to expand...

I have the mixture set at 2.5 turns (default setting for moment) until I get the gunsons colourtune on it, maybe worth a try a little richer for sure. I would almost say that when I back off on the throttle she speeds up, but maybe my imagination. tbh.
I have 20/50 in the dashpots, so will also swap this out, maybe try something available like 3 in 1 oil, as the nearest I have to the spec is 30w for lawnmowers.
Will also look at the others things you suggest, thanks.
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

Lovel said:
when I back off on the throttle she speeds up, but maybe my imagination. tbh.
Click to expand...
Mine has come out of sync a bit since I made tonnes of adjustments a couple of weeks ago, and I'm getting a little bit of "second wind" just as I come off light throttle in some scenarios. One carburetter is shutting off while the other is still fuelling. I might check that the acc pedal upright rod by the heater box is adjusted for full pedal travel. Manually activate the throttle mechanism to see if it brings both carbs off the idle screws at the same time, and as much.

But the rest of this makes me think your coil or ignition wiring may be out of sorts?
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

Adjusting the carbs from your current setting is easy,just turn it 1/4 turn either way,and it will speed up or slow down,what you need to find is the fastest point!
I have mine set 1/4 turn weaker from the fastest point,and on a nice steady M-way 55mph run return about 25mpg!!
If I really boot it and also using it for work and back short trips I get circa 15-18mpg!!
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

Hmm, the plot thickens. Changed out dashpots to 3in1 oil no real discernible difference tbh. However the lack of revving or lethargy kicks in nearer 2500rpm (I said 3500rpm (mistakenly) originally, I dont think the viscosity of the oil has caused this) and seems to get worse the more I floor it. Now I know it was doing this previously with the points installation too, but the ballast coil was wired incorrectly for polarity. I installed the new elec ign and 12v 3ohm coil as per the way it should be.

Once I reach revs over 2500 ish the rev counter starts dropping and swaying around, suggesting power supply issues?, but when I check for intermittent supply with my multimeter all seems well. To get around this I temporarily took the 12v supply direct from washer supply and get same results.
When I installed the elec ign system I spliced in (soldered and heat shrink) a wire to run to the coil which bypassed the ballast wire behind the rev counter, but left the ballast wire intact so I could swap back to original if I wanted to.

Have fitted new vacuum, leads, plugs, dist cap previously.

With sudden cutting out at higher revs it looks on the surface it could be something like electrical continuity issue, but the harder I try to push it the worse it gets, possibly need to look at fuel starvation, blocked breathers, fuel cap etc, will take a look at fuel filter (was new) and fuel pump (old not renovated but is putting out 4-5psi which seems to be within specs). Noted that with engine stopped the fuel pressure in the pump drops off over two mins to 0psi or so which suggests possible check valve leak in pump.
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

that is coil or condenser symptoms... there are some dodgy ones about!

Whats the other low tension wiring like?
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

rockdemon said:
that is coil or condenser symptoms... there are some dodgy ones about!

Whats the other low tension wiring like?
Click to expand...

The low tension wiring is in good shape for its age, although I have bypassed some of it for the elec ignition kit. There are no dodgy splices insulating tape etc. If I do need to splice in it is soldered into loom and good quality heat shrink tube and braised sleeving used where appropriate, and rather than crimp connectors on I always solder them at the ends too, then push the rubber boot over the end.

The coil I'm using with the elec ignition is 3.0 Ohm resistance, I need to check primary and secondary resistance to see what I get there.

With the engine running I am getting a bit over 14v, I have just noticed that the electronic ignition that ive fitted specifies 13.5V max. According to the supplier, damage can occur if you have 14.2V at 2500rpm. However it must be said I had the same issue when the points were fitted albeit with the coil polarity incorrectly fitted. Perhaps an adjustment of the alternator regulator is on order to bring it down a bit?
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

I was thinking coil too. So you've fitted a different one now?

Fuel starvation? Surely not if the symptom appears straight away? It'd take a couple of secs of deep pedal before you drain the float chambers and lose power, and then it'd be one carb first making the engine run like an asthmatic four.

But an automotive electronics product that can fail over 13.5V when cars tend to charge at 13.8V and such - added to fluctuations? I dunno about that.
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

Tor said:
I was thinking coil too. So you've fitted a different one now?

Fuel starvation? Surely not if the symptom appears straight away? It'd take a couple of secs of deep pedal before you drain the float chambers and lose power, and then it'd be one carb first making the engine run like an asthmatic four.

But an automotive electronics product that can fail over 13.5V when cars tend to charge at 13.8V and such - added to fluctuations? I dunno about that.
Click to expand...

Yes I think I will try another coil, maybe have old one somewhere to substitute.

They specify that above 14.2v 2500rpm can have issues, I agree with you, may be a very tight tolerance tbh, it is such a small margin. Possibly no worse than some of the other suppliers of these things. I guess this is where the old points and condenser system is more tolerant.
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

I removed the distributor cap to check for movement of the vacuum advance and discovered that the electronic ignition module was fouling the rotating magnet after a certain amount of movement of the baseplate and rpm's, stopping any further advance above 2500rpm.

The electronic ignition I fitted does not fit the distributor base plate so it was modified to fit, which seemed to go fine initially, its just that when the triangular base plate advances the module starts to strike the rotating magnet as they do not move relative to one another.

The original points, condenser and coil have been refitted for now, and work fine. The reason I changed them out in the first place was because I had serious misfiring. This was actually down to the coil polarity being incorrectly wired by a previous owner, but by the time I had realised this error the electronic ignition was almost fully fitted.

I checked the vacuum advance readings and found it begun to move at 5hg and was fully maxed out at 14hg. Thumbing through the workshop manual there seems to be quite a few different value vacuums with some that have a full advance of 26hg. I purchased mine new recently from a recognised specialist, maybe it is too low, as I still think the vehicle is lethargic at higher rev range ?

With engine running I am getting 13.8v at 2500rpm so I think the regulator is ok tbh, so will not adjust for now.
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

Lovel said:
With engine running I am getting 13.8v at 2500rpm so I think the regulator is ok tbh, so will not adjust for now.
Click to expand...

Can the alternator output voltage be adjusted easily? Our old 1972 2000Auto had the 18ACR, so all the control electrics were within the alternator unit itself. It would be quite a job to adjust the output voltage! Your might have the older 11AC with external control electrics, but still... I'd think adjusting the output voltage is quite a job with an 11AC too!

To be honest, I'd say the module you have with a 13.5V max rating and not liking 14.2V at 2500rpm is not really fit for purpose. I'm almost certain that the optimal generator voltage for batteries of the P6 era (maybe not so with modern batteries, I hasten to add!) was quoted to be 14.2V or thereabouts.
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

Lovel said:
Once I reach revs over 2500 ish the rev counter starts dropping and swaying around
Click to expand...

This happened exactly the same to mine. After a lot of setting up, timing, new leads, which all made it run better below the misfire but didn't fix it, it turned out to be the coil, I swapped it out for another one and it was (and still is) completely fixed.
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

Pilkie said:
Looking down the carbs,do both throttle butterflies open fully at full foot down on throttle pedal?
Click to expand...

Had a litle check of the throttles and found that they do not open fully, I'd say about 3/4 open, but its is hard to say. Looking at the throttle levers on the carb when fully open a tab on the lever hits the carb body. I ended up removing a small right angled plate which sits next to one of the brass bushes, this plate restricted movement of the lever somewhat and I can now get full movement of the throttle. I swapped the bushes around as one of them was more worn, but will order new if available.
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

Pilkie said:
With eng ticking over turn/adjust dizzy everso slightly left or right to get the fastest tickover point!
Click to expand...

I tried this method at weekend, only to find that the engine sounded as though it ran better the more advance I put on it. It was set up for 6 deg BTDC with the strobe and also tried at TDC on some occasions in the past. I also run it on castrol valvemaster with octane booster

When I advanced the timing by listening method I found that I got to 20 deg BTDC before it started to sound poorly. I tried a road test at 12 deg BTDC and it was slightly worse performance than at 6 deg, so back to drawing board.

Although the misfire has gone after my fun with the distributor, the ooomph at top end is still not there yet. Next step is to rebuild carbs with new jets, spindles, float hieght etc and rebuild mechnical fuel pump.

Again, compression is consistant at 180psi and vacuum is very good indicating good healthy timing.

I keep thinking that the 3.5 EFI cam (ETC6099) is too much for the standard P6 heads and not allowing it to breathe correctly ?
 
Re: Lack of Free Reving beyond 3500RPM 1972 V8 3500S Manual. Nee

I have been following this saga without putting my oar in as I thought you were getting somewhere with the problem. However having got to this stage, putting myself in your shoes I would be tearing my hair out (not that there is much left anyway !)

Seems there is a lot that is not quite as it should be from the throttle butterflies not opening properly to the EFI cam. I have rebuilt my 10.5 CR engine twice in the last 40 years with my car, prior to the last rebuild in 2008 I did contemplate fitting a hotter cam (fast road spec) but was advised against it by a friend of a friend who prepared Buick V8 engines for racing – I quote from his e.mail “ unless you are going to go the whole hog and race spec the engine and obviously upgrade everything else, brakes etc, stay with the standard “S” cam which – if in good unworn condition, will give you all the breathing you want for spirited road use” . This advice was 4 years ago and better information may be forthcoming now, but my engine has always reved to the red line (down the Silverstone straight when I got a bit carried away on the Classic in 2010)

I would not have changed so many things at the same time as you have done, doing so fogs the positive and an advantage, and whats a negative, sorry not to be more constructive but I think you need to start from the basics in a logical way.
 
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