Feels low on power

sdibbers

sdibbers

Well-Known Member
So, I've been asked to look at a friend's 3500S (Fed model). It feels a little under-powered. Set up is as follows:

  • 1970 3500 engine with SD1 cylinder heads
  • Rebuilt in last 2-3k miles by persons unknown.
  • LT77 5 speed conversion
  • HS6 carbs with dodgy manual choke conversion (no AED now)
  • Mallory dissy and electronic ignition.

I've found rectified the following:

  • Replaced crumbly HT leads (spark plug wires)
  • Tuned carbs as best I could (see follow on) - found one needle set inside piston on one carb. Idle set on other carb with fast idle screw. Needles are BAK
  • Replaced all vacuum/breather hoses
  • Replaced spark plugs (They're not bad, but as I was there)
  • Set ignition timing
Power is a little better, idle is much improved. But still not happy. I did find that the carb spindles and bushes are worn. You can move them slightly, biggest indicator was that if I sprayed carb cleaner near the bushes the idle drops. I've sent the carbs off to get spindle bushes machined and replaced. Could they have that much of a negative impact on performance?

The ignition timing seems good with no scatter (I know the timing chain/sprockets wear on these engines), I can't tell if the cam lobes are worn, but with so few miles on the engine I'd be surprised if they were. Idle is pretty smooth at 650rpm.

Can anybody think of something I might of missed?

Many thanks,

Steven
 
The Nada cars are weighed down with air con and power steering which all use the engine to run i believe aswell as all the extra smog gear.... will not be as fast as a GB spec 3500s... more suffocated
 
When you did the timing did you confirm TDC i.e. from #1 cylinder not the mark?

What was done in the engine rebuild? Timing gear/cam? How much slop is there in the dizzy off the cam?

Did you check the advance at the four engine speeds given?

How much compression are you making? Are the cylinders equal?

Is water circulating through the tower properly?

Are the plugs burned or fouled?
 
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I used the mark.

I believe it was a full rebuild, but I'm not sure I trust the quality of the work.

Dissy shaft has little play fore and aft. No scatter when using a timing strobe either

I just tested at two speeds, but that's an excellent point.

Not sure on compression. That'll be next on the list

Water is circulating fine. I removed the bleed tube and a good stream comes out when running.

Plugs look fine other than soot from running so rich from poor tuning before I looked at it last time.

Thanks for the replay!
 
Sorry it was a not particularly original "state the obvious"but I've usually forgotten the obvious at some point, one being the maximum advance being set far too low and the vacuum mechanism being stuck despite going to all the effort to actually measure the vacuum itself. Anyway you have a far superior dizzy so hopefully none of the above applies.
 
PeterZRH said:
Sorry it was a not particularly original "state the obvious"but I've usually forgotten the obvious at some point, one being the maximum advance being set far too low and the vacuum mechanism being stuck despite going to all the effort to actually measure the vacuum itself. Anyway you have a far superior dizzy so hopefully none of the above applies.
Click to expand...
Not at all! I'm not fully versed on the RV8 installed in an early P6, so all of the above is very helpful. I will look at the advance once I have the carbs back. I think they will have a pretty good affect on things. Carb cleaner squirted near one of the spindles would almost stall the engine.
 
low on power on a v8 powered car ...hmmm...ignition timing or worn camhaft.....those are the most common issues....
 
It's a tool that you use to determine exact TDC on the engine. Basically, you take an old spark plug and remove the innards and fit a bolt with a rounded end into it that protrudes down into the bore.
You then turn the engine over by hand and mark the point where it contacts the stop, turn it back the other way and repeat. Midway between the 2 points is exact TDC.
piston stop tool - Google-Suche
 
I did find some play in the carb spindles when looking over the car. So they've gone off for machining and bushes. Doubt they would cause such a loss of power, but it would certainly explain how it wouldn't hold a tune or the drinking habit it has (even by V8 standards).

Once I get a chance to go back up to fit the carbs and look her over I will report on timing accuracy.
 
carb bushes are overvalued....the engine will still run well unless you can stick your finger between spindle and bush....this doesnt have a severe effect on the general power output


drinking-habbit has to do with worn camshaft and vacuum-advance on dizzy....are you sure the vacuum-capsule is still functioning?

camshaft by the way: do you have at least a bill of the engine overhaul where a camshaft change has been mentioned?

otherwise remove the plenum....ypu already have the carbs off for overhaul...so a couple of bolts more and you can view down the valley and check the cam-lobes
 
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How much bhp down do you reckon it is? Which compression engine is it?
Since it was an unknown recent rebuild there is always the possibility of the camshaft not being run in correctly and some of the lobes have worn prematurely.
Is the electronic Mallory a converted dual point or a complete electronic version?
+1 for compression check
Going back to basics it would be worthwhile checking for any restrictions in the air intake etc, you mention dodgy manual choke-is this remaining slightly on perhaps?
Jim
 
I would say its down a fair amount. with mods to my 2000TC I reckon on about 135bhp using mine and an experienced friend's bum dynometer. The V8 feels lackluster in comparison with the LT77 five speed fitted. The LT77 is out of a TR8 so gearing should be correct.
Complete electronic Mallory system.
Not sure on compression, I will look when I get a chance to head out to his garage next time. Paperwork is patchy at best.
I will do a compression check when there.
Choke I was able to rectify so I think its clear now.
 
hi there all,
Thank you for explaining the use and a picture of a piston stop.
I investigated further and found this article on checking the timing on a block, a piston stop could also be used if the heads were on the engine.
Also found that there is a timing light called a "readback " timing light, somehow you can adjust the distributor via the timing light? seems like an innovative idea?

How to Accurately Determine Piston TDC - Hot Rod Network

Peter
 
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ButterFingers said:
Also found that there is a timing light called a "readback " timing light, somehow you can adjust the distributor via the timing light? seems like an innovative idea?
Click to expand...

That's not quite how it works, how could it? Basically if you have an engine with just one mark on the pulley at TDC, and a pointer that lines up with it, if you want to set the timing at 6BTDC you set the timing light at 6 degrees, and then line up the pulley TDC mark with the pointer and the light calculates the 6 degree point for you.
 
Quick update. So, I fitted the rebuilt carbs yesterday. I did buy an inlet manifold gasket to inspect the camshaft and promptly left it at home before driving 45 mins to my friend's garage.

Good news is a lot of power has come back. The chap who rebuilt them said both spindles were badly worn. Even before the test drive and after tuning the engine was far more responsive. Before hand if you blipped the throttle the body hardly moved. Now you can feel the body roll in from the torque of revving the engine. Took it for a drive and she feels much more responsive, much more tractible. Able to cruise in 5th (it has an LT77 conversion) at 1,200 rpm and pull away cleanly from there.

I think the case is closed for now.
 
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