Kizmit99

Hi All,
I was helping a friend try to diagnose some issues with his '83 GSL this past weekend and we identified some symptoms that I would like to get your thoughts on...
The car does not want to exceed about 50-60 MPH. It seems to pull strong at the low end, but just sort of (softly) hits a wall at around 60 mph. It doesn't seem to matter whether it's in 4th or 5th gear, mph seems to be limiting factor.

We did a compression test (using a digital tester) and saw numbers like:
92, 90, 89 and 92, 60, 90 (after normalization to 250RPM, with engine at operating temp)

The 60 psi reading would seem to indicate an issue with the rear rotor. I'm assuming a side seal since it's only one low reading on the rotor. Is that a valid assumption?
Would you expect these compression numbers to be the ultimate cause of the "high speed" performance issue? Or is this a red herring? The car does start up just fine.

The motor was recently torn down, all components spec'd out and rebuilt. However, I'm pretty sure only parts that were out of spec were actually replaced though.
Would the next step be to pull the motor and open it up again, or would another course of action be indicated?

Thanks in advance for any suggestions.

vipernicus42

iTrader: (4)

Those compression numbers definitely look like a side seal issue, but before you tear the engine down check all of the easier stuff first just to make sure you don't have two separate problems.

You've checked compression, so take a second look at air/fuel and spark. Is the timing in the right spot? If it's got vacuum advance, are those tubes hooked up properly and not cracked? How does it sound at idle, does it sound lumpy or smooth?

If all the usual suspects look good I'd say that sadly it's teardown time again. For an engine that was recently torn down, even the "good" chambers aren't putting out great pressure. Did the side-seals get replaced or were the originals put back in? If it's the originals, how good of a job was done at keeping track of which side seal went where? I learned the hard way that rotors aren't machined to super high precision when it comes to side seals, so new ones are too long by default and have to be shaved down to fit in their respective grooves. If you re-use them and you don't put them in the same spots you can introduce a gap (although that usually means that one of the other seals was too long to fit and didn't want to go in right...)

If you guys tear it down again, get new side seals and springs, and measure the apex and corner seals to make sure they're within spec. If you don't have the crazy measuring tools needed for some of the stuff in the manual, at least grab a measuring caliper from the local parts store and just assume that if it measures "barely" in spec, it probably isn't.

Good luck,

Jon

Kizmit99

Thanks for that info Jon!

The engine idles smoothly at about 800RPM and doesn't seem to stumble as it revs up (which is one of the things I've seen on mine with a vacuum issue).

We didn't check the timing when I was there - definitely something to check before pulling the engine again.

The more I think about this, the more it seems to me like a fuel issue. Again, it seems to have plenty of power until it just runs out so much earlier than we would expect. I know he dropped the gas tank and had it cleaned, so wouldn't expect any crud coming from there. Probably wouldn't hurt to try replacing the filter again though, just in case... I'm not sure of the carb status - whether it was rebuilt or not (it is a stock Nikki though).

On the rebuild. The original engine lost a rotor and cratered the housing. This engine is a replacement that I believe he received completely disassembled and he reassembled it. I wasn't there, but from what he's said I believe he followed rebuild instructions in the FSM to the letter. However, I'm pretty certain there's no way that the seals ended up in their original positions (I think he replaced some, but not all of them, with new). If the engine gets opened again I'm pretty sure the plan will be new seals all around. (I'm aware that means a lot of fitting)

vipernicus42

iTrader: (4)

Fuel is a good thing to look at for the kind of hitting-a-wall you're talking about. The FSM (or the Heynes) have tests you can use to make sure that the fuel pump is putting out enough fuel, especially if it's an old stock pump. Worth checking it out! That would be a much easier fix than tearing the engine down.

I don't know how many miles he's got on the engine since the rebuild, but breaking it in a bit further might shake lose the side seal if it's just kind of stuck. So yeah, check ignition and fuel, put some more miles on it, and see what happens.

Keep an eye on the oil - bad side seals would lead for combustion gasses blowing by the side seal and would show up in the oil. Might be worth doing a cheap and easy oil change, put a tank or two worth of miles on it, then change it again and have a look.

Jon

peejay

That is characteristic of a plugged fuel filter, especially if it still has the stock pump.

On my '80, after about three thousand miles, it would simply run out of oomph to go over 80mph. Just run out of steam, as that was about the speed where fuel flow could not keep up with demand and the carb would run dry. Replace the filter and it was good for 124mph again, as I used to test every day on the way to work

Another thing to check is the carburetor return spring. If the spring breaks, a trapdoor like mechanism sets and prevents the throttle from opening all the way. I think it also prevents the secondaries from opening. It is EXTREMELY COMMON for this to get sprung when messing with the carb.

Here's a good image of it. Note that the return spring attaches to a rocker and the rocker is held in place by a long rod with a pretensioned compression spring and a T fitting on the end. The issue happens when the rod is NOT on the rocker. The compression spring pushes that rod down and it becomes an emergency backup return spring that intentionally interferes with throttle opening so you know there is a problem. (Ignore the captions, they are relevant to whatever thread this came from)



Your compression numbers are not great but not the source of your problem. Really all the numbers are fine except for that one on the rear rotor. When an engine has low compression it loses low end and midrange power but still makes a good amount of power up top because there is less time for compression to bleed out.

Kizmit99

Thanks peejay - lots of good info there!
The return spring assy sounds like a likely culprit here. I know when I rebuilt my carb the first time I totally screwed up that bit of it. We'll definitely have to take a look at that to see if it's assembled correctly.
Also very good to hear confirmation that the compression numbers we're seeing *shouldn't* necessarily be causing the performance issue he's seeing.