Mid Range RPM Hesitation

[beermanpete]

The engine speed will affect the output volume. Did you have the engine speed at 1,000 RPM per the manual's instructions?

If you race the engine does the pressure increase or does it stay as 3 PSI?

I am wondering if you have an air leak in the fuel line. When you measured the flow rate did the fuel look foamy of bubbly?

Another thought is perhaps the return line is returning too much fuel. There is a restrictor orifice in the end of the hard line. Perhaps it is too big.

[landmizzle]

Hi beermanpete,

Yes, the engine was running at 1000 rpm. I actually ran another test today and put an external tach on the engine. With the rpms reading ~1100 rpm on both the inside and external tachs, I got repeat flow rates of 6.5 cups/min or 3.25pts/min. Just under the 3 3/8 pts/min called for.

If I race the engine, the fluctuating readings stabilize and I typically get 3.1 psi front and 3.5 psi rear, but the pressure does not rise.

Fuel did not look foamy or bubbly.

The return fuel line is certainly thinner than the fuel line to the carbs. Not sure if the end orifice is too big on the return line. However, it would not have been something that changed since before this issue began.

Thanks for the response!

[beermanpete]

It sounds like the fuel delivery to the carbs is ok to me. I doubt that 1/8 pint per minute low would cause a problem. The only thing left would be to attach a fuel pressrue gauge to the car in such a manner that you can watch the fuel pressrue while driving to see if ti drops when the car is running poorly. If you try this DON"T run fuel into the car, mount the gauge outside the cabin.

Have you check the fuel lines that connect the float bowls to the jet nozzle? Perhps they are kinked or restricted for some reason.

[landmizzle]

Hi beermanpete,

Ok, I think we can rule out the fuel pump of fuel pressure. I hooked up he fuel pressure gauge as you suggested by taping it to the outside windshield while I drove around. Fuel gauge showed no change even while the hesitation was occurring.

I'll really check the fuel lines below the bowls to the nozzles, but the last time I looked at them nothing seemed unusual.

[landmizzle]

Hi all,

Did two things this morning.

First pulled the spark plugs and gapped theme to .34 (they were all at .31 ~).

Then I jumpered the ballast resister. I'm running the Pertronix ignition right now with the a stock spec coil so I think this is safe to do. If I were running points, it's my understanding that this configuration would eventually fry the points.

Took the car for a quick trial run and the flat spot at around 4500 and beyond is gone! I was able to wring her out too 6000 rpm without issue. Nice!

I'm going to take her out for a longer test drive and I'll report back, but I'm so jazzed right now I wanted to put up a quick post.

[llouie]

That's great news. I've had a similar problem at 3500 rpm for almost a year now....still can't seem to figure it out, but I haven't tried all the things that have been mentioned. Curious to see how your longer test drive turns out. Hope is all good.

[landmizzle]

Hi all,

The longer test drive confirmed that the 4500 rpm + hesitation was mostly gone with the ballast jumpered. There was still a little flat spot in the power range but it was much less pronounced. Now it seemed more like a tuning issue (A/F, SU damping oil, etc..).

I emptied the SU damping oil which was ATF and reverted back to straight 20w. Next test drive the car performed better especially upon acceleration. Revved nicely to 6000 rpm, and wasn't as flat in the powerband, but still could be better.

I don't recall if I mentioned but the stock engine has been rebuilt to a 2.8 stroker by Rebello Racing about nine years ago. Consequently, she likes to breath a little deeper. I have a spare air housing that I cut openings in on the top surface to allow more air flow. I placed that on the car and took her for another drive. Nice! She really picked up some steam in the upper RPM range now. Still may need to adjust the A/F or put in SM needles, but all in all much better than before.

So, just to confirm that the ballast jumpering was the main "fix", removed the wire that was connecting the +/- poles. Symptom returned.

I took the ballast off and inspected it more closely. There's not much to it. A ceramic housing with a thin coiled spring enclosed. The spring is attached to two bolts with washers and the bolts protrude up through the housing and are what various ignition circuit wires attach to. There was some tarnish/rust on these bolts and washers so I took a Dremel to them and cleaned them up. I then cleaned all the connectors on the ballast. Unfortunately, non of this helped. The 4500+ hestitation will persist if I don't jumper the ballast.

So, on one hand I'm happy, the car drives much better. Actually it drives like it's supposed to. On the other hand, I'm perplexed that the ballast has to be jumpered even after the cleaning. I'll go ahead a get another (they're cheap), just in case I'm missing something and the cleaning wasn't enough.

So, it seems clear to me that this essentially came down to a weak spark issue (as many of you suggested) and that the ignition circuit has too much resistance in it unless the ballast is jumpered (which of course removes the resistance of the ballast). If a new ballast doesn't resolve the problem (without the ballast jumpered) then there has to be additional uncalled for resistance elsewhere in the ignition circuit.

Of course I'm getting ahead of myself here, but the fact of the matter is the car did not need a jumpered ballast in the past to run properly. So while my fix works, it is a band aid. It's a band aid that prevents me from going back to points if I wanted to.

I'm thinking maybe it's time to close this thread and open up another one that focuses just on the ignition/weak spark issue.

While this issue is not closed, I feel much better. At least now, I see the logic (for the most part) of the problem. I've learned a lot in this process and want to thank everyone that took the time to post replies and put up with my ignorance and sometimes thick headed nature. If anything, this ordeal has reinforced the importance of being open minded, make limited changes during trouble-shooting, and probably most of all, continuity of approach (don't let long time spans occur in between trouble-shooting sessions).

[Zedyone_kenobi]

Well congrats man!

I am always happy to hear a problem resolved. I am still trying to fix my issue, but I have jumped my resistor and still have my weird issue. Best of luck man!