RPM surges - revisited

[Chris Foster]

Started having a similar problem to one discussed here last fall - an RPM surge in the stbd engine every few minutes.* It also would idle 200-300 RPM high for 20-30 seconds at a time.* This is on a T6.354 Perkins.

The consensus here was a plugged filter or air leak.* Vacuum gage on the Racor showed the filter OK even after a full RPM run, so a leak seemed likely.* I plumbed in a short section of clear vinyl tubing on the fuel return, and every 30 seconds or so, a small bubble would appear.* So definitely an air leak, but seemingly a pretty tiny one.

The primary filter has a fuel bleed at a high point on its inlet side*that should bleed off any small bubbles long before they get into the injection pump, so I decided to check that for obstructions while changing the primary filter.* The bleed circuit has a check valve and then a banjo bolt that feeds a collar and a bit of copper tube to the return outlet.* I pulled out the banjo bolt and blew through it - blocked solid.* What the ???* The banjo bolt was drilled axially (through the center along the axis) but not radially (where fuel would exit to the collar).*

The parts manual shows a hole there, but there was*none on the part.* A minute on the drill press changed that.* I reassembled the whole business, fired up, and nary a surge.

This is a 30+ year old engine and has presumably been this way forever.* Presumably it never leaked enough air to be a problem.* I still need to determine the leak source and fix it (likely the lift pump) before it gets worse.* Still, a minor unporting of a fuel tank shouldn't be a big problem, and surely would have been on this engine.

The banjo bolt is item 5 on the attached drawing - you can see the hole drawn on the unthreaded part of the shank.

 

[Tom.B]

That is ODD! Is there a chance someone replaced it with the wrong part in the past? My CAV filter assembly has a 90 banjo fitting where your #5 is. One side it the primary fuel inlet to the filter (from the lift pump) and the other side is the return to the tank. At least that is the way I think it works.

 

[Chris Foster]

Anything's possible, I suppose.* But because the axial hole had been bored, I don't think that it would be an 'incorrect' part -- what would be the purpose of such a part?* So it seems likely that it was defective.

The heritage of the engines is unknown.* They are 8 years younger than the hull, but no records exist as to whether they were new or used when installed.

The washers weren't correct when I disassembled it, either.* Washer 6 was below the collar fitting instead of above.

I haven't checked the*port engine yet to see if it's set up the same way.

The manufacturing year of the engines is 1979, which as I recollect wasn't exactly a banner year for English manufacturing quality - who remembers (or doesn't want to forget) British Leyland??*

 

[captal]

I had the same problem with the same year and model engine. My lift pump was damp with fuel but no drips. I replaced it and the surge went away.

 

[Chris Foster]

Update:

Took the first multiday trip with the fix in this weekend and nary a surge.*

One other difference:* I've always had problems maintaining engine sync - I'd get them set just right and five minutes later, THRUM-THRUM-THRUM.* Set them up again and ten minutes later... THRUM THRUM THRUM.* Assumed it was probably a bit of friction in the throttle cables that I'd fix someday.* That's gone now - I set the engines up and three hours later they sound just the same.

Going to replace the lift pump this winter in any case - I had problems with the port engine pump previously, and they're not real expensive.* Will see if that fixes the air issue.

 

[FF]

"Assumed it was probably a bit of friction in the throttle cables that I'd fix someday."

Your local motor cycle shop has a cable lube lit that only requires access to one end of the cable.