Replaced filter, engine no longer revs high. Why?

[TomLewis]

Good call Dan, any change in air affecting vacuum will cause a lowered RPM that is consistent, like yours at 2000. Mine went down when I installed a new tank. I assume it was the vacuum difference because of the new setup. I live with it. Tighten up everything, even things that you didn't touch.

[antoha]

transmitterdan: agreed, and racor explicitly warns against this.

i just bought goop/dope: hercules megaloc (MaineSail recommended it in one of his posts about fuel tanks) from home depot, plumbing section. is this good enough - or do i need to go with smth from a marine store, like permatex high performance thread sealant (defender sells it)?

thanks again for all the advice!

[transmitterdan]

Originally Posted by antoha

transmitterdan: agreed, and racor explicitly warns against this.

i just bought goop/dope: hercules megaloc (MaineSail recommended it in one of his posts about fuel tanks) from home depot, plumbing section. is this good enough - or do i need to go with smth from a marine store, like permatex high performance thread sealant (defender sells it)?

thanks again for all the advice!

Megaloc Multipurpose is fine for diesel and gasoline lines.

[antoha]

alright, spent saturday troubleshooting. easy things first:

1. prop was not dirty enough to cause problems. but i scrubbed it nonetheless, since i was in the water already.

2. air filter is not pristine, but i don't think it's really an issue (i will change it since it's cheap enough to do even if i don't need to).

3. fittings: this is where the problem may come from. i have those swivel fittings, with hose barb (no clamps) on one side and female nut + compression thingie on the other side. these look like parker/racor push-lok style fittings (straight, with a yellow plastic ring at the end of the hose), but i'm not entirely sure. i couldn't figure out if the thread is SAE (straight) or NPT (tapered), so i may have put the elbow into these fittings the wrong way around (elbow has one side jic/straight, the other side NPT). so the threads are now partially stripped. i managed to put them back together and it holds (and i got the engine to 2400 rps umder load, getting boat to hull spead, but couldn't rev higher under load - i wonder why), no leaks, with megaloc as goop.

4. i put in a gauge via a regular t+insert (since i couldn't find a street t), and the gauge is working properly: first when i started the engine i forgot to open the fuel supply line, and although the lift pump clicked and the engine actually ran fine (out of gear, idling only) the gauge got to about 15 which is in the red zone - when i opened the valve it dropped to almost zero which is how it's running now, so the lines and filter are probably fine.

given this behavior, i would guess there should be some air leak in the system at the fittings (otherwise how else would the engine manage to run for half a minute with the fuel shut-off valve closed). i would also like to replace the fittings.

the problems with the fittings are these though: the hose size is 5/16, and i can't seem to find fittings for it easily anywhere (on the web or locally; autozon/napa etc automotive stores are not helpful here). and the other issue - how do i remove the existing fittings? i could of course go the barbs+clamps route, but i guess whoever installed this originally decided not to, so there must have been a reason for that.

thanks again, any suggestions greatly appreciated!

[DeepFrz]

Look for minute cracks in the hose at the fittings and for tiny punctures/wear/abrasion along the length of the hose. The hose between the filter and the lift pump are still under negative pressure so could leak air in without leaking diesel out.

Have you tried setting up a day tank type of thing with a bottle of filtered fuel and bypassing the racor filters? (I haven't gone back over the complete thread so don't remember if you did that) A clean gallon jug of filtered fuel should give you enough fuel to give the system a good test.

[Limitless37]

Check with Defender Industries online, they sell the correct fittings for all Racor fuel/water separators/ filters. They come straight and 90's.