7.5hp fuel pressure

[mikeya4]

Hi, i started a post before about my 7.5 red band dying after half throttle under load, I changed the carb for one I know is good and it didn't change a thing, I've read a few posts and im 99.9% sure its a fuel prob.I thought it might be the fuel pump, I have put new fuel lines in because the old ones were cracked and I have swapped the release conectors for the tank becuase they felt quite loose fitting, I got a fuel pressure gauge after I done all this and I tee'd it just by the carb, the pressure gauge shows .02 bar @ idle then moves up to and over .4 bar which I believe is near 6psi, I think this is good? would this rule out the fuel pump, lines tank ect(everything upto the carb?) Im hoping this has solved the problem, i will try it out later on the boat. let me know your opinions please

 

[Laddies]

Re: 7.5hp fuel pressure

Thats good pressure for the pump, if the engine is CD ign. I would suspect the high speed stator winding.

 

[mikeya4]

Re: 7.5hp fuel pressure

just took it out and still only half throttle, got pissed off so took one of the leads of an guess what, no difference its only been running on 1 pot, anyone have any ideas where to start? its a 1971 so not sure what type of ignition etc
thanks

 

[Laddies]

Re: 7.5hp fuel pressure

If it's a 71 the stator is a single winding but if it,s a 72 it has a high and low speed winding and looks like this

 

[emckelvy]

Re: 7.5hp fuel pressure

On either type of ign, check the points, if they're burnt they can be cleaned up and re-gapped to .020".

Also on either type ign, a very common failure is with the plastic insulating blocks where the points wires pass thru the 'breaker box' to connect to the external wiring. These insulators crack then will pass hi-voltage to ground and kill the spark.

The official name is "Insulator, Cable to Contact Housing", part # 336-4460, or just "4460" at this site:

http://www.crowleymarine.com/omc_mercury_parts.cfm

At $3.24 ea it might be a good idea to replace both.

HTH..........ed

 

[mikeya4]

Re: 7.5hp fuel pressure

well im pretty sure from the guides its a 71 the serial is 3102338
thanks for the helpful info

 

[mikeya4]

Re: 7.5hp fuel pressure

well I just went out and changed the coils round and the working cylinder swapped, I took the coil that does worked and this is what it looked like

had a chunck out of it, had loads of hairline crack and loads of rust, nothing like the other coil! I guess this means the rest of the ignition is fine?
thanks for everyones help

 

[mikeya4]

Re: 7.5hp fuel pressure

Just one more plee for help, I've just ordered a 2nd hand coil for a 7.5 mercury and just realised mine is classed as a mercury 75, and have different coil part numbers but look almost identical, will they be ok? or is there much variation in coils for the same engine size?
thanks in advance

 

[Laddies]

Re: 7.5hp fuel pressure

There are at lest 3 differant coils for the CD ign systems on the 7.5 engine and you must have the correct part for the system.

 

[8075mercury]

Re: 7.5hp fuel pressure

The brown ignition coil for a 71 aluminum donut stator is:

336 4409

Hope this helps, good luck.

(Oops, I see you already have the coil number.)

 

[mikeya4]

Re: 7.5hp fuel pressure

thanks for that, trying to find a replacement one is a bit of a nightmare in the uk, just got a 2nd hand one for next to nothing but is a different part number 339 5258 585, am only going to use it for testing though, not sure how different they are, but if the resistance is the same as my good one it should be ok?
thanks