Griff

Stock Magnum cam is .510/.510, 224/224 and 115.5 lob sep

articfriends

iTrader: (1)

Im rethinking your "dieseling/run on issue" and here is my guess at WHY based on my own experiences : IF base timing was actually set at ZERO on top of having a bigger cam, motor would NOT idle/stay idling when fired up. The first thing that was done at that point was the idle stop screw was turned IN until it at least would idle, since timing was LOW, the idle wasnt strong and the throttle blades/idle stop got turned in quite a ways to get a decent idle due to lack of idle timing, (seen this on my dyno) so, when engine was ran then shut off, it was getting all kinds of air , hence the run on. Ive had to drag my dyno brake when shutting motors down on fire up as we get all the adjustments balanced to shut down a motor that wanted to run on, at least thats my guess, Smitty

CPFITNESS

Today's update
this boat has been a project that has now gone on for over a year. I thought it was Done in January of 2021 and I put a full tank of 87 octane in then. I'll skip the particulars but the boat ran for a total of about half a tank of fuel before motor blew up and we did this rebuild. So I have a half tank of 87 octane fuel in there (it was treated with stabilizer when the tank was filled and we have never seen evidence of water in the fuel) and the mention of the higher compression and the low octane was bothering me. I dont think it's responsible totally but it certainly can't be helping. So I bought some "race gas concentrate" to up the octane to approximately 93. We dumped it in and allowed the drive to the ramp to hopefully mix it up. Got to the ramp and the same high idle condition persisted. That had a vaccum gauge hooked up and we're doing diagnostics ultimately just trying to get the idle down low enough so they could check and set the base timing. We got it to idle around 800-850 and were able to get base timing in the ballpark. The carb was very sensitive to many adjustments they were making, particular the idle screw. It was like the slightest movement would be too much and either stall out or rev WAYYYYY to high.

Ultimately we had it idling decent enough that we decided to go run it and see if we had any overheat issues. We didn't overheat and particularly under load it seemed to stay quite cool, if anything, at idle the temp would creep up and there was definitely varying temps side to side. I know it's common for one side to be hotter than the other on the hose but I thought I. The water it would be more balanced than that. Curious if anyone has any thoughts on that.

When we got back to the dock or anytime we try to put her in neutral she wasn't idling great and would still sometimes stall out. The good thing is we didn't have any dieseling this time out.

I stopped in at the engine rebuilders shop as he is in the same industrial park as the mechanic and he thinks it's as simple as something buggered up in the carb. I can across another thread in my research where someone had rebuilt their carb multiple times and it never worked right but finally he just bought a new carb and that solved his problem. I'm not opposed to that except that apparently Edelbrock marine carbs are just another of the .a y things on crazy backorder

CPFITNESS

Another night losing sleep researching everything I can on carburetors. I think I may have stumbled onto the problem. It sounds like the cam change is giving us low vaccumn at idle and we have too much spring pressure so its opening opening and allowing more fuel than desired. Kind of explains the sensitivity with trying to adjust idle and getting no middle ground on the adjustment. It would be awesome if it's as simple as that spring change.

I like my mechanic but again as a younger guy he's more about fuel injection but if he's gonna market himself as a mercruiser guy he needs to get familiar with carbs. I think after working on my boat he may become the next mechanic to go "outboards only, 10 years old or less" LOL

liberator221

I don’t think the cam is big enough to mess up the fuel enrichment system in carb. Sounds like it is not running on the idle circuit. Haven’t found the vac leak, or carb needs rebuilt.

CPFITNESS

I don't disagree but I will say this....Carb was just rebuilt in August and literally only had 5 hours use them motor blew and had to be rebuilt. So it's possible it got some crud in it sitting on a shelf in the shop I guess but I'm thinking that this carb was rebuilt to the specs for a factory 330hp 454, not the magnum. In fact, research appears to show that the magnum usually uses a qjet carb. So I really think next step is to definitely at least try the metering rod spring change because the symptoms are exactly as described in the manual and as mentioned in this YouTube video.

SB

iTrader: (1)

Thanks for the pic, however looking for pic with engine installed in boat because rigging is more often than not , people’s issues. Sometimes we catch these mistakes on ohotis.

Did you use the factory weber carb to quadrajet intake manifold in which one side of adapter look like this ? If so, which gaskets did you use ?

CPFITNESS

No carb adapter it's a factory intake that has both types of bolt holes

SB

iTrader: (1)

You have pic of top of intake without carb installed ? And casting # ?( My phone can’t clearly show me cast # from your last pic.)

CPFITNESS

I don't think I do. I did find this pic of motor in the boat though