Thread: RPM Loss
10-09-2007, 05:56 AM #1
Jim I know this topic has come up before but I am really starting to scratch my head about the RPM loss we are seeing. We happened to go out this weekend Gphotrod, Voodoo and myself for a little test and tune and to see if we had gained back any of our lost RPM's from being in the hot southern heat where it was about 100+ and very humid and to be quite honest I was a little supprised at our findings. Now up here in Michigan most of the summer I had been running with lets call it boat no#1 7080 to 7090 and mid to high 75 mph.
GPhotrod was running exactly the same untill the middle of summer when lets call it boat no#2 had a sudden loss of about 100 rpm's. Now is this is when I think it starts getting interesting this weekend we go out like I said and even though the weather was very nice for October in Michigan It wasn't any hotter than most of the summer and not even as close to as hot and humid as it was in Alabama and Kentucky in late August and now both boats are down on RPM's. From what I see boat no#1 is down at least 100 RPM's at 6990 from 7090 and boat no#2 is now in the dangerously low 6910 range from 7080. I was running 7050 in 105 deg. heat in Alabama with 96% humidity at 75.1 and was thrilled that I had that much RPM with boat no#1. Gphotrod was running around 6990 but had an 87mm nozzel on because he had already had some RPM loss and didn't want to risk detonation. I figured that I would easily be up to about 7120 back in Michigan this time of year.
Now we get home and deside to do some checking because we keep hearing about all these cranks coming apart and or going out of phase and our first check of boat no#2 it looked like we had a winner because it looked like no.3 Cyl was out about .163 with no.1 on what we think is TDC but this was just a quick check. So this required a deeper look so it was off with the head and time to get a a real good read on this thing. After bringing up no.1 on TDC with a dial indicator and using a digital caliper to measure down no.2 and 3 they were within .020 which still seems bad to me but not as bad as .163.
I know that you just went through this RPM loss on one of your boats it is pain when you think you have checked everything and still can't find it. Did you find any other possible RPM grabbing bugs when you went through yours?
We have already checked the following on boat no.2 .
Head gasket. OK
jet works. OK
Power Valves. OK
EFI Controller. OK
Pump bearings. OK
Pump Shoe. OK
All ride plate and trim tab bolts. OK
This thing is driving the two of us absolutely crazy even though we are getting much faster, you know faster at pulling it apart and putting it back
together again. ha. One other thing I have noticed the idle seems a little loppier than normal. Any imput would be appreciated.
10-09-2007, 11:03 AM #2
By any chance have you had your injectors out to pressure test them and make sure they are not clogged because that will cause rpm lost and rough idle to. Keep me up to date on your ski's, Feel free to give me a call if you can't find the cause. Thank you, Jim
10-09-2007, 12:03 PM #3
Jim thanks, I did take a mirror and tried to get a good look at the piston skirts but the thrust bearing never even came to mind. We have been mixing Amsoil interseptor with the gas along with the injection up untill just resently and I was wondering about the injectors. Did you preasure test them yourself or did you send them out? We are planning to go through both engines over the winter but we would sure like to find this little bug first if you know what I mean. By the way I am glad yours is running good since you got it back together? We really like these things but man sometimes they drive us nuts.
10-09-2007, 12:10 PM #4
10-10-2007, 04:16 AM #5
Jim thanks I appreciate the offer. When we get them apart we will take you up on that offer. When we pulled the head off of Rons boat we noticed that no. 1 cylinder seems leaner than 2 and 3 and we aren't sure why. I will keep you informed of the progress.
Last edited by injected2stroke; 10-10-2007 at 04:42 AM.
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