Knock when i decellerate 100HP Chrysler

[MickLovin]

I took my boat out with the 100hp chrysler and found it wouldn't go onto plane, it felt like I had an electrical problem as sometimes it would not even start as well. It seemed to idle ok but as soon as I opened the throttle it bogged down and had no power. I took it home and replaced the cdi and coil with the one from my 75hp as I suspected I was getting an intermittent problem with the cdi. Ok after doing this it would start straight away with no hesitation, but now when I decellerate I hear a knocking short knocking noise untill it goes back to idle, it only does it when you decellerate fast and I thought I had gotten rid of it with a carbie adjustment, but I haven't. In a tub it seems to be accelerating fine, but when I had it in the water it went nowhere. I am hoping I may have fixed it with the coil and cdi, but I afraid I may have bigger issues with the knock.
Please help with any suggestions

 

[MickLovin]

Re: Knock when i decellerate 100HP Chrysler

ok just an update, I adjusted the Carbs as per Frank's vid Thank you Frank, I throttled up and down and now no noise. I have also been able to start it with no problems, my only concern will be when I take it to the water and it does the same thing, fingers crossed it doesn't.

 

[Justinde]

Re: Knock when i decellerate 100HP Chrysler

Hi Mick

My 125 Force had the same problem, and with Frank's help it was diagnosed as a broken engine mount ( lower) which I am still yet to replace. With my Force, with the engine tilted up, I can go to the back of the boat, and lift the leg ever so slightly more, causing a knock.

Maybe try with yours?? I am unsure if they are exactly the same, but it should be the same test to check your engine mount.

Hope this helps.

 

[MickLovin]

Re: Knock when i decellerate 100HP Chrysler

ok new update, after checking the motor again doing carbs etc etc as previously stated, I think I may have found the problem. I thought after tuning the carbies I had gotten rid of a knock on decelleration, I did seem to have gotten rid of it, BUT I was checking things again and for some reason decided to pull the top and lower carbie bowls and to my surprise here was the bottom screw-in jet loose in the bowl of the top carbie. Also after I changed out what I thought to be a problem cdi and coil from my old 75hp, I found after further investigations that my ignition switch had a loose red positive. I have tighten the carbie jet back in position and also the ignition positive and I am hoping to have no problems as this motor is immacualte inside and has had little use as every bolt I have taken out has been easy and near new, excepting the thermostat bottom bolts, the top bolts were new with a slight bit of corrosion, but I'd say the bottom are more corroded as I haven't been able to budge them either with a penetrating spray or heat or impact drill.

 

[Frank Acampora]

Re: Knock when i decellerate 100HP Chrysler

Mick: The bottom two thermostat screws are longer and go into the water jacket on the head. Thus, they are almost always corroded to some degree. Unless you have a lot of time and are willing to heat them significantly, don't bother to try to remove them. If they are that tight, chances are decent that they will snap. The good news is that since they are not blind holes, if you do snap a bolt, it is easier to drill out and re-tap.

If the thermostat appears to be working--leave it alone.

The jet in the bottom of the carb LIMITS how much fuel will be delivered at full throttle so, if you were running at 3/4 or so, then it would have no effect on performance.

 

[MickLovin]

Re: Knock when i decellerate 100HP Chrysler

Thanks frank, the thermostat is running fine so I am not touching it . Also I have replaced the jets back to original cause I am a numbskull and swapped them with the 75hp ones which I shouldn't have done.
Frank, may I ask, if the jet was out completely floating in the bowl, would that stop me from planing?

 

[Frank Acampora]

Re: Knock when i decellerate 100HP Chrysler

Remember: The carb venture can deliver way more fuel than the engine needs. The purpose of the jet is to LIMIT fuel delivery at full throttle.

If you were running at full throttle in order to plane, YES! With the jet missing, the orifice now becomes about 1/4 inch, allowing as much fuel flow as the venturi and dip tube can deliver. The top carb would be so rich that power would be severely reduced. It would have very little effect at idle, fooling you into thinking the engine is running well, because the low speed needle and metered idle /off idle orifices control fuel flow. At idle/off idle, there is not enough air flow for the venturi to function. THEN, once the venturi does come on, it will get progressively richer as airflow increases--until limited by the high speed jet.

The jets in the 75 HP carbs are either .064 or .066 depending upon model. The jets in the 100 HP carbs are .080. Swapping 75 HP jets into 100 HP carbs will be so lean that the engine will lose power--so much so that it probably will not even detonate and cause damage. Even above 6000 feet altitude, the recommended jet size is .072. I don't want to do the math but if you want to do so, I would bet that .072 is probably twice the area of .066

In both instances, there will not be enough power to plane.

How's that for a little carb tutorial?

 

[MickLovin]

Re: Knock when i decellerate 100HP Chrysler

ok new update, took boat on water with proper jets back in and it went to plane fine, ok heres the thing, it still has a like a piston slap when decelerating out of gear, and it was surging sometimes off and on every 10 - 20 seconds. I took off the hood and found some water condensated in the bowl and the carbies were really cold??? At the moment though I am more concerned about the knock on decelleration

 

[MickLovin]

Re: Knock when i decellerate 100HP Chrysler

Hey Thank you frank as usual you are wealth of knowledge. Thanks for the sizing I am going to find some jets as even the ones in it look smaller than the 75hp ???

 

[MickLovin]

Re: Knock when i decellerate 100HP Chrysler

ok new update found jets which were actually still on my 75hp so I hadn't changed them properly anyway, I noticed that they are a fair bit larger and one had the 080 mark still on it, but one didn't and it happened to be the one which was half clogged full of white crud, so I cleaned the jet with a suitable drill bit which was perfect size for jet and got all the white gunk out so I am having another test today and Ill let ya know what happened, stay tuned!

 

[MickLovin]

Re: Knock when i decellerate 100HP Chrysler

Thank you frank for letting me know the sizing.

 

[MickLovin]

Re: Knock when i decellerate 100HP Chrysler

I think I am in love with my 100hp Chrysler, after all the mucking around and 2 water tests, I have finally got this baby running correctly with ample power now. might be chewing a little petrol so maybe a fine tune of the carbies next run.

 

[jerryjerry05]

Re: Knock when i decellerate 100HP Chrysler

If it's using too much gas???
The first place to look is the prop.
Right size? too much prop will make it use too much.
I tried a change from a 17 to a 19. Wanted more speed.
I changed and got about 3 mph more but my usage went up by almost 20-25%
Boat load,and prop and position of motor can affect the usage.
You need to do a run and check the RPM's at WOT.
Your 100 should run at about 5300-5500 at WOT.
No trim/tilt can hurt you too.
And if you don't really need it leave it at home.(the wifes not an option)

 

[MickLovin]

Re: Knock when i decellerate 100HP Chrysler

Thanks Jerry, I actually had a problem out there with the tilt trim not trimming up enough, I have a spare kit from my 1980 75hp chrysler which I didn't use so looks like new O rings for the tilt trim, I will firstly change the oil as I noticed some brown oil coming out the other day after a refill, so I will either bleed it out or just open up the valve body screws and drain the lot, it had a spongie sound a couple of times would it be the o rings, I actually haven't seen any oil leaks. It tilt trims fine when not at plane, seems to struggle a lot when planing or 3/4 throttle and actually dropped down a bit after trying to raise it. Also I had the original Chrysler taco but it doesn't seem to be reading right as it was at 5000 rpm at 3/4, I have a digital which will work so am going to change it.
One last thing, it was a little lumpy towards the end of my last run (only at idle) after running well at full speed, didn't want to do any more adjustments on carbies at this point as it ran really good otherwise. Think I hit around 45mph WOT but a bit inconclusive as the old pitot isn't very reliable, I have a 17" Half Cabin Carribean 1980 and my 75hp was pulling 35 - 40mph but again pitot values.
I also have the motor set up in line with the bottom of the V to the Cav plate should I try lifting it, I did notice however it trying to rev out a bit on a quick turn so not sure here. I did have a lot of spray from my hummingbird sender which I will place deeper down on the V as it is to the outer edge atm, can you put a deflector plate above the pitot and sender of hummingbird to stop spray to motor?
Thanks again Jerryjerry for the check list, will have all this checked out and report back

 

[MickLovin]

Re: Knock when i decellerate 100HP Chrysler

Also I will check my 75hp prop its a 1980 model so will fit just not sure of pitch atm

 

[steelespike]

Re: Knock when i decellerate 100HP Chrysler

It's not unusual for a finely tuned setup to ventilate a little in turns and it is usual to trim down for turns and hole shot.
You could lower the motor but it may cost some speed.If you want really accurate speed indication a gps is best .
I think most cells can get a gps tab.
Do you know your gear ratio?If you get us your prop size,gear ratio,accurate rpm and gps speed we can evaluate your setup.
Even finely tuned your 100 will use about 10 gallons per hour at wot.
Usually it is suggested to make one change at a time and test.To better evaluate the changes.

 

[Frank Acampora]

Re: Knock when i decellerate 100HP Chrysler



The speed increase of 5 MPH is just about what you would expect going from 75 to 100 HP.

When I went from 90 to 125 on my go-fast test bed boat speed increased from 55 to 60. When I went from 90 to 120 on my 15 foot Glastron, I gained 3 MPH.

 

[MickLovin]

Re: Knock when i decellerate 100HP Chrysler

The power is fine as I now have a better speed at 3/4 which is what I wanted, I need to invest in a GPS.
Thanks Steelespike and Frank for responding to my questions and info, it is great to be able to put it out there and have some experience guide you.
The boat started with 60 litres in it, and after 2 short trips I had about 25 litres, this usage could also have been more due to the problems I had to fix up to this point with the carbies. I will first get a gps for accurate speed, change my taco for true rpm. Also what do you mean by a gps tab?
Pretty sure but not positive that my gear ratio is 2:1, ill let you know for sure soon about what prop I have.
Thanks again guys for your help. I am looking forward to playing with this one till its running perfect.

 

[MickLovin]

Re: Knock when i decellerate 100HP Chrysler

Hey frank that seond pic, the boat looks like its hardly touching the water, reminds me of my last boat, a Fletcher Arrowflyte 14" with a 115HP suzuki, when I had it running nice it would hardly touch the water, but the motor was old with bad compression and could never get it to more than 38mph, the Fletcher was a British boat, nice boat though, the boat I have now is a Carribean half cabin an oldy but a goody

 

[MickLovin]

Re: Knock when i decellerate 100HP Chrysler

Hey frank that seond pic, the boat looks like its hardly touching the water, reminds me of my last boat, a Fletcher Arrowflyte 14" with a 115HP suzuki, when I had it running nice it would hardly touch the water, but the motor was old with bad compression and could never get it to more than 38mph, the Fletcher was a British boat, nice boat though, the boat I have now is a Carribean half cabin an oldy but a goodyView attachment 221411View attachment 221412View attachment 221413View attachment 221414