Recently my 670 pilot started to run lean and would occasionally hang and not immediately come back down to idle. I suspected the intake to be cracked again, and sure enough i was right. They all end up cracking in the same spot, and i cant figure out what exactly is causing it. I believe i have posted here before about this same issue, but this time i wanted it to have its own thread. Ive got 2 theorys.

1. The weight of the carburetors is too much sitting on top of this thin intake manifold causing the cracks. When this intake was used in the SeaDoo, they had a brace that ran from the top of the carburetors to the head. I have tried to copy this on my setup using a bent piece of aluminum that bolts to the top of the carbs, and uses longer studs on the thermostat housing to attach to the head. I thought this solved my issue since it worked great for quite a while, so maybe the weight is not the issue?

2. The 670 Engine in a buggy has a greater amount of Engine harmonics that are rattling the aluminum intake apart. Ive never road a JetSki, but i assume the water makes a fairly good vibration damper?



Since this is not my first time breaking one of these, its one of the spare parts i always keep on the shelf so i wont have much downtime, but i would like to try and remedy this issue so it does not happen again. Ive been thinking of filling the upper center cavity (top in 3rd picture) with an epoxy or JB weld to try and hold it all together.

What are your thoughts? Am i overlooking something? Im taking some time to rewire some things and build an input shaft support bearing for my gearbox, so i have a little time before i will be bolting the Engine back in the frame.

When I look at those pictures I see a design issue.
I see a carb intake that is high up and when you mount the carbs there is a lot of weight there up high.
The G forces when you hit a hole at 60 mph is tremendous --- ask me how I know .
I see two bolts that hold down this intake onto a plate and there is a big gap between them. There should have been another bolt in between.
I see bolt holes that have NO ribs on the side to offer support and therefore the bolt flange will just snap off from the weight of the carbs and the G forces. That flange needed ribs for support and fracture resistance.

You also stated that you had a bent piece of aluminum brace to copy the original factory seadoo set up.
I bet money your bent aluminum just straightened out from the G forces and had limited holding power.
The way I see it is that it would have to be steel and it would also have to be mounted to the top of the flange and then to something on the Engine. Aluminum aint gona cut it unless it is perfectly straight.

JB Weld
I love this stuff but I don't feel it's gona work in this application.
It's just gona crack and flake off. Waste of time in my opinion.
CO

It’s the harmonics combined with the unsupported weight. You’re brace needs to be H shaped with two mounting points on the intake and two on the block to give u the proper bracing. I’d go one step more and use a couple vibration reducing dampers where it mounts to the intake.

I have a Polaris 1050 triple jet ski and u can’t feel the harmonics however I would put my Zune “digital music player” in my glove box to to protect it. Well the harmonics would actully cause an internal fault in my zune. I wrapped it in foam and it never happened again.

was the cracked intake resolved with a fix and what was the fix, last trip out I found a crack in my manifold, no problem I've got my spare Engine with the same setup, got home and checked it out and that one was broken also.
Was hoping you found the cure but I did not find another post.

Its a problem that Seadoo had back in the 90s with Jetskis (Intake manifolds cracking etc). Seadoo had a recall out back then and installed braces to deal with the excessive weight and cracking issue. Some folks didn't get the bracket and so their motors failed later on when the intakes finally cracked and they didn't know.

Here is a photo I believe of Seadoo's aftermarket bracket, for a Seadoo jetski using aftermarket flame arresters (Air filters).

Note, even the single carb'd Seadoo jetski twin engines had cracking issues.

AND - on a jetski, sometimes they ride on smooth glass water, and other times they jump off 8' waves. Slamming into wakes and waves no doubt breaks motor (( Internal Combustion Engine ? )) mounts a LOT.

thanks, I'll have to see what I can setup, not running the seadoo head and different intake will make it a bit more of a challenge