OSO Guru's

My friend and I did a shake down run in his Donzi tonight.

The results were less than spectacular.

The motor is a 350 mag that was just freshened up and a little bigger cam was added. The Timming is set on 32 degrees total advance. (GM HEI Distributor)

The carb is the same 650 double pumper used last season.

Here is what happened in the water tonight.

The engine starts up and idles like a champ. We took it from 700rpms to 3100rpms with a hint of a miss or bobble.
As soon as he tried to get past 3100rpms the the engine bobbled miss fired and made one heck of a backfire thru the air cleaner. We took it back down to 3100rpms and it showed no signs of running poorly.

We have absolutely no idea what would cause this to occur at this particular rpm.

I think that its either the timming or something internal in the carb. Could it be a jetting problems??? We stopped and pulled a plug out, but i dont think we were running hard enough to determine anything from plug color.

I consider myself to be a novice by all means when it comes to diagnosing engine problems, so any help would really be appreciated and well needed.


Thank you
Brian

Hi Brian
I had a freshened 454, told I should haVe kept the RPMs over 2000 on start-up. Had the same problem. It turned out to be the exhaust lobes on the cam. Hope this helps.

It sounds like a lean condition. Here is what I would look at. First , you say a little bigger cam, did you jet up a bit with for the cam? It also may be the power valve. Check what valve is in the carb. Then take some vacuum readings while under way to see where the valve opens up. It may be opening to late causing your lean condition. If it pops at 3100 rpm, I would concentrate my readings there. The motor may want the valve to be opened there.

I would put a fuel pressure gage on it. Sounds like an obstruction in fuel delivery and the float is going dry. Gage will show that. Could also be a low float level which is easy enough to check. You should replace the power valve now no matter what due to the backfire through the carb. That is unless it has the blow out preventing ball installed.

BT .

Also if you made no carb changes.
I would say also strong chance you may have the lifters to tight.
Go through all the rocker arms and recheck your clearances as per cam manufacturers recommendations.

Sound lean to me. Check fuel pressure. Second guess would be timing , even though you say it's 32*total. I would recheck and make sure all there marks are correct. But really sounds like it's starving for gas.

Since it's anything over 3200 rpms my guess would be that you have a lean condition when the secondaries open up. I found a problem on a boat I worked on a few years ago, it was a clogged needle/seat

How big of cam did you put in this new motor?
To much cam and to little compression will give you this kind of results.

454 would backfire after being at WOT for 1-2 mins ... I put merc fuel treatment in tank ..one quart of MMO in the oil ...changed out the fuel filter ... installed new plugs .. and prob went away ....

Thank You All!!!

The ideas and input are a real help.
After hearing your input is seems as if when the secondaries of the carb start to open it leans out the engine.

I know my buddy left his carb sitting on a shelf all winter in his garage, so mabey it got knocked around or out of adustment.
Or some crap settled in seat and spring.

The cam change was from a stock 350 mag cam to a extreme marine , 490 lift cam.

I really think that his carb has to be taken apart. I'm going to let him borrow my 750 double from my old engine and have him hook it up and see if that changes anything.

Thanx again everybody

Brian