350 backfire question

[gwhjr98]

350 Vortec 310HP Indmar engine. When approaching 4400 RPM. Single backfire in engine. New install. What would be cause of problem? <br /> 2nd question.. I installed this engine to replace a Merc 305 220HP.(outdrive did not change its an Alpha 1 1998) With the increase in HP I would assume that I would have seen a higher RPM therefore an increase in speed that I would have to adjust with a different prop but it seems the RPM may be lower/equal to my old motor which ran around 4800 RPM.<br />Thanks...

 

[Elk Chaser]

Re: 350 backfire question

First I would double check the timing at idle and then check the advance timing at full RPM (WOT). Make sure they are within spec. <br /><br />Is the carb and distributor brand new also? That would be the next things to check.<br /><br />I think you may need to regear your outdrive for the extra HP, each outdrive is geared for the specific HP of the engine.... Not real sure how much of an increase in HP would require the change in gears. I'd check into this before changing props.

 

[tylerin]

Re: 350 backfire question

1 Lean, and or timing<br />2 Lean and or timing,Let her break in

 

[gwhjr98]

Re: 350 backfire question

Thanks for the response.. The engine was a complete replacement. The engine has a new holley 4 barrel. carb and dist.

 

[cc lancer]

Re: 350 backfire question

You may have 1 intake valve that is to tight, if the engine cranks O.K. and runs good up through the RPM band, then I would lean towards the valve.<br />The valve was adjusted a little to tight when the engine was built, when the engine get towards the WOT throttle position, the valve does not have time to close,which will cause it to backfire.

 

[gwhjr98]

Re: 350 backfire question

Thanks for the pointers..... I guess I will start with the timing first. The engine seems to run fine. After the first 2 times it backfired I have have been running it below the 4400 rpm (ok as long as the 4 barrel does not kick in)for the past few running hours as I did not want to damage the engine. If it is the valve then I assume I will have to find out which side... Any chance it would loosen up on its own? I would like to avoid pulling the heads on a new engine.

 

[cc lancer]

Re: 350 backfire question

If it was my engine, or one that I had installed, I would keep the RPM's to a max of 3800, and put some time on the engine.<br />The reason I said the valve, is because I have seen this occur before, and some times it is a pain to find the one that is leaking.<br />Remember this, the cam, lifter, push rod, rocker is new and we are talking maybe 15 thousands of an inch.<br />All you have to do is pull the valve cover, find the one that is tight and back off half a turn.<br />Putting a load on the engine (starting from stopped] and running the engine up, will make every thing "wear" in quicker than just running along at mid range RPM.<br />Personally I think the problem will go away. If it doesn't, post again.

 

[gwhjr98]

Re: 350 backfire question

Will do.. Thanks for the info... I will post my results.. Thanks again..

 

[ron7000]

Re: 350 backfire question

I agree with everything stated,<br />on a new motor install you should verify the timing is set and working properly. Don't trust it was set perfect at the factory. You should find that statement somewhere in the break-in section of the owners manual, if you got one with the engine.<br /><br />How many hours you have on the new motor? It's usually recommended to avoid wide open trottle for the 1st 20 hours or so. Maybe now we know the reason?<br /><br />Where'd you get the new motor, and how much?<br />What make distributor?<br />What kind of boat is it in?<br />Sorry for the questions, I'm curious.<br /><br />With the backfire situation, and break-in maintenance in general, it's a good idea to pull spark plugs and check for proper combustion. If you find a white or super clean plug and the rest are sootier, then you know which cylinder is the culprit. In my opinion the holley carb runs rich so unless you have something wrong with the carb, I'd put my money on the valve.

 

[ron7000]

Re: 350 backfire question

just came across a couple memories from my install. Check your fuel pump, fuel pressure at w.o.t. and double check your fuel lines, filters, and pickup tube in tank. I had gunk partially clogging my pickup tube which was enough to restrict fuel flow at w.o.t. and caused heavy backfiring. Drove me nuts trying to find that one.<br />I mention the fuel pump because my replacement motor did not come with new fuel pump. I had to use my old one, which had been rebuilt though.<br /><br />Rig up a fuel pressure gauge if its not too hard, between the fuel pump and carb, and verify you're still getting at least 3-4 psi at w.o.t. and 5psi or better everywhere else. If not, you might have a bad pump- not able to supply enough fuel and causing the backfire at wot.<br /><br />If the floats are not set correctly, drop in particular, I believe that may cause the lean condition; not letting enough fuel in at wot. I'd have to reference my holley books to be sure, just thought I'd mention in case the problem doesn't correct itself and you go troubleshooting.

 

[gwhjr98]

Re: 350 backfire question

Ok.. checked the timing today. It was way off.. high end. Reset to 8 degrees and everything is running great... Now I do need to look at the next step up in prop. 19 pitch is not going to do it. I am holding a lot back at 4800 RPM.<br />To reply to your questions.. around 22 hrs on the motor. <br />Where'd you get the new motor, and how much?<br />Indmar 350 - ebasic.com. $3300. Its a complete engine by Barr marine. Barr has been a great deal of help.<br />What make distributor? Not sure.. its a holley 4150 carb. <br />What kind of boat is it in? Larson 226 sei. Going to try a 21 pitch. <br />Thanks to everyone for your help..