WOT RPM Loss

[Don S]

Could be timing, have you checked the base timing and the advance timing to make sure the mechanical advance is working? ? ? ? ?
May also be a lean fuel condition. Here is a method of testing the fuel system.

http://www.iboats.com/bbboard/bbBoard.cgi?a=viewthread;fid=24;gtid=1107009

 

[baykrboy]

WOT RPM Loss

hey all, I'm back again!!! My boat is timed perfectly and sounds great!!!! I have one issue though. I am still backfiring through the carb. I rebuilt the carbs and replaced plugs, rotor, condenser, points, wires etc. This is the only issue. I have a feeling it is a carb issue but can not for the life of me figure it out. Any ideas I am able to get the boat to 4100 RPMs then it planes and after about 2 minutes it begins to throttle down to 3100 RPMs and then if I crank the throttle to WOT it backfires through the carb. Need help. Thanks!
It is an OMC 460 7.5 Liter engine with an OMC KIng Cobra drive. The carburetor is a 4 barrell Holley.

 

[Bondo]

Re: WOT RPM Loss

Backfires at WOT is Usually caused by a Lean condition.......

I'd say your Carb Might be Junk........

 

[baykrboy]

Re: WOT RPM Loss

Thanks for the help. I took the carb off once more and replaced the Power valve and the secondary Vacuum diaphram. The boat runs great at low rpms and sounds great. I will try it at WOT today again and repost. Thanks!

 

[jason9080]

Re: WOT RPM Loss

i have the same motor and the same problem. it comes and goes and i dont understand it. i have rebuilt the carb twice. i only get about 3500 rpms and 40 mph from a boat that is supposed to go 60mph. and i do get random backfires out of the carb. i have tried everything im lost.

 

[speedholes]

Re: WOT RPM Loss

Sounds to me like maybe a float or fuel pump problem. It can keep up at lower speeds, but at WOT it sucks the bowls dry, causing the lean condition, and hence the backfire. Make sure the pump is supplying sufficient pressure all the time. Might be a float adjustment as well, I had this problem on my ATV and it went away with a carb rebuild.

 

[Mahoney]

Re: WOT RPM Loss

I had this problem as well and have seen that it is definitely caused by a lean condition when your secondaries are engaged.

Is there a tension you can set for your secondary butterfly valves? its possible these are opening to easily when your throttle engages the secondarys and sucking in too much air. I also had to bend my hanger( I have a quadrajet) so that the secondary needles set up a bit higher in the jets allowing more fuel.

 

[baykrboy]

Re: WOT RPM Loss

Ok here is the latest....
I took it out and same problem. Can't get to full throttle. No backfiring though. Timing is perfect. Everything in the motor is awesome. I am going to change the float levels this evening until it floods a bit then back just a bit and will try again. Stay tuned. If this doesn't work I am going to connect my electric fuel pump to erradicate fuel delivery issues.

 

[baykrboy]

Re: WOT RPM Loss

OK just returned. Still no resolution. I increased the float measurement and it didn't help. I connected up my electric fuel pump and same issue. I am completely stumped.....any ideas????

 

[Don S]

Re: WOT RPM Loss

I am completely stumped.....any ideas?

Yes, I have an idea. Check the fuel system like I linked you to in my first reply. All you accomplished was to try some hit and miss techniques that missed.

 

[baykrboy]

Re: WOT RPM Loss

Here is one more piece of info. I remove the carb again today and the seat and needle in the primary is different from the seat and needle in the secondary. I going to the auto parts store tomorrow to get a fuel pressure/vacuum gauge as you suggested and will post any findings

 

[WanabeBoater]

Re: WOT RPM Loss

Here is one more piece of info. I remove the carb again today and the seat and needle in the primary is different from the seat and needle in the secondary. I going to the auto parts store tomorrow to get a fuel pressure/vacuum gauge as you suggested and will post any findings

 

[baykrboy]

Re: WOT RPM Loss

Ok here is the latest.
I am geting good vacuum and fuel pressure. The only aspect I wasn't able to test is the fuel pressure and vaccum at WOT. Vacuum at that point should be nil. I tried the carb cleaner spray on the Intake mainfold seamsand carb base and no increase in RPMs which indicates no vacuum leakage. I am now leaning toward the lean mixture in the secondaries due to a possible metering issue of the fuel pickup may have a partial block whereby at WOT not getting enough fuel.
The process I will employ is fuel pressure check at WOT first tonight and then I will see if I use my compressor to blow out the channels to the secondaries to see a difference can be made. Any other ideas?

 

[Mahoney]

Re: WOT RPM Loss

Like I said before I had a problem just like this with my boat and it was centered around the secondary circuit on my Rochester Quadrejet. I am unfarmiliar with Hollies, and I believe those "should" actually be less finicky than the Rochesters, but again I dont know about Hollys.

Have you pulled some plugs recently immediately after encountering the problem and shutting down? This may or may not show your lean condition, but is an easy check.

You also didn't say if your timing advance was working properly. If you have mechanical timing advance those weights can get gummed up pretty good and not provide the correct max advance at throttle.

 

[baykrboy]

Re: WOT RPM Loss

It is definately a lean condition that is causing it. It is the secondaries aren't getting enough fuel. I thought that the weights and springs in the distributor might be a problem initially but then ruled it out because I can get to 4100 RPMs for a bit before the engine starts to bog down and drop me to 3000 RPMs. Do you know of a way to identiofy if the springs and wieghts are good? I did however increase the throttle and noticed the advance happening. Is that enough?

 

[calwldlif]

Re: WOT RPM Loss

I read this post a couple of times
You recieved good advice and.......................

Reminds me of so many other posts
where someone asks for help and
DOESN'T do what is advised:%

You have in your mind something and are blind to anything else.:'(

You start out saying your back?
Is there some prior problem you asked about?

I haven't read where you tested anything..........
Put some parts in and still don't know whats what.

I imagine that not to many people will help when
you are going to do what you want anyway:/

There are more than 1 system that can
influence performance.....................

That combo drive/eng are not new...............
I recommend you regroup and post a systematic
recap of what you have checked and what you
have not

 

[baykrboy]

Re: WOT RPM Loss

Ok here it goes.
I p/u the boat in Sept of last year, and this year it drove fine when I put it in the water with the exception that the timing was too advanced and once in a while I would get a backfire when accelerating. I attributed it to a carb issue. The next day I went down to the boat and it wouldn't start. No fuel delivery. I then verified delivery to the carb but still none into the bowl. I pulled off the carb and found the seat blocked. I then rebuilt the carb. Once rebuilt I figured it best to do some proactive maintenance. I replaced the plugs, rotor, condenser, points, cap and set the timing. This thinkgs ran amazing and sounded awesome as long as I didn't go WOT. Once I tried to go to 4000RPMs or close it would bog down and sputter after almost planing off.

This is where I started this thread. I have run the tests outlined above in the prior posts. I am still having these issues with WOT and planing. I will tonight do the following:
1. Empty the tank to eliminate any bad fuel issues
2. Remove the tank Pickup and inspect
3. I will then also remove the carb once more and using a compressor blow out all channels.
4. I will then reinstall all the components one last time and take her for a spin with the fuel pressure gauge and verify the proper reading is being achieved at WOT

If none of that shows any issues I will then start looking at the distributor as the issue.
How does that all sound to someone out there? Is it a good plan? Is there a way to identify the weights and springs in the distributor easily?

Thanks for all the help!

 

[Bondo]

Re: WOT RPM Loss

Well,.............#3 Isn't going to cut it.........

When dealing with Old Boats,.........
REBUILDing the Carb is a 50/50 Shot,....... At Best.......
This is due to the Fact that Water tends to find it's way into the Damnedest Places on a Boat......... Especially the Carb.......
And Water does Strange Things to Aluminum,..... Which of course your Carb is made of......
With a Total Disassembley of the Carb,+ a Good Soaking, Over Night, in a Quality Carb Cleaner Bath,..........
It Might Possibly free up all the Crud that builds up in Old Carbs on Old Boats,..... Then you could Blow Out ALL of the passages in the Carb,+ Reassemble it with New Parts(Ayuh,.. the Kit)........
With ALL that Work,....... The Odds are Still 50/50............

Leave the Ignition Alone...........
You've Been Fighting a Fuel Delivery Issue from the Start,......
And it's Beating You......
It Runs Perfect til you Need Lots of Gasoline........ You Already Know the Ignition is Right........
I Think you Might need a New Carb,.......... Or,... Maybe a Rebuild at the Least,........
But,..... Until you Scatter that Carb across the Bench..........
There Ain't Nothing gonna go WOT.............

 

[calwldlif]

Re: WOT RPM Loss

I would agree with Bondo but
it would be Useless.

I see where you have rebuilt the carb
and believe you know how.
Now that you give more symptoms I agree
that fuel starvation sounds possible.
Quick check with external tank and fresh gas
would help your cause.
I don't see you talk about filters?
If none get one or two.

 

[baykrboy]

Re: WOT RPM Loss

Hey, I baaack. I wanted to share what happened last night as I think to have resolved the issue. I was in the boat and checked the gas and it was fine. No water separation when dumped into a glass jar so I didn't empty the tank. I thought to then adjust the floats slightly higher than the manufacturer settings. Once I pulled off the carb I decided to also check all the filters one more time. The fuel inlet port of the carb has a small screen in it as the fule enters into the seat. It was gummed up beyond belief. I had changed it when I did the carb rebuild but somehow crap got in there. I only checked it again because I saw small pieces of crud in the bowls again and flipped out.
Once I cleaned it ( and boy did I spray the **** out of it) I reassembled the whole thing and this thing flew!!!!!!
Thanks to all for all your help and advice. I think it was a combination of efforts that helped to resolve this and appreciate everyone's assistance.

Who'da thought I had a problem wit gas..................d